CHARACTERIZATION OF MICRO-CAPILLARY WICKING EVAPORATORS
TIFFANY ANNE QUY
A thesis submitted in partial fulfillment of
the requirements for the degree of
MASTER OF SCIENCE IN MECHANICAL ENGINEERING
WASHINGTON STATE UNIVERSITY
School of Mechanical and Materials Engineering
DECEMBER 2006
To the Faculty of Washington State University:
The members of the Committee appointed to examine the thesis of TIFFANY ANNE
QUY find it satisfactory and recommend that it be accepted.
Chair
ii
ACKNOWLEDGEMENTS
I would like to thank my office mate Leland for his RTD circuitry design,
modeling, and engine testing. Dr. D.J. Morris helped me with chrome mask designs and
fabrication procedures. Nazmul Mamun also helped me develop fabrication procedures
for thick, high aspect ratio SU8 resists. I would like to thank Steve Brown and Dawn
Findley who have been great at their efforts to keep the cleanroom up and running. I
would like to thank Coralee McCarter who walked me through the SEM to take wick
pictures. The MME machine shop has been continually helpful in the fabrication of
different testing devices, specifically the machinists Henry Ruff and Robert “Kurt”
Hutchinson who always take their time out to help. I would like to thank Dan Carpenter
for his training and getting me started in this work. I would like to thank HoKi Lee for
his modeling work and writing. I hope he has learned from me as much as I learned from
Dan. I would like to thank the rest of the P3 team, past and present, for being there.
Finally, I would like to thank my advisors, Dr. R.F. Richards, Dr. C.D. Richards, and Dr.
D.F. Bahr, for the support and encouragement through my master’s program.
iii
CHARACTERIZATION OF MICRO-CAPILLARY WICKING EVAPORATORS
Abstract
Tiffany A. Quy, M.S.
Washington State University
December 2006
Chair: Robert F. Richards
Micro-capillary channels can be used as an effective heat dissipation method. In
this work, concentric resistance temperature detectors (RTDs) are used in conjunction
with cooling channels to measure steady state evaporative efficiencies. The geometry
and dimensions of the channels are varied to determine how these factors affect the
evaporation capacity of the channels. Engine assembly tests are performed to determine
how channel dimensions affect the dynamic operation of evaporator membranes with
cooling channels. The channels are fabricated from SU8 at heights of 10, 40, and 70 μm.
The SU8 walls have widths of 5 and 10 μm. Channel widths range from 10-90 μm. In
general, the 40 μm high SU8 channels outperform the 10 and 70 μm heights in both
evaporative and dynamic efficiencies. The 5x40 μm SU8 features with 70 μm channels is
found to have the highest overall performance. These dimensions yield a mass
evaporation rate of 8.3 mg/min and dynamic efficiency up to 0.132% at a frequency of 40
Hz and input energy of 14.4 mJ. For 70 μm high SU8 structures, the 90 μm channels
show the highest performance. These dimensions yield maximum mass evaporation rate
iv
of 7.2 mg/min and a dynamic efficiency up to 0.011% at a frequency of 40 Hz and input
energy of 11.8 mJ. For 10 μm high SU8 structures, the 35 μm channels show the highest
performance. These dimensions yield a mass evaporation rate of 9.2 mg/min with
dynamic efficiency up to 0.021% at a frequency of 40 Hz and input energy of 10.7 mJ.
v
TABLE OF CONTENTS
ACKNOWLEDGEMENTS.............................................................................................iii
ABSTRACT.....................................................................................................................iv
TABLE OF CONTENTS.................................................................................................vi
LIST OF FIGURES .........................................................................................................ix
LIST OF TABLES...........................................................................................................xii
CHAPTER ONE: INTRODUCTION.............................................................................. 1
1.1
Motivation................................................................................................. 1
1.2
Background ............................................................................................... 2
1.3
1.2.1
Capillary action in rectangular channels....................................... 4
1.2.2
SU8 fabrication ............................................................................. 6
1.2.3
Visualization ................................................................................. 10
Research Objectives.................................................................................. 11
CHAPTER TWO: FABRICATION ................................................................................ 12
2.1
Fabrication Steps....................................................................................... 12
2.2
Processing Bulk Wafers............................................................................ 12
2.3
Patterning Wafers for Membrane Etching ................................................ 13
2.4
Front side Patterning of Measurement Tools............................................ 15
2.5
Annealing Wafer....................................................................................... 17
2.6
Fabricating Wicking Structures ................................................................ 17
2.7
Wet etching to Define Membrane Structures............................................ 23
CHAPTER THREE: EXPERIMENTAL SET-UP AND PROCEDURES ..................... 25
3.1
Experimental Set-Up and Equipment ....................................................... 25
vi
3.2
3.1.1
Die carrier set up........................................................................... 25
3.1.2
Calibration..................................................................................... 26
3.1.3
Energy dissipated in resistance heater .......................................... 26
3.1.4
Conduction across membrane ....................................................... 27
3.1.5
RTD testing box............................................................................ 29
3.1.6
Energy into evaporation................................................................ 29
3.1.7
Visualization ................................................................................. 30
Experimental Procedures .......................................................................... 32
3.2.1
Die carrier assembly ..................................................................... 32
3.2.2
Calibration procedure.................................................................... 34
3.2.3
Steady state conduction tests ........................................................ 36
3.2.4
Steady state evaporation tests ....................................................... 38
3.2.5
Visualization ................................................................................. 39
3.2.6
Uncertainties ................................................................................. 40
CHAPTER FOUR: RESULTS ........................................................................................ 44
4.1
Summary of Tests Performed ................................................................... 44
4.2
Steady State Evaporation Tests................................................................. 44
4.2.1
Variation of wick geometry .......................................................... 44
4.2.2
Increasing channel width .............................................................. 49
4.2.3
Increasing channel height ............................................................. 50
4.2.4
Silicon nitride membranes ............................................................ 55
4.3
Engine Efficiencies ................................................................................... 56
4.4
Flow Analysis ........................................................................................... 58
vii
4.5
Visualization ............................................................................................. 61
CHAPTER FIVE: CONCLUSIONS ............................................................................... 64
APPENDIX A: SU8 FABRICATION............................................................................. 66
APPENDIX B: CALIBRATION TEST RESULTS ........................................................ 72
APPENDIX C: CONDUCTION TEST RESULTS......................................................... 80
APPENDIX D: EVAPORATION TEST RESULTS ...................................................... 85
APPENDIX E: VISUALIZATION ................................................................................. 89
REFERENCES ................................................................................................................. 92
viii
LIST OF FIGURES
Figure 2.1: KOH pattern for 5mm square membranes .................................................... 15
Figure 2.2: Illustration of platinum lift-off process ......................................................... 16
Figure 2.3: Photolithography mask for resistance heater and dual RTDs ....................... 17
Figure 2.4: SEM example of SU8 wicks .......................................................................... 18
Figure 2.5: AutoCad wick template ................................................................................. 19
Figure 2.6: SEM wick picture with 40 μm high, 5 μm thick SU8 walls and 70 μm wide
fluid channels .................................................................................................................... 21
Figure 2.7: SEM wick picture with 70 μm SU8 height, 5 μm thickness and 35 μm fluid
channels............................................................................................................................. 22
Figure 2.8: Overhead view of completed silicon membrane ........................................... 24
Figure 3.1: Evaporation experiment schematic .............................................................. 26
Figure 3.2: Circuit schematic for measuring total power dissipation .............................. 27
Figure 3.3: RTD and resistance heater schematic............................................................ 28
Figure 3.4: Schematic of RTD testing box circuit ........................................................... 29
Figure 3.5: Visualization set-up....................................................................................... 31
Figure 3.6: Completed carrier assembly .......................................................................... 34
Figure 3.7: Picture of calibration experiment .................................................................. 35
Figure 3.8: Example of test calibration............................................................................ 36
Figure3.9: Evaporation experiment test set-up ................................................................ 38
Figure 3.10: Picture of visualization tests........................................................................ 40
Figure 4.1: Wick Pattern A – Combination of radial and annular fill channels .............. 45
Figure 4.2: Wick Pattern B – Staggered radial fill channels............................................ 45
ix
Figure 4.3: Summary of evaporation results for annular and radial wick geometries..... 47
Figure 4.4: Visualization of wick dry out on wick pattern A with 22 mW of power input.
For this picture, wick dimensions are all 10 microns ....................................................... 48
Figure 4.5: Visualization of wick dry out on wick pattern B with 22 mW of power input.
Channels are 35 μm wide with 5x10 micron SU8 structures............................................ 50
Figure 4.6: Calculation of wick efficiencies for 40 and 70 μm wall thicknesses for 50
mW of power input assuming constant evaporation rates ................................................ 52
Figure 4.7: Comparison of mass evaporation to power dissipation from the internal
resistance heater throughout experimental range. Legend shows wick dimensions (SU8
thickness – channel width/height)..................................................................................... 53
Figure 4.8: Comparison of average mass evaporation rates ............................................ 54
Figure 4.9: Engine efficiency tests at 20 Hz .................................................................... 57
Figure 4.10: Engine efficiency tests at 40 Hz .................................................................. 58
Figure 4.12: Variation of fluid channel dimension and its effect on fluid fill Rates ....... 61
Figure 4.13: Visualization of wick dry out at various power inputs, wick pattern A...... 62
Figure 4.14: Visualization of 35 μm Channels for 10 μm SU8 Thickness...................... 63
Figure B1: Calibration of die 1093 .................................................................................. 72
Figure B2: Calibration of die 1124C ............................................................................... 72
Figure B3: Calibration of die 1129F................................................................................ 73
Figure B4: Calibration of die 1307J................................................................................. 73
Figure B5: Calibration of die 1307M............................................................................... 74
Figure B6: Calibration of die 1307N ............................................................................... 74
Figure B7: Calibration of die 1421B ............................................................................... 75
x
Figure B8: Calibration of die 1397A ............................................................................... 75
Figure B9: Calibration of die 1397B ............................................................................... 76
Figure B10: Calibration of die 1397D ............................................................................. 76
Figure B11: Calibration of die 1396F.............................................................................. 77
Figure B12: Calibration of die 1411A ............................................................................. 77
Figure B13: Calibration of die 1411B ............................................................................. 78
Figure B14: Calibration of die 1411C ............................................................................. 78
Figure B15: Calibration of die 1411D ............................................................................. 79
Figure B16: Calibration of die 1424A ............................................................................. 79
Figure E1: 5x10 μm SU8 walls with 50 μm fluid channels at 50 mW power input........ 89
Figure E2: 5x40 μm SU8 with 70 μm channels at 100mW power input......................... 89
Figure E3: 5x40 μm SU8 with 70 μm channels and 120mW power input...................... 90
Figure E4: 5x70 μm SU8 with 70 μm channels – Fluid channels remain filled throughout
77 power input ranges....................................................................................................... 90
Figure E5: 10x10 μm SU8 with 10 μm channels – Overview of completed wicks and
measurement tools ............................................................................................................ 91
xi
LIST OF TABLES
Table 4.1: Table of wick efficiencies for varying geometries ......................................... 46
Table 4.2: Effect of small channel width variation.......................................................... 49
Table 4.3: Wide channel wick efficiencies with SU8 dimensions 5 μm wide and 10 μm
high
............................................................................................................................... 49
Table 4.4: Wick efficiencies for 40 and 70 μm high channels ........................................ 51
Table 4.5: Conduction result for silicon nitride membrane ............................................. 55
Table 4.6: Evaporation result for silicon nitride membrane ............................................ 55
Table 4.7: Results of fluid fill rate analysis for varying geometries................................ 60
Table C1: Conduction tests for die 1124C....................................................................... 80
Table C2: Conduction tests for die 1129F ....................................................................... 80
Table C3: Conduction tests for die 1307J........................................................................ 80
Table C4: Conduction tests for die 1307M...................................................................... 81
Table C5: Conduction tests for die 1307N ...................................................................... 81
Table C6: Conduction tests for die 1308G ...................................................................... 81
Table C7: Conduction tests for die 1421B....................................................................... 81
Table C8: Conduction tests for die 1421A ...................................................................... 82
Table C9: Conduction tests for die 1423A ...................................................................... 82
Table C10: Conduction tests for die 1397A .................................................................... 82
Table C11: Conduction tests for die 1397B..................................................................... 82
Table C12: Conduction tests for die 1397D .................................................................... 83
Table C13: Conduction tests for die 1396E..................................................................... 83
Table C14: Conduction tests for die 1411A .................................................................... 83
xii
Table C15: Conduction tests for die 1411B..................................................................... 83
Table C16: Conduction tests for die 1411C..................................................................... 84
Table C17: Conduction tests for die 1411D .................................................................... 84
Table C18: Conduction tests for die 1434A .................................................................... 84
Table D1: Evaporation tests for die 1124C...................................................................... 85
Table D2: Evaporation tests for die 1129F ...................................................................... 85
Table D3: Evaporation tests for die 1307J....................................................................... 85
Table D4: Evaporation tests for die 1307M..................................................................... 85
Table D5: Evaporation tests for die 1307N ..................................................................... 86
Table D6: Evaporation tests for die 1308G ..................................................................... 86
Table D7: Evaporation tests for die 1421B...................................................................... 86
Table D8: Evaporation tests for die 1421A ..................................................................... 86
Table D9: Evaporation tests for die 1423A ..................................................................... 86
Table D10: Evaporation tests for die 1397A ................................................................... 87
Table D11: Evaporation tests for die 1397B.................................................................... 87
Table D12: Evaporation tests for die 1397D ................................................................... 87
Table D13: Evaporation tests for die 1396D ................................................................... 87
Table D14: Evaporation tests for die 1411A ................................................................... 87
Table D15: Evaporation tests for die 1411B.................................................................... 88
Table D16: Evaporation tests for die 1411C.................................................................... 88
Table D17: Evaporation tests for die 1411D ................................................................... 88
Table D18: Evaporation tests for die 1424A ................................................................... 88
xiii
CHAPTER 1 INTRODUCTION
1.1
Motivation
The increasing power density of microelectronic devices has driven the search for
a compact and efficient means to transfer heat in micro-scale devices. As the
performance of microelectronics increases and their size decreases, the demand for more
compact and efficient heat dissipation devices increases [1-3]. Different methods being
studied to find effective methods to dissipate heat from electronic devices include aircooling, heat sinks, and spray cooling. Air-cooling is currently used in many devices in
the computer industry. However, these devices are reaching their maximum potential
while chip dissipation levels continue to increase [4]. Finned metallic heat sinks have
been added to these systems to help with heat flow, but they are also becoming
insufficient for projected heat loads [5]. Spray cooling has been proven as a very
effective heat dissipation method. It works by using a micro-pump to supply a
continuous liquid droplet array to cool the processor [5]. However, these devices are
complex, expensive, bulky, and require a power draw from the overall system to operate
the spray pump. Another approach uses the idea of micro-groove channels for electronic
cooling. The channels can be fabricated directly onto the device and use capillary
pressure to draw in fluid directly in contact for cooling. Micro-channels have been
shown as effective heat dissipation devices for electronic components [1]. They are also
relatively easy to fabricate, compact, low cost, and maintain relatively high dissipation
capacities [6-7]. For these reasons, the development of microelectronic components has
been a major contributor to the continued study and development of micro-fluidic
channels.
1
An application example for the current study of these micro-channels is the P3
micro-engine under development at Washington State University (WSU). This device
consists of a cavity contained between two silicon membranes. The lower (evaporator)
membrane transfers heat to the working fluid within the cavity. As the fluid is heated, it
evaporates. The evaporation increases the pressure inside the cavity and deforms the
upper (generator) membrane. As the generator membrane expands, it does work by
mechanical deformation. Micro-fluidic wicking channels are fabricated on the surface of
the evaporator membrane to control the placement of the working fluid in the engine
cavity. These micro-channels are effective in this function because they are small,
inexpensive, and can be fabricated through standard UV lithography directly on the WSU
campus [8-9].
This work is focused on the characterization of two-phase capillary flow through
open, rectangular capillaries. The capillaries under investigation are made from an SU8
photoresist. The rest of this chapter will focus on some of the major challenges faced in
the study of micro-channel devices including their heat transfer capacity, flow analysis,
SU8 properties and fabrication, and visualization.
1.2
Background
Although there are many methods of thermal management in micro-electrical
devices, micro-channels have been shown to be very effective for heat transfer [10]. One
of the benefits of this heat transfer method is that the micro-channels can be integrated
directly into the heat generating substrate. Integration allows the contact resistance
between the channel and substrate to be ignored and is advantageous in modeling and
numerical analysis [10]. The study of heat transfer through micro-channels is still not
2
completely understood, and there are many different methods being explored to expand
the knowledge of these devices. The heat transfer through micro-channels is dependent
on flow method, flow phases present, channel geometry, physical properties, and heat
flux.
Since the early 1980’s, there has been much interest in using micro-channels for
heat dissipation [11]. The initial research of the heat transfer in micro-channels focused
almost completely on single-phase flows. It has been shown that electronics can be
cooled effectively through the forced convective flow of water through micro-channels
fabricated on silicon [3, 12]. However, a major drawback in single-phase flow is that
there is a large temperature gradient in the device from the rise in coolant temperature
[11, 13]. Two-phase flow is promising because it takes less pumping power than singlephase liquid convection to maintain a given thermal resistance [12]. Two-phase flows
also use the latent heat of vaporization of the fluid to increase the convective heat transfer
coefficient and to help maintain a relatively uniform surface temperature of the heated
device [3, 11]. The surface temperature maintained is dictated by the saturation
temperature of the fluid [11]. Two-phase flow therefore can be a more efficient heat
dissipation method when compared with two-phase flows. However the more difficult
physics involved complicates the numerical analysis of these systems.
Past research has also mainly focused on micro-channels with circular or vgroove cross-sections. Changing the cross-sectional geometry of the channel can change
both the fluid flow and the heat transfer characteristics of the channels [13]. The aspect
ratio of a rectangular channel influences flow friction and convective heat transfer
whether the flow is turbulent or laminar [13]. The thermophysical properties of the
3
working fluid can also greatly affect the heat transfer characteristics of the device as
shown in a detailed numerical simulation performed by J. Li et al. [14]. R.H. Nilson et
al. [15, 16] have recently performed a detailed analysis of micro-channels with
rectangular cross sections. In these studies, rectangular capillary action is described and
the affects of aspect ratio, size, and fluid contact angles on the capillary action are
defined. Capillary flow through rectangular micro-channels is discussed more in the next
section.
1.2.1
Capillary action in rectangular channels
Wicks are defined as channels that can pull in fluids through capillary action.
Capillary pumps eliminate the need for active pumping by using the capillary suction of
the wick material [17-18]. This capillary action is a phenomenon associated with the
surface tension energy between the fluid and capillary walls that results from a liquidvapor phase interface pressure differential [5]. The microstructure of a wicking channel
strongly affects the performance of that wick. In general, a material possessing small
pores is desired because it will reduce the minimum radius of curvature of the phase
interface and increase the capillary pressure available to draw fluid. However, small
pores also result in slower liquid transport due to greater frictional resistance [18].
Wicking devices are currently being made through many different microfabrication techniques including electrical discharge machining (EDM), wet and dry
etching, and different lithographic methods. EDM specifically has the ability to create
silicon micro-grooves of triangular, trapezoidal, sinusoidal, and near rectangular crosssections [15]. To date, research has focused on triangular cross-sectional grooves. This
focus has been due to the monotonic decrease in meniscus radius and capillary pressure
4
the geometry provides as the meniscus flows down the capillary channel. However, the
cross-sectional area of the triangular groove is only half that of the rectangular crosssection. The viscous friction is also greater in a triangular groove which reduces the
maximum axial flow rate [15, 16].
Lithography is well suited for wick fabrication because it can form very detailed
and diverse micro-scale features. Lithography process is also suited to make rectangular
or near rectangular cross-sections. Straight, rectangular micro-channels specifically have
a primary disadvantage to triangular grooves in capillary action. The capillary pressure
in a rectangular channel varies with the liquid height in the channel only if the meniscus
remains attached to the top corners of that channel [15]. This attachment occurs at the
capillary opening where the fluid completely fills the channel. As the fluid travels
through the capillary, the fluid surface forms a meniscus at the top of the channel. The
angle between the fluid and the channel increases during the meniscus formation until the
optimum contact angle is reached. Therefore the radius of curvature of the interface can
range anywhere from zero to a minimum wetting angle associated with the fluid to solid
interface energy. At this point, the angle remains constant and the meniscus moves down
the channel wall. In this region a “dead zone” is formed in which there is no capillary
pressure differential and the fluid may be no deeper than half the channel width. At the
end of this region, the meniscus reaches the channel bottom and the capillary pressure
gradient is reestablished in the corner flow regime [15, 16]. Corner flow continues until
the channel dries out.
Rectangular wicks are therefore better suited for cooling compared to triangular
devices due to their greater surface area and capillary pumping potential. Current work is
5
focused on rectangular cross-sections made from SU8 through a lithographic process.
Mean axial speeds of these rectangular wicks with a low porosity sidewall are analyzed,
and the affect of channel height and aspect ratio on capillary pumping is determined.
1.2.2
SU8 fabrication
SU8 resists are widely used in the development and fabrication of a variety of
MEMS devices. They are useful due to their thermal stability, chemical stability, and
processing ability [18]. Depending on its cure temperature, SU8 can be stable up to
200ºC and decomposes near 340ºC [19]. It is strong, possesses high resolution potential,
high aspect ratio ability, and can form single layers from 2 - 200μm thick. The thin films
can also be layered to form much thicker features. SU8 is near UV sensitive with a high
degree of cross-linking. The specific properties of SU8 enable aspect ratios up to 25:1
with standard UV lithography while maintaining high uniformity with nearly vertical
sidewalls. These abilities make SU8 useful for a variety of applications including coils
and dielectrics for capacitors in micro-electronics, sensors and fast prototyping in micromechanics, biochips, microchips, micro-pumps for micro-fluidics, and stop layers for
electroplating.
SU8 is a photosensitive epoxy based resist. It is considered an epoxy-based resin
because its structure contains at least one 1,2-epoxy group or epoxide. An epoxide group
refers to two carbon atoms that are bridged by one oxygen atom forming an epoxy ring.
This functional group is highly reactive and allows for molecules to convert to a
thermoset or a three-dimensional network structure through a curing process [19].
The fabrication of SU8 is done through a lithography process. SU8 is a negative
resist. In a negative resist, the exposed portion of the material cross-links to become
6
insoluble in the developer solution. The unexposed portion remains soluble in the
developer solution and is removed during this step. The final features on the substrate
therefore become the features exposed to the UV light.
Although the photolithography process itself is well known and common in MEMS
fabrication, SU8 has its own fabrication challenges. Every processing step is critical to
the final product and each step must be individually tailored to the final application.
Over-baking or over development, for instance, can cause internal stresses. These
internal stresses cause weakening of the epoxy layer and cracking can occur. Underbaking yields a low degree of cross-linking which can result in adhesion problems.
Under exposure can cause features to thin out toward their base on the substrate. Such
features are susceptible to over development and failure in sheer.
The resist’s sensitivity to its fabrication process increases with increasing layer
thickness. Optimizing the design processes includes trade-off for various structural
features due to the interrelation of different processing steps. For instance, the initial preexposure or soft bake is important because the surface must be sufficiently dry for the
mask not to stick to the substrate. However, the photo acid formed during development
must be mobile enough to provide uniform cross-linking across the surface of the
substrate. The post-exposure bake is necessary to transfer the exposure image to a stable
structure. The degree of cross-linking is controlled by both of the pre- and post- exposure
bake times and the exposure dosage. The cross-linking is necessary to develop the
desired mechanical properties. However, a material that is too highly cross-linked
becomes brittle and looses adhesion. The bake and exposure times are also influenced by
the structure geometry and are varied accordingly. Relaxation or bench cooling time is
7
required to reduce internal stresses and must be increased with increasing layer
thicknesses [18]. Stepping down the substrate temperature to slow the cooling of the
resist can also help reduce internal stresses.
J.H. Daniel et al. discuss some of the fabrication challenges of SU-8 resists. The
processing of SU8 can cause a variety of difficulties despite the good performance and
many advantages of the material [20]. The problems inherent to the processing include
poor adhesion to the substrate, shrinkage, and brittleness. There is currently much
research involving the modification of SU-8 resists to minimize these issues [20, 21].
The internal stresses inherent in an SU-8 microstructure can result in the cracking
of lithographic features limiting fabrication. These internal stresses are developed in the
photo-generated cationic polymerization process. This process also generates the
desirable high-contrast features and characteristically rigid structure [22]. Much research
is currently looking at different methods to process and develop SU-8 to maintain its
desirable thermal, electrical, and mechanical properties while decreasing the inherent
internal stresses developed during processing. Some studies are focusing on developing
composites [22] while others are focused on developing different fabrication procedures
and studying different property results [21].
Johnson et al [22] researched different ways to improve the process capabilities of
SU-8 resists. They looked at adjusting the compositions of SU-8 epoxies to reduce stress
cracking before and after thermal curing while maintaining electrical properties, low
moisture absorption, thermal properties, and high-contrast ability. For this study, low
molecular weight aromatic/aliphatic epoxies were mixed with SU-8 and developed.
Some of the major properties tested in these composite mixtures included adhesion,
8
resolution, aspect ratio, and feature cracking. Of all the materials tested by this group,
only the polybutadienes functionalized with epoxy groups exhibited photolithographic
properties while having the ability to enhance the desirable SU-8 properties. Other
substances tested included multifunctional urethanes, Bisphenol-A, and aliphatic
polyethers. However all of these substances were unsuccessful in that they formed brittle
materials or lacked photosensitivity.
Another study published by R. Feng and R.J. Farris studied how different
processing conditions can affect the thermal and mechanical properties of SU-8 resists.
Five different variables were varied including soft-bake time, exposure time, postexposure bake time, development time, and different substrate materials. They looked at
SU-8 thicknesses of 50 mm, 100 mm, and 220 mm, and looked to optimize properties
including sidewall profile and film adherence. Feng and Farris found that the SU-8
molecules cross-linked to form a network structure during thermal baking and that the
material transformed from ductile to brittle during this process. They also found that
material shrinkage was a function of baking temperature so that the temperature could
affect internal stresses. Residual stress was also determined to be dependent on
environmental humidity. This effect was reversible by increasing the post exposure bake
time before hard baking. The post exposure bake, in effect, decreased the sensitivity of
the material to humidity. The post exposure bake time also increased the final crosslinking density that decreased the overall toughness of the material [21].
It should also be mentioned that deep X- ray lithography processes such as LIGA
are also being explored for processing SU-8 epoxies. These processes are desired
because they enable deposition on curved surfaces and even higher aspect ratios (up to
9
100:1). However the equipment for LIGA processing is more expensive and less
available than standard UV lithography [1.18]. Therefore, standard lithography is still
the main development process for these resists.
Currently, much research is being done to further develop the processing of SU-8
epoxies. However, since the process is pattern sensitive, trial and error is required for
each individual pattern. The basic recipe described by the chemical manufacturer
(MicroChem) is a good starting point. With small changes to bake times, the adhesion
can be improved and the cracking minimized. The effects of these process sensitivities
are the most dramatic for thicker layers and small features (down in the 5 μm range). If
the Omnicoat release/adhesion layer is used, the same substrate can also be used multiple
times to perfect the process.
1.2.3
Visualization
Flow visualization in micro-channels is an important step toward understanding
what is happening within the channels. It is especially useful in two-phase flow when
attempting to see the motion of the liquid-vapor interface. It becomes even more
important in passive micro-fluidic systems (such as capillary wicking structures) [13].
As the use of micro-fluidic devices becomes more widespread, the need for visualization
tools with spatial resolutions down to the μm level becomes more important. There are
many obstacles involved in obtaining worthwhile images of micro-channel fluid flow.
Problems include the small channel size, lighting requirements, and internal reflection
within the walls of the channels themselves [11].
Visualization of these devices is still being researched and developed to increase
the spatial and time resolution ability. N. Ichikawa et al. [23] used a CCD Camera to
10
visualize capillary wicking motion of rectangular micro-channels of varying sizes. They
were limited to spatial resolutions of 10 μm and time resolutions of 0.033 second. C.D.
Meinhart et al. [24] used an established macro-visualization technique called particle
image velocimetry (PIV). In this experiment, an Nd: YAG laser was pulsed at 5 ns to
fluoresce a fluid mixed with a laser dye. This was used in conjunction with a microscope
and CCD camera. With this experimental set-up, they were able to obtain spatial
resolutions approaching 0.9 μm. This resolution was limited by the diffraction limit of
the recording optics. There was also a higher spatial resolution close to the channel walls
and lower resolution away from the wall.
Although visualization is important in micro-channel analysis, especially
concerning multi-phase flow, very little research is found in this area. Current works are
focused on increasing resolution capabilities and perfecting the visualization process of
micro-fluidic devices.
1.3
Research Objectives
The objective of this study is to use a heat flux meter working concurrently with
micro-capillary wicking devices to determine their overall efficiencies. The operation of
the device is determined through dry conduction tests, and the efficiencies of the wicks
are determined through evaporation tests. Efficiencies are calculated for wicks fabricated
on both silicon and silicon nitride substrates. The wall thicknesses range from 5-10 μm
in width and with heights of 10, 40, and 70 μm. The fluid channels tested have widths
ranging from 10 to 90 μm. Engine assembly efficiencies are also determined for the
varying wick dimensions. The results of the wick and engine efficiencies are finally
compared to recommend the overall most effective wick geometry and design.
11
CHAPTER 2 FABRICATION
2.1
Fabrication Steps
Micro-capillary evaporators are fabricated with an internal platinum resistance heater
and concentric platinum RDTs. To accomplish this, micro evaporators are fabricated
over the platinum features onto a silicon wafer. The complete processing list for the
fabrication of these devices includes:
1) Process bulk wafer;
2) Pattern wafer for membrane etching;
3) Pattern wafer with desired dual RTD with heater design;
4) Sputter wafer with platinum and perform lift-off;
5) Anneal wafer;
6) Fabricate wick structures;
7) Wet etch to define membranes.
Each of these processes is explored in more detail in the following sections. For clarity,
the front side of the wafer refers to the boron doped-side which contains the platinum
RTDs, resistance heater, and wicks.
2.2
Processing Bulk Wafers
Two different processes are used to initiate the fabrication of membranes from 3-
inch stock silicon wafers. One process is required for the fabrication of silicon
membranes. The other process is used to fabricate silicon nitride membranes.
To fabricate silicon membranes, first the front side of the wafer is doped with
boron to act as an etch stop. The thickness of the boron doped silicon defines the final
thickness of the silicon membrane. For these experiments, the final membrane thickness
12
is 2 μm. Next a low temperature oxide (LTO) is grown onto the surface of the wafer to
act as an electrical insulation layer. The silicon wafer is then ready for the next step,
patterning the membranes.
To fabricate silicon nitride membranes, the desired thickness of silicon nitride is
first grown on the wafer. This process is not available at WSU so the bulk wafers are
ordered from either the University of South Florida or WTC. For these tests, a membrane
thickness of 300 nm is used. Silicon nitride is grown on both the front and back sides of
the wafer. The back side acts as an etch stop for KOH, and the front side forms the final
membrane.
2.3
Patterning Wafers for Membrane Etching
Again two different processes are used to pattern membranes on the silicon and
silicon nitride membranes. For the silicon membranes, the backside of the wafer is first
sputtered with 500 nm gold along with a TiW adhesion layer. Both sides of the wafer are
spin coated with AZ5214 photoresist, and the backside is patterned with the desired
membrane design using standard UV lithography. AZ5214 is a positive resist. This
means the exposed portion of the wafer is removed during development. The photoresist
is developed and the gold and TiW layers are etched away from the membrane pattern.
The wafer is then soaked in buffered oxide etch (BOE) for 15 minutes to etch away the
silicon dioxide layer from the membrane squares. The wafer is finally cleaned in a spin
rinse dryer. The photoresist acts as an etch stop for etching through the gold, TiW, and
silicon dioxide layers. The back side gold acts as an etch stop during KOH membrane
etching.
13
To pattern silicon nitride membranes, the wafer is first spin coated with AZ5214
photoresist on both sides. The photoresist is exposed and developed for the desired
backside membrane pattern. The wafer is exposed to a reactive ion etcher (RIE) plasma
for 10 minutes with 60 W of power, 9 cm3/sec CF4, and 1 cm3/min O2 to etch the backside silicon nitride layer from the membrane squares. The photoresist acts as an etch stop
for the RIE plasma. The back side silicon nitride acts as an etch stop during KOH
membrane etching.
The membrane pattern is the same for both of these processes and can be seen
below in Figure 2.1. The figure shows the positive field of this design; however the
negative of this mask is actually used for fabrication. The pattern shown consists of
fourteen 5 mm square membranes. This pattern maximizes the functional surface area of
a 3-inch wafer for the given PRT/heater/wick design. Each membrane has a 10mm by
1.18mm rectangle around it, defined by thin lines. These lines are required for dicing the
wafer into 14 individual die. The edges of the wafer are left unpatterned too allow the
wafer to be properly sealed in the KOH carrier during membrane etching. The four small
crosses are used as alignment marks.
14
10 mm
KOH/oxide Mask - 5mm membranes
Figure 2.1: KOH pattern for 5mm square membranes
Once the membrane patterning is complete, the wafer is ready for front side
fabrication. The remaining fabrication steps are the same for both silicon and silicon
nitride membrane types.
2.4
Front Side Patterning of Measurement Tools
A lift-off process is used to fabricate the internal platinum resistance heater and
concentric RTD structures. In this process, positive photolithography is used to pattern
the PRT/heater design on the wafer using AZ5214 photoresist. The substrate is patterned
so that the RTD/heater design is the only fraction of the wafer uncovered with
photoresist. Platinum is sputtered over the resist at a thickness of 175 nm. The wafer is
then soaked in acetone for at least 30 minutes to lift off the sacrificial photoresist layer.
15
This process leaves the desired platinum structures. Any remaining platinum is removed
by “blasting” the wafer with acetone. The wafer is finally cleaned and inspected under a
microscope to determine that the entire sacrificial layer has been removed. If there is still
a substantial amount of unwanted platinum present, the process can be repeated until liftoff is complete. This lift-off process is illustrated in Figure 2.2. The dual concentric
RTD/heater design can also be seen in Figure 2.3.
Figure 2.2: Illustration of platinum lift-off process
16
10 mm
Figure 2.3: Photolithography mask for resistance heater
and dual RTDs
2.5
Annealing Wafer
The magnetron sputtering process leaves internal compressive stresses in the
metal deposition layers. To relieve these stresses, the wafer is annealed at 650ºC for ten
minutes in a vertical furnace. This process lowers the internal resistance of the platinum
structures and homogenizes the gold to help with KOH membrane etching.
2.6
Fabricating Wicking Structures
The wicking structures are fabricated concentrically over the platinum structures
on the substrate. For this reason, the wicks need to tolerate high temperatures
experienced during the heat addition process to the resistance heater. SU8 is relatively
easy to fabricate, demonstrates sharp features down to the 5 μm range, high aspect ratios,
17
and good thermal properties. These abilities make SU8 a good choice for fabricating
micro wicking structures. Unlike AZ5214, SU8 is a negative resist. In a negative resist,
the exposed chemical remains on the wafer while the unexposed portion dissolves during
development. The SEM photograph seen in Figure 2.4 shows an example of 10 μm SU8
wicking structures.
100 μm
Figure 2.4: SEM example of SU8 wicks, Courtesy J. Martinez
Although SU8 is fabricated using standard lithography methods, the material is
very sensitive to each process. Bake times, bake methods, exposure times, and
development times all affect successful fabrication. MicroChem publishes fabrication
guidelines for all of their SU8 resists, but these processes vary with application
parameters such as exposure equipment, substrate, spin coating, heating, bake methods,
18
and feature geometry. Therefore each fabrication step is reanalyzed for each channel
thickness and geometry. MicroChem’s guidelines are used as a start point for fabrication,
but the final exposure times, bake times, spin rates, and spin times are all determined
empirically. High-resolution chrome masks are used for UV exposure of the SU8 resist.
The chrome masks are made at the University of Minnesota from an AutoCad template as
seen in Figure 2.7.
10 mm
Figure 2.5: AutoCad wick template
To determine exposure times for this application, an exposure matrix is
constructed using a single sided silicon wafer, a chrome plated wick mask, and aluminum
foil. A small square, the size of a wick pattern, is cut away from the aluminum foil to
allow light through. The foil is then placed between the ultraviolet light source and the
chrome on glass mask. The single sided wafer is coated with SU8. Instead of one
19
exposure for the entire wafer, the aluminum foil is used to test several different exposure
times on the same wafer by exposing each wick pattern separately. Fabrication is
completed and the results are checked under a microscope.
A large spread of exposure times is used attempting to make steep, high aspect
ratio sidewalls. Over exposure produces pyramidal features with sloping instead of
vertical walls where under exposure produces features with significant undercutting
below the surface of the SU8. This undercutting yields very poor SU8 adhesion. The
exposure process is repeated until the ideal exposure times are found for each wick
thickness and geometry (See Appendix A).
The MicroChem fabrication guidelines are also used as a start point to determine
optimum spin speeds for each desired SU8 thickness. SU8-2010 is used to fabricate
wicking structures10 μm in height. SU8-2025 is used to fabricate wicking structures 40
and 70 μm in height. After processing, profilometry is used to measure the final height of
the wicks at varying spin speeds. SU8 2010 is found to produce a sidewall thickness of
8-11 μm for a spin rate of 2000 rpm. SU8 2025 produces 38-41 μm sidewalls at a spin
rate of 2000 rpm and 68-70 μm at a spin rate of 1000 rpm. These thickness values are
consistent with the MicroChem guidelines. SEM photographs of both 40 and 70 μm
channels can be seen in Figures 2.6 and 2.7.
20
50 μm
Figure 2.6: SEM wick picture with 40 μm high, 5 μm thick SU8 walls
and 70 μm wide fluid channels
21
25 μm
Figure 2.7: SEM wick picture with 70 μm SU8 height, 5 μm
thickness and 35 μm fluid channels
Surface cracking and adhesion are recurring problems in SU8 fabrication. This is
accounted for by adjusting the times and methods for both the pre-exposure (soft bake)
and post-exposure (PEB) bakes. If the sample is under baked, there is a lack of polymer
cross-linking which will result in poor adhesion. If it is over-baked, the material can over
crosslink resulting in both surface cracking and poor adhesion. Rapid heating or cooling
of the substrate can also result in surface cracking. Rapid thermal changes are minimized
for the 2010 SU8 by adding an extra minute to the 65ºC PEB. There is also a thermal
step down added to the process after the 95º PEB. The step down is accomplished by
adding another minute of 65ºC baking before bench cooling. For the thicker 2025 SU8,
the bake times were also varied to maximize adhesion. Also, the 95ºC soft bake was
22
performed in a convection oven instead of a hot plate to minimize rapid heating of the
substrate. The complete finalized fabrication process for these designs for each thickness
is shown in Appendix A.
2.7
Wet Etching Wafer to Define Membrane Structures
The final step in the fabrication process is to etch through the backside of the
wafer to define the membrane structures. This process is accomplished through an
anisotropic wet etch using a potassium hydroxide (KOH) solution. The KOH solution
consists of 250 g KOH pellets dissolved in 400 mL deionized water and is heated to
approximately 80ºC. The wafer is placed in a carrier device, and the backside is exposed
to the solution for at least four hours. The wafer is pulled when the etching slows and the
membranes become translucent. A top view of a completed membrane can be seen in
Figure 2.8. In this figure, the black annular lines in the middle make up the resistance
heater, and the two outer concentric annular sets of rings make up the two RTDs. The
radial lines form the wick structures, and the liquid-vapor interface can be seen between
the resistance heater and inner RTD.
23
Outside RTD2
Inside RTD1
Internal Resistance
Heater
1000 μm
Figure 2.8: Overhead view of completed silicon membrane
24
CHAPTER 3 THE EXPERIMENT
3.1
Experimental Set Up and Equipment
The evaporative efficiencies of capillary channels are determined by performing
an energy balance. The energy balance is used to track the heat transfer across a
membrane. The evaporation efficiency is calculated using the measured energy into
evaporation. The experimental set-up includes a die carrier, a resistance heater, dual
concentric RTDs, a scale, and a timer. RTD testing boxes are used to record RTD
outputs. A hot plate and hot water bath are used to calibrate the RTDs. Finally, a
cylindrical conduction model is used to analyze the experimental results. Each of these
components is described in more detail through out this section.
3.1.1
Die carrier set-up
During testing, the die is mounted in an acrylic carrier. The carrier is used to
maintain electrical contact with the platinum components and contain a fluid reservoir
during evaporation. The carrier consists of two 3” x 3” square acrylic pieces, probes,
probe carriers, wires, and two o-rings. The probes are bonded into the carrier with epoxy
to allow for electrical contact to the heater and RTDs. The two o-rings control the fluid
flowing from the reservoir. The inner o-ring has a 7 mm inner diameter and encompasses
the membrane so that the wicks are not flooded over with fluid. The outer o-ring has at
least a 4 cm inner diameter and contains the fluid reservoir. The inner o-ring sits on top
of the evaporator channels leaving small gaps under the o-ring to allow capillary
pumping to cover the membrane. The fluid reservoir produces a continual flow of fluid
in toward the heater to cool the membrane. A schematic of this set up can be seen in
Figure 3.1.
25
Figure3.1: Evaporation experiment schematic
3.1.2 Calibration
The voltage changes of the RTDs are calibrated to measure a temperature profile
across the membrane. The calibration uses the carrier assembly, RTD testing boxes,
water bath, hot plate, stir bar, acrylic ring, thermometer, and multimeters. The
multimeters are Fluke 189 True RMS Multimeters. They are used to read the RTD
potential changes. The hot plate is a Corning Stirrer/Hotplate and is used to heat the
water bath to impose the potential difference. The hollow acrylic ring is ¼” thick, 1.5”
high, and 2” in diameter. It is needed to raise the carrier assembly from the surface of the
hotplate to stabilize the RTD temperature rise.
3.1.3
Energy dissipated in resistance heater
The electrical power dissipated by the resistance heater is measured directly. An
Agilent E3610A DC power supply provides the power input to the resistance heater. A
10 Ohm power resistor with a 10 Watt power rating is placed in series with the heater to
calculate the current through the resistance heater. Multi-meters are used to calculate the
voltage across the heater and the voltage across the power resistor. Finally, Ohm’s law is
26
used to calculate both the current through the heater and the power dissipated by the
platinum resistance heater where V = IR and P = VI. A schematic of this circuit can be
seen in Figure 3.2.
Figure 3.2: Circuit schematic for measuring total power
dissipation
3.1.4
Conduction across membrane
Two RTDs are required to determine the radial temperature profile of the
membrane. The temperature profile is used along with a conduction model to evaluate
the overall heat flux across the membrane. For these tests, the two RTDs are situated
concentrically with the internal resistance heater as seen in Figure 3.3.
27
Figure 3.3: RTD and resistance heater schematic
The RTD design had to meet specific criteria. The overall resistance of an RTD is
determined by its material resistivity, length, and cross-sectional area. The final RTD
resistance is needed to be within range of the RTD testing box to enable the circuit to be
balanced. The RTDs are placed far enough apart to register a temperature differential.
They are also moved far enough out across the membrane to remain covered with
working fluid during evaporation. The RTDs are designed in a circular shape to give
accurate temperature measurements by maintaining radial symmetry along the
membrane. Finally to minimize error, the resistance ratios between the leads to the
electrodes and the leads to the RTDs are maximized. This is to ensure that the leads do
not affect RTD temperature measurements in any way.
The heater and RTDs are all fabricated from platinum and are designed to fit
within a 5 mm membrane square. The outside radius of the internal heater is 1.6mm.
The average radii of the inner and outer RTDs are 1.7 and 2.35 mm respectively. The
standard resistor design consists of serpentine annular rings connected to large electrodes.
The ring features are made of 50 μm wide lines separated by 50 μm wide gaps. These
dimensions are printable at WSU on a standard transparency mask.
28
3.1.5
RTD testing box
Wheatstone bridges were used to measure changes in RTD resistance. Each
bridge circuit consists of an input RTD signal, an output to the voltage gage, two
potentiometers used to zero the circuit, an outside DC power source, an amplification
chip, and four nine volt batteries. The amplification chip is necessary to increase the
signal to noise ratio of the RTD signal. The nine volt batteries are used to power the
amplification chip, and the box is powered by converting an AC wall supply through
1.5V, 700mA power adapters. A schematic of this circuit is shown in Figure 3.4.
Figure 3.4: Schematic of RTD testing box circuit
3.1.6
Energy into evaporation
To complete the energy balance, the amount of evaporated working fluid is used
to calculate the power into evaporation. The energy into evaporation is calculated using
29
the latent heat of vaporization and the mass evaporation rate. The mass evaporation rate
is found using a timer and scale. The mass loss is tracked using an Acculab VI-1mg
digital scale with milligram range accuracy. The evaporation is timed by a Presto
Electronic Clock/Timer to find the rate of evaporation. The final mass evaporation rate is
determined as the ratio of mass loss to time.
3.1.7
Visualization
A TSI Particle Image Velocimetry (PIVCAM 13-8) camera is hooked to a Questar
QM1-10126-MKIII long distance microscope to observe the positioning of the liquid
vapor interface. The PIVCAM 13-8 has a scan rate of 12.5 MHz and a maximum frame
rate of 8 fps. It is capable of 200 nm frame straddling using laser synchronization.
However, for free exposure mode, it is limited to a 125 msec shutter speed.
For the visualization set-up, the carrier needs to remain horizontal to ensure that
the fluid reservoir remains filled and that the liquid naturally wicks across the membrane.
This leads to different challenges with both the set-up and lighting. The best results
eventually came from lighting the membrane from above and imaging from below. A
yellow filter also helped to clarify the image. A visualization schematic can be seen in
Figure 3.5.
30
Figure 3.5: Visualization set-up
The visualization set-up shown consists of the camera, light source, magnification
lenses, long distance microscope, and a carrier component. The camera includes a Nikon
5 bayonet mounting and is attached to a Questar long distance microscope through a
series of Questar extension tubes and insertion rings. Barlow 1.5x and 2.0x lenses are
used for extra magnification. They are mounted by screwing the back of the Barlow
lenses to the inside of the insertion rings. The extension rings are used to compress the
lenses and mount them between the camera and the long distance microscope. More
lenses may be attached for greater magnification, but can compromise picture resolution.
The carrier component is mounted on a sled and positioned within the field of
view of the microscope and camera assembly. For the assembly described, this is
approximately 23.5 inches from the end of the microscope. The carrier component is
mounted on a sled and contains a 90º angled mirror which directs the picture placement
to the under side of the membrane. The carrier component also contains an x-,y-,z- stage
31
on which the die carrier is placed. The die carrier lies horizontal and the view can be
moved around the membrane using the x-y stage. The z-stage is used for fine positioning
of the carrier for focusing the image.
A Fiber-Lite High Intensity Illuminator is used to light the membrane from above.
This is a fiber optic light with adjustable intensity. The light cord is placed directly over
the opening above the membrane using a separate clamp and post. Yellow tissue paper
and lens papers are used to filter and diffuse the optical light. These additions help
improve the imagablity of the liquid vapor interfaces.
3.2 Experimental Procedures
The goal of these experiments is to determine the efficiencies of wicking
evaporators though an energy balance. To accomplish this, four different experiments are
performed. These experiments include RTD calibrations, steady state conduction tests,
steady state evaporation tests, and visualization. RTD calibrations are used to convert the
RTD outputs to temperature. The steady state conduction tests are used to confirm
operation of the heat flux meter. The evaporation experiments are performed to analyze
the wicking evaporator efficiencies, and the visualization is to view the placement of the
liquid vapor interface in the capillary channels.
3.2.1
Die carrier assembly
Before any tests are run, the membrane to be tested must be operational. The
membrane is first examined to verify that a full array of wicks cover the membrane.
Each platinum feature is also tested to assure that the resistance is within testing limits. It
is preferred that both RTDs and the resistance heater have resistances between 500 and
1000 Ohms. If the resistances are above 1000 Ohms, then standard resistors are placed in
32
the Wheatstone circuit in parallel with the RTD outputs to lower their equivalent
resistances.
When the operation of the testing components is confirmed, the die is ready to be
mounted in the die carrier. The die is placed on a semiconductor tape pad on the bottom
piece of the acrylic carrier, and a small o-ring is placed on the die surrounding the
membrane. A large o-ring is placed on the bottom of the carrier surrounding the tape pad
to contain the liquid reservoir. The top of the carrier is positioned so that the probes line
up with the concurring electrode pads. The liquid fill hole on the top of the carrier is
circumscribed by the large o-ring on the bottom of the carrier when the pieces are
aligned. Once positioning is correct, the four screws are tightened to complete the
assembly. The screws are torqued tight enough to form a firm seal. A good seal is
required to provide good electrical connections and prevent fluid leakage. However, if
the carrier is too tight, the pressure can break the membrane. Once the assembly is
complete, the electrode wires are checked to verify electrical contact. When the
resistance reading is stable and similar to the initial resistance of the heater and RTDs,
then the assembly is complete and ready for further testing. If contact is poor, then the
assembly is broken down and realigned. A completed assembly can be seen in Figure
3.6.
33
Figure 3.6: Completed carrier assembly
3.2.2
Calibration procedure
The voltage changes of the RTDs are calibrated to record the temperatures of
RTD. The calibration uses the carrier assembly, RTD testing boxes, deionized water
bath, hot plate, stir bar, acrylic ring, thermometer, and multimeters. The acrylic ring and
stir bar are placed into the water bath. The magnetic stir bar is kept in the center of the
acrylic ring to maintain constant stirring without disruption of the carrier. Next, the
carrier is tilted, slowly lowered into the water bath, and centered on the acrylic ring.
Dropping the carrier can put undue strain on the membrane and break it. The water bath
is placed onto the hotplate. The RTD electrode wires are attached to their corresponding
RTD box inputs, and the box outputs are attached to multimeters. The heater electrodes
are attached directly to a third multimeter. Once everything is attached, the
34
potentiometers are used to zero out the RTD outputs from the multimeters. A picture of
this experiment can be seen in Figure 3.7.
Figure 3.7: Picture of calibration experiment
Once the set-up is complete, the room temperature of the water is recorded along
with the zeroed output before the hotplate and stir bar are turned on. The hot plate is then
turned on to approximately three and the stir bar to two. Slow stirring is recommended to
decrease both electrical noise in the output and physical disturbance of the die carrier
assembly. Slow heating is desired to decrease the variance in the calibration and the
uncertainties in the RTD measurements. The temperature of the bath is checked regularly
and measurements taken at approximately every degree of temperature increase. Once
the bath reaches about 35ºC, the hot plate is turned up gradually to continue slowly
increasing the temperature of the bath. Preferably the die is calibrated over at least a 1520ºC temperature differential, from approximately 22-40ºC. These results are plotted to
35
make a calibration curve for the two RTDs. A sample calibration curve can be seen in
Figure 3.8. A linear curve fit is used to determine the voltage to temperature relationship.
An R2 value of at least 0.99 is desired.
Figure 3.8: Example of test calibration
3.2.3
Steady state conduction tests
Conduction tests are performed on each membrane to verify the operation of the
heat flux meter with the conduction equation (equation 3.1). These tests call for a dry
carrier assembly, the RTD boxes, timer, multimeters, power resistor, and power supply.
Once again, the RTDs are attached to the RTD box inputs, the outputs are attached to
multimeters, and the multimeters are zeroed at room temperature. The heater is then
placed in series with both the power supply and power resistor. Once the set-up is
complete and the zero marks recorded, the power supply is turned on to the desired
voltage setting, and the RTD outputs are recorded again. These output measurements are
36
taken in five minute time increments for 30 minutes and recorded in a spreadsheet.
Measurements of the voltage across the heater and the voltage across the power resistor
are also recorded.
The RTD measurements are used with the cylindrical conduction heat transfer
equation to calculate the heat flux conducted across the membrane. The temperatures
taken by the two annular RTDs are used to measure the radial temperature differential
across the membrane. This temperature differential is used in Equation 3.1.
qr =
2πLkΔT
ln(r2 / r1 )
(3.1)
For equation 3.1, k represents thermal conductivity (k= 153 W/mK for silicon and 30.1
W/mK for silicon nitride), L is the thickness of the membrane (L = 2 μm for silicon and
300 nm for silicon nitride), and r1 and r2 are the respective average radii of the
temperature measurements from the center (r1 = 1.7 mm r2= 2.35 mm).
The voltage across the heater and power resistor are used to calculate the power
input to the system from Ohm’s Law (equation 3.2 and 3.3).
V = IR
(3.2)
P = VI
(3.3)
In equations 3.2 and 3.3, V = voltage, I = current, R = resistance, and P = power. The
power input is compared to the conduction results to balance the energy flow through the
system. The energy balance error is taken as the difference between the input power and
the conduction power divided by the total power input (equation 3.4). The error should
remain below ±5% throughout testing.
⎛ P − Pcond
% Error = ⎜⎜ in
Pin
⎝
⎞
⎟⎟ * 100
⎠
37
(3.4)
3.2.4 Steady state evaporation tests
Evaporation tests are performed to calculate the efficiencies of the micro-wicking
evaporators. These tests are very similar to the conduction tests except that a scale and
timer are used to measure the mass evaporation rate of the system. These tests call for a
carrier assembly, the RTD boxes, multimeters, power resistor, power supply, FC77
working fluid, scale, and timer. Once again, the RTDs are attached to the RTD box
inputs, the outputs are attached to multimeters, and the multimeters are zeroed at room
temperature. The heater is placed in series with the power supply and power resistor.
The carrier assembly is placed on the scale and the working fluid (FC77) is added to fill
the liquid reservoir of the carrier. The power supply is turned on and the mass of the
system, RTD outputs, voltage across the heater and power resistor, and time are measured
in five minute intervals for 30 minutes. A picture of this experiment can be seen in
Figure 3.9. This set-up is identical to that for conduction tests except for the use of the
working fluid.
Figure 3.9: Evaporation experiment test set-up
As these tests are run, the results are recorded in an evaporation test template. A
graph is made of the mass versus time and a linear fit is done on those results. This
calculation gives the average evaporation rate of the system throughout the test in grams
38
per second. The mass evaporation rate is then multiplied by the latent heat of
vaporization of the fluid to calculate the average power into evaporation (equation 3.5).
Pevap = h fg m& evap
(3.5)
For equation 3.5, Pevap = power into evaporation, m& evap = mass evaporation rate, and h fg
= latent heat of vaporization (83.736J/g for FC77). The average power into evaporation
is added to the conduction power and balanced with the total heat input to complete the
energy balance. Once again, the error should remain below ±5%. The wicking
efficiency is calculated through the ratio of evaporation power to power input as in
Equation 3.6.
η wick =
Pevap
Pelec
(3.6)
For equation 3.6, η wick = wicking efficiency. The efficiency results are finally compared
with different wick dimension and geometries to maximize their functionality.
3.2.5 Visualization
These tests are performed to track the movement of the liquid/vapor interface at
different power inputs. In these tests, the focal length of the telescope is first determined.
To do this, a printout of small numbers (4 point font) is taped to a square piece of acrylic
and set in the carrier component described in the set up. The paper is back lit by the fiber
optic light. The camera and Insight data collection software are then turned on. Insight
should be set to Exposure Mode “Free” and Capture Mode “Continuous” to capture real
time data. The camera icon is pressed to start collecting images. The sled and focus are
adjusted until the center of the paper is clearly read on the screen. This determines the
placement of the carrier component.
39
To take wick pictures, the carrier assembly is placed on the carrier component of
the visualization set-up and the fiber optic light is placed directly over the membrane
opening. The heater electrode is connected to the power supply and the liquid reservoir is
filled with working fluid. Yellow tissue paper and filter paper are also placed above the
membrane to diffuse the intensity of the light source. The voltage setting of the power is
varied to observe the movement of the interface at varying power inputs. To collect still
pictures of the interface, the Insight data collection software should be set to Exposure
Mode “Free” and Capture Mode “Single.” The camera icon is selected to obtain the
single wick image and saved to a memory device. A picture of these tests can be seen in
Figure 3.10.
Figure 3.10: Picture of visualization tests
3.2.6 Uncertainties
Uncertainties are determined for all measurement taken including the power
input, conduction power, and evaporation rate. Uncertainties are possible from different
40
components including equipment inaccuracies, measurement tools, and data analysis
software.
The power input to the resistance heater is measured directly with Fluke 189
multimeters which have resolutions of ±0.5mV and basic DC uncertainty of 0.025%.
This uncertainty is determined using the roots sum of squares method or RSS as seen in
equation 3.7.
2
⎡ n ⎛
∂R ⎞ ⎤
⎟ ⎥
wR = ⎢∑ ⎜⎜ wxi
∂xi ⎟⎠ ⎥
⎢⎣ i =1 ⎝
⎦
1
2
(3.7)
For equation 3.7, R is the result and xi is the uncertainties of the components. Both the
current and the power into the heater are measured using these multimeters along with
Ohm’s Law where I = V/R and P = VI. Due to the high accuracy of the multimeters, the
resulting uncertainties in the current and power calculations are less than 0.5% and
considered an insignificant contributor to the overall experimental error.
The next uncertainty measurement under consideration is that of the heat flux
across the membrane. This uncertainty is calculated using the RTD calibration data and
RTD dimensions. The residual temperatures between the thermometer and RTD
calibrations are used. The residual temperature difference between the calibration results
and the measured temperature (ΔR) is calculated first. The average of the residual
change (ΔRavg) and the sum of the squares (SS) are also found. These equations are
shown below.
ΔR = xi − x m
(3.8)
(ΔR) avg = ( xi − x m ) / n
(3.9)
41
ΔR = xi − x m
(3.10)
SS = ∑ ( xi − x m ) 2
(3.11)
SS
(n − 1)
(3.12)
σ =
CI = (ΔR) avg ±
1.96σ
n
(3.13)
For equations 3.8-3.13, xi = temperature calculation, xm = measured temperature, and n =
the number of measurements. The standard deviation of ΔR is found by using equation
3.12. The number of calibration measurement taken remains below 20. This sets up a
small sample statistical analysis. To find the measurement error, a t-value for small
sample sizes and a 95% confidence interval are used. The resulting t-value is 1.96. The
upper and lower confidence intervals are found by the equation 3.10. The upper and
lower bound for each RTD is used to calculate for the maximum error possible in the
power calculation across the membrane. The uncertainties of the individual temperature
measurements are added together when calculating the temperature differential across the
membrane. The maximum acceptable uncertainty for these measurements is taken as
±0.5ºC. The RTD radii measurements come from their location on the transparency
mask. The RTD masks are printed at Washington State University, and the printing
technology is limited in resolution. Prints have shown that mask dimensions such as
thickness of a line are accurate down to 20μm and smaller features can bleed together.
This error is less than 1% of the RTD distance from the center of the membrane and not a
significant contributor to the heat flux uncertainty. The overall heat flux error determined
by these factors results in a ±5% of the measured values.
42
Finally the evaporation error is accounted for. The Acculab scale used possessed
a resolution of ±0.5mg. To determine the uncertainty, a series of calibration
measurements are taken with a 100 mg calibration weight. Once again, equations 3.8 and
3.9 were used to find the deviation or precision index of these measurements. The
precision limit is calculated using equation 3.14.
Pxi = tS x
(3.14)
In this case, a confidence interval of 95% was used with a sample size of 60. This
calculation found the uncertainty to be approximately ±5 mg.
To find the drift of the scale over time, the calibration weight is left on the scale
for two hours while the mass is measured every 30 minutes. This test is repeated 10
times. The resulting drift uncertainty is ±0.2 mg/min.
43
CHAPTER 4 RESULTS AND ANALYSIS
4.1
Summary of Tests Performed
To complete each efficiency test, a series of experiments including calibration,
dry conduction, and evaporation tests are performed. Calibrations are used to fit RTD
potential changes to temperature. Conduction tests are used to verify the operation of the
RTDs using an energy balance. Evaporation tests are used to find the efficiency of each
style of micro-capillary structures. These tests are performed for both silicon and silicon
nitride substrates. The calibration and conduction results are found respectively in
Appendices B and C. The evaporation results are summarized in Section 4.2 and the
complete tables are found in Appendix D. In addition, engine tests are performed to
study the effect of wick geometry on the dynamic operation of the evaporator membrane.
Finally, an analysis of fluid flow through rectangular channels is performed. The flow
results show how height and aspect ratio affect the fill rates of the wicking evaporators.
4.2
Steady State Evaporation Tests
A series of evaporation tests are performed. These tests vary different aspects of
the membrane and micro-channel structure to determine how they affect wick
efficiencies. The factors varied include channel geometry, channel width, SU8 height,
and membrane material.
4.2.1 Variation of wick geometry
Channel geometry is considered in the first set of evaporation tests. Two wick
geometries are under investigation. Wick pattern A is made from a combination of radial
and annular fill channels seen in Figure 4.1. Wick pattern B is a staggered formation of
radial channels seen in Figure 4.2. The purpose of the staggered design is to decrease the
44
SU8 mass on the membrane and maintain a constant channel width. For both designs, all
channel widths are kept at 10 μm to maintain a 1:1 aspect ratio. The SU8 features are
also 10 μm in width. This dimension was used because it is within the fabrication ability
of the WSU cleanroom.
Figure 4.1: Wick Pattern A – Combination of radial
and annular fill channels
Figure 4.2: Wick Pattern B – Staggered radial fill
channels
45
In the evaporation experiments for these geometries, the power varies from 27.1
to 41.3 mW. The RTD temperatures, power input, power into conduction, and power
into evaporation are tracked throughout these tests. Finally the wick efficiencies are
calculated for each test from measurements of the power in and power into evaporation.
These results are shown in Table 4.1 and Figure 4.3.
Table 4.1: Table of wick efficiencies for varied geometries
46
Figure 4.3: Summary of evaporation results for annular and
radial wick geometries
These test results show that anywhere from 91-95% of the total power input
conducts across the membrane while only 5-9% goes toward evaporation. As seen in the
table, there is a slight decrease in efficiency with increasing power input. This decrease
in efficiency occurs due to dry out which lowers evaporation rates. Dry out is caused by
the heat input forcing the liquid vapor interface out across the membrane. As the
working fluid moves out, less of the membrane is covered with liquid and less power
goes toward evaporation. The result is a decrease in mass evaporation rate with
increasing power input. Picture of dry out can be seen in Figure 4.4.
47
Liquid Vapor
Interface
Internal Resistance
Heater
50 μm
Inside RTD1
Figure 4.4: Visualization of wick dry out on wick pattern A with
22 mW of power input. For this picture, wick dimensions
are all 10 microns.
Only a minimal difference in efficiency is found between the two geometries
considered. However visualization of the evaporators has shown that the staggered radial
wicking structure shown in Figure 4.2 possesses faster fluid fill rates compared with wick
pattern A. Fast filling of the micro-channels is required for dynamic engine operation.
Therefore the study of this design is continued with the examination of how varying wall
heights and channel thicknesses help to maximize efficiencies.
4.2.2 Increasing channel width
Decreasing SU8 mass and increasing channel widths is considered next. The SU8
wall thickness is decreased to 5 μm. The fluid channel width is increased up to 20 μm.
An aspect ratio close to 1:1 is maintained. The evaporation results are shown in Table
4.2.
48
Table 4.2: Effect of small channel width variation
This table summarizes evaporator efficiencies at similar power inputs. The results
show that as fluid channel width increases, wicking efficiencies increase. Also, as the
SU8 wall thickness increases, the efficiency decreases.
The effects of further increases in channel width is explored by fabricating wicks
with 5 μm SU8 wall thicknesses with fluid channel widths of 35, 50, 70, and 90 μm. The
evaporation results for these tests are summarized in Table 4.3.
Table 4.3: Wide channel wick efficiencies with SU8 dimensions
5 μm wide and 10 μm high
Table 4.3 shows that there is a maximum channel thickness over which
evaporator efficiencies begin to decrease. The maximum efficiency in these tests
corresponds with the 35 μm channels. Images of the liquid vapor interface for the 35 μm
channels are shown in Figure 4.5. This image illustrates both dry out and corner flow
present in these tests. Comparison of Figures 4.4 and 4.5 also shows that it takes a much
higher power input to dry out the larger channels. However, as the fluid channels
continue to increase in width, the dry out and corner flows occur at lower power inputs.
49
This effect is mirrored in Table 4.3 by the decrease in efficiency as channel width
increases past 35 μm.
Liquid Vapor Interface
with Corner Flow
Inside RTD1
Internal Resistance
Heater
Figure 4.5: Visualization of wick dry out on wick pattern B with
22 mW of power input. Channels are 35 μm wide with
5x10 micron SU8 structures
4.2.3 Increasing channel height
The effect of channel height is considered next. Fluid channels with 5 μm SU8
walls and heights of 40 and 70 μm are fabricated. These higher channels decrease aspect
ratio fluid channels and increase the fluid mass over the membrane. A summary of these
results are shown in Table 4.4.
50
SU8
Height
(μm)
40
70
Channel
Width
(μm)
35
35
50
50
70
70
90
35
35
50
70
90
Power
into
Heater
(mW)
37.7
42.1
36.9
35.6
41.2
31.1
33.5
46.2
50.7
47.8
43.6
40.3
Inside
RTD
Temp
(ºC)
35.2
42.0
36.3
31.5
34.0
30.7
29.8
34.9
35.6
35.0
34.0
33.3
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
30.0
30.6
35.9
35.8
29.8
38.9
27.4
24.6
29.0
29.7
27.5
19.2
26.1
22.2
29.0
35.1
29.8
38.7
29.1
38.7
28.1
38.7
27.7
33.1
Power
into
Evap
(mW)
6.3
6.6
10.2
10.0
11.6
11.4
10.3
9.4
9.2
9.2
9.2
7.7
Evaporation
Efficiency
Rate
(%)
(mg/min)
4.5
4.7
7.1
7.2
8.3
8.2
7.4
6.7
6.5
6.7
7.0
7.2
16.6
15.5
26.8
28.3
27.8
36.7
30.8
20.2
18.4
19.6
22.4
24.9
Table 4.4: Wick efficiencies for 40 and 70 μm high channels
The evaporation experiments summarized in Table 4.4 show evaporator
efficiencies ranging from 16.6-30.8%. For the most part, the efficiencies of the 40 μm
high channels are larger than the efficiencies of the 70 μm high channels. Efficiencies
also tend to increase with increasing power inputs. However, the power dissipation
varies from test to test due to resistance changes between heaters. Equation 3.6 defines
wicking efficiency as the ratio of evaporation power over power input. Because of this,
the power input should be consistent between tests to accurately compare the
performance of the evaporators. To resolve this, the mass evaporation rate is used. The
mass evaporation rates remain constant in the 40 and 70 μm high channels throughout
each series of tests (see Appendix D for full data series). The constant mass evaporation
rate is used to determine the wick efficiency at a power input of 50 mW using equations
3.5 and 3.6. The choice of 50 mW is made because it is within the power dissipation
range for all experiments performed on 40 and 70 μm high SU8 structures. The results of
this comparison are shown in Figure 4.6.
51
Calculation of Wick Efficiencies at 50 mW Power Input
Efficiency (%)
30
28
40 μm high SU8
26
70 μm high SU8
24
22
20
18
16
14
12
10
30
40
50
60
70
80
90
100
Fluid Channel Width (μm)
Figure 4.6: Calculation of wick efficiencies for 40 and 70 μm wall thicknesses
for 50 mW of power input assuming constant evaporation rates
Figure 4.6 shows that the efficiencies of the 70 μm high wicks slowly increase
with increasing power inputs. The 40 μm high wicks reach a maximum efficiency of
23% at a channel width of 70 μm thickness. As the channel thickness continues to
increase, the efficiency decreases to 20%.
Figure 4.6 is an interpolation made using the assumption of constant evaporation
rates throughout the tests. This assumption is made to compare efficiencies at equal
power inputs and is not exact. The mass evaporation rates did remain relatively constant
throughout each experiment. However, there is a general trend of increasing evaporation
rates with increasing power dissipation over the range of experiments. This trend can be
seen in Figure 4.7.
52
Comparison of Mass Evaporation Rate to Power Input
10-10/10
10.0
10-10/10
10-10/15
Mass Evaporation Rate (mg/min)
9.0
5-10/10
8.0
5-10/20
5-10/35
7.0
5-10/50
5-10/70
6.0
5-10/90
5-70/35
5.0
5-70/50
4.0
5-70/90
5-40/35
3.0
5-40/50
5-40/50
2.0
5-40/70
5-40/70
1.0
5-40/90
0.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
110.0
Pow er into Resistance Heater (m W)
Figure 4.7: Comparison of mass evaporation to power dissipation from the internal
resistance heater throughout experimental range. Legend shows wick
dimensions (SU8 thickness – channel width/height)
Figure 4.6 plots the mass evaporation rates with power input for a range of tests.
Although each experiment remains constant, there is a general trend of increasing
evaporation rate with increasing power input. Mass evaporation also tends to increase
with decreasing SU8 coverage. These results show that mass evaporation rates are
dependent on both wick dimensions and power dissipation levels.
To compare the performance of the 10 μm high wicks, the mass evaporation rates
are used. The 10 μm high wicks are not tested at as high of power inputs as the 40 and 70
μm high wicks because they dry out at lower power inputs (see Figures 4.4 and 4.5).
53
Therefore they can not be compared at the same power inputs. An average rate of
evaporation throughout the tests is calculated and graphed in Figure 4.8.
Comparison of Evaporation Rates
Rate of Evaporation (mg/min)
11
40 μm high SU8
10
70 μm high SU8
10 μm high SU8
9
8
7
6
5
4
3
30
40
50
60
70
80
90
100
Fluid Channel Width (μm)
Figure 4.8: Comparison of average mass evaporation rates
The average mass evaporation rates vary for each wick dimension. The
maximum average evaporation rate found is 9.15 mg/min. This corresponds to 35 μm
channel made with 10 μm high SU8. However, as channel dimensions continue to
increase, the evaporation rate drops down to 4.7 mg/min. As channel widths continue to
increase, evaporation rates decreases. Visualization shows that more power is needed to
dry out larger wick channels to a point. As aspect ratios continue to increase, the power
needed to dry out the channels begins to decrease. This effect can be seen from the
visualization of 10, 35, and 50 μm channels with 5x10 μm SU8 structures. The 10 μm
channels dry out to the inside RTD at 32 mW of power input. It takes 85 mW for the 35
54
μm channels and 50 mW for the 50 μm channels to dry out to the same RTD. The dry
out effect is also seen in mass evaporation rates. As more power is needed to dry out the
wicks, the mass evaporation rates also increases.
The 40 μm high SU8 wicks see a maximum rate of evaporation of 8.25 mg/min
corresponding with the 70 μm channels. As the channels width increases to 90 μm, the
evaporation rate decreases to 7.4 mg/min. The 70 μm SU8 channels show a gradual
increase in evaporation rate throughout the experiments. The 35 x 70 μm channels show
an evaporation rate of 6.7 mg/min. As the channels increases in width, the evaporation
rate increases to a maximum 7.1 mg/min corresponding with the 90 μm channels.
4.2.4 Silicon nitride
Conduction and evaporation tests are performed on different wick structures
fabricated on silicon nitride membranes. Evaporation tests show some improvement in
evaporation rates when compared evaporation on silicon membranes. However, there are
consistently high errors associated with both the conduction and evaporation results.
Examples of these tests are seen in Tables 4.5 and 4.6.
Power
Power Inside Outside
SU8 Channel
RTD
RTD
across
into
Height Width
Heater Temp Temp Membrane
(μm)
(μm)
(mW)
(ºC)
(ºC)
(mW)
10
35
33.605 51.8
34.3
3.1
Percent
Error
90.8
Table 4.5: Conduction result for silicon nitride membrane
Power
SU8 Channel
into
Height Width
Heater
(μm) (μm)
(mW)
10
35
33.6
Inside Outside Power
Power Evaporation Power into Power into
RTD RTD across
Percent
intoEvap
Rate
Evaporation Conduction
Temp Temp Membrane
Error
(mW)
(mg/min)
(%)
(%)
(ºC) (ºC)
(mW)
44.2 32.8
3.3
17.6
12.6
52.3
9.9
37.7
Table 4.6: Evaporation result for silicon nitride membrane
55
Tables 4.5 and 4.6 summarize results for 35 μm wick channels 10 μm high conducted on
300 nm silicon nitride membranes. The evaporation rates for these tests are consistently
higher than tests conducted on silicon membranes. The silicon nitride membranes
evaporate 12.6 mg/min where the same wick geometry on silicon membranes only
evaporates up to 9.15mg/min. However, the results are inconclusive due to the high
errors associated in the energy balance of the silicon nitride membranes. Evaluation of
the results shown in Figures 4.5 shows that a temperature difference of almost 200ºC
must be observed for the test error to be within acceptable limits (±5%). The evaporation
results show a required temperature difference of 85ºC. Further analysis includes the
calculation of radiation heat loss (Equation 4.1). Radiation is able to account for
approximately 10% of the power input. However, results remain well outside of the
desired operational error. The required temperature differences indicate that the
conduction model (Equation 3.1) used to evaluate the silicon membrane is not valid for
silicon nitride substrates.
4
Prad = eσATavg
(4.1)
For equation 4.1, P = power into radiation, e = emissivity (1 for SiNx), σ = StefanBoltzman constant = 5.67*10-8 W/m2K4, A = membrane area (25 mm2), and Tavg =
average temperature measured across the membrane in Kelvin.
4.3
Engine Efficiencies
Engine assemblies are tested for varying wick dimensions to determine the effect
of wick geometry on dynamic engine operation. All tests use a 5mm silicon nitride upper
membrane. The tests are performed at frequencies of 20 and 40 Hz. The results are
shown in Figures 4.9 and 4.10.
56
Figure 4.9: Engine efficiency tests at 20 Hz (courtesy L. Weiss). Legend shows
wick dimensions (SU8 thickness – channel width/height)
57
Figure 4.10: Engine efficiency tests at 40 Hz (courtesy L. Weiss). Legend shows
wick dimensions (SU8 thickness – channel width/height)
In Figures 4.9 and 4.10, the 40 μm high wicking evaporators outperform the 10
and 70 μm heights. The highest efficiency found is .132%. This comes from 40 μm
thick SU8 with 70 μm channel widths at a frequency of 40 Hz. Also, as channel
thickness increases, the efficiency also increases. In other words, as the percent coverage
of SU8 decreases, the dynamic engine efficiency increases.
4.4
Flow Analysis
Fast fluid fill rates are required to improve cooling for the dynamic operation of
the evaporator membrane. To study the effects of channel geometry on fluid fill rates in
rectangular channels, the approach taken by Nilson et al. [15, 16] is taken. This approach
takes into account the channel geometry as well as the effects of viscosity, aspect ratio,
58
and liquid/solid contact angle on axial capillary flow. The study is done for the dead
zone where there is no capillary pressure differential at the liquid/vapor interface. To
complete this study, the following equations are used [15, 16].
∂
(ρAcs s ) + ∂ (ρu o Acs s ) = q"Wb
∂t
∂x
h fg
u=
(4.2)
ρgW 2
βμ
(4.3)
⎤
⎡
(U *U * )
=U* = ⎢ * m1 2 * m ⎥
β
⎣ (U 1 ) + (U 2 ) ⎦
1
(4.4)
⎡
⎤
U cp, 0
*
Uc = ⎢
2
k p ⎥
⎣⎢1 + (U c , 0 )[7(Λ − 2) + b(Λ − 2) ] ⎦⎥
Λ=
W
2 cos α
=
h − hc 1 − sin α
1/ p
(4.5)
(4.6)
For equations 4.2-4.6, U 1* = 1 / 12 , λ = W / hc , a = 2.6, n = 0.82, m = 1.31, u = mean axial
speed, W = channel width, μ = dynamic viscosity = ν*ρ, g = gravitational constant, ν =
kinematic viscosity, ρ = density of liquid phase, β = 1/U*, and U* = normalized mean
fluid speed, h = liquid depth at sidewall, hc = liquid depth at center, Uc,o = 0.0027 = mean
speed of corner flow with α = 0º, b = 150, k = 0.87, and p = 1.88. For clarity, dimension
variables for equations 4.2-4.6 are illustrated in Figure 4.9.
59
Figure 4.11: Illustration of flow regions and channel
dimension variables
These equations find a mean axial speed with a maximum relative error of 10%.
The parameters are for a rectangular geometry. The equation is only valid for contact
angles between 0 and 60º. In the dead zone, the contact angle remains constant at the
desired liquid/solid contact for the given fluid and surface. The results of these equations
for varying geometries are shown in Table 4.7 and Figure 4.12.
Table 4.7: Results of fluid fill rate analysis for varying geometries
60
Mean Axial Velocity (mm/s)
Variation of Channel Dimensions
500
W = 10 μm
W = 20 μm
400
W = 40 μm
W = 70 μm
W = 90 μm
300
200
100
0
5
15
25
35
45
55
65
75
Channel Height (μm)
Figure 4.12: Variation of fluid channel dimension and its
effect on fluid fill rates
Figure 4.12 shows that as either height or width is increased, the mean axial
velocity is also increased. This effect diminishes as the channel dimensions become
equal (aspect ratio becomes 1). Once the height and width are equal, further increases in
either height or width no longer help improve axial velocity.
4.5
Visualization
To track the movement of the liquid vapor interface through the experimentation,
pictures were taken of various wick dimensions and power levels. The first examples are
the 10 μm thick SU8 wicks with 10 μm features. For these, geometries (wick patterns A
61
and B) maintained the same liquid/vapor interface position for similar power inputs.
Examples of these visualization results are shown in Figure 4.13.
100 μm
Internal Resistance
Heater
Inside RTD1
a)
b)
100 μm
Liquid Vapor
Interface
100 μm
c)
Figure 4.13: Visualization of wick dry out at various power inputs on wick pattern A.
a) 32 mW power in; b) 29 mW of power in; c) 22 mW power in
In Figure 4.13, the inner black rings consist of the internal resistance heater while
the outer rings form the inner RTD. The stages show how the capillary pressure draws
fluid in toward the center of the membrane while the heat input forces the liquid radially
out away from the center causing the channels to dry out. It is desired for both RTDs to
remain covered throughout experimentation. Therefore wick dry out is used to limit the
power input to these membranes. Dry out is most significant in smaller fluid channels.
62
Figure 4.14 shows 35 μm fluid channels at different power dissipation levels. The SU8
wall dimensions for this figure are 5 μm wide and 10 μm high.
Inside RTD1
a)
Liquid Vapor Interface
with Corner Flow
Internal Resistance
Heater
100 μm
100 μm
b)
Figure 4.14: Visualization of 35 μm channels for 10 μm SU8 thickness. a) 45 mW
power in; b) 78 mW power in.
As illustrated, the larger channels shown in Figure 4.14 can maintain greater fluid
coverage at much higher power dissipation levels. The spots in front of the interface
illustrate the appearance of corner flow. The higher power dissipation potential makes
the fluid interface limitation less important than temperature limitations. For steady state
tests, the temperature should be kept below about 65ºC to assure that there is no
deterioration of the tape pads or the acrylic carrier. As the channel heights are increased,
more fluid is kept on the membrane. In fact, the power required to move the interface out
results in temperature too high to test at steady state.
63
CHAPTER 5 CONCLUSIONS
The goal of this study is to determine the affect of geometry and dimensions on
the performance of micro-capillary evaporators. The geometries compared are a
staggered radial wick versus a combination radial/annular channel wick. The wall
dimensions range from 10-70 μm in height and 5-10 μm in width. The fluid channel
widths range from 10-90 μm. Steady state evaporation tests and dynamic engine tests are
performed to compare the performances of the different channels. A comparison of
capillary fill rates on different channel dimensions is also performed.
Characterizations of capillary fill rates show that increasing either channel height
or width will increase axial fill rates until the dimensions become equal. Once the
channels reach a 1:1 aspect ratio, fill rates level out. Further increases in either height or
width no longer increase fluid fill velocity.
Evaporation tests show the wick dimensions, SU8 mass, and power dissipation
levels can all affect the evaporative potential of the channels. The two different
geometries tested show the same evaporative potential. For the 10 μm high SU8, there is
a maximum evaporation rate of 9.2 mg/min corresponding with 35 μm fluid channels.
Further increases in channel width led to a decrease in evaporation rate. This effect can
be seen in the visualization of wick dry out. The 35 μm channels require a greater power
input to accomplish the equivalent dry out effect when compared with 10 or 50 μm
channels of the same height. The 40 μm high SU8 structures show a maximum
evaporation rate of 8.3 mg/min corresponding with 70 μm fluid channels. The 70 μm
high SU8 structures show a continuous increase in evaporation rates for increasing
64
channel widths. The maximum evaporation rate for 70 μm high SU8 is 7.2 mg/min
corresponding with 90 μm fluid channels.
The 40 μm high SU8 structures show the maximum performance in dynamic
engine efficiency testing. In general, the engine efficiencies tend to increase with
increasing channel width. The highest measure of efficiency corresponds to 40x70 μm
channels. They measure 0.132% efficiency at a power input of 14.4 mJ and 40 Hz
operation frequency. The 70 μm high SU8 structures correspond to a dramatic decrease
in dynamic efficiency.
Although further work is needed to complete the characterization of the microcapillary evaporators, a 40 μm SU8 thickness is recommended. These channels perform
well both in steady state evaporation and dynamic assembly testing. They possess
relatively high axial fill rates which improve both dynamic operation and evaporation
potential. The 70x40 μm channels posses one of the highest evaporation rates as well as
the highest tested dynamic efficiency.
65
APPENDIX A
SU8 FABRICATION
SU8 Fabrication Steps – 2010 for 10 μm thicknesses
•
Clean wafer with five step process. Acetone, IPA, DI water, Acetone, IPA +
canned air
•
Place on hotplate at 200 °C for 5 minutes to dehydrate wafer
•
Spin on Omnicoat at 3000 rpm for 30 sec
•
Bake Omnicoat layer 1 minute at 200ºC and let cool to room temperature **
o While letting wafer sit, change hotplate to 65ºC. Also be sure the second
hotplate is set for 95ºC (approximately 116)
___________________________________________________________________
•
Spin coat the wafer with 2010 SU8 at 2000 rpm for 30 seconds for a 10 μm
thickness (set acceleration and deceleration at about 1 o’clock)
•
Soft bake the wafer at 65 °C for 1 minute and immediately following 95 °C for 2
minutes (on second hot plate)
•
Expose wafer using predetermined time for substrate and desired pattern (15
seconds for Pt dual RTD pattern)
•
Post exposure bake for 2 minutes at 65 °C and 2 minutes at 95 °C, then again at
65ºC for 1 minute
•
Let wafer cool on work bench for 5 minutes after baking to help prevent cracking
•
Develop wafer using a full immersion in 2010 SU8 Developer for 3 minutes with
strong agitation
•
Rinse with IPA + DI + canned air
66
•
Optional cure on hotplate at 200 °C for 3-5 minutes if the wafer is to be used in a
high temperature device
** Note that Omnicoat layer may be spun in advance for SU8 fabrication
___________________________________________________________________
Omnicoat removal (not necessary for evaporators):
•
Place wafer in oxygen plasma at 190 mTorr, 100W for 30 seconds to remove the
Omnicoat not covered by SU8
___________________________________________________________________
SU8 Removal:
Place wafer in Remover PG solvent at 80ºC for 30 min to remove SU8 if needed
67
SU8 Fabrication Steps – 2025 for 40 μm thickness
•
Clean wafer with five step process. Acetone, IPA, DI water, Acetone, IPA +
canned air
•
Place on hotplate at 200 °C for 5 minutes to dehydrate wafer
•
Spin on Omnicoat at 3000 rpm for 30 sec
•
Bake Omnicoat layer 1 minute at 200ºC and let cool to room temperature
o While letting wafer sit, change hotplate to 65ºC. Also be sure the second
hotplate is set for 95ºC (approximately 116). Also check that convection
oven is set to 95ºC ± 5ºC
___________________________________________________________________
•
Spin coat the wafer with 2025 SU8 at 500 rpm for 7 sec with acceleration of 100
rpm/s
o Check to see if any bubbles have occurred. If so, pop with razor blade.
•
Spin wafer for another 30 seconds at 2000 rpm with an acceleration of 300 rpm/s
for a 40 μm thickness
•
Soft bake the wafer at 65 °C for 2 1/2 minutes on hot plate
•
Bake for one hour at 95 °C in convection oven
•
Expose wafer using predetermined time for substrate and desired pattern – for
large wick channels, 55 sec for 35μm channels and 65 second for the rest of the
wafer.
•
Post exposure bake for 1 minute at 65 °C on first hot plate, then 4 minutes at 95
°C on the second hot plate
68
•
Let wafer cool on work bench for at least 10 minutes after baking to help prevent
cracking
•
Develop wafer using a full immersion in SU8 Developer for 5 1/4 minutes with
constant strong agitation
•
Rinse with IPA + DI + canned air
69
SU8 Fabrication Steps – 2025 for 70 μm thickness
•
Clean wafer with five step process. Acetone, IPA, DI water, Acetone, IPA +
canned air
•
Place on hotplate at 200 °C for 5 minutes to dehydrate wafer
•
Spin on Omnicoat at 3000 rpm for 30 sec
•
Bake Omnicoat layer 1 minute at 200ºC and let cool to room temperature
o While letting wafer sit, change hotplate to 65ºC. Also be sure the second
hotplate is set for 95ºC (approximately 116). Also check that convection
oven is set to 95ºC ± 5ºC
___________________________________________________________________
•
Spin coat the wafer with 2025 SU8 at 500 rpm for 7 sec with acceleration of 100
rpm/s
o Check to see if any bubbles have occurred. If so, pop with razor blade.
•
Spin wafer for another 30 seconds at 1000 rpm with an acceleration of 300 rpm/s
for a 70 μm thickness
•
Soft bake the wafer at 65 °C for 2 1/2 minutes on hot plate
•
Bake for one hour at 95 °C in convection oven
•
Expose wafer using predetermined time for substrate and desired pattern – for
large wick channels, 60 seconds for 35μm channels and 70 seconds for the rest of
the wafer.
•
Post exposure bake for 1 minute at 65 °C on first hot plate, then 5 1/2 minutes at
95 °C on the second hot plate
70
•
Let wafer cool on work bench for at least 15 minutes after baking to help prevent
cracking
•
Develop wafer using a full immersion in SU8 Developer for 5 1/2 minutes with
constant strong agitation
•
Rinse with IPA + DI + canned air
71
APPENDIX B
CALIBRATION TEST RESULTS
Figure B1: Calibration of die 1093F
Figure B2: Calibration of die 1124C
72
Figure B3: Calibration of die 1129F
Figure B4: Calibration of die 1307J
73
Figure B5: Calibration of die 1307M
Figure B6: Calibration of die 1307N
74
Figure B7: Calibration of die 1421B
Figure B8: Calibration of die 1397A
75
Figure B9: Calibration of die 1397B
Figure B10: Calibration of die 1397D
76
Figure B11: Calibration of die 1396F
Figure B12: Calibration of die 1411A
77
Figure B13: Calibration of die 1411B
Figure B14: Calibration of die 1411C
78
Figure B15: Calibration of die 1411D
Figure B16: Calibration of die 1424A
79
APPENDIX C
CONDUCTION TEST RESULTS
Conduction Test Die #1124C - Radial Wicks
Inside Outside
SU8 Wall Channel
SU8
RTD
RTD
Width
Width
Height
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
38.1
33.2
39.3
33.9
10
10
10
40.4
34.4
41.5
34.9
42.6
35.3
Power
into
Heater
(mW)
30.2
33.7
37.0
40.8
44.4
Power
across
Membrane
(mW)
29.0
32.2
35.8
39.1
42.9
Percent
Error
4.1
4.3
3.3
4.3
3.4
Table C1: Conduction tests for die 1124C
Conduction Test Die #1129F - Annular Wicks Wicks
Inside Outside Power
SU8 Wall Channel
SU8
RTD
into
RTD
Width
Width
Height
Temp
Heater
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
(mW)
27.9
37.8
32.4
31.0
38.7
32.8
10
10
10
34.8
39.7
33.3
37.9
40.7
33.7
41.3
41.9
34.2
Power
across
Membrane
(mW)
29.2
31.4
34.4
37.6
41.4
Percent
Error
4.6
1.4
1.1
0.7
0.4
Table C2: Conduction tests for die 1129F
Conduction Test Die #1307J - Radial Wicks
Inside Outside
SU8 Wall Channel
SU8
RTD
RTD
Width
Width
Height
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
37.8
32.4
39.1
33.2
10
10
15
40.4
34.0
41.5
34.5
42.9
35.0
Power
into
Heater
(mW)
31.0
34.5
38.6
42.1
45.9
Power
across
Membrane
(mW)
32.4
34.8
37.9
41.5
46.8
Table C3: Conduction tests for die 1307J
80
Percent
Error
4.6
0.9
1.9
1.4
2.0
Conduction Test Die #1307M - Radial Wicks
Inside Outside
SU8 Wall Channel
SU8
RTD
RTD
Width
Width
Height
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
31.1
26.1
39.1
33.6
10
5
10
40.3
33.9
41.5
34.4
43.0
35.5
Power
into
Heater
(mW)
31.1
33.8
38.8
43.8
45.9
Power
across
Membrane
(mW)
29.5
32.6
38.0
42.4
44.4
Percent
Error
5.0
3.6
2.1
3.2
3.2
Table C4: Conduction tests for die 1307M
Conduction Test Die #1307N - Radial Wicks
Inside Outside
SU8 Wall Channel
SU8
RTD
RTD
Width
Width
Height
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
31.1
26.1
34.9
28.8
10
5
20
36.7
29.8
38.6
31.2
41.9
34.0
Power
into
Heater
(mW)
31.0
34.7
39.0
42.7
45.9
Power
across
Membrane
(mW)
29.6
36.2
40.6
43.9
47.3
Percent
Error
4.4
4.3
4.2
2.9
3.1
Table C5: Conduction tests for die 1307N
Conduction Test Die #1308G - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
31.9
26.9
10
5
35
39.8
33.0
58.4
49.4
Power
into
Heater
(mW)
27.9
39.7
51.5
Power
across
Membrane
(mW)
29.8
40.1
53.5
Percent
Error
6.7
3.5
4.0
Table C6: Conduction tests for die 1308G
Conduction Test Die #1421B - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
32.5
27.2
10
5
70
38.0
29.1
Power
into
Heater
(mW)
32.3
54.0
Power
across
Membrane
(mW)
31.5
52.8
Table C7: Conduction tests for die 1421B
81
Percent
Error
2.6
2.3
Conduction Test Die #1421A - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
29.2
26.8
10
5
90
31.3
26.2
Power
into
Heater
(mW)
14.1
31.1
Power
across
Membrane
(mW)
13.9
30.4
Percent
Error
1.5
2.2
Table C8: Conduction tests for die 1421A
Conduction Test Die #1423A - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
29.1
25.9
10
5
50
33.2
28.5
37.6
31.0
Power
into
Heater
(mW)
19.4
27.8
38.1
Power
across
Membrane
(mW)
19.0
27.4
38.8
Percent
Error
4.2
1.6
1.8
Table C9: Conduction tests for die 1423A
Conduction Test Die #1397A - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
32.1
27.5
36.6
29.8
70
5
70
42.3
33.2
46.0
33.4
Power
into
Heater
(mW)
26.1
40.3
57.1
76.9
Power
across
Membrane
(mW)
27.4
40.1
54.2
75.3
Percent
Error
4.9
0.5
5.1
2.1
Table C10: Conduction tests for die 1397A
Conduction Test Die #1397B - Radial Wicks
Inside Outside
SU8 Wall Channel
SU8
RTD
RTD
Width
Width
Height
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
33.1
28.8
37.9
30.4
70
5
35
42.7
33.3
47.5
35.0
Power
into
Heater
(mW)
26.7
46.6
58.4
76.9
Power
across
Membrane
(mW)
25.6
44.5
56.1
74.2
Table C11: Conduction tests for die 1397B
82
Percent
Error
4.0
4.5
3.8
3.4
Conduction Test Die #1397D - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
38.2
30.2
70
5
35
45.4
32.6
53.5
36.0
Power
into
Heater
(mW)
49.6
77.6
108.2
Power
across
Membrane
(mW)
47.4
76.2
104.0
Percent
Error
4.6
1.9
3.8
Table C12: Conduction tests for die 1397D
Conduction Test Die #1396E - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
33.1
28.6
38.5
31.9
40
5
70
42.8
33.3
47.8
35.3
Power
into
Heater
(mW)
27.7
40.7
58.3
77.0
Power
across
Membrane
(mW)
26.7
39.0
56.2
74.1
Percent
Error
3.8
4.2
3.5
3.8
Table C13: Conduction tests for die 1396E
Conduction Test Die #1411A - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
32.2
28.2
40
5
70
35.1
29.3
41.1
32.4
Power
into
Heater
(mW)
23.8
35.0
49.6
Power
across
Membrane
(mW)
23.9
34.8
51.9
Percent
Error
0.8
1.5
4.7
Table C14: Conduction tests for die 1411A
Conduction Test Die #1411B - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
31.4
27.3
40
5
50
35.3
29.3
39.9
31.8
Power
into
Heater
(mW)
25.3
36.5
50.2
Power
across
Membrane
(mW)
24.1
35.2
48.2
Table C15: Conduction tests for die 1411B
83
Percent
Error
4.6
3.4
3.9
Conduction Test Die #1411C - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
30.4
26.4
40
5
50
34.7
28.6
39.1
31.1
Power
into
Heater
(mW)
23.3
35.5
48.4
Power
across
Membrane
(mW)
24.1
36.8
47.2
Percent
Error
3.3
3.6
2.5
Table C16: Conduction tests for die 1411C
Conduction Test Die #1411D - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
29.7
26.1
40
5
90
32.9
27.3
36.9
29.4
Power
into
Heater
(mW)
22.2
33.5
45.6
Power
across
Membrane
(mW)
21.3
33.1
44.3
Percent
Error
4.2
1.1
2.8
Table C17: Conduction tests for die 1411D
Conduction Test Die #1424A - Radial Wicks
Inside Outside
SU8
SU8 Wall Channel
RTD
RTD
Height
Width
Width
Temp
Temp
(μm)
(μm)
(μm)
(ºC)
(ºC)
38.5
32.3
40
5
35
42.4
34.7
46.0
36.2
Power
into
Heater
(mW)
37.7
47.8
59.1
Power
across
Membrane
(mW)
36.9
45.7
58.1
Table C18: Conduction tests for die 1434A
84
Percent
Error
2.1
4.3
1.7
APPENDIX D
EVAPORATION TEST RESULTS
Evaporation Test Die #1124C - Radial Wicks
Inside Outside
Power
Power
SU8
SU8 Wall Channel
Evaporation
RTD
RTD
across
into
Power into
Efficiency
Height
Width
Width
Rate
Temp
Temp Membrane Evap (mW)
Heater
(%)
(μm)
(μm)
(μm)
(mg/min)
(mW)
(ºC)
(ºC)
(mW)
27.1
36.9
32.7
22.7
2.4
1.7
8.7
30.3
37.9
33.2
25.2
2.6
1.8
8.5
10
10
10
34.1
38.8
33.6
27.7
2.5
1.8
7.5
37.2
38.8
33.6
30.3
2.4
1.7
6.5
41.3
40.6
34.5
36.2
2.4
1.7
5.9
Table D1: Evaporation tests for die 1124C
Evaporation Test Die #1129F - Annular Wicks
Inside
Power
SU8
SU8 Wall Channel
RTD
into
Height
Width
Width
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
28.2
37.2
31.0
38.4
10
10
10
34.4
39.6
38.1
40.8
41.3
41.9
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
32.5
25.3
33.0
28.8
33.5
32.6
34.0
36.6
34.4
40.2
Power into
Evap (mW)
2.0
1.9
2.0
2.3
2.0
Evaporation
Efficiency
Rate
(%)
(mg/min)
1.4
1.4
1.4
1.6
1.4
6.9
6.1
5.9
6.2
4.9
Table D2: Evaporation tests for die 1129F
Evaporation Test Die #1307J - Radial Wicks
Inside
Power
SU8
SU8 Wall Channel
RTD
into
Height
Width
Width
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
31.4
36.3
34.4
38.0
10
10
15
38.2
39.2
42.3
40.6
45.9
41.3
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
31.8
27.0
33.0
30.0
33.5
33.1
34.0
39.9
34.4
40.5
Power into
Evap (mW)
3.8
3.9
4.0
4.1
4.0
Evaporation
Efficiency
Rate
(%)
(mg/min)
3.0
2.9
2.9
2.9
2.8
12.0
11.4
10.6
9.7
8.8
Table D3: Evaporation tests for die 1307J
Evaporation Test Die #1307M - Radial Wicks
Power
SU8
SU8 Wall Channel
into
Height
Width
Width
Heater
(μm)
(μm)
(μm)
(mW)
30.1
33.6
10
5
10
37.8
41.3
45.9
Inside
RTD
Temp
(ºC)
30.3
37.6
38.7
39.2
41.5
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
26.0
25.7
32.7
29.2
33.2
32.5
33.3
35.5
34.9
39.5
Power into
Evap (mW)
3.6
3.8
4.1
4.2
4.5
Table D4: Evaporation tests for die 1307M
85
Evaporation
Efficiency
Rate
(%)
(mg/min)
2.6
2.7
2.9
3.0
3.3
12.0
11.2
10.8
10.1
9.9
Evaporation Test Die #1307N - Radial Wicks
Inside
Power
SU8
SU8 Wall Channel
RTD
into
Height
Width
Width
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
31.0
30.1
34.7
32.3
10
5
20
39.3
34.3
42.8
36.7
45.9
37.1
Outside
Power
Power into Evaporation
RTD
across
Efficiency
Evaporation
Rate
Temp Membrane
(%)
(mW)
(mg/min)
(ºC)
(mW)
25.7
25.7
4.6
3.3
14.9
27.4
28.9
4.5
3.3
13.1
28.2
36.1
4.9
3.5
12.4
30.1
39.0
5.3
3.5
12.4
30.5
38.9
5.3
3.5
11.6
Table D5: Evaporation tests for die 1307N
Evaporation Test Die #1308G - Radial Wicks
Power
Inside
SU8 Wall Channel
SU8
into
RTD
Width
Width
Height
Heater
Temp
(μm)
(μm)
(μm)
(mW)
(ºC)
46.4
34.2
10
5
35
64.9
40.9
86.8
51.4
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
28.8
31.7
32.7
49.0
39.5
71.0
Power into
Evap (mW)
13.3
13.0
12.8
Evaporation
Efficiency
Rate
(%)
(mg/min)
9.5
9.3
9.2
28.6
20.0
14.8
Table D6: Evaporation tests for die 1308G
Evaporation Test Die # 1421B- Radial Wicks
Inside
Power
SU8
SU8 Wall Channel
RTD
into
Height
Width
Width
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
32.3
32.0
10
5
70
54.0
35.3
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
27.7
25.4
27.6
46.3
Power into
Evap (mW)
6.1
6.0
Evaporation
Efficiency
Rate
(%)
(mg/min)
4.4
4.3
19.0
11.1
Table D7: Evaporation tests for die 1421B
Evaporation Test Die #1421A - Radial Wicks
Inside
Power
SU8 Wall Channel
SU8
RTD
into
Width
Width
Height
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
31.1
28.4
10
5
90
53.3
32.8
65.2
37.9
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
24.2
24.8
25.0
46.4
28.1
58.2
Power into
Evap (mW)
5.9
5.6
5.4
Evaporation
Efficiency
Rate
(%)
(mg/min)
4.2
4.0
3.9
18.8
10.5
8.3
Table D8: Evaporation tests for die 1421A
Evaporation Test Die #1423A - Radial Wicks
Inside
Power
SU8 Wall Channel
SU8
RTD
into
Width
Width
Height
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
27.8
29.8
10
5
50
38.1
32.9
49.9
35.0
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
26.3
20.9
27.7
31.1
29.9
30.3
Power into
Evap (mW)
6.6
6.4
6.4
Table D9: Evaporation tests for die 1423A
86
Evaporation
Efficiency
Rate
(%)
(mg/min)
4.7
4.7
4.7
23.6
16.9
12.9
Evaporation Test Die #1397A - Radial Wicks
Inside
Power
SU8
SU8 Wall Channel
RTD
into
Height
Width
Width
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
40.3
33.3
70
5
70
57.1
39.1
76.9
42.5
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
27.7
33.1
31.0
47.8
31.4
65.9
Power into
Evap (mW)
7.7
7.7
7.5
Evaporation
Efficiency
Rate
(%)
(mg/min)
5.5
5.5
5.4
19.0
13.4
9.8
Table D10: Evaporation tests for die 1397A
Evaporation Test Die #1397B- Radial Wicks
Inside
Power
SU8
SU8 Wall Channel
RTD
into
Height
Width
Width
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
46.2
34.9
70
5
35
65.9
40.7
87.4
44.9
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
29.0
35.1
30.8
58.9
32.2
75.7
Power into
Evap (mW)
9.4
9.4
9.4
Evaporation
Efficiency
Rate
(%)
(mg/min)
7.0
7.0
6.7
0.2
3.6
2.7
Table D11: Evaporation tests for die 1397B
Evaporation Test Die #1397D- Radial Wicks
Inside
Power
SU8 Wall Channel
SU8
RTD
into
Width
Width
Height
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
50.8
34.6
70
5
35
77.2
41.1
108.2
48.9
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
28.1
38.7
30.1
65.8
33.1
94.1
Power into
Evap (mW)
9.2
9.2
9.2
Evaporation
Efficiency
Rate
(%)
(mg/min)
6.6
6.6
6.4
18.1
11.9
8.5
Table D12: Evaporation tests for die 1397D
Evaporation Test Die #1396E- Radial Wicks
Inside
Power
SU8 Wall Channel
SU8
RTD
into
Width
Width
Height
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
41.7
35.5
40
5
70
67.5
47.4
96.9
57.2
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
30.5
29.7
37.6
58.1
42.1
89.4
Power into
Evap (mW)
11.6
11.6
11.6
Evaporation
Efficiency
Rate
(%)
(mg/min)
8.3
8.3
8.3
27.8
17.2
12.0
Table D13: Evaporation tests for die 1396D
Evaporation Test Die #1411A- Radial Wicks
Inside
Power
SU8 Wall Channel
SU8
RTD
into
Width
Width
Height
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
31.1
30.7
40
5
70
50.2
36.1
66.8
43.4
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
27.5
19.2
29.4
40.1
34.4
53.5
Power into
Evap (mW)
11.4
11.6
11.6
Table D14: Evaporation tests for die 1411A
87
Evaporation
Efficiency
Rate
(%)
(mg/min)
8.2
8.3
8.3
36.8
23.1
17.4
Evaporation Test Die #1411B- Radial Wicks
Inside
Power
SU8
SU8 Wall Channel
RTD
into
Height
Width
Width
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
36.9
32.2
40
5
50
51.3
36.3
67.4
41.0
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
27.8
25.8
29.8
38.9
31.6
55.7
Power into
Evap (mW)
9.9
10.2
9.9
Evaporation
Efficiency
Rate
(%)
(mg/min)
7.1
7.3
7.1
26.8
19.9
14.7
Table D15: Evaporation tests for die 1411B
Evaporation Test Die #1411C- Radial Wicks
Inside
Power
SU8
SU8 Wall Channel
RTD
into
Height
Width
Width
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
35.5
31.5
40
5
50
51.3
36.2
74.7
41.4
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
27.4
24.6
28.9
43.5
30.9
62.1
Power into
Evap (mW)
10.0
9.9
10.0
Evaporation
Efficiency
Rate
(%)
(mg/min)
7.2
7.1
7.2
28.3
19.3
13.4
Table D16: Evaporation tests for die 1411C
Evaporation Test Die #1411D- Radial Wicks
Inside
Power
SU8 Wall Channel
SU8
RTD
into
Width
Width
Height
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
33.5
29.8
40
5
90
45.6
34.0
74.7
40.8
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
26.1
22.2
28.3
34.3
30.3
62.8
Power into
Evap (mW)
10.3
10.2
10.5
Evaporation
Efficiency
Rate
(%)
(mg/min)
7.4
7.3
7.5
30.8
22.4
14.0
Table D17: Evaporation tests for die 1411D
Evaporation Test Die #1424A- Radial Wicks
Inside
Power
SU8 Wall Channel
SU8
RTD
into
Width
Width
Height
Temp
Heater
(μm)
(μm)
(μm)
(mW)
(ºC)
37.7
35.2
40
5
35
47.8
40.4
62.5
42.2
Outside
Power
RTD
across
Temp Membrane
(ºC)
(mW)
30.0
30.6
33.2
42.9
33.2
53.3
Power into
Evap (mW)
6.3
6.3
6.3
Table D18: Evaporation tests for die 1424A
88
Evaporation
Efficiency
Rate
(%)
(mg/min)
4.5
4.5
4.5
16.6
13.1
10.0
APPENDIX E
VISUALIZATION
Figure E1: 5x10 μm SU8 walls with 50 μm fluid channels with
50 mW power input
Figure E2: 5x40 μm SU8 with 70 μm channels at 100mW power input
89
Figure E3: 5x40 μm SU8 with 70 μm channels and 120mW power input
Figure E4: 5x70 μm SU8 with 70 μm channels – Fluid channels
remain filled throughout power input ranges
90
Figure E5: 10x10 μm SU8 with 10 μm channels – Overview of
completed wick and measurement tools
91
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