TS TOC
Proceedings of IMECE2002
ASME International Mechanical Engineering Congress & Exposition
November 17-22, 2002, New Orleans, Louisiana
IMECE2002-32043
EVOLUTION OF MICROCHANNEL FLOW PASSAGES – THERMOHYDRAULIC PERFORMANCE
AND FABRICATION TECHNOLOGY
Satish G. Kandlikar
[email protected]
Mechanical Engineering Department
William J. Grande
[email protected]
Microelectronic Engineering Department
Rochester Institute of Technology
Rochester, NY 14623
ABSTRACT
This paper provides a roadmap of development in the thermal
and fabrication aspects of microchannels as applied in the
microelectronics and other high heat-flux cooling applications.
Microchannels are defined as flow passages that have hydraulic
diameters in the range of 10 to 200 micrometers. The impetus
for microchannel research was provided by the pioneering work
of Tuckerman and Pease [1] at Stanford University in the early
eighties.
Since that time, this technology has received
considerable attention in microelectronics and other major
application areas, such as fuel cell systems and advanced heat
sink designs.
After reviewing the advancement in heat transfer technology
from a historical perspective, advantages of using
microchannels in high heat flux cooling applications is
discussed, and research done on various aspects of
microchannel heat exchanger performance is reviewed. Singlephase performance for liquids is expected to be still describable
by the conventional equations; however the gas flow may be
influenced by the rarefaction effects. Two-phase flow is another
topic that is still under active research.
The evolution of research into microchannel heat sinks has
paralleled the advancements made in microfabrication
technology. The earliest microchannels were built using
anisotropic wet chemical etching techniques based on alkali
solutions. While this method has been exploited successfully, it
does impose certain restrictions on silicon wafer type and
geometry. Recently, anisotropic dry etching processes have
been developed that circumvent these restrictions. In addition,
dry etching methods can be significantly faster and, from a
manufacturing standpoint, create fewer contamination and
waste treatment problems.
Advances in fabrication technology will continue to fuel
improvements in microchannel heat sink performance and cost
for the foreseeable future. Some fabrication areas that may spur
advances include new materials, high aspect ratio patterning
techniques other than dry etching, active fluid flow elements,
and micromolding
NOMENCLATURE
A
Dh
f
G
h
Kn
L
Nu
P
R
q
1
Surface area, (m2)
Hydraulic diameter ( m )
Fanning friction factor
Mass flux ( kg/m2s )
Heat transfer coefficient ( W/m2K )
Knudsen number
Length ( m )
Nusselt number ( = q / AHT*∆TLMTD )
Pressure ( kPa )
Gas Constant
Heat transfer ( W )
Copyright © 2002 by ASME
Re Reynolds number ( = G*Dh / µ )
T Temperature ( °C )
Greek
λ mean free path, (m)
µ Viscosity ( N / sm2 )
ρ Density ( kg/m3 )
Subscripts
f fluid
s surface
1. INTRODUCTION
1.1 Historical Advancements in Heat Transfer
Technologies
Heat, or thermal energy, represents the ultimate
manifestation of all forms of energy. Transfer of heat from one
location to other, from one medium to another, and meeting the
challenges of accomplishing this transfer under a variety of
constraints, have been the objectives of heat transfer research
ever since fire was “domesticated.”
Heat transfer by convection provides a means of transferring
heat quickly away from heat exchange surfaces. Fluids
employed in engineering systems and processes undergo the
changes in their thermal state in heat exchangers. The basic
equation of heat transfer by convection is expressed as follows:
q = h A Ts − T f
(1)
(
)
The early developments in the 19th and early 20th centuries
focused on increasing the surface area to accommodate higher
heat transfer rates. Shell and tube heat exchangers dominated
the scene with their ability to scale up in size to individual units
rivaling in some cases the sizes of modest single-family homes.
The era of compact heat exchangers began its earnest drive
through demands from the transportation sector – automotive,
aircraft, submarine, and spacecraft. Equation (1) was revisited,
this time the emphasis being on improving h and A
simultaneously, but with added constraints of the overall
volume and weight. Plate-fin exchangers utilizing small size
passages, on the order of a few mm, were developed for gas
applications. Applications of novel fins, including microfins,
became prevalent in single-phase and two-phase applications,
use of twisted tapes and other enhancement devices were
developed in an effort to provide a major facelift to older
generation technology utilizing large hydraulic diameters, on
the order of several inches (use of English units is intentional to
reflect the true state of art; although it will go undoubtedly
unnoticed by many US industries who are still refusing to
translate themselves, perhaps for some valid reasons, into the SI
units!)
The process industry, with somewhat liberal views on the
space constraint, embraced the flexibility of the plate heat
exchangers. The cryogenic industry, with its eye on the heat
exchanger effectiveness, were ahead of the pack through the use
of compact regenerators using sub-millimeter sized flow
passages. The refrigeration industry, realizing the benefits of
economics alone, embraced the microfin tubes in residential and
commercial evaporators and condensers. Single-phase and twophase applications received the same aggressive treatment
throughout the heat transfer industry.
1.2 Heat and Mass Transfer in Biological Systems
Nature provides us with some important clues regarding the
heat and mass transfer processes. For example, the African
elephants have larger ears than those in Asia; the higher
temperature in the desert environment in Africa requires a larger
surface area for the ears, which are the main heat dissipation
devices for elephants. Looking at the biological systems, such
as a human body, Chen and Helmes [2] found that the blood
vessels that are largely responsible for thermal exchange have
sizes on the order of hundreds of micrometers, 175 µm diameter
being typical, (known as thermally significant blood vessels).
The mass transfer processes on the other hand take place in
much smaller sized vessels, such as alveoli, which are on the
order of a few micrometers, and form the air sacs at extremities
of the air passageways in the lungs. The arterioles and venules,
which are smallest vessels for blood transportation are only 10
to 15 µm in diameter. The capillaries, where most of the mass
transfer processes occur, are only 4 µm in diameter. The mass
transfer effectiveness of these three units, arterioles, venules
and capillaries are over three orders of magnitude higher than
the larger vessels, (Lightfoot and Duca [3]).
1.3 Channel Classification
The thermal scientists in the last two decades took another
look at eq. (1) in their attempt to address the challenges posed
by the high heat flux devices. High heat fluxes coupled with the
small device sizes led to smaller channel dimensions. The word
“micro” was embraced enthusiastically with the opening of its
newest branch in microscale heat transfer. The classification of
small channel dimensions as proposed by Mehendale et al. [4]
divides the range from 1 µm to 100 µm as microchannels, 100
µm to 1 mm as meso-channels, 1 mm to 6 mm as compact
passages, and > 6 mm as conventional passages.
This
classification is based simply on the dimensions of the channels.
The classification provided by Kandlikar [5, 6], further refined
below, is based on the flow considerations.
Conventional techniques are applied in making the channels
of 3 mm or larger hydraulic diameter. The channel sizes below
about 3 mm are formed as narrow fin passages, as in plate-fin
heat exchangers. The regenerative heat exchanger matrix and
plate heat exchangers belong to this category. The lower limit
for manufacturing smaller channels is really imposed by the
major changes in fabrication technology warranted below about
200 µm. The range for compact heat exchanger passages is
expected to decrease in the future, with the hydraulic diameter
range 200 µm to 3 mm classified as compact heat exchangers.
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Copyright © 2002 by ASME
As will be seen in later sections, no fundamental change occurs
in the single phase liquid and gas flows (incompressible and in
the absence of rarefaction effects) or two-phase flows in
channels up to 200 µm. Below 200 µm, the manufacturing
techniques and operational considerations for cleanliness
become extremely important. The next range under this
classification, termed as microchannel, is influenced by the
rarefaction effects for gases, as described by the Knudsen
number, Kn:
Kn = λ / Dh
(2)
where λ is the mean free path for the gas calculated from the
following equation:
µ π
λ=
(3)
ρ 2 RT
where R – gas constant, J/kgK, µ - dynamic viscosity, N/ms, ρdensity, kg/m3, and T is absolute temperature in K.
Table 1 gives the values of mean free paths for different
gases at 300 K. As an example, the mean free path for air at
300 K is 0.068 µm. The microchannel range, covering 10 µm
to 200 µm, is generally affected by the rarefaction effects for
many gases. The continuum approach with no wall slip is
modified in these channels; this approach being valid for
0.1>Kn>0.001, the region identified as the slip region (further
discussed in section 2.3).
Below 10 µm, depending on the gas and the pressure, the
transitional region is encountered, where rarefaction effects are
more severe and approach the molecular flow. The range 10 ≥
Kn ≥ 0.1 is referred to as the transitional region. Tentatively,
we may assign the channel dimensions from 10 µm ≥ Dh ≥ 0.1
µm (1000 Å) to a newly defined region as Transitional
Nanochannels.
Table 1 Mean free path calculations for gases at atmospheric
pressure
ρ,
T,
K
R,
J/kg K
kg/m3
Air
300
287.0
1.1614
Helium
300
2077.03
Hydrogen
300
4124.18
Gas
Nitrogen
300
296.8
µ,
kg/m s
1.846×10-
λ,
µm
5
0.068
0.1625
1.99×10-5
0.194
0.08078
8.96×10-6
0.125
1.1233
1.782×10
5
-
0.066
Conventional channels Dh > 3mm
Minichannels –
3mm ≥ Dh > 200 µm
Microchannels –
200 µm ≥ Dh > 10 µm
Transitional Channels10 µm ≥ Dh > 0.1 µm
Transitional Microchannels -10 µm ≥ Dh > 1 µm
Transitional Nanochannels - 1 µm ≥ Dh > 0.1 µm
Molecular Nanochannels - 0.1 µm ≥ Dh
Although the above criteria are developed mainly from gas
flow considerations, they are recommended for both liquid as
well as two-phase flow applications to provide a uniformity in
channel classification. As will be seen later in section 2.4,
thermal and flow characteristics of minichannels, as classified
above, in the flow boiling region seems to be only slightly
affected as compared to the conventional channels.
1.4 Applications of Microchannels in Heat Transfer Devices
Owing to their higher thermal performance, minichannels are
being increasingly employed in process applications; the higher
pumping power requirements are offset with the overall size and
cost reductions.
In the automotive and aero industry,
minichannels proved to be valuable in addressing the severe
space constraints.
Compact heat exchangers employ
minichannels in these and many other applications.
The microelectronics engineers, conversant with the submicron scale in their IC designs, and the mechanical engineers,
familiar with the minichannels in compact heat exchangers,
found the microchannel range as a desirable compromise in
microelectronics cooling application. In biomedical and optical
applications, transitional nanochannels are often employed.
The race is far from over, mechanical engineers moving from
the comfortable confines of heat sinks mounted on electronics
devices, components and assemblies, to the uncharted territories
of microchannels, partnering with the microelecectronics
engineers for their on-chip flight to meet the new micron and
sub-micron sized thermal and fabrication challenges.
Understanding of these systems will enable us to proceed to the
next level of nanochannels in biological applications.
1.5 A Note on the Historical Perspective
The paper is intended to provide a historical perspective to
the developments in microchannels from different viewpoints.
The task is very much complicated by the fact the
microchannels represent a relatively new technology in the heat
transfer application, tracing its roots with the pioneering work
of Tuckerman and Pease [1] just over twenty years ago. Active
research on establishing its thermal hydraulic performance in
single and two-phase applications has started in the earnest only
in the last five years. The major landmarks are being currently
in the making, and it is difficult to identify them as they are
being presented in various technical conferences and journals.
On the basis of the above discussion, the following
classification is presented.
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Copyright © 2002 by ASME
It is therefore hoped that the present paper would provide a
good reflection of the twenty years of development in this field
to facilitate an accurate historical account a decade later when
the field would have matured to some extent.
2. THERMOHYDRAULIC PERFORMANCE OF
MICROCHANNELS
2.1 Basic Heat Transfer and Pressure Drop Relationships
The effect of hydraulic diameter on heat transfer and
pressure drop is illustrated in Fig.1 for water and air flowing in
a square channel under a constant heat flux and fully developed
laminar flow conditions. The heat transfer coefficient is
unaffected by the flow Reynolds number in the fully developed
laminar region. It is given by
Heat transfer coefficent, W/m 2K
h = Nu
k
D
(4)
2 f G2
=
(5)
L
ρD
where ∆p f / L is the frictional pressure gradient, f is the
Fanning friction factor, G is the mass flux, ρ is the density,
∆p f
and D is the hydraulic diameter. For fully developed laminar
flow, we can write-
1000000
100000
Water
10000
Air
1000
100
10
10
100
1000
10000
Hydraulic diameter (=side) of a square channel, µ m
Fig. 1a Variation of heat transfer coefficient with
channel size for a square channel under laminar flow,
constant heat flux boundary condition, assuming no
rarefaction and compressibility effects
1.00E+08
Pressure Gradient, Pa/m
Nusselt number for fully developed laminar flow in a square
channel under constant heat flux conditions is 3.61. Figure 1a
shows the variation of h with channel hydraulic diameter under
laminar fully developed flow conditions.
The dramatic
enhancement in h with a reduction in channel size is clearly
demonstrated.
However f varies inversely with Re, since the product f*Re
remains constant during fully developed laminar flow. The
frictional pressure drop per unit length for the flow of an
incompressible fluid is given by-
1.00E+07
Water
1.00E+06
Air
1.00E+05
1.00E+04
1.00E+03
1.00E+02
1.00E+01
10
100
1000
10000
Hydraulic diameter (=side) of a square channel, µ m
Fig. 1b Variation of pressure gradient for fully
developed laminar flow in smooth circular tubes at
2
300 K for water at G=200 kg/m s (V=0.20 m/s), and air
2
at 5 kg/m s (V=4.25 m/s), assuming no rarefaction and
compressibility effects.
f Re = C
(6)
where Re is the Reynolds number, Re=GD/ µ , and C is a
constant, =14.23 for a square channel. Figure 1b shows the
variation of pressure gradient with the channel size for a square
channel with G=200 kg/m2s, for air and water assuming
incompressible flow conditions. These plots are for illustrative
purposes only, as the above assumptions may not be valid for
the flow of air, especially in smaller channel sizes. It is seen
from Fig. 1b that the pressure gradient increases dramatically
with a reduction in the channel size.
2.2 Single Phase Liquid Flow in Microchannels
Single phase flow is expected to be unaffected for liquids, as
the hydraulic diameter is reduced in the range from 200 to 10
µm. These channel dimensions are still a few orders of
magnitudes higher than the molecular mean free flow path. The
studies by Richter et al. [7] and Pfahler et al. [8] support these
observations.
Richter et al. [7] etched channels by KOH solution
producing 54.7° side angles for the triangular channels. The
top width was between 28 and 182.7 µm and the length of the
flow channel was set at 2 mm. The flow rate was between 0.01
to 1000 µl/min. The flow was laminar with Re less than 1.
Richter et al. compared their experimental results with the
predictions using the standard triangular channel friction
factors. The agreement was very good over the entire range.
They also noted that the flow rate was quite sensitive to the
temperature as the viscosity of water changed considerably with
temperatures over the experimental range of 20 to 50°C.
Pfahler et al. [8] conducted experiments with N-propanol in
two different size rectangular microchannels. The larger ones
were made of silicon with <110> orientation, 53 µm deep by
135 µm wide, while the smaller channels were made of silicon
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Copyright © 2002 by ASME
with <100> orientation, only 1.7 and 0.8 µm deep, and100 µm
wide. Their results indicate that for all the test sections with the
exception of the smallest depth of 0.8 µm, the conventional
theory predicted the friction factor quite well. For the test
section with 0.8 µm depth, a threefold increase in the friction
factor was noted. The results indicated a large contribution due
to developing length. For such small channels, accurate height
measurement was difficult. From this study, it can be
concluded that the conventional theory is applicable to liquid
flow in channels as small as 1.7 µm in depth.
Phillips et al. [9] conducted extensive studies on the
application of liquid cooled microchannels for chip cooling
applications.
The study included numerical as well as
experimental work, including incorporation of longitudinal fins.
An experimental study conducted by Kandlikar et al. [10]
with water flowing in 200 µm square channels yielded excellent
agreement with the conventional theory for friction factor. In
the case of diabatic flow, the property correction method for
friction factor was seen to work quite well. Although the heat
transfer results are expected to yield similar agreement with the
conventional channels, considerable discrepancy was noted,
perhaps due to the heating from only three sides of the channel.
Further confirmation in this regard may be warranted.
2.3 Single Phase Gas Flow in Microchannels
The effect of rarefaction is expressed by the Knudsen
number defined in eq. (2). At high values of Kn, the continuum
assumption of no slip at the wall no longer holds. Harley [11]
provides the classification described in Table 2 on the basis of
the Knudsen number.
The range of channel dimensions for different types of flow
conditions is presented in Table 3 for three gases. It is seen that
for all gases listed here, the flow will enter into the slip flow
condition for microchannels, which are defined as channels in
the hydraulic diameter range of 10 to 200 µm.
For Kn<0.1, rarefaction effects become important. In the
slip flow region, 0.1>Kn>0.001, which is generally of interest
in the microchannel and fluidics MEMS devices, the continuum
theory can be modified by applying a slip ratio at the wall.
Ebert and Sparrow [12] presented a wall slip model using the
first order derivative of the velocity profile at the wall. Harley
[11] and Harley et al. [13] presented a comprehensive model
accounting for the wall effect during flow through parallel
channels and rectangular microchannels.
In their formulation, Ebert and Sparrow [12] modeled the
wall slip condition using the first derivative of the velocity
profile at the wall. Aubert and Colin [14] point out that the
Taylor series form of the velocity profile proposed by Ebert and
Sparrow does not converge with the second order boundary
condition. Aubert and Colin [14] used a second order boundary
condition at the wall. Although their results provided a better
fit with the experimental data, they yield accommodation
factors of greater than 1, suggesting that these be treated as
empirical constants at this time.
Table 2 Knudsen number ranges for various types of flow
Range of Knudsen Numbers
0.001>Kn
0.1>Kn>0.001
10>Kn>0.1
Kn>10
Type of Flow
Continuum
Flow:
rarefaction effects
no
Slip flow: rarefaction effects
that can be modeled with a
modified continuum theory
accounting for wall slip.
Transition Flow: a type of
flow between slip flow and
free molecular flow that is
analyzed statistically; i.e.
with Boltzman equation.
Free
Molecular
Flow:
motion
of
individual
molecules must be modeled
and then treated statistically
Table 3 Channel dimensions in µm for different types of
flow for gases at one atmospheric pressure
Channel Dimensions in µm
Gas
Continuum
Flow
Slip
Flow
Transition
Flow
Free
Molecular
Flow
Air
>67 µm
0.6767 µm
0.00670.67 µm
<0.0067
µm
Helium
>194 µm
1.94194
µm
0.01941.94 µm
<0.0194
µm
Hydrogen
>123 µm
1.23123
µm
0.01231.23 µm
<0.0123
µm
The wall effects also influence the heat transfer
characteristics in microchannels. In the case of fluid flow, the
friction coefficient was reduced, yielding a higher mass flow
rate of gases as compared to the predictions from conventional
correlations. In case of heat transfer, a decrease in the heat
transfer coefficient is expected as channel dimensions become
smaller, or Knudsen number increases beyond 0.001.
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Copyright © 2002 by ASME
The approach proposed by Li et al. [15] represents an
alternative model for analyzing the wall effect.
The
temperature and velocity jumps at the wall proposed in the slip
flow models are replaced with a continuous variation of
viscosity and thermal conductivity in the layer close to the wall
within a several mean free path distance. Further evaluation of
this model is needed before it can be applied to other systems.
This approach however seems to be promising as the wall
effects are modeled through a continuously variable property
rather than a jump condition, which is not clearly understood.
Hadjiconstantinou and Simek [16] analyzed the fully
developed flow in the slip flow and transition regions for the
case of constant wall temperature. They included the axial heat
conduction term in the slip flow model. Their results indicate
that axial conduction increases the Nusselt number on the order
of 10 percent.
2.4 Flow Boiling in Microchannels
Surface tension plays an important role as channel
dimensions become smaller. An exhaustive review of literature
was conducted by Kandlikar [5, 6] on the fundamental issues
related to flow patterns in microchannels and minichannels.
Readers are referred to those articles for further details.
Briefly, the following important points can be summarized.
• The flow patterns observed in the microchannels are
strongly dependent on time, with a sequence of
different flow patterns passing through the channel as a
result of flow instabilities. The results are more
pronounced in multichannels.
• Occasional flow reversals are commonly experienced
in a channel. The visual studies have confirmed that
the fluctuations result from the rapid expansion of a
bubble into a slug, which pushes the liquid away both
upstream as well as downstream of the flow.
• The heat transfer coefficient during flow boiling in
microchannels can be predicted with the existing
correlations, such as Kandlikar (1990) correlation, for
large diameter tubes.
A recent study reported by Kandlikar et al. [17] shows that
the flow boiling heat transfer coefficient with liquid flow
Reynolds numbers considerably smaller than the transition
value of 2300 are predicted well by using the conventional flow
boiling correlation, such as by Kandlikar [18], with the fully
developed laminar flow value for the all liquid heat transfer
coefficient, instead of a turbulent flow correlation. Further
work is continuing in this area.
In the author’s laboratory, we have recently obtained high
speed images of the flow patterns in 200 µm square
microchannels. The presence of the churn flow, as shown in
Fig. 2a, has been seen perhaps for the first time, in such small
diameter channels. Flow reversal behavior for these channels
was also observed. Figure 2b shows a sequence with a bubble
expanding into a slug that pushes the liquid flow back into the
inlet manifold.
Fig. 2a Churn Flow sequence observed during flow
boiling of water in a 200 µm square microchannel
Figure 2b Expanding bubbles into slugs that create a
reversed flow, flow from left to right, 200 µm square
microchannels with flow boiling of water near
atmospheric pressure
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Copyright © 2002 by ASME
dimensional scales. The groupings are meant to highlight
contrasts and similarities and are not absolute. For reasons of
space and clarity many minor technologies have been omitted.
Some techniques such as lithography, laser exposure,
electroplating and molding have been widely adopted and are
encountered in multiple fabrication methodologies.
The first major division identified in the taxonomic hierarchy
is between miniaturized traditional and “modern” technologies.
Miniaturized traditional techniques are rooted in conventional
machine shop and manufacturing practices but adapted to
achieve micro-scale features. The “modern” technologies are
more difficult to characterize but are generally based on
advances that occurred in the latter half of the Twentieth
Century, such as lasers and micron-level photolithography.
Semiconductor and allied fabrication methods account for the
bulk of the “modern” technologies.
Miniaturized traditional techniques are in some respects the
most straightforward approach to creating micro-features. These
miniature techniques often use conventional machine tools that
are specially adapted to operate in the micro regime. The
adaptations range from shrinking the machine tool itself to the
introduction of lithographic patterning. Small machine tools
such as miniature milling machines [19] have been
demonstrated. Sawing has been taken into the micro realm
especially in the form of wafer dicing. Saw cuts on the order of
25 µm width with a placement accuracy of 4 µm at 3 sigma can
be obtained with commercially available equipment [20].
2.5 Condensation in Microchannels
Condensation heat transfer is significantly enhanced in
microchannels. Although condensation heat transfer is studied
extensively in minichannels, it has received little attention in
microchannels because of difficulties associated with
experimentation and testing. The space constraints during
condensation are not as severe as in boiling applied to high heat
flux removal situations.
For this reason, the topic of
condensation is not reviewed in this paper.
3. FABRICATION TECHNIQUES FOR MICROCHANNELS
Heat transfer flow paths having a characteristic dimension in
the range of 10-200 µm were classified above as microchannels.
The evolution of microchannel-based heat exchangers has
largely been paced by advances in microfabrication technology.
In this section we give an overview of existing microfabrication
techniques with an emphasis on those particularly suited for
building microchannel devices. It should be noted that various
subsets of these technologies can be usefully applied to the
fabrication of channels of both greater and lesser dimensions,
covering the range from minichannels to molecular
nanochannels.
3.1 Microfabrication Taxonomy
A group of microchannel fabrication technologies is depicted
taxonomically in Fig. 3. This collection of microfabrication
techniques covers a broad range of machining principles and
Taxonomy of Microfabrication
Modern
Miniaturized Traditional
Shop Techniques
Manufacturing Techniques
Printed Circuit Board
Stereolithography
Electroforming
Molding
Milling and Sawing
Electro-discharge
Ultrasonic/Waterjet Cutting
Silicon-based
Bulk
LIGA
UV-LIGA
Surfac
Wet Chemical
Etching
Batch
Micromolding
Microstamping
System-onChip
Serial
Laser Machining
Focused Ion Beam
Other
Material
Hybridization
Wafer
Bonding
Glasses
Ceramics
Metals
Disruptive Technologies
Assembly
Packaging
New Materials
New Processes
New Designs
Nanotechnology
Fusion
Anodic
Adhesive
Fig. 3 Taxonomic Chart of Microfabrication Technologies
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Copyright © 2002 by ASME
Micro-electro-discharge machining [21-23] has been
demonstrated using very fine wires as electrodes. Other cutting
techniques such as ultrasonic and waterjet machining are
especially effective on hard brittle materials and are being
practiced at ever finer scales.
Included as miniaturized traditional techniques are a number
of manufacturing methodologies. Commercial electroforming,
molding, and stereolithographic fabrication have been brought
into the micro regime through the incorporation of lithographic
and laser-based patterning. The printed circuit board industry,
for example, is routinely producing micro via holes at the 25
µm scale.
The “modern” technologies can be distinguished as either
serial or batch and have been reviewed by a number of excellent
sources [24-31]. Because serial techniques machine objects in a
point by point fashion they tend to have low material removal
rates and low throughput. However, these techniques are often
used for specialized high-value low-repetition operations like
micro-feature repair and via formation. Laser machining has
become an increasingly powerful tool that can handle a wide
variety of difficult materials. Focused ion beam machining
offers many similar benefits and can operate in the sub-micron
regime.
3.2 Semiconductor-like Fabrication Techniques
Much of the current research and excitement in the fields of
microsystems technology (MST) and microelectromechanical
systems (MEMS) is centered among a group of batch
microfabrication methods that arose from the semiconductor
sector. Silicon-based variants are the most prevalent because of
the enormous installed infrastructure that supports the
microelectronics industry.
Silicon-based micromachining techniques can be broadly
divided into two groups. In bulk micromachining the final part
is made by selectively removing portions of the starting
substrate. Because the final part is made from the original single
crystal silicon it has tremendous strength and is virtually stressfree. In contrast surface micromachining creates the final part
on or above the starting substrate through a series of deposition,
patterning, and etching steps. In many cases the substrate is
completely irrelevant to the fabrication process and silicon
becomes the substrate of choice for reasons of cost and process
compatibility.
The major issue in bulk micromachining is the etch process.
Bulk etching can be carried out in either a wet chemical or dry
plasma format and both techniques have isotropic and
anisotropic variants.
Anisotropic wet chemical etching (WCE) of silicon has been
the workhorse technique since the original work of Tuckerman
and Pease [1]. A number of etchants including potassium
hydroxide and ethylene diamine pyrocatechol will etch the
{111} planes of silicon at such a slow rate compared to other
crystal directions that the {111} planes can be thought of as etch
stops [32, 33]. In silicon with a (100) surface the {111} planes
can be used to create V-grooves with a 54.74° angle to the
surface. In (110) oriented silicon the {111} family has two pairs
of plane that are perpendicular to the wafer surface and intersect
each other at 70.53°. (The other two pairs shallowly intersect
the surface at 35.26° and help determine the shape of the
bottom of the etch trench.) The dependence on crystal
orientation means that WCE can produce only a very few
specific microchannel device designs. Care must be taken when
aligning the etch mask to the crystal planes in order to avoid
unwanted artifacts [34]. Anisotropic WCE etch rates are
typically in the range of 1 µm/min, so that, etch times of many
hours are common. Wafers can, however, be etched in batches
to improve throughput. Another constraint of anisotropic WCE
is that features formed by the intersection of (111) planes are
stable only where the angle of the corner is less than 180°. This
means that properly bounded isolated trenches can be etched to
arbitrary depths; whereas, isolated mesas will undergo attack at
the corners and may require compensation techniques to
achieve the desired shape [35].
Wet chemical etching can also be carried out in an isotropic
manner, most commonly using the “HNA” system of
hydrofluoric acid, nitric acid, and acetic acid. Isotropic WCE
can exhibit high etch rates – greater than 100 µm/min – and
results in hemispherical etch profiles.
A recent advance in etch technology has been the emergence
of dry etch techniques [36-39]. Of particular interest are the
deep reactive ion etch (DRIE) processes which can produce
vertical etch profiles in silicon. The most commercially
prominent technique is the so-called “Bosch etch” [40] which
uses alternating etch and polymer passivation chemistries [41].
The reaction chamber is typically fitted with an inductively
coupled energy source to create plasmas that are one to two
orders of magnitude denser that those obtained by conventional
parallel-plate reactive ion etching. The substrate is either
mechanically or electrostatically held to a cooled platen with a
separate bias to control incident ion energy. Typical etch rates
are in the range 2 – 8 µm/min; however, etch rates greater than
20 µm/min have been reported in a specially designed tool [42].
An attractive feature of DRIE is that the etch process is readily
masked with a variety of dielectric and polymer films such as
silicon dioxide and photoresist. Etch rate ratios between silicon
and the masking materials can be in the range 50 –150. Thus,
deep trenches and through-wafer features can be readily
obtained. Fig. 4 shows a set of microchannels with 42 µm
trenches that were etched to greater than 100 µm. The strong
interplay among process responses such as etch rate, sidewall
angle, and sidewall roughness has been characterized by Ayon
and co-workers [43]. Dry isotropic etching can be obtained
without a plasma by exposing silicon to the gas xenon
difluoride with which it reacts spontaneously at room
temperature [44-45]. Bulk micromachining etch processes are
summarized in Fig. 5.
8
Copyright © 2002 by ASME
(a) Anisotropic etch and sidewall protection
1 mm
(b) Isotropic etch to form microchannels
Fig. 4 Microchannel array formed by silicon DRIE.
(c) Microchannels sealed by conformal film deposition
Fig. 6 Construction of buried microchannels by
combined anisotropic and isotropic etching.
(100) Silicon
(110) Silicon
(a) Anisotropic wet chemical etch profiles.
(b) Anisotropic deep reactive ion etch profiles.
(c) Isotropic profiles obtainable from either
wet or dry etching.
Fig. 5 Bulk micromachining etch profiles.
Etch techniques can be combined in unique ways to create
complex microchannel structures. Fig. 6 schematically
illustrates a process [46] that can create microchannels buried
within the interior of a wafer. It is also possible to build sets of
microchannels at different depths and have them cross over
each other. The technique uses an initial anisotropic etch to
create a deep narrow trench in a silicon substrate. A mask layer
is coated on the sides of the trench but not the bottom and a
subsequent isotropic etch step forms the channel. The initial
trench can then be plugged by conformal deposition of a thin
film.
Bulk micromachining and surface micromachining are
complementary techniques and can be combined to build multifunctional systems. Zohar and co-workers [47, 48] have built
microchannel devices with integrated heaters and temperature
sensor arrays. It is anticipated that true systems-on-chip that
combine microcooling with sensing, computation, active fluidic
components, on-board power sources and communication to the
outside world will evolve in the future.
3.3 High Aspect Ratio Lithography and Molding
A class of very high aspect ratio fabrication processes based
on the lost wax molding technique have come to be known by
the term LIGA (a German acronym for lithographie,
galvanoformung,
abformung
meaning
lithography,
electroplating and molding). As shown in Fig. 7 LIGA uses
highly collimated x-rays projected through a special x-ray mask
to provide near diffraction-free exposure of a thick photoresist.
The developed features in the photoresist can then be filled with
a variety of materials and planarized. The technique can create
structures with aspect ratios in excess of 100:1 and can hold
submicron tolerances over many hundreds of microns of
vertical height [49]. Final parts can be obtained in three distinct
ways. First, the patterned resist can be separated from the
substrate and used as a precision machined polymer. Second,
the molded deposit, which is typically an electroformed metal
such as nickel or copper, can be separated from the substrate.
Lastly, the substrate and deposited metal can be used in
combination as a high precision molding master. High aspect
ratio electrodeposited features were first obtained using x-ray
exposure by Romankiw and coworkers in the mid-1970’s [50].
In 1982 Ehrfeld and co-workers recognized the potential of the
process to build molding masters that could be used to
inexpensively create ultra-precise high aspect ratio parts [51].
9
Copyright © 2002 by ASME
Optimal exposure wavelengths for LIGA are in the range of 0.20.5 nm and are obtained as bremstrallung radiation from a
synchrotron source. A wide variety of structures and
X-rays
Resist
Substrate
(a) Exposure of x-ray resist
(b) Development of resist
(c) Creation of a molding master
(d) Creation of a separable part
Fig. 7 The LIGA process.
(a) Wafers before bonding.
(b) Wafers after bonding.
Fig. 8 The wafer bonding Process
devices have been demonstrated using LIGA including fluid
channels and fluidic components [52], geared micromotors
[53], and high precision connectors [54]. The technique has
been extended to multiple feature levels and applied to nonplanar work pieces [55]. However, LIGA has failed to become
widely accepted because of the difficulty in making suitable xray masks and the cost and limited availability of the exposure
equipment.
There is great interest in alternative techniques that can
provide high aspect patterning using conventional ultraviolet
sources. These so-called ultraviolet-LIGA or UV-LIGA
processes have become increasingly viable with the
development of multiple coating techniques [56], thick layer
coating equipment [57], and chemically amplified resists. At
present the most widely used UV-LIGA material is a negativeworking, epoxy resin-based resist known as SU-8. Developed
originally by IBM [58] and offered commercially by
MicroChem [57] and SOTEC [60], SU-8 can be applied in
thicknesses up to 2 mm and exposed with standard
photolithographic tools. Aspect ratios greater than 20 have been
reported.
The existence of fabrication techniques such as DRIE and
LIGA has enabled the creation of high aspect ratio masters from
silicon or metal that can be used for micromolding and
microstamping a wide variety of parts and features [61-64].
3.4 Wafer Bonding Techniques
None of the technologies described above can individually
produce a complete microsystem. Hybridization is the process
of combining all the necessary disparate substrates, structures,
components, and sub-assemblies into a final product. An
extremely versatile variant of hybridization is wafer bonding,
where two flat substrates of nearly arbitrary composition can be
permanently attached. Direct wafer bonding is a collection of
processes whose exact details vary with material but the
technique can generally be tailored to obtain a wide range of
adhesive bond strengths. Plöβl and Kräuter have extensively
reviewed wafer bonding and its application to microsystems
construction [64].
Three bonding techniques of particular interest are fusion
bonding, anodic bonding, and adhesive bonding. In fusion
bonding two wafers whose surfaces are silicon or silicon
compounds such as the oxide and nitride can be covalently
bonded through a combination of chemical surface treatments,
pressure, and annealing at elevated temperature. When properly
performed the bond strength is at least as great as the bulk wafer
strength. Wafer stacks of greater than two wafers can be bonded
in either a serial or parallel fashion. Prior to fusion bonding the
wafers can be extensively machined. A variety of fusion bonded
microsystems with complex internal cavities and moving parts
have been realized, including accelerometers [64], microfluidic
valves [67], and micro turbine engines [68]. In anodic bonding
silicon and ionic glass surfaces are joined through a
combination of pressure, temperature, and electric field. While
both fusion and anodic bonding can produce interfaces with
10
Copyright © 2002 by ASME
great strength, they are quite material specific. For generic
hetero-bonding, adhesive techniques are the most general. Fig.
8 shows how microchannel devices can be formed by wafer
bonding.
4. FABRICATION TECHNIQUES FOR TRANSITIONAL
CHANNELS AND MOLECULAR NANOCHANNELS
Heat transfer flow paths in transitional channels and
molecular nanochannels have characteristic dimensions in the
10 – 0.1 µm and below 0.1 µm ranges, respectively. At these
dimensions many of the fabrication technologies described in
the preceding section can still be practiced with little
modification, while others must be modified or even abandoned
at very small dimensions. However, the exploding interest in
nanotechnology is beginning to offer unique fabrication tools
for the nanometer regime.
The dimensional range from 10 – 1 µm represents a
transition between MEMS fabrication and standard
semiconductor fabrication. In general both additive (i.e.
deposition) and subtractive (i.e. etching) fabrication
technologies become mainstream and widely available at
dimensions of a few microns down to approximately 0.1 µm.
Thus, products based on transitional channels can readily
leverage
the
global
microelectronics
manufacturing
infrastructure. At ultra-small dimensions the salient question
becomes: what is technologically possible and what is
commercially viable? Leading edge lithography today is
practiced down to 0.13 µm and is projected to reach 0.022 µm
by 2015 [69]. The ultimate feat of patterning was Eigler and
Schweizer’s spelling out of “IBM” in xenon atoms using a
scanning tunneling microscope [70]. For deposition and etching
techniques at ultrasmall dimensions the main issue is control
and accuracy. Advanced processes such as atomic layer epitaxy
and digital etching represent the ultimate in dimensional control
and are able to add or remove monatomic layers. Thus, the
physical limit to building molecular nanochannels is the atomic
structure of the channel materials themselves.
5. CONCLUDING REMARKS
Reducing channel dimensions yields larger surface area per
unit flow volume, and a larger heat transfer coefficient. With
these features, significantly higher rates of heat transfer, on the
order of several hundreds of Watts/cm2, are possible. Over the
last century, the flow passage dimensions have been consistently
moving toward smaller hydraulic diameters to meet the
demands of evolving technologies. To provide a new reference,
a new channel classification is presented in this paper on the
basis of molecular mean free path considerations for single
phase flow, and surface tension effects in two-phase flow. Two
new classification types are introduced: transitional
microchannels and nanochannels, and molecular nanochannels.
The single phase performance in microchannels is seen to be
similar to the conventional channels for Knudsen numbers
below 0.001. However, the wall roughness effects need to be
considered carefully, as the relative roughness may become very
large when the channel dimensions become comparable to the
roughness features.
An understanding of the liquid and vapor phase interactions
during two-phase flow in microchannels is emerging as a topic
of intense current research interest. Preliminary studies indicate
that the surface tension effects modify the flow structure
somewhat, such as the absence of stratified flow indicating
minimal effect due to gravity, and flow pattern fluctuations.
However, the basic features of two-phase flow seem to be still
preserved, with surface tension becoming a dominant force in
the flow field. A need for accurate experimental flow boiling
data is seen, as currently there are no reliable data sets available
in the literature.
From a historical perspective, currently microchannels are at
an infancy stage. We are still trying to understand their
characteristics. Their widespread usage is expected to begin
with advances in MEMS devices and systems, microscale
sensors and actuators, advanced high heat flux removal systems,
and biomedical applications.
As an example, use of
microchannels and nanochannels is critical in developing highly
efficient heat and mass transfer devices such as artificial
kidneys or artificial lungs suitable for human implant. These
developments are expected to make headlines in the coming
decades.
From a fabrication perspective, we have considered channels
with characteristic dimensions that range over more than 5
orders of magnitude, from several millimeters to below 0.1 µm.
Standard machining techniques can readily produce channels at
the larger dimensions and down to a few hundred microns.
Semiconductor manufacturing technology can accommodate
channels at dimensions of a few microns to below 0.1 µm.
Microchannels occupy the region between a few microns and a
few hundred microns. This dimensional range has become a
focus of interest for thermal transfer research over the last two
decades since the work of Tuckerman and Pease. Over the same
time period the microfabrication community has also focused
on this dimensional range. Today, a wide variety of specialized
processes and fabrication tools, some derived from traditional
machining, some derived from the semiconductor industry, and
some clever adaptations of other technologies, are optimized for
the construction of microchannels. Thus, from a fabrication
standpoint there are virtually no dimensional limits on building
channels for thermal transfer applications.
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