MECHANICAL CHARACTERIZATION OF KMPRTM BY NANO-INDENTATION
FOR MEMS APPLICATIONS
1.
Kuang-Shun Ou1, Hong-Yi Yan1, Kuo-Shen Chen2+
Graduate student, 2. Professor, +Corresponding author
Department of Mechanical Engineering
National Cheng-Kung University
Tainan, Taiwan, 70101
Abstract
This paper presents the elastic properties characterization for a novel negative photoresists: KMPR for MEMS applications.
Structures with very high aspect ratio are possible created using KMPR along with the standard lithography technology.
However, up to now, the mechanical properties of KMPR has not been reported yet. In this work, nanoindentation technique is
used to characterize the modulus and hardness of KMPR thick films after heat treatment with different temperatures. The
elastic modulus and hardness are measured as 6.5-7.5 GPa and 0.26-0.32 GPa, respectively. On the other hand, the elastic
modulus of KMPR under tensile loading is characterized as 1.07 GPa via uniaxial testing. The additional vibration test
confirms that KMPR exhibits strong anisotropy under different loading manners. The test results obtained in this work would
be useful for polymer-based MEMS structure design. With the test data reported in this work, MEMS engineers should be able
to adjust their fabrication parameter for structural design optimization of KMPR structures.
Keywords: KMPR, Nanoindentation, Modulus, Hardness
I. INTRODUCTION
The ability to create high-aspect ratio structures (HARS) is extremely important for many microelectromechanical system
(MEMS) sensors and actuators applications such as gyros, accelerometers, and pumps [1]. Traditionally, HARS are fabricated
using LIGA [2] and ICP techniques [3]. However, the expensive and time consuming LIGA process are usually unaffordable
for general users and the silicon ICP process usually suffers from poor sidewall quality. As a result, the UV based LIGA-Like
process becomes a competitive fabrication process for creating HARS in polymeric materials. During the past decade, with
the negative photosensitive resin, SU-8, UV LIGA-Like process has successfully demonstrated its capability in shaping HARS.
However, SU-8 needs stringent temperature and time control in baking cycle to avoid cracking and the reliability of the
TM
fabricated structure could be a serious concern [4,5]. Recently, a new negative photosensitive resin, called KMPR , has
been reported to have better characteristics in fabrication and it does not require tight baking control [6]. In addition, unlike SU8, after hard baking, KMPR can still be removed using solvents. As a result, it has the potential to replace SU-8 as the
structural material for HARS in the future. However, to our best knowledge, its mechanical properties are not yet reported and
this represents a gap to be filled for related MEMS structural design. The purpose of this paper is therefore to provide specific
material properties, namely hardness and elastic modulus of KMPR after different thermal treatments via nano-indentation and
other associated testing such as uniaxial tensile and vibration testing for MEMS design applications.
The rest of the article presents the characterization and the discussion of the test results in detail. In Section II,
nanoindentation theory is briefly stated. The fabrication of specimens and the preparation of the test plan are introduced in
Section III. Detailed experimental characterization and the results are presented and discussed in Section IV. Finally, Section
V concludes this work.
II. Nanoindentation Theory
A typical instrument nanoindentation system, schematically shown in Figure 1, uses a diamond tip to indent the specimen. By
simultaneously recording the applied load and the penetration depth, associated material properties such as hardness and
modulus can be extracted from the test data. A schematic plot of the nanoindentation test data is shown in Figure 2. By
contact mechanics, it is possible to correlate the initial unload slope S with the reduced elastic modulus Er as [7]
S=β
2
π
Er A(hc ) .
(1)
For Berkovich indentor, β equals 1.034 and Er is defined as
1 −ν i
1 1 −ν s
,
=
+
Er
Es
Ei
2
2
(2)
where the subscripts i and s represent the indentor and the substrate, respectively. As a result, once the modulus of the
indentor is known, the plane strain modulus of the substrate can be deduced from the nanoindentation test data. In addition,
the hardness of the material can also be determined by divided the maximum applied load with the maximum contact area.
Magnet
Coil
Springs
Capacitance
dsplacement
gage
KMPR
Wafer
Indenter
Stage
FIGURE 1.
Schematic of a nanoindentation system.
III. SPECIMEN FABRICATION AND INDENTATION EXPERIMENT
KMPR, obtained from MicroChem Corp, is used to fabricated the test structures, which are firstly fabricated via spin-coating
usng a (MSC300T Spin Coater) with a speed of 4000 rpm and an exposure energy of 400mJ at a wavelength of 365nm (ILine). With the associated baking process, the design result in a KMPR thickness of 20μm. Table I shows the detail
processing parameters. Finally, a post-fabrication thermal treatment period (various temperature, 100, 140, and 180°C,
keeping at the temperature with a period of 100 minutes) is performed using an oven baker. After heat treatment, the
specimens are cooled to room temperature.
The MTS nano indentor XP with a Berkvitch indentor is used to carry on the experiment. Under the force control mode, the
peak loads are set as between 1-20 mN. The penetration and the applied load in both loading and unloading periods are
recorded, as well as the final indentation images. In parallel, a self- designed uniaxial tensile characterization system is also
used to characterize the modulus of KMPR. As shown in Figure 2, this micro tester uses a step motor to drive the specimen.
The loading and displacement are measured by a load cell and a capacitive sensor, respectively. The uniaxial tensile
specimen, designed as a dog-bone shape is also shown in Figure 2. The fabrication process is schematically shown in Figure
3, a PCB board is used as the substrate. After spinning KMPR, baking, and the lithography process, the copper layer on the
PCB is sacrificially released using FeCl3 and this process usually takes 48-72 hours.
TABLE I. The processes parameters of KMPR
Soft bake
Expose energy
o
365nm, 400 mJ
Step I: 65 C, 5min
o
Step II: 95 C, 20min
Spin speed
4000rpm
(film thickness =20μm )
Post exposure bake
o
Step I: 65 C, 2min
o
Step II: 95 C, 5min
Specimen
1.6mm
10.8mm
Probe
5mm
Stage
Motor
Figure 2.
The uniaxial tensile testing system and the schematic plot of the specimen
Mask
PCB board
Spin coating
KMPR
Photolithography
Etch Cu
Cu
Figure 3.
Release
KMPR
Schematic fabrication flow for the dog-bone specimen for uniaxial testing
IV. EXPERIMENTAL RESULTS
„
Nanoindentation Test
Figure 4 shows the original test data without additional heat treatment with several peak penetration forces. The surface
profile after indentation is shown in Figure 5 obtained by Nano Vision. The same test procedure is also applied for those
specimens with additional heat treatment and similar results are recorded. By Figure 5, it can be observed that the specimen
is slightly piled up after indentation. However, the situation is still acceptable and the test data can still be reduced by using
the ideal formula proposed by Oliver and Pharr[7] to find the corresponding modulus and hardness.
Figure 6 shows the relationship between the elastic modulus, hardness, the indentation depth, and the heat treatment
temperature. It can be found that the modulus decreases with the heat treatment temperature. On the other hand, the
hardness shows a different tendency. That is, it increases with the heat treatment temperature. Notice that the film thickness
is 20μm and by the rule of thumb, the indentation depth should not exceed 2μm (2000 nm) to avoid the substrate effect. On
the other hand, if the indentation is too shallow, the results are also vulnerable to the surface roughness effect. Both hardness
and modulus shows depth-dependent results and this is believed to be caused by the pile-up effect shown in Figure 5.
It is also important to examine the possible cracks induced during the indentation. By examine the crack length after
indentation; it is possible to estimate the fracture toughness of a material. However, by inspecting the optical micrograph, it
cannot find any cracks propagated from the indent. As a result, the toughness of KMPR should be up to a certain level and is
better than that of SU-8. And this explains that why KMPR is more robust than SU-8 in terms of its fracture behavior after heat
treatments.
„
Uniaxial and Vibration Testing
The uniaxial test data for KMPR is shown in Figure 7 after data reduction and eliminating the machine compliance from the
data. The Young’s modulus is estimated as 1.07GPa, which is only 1/6 of the value obtained by the indentation test.
Therefore, we suspect that KMPR is an anisotropic material with different modulus in tensile and in compressive loadings. In
order to verify this assumption, a vibration test is conducted. As schematically shown in Figure 8, the dog-bone specimen is
clamped as a cantilever and its free vibration is characterized using a laser position sensor. The test data in both time and
frequency domain are presented in Figure 9. By finite element simulation using ABAQUS [8] (model shown in Figure 8), with
3
the measured density 1380Kg/m in hand, the Young’s modulus is estimated as 4.81GPa. Since the structure experiences
both tensile and compressive loadings during the vibration test, it is reasonable to treat the obtained Young’s modulus as an
averaged value between tensile and compressive tested modulus. The magnitude of the elastic modulus tested via vibration is
between that performed by uniaxial tensile and nanoindentation test and this qualitatively verify our suspension and the result
suggests that KMPR has strong anisotropy in its elastic behavior.
Figure 4.
The nano-indented load deflection curves for KMPR specimens without thermal treatment
Figure 5.
The Nano Vision images of the indentation. Pile up can be found at the edge of the indentation.
8
Elastic modulus (GPa)
(a)
100℃
7.5
Un-heat treated
7
6.5
140℃
6
180℃
5.5
0
500
1000
1500
2000
Indenting depth (nm)
0.34
Hardness (GPa)
(b)
0.32
180℃
140℃
0.3
100℃
0.28
0.26
Un-heat treated
0.24
0
500
1000
1500
2000
Indenting depth (nm)
Mechanical properties of KMPR obtained by nanoindentation at different thermal treatment temperature (a)
elastic modulus and (b) hardness.
3.5E+07
3.0E+07
y = 1.07E+09x - 2.82E+06
2.5E+07
2.0E+07
stress
Figure 6.
1.5E+07
1.0E+07
5.0E+06
0.0E+00
-5.0E+06
0
0.005
0.01
0.015
0.02
0.025
0.03
Strain
Figure 7.
The uniaxial tensile test results
0.035
(a)
Specimen
(b)
Laser
Laser
Figure 8.
(a) Schematic plot of the vibration test and (b) the finite element mesh for the cantilever specimen
10
(a)
1
(b)
X: 91.21
Y: 0.05329
-1
10
0.8
0.6
Magnitude
D is p la c e m e n t (m m )
Spectrum Analysis
0
1.2
0.4
-2
10
0.2
-3
10
0
-0.2
0
1
2
3
4
5
6
-4
10
0
50
100
Time (s)
Figure 9.
150
Frequency (Hz)
200
250
300
The free vibration test data in (a)time domain and (b) frequency domain.
V. SUMMARY AND CONCLUSION
The ability to create high-aspect ratio structures is extremely important for many MEMS sensors and actuators applications
such as gyros, accelerometers, and pumps. Polymer-based LIGA-Like technique represents an opportunity to achieve the
structure. However, the traditional material used for LIGA-Like, SU-8, needs stringent temperature and time control in baking
cycle to avoid cracking and potentially, the reliability of the fabricated structure become a concern and new materials are
sought. This paper presents the elastic properties characterization for a novel negative photoresists: KMPR for MEMS
applications. Structures with very high aspect ratio are possible created using KMPR along with the standard lithography
technology. However, up to now, the mechanical properties of KMPR has not been reported yet. In this work, nanoindentation
technique is used to characterize the modulus and hardness of KMPR thick films after heat treatment. The elastic modulus
and hardness are measured as 6.5-7.5 GPa and 0.26-0.32 GPa, respectively. On the other hand, the elastic modulus of
KMPR under tensile loading is characterized as 1.07 GPa. The additional vibration test confirms that KMPR exhibits strong
anisotropy under different loading manner. The test results obtained in this work will be useful for polymer-based MEMS
structure design. With the test data reported in this work, MEMS engineers should be able to adjust their fabrication parameter
for structural design optimization of KMPR HARS.
ACKNOWLEDGEMENT
This work is supported by Nationa Science Council of Taiwan under contract No. NSC-94-2212-E-006-046.
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