Purdue University
Purdue e-Pubs
Birck and NCN Publications
Birck Nanotechnology Center
10-2009
Impact of Sacrificial Layer Type on Thin-Film
Metal Residual Stress
Anurag Garg
Purdue University - Main Campus, garg@purdue.edu
Joshua A. Small
Purdue University - Main Campus, jsmall03@purdue.edu
Xiaoguang Liu
Purdue University - Main Campus, liu79@purdue.edu
Ajit Mahapatro
Purdue University - Main Campus
Dimitrios Peroulis
Birck Nanotechnology Center and School of Electrical and Computer Engineering, Purdue University, dperouli@purdue.edu
Follow this and additional works at: http://docs.lib.purdue.edu/nanopub
Part of the Nanoscience and Nanotechnology Commons
Garg, Anurag; Small, Joshua A.; Liu, Xiaoguang; Mahapatro, Ajit; and Peroulis, Dimitrios, "Impact of Sacrificial Layer Type on ThinFilm Metal Residual Stress" (2009). Birck and NCN Publications. Paper 548.
http://docs.lib.purdue.edu/nanopub/548
This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for
additional information.
Impact of Sacrificial Layer Type on Thin-Film
Metal Residual Stress
Anurag Garg, Joshua Small, Xiaoguang Liu, Ajit K Mahapatro, and Dimitrios Peroulis
Birck Nanotechnology Center
Department of Electrical and Computer Engineering
Purdue University, West Lafayette, IN 47906
Email: garg@purdue.edu, smallj@purdue.edu, liu79@purdue.edu, ajit@purdue.edu, dperouli@purdue.edu
Abstract— In this paper we study the impact of two sacrificial
layers on the final residual stress of thin gold films. In particular,
we comapre a typical photoresist layer (Shipley SC1827) to singlecrystalline silicon. We fabricate and measure cantilever beams
on both sacrificial layers and study their residual stresses by
analyzing the final displacement profile of the released beams.
All samples were fabricated at the same time and under identical
conditions. The study clearly shows that the induced stress on
thin films is dependent on the sacrificial layer. The gold film
deposited over the single-crystalline silicon shows nearly zero
gradient stress after release. On the other hand, gradient stress
dominates the gold film deposited during the same run but over
a photoresist layer. Such results are very useful in designing and
fabricating a wide variety of low-stress actuators and sensors.
I. I NTRODUCTION
Micromachining and Micro-Electro-Mechanical-Systems
(MEMS) have emerged as very pomising enabling technologies for the development of miniaturized low-cost and lowpower switches, actuators, and sensors. Applications include
RF components for high frequency systems [1], [2], mechanical sensors [3], and biochemical sensors [4] to name a few.
The fabrication of these typically involves the patterning of
some material over one or more sacrificial layers that are
later etched away, leaving behind a released microstructure.
Current fabrication technologies offer an array of choices for
this sacrificial layer, including metals, metal-oxides, photoresists and other polymers.
In the case of electromechanical switches and components
for RF systems and sensors, metals are often used as structural
materials [1], [5]; and one major challenge during thier fabrication is maintaining planarity after release. The interaction
between the metal and sacrificial layer atoms at the interface
of the two films induces stresses in both films. This is a result
of the difference in the thermal expansion coefficients of the
two materials; both films expand and contract at their own
respective rates, but are forced to comply at their interface. The
effects of each thermal cycle - both at the time of deposition,
and heating/cooling steps during fabrication - are stored in
the materials as film stresses. The adverse effects of residual
stresses in the metal films cause unwanted deformations and
curling of the metal beams and cantilevers resulting in large
variations in spring constant and therefore pull-in voltage [6].
Studies have been conducted to model the effects of residual
stress on device performance [7], [8], with the attempt to cor-
978-1-4244-5335-1/09/$26.00 ©2009 IEEE
rect for it in the final device design. Others have attempted to
optimize fabrication parameters to be able to fabricate planar
thin-films [6], [9], [10]. No study however has addressed the
effects of sacrificial layer type on the induced residual stress
in metal films.
In this paper we study the impact of two different sacrificial
layers on the final residual stress of thin gold films. In
particular, we compare a typical photoresist layer (Shipley
SC1827) to single-crystalline silicon. We fabricate and measure cantilever beams on both sacrificial layers and study their
residual stresses by analyzing the final displacement profile of
the released beams. The results presented here show that the
induced stress on thin films is dependent on sacrificial layer.
They also suggest that conventional fabrication processes
using polymers as sacrificial layers [2], [11] may inherently
introduce higher residual stress than if they had used solid
films. These results can be crucial to the succesful fabrication
of consistent gradient-stress free planar metal films.
II. FABRICATION AND M EASUREMENT
Figure 1 and Figure 2 outline the fabrication processes
for the photoresist (Case A) and single-crystalline silicon
(Case B) sacrificial layers. Variations of these steps have
been employed in a wide variety of applications [11], [12].
In summary, the steps are
Case A - Photoresist sacrificial layer
1) Begin with an oxidized silicon wafer,
2) Spin and pattern sacrificial layer resist,
3) Sputter gold film,
4) Pattern structures,
5) Photoresist strip using PRS2000, followed by critical point
drying (CPD)
Case B - Silicon sacrificial layer
1) Begin with oxidized silicon wafer,
2) Pattern oxide and perform bulk etch,
3) Strip remaining oxide, and re-oxidize,
4) Pattern oxide,
5) Sputter gold film and pattern structures,
6) Dry isotropic etch of Si using XeF2 for release .
It is important to note that the gold films are sputter
deposited during the same fabrication run for both processes.
1052
IEEE SENSORS 2009 Conference
Fig. 3. SEM image of cantilevers fabricated with a photoresist sacrificial
layer; Case A. All beams are 10-µm wide. Lengths range from 20-µm (top)
to 500-µm (bottom) in steps of 20-µm
Fig. 1.
Fig. 2.
Fabrication for Case A - Photoresist sacrificial layer
Fig. 4. SEM image of cantilevers fabricated with a silicon sacrificial layer;
Case B. All beams are 10-µm wide. Lengths range from 20-µm (top) to
500-µm (bottom) in steps of 20-µm
Fabrication for Case B - Silicon sacrificial layer
Therefore we ensure identical deposition parameters to the
maximum extent possible. Both fixed-fixed and catilever
beams are fabricated and released following both processes.
Figure 3 and Figure 4 show representative SEM (Scanning
Electron Microscope) images from both fabrication processes.
Both images show 10-µm wide beams with lengths ranging
from 20-µm to 500-µm in steps of 20-µm. Figure 5 and Figure
6 show SEM images of the cantilever anchor profiles for both
processes.
The beam profiles were also measured using an optical
microscope. Height information was obtained by manually
adjusting the focus knob on the microscope and reading the
stage height change; the uncertainty of each measurement is
±1-µm. Figure 7 and Figure 8 show several measured curves
from beams with identail dimensions. For each beam type we
meausure at least 4 samples on the wafer. Two very different
profiles are observed; the beams deposited over a singlecrystalline silicon sacrificial layer display a very linear profile
with maximum deflection of approximately 60-µm, whereas
the beams deposited over photoresist have a very dominant
curvature and maximum deflection of over 100-µm.
III. R ESULTS AND D ISCUSSION
Following the analysis in [13], it is possible to isolate
the mean stress and gradient stress components from the
beam deflection profiles. For the anchor type discussed, the
final deflection profile of the cantilever beams can be broken
down into ‘rotation’ and ‘curvature’ components. A tensile or
compressive (mean) residual stress causes the beam to ‘rotate’
linearly about the anchor, above or below the wafer plane
respectively. A stress gradient through the thickness of the
beam results in a curved profile. The radius of curvature is
a function of the stress gradient. The final beam shape is a
superposition of the displacements caused by the two stress
components.
1053
Fig. 5. SEM image of the beam anchor when using a photo-resist sacrificial
layer
Beam deflection profiles for four beams fabricated on silicon.
Fig. 8.
Measured using an optical microscope
Fig. 6. SEM image of the beam anchor when using a silicon sacrificial layer
Fig. 9. A comparison of the beam deflection profiles for cantilevers fabricated
using silicon and photo-resist sacrificial layers
The gold film deposited over the single-crystalline silicon
sacrificial layer show nearly zero gradient stress after release.
This is clearly seen by the beam deflection profile that follows
the form y = Ax where A was measured to have an
average value of 0.0914 and standard deviation of 0.0135 for
our deposition conditions. On the other hand, gradient stress
dominates the gold film deposited over the photoresist layer
where the beam profile follows the form y = Bx3 +Cx2 +Dx.
The average values and standard deviations measured for
the current deposition conditions are as follows: B(2.975E5µm−2 , 2.039E5-µm−2 ), C(292.5-µm−1 , 64.95-µm−1 ), and
D(2.3, 0.95).
Fig. 7. Beam deflection profiles for four beams fabricated on photoresist.
Measured using an optical microscope
Figure 9 summarizes the difference in the deflection profile
and stress for the two sacrificial layers studied.
1054
IV. C ONCLUSION
An experimental analysis has been presented to study
the variation in induced residual stress when using singlecrystalline silicon versus photoresist as sacrificial layers in the
fabrication of MEMS cantilevers. Two different set of fixedfixed beams and cantilevers were fabricated and the beam
displacement profiles were studied. The gold film deposited
over the single-crystalline silicon shows nearly zero gradient
stress after release. On the othe hand, gradient stress dominates
the gold film deposited during the same run but over a photoresist layer. Such results are very useful in designing and
fabricating a wide variety of low-stress actuators and sensors.
R EFERENCES
[1] Peroulis, D., Margomenos, A., Katehi, L.P.B., “RF MEMS and Si Micromachining in High Frequency Applications”. IEEE Radio and Wireless
Conference Digest, pp.265-268, Boston, MA, August 2002
[2] Peroulis, D., Pacheco, S.P., Sarabandi, K., Katehi, L.P.B., 2003, “Electromechanical Considerations in Developing Low-Voltage RF MEMS
Switches”. IEEE Transactions on Microwave Theory and Techniques,
51(1), January pp.259-270.
[3] Piyabongkarn, D., Rajamani, R., Greminger, M., 2005, “The Development of a MEMS Gyroscope for Absolute Angle Measurement”, IEEE
Transactions on Control Systems Technology, 13(2), March, pp.185-195
[4] Yoo, S.K., Lee, J.H., Yun, S.S., Gu, M.B., Lee, J.H., 2007, “Fabrication
of a bio-MEMS based cell-chip for toxicity monitoring”, Biosensors and
Bioelectronics, 22, pp.1582-1592
[5] Goldsmith, C.L., Yao, Z., Eshelman, S., Denniston, D., 1998, “Performance of Low-Loss RF MEMS Capacitive Switches”. IEEE Microwave
and Guided Wave Letters, 8(8), August pp.269-271
[6] Horsfall, A.B., et.al., 2004, “Dependence of Proccess Parameters on Stress
Generation in Aluminum Thin Films”. IEEE Transactions on Device and
Materials Reliability, 4(3), September pp.482-487
[7] Chokshi, T., “Stress-Induced Failure Modes in High-Tuning Range RF
MEMS Varactors”. GaAs and Other Semiconductor Application Symposium, pp.517-520., Paris, France, October 2005
[8] Garg, A., Small, J., Liu, X., Scott, S., Peroulis, D., “Post-Release Displacement Uncertainty of Micro-Cantilevers Due to Anchor Over/Under
Etching”. Proceedings of the ASME International Mechanical Engineering Congress and Exposition, Boston, MA, November 2008
[9] Stanec, J.R., Smith III, C.H., Chasiotis, I., Barker, N.S., 2007, “Realization of low-stress Au Cantilever Beams”. J. Micromech. Mircoeng, 17,
pp.N7-N10.
[10] Stanec, J.R., Begley, M.R., Barker, N.S., 2006, “Mechanical properties
of sacrificial polymers used in RF-MEMS applications”. J. Micromech.
Mircoeng, 16, pp.2086-2091.
[11] Rebeiz, G.M., “RF MEMS Theory, Design, and Technology.”, New
Jersey: Wiley, 2003.
[12] [!h] Gardner, J.W., Varadan, V.K., Awadelkarim, O.O., “Microsensors,
MEMS and Smart Devices”, West Sussex, England: Wiley, 2002.
[13] Fang, W., Wickert, J.A., 1996, “Determining mean and gradient residual
stresses in thin films using micromachined cantilevers”. J. Micromech.
Microeng, 6, pp.301-309.
1055