This paper presents a preliminary study into stiction between parylene C and substrate surfaces for biocompatible check-valve applications. During fabrication, parylene C is used as the structural material for the check-valve. The substrate surfaces studied include Au, Al, Si, parylene C, XeF_2 treated Si, and silicon dioxide. Stiction between different surfaces is created after sacrificial photoresist etching. Then, the stiction is measured using blister tests, and stiction mechanisms for different materials are investigated. The devices are released with different recipes to examine their effects. Finally, the results of the study reveal methods to control the cracking pressure of parylene check-valves

Chen, Po-Jui

Lin, Jeffrey Chun-Hui

Tai, Yu-Chong

Yu, Feiqiao

English

Caltech Authors - Main

PARYLENE STICTION 
 
Feiqiao Yu1, Jeffrey Chun-Hui Lin1, Po-Jui Chen2, Yu-Chong Tai1 
1California Institute of Technology, USA 
2Robert Bosch LLC, USA 
 
ABSTRACT 
 
    This paper presents a preliminary study into 
stiction between parylene C and substrate surfaces for 
biocompatible check-valve applications. During 
fabrication, parylene C is used as the structural 
material for the check-valve. The substrate surfaces 
studied include Au, Al, Si, parylene C, XeF2 treated 
Si, and silicon dioxide. Stiction between different 
surfaces is created after sacrificial photoresist 
etching. Then, the stiction is measured using blister 
tests, and stiction mechanisms for different materials 
are investigated. The devices are released with 
different recipes to examine their effects. Finally, the 
results of the study reveal methods to control the 
cracking pressure of parylene check-valves. 
 
INTRODUCTION 
 
    Parylene C is becoming a popular material for the 
fabrication of implantable MEMS 
(Microelectromechanical Systems) devices. Due to 
its biocompatibility, flexibility, and low permeability 
to fluids, parylene represents an ideal coating for 
otherwise non-biocompatible structures [5]. In 
addition to serving as a protective coating, parylene C 
has also been used in the fabrication of biocompatible 
check valves [2]. Such check valves are then used in 
implantable microfluidic flow control systems such 
as a glaucoma drainage valve [6]. However, since the 
cracking pressure of these check valves is governed 
by stiction between the parylene film and the 
underlying substrate, a comprehensive study of 
stiction between parylene C and different surfaces is 
required in order to understand, design, and create 
valves with specific cracking pressure and desired 
flow rate profiles (Fig.1). 
 
 
Figure 1. Theoretical flow rate vs. pressure graph of 
check valves with different cracking pressure 
    Stiction is an attraction that occurs between free 
standing surface micro-machined features and the 
substrate after the release of sacrificial photoresist 
[8]. Even though stiction is often an undesirable 
phenomenon, it can be employed to control the 
operation regime of thin film parylene check valves. 
Attempts have been made to reduce stiction for 
specific check valve geometries. For example, the 
cracking pressure of a polyimide check valve with 
C4F8/Ar non-stiction coating changed from 210kPa to 
59kPa [3], and SAM (self-assembled monolayer) is 
also used to reduce stiction [7].  
    The current study presents a comprehensive 
investigation of stiction between parylene and a 
variety of different surfaces using the blister test. The 
surfaces under investigation include Au, Al, Si, 
parylene C, XeF2 treated Si, and silicon dioxide. After 
quantifying surface stiction, possible mechanisms 
that lead to stiction between parylene and various 
materials are explained. In addition, different recipes 
for sacrificial photoresist release that may affect the 
resulting stiction are also experimented with. The 
stiction results for different surfaces under different 
photoresist releasing methods show that surface 
coating and releasing procedures used in this 
investigation can be used to control characteristics of 
parylene check valves. 
 
DESIGN AND FABRICATION 
 
    An outline of the fabrication procedure of stiction 
test devices is depicted in Fig.2. A backside circular 
trench 300µm in diameter is created using DRIE 
(deep reactive ion etching) until only a thin silicon 
membrane remains. Front side surface treatment is 
performed. These treatments include XeF2 
roughening, gold (0.2µm) and aluminum (0.25µm) 
deposition, parylene C coating (2.5µm), and silicon 
dioxide growth (1µm).  After surface treatment, 
sacrificial photoresist and 5µm parylene layers are 
coated. Finally, DRIE is used again to remove the 
thin silicon film. Top views of the finished valves are 
shown in Fig.3.  
    After dicing the wafer, different photoresist 
releasing methods are used. The sacrificial 
photoresist of all devices is released using ST-22, 
after which acetone is used to remove the ST-22 
residue. Then, some valves are dipped in IPA 
(isopropyl-alcohol) and air dried. These valves are 
used for surface stiction characterization. Some 
valves are dipped in a mixture of acetone and 5ml of 
 
 
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silicone oil before air drying. These devices are used 
to study the effect of oil coating on parylene stiction. 
In addition, to test the hypothesis of stiction 
mechanisms, some devices are subjected a 30-second 
HF (hydrofluoric acid) dip followed by a quick rinse 
in water to remove the acid. All devices are 
characterized using blister test. 
 
BLISTER TEST 
 
    Blister test is usually performed on thin films 
overlying a solid substrate. Through holes are used to 
apply pressure to the film from the back side (Fig.4). 
For parylene check valves, when pressure is applied, 
plastic deformation occurs in the parylene film that 
causes it to bulge to a distance d that is dependent on 
the Young’s modulus, Poisson’s ratio, geometry of 
the substrate opening, and thickness of the parylene 
film.  
    Due to the circular via in silicon, it can be assumed 
that the blister has a semispherical profile. With this 
assumption, stiction for can be calculated using Eqs. 
(1) and (2) [1]. 
 !" # $%&'()*+ ,"-. / 012)*3 ,"---------------------------------456 
7 # 8%88() 9:;< =+ / 8%>>12) 9:;< =3 -----------------------486 
 
Where !" is the critical debonding pressure, ," is the 
maximum vertical displacement of the parylene film, 
E is the elastic modulus of parylene C (~ 4 GPa), t is 
the thickness of the parylene film (3 !m), " is the 
radius of the blister (100 !m), and 7 is the stiction 
between parylene and silicon. The constant 12 
represents residual stress within the parylene film. 
For this particular experiment where parylene is 
annealed at 100°C, 37.8 MPa is used as the residual 
stress [4]. As pressure inside the blister exceeds the 
critical pressure-!", parylene film debonds from 
silicon. When this event occurs, !" is used to 
calculate 7.  
 
 
 
Figure 4. Theoretical blister formation during 
experimentation 
Figure 3. Fabrication process 
for circular parylene check 
valve with different valve-seat 
surface treatments 
Figure 2. Top view of 
finished parylene check 
valves 
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    During experimentation, each die is placed in a 
testing jig that allows fluid (N2 gas) to apply pressure 
to the parylene membrane. The jig is then connected 
to a fluidic setup consisting of a valve, a pressure 
regulator, and a pressure gauge (Fig.5). The setup is 
placed under a microscope for observation. Pressure 
inside the tubing is gradually increased by adjusting 
the pressure regulator. The pressure gauge reads out 
the current pressure inside the blister. The critical 
pressure is recorded when debonding occurs. 
 
 
 
Figure 5. Stiction test setup along with a schematic 
diagram of the side view of the testing jig 
 
 
Figure 6. Stiction of parylene with different substrate 
surfaces after three kinds of releasing processes. Blue: 
acetone soak followed by IPA and water soak. Green: 
acetone soak followed by HF dip and water rinse. Red: 
soaking in mixture of acetone and silicone oil followed by 
direct air drying. 
 
RESULTS AND DISCUSSION 
 
    Figure 6 illustrates stiction between parylene and 
different surfaces after each device is soaked in 
acetone, IPA, and allowed to dry in air. In addition, 
the average cracking pressure of the valves is also 
recorded (Table 1). The results show that Si has the 
greatest tendency to stick to parylene after drying 
(2.59 J/m2). The high stiction of silicon to parylene 
can be explained by surface passivation. Even though 
silicon and parylene are inherently hydrophobic, 
when they are subjected to water during photoresist 
release, the dangling bonds on the surface of the 
materials tend to bond to OH- groups in water. Such 
bonds make surfaces slightly hydrophilic. As the 
device dries, decreasing water content between the 
parylene film and silicon surface pulls the two 
surfaces together through hydrogen bonding (Fig.7). 
When the surfaces are extremely close from each 
other, Van der Waal’s forces result in adhesion. 
Compared to Si, Au, oxide, and parylene all had 
gradually decreasing stiction to parylene. The 
decreasing values could be attributed either to 
increasing surface roughness or decreasing reactivity 
to OH- groups. Al and XeF2 treated Si surfaces show 
almost no stiction to parylene. The former result can 
be attributed to the high surface energy of aluminum, 
which precludes effective adhesion to most materials. 
On the other hand, XeF2 treated Si surface display 
huge surface roughness. As a result, very little silicon 
surface actually come into contact with parylene. 
Thus, Van der Waal’s force is not great enough to 
cause significant stiction. 
 
Table 1. Cracking pressure of parylene check valves 
under different releasing procedures. 1: actone and 
IPA soak followed by air drying 2. HF dip, water 
rinse followed by air drying 3.  soak in a mixture of 
acetone and silicone oil before air drying. Zero 
stiction means that the stiction is too small to be 
measured effectively 
Release 
method 1 (J/m
2) 2 (J/m2) 3 (mJ/m2) 
Si 2.59 1.86 18.5 
Au 1.54 1.07 12.5 
Al 0 0 13.2 
XeF2 0 0 8.2 
Parylene 0.22 0.62 95.6 
Oxide 0.76 1.06 13.9 
 
 
 
Figure 7. Hydrogen bonding that occurs between 
water molecules and the passivated silicon surface 
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Figure 8. Stiction between parylene and various 
surfaces after the releasing in a mixture of acetone 
and silicone oil 
 
    Following the second photoresist releasing method 
where devices are air dried after soaking in a mixture 
of acetone and silicone oil, stiction between parylene 
and all surfaces are reduced (Fig.8). Results show 
that, other than parylene, all stiction values are 
between 0.01 and 0.02 J/m2. These results suggest 
that, after silicone oil coating, stiction is no longer 
caused by the interaction between parylene and 
substrate surfaces but by the adhesion between 
parylene and the oil layer. Since this oil layer reduces 
surface passivation and the proximity between 
surfaces, stiction is decreased. The relatively high 
stiction value between two parylene surfaces can be 
explained by the roughness of the parylene surface, 
which increases its effective area to interact with 
silicone oil. 
    In order to verify the proposition that surface 
passivation contributes to stiction, some devices are 
subjected to a short HF dip before drying. This HF 
dip should remove some surface –OH bonds and thus 
reduce stiction. Results from blister tests done on 
these valves reveal that for certain material surfaces 
(ie. Si and Au), HF dip does decrease stiction slightly 
(Fig. 6). Even though HF dip removes much of the 
hydroxyl groups, the ensuing rinse in water probably 
introduces some –OH groups back. As a result, 
stiction still remains.  
 
CONCLUSION 
 
    This study successfully quantifies stiction between 
thin film parylene C and various surfaces. Devices 
with check valve configurations were fabricated and 
released using different procedures. After performing 
blister tests, stiction values were recorded. The 
mechanisms that lead to stiction or the reductions 
thereof include surface passivation with hydroxyl 
groups, surface roughness, and surface proximity. 
Experiments show that mechanisms that can reduce 
the proximity between parylene and other surfaces 
during drying will likely reduce stiction. In addition, 
since different surface treatments result in different 
stiction, all processes and photoresist releasing 
methods from this study can be used to design 
parylene check valves with different cracking 
pressures.  
 
REFERENCES 
 
[1] M. G. Allen and S. D. Senturia, "Analysis of 
critical debonding pressures of stressed thin 
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303-315, 1988 
[2] P. J. Chen, D. C. Rodger, M. S. Humayun, and 
Y. C. Tai, "Floating-disk parylene microvalves 
for self-pressure-regulating flow controls" J. 
Microelectromechanical Systems , vol. 25, pp. 
1352-1361, 2008 
[3] J. Han, B. Flachsbart, R. Masel, and M. A. 
Shannon. "Micro-fabricated membrane gas 
valves with a non-stiction coating deposited by 
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18, pp. 1-9, 2008 
[4] T. A. Harder, T. Yao, Q. He, C. Shih, and Y. C. 
Tai, "Residual stress in thin-film parylene-C" 
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[6] J. C. Lin, P. J. Chen, B. Yu, M. S. Humayun, and 
Y. C. Tai, "Minimally invasive parylene dual-
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25-29, 2009, pp. 196-199 
[7] R. Maboudian, W. R. Ashurst, and C. Carraro 
"Self-assembled monolayers as anti-stiction 
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Parylene stiction

Feiqiao Yu

Jeffrey Chun-Hui Lin

Po-Jui Chen

Yu-Chong Tai

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http://authors.library.caltech.edu/19080/1/Yu2010p10693Mems_2008_21St_Ieee_International_Conference_On_Micro_Electro_Mechanical_Systems_Technical_Digest.pdf

Parylene stiction

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