This paper describes the first flexible parylene electrostatic actuator valves intended for micro adaptive flow control for the future use on the wings of micro-air-vehicle (MAV). The actuator diaphragm is made of two layers of parylene membranes with offset vent holes. Without electrostatic actuation, air can move freely from one side of the skin to the other side through the vent holes. With actuation, these vent holes are sealed and the airflow is controlled. The membrane behaves as a complete diaphragm. 



We have successfully demonstrated this function using a 2-mm x 2-mm parylene diaphragm electrostatic actuator valves. This work also includes the novel anti-stiction technology that is crucial to make such large-area parylene actuator diaphragm with the combined use of anti-stiction posts, self-assembled monolayers (SAM), surface roughening, and bromine trifluoride (BrFe) dry etching. With the help of SAM treatment, the operating voltage is lowered from 30 volts to 13 volts. The load deflection method is then used to measure the effective thickness of the composite 

diaphragm. The flexible parylene diaphragm can be deflected up to 100 μm when 150 Torr of pressure is applied. The result is fitted into a theoretical model and yields an effective thickness of 5.9 μm, which is agreeable with the actual thickness of 5.6 μm, thus proves the functionality of the device

Ho, C. M.

Nassef, H.

Pornsin-Sirirak, T. N.

Tai, Y. C.

English

Caltech Authors - Main

FLEXIBLE PARYLENE ACTUATOR 
FOR MICRO ADAPTIVE FLOW CONTROL 
T. N. Pornsin-Sirirak, Y.C. Tai 
Caltech Micromachining Laboratory, 
Electrical Engineering Department, California Institute of Technology 
Pasadena, CA 9 1 125 
H. Nassef, C.M. Ho 
Mechanical and Aerospace Engineering, 
University of California, 
Los Angeles, CA 90095 
ABSTRACT 
This paper describes the first flexible parylene electrostatic 
actuator valves intended for micro adaptive flow control for the 
future use on the wings of micro-air-vehicle (MAV). The actuator 
diaphragm is made of two layers of parylene membranes with 
offset vent holes. Without electrostatic actuation, air can move 
freely from one side of the skin to the other side through the vent 
holes. With actuation, these vent holes are sealed and the airflow is 
controlled. The membrane behaves as a complete diaphragm. 
We have successfully demonstrated this function using a 2- 
mm x 2-mm parylene diaphragm electrostatic actuator valves. 
This work also includes the novel anti-stiction technology that is 
crucial to make such large-area parylene actuator diaphragm with 
the combined use of anti-stiction posts, self-assembled monolayers 
(SAM), surface Toughening, and bromine trifluoride (BrF,) dry 
etching. With the help of SAM treatment, the operating voltage is 
lowered from 30 volts to 13 volts. The load deflection method is 
then used .to measure the effective thickness of the composite 
diaphragm. The flexible parylene diaphragm can be deflected up 
to 100 pm when 150 Torr of pressure is applied. The result is 
fitted into a theoretical model and yields an effective thickness of 
5.9 pm, which is agreeable with the actual thickness of 5.6 p n ,  
thus proves the functionality of the device. 
INTRODUCTION 
Previous year in MEMS 2000, we reported a MEMS wing 
technology using titanium-alloy and parylene as wing spars and 
membrane, respectively [l]. Our further wind tunnel experiments 
have indicated that aerodynamic thrust force produced by flapping 
are intimately related to the flexibility and the location of the wing 
membrane [2]. The comparison of the thrust performance of 
flexible wing membrane to the rigid wing membrane is shown in 
Figure 1. The mylar wing, which is less rigid and has better 
flexibility than the paper wing, shows to have higher thrust 
coefficient, hence higher thrust force, when it is compared to the 
more rigid paper wing. 
We have also discovered the effect of the inboard and 
outboard regions of the wing in relation to thrust and lift 
generations. This effect is shown in Figure 2. Two wings are 
compared where the inboard region of one wing was arbitrarily 
0-7803-5998-4/01/$10.00 (32001 IEEE 511 
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i m  8 5 
-0.05 ..................................... ..................................... ! ................. j .................. 1 
-0.10 i 2 3 4 5 6 7  
Advance Ratio, J = U / ( 2 @ ? )  
Figure 1. Aerodynamics thrust performance of wings with 
different membrane rigidity 
removed. Because the wing speed varies along the wing span, the 
strength of the dynamic stall vortex also varies, and thus the lift 
force. The rotational speed of the wings is higher towards the tips 
and this leads to stronger amounts of vorticity in the outboard 
region of the wings. Therefore, it can be expected that the bulk of 
the lift is produced in the outboard region of the wings. Removal 
of the inboard region does not affect the lift coefficient as shown 
in Figure 2a) because the outboard vorticity is not affected. 
However, the thrust production is affected dramatically as seen in 
Figure 2b). The thrust performance of the wing without the 
inboard region deteriorates when compared to that of the wing 
with the inboard region, which is yet another indication of the 
dependence of thrust on the vortex shedding. This indicates the 
area where the actuators can be distributed to control the flexibility 
of the membrane and how the vortices are shed. Hence, the thrust 
performance can be improved. 
This is the motivation to investigate the flexible parylene 
MEMS actuator membrane for micro adaptive flow control. By 
distributing actuators on the wing membrane, it is possible to have 
With inboard region Without inboard region 
4 . 5 1 . .  , . . , . , I 
4 -  c;, 3.5 
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g 2  
+. 
'5 2.5 
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y -1  2 -1.2 
2 -1.4 
-1.6 
Advance Ratio, .I = U/(ZWb) 
Figure 2a) 
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Fi,yure 2. Aerodynamics thrust performance of wings with 
dgferent membrane rigidity 
I 
Electrostatic Actuators 
Membrane before actuation 
Pi-A' 
Membrane after actuation 
(double thickness) 
Figure 3: Active wing actuator membrane concept 
an active control of the flexibility of the wings to achieve optimal 
aerodynamic performance. This concept is illustrated in Figure 3. 
Before actuation, the offset vent holes on the diaphragm let the air 
move freely through the membrane; the membrane remains 
flexible. Once actuated, the holes are sealed and the membrane 
behaves as a complete diaphragm. The thickness is also doubled. 
Therefore, the rigidity of the membrane increases; this will affect 
the thrust performance. This kind of membrane will have 
important application for adaptive airfoil thai. its wing loading can 
be controlled by simple electrostatic actuation. To demonstrate 
this feasibility, flexible parylene actuator membrane is built on a 
silicon chip. It will be demonstrated that the effective thickness of 
the parylene composite membrane changes after an actuation is 
performed. The load deflection test setup is built to collect data 
that will be fitted into the load deflection model [3].  
DESIGN AND FABRICA'TION 
The large-area of 2-mm x 2-mm parylene electrostatic 
actuator diaphragm is fabricated and shown in Figure 4. The 
diaphragm is consisted of two metalized parylene membranes, 
each with 2.8-pn thickness. Photoresist (4  km) is used as a 
sacrificial layer by acetone dissolution. Because parylene has a 
low Young's modulus (-4.5 GPa), acetone surface tension can 
easily pull down the structure to substrate to cause stiction. These 
challenges must be overcome. The anti-stiction technology we 
developed is a combination of steps that use both wet photoresist 
dissolution and silicon dry etching in BF,. Photoresist dissolution 
allows large-area parylene membrane release and BF, dry etch 
prevents stiction. We also utilize anti-stiction posts to reduce the 
collapse of the top parylene membrane after wet releasing of 
photoresist. Surface of parylene is roughened in 0, plasma to 
reduce the contact area of stiction. In addition, after releasing 
photoresist, SAM coating of octadecyltrichlorosilane (OTS) [4, 51 
is used to treat the parylene surface to fuIther reduce possible 
stiction 
Figure 4a) Figure 4b) 
Figure 4: a )  fabricated 2-mm x 2-mm parylene electrostatic 
actuator diaphragm; b )  anti-stiction posts and photoresist 
dissolution holes 
The actuator diaphragm fabrication process is shown in 
Figure 5. First, 1.8-pm thermal oxide is grown at 105OOC by wet 
oxidation. This layer is used as a mask during formation of the 
diaphragm by KOH etching. The oxide is pattemed from the 
backside. Then the substrate is time-etched in KOH solution at 
58OC for 23 hours to form a thin 25-um-thick silicon diaphragm. 
This is followed by the front-side patteming of the remaining 
oxide to create membrane area of 2-mm x 2.". The surface is 
then roughened with a 3 mTorr pulse of BrF,. This helps improve 
the adhesion of parylene to the substrate surface. Next, the 
solution of 0.5% A-174 silane adhesion promotion (DI H20: IPA: 
A-174 = 1OO:lOO:l in volume) is used to further improve the 
adhesion. A-174 is commercially available and can be obtained 
512 
Oxide Parylene A1 W P R  
+Before actuation 
100 
90 
80 
70 
60 
50 
$ 40 
30 
20 
10 
0 
h 
.A 
I 
0 50 100 150 200 
Pressure (Torr) 
1) Oxide; pattem oxide backside; 4) Parylene; pattem; deposit Au; 
KOH; pattem 'oxide frontside; pattern Au 
rouehen surface 
2) Parylene; pattem; deposit Au; 5 )  Parylene; pattem 
pattem Au 
3) Parylene; pattem; spin PR; 
pattem PR backside 
6) Release P R  BrF3 etch Si 
Figure 5: Fabrication process for flexible parylene electrostatic 
actuator diaphragm 
from Specialty Coating Systems, Inc. [6] The solution is stirred for 
at least 30 seconds and allowed to settle for a minimum of 2 hours. 
The wafers are then submerged in the prepared solution for 15 
minutes and air dried for another 15 minutes. Then they are rinsed 
with IPA for 15-30 seconds and dried in a convection oven for 30 
minutes at 70°C. 
Next, the first thin film layer of parylene C is deposited and 
pattemed, followed by parylene surface roughening by 0, plasma. 
Cr/Au/Cr layers, as ground electrodes, are evaporated and 
pattemed. We use Cr/Au/Cr layers because A-174 silane adhesion 
promotion will create a good bonding strength between CrO, and 
parylene interface to improve adhesion. The use of 0, plasma to 
roughen the parylene surface before depositing Cr layer is also 
important because without roughening the surface, we have 
experienced the delamination of metal layers due to poor adhesion. 
The next layer of parylene is then deposited to seal the ground 
electrode. This helps prevent short-circuiting the ground and top 
electrodes. 
After spinning on the 4ym-thick sacrificial layer photoresist, 
the resist is pattemed to form anti-stiction posts. The post size is 
20-pm in diameter and 200-pm apart. The third layer of parylene 
is then deposited and the surface is roughened. Top Cr/Au/Cr 
layers are evaporated and pattemed, followed by the deposition of 
the last parylene layer. Finally, the top parylene membrane is 
patterned to open up areas for electrode contacts and holes for 
photoresist dissolution in acetone. The wafers are then submerged 
in acetone to release the diaphragm. After releasing the 
diaphragm, we also submerge the wafers in SAM solution of 
octadecyltrichlorosilane (OTS) to further prevent stiction. 
It is important to note that if there are no posts between the 
top and the bottom parylene membranes, we have yet been able to 
successfully free a large area of parylene without stiction. As 
shown in Figure 6, without anti-stiction posts, the top layer of 
parylene on a test structure is pulled down by meniscus force after 
the dissolution of photoresist in acetone. The anti-stiction posts, 
which are rested on a thin 20-pm-thick silicon diaphragm, prevent 
the stiction. This thin silicon diaphragm is then released by BrF, 
dry etching [7]. The dry etching is necessary at the last step 
because the membranes will not be in contact with any wet 
chemicals that can cause stiction to occur. 
I 
Figure 6a) Figure 6b) 
Figure 6: a)  stiction occurred on a 400-pn x 400-pm parylene 
diaphragm when no posts and any anti-stiction methods used; b )  
no stiction occurred for a 2-mm x 2-mm parylene diaphragm when 
no.st.7 and other stiction nreventive methods are used. 
RESULTS AND DISCUSSION 
The actuation test results show that without SAM coating, a 
higher voltage must be applied in order to actuate the parylene 
diaphragm actuators. This actuation voltage is 30 volts. With 
SAM coating, the actuation voltage is reduced to 13 volts. The 
load deflection test setup is constructed. By applying pressure to 
the backside of the membrane, the deflection distance can be 
measured by the load deflection setup before and after actuation. 
Before actuation, the vent holes open; the membrane is flexible. 
After actuation, the vent holes are closed. Thus the airflow is 
ceased and can be controlled. The membranes become more rigid. 
The result is shown in Figure 7. The actuation effect on the 
stiffness of the membrane can be seen clearly. 
Figure 7: Load diflection test result indicates the stiffness 
chanze qfter actuation 
Figure 8 shows the after-actuation data fitted by the load 
deflection for rectangular membrane model. We assume that our 
513 
membrane behaves as a rectangular membrane once the actuation 
occurs. The relationship between the pressure, p, and the 
deflection height, h, is shown in the following equation: 
a‘ a4 
where 
n4(1+ n 2 )  c, = ~ 
64 ’ 
c -. ~ 7c6 ( 9 + 2 n 2 + 9 n 4  
[- 11, 
n = ;I; 
32(1:v4) 256 2 -. 
(4 + n + n2 + 4n3 - 3nv(i  + n))’ 
2[81n2(1+ n 2 )  + 128n +v[(128n -97c2(1 + n’)]) 
v = 0.25 
CT = int ernal stress ; t = the membrane thickness 
a = half - length of the membrane 
E = Young ’ s mod ulus 
150 
2 125 
8 100 
g 75 
y1 
2 a 
25 
Figure 8: Plot of pressure, p .  vs. deflection height,h, of the 
after-actuation for flexible uarvlene actuator membranes 
From Figure 8, the model fit yields, 
(2) P = 1.561 *h + 0.000363 * h3 L 7 - - ’ Y  
A B 
The effective thickness after actuation can be calculated by , 
Ba4 
C2E Effective thickness t = (3) 
where B = 0.000363, a = 1 mm, C, = 1.83, and E = 4.5 GPa 
(Young’s modulus of parylene). This yields the calculated 
effective thickness of 5.9 p compared to the actual diaphragm 
thicknms of 5.6 p. which is agreeable and the functionality of 
this flexible parylene actuator diaphragm is proved. 
CONCLUSIONS 
We have successfully fabricated 2-mm x 2-mm parylene 
diaphragm electrostatic actuator valves and used this flexible 
parylene actuator for micro adaptive flow control. This work also 
inc1udt:s the novel anti-stiction technology that is crucial to make 
such large-area parylene actuator diaphragm with the combined 
use of anti-stiction posts, SAM layers, surface roughening, and 
bromine trifluoride (BrF,) dry etching. The operating voltage of 
this device can be initiated at 13 volts. The effective thickness of 
the composite diaphragm is calculated bly fitting the load- 
deflection test data into a theoretical model and it yields an 
effective thickness as 5.9 pm, which is very close with the actual 
measured thickness of 5.6 p. 
To further improve the aerodynamic performance of MEMS 
wings, the selective stiffness distribution conlrol on the membrane 
is desired. This will help control the flexibility of the wings and 
how vortices are shed. Thus our future work is to integrate these 
flexible actuator membranes onto the membrane of MEMS wings. 
ACKNOWLEDGEMENTS 
This work is supported under DARPNTTO MAV program 
DABT63-98-C-0005. The authors would like to thank Dr. Xing 
Yang from Caltech for his assistance in load deflection test and 
Steve Ho from UCLA for his assistance in the wind tunnel testing 
and his contributions to the project. 
REFERENCES 
1. T. N. Pomsin-Sirirak, S.W. Lee, H. Nassef, J. Grasmeyer, Y.C. 
Tai, C. M. Ho, M. Keennon, “MEMS Wing Technology for a 
Battery-Powered Omithopter,” Proceedings of the 13’h IEEE 
Annual International Conference on M E W  2000, Miyazaki, 
Japan, 1/23-27/00, pp. 799 - 804 
2. T. N. Pomsin-Sirirak, Y.C. Tai, H. Nassef, C. M. Ho, 
“Unsteady-State Aerodynamic Performance of MEMS Wings,” 
International Symposium on Smart Structures and Microsystems 
2000 (IS3M), The Jockey Club, Hong Kong, 10/19-21/00, pp. G1-2 
3. 0. Tabata, K. Kanahata, S. Sugiyama, I. Igarashi, “Mechanical 
Property Measurements of Thin Films Using Load-Deflection of 
Composite Rectangular Membranes,” Sensors and Actuators, 20 
(1989), pp. 135 - 141 
4. R. Maboudian, “Self-Assembled Monolayers as Anti-Stiction 
Coatings for Surface Microstructures,” Digest of Technical 
Papers, The 10“ International Conference on Solid-state Sensors 
and Actuators (Transducer’99), Sendai, Japan, 6/7-10/99, Vol 1, 
pp. 22-25 
5. M.R. Houston, R. Maboudian, R. T. Howe, “Self-Assembled 
Monolayer Film as Durable Anti-Stiction Coatings for Polysilicon 
Microstructures”, Solid-state Sensor and Actuator Workshop, 
Hilton Head, South Carolina, 6/2-6/96, pp. 42 - 47 
6. Product Specifications, “ A-174 Silane Promotion”, Specialty 
Coating Systems, Inc. Indianapolis, IN, 1-800- 356-8260 
7. X. Q. Wang, X. Yang, K. Walsh, and Y. C. Tai, “Gas-Phase 
Silicon Etching with Bromine Trofluoride,” Digest of Technical 
Papers, The 9th International Conference on Solid-state Sensors 
and Actuators (Transducers ’97). Chicago, IL, 6/ 16-19/97, Vol. 2, 
pp. 1505-1508 
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Flexible parylene actuator for micro adaptive flow control

Gas-Phase Silicon Etching with Bromine Trofluoride,” Digest of Technical Papers,

Mechanical Property Measurements of Thin Films Using

MEMS Wing Technology for a Battery-Powered Omithopter,”

Self-Assembled Monolayer Film as Durable Anti-Stiction Coatings for Polysilicon Microstructures”, Solid-state Sensor and Actuator Workshop,

Self-Assembled Monolayers as Anti-Stiction Coatings for Surface Microstructures,” Digest of Technical Papers,

Unsteady-State Aerodynamic Performance of

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Flexible parylene actuator for micro adaptive flow control

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