This paper describes the first work of using wafer-sized flexible parylene-valved actuator skins (total thickness ~20 µm) for micro adaptive flow control. The check-valved actuator skins feature vent-through holes with tethered valve caps on the membrane to regulate pressure distribution across the skins. The skins were integrated onto MEMS wings and were tested in the low-speed wind tunnel for aerodynamic evaluation. The test results have shown very significant effects on the aerodynamic performances. Compare to the reference MEMS wings (no actuators), both the lift and thrust of the parylene check-valved wings were improved by more than 50%. This is the first experimental result to demonstrate that the application of MEMS actuator skins for flow control is very promising

Ho, Chih-Ming

Ho, Steve

Liger, Matthieu

Pornsin-Sirirak, Nick

Tai, Yu-Chong

Caltech Authors - Main

FLEXIBLE P ARYLENE-V AL VED SKIN FOR ADAPTIVE FLOW CONTROL 
Nick Pornsin-Sirirak, Matthieu Liger, Yu-Chong Tai 
Caltech Micromachining Laboratory 
Electrical Engineering Department, California Institute of Technology 
Pasadena, CA 91125 
Steve Ho and Chih-Ming Ho 
Mechanical and Aerospace Engineering 
University of California 
Los Angeles, CA 90095 
ABSTRACT 
This paper describes the first work of using wafer-sized 
flexible parylene-valved actuator skins (total thickness ~ 20 
µm) for micro adaptive flow control. The check-valved 
actuator skins feature vent-through holes with tethered valve 
caps on the membrane to regulate pressure distribution 
across the skins. The skins were integrated onto MEMS 
wings and were tested in the low-speed wind tunnel for 
aerodynamic evaluation. The test results have shown very 
significant effects on the aerodynamic performances. 
Compare to the reference MEMS wings (no actuators), both 
the lift and thrust of the parylene check-valved wings were 
improved by more than 50%. This is the first experimental 
result to demonstrate that the application of MEMS actuator 
skins for flow control is very promising. 
INTRODUCTION 
From our previous study of MEMS wings for MA V 
applications [l], titanium-alloy wings with parylene 
membrane were fabricated to test unsteady aerodynamics 
mode of flapping flight. We have learned that the control of 
the wings' pressure distribution is very important in order to 
achieve an optimal aerodynamic performance. Thus, it is 
logical to fabricate devices on the membrane, passive or 
active, that can regulate the wing loading. To achieve this, 
we reported last year a chip-sized flexible parylene 
technology [2]. The more challenging next step, hence this 
work, is to make wafer-sized flexible parylene actuator 
membranes or "skins," and integrate these skins onto the 
MEMS wings. The goal is to distribute actuators on the 
wings to regulate pressure distribution during downstroke 
and upstroke. This concept is illustrated in Figure 1. The 
pneumatic actuation relies upon the wing loading during 
flapping. For each flapping cycle, the pressure loading 
either pushes the valve caps to open or close, controlling the 
air movement through the vent holes. The opening of the 
valves equalizes the pressure between the upper and the 
lower surfaces of the wings. When the valves are closed, 
the existence of the pressure difference can affect the 
aerodynamic performance, i.e., lift and thrust. 
0-7803-7185-2/02/$10.00 ©2002 IEEE 101 
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Pneumatic Actuators 
Check Valves Open 
i 
Check Valves Close 
Figure 1: Integrated actuator MEMS wing concept 
In a fluidic system, a check-valve is a device used to 
control liquid or gas flow in one direction. The valve is 
opened by a forward fluid flow and is closed by the reverse 
flow. It is analogous to a diode used to rectify the direction 
of current flow in an electric circuit. Recently, mrious 
structures of MEMS micro check-valve such as cantilevers, 
floats, gates, ring mesas, and membranes that were 
fabricated from silicon, metal, silicone, polyimide, and 
parylene have been developed [3-8]. However, they were 
mainly based on bulk microrrnchining of silicon or glass 
structure. There has been no previous work of check-valve 
presented using freestanding parylene skin before. 
This paper describes the first flexible, wafer-sized 
parylene check-valved skin intended for micro adaptive 
flow control of the MEMS wings. Photoresist is used as a 
sacrificial layer to release the skin. We have also integrated 
anti-stiction technologies using amorphous silicon as an 
anti-stiction layer [9]. At the end of the process, dry gas 
etching of BrF 3 [10] is used to etch the amorphous silicon to 
completely free the valve caps. Aerodynamic test results in 
a low-speed wind tunnel will be presented and analyzed. In 
addition, challenges and future work will be discussed. 
DESIGN AND FABRICATION 
The parylene-valved skin consists of many small check-
valved actuators that are made of two layers of parylene 
membranes. Each layer of parylene is approximately 10-µm 
thick. There are 107 dice on one wafer skin. On each 
individual die, there are 5 rows and columns cf parylene 
check-valves. Each valve consists of a circular vent hole 
and a valve cap. Each vent hole is 1.5 mm apart. The vent 
holes and valve caps have diameters of 500 µm and 900 µm, 
respectively. The valve cap is connected to the skin by four 
flexible reverse "S" arms which allow the valve cap to open 
or close depending on the direction of the air flow. Once 
the check-valve is closed, the valve cap can cover the entire 
vent holes completely to minimize any air leakage. The 
fabrication process of the flexible parylene-valved skin is 
shown in Figure 2. 
(a) Sacrificial PR; parl(lO µm); sputtered Si, 
pattern# 1 (Si) 
(b) Evap Al, pattern#2(Al); etch parylene; strip Al 
( c) Spin 2nd sacrificial PR; pattern#3(PR) 
ID 
(d) Par2(10 µm); evap Al; pattern#4(Al); etch 
parylene; strip Al 
( e) Release PR; SAM 
(f) BrF3; release the skin 
Ill Silicon D Photoresist D Parylene 
Figure 2: Summary of the fabrication process 
The process begins with the first sacrificial layer of 
photoresist on a wafer. The resist is hardbaked at 120 °C 
102 
for 30 minutes to completely bake out the solvent. Next, the 
first layer ofparylene is deposited and then roughened in the 
oxygen plasma. The wafer is then annealed at 120 °C for 30 
minutes to reduce stress in the parylene film. Afterwards, a 
thin film of amorphous silicon is deposited and patterned. 
This layer is used as an anti-stiction layer. Aluminum film 
is evaporated as a mask to pattern parylene. After parylene 
patterning, the sacrificial layer of photoresist is spun on and 
patterned. Then the top layer of parylene is deposited and 
patterned to form the valve caps. Next, the photoresist is 
released in an acetone bath to free the skin and then SAM 
layer is coated [11]. Finally, the amorphous silicon is 
etched in BrF3 gas to complete the fabrication process. 
Figure 3 shows the released parylene-valved actuator skin. 
Figure 3: Flexible parylene-valved actuator skin 
There are several processing challenges during the 
fabrication that are worth noted. For example, because the 
corner step of the vent hole is the thickness of the bottom 
parylene (-10 µm), it is important that the fd sacrificial 
photoresist must be thick enough to cover this step corner. 
Otherwise the top and bottom parylene films will fuse 
together at the corner. This can prevent the valves from 
opening. Figure 4 displays other problems that are also 
encountered. Figure 4 (a) shows bubbles generated the 1st 
sacrificial photoresist during the annealing of parylene film. 
This problem occurs because the 1st sacrificial photoresist is 
not hardbaked long enough to get rid of the solvent. Thus 
the remedy is to hardbake the photoresist at 120 °C for at 
least 30 minutes. Figure 4 (b) shows the buckling of the 
thin amorphous silicon film due to the stress mismatch 
between the silicon and parylene films. By annealing the 
parylene film before the amorphous silicon deposition, the 
stress in parylene becomes tensile. The buckling effect can 
be reduced. Lowering the deposition power also helps 
alleviate this problem. Finally, Figure 4 (c) shows the 
stiction of the valve after photoresist is released in acetone. 
After the skin is dry, the valves are stuck and cannot be 
opened. By having an amorphous silicon layer underneath 
the caps, dry etching of BrF3 gas can etch the thin silicon 
layer away and helps release the valves without any stiction. 
• 
. 
. 
. ........ ~ 
..... ,.., •t 
(a) Resist bubbles (b) Buckling ( c) Stiction 
Figure 4: Examples of processing challenges 
RESULTS AND DISCUSSION 
The actuator skins are integrated onto the MEMS wings 
and tested in a low-speed wind tunnel as seen in Figure 5. 
In addition to the actuator skins, parylene pin-holed skins 
with 500-µm hole diameter are also fabricated. The design 
of these skins is identical to those of the check-valved type 
except that there are no valve-caps. The pin-hole-integrated 
wings, as well as unmodified MEMS wings, are used as 
references for comparison of the effect of the check-valved 
actuators. Table 1 summarizes the phase-average results for 
reference, pin-holed, and check-valved integrated MEMS 
wings. Figure 6 and 7 show the plots of lift and thrust 
forces of those wings. Phase averaging is the method used 
to achieve instantaneous lift and thrust measurements by 
collecting the entire lift and thrust history of a complete 
downstroke and upstroke cycle. It also reduces the noise 
levels as many flapping cycles are averaged. The final data 
is the result of the integration of the lift md thrust values 
over the entire period of flapping cycle. 
Figure 5: Wind tunnel aerodynamic test 
Table 1: Summary of phase-average results 
Reference 
Pin-holes 500-
Check"valves open on 
downstroke 
Check-valves close on 
downstroke 
Lift (g] 
2.190 
1.376 (-37%) 
2.996 (+37%) 
3.358 (+53%) 
Thrust (g] 
0.454 
0.223 (-51%) 
0.718 (+58%) 
0.663 (+46%) 
103 
60 
40 
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<::::: 
:.::5 0 
-20 
-40 
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 
Flapping Period 
:§ 10 .............. , ............ ., .......... ,,. 
] 0 1-···········'•············ll'····· .. ····~············~···········+···········;···········\' 
£-< -10 
0.1 0.2 0.3 0.4 0.5 0.6 0. 7 0.8 0.9 
Flapping Period 
Figure 6: Phase-average results of the reference and pin-
holed integrated wings 
60 
-40 
30 
-20 
Skin 
Area 
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 
Flapping Period 
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 
Flapping Period 
Figure 7: Aerodynamics and phase-average results of 
the check-valved integrated wings 
The phase-average results of the reference and pin-
holed integrated wings in Figure 6 indicate that the pin-
holed integrated wings do not perform as well as the 
reference wings. With only vent holes on the wings, the air 
can flow through the holes freely in both downstroke (0-0.5) 
and upstroke (0.5-1) cycles. The holes equalize the pressure 
on the upper aid lower surfaces of the wings. The wing 
loading is reduced in both upstroke and downstroke, hence 
the reduction in aerodynamic performance. The phase-
average of lift and thrust are 3 7% and 51 %, respectively, 
lower than those of the unmodified reference wings. 
On the other hand, the test results of the pneumatic 
check-valved type have shown a dramatic effect on the 
aerodynamic performance. The MEMS wings integrated 
with flexible parylene-valved actuator skin are shown in 
Figure 7. The actuators are placed in the leading-edge and 
trailing-edge areas where they are believed to significantly 
affect the aerodynamic performance of the wings. The 
check-valved actuators are mounted onto the MEMS wings 
in two different directions. First, the valves open during 
downstroke and close on upstroke. Second, the valves close 
during downstroke and open on the upstroke. Aerodynamic 
test results show lift has increased significantly by 3 7% and 
53% for the first and second ways, respectively. Similarly, 
the thrust has also increased by 58% and 46%, respectively. 
These numbers show tremendous improvements. The plots 
also show that there is a phase delay between opening and 
closing of the valves. The effect moderates the extreme lift 
peaks and in particular the negative lift peak that happens 
during the downstroke. The aerodynamic performance is 
highly dependent on the formation of the shedding vortices. 
Disturbing the leading-edge vortices will vary the pressure 
distribution and alter the aerodynamic performance. This 
delay also causes the upstroke peaks to change and phase 
shift relative to the unmodified reference wing. 
CONCLUSIONS 
We have successfully demonstrated the dramatic effect 
on the aerodynamic performance of the MEMS wings by 
integrating with parylene-valved skin. Compare to the 
reference MEMS wings, both lift and thrust of the parylene-
valved wings, mounted such that the check-valves close 
during downstroke, can be improved by almost 50%. This 
is the first experimental result to demonstrate the promising 
application of MEMS actuator skins for flow control. 
This work also includes the novel anti-stiction 
technology that is crucial to release such large-area parylene 
actuator skin with the combined use of SAM, amorphous 
silicon layer, and BrF 3 dry etching. The future work will 
include the design, fabrication, and test results of active 
check-valved electrostatic actuator skin. 
ACKNOWLEDGEMENTS 
The authors would like to thank DARPA (DABT63-98-
C-0005) and NSF ERC for their generous supports. 
104 
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Flexible parylene-valved skin for adaptive flow control

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Flexible parylene-valved skin for adaptive flow control

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