We report here a surface micromachined leakage proof Parylene MEMS check valve that has nearly ideal performance. This new check valve has a unique design that its valve membrane has a sealing ring directly deposited on, and hence in direct contact with, the gold-coated valve seat. The adhesion between Parylene and gold is studied along with the reduction of adhesion of the sealing ring to the valve seat when self-assembled monolayer (SAM) coatings are used on gold. Testing results clearly demonstrate the effectiveness of SAM coating. Experiments show that the valves have no observable leakage in the reverse flow direction up to 30 psi and a cracking pressure less than 1 psi in the forward flow direction. Integration of this valve with microchannels in a microfluidic system is also demonstrated. Testing shows the in-channel check valve also has nearly ideal performance under both forward and reverse flow

Tai, Yu-Chong

Wang, Xuan-Qi

Xie, Jun

Yang, Xing

Caltech Authors - Main

SURFACE MICROMACHINED LEAKAGE PROOF PARYLENE 
CHECK VALVE 
Jun Xie, Xing Yang, Xuan-Qi Wang, and Yu-Chong Tai 
Electrical Engineering, 136-93 
Caltech Micromachining Laboratory 
California Institute of Technology 
Pasadena, CA, 91 125, U S A .  
ABSTRACT 
here a surface micromachined leakage proof 
Parylene MEMS check valve that has nearly ideal performance. 
This new check valve has a unique design that its valve membrane 
has a sealing ring directly deposited on, and hence in direct contact 
with, the gold-coated valve seat. The adhesion between Parylene 
and gold is studied along with the reduction of adhesion of the 
sealing ring to the valve seat when self-assembled monolayer 
(SAM) coatings are used on gold. Testing results clearly 
demonstrate the effectiveness of SAM coating. Experiments show 
that the valves have no observable leakage in the reverse flow 
direction up to 30 psi and a cracking pressure less than 1 psi in the 
forward flow direction. Integration of this valve with 
microchannels in a microfluidic system is also demonstrated. 
Testing shows the in-channel check valve also has nearly ideal 
performance under both forward and reverse flow. 
a new check valve design that has a sealing structure that 
completely eliminates the gap between the sealing plate and the 
valve seat. By using this method, the in-channel check valve only 
needs 2 layers of Parylene. The new sealing structure is a Parylene 
sealing ring directly deposited onto valve seat surface. Because of 
the direct deposition of Parylene on the valve seat surface, the 
sealing plate needs to be pneumatically released from the valve 
seat before normal operation. It is found that this initial release 
pressure has a great effect on the performance of the check valve. 
To reduce the release pressure, we have put gold on the valve seat 
surface and used self-assembled monolayer (SAM) coating for 
gold [4] to further reduce the adhesion between Parylene and gold. 
As shown in the following, the testing results prove the 
effectiveness of SAM coating on gold. Flow characteristics also 
show that both out-of-channel check valve and in-channel check 
valve have nearly ideal performance in both forward and reverse 
directions. 
INTRODUCTION 
In the past few years, many MEMS fluidic handling devices 
such as valves and pumps have been developed with the ultimate 
goal of achieving integrated microfluidic systems for chemical and 
biological applications. As a fundamental component in such a 
system, a check valve is a passive device that allows fluid flow in 
forward direction and prevents the flow in the reverse direction. 
An ideal check valve should have zero cracking pressure and zero 
flow resistance in the forward direction and infinite flow resistance 
in the reverse direction. 
There are many published MEMS check valves [l-31. 
Various materials (including silicon, silicone, polyimide, and 
metal) along with various technologies have been used to construct 
check valves. Previously we developed surface micromachined 
check valves using Parylene, a CVD thin-film polymer, as the 
structural material with photoresist as the sacrificial layer. We 
reported that Parylene has the advantages of low modulus, good 
sealing, and good biocompatibility. 
Although our previous Parylene check valve [2] shows good 
performance, it has a gap created by the sacrificial layer between 
valve membrane and valve seat. Fluid can then leak through this 
gap in the reverse flow direction, which could have even more 
serious leakage problems when the check valve is integrated 
within microchannels. In a microchannel, the pressure drop across 
the check valve may be too small to close the valve. It is the goal 
of this work to achieve a check valve with zero leakage in the 
reverse flow direction even under very low reverse pressure drop 
and demonstrate it inside a microchannel. As a result, we propose 
0-7803-5998-4/01/$10.00 @200 1 IEEE 
Shown in Figure 1, the design for the check valve outside a 
channel is a Parylene disk suspended by four tethers. Unlike our 
previous check valve, there is a sealing ring structure on the edge 
of the Parylene disk. This sealing ring structure is formed by 
patteming a ring window in the photoresist sacrificial layer 
resulting in direct deposition of Parylene in the ring area. With 
such a sealing structure, there is no initial gap between the sealing 
plate and the valve seat. This no-gap design helps to achieve 
leakage proof in the reverse flow direction even under low- 
pressure drop. The radius of the valve sealing membrane varies 
from 60 pm to 120 pm and 5 pm to 20 pm for the width of the 
sealing ring. 
After the check valve is fabricated, the sealing ring part of the 
valve membrane has to be pneumatically released from the valve 
seat before it can be used. It is found that this initial release 
pressure has a great effect on the performance of the check valve 
Parylene 
Tether  Valve Membrane Sealing 
\ ,Ring 
Figure 1. Conceptual Design of the Check Valve 
539 
Check Valve Check Valve 
Outside a Microchannel Inside a Microchannel 
Pasylene n PR Cr/Au 
Figure 2. Check Valve Outside and Inside a Microchannel 
because high initial release pressure may cause permanent plastic 
deformation in the valve membrane and then compromise the 
valve sealing. To solve this problem, one can use a surface with 
poor adhesion to Parylene under the sealing ring structure. Gold 
typicdly has poor adhesion to Parylene compared to other 
surfaces, such as SiO, and Si. Experimentally, we found that the 
adhesion between Parylene and gold is still fairly strong so the 
large release pressure did damage the Parylene sealing membrane. 
As a result, we have used octadecanethiol SAM coating on gold. 
This SAM coating on gold is proved to be very effective in 
reducing the adhesion. Test results are shown in next section. 
'To integrate the check valve in a microchannel, some 
modification has to be made. In the case of check valve outside the 
micrcchannel, all the pressure is applied across the valve. But 
when the valve is placed inside a channel, the pressure drop across 
the valve is much smaller if the dimensions of the channel and the 
valve are similar. To increase the pressure drop, check valve with 
smaller dimensions should be used. In another design, a sealing arc 
(15 pm long and 60 pm wide) instead of the sealing ring is used. 
The design of the check valve inside and outside a microchannel is 
shown in Figure 2 .  
FABRICATION 
The check valves are fabricated using surface 
micrclmachining technology with Parylene-C as the structural layer 
and PZ photoresist as the sacrificial layer. 
Figure 3 shows the fabrication process for the check valve. 
The process starts with 4" silicon wafers. First, a layer of 1.6 pm 
thick thermal oxide is grown on both sides of the wafers. Etching 
windows are patterned on the wafer backside. KOH is used to etch 
holes in the silicon substrate until a 15 p n  thick silicon membrane 
is left on the front side of the wafer. These holes serve as the fluid 
inlet/outlet for the valve. Then a layer of 100 Cr and 2000 A Au 
is thermally evaporated on the wafer front side. The tri-layer of 
Au/Cr/SiO, is patterned to form the sealing ring area where the 
Parylme sealing ring is directly deposited. The wafers are then 
etched for one minute in BrF, vapor to roughen the silicon surface. 
During the BrF, etching, the silicon surface is etched away by 
0.5 pin. 
This BrF, roughening step serves two purposes. First it helps 
to improve the adhesion of Parylene layer to the silicon substrate. 
Double Side Alignment Thermal Oxidation 
KOH Etching 
b : m  
CrlAu, Deposition 
CrlAulSi0,Patterning Br& Roughening 
S A M  Coating 
1st Parylene-C Deposition and Patterning 
1st Sacrificial Photoresist Patterning 
2nd Sacrlficial Photoresist Patterning 
2nd Parylene-C Deposition and Patterning 
Si Membrane Etching 
Acetone Photoresist Releasing 
Figure 3. Check Valve Fabrication Process Flow 
Adhesion promoter (OS1 A-174) could not be used because of the 
presence of the photoresist sacrificial layer. Second, the area under 
the sacrificial photoresist layer is also roughened to reduce the 
stiction that may happen during the final acetone releasing of the 
photoresist sacrificial layer. For valves with large dimensions, an 
array of posts is fabricated in the valve membrane to further reduce 
stiction. 
Before the SAM coating for gold, gold surface is cleaned in 
200 mTorr oxygen plasma with 200 W power for 2 minutes. 
General instructions and principles on organic thiol SAM coating 
for gold are given in [4]. In our experiment, octadecanethiol is 
used as the coating material. 0.4 g octadecanethiol is dissolved in 
800 mL ethanol (-2 mM)'. Then the wafers are immersed in the 
solution for 10 hours. After the wafers are taken out of the 
solution, they are air dried for 15 minutes and followed by a 30 
seconds rinse in ethanol and a 10 minutes bake at 50°C. Water 
contact angle measurement is used to monitor the coating process. 
After SAM process, a 1 pm AZ 1518 photoresist layer is spun 
and pattemed to form the first sacrificial layer. This sacrificial 
layer separates the valve membrane from the valve seat except in 
the sealing ring region. A 0.5 Fm thick Parylene-C layer is 
deposited and then pattemed by using oxygen plasma with 4 pm 
thick soft baked AZ 4400 photoresist as the etching mask. Finally 
the valves are freed by etching away ihe remaining silicon 
membrane and oyemight acetone releasing of the photoresist 
sacrificial layer. 
To fabricate the check valve inside a microchannel, a second 
sacrificial layer of 8 pm AZ 4620 photoresist is spun and pattemed 
to form the channel pattem. Before the second Parylene-C 
deposition, the wafers are cleaned and roughened by oxygen 
plasma in order to increase the adhesion between the two 
540 
(a)  Outside a Microchannel (b) Inside a Microchannel 
Figure. 4 Pictures of the Fabricated Check Valves 
Parylene-C layers. A second layer of 5 pm Parylene-C is deposited 
to form the channel wall. The final releasing steps are similar to 
the case of check valves outside the channel. 
Shown in Figure 4 are pictures of fabricated check valves 
both outside a channel and inside a channel. 
EXPERIMENTAL RESULTS 
SAM Coating Process 
Contact angle measurement of water on the gold surface is 
used to monitor the SAM coating process. Right after the oxygen 
plasma cleaning, gold and roughened silicon surface have contact 
angles of water less than 10". After SAM coating, the contact angle 
on gold surface increases to 95" while the contact angle on 
roughened silicon surface is still less than 10". This confirms that 
SAM layer of octadecanethiol only coats gold surface as the 
principle of SAM coating implies. Also a measurement is made on 
gold surface before and after one cycle of photolithography 
process (spin coating, soft bake, exposure, development, rinse, and 
hard bake) and the contact angle on gold remains above 90". Even 
after the whole fabrication process is finished the contact angle on 
gold still remains above 90". This shows that the SAM coating of 
organic thiol on gold is very stable during the whole fabrication 
process. 
Table 1 compares the release pressure of the valves with and 
without SAM coating. The initial release pressure is defined as 
pneumatic pressure needed to separate the valve membrane from 
the sealing ring contact and get a forward flow through the valve. 
During testing, it is found that if this release pressure is too high, it 
can cause plastic deformation on the Parylene valve membrane and 
thus seriously affect the valve performance. Table 1 shows that for 
the same design, the release pressure of those valves with SAM 
coating is on average 10 times smaller than those without SAM 
coating. This proves the effectiveness of SAM coating in reducing 
the adhesion between Parylene and gold. 
Figure 5 shows a check valve that was initially released at a 
pressure of 27 psi. After releasing, the plastic deformation in the 
With SAM 2psi 
Without SAM 
Table 1. Comparison of Release Pressure of Check Valves 
With and Without SAM Coating. The valves have radius of 
120pum and sealing ring width of IO and 20 pm. 
Figure 5. Plastic Deformation of the Valve Membrane 
and Tethers 
Parylene membrane c& be clearly seen as indicated by the arrows, 
and testing shows that the valve has severe leakage in the reverse 
flow direction. 
Valve Performance 
The flow performance of both types of check valve has been 
tested with air and water as the working fluid. A simple testing 
setup is built with compressed air as the pressure source. The 
applied pressure is controlled with pressure regulator (0.1 psi 
accuracy) and pressure gage (0.01 psi accuracy). In the case of 
water testing, a column of water driven by the compressed air 
provides the working fluid. Flow rate is measured in a few 
different methods depending on the flow rate range. Air flow rates 
larger than 0.1 mL/min and water flow rates larger than 1 pL/min 
are measured with commercial available flow meters. For smaller 
air flow rates, a calibrated pipette with a small water droplet is 
used. The flow rate can be obtained by measuring the droplet 
movement in a given time.-Small water flow is measured in the 
similar way by following the movement of the water column front. 
Figure 6 shows the measured air and water flow 
characteristics of a check valve outside a microchannel. Since the 
Parylene valve membrane is very thin (0.5 pm), it tends to be 
plastically deformed or even break at the joint of the membrane 
and the tethers if the applied forward pressure is too large. Due to 
this, all the forward flow testing is performed at the pressure less 
than 10 psi. For reverse flow direction, valves are tested in both 
high-pressure region (up to 30 psi) and low-pressure region (down 
to 0.1 psi). Testing results in Figure 6 shows the valve has a 
forward flow rate of 2.5 mL/min for air and 20 pLmin for water at 
5 psi pressure. In the reverse flow direction, it shows no 
observable leakage up to 30 psi. The leakage is tested by 
observing the volume change of a water droplet on the valve outlet 
under the microscope. No significant volume change can be 
observed over 10 minutes. 
The testing results of an in-channel check valve for both air 
and water are shown in Figure 7. Due to the presence of the 
microchannel (80 pm wide, 8 pm high, and 7 mm long), the check 
valve has a much smaller flow fate compared to the check valve 
outside a channel. At 5 psi forward pressure, this valve has a flow 
rate of 3 @,/min for air and 150 nL/min for water. In the reverse 
flow direction, leakage is tested in a similar way as described 
above and no observable leakage can be found up to 30 psi. 
541 
-10 -8 -6 -4 -2 0 2 4 6 8 10 
Pressure (psi) 
(a)  Air 
3
-10 -8 -6 -4 -2 0 2 4 6 8 10 
Pressure (psi) 
(b) Water 
Figure 6. Air and Water Flow Characteristics of a Check 
Valve Outside a Microchannel 
CONCLUSIONS 
By using a unique sealing structure and surface 
micromachining technology, leakage proof Parylene check valves 
both outside and inside a microchannel have been successfully 
developed. The experimental results show the effectiveness of the 
design and the nearly ideal performance of the check valve under 
both forward and reverse flow. SAM coating is also demonstrated 
to be an effective way to reduce the adhesion between Parylene 
and gold. For future work, the integration of this check valve with 
other microfluidic devices, such as micro-nozzles, will be pursued. 
ACKNOWLEDGMENTS 
‘The authors would like to thank Dr. Terry D. Lee from 
Beckinan Research Institute of the City of Hope for helpful 
discussion and support. Also the authors would like to thank 
Mr. Yong Xu and Mr. Trevor Roper for their assistance in the 
fabrication. This project is co-supported by NIH under Grant R01 
RRO6217-06 and NSF ERC Center of Neuromorphic Systems 
Engineering (CNSE). 
-10 -8 -6 -4 -2 0 2 4 6 8 10 
Pressure (psi) 
(a)  Air 
-10 -8 -6 -4 -2 0 2 4 6 8 10 
Pressure (psi) 
(b) Water 
Figure 7. Air and Water Flow Characteristics of a Check 
Valve Inside a Microchannel 
REFERENCES 
1. S. Shoji, “Fluids For Sensor Systems,” Topics in Current 
Chemistry, Vol.194, 163 (1998). 
2. X. Q. Wang, Q. Lin, and Y. C. Tai, ”A F’arylene Micro Check 
Valve,” Proc. of the 12‘h IEEE MEMS 1!)99 Technical Digest, 
Orlando, Florida, 1/17-21/99, IEEE (1999), pp. 177-182. 
X. Q. Wang and Y. C. Tai, “A Normally Closed In-Channel 
Micro Check Valve,” Proc. of the IEEE MEMS 2000 
Technical Digest, Miyazici, Japan, 1/23-27/00, IEEE (2000), 
C. D. Bain, E. B. Troughton, Y. T. Tao, J. Evall J, G. M. 
Whitesides, and R. G. Nuzzo, “Formation of Monolayer 
Films by the Spontaneous Assembly of Organic Thiols from 
Solution onto Gold,” J .  Am. Chem. Soc., Vol.111, 321 
(1989). 
3. 
pp. 68-71. 
4. 
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Surface micromachined leakage proof Parylene check valve

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Surface micromachined leakage proof Parylene check valve

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