This paper describes parylene as an emerging bioMEMS material. Parylene has the unique feature of room-temperature, pinhole-free conformal CVD deposition. It is chemically inert and biocompatible. More interestingly, it is found that parylene thin film usually possesses a tensile intrinsic stress, controlled by the last thermal steps. These features allow free-standing parylene MEMS structures in many designs. Parylene MEMS is also a suitable technology for post-CMOS integration. As a result, multi-layer parylene MEMS technology has been developed, especially for bioMEMS applications. This paper also gives examples of integrated parylene microfluidics and HPLC on-a-chip

Tai, Yu-Chong

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2005 IEEE International Conference on Robotics and Biomimetics 
Parylene BioMEMS 
Yu-Chong Tai 
Electrical Engineering and Bioengineering 
California Institute of Technology, 
Pasadena, California 91125 
Abstract 
This paper describes Parylene as an emerging BioMEMS material. Parylene has the unique 
feature of room-temperature, pinhole-free conformal CVD deposition. It is chemically inert and 
biocompatible. More interestingly, it is found that Parylene thin film usually possesses a tensile 
intrinsic stress, controlled by the last thermal steps. These features allow free-standing parylene 
MEMS structures in many designs. Parylene MEMS is also a suitable technology for post-CMOS 
integration. As a result, multi-layer Parylene MEMS technology has been developed, especially for 
BioMEMS applications. This paper also gives examples of integrated Parylene microfluidics and 
HPLC on-a-chip. 
1. Parylene as a MEMS material 
Parylene is the generic name for members of a unique family of thermoplastic polymers that 
are deposited by using the dimer of para-xylyene (di-para-xylylene, or DPXN). Parylene coating 
processes were originally developed in the 1950s by William F. Gorham and then commercialized by 
the Union Carbide Corporation in 1956. Parylene is widely used for coating of electronic components, 
medical instruments, and has recently been explored as the interlayer dielectrics (ILD) in the 
microelectronics industry. The main reason for studying Parylene material to be used in MEMS 
applications lies in its unique room-temperature CVD conformal deposition characteristics. As 
demonstrated, the Parylene MEMS technology using photoresist as the sacrificial material requires 
processing temperatures no higher than 120°C, which is a great advantage in terms of cost 
consideration and. As shown in Fig.l, there are three different kinds of Parylene commercially 
available on the market. Parylene N is poly-para-xylylene, a completely linear and highly crystalline 
polymer. Parylene C, which is the main focus of study in this thesis, is basically Parylene N with a 
chlorine atom replacing one of the aromatic hydrogens. Parylene D is similar to Parylene C but with 
two aromatic hydrogens being replaced with chlorine atoms. The benzene backbone of the parylenes 
makes them very chemically inert. At room temperature, there is no known chemically that will 
significantly attack the parylenes. 
Parylene N Parylene C Parylene D 
Figure 1 The three most popular Parylenes. 
Parylene is deposited at room temperature around 0.1 torr where the mean free path of the molecules 
is on the order of 0.1 cm. The first step of the deposition process is the vaporization of the solid dimer 
at approximately 150°C. The second step is the pyrolysis of the dimer at about 650°C, in which the 
dimer is cleaved at the two methylene-methylene bonds to yield the stable monomeric diradicals, 
para-xylylene. Finally, the monomers enter the deposition chamber at room temperature where they 
polymerize on the substrate. The cold trap at approximately -70°C is used to collect the un-reacted 
monomers before they enter the mechanical pump. Although Parylene can be deposited selectively 
using heated resistors to create local heating, Parylene is best patterned with plasma etching, instead 
of chemical etching. Because Parylene deposition is conformal, lift-off process to pattern the Parylene 
is difficult. Therefore, O2 plasma etching with photoresist as the mask is used in most cases. Other 
metals such as sputtered silicon have been explored as etching masks. However, the most important 
criteria for selecting a mask material is the adhesion between the etch mask and the material itself. 
Adhesion of photoresist and Parylene is excellent because they are both polymers, whereas some 
metals such as evaporated aluminum do not guarantee good adhesion and therefore have to be used 
0-7803-9315-5/05/$20.00 ©2005 IEEE 26 
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with great care. A-174 is the adhesion promoter suggested by the manufacturers in order to enhance 
the adhesion between the Parylene film and many substrates (Liger, Rodger and Tai, 2003). The ease 
of Parylene process is further demonstrated by a multi-layer Parylene MEMS technology with as 
many as 5 layers of Parylenes (Wang and Tai, 1999) without special planarization techniques. 
Mechanically, parylene has a high electrical resistivity and low Young's modulus (2-5 GPa). 
2. Biocompatible Parylene for BioMEMS 
It is known that materials like silicones (e.g., PDMS) and polyimides could be "short-term" 
(e.g., for 3 months) biocompatible, but they are not known to be "long-term" biocompatible materials. 
On the other hand, Parylene is a much better biocompatible as it is an US Pharmacy (USP) officially-
approved class-VI implantable plastic material. It has a long history of being used in many biomedical 
devices such as pacemakers (for 3 decades) and even implantable electronic packages. More 
importantly, a complete set of useful Parylene MEMS micro fluidics devices including valves, pumps, 
reaction chambers, and even HPLC have been developed for the last 10 years (Tai, 2003). This 
Parylene micro fluidics technology can now be fully available. 
3. Integrated Parylene BioMEMS 
MEMS can have very interesting microflow control applications (Ho and Tai, 1998), but time 
may prove that BioMEMS may be the biggest winner. Here, then, some important examples of 
Parylene micro fluidics valves and pumps developed for BioMEMS applications are described. 
3.1 In-channel Check Valves 
Fig.2 shows the first surface-
micromachined, normally closed, in-channel, 
Parylene check valve (Wang and Tai, 2000). 
This device is fabricated monolithically on a 
silicon substrate using a five-layer Parylene 
process. The operating structure of the check 
valve is a circular sealing plate on top of a ring-
shaped valve seat. The sealing plate is center-
anchored on top of a chamber diaphragm that is 
vacuum-collapsed to the bottom of the chamber 
in order to achieve a normally closed position. 
A thin gold layer on the roughened valve seat 
surface is used to reduce stiction between the 
sealing plate and the valve seat. We have 
Forward 
Micro Channe 
Membrane Cap 
Figure 2 a surface-micromachined normally closed 
in-channel check valve using 5-lavers ofParvlene. 
achieved an in-channel check valve with a cracking (opening) pressure of 20~40 kPa under forward 
bias and no measurable leakage under reverse bias up to 270 kPa. Using this design, this valve 
performs well in two-phase micro fluidic systems (i.e. microchannel flows containing gas, liquid, or 
gas/liquid mixture). 
3.2 Integrated Mass Flow Controller 
An integrated surface-micromachined 
mass flow controller (MFC) that consists of an 
electrostatically actuated microvalve and a 
thermal flow sensor is shown in Fig. 3 (Xie and 
Tai, 2003). With a special design and a 
multilayer Parylene process, the active 
microvalve and the flow sensor are integrated 
onto a single chip to perform closed-loop flow 
control. Sensitivity of the flow sensor is 55 
/lV/(/lL/min) for air flow and 12.2 /lV/(nL/min) 
for water. Valve is actuated with 10 kHz AC 
voltage and 200 Volts can close the valve while 
Figure Fabricated mass flow controller 
27 
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3 psi pressure is applied. For flow control, both Pulse Width Modulation (PWM) and actuation 
voltage adjustment are used. The testing results show that PWM has better control capability and 
linearity. 
3.3 Electrostatic Parylene Peristaltic Pump 
Fig. 4 presents a new 4-Parylene-Iayer 
surface-micromachining platform technology 
and its application for an electrostatic 
peristaltic micropump (Xie, Shih and Tai, 
2004). CVD Parylene is used as the structural 
material, while normal photoresist is the 
sacrificial layer and Crl Au the electrode. This 
versatile process enables a peristaltic pump 
design, in which electrical field does not exist 
inside the fluid. Testing shows an electrostatic 
pull-in voltage of 150V for a 200-)lm-diameter 
membrane. 3 and 6-phase peristaltic pumping Figure 4 Fabricated micro peristaltic pump. 
actuations are used and pumping of either water or ethanol is demonstrated. A pumping rate of 
2nL/min (flow velocity of lOO)lm/sec) is achieved at 60Hz for ethanol. Higher flow rate (> lOnL/min) 
is feasible with improved design. 
4 Integrated Chip for Protein Mass Spectral Analysis 
Finally, Fig. 5 presents the first fully integrated gradient-elution liquid chromatography-
electrospray ionization (LC-ESI) system on a chip (Xie et aI, 2005). This chip integrates a pair of 
high-pressure gradient pumps, a sample injection pump, a passive mixer, a packed separation column, 
and an ESI nozzle. We also present the successful on-chip separation of protein digests by reverse 
phase (RP)-LC coupled with on-line mass spectrometer (MS) analysis. Fig. 6 shows an example of 
BSA peptides that are separation by the HPLC chip. Therefore, we have demonstrated the first fully 
integrated LC-ESI system on a chip. The chip consists of (gradient and sample injection) pumps, a 
mixer, a column, filters, and an ESI nozzle to create a functional device. Current work involves 
integrating sensors for feedback control and improving separation performance. 
Figure 5 A photograph of the fabricated HPLC lab-on-a-chip. The picture on the right is a fully 
packaged HPLC system that can be connected directly to mass spectrometers through electro spray 
ionization (ESI). 
Intens . r--------------------, 
xl07 
2.5 
2.0 
1.5 
1.0 
0.5 
10 15 20 25 30 35 40 45 50 Time [min] 
Figure 6 Separation ofBSA Digest using commercial HPLC system. 
5. Conclusions 
28 
Authorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on April 13,2010 at 22:02:31 UTC from IEEE Xplore.  Restrictions apply. 
This paper describes Parylene as an emerging BioMEMS material. In addition, multi-layer 
Parylene MEMS technology has also been developed, which enables many important BioMEMS 
applications. This paper gives examples of integrated microfluidics. Moreover, HPLC on-a-chip is 
also described as an example of fully integrated and complex BioMEMS system. 
Acknowledgements 
The authors would like to thank various graduate students at Caltech for their contribution to 
produce the excellent results. We also like to thank many US funding agencies, including DARPA, 
AFOSR, NASA, NSF and NIH for their support of our research. 
References 
Ho CM and Tai YC (1998), Micro-electro-mechanical systems (MEMS) and fluid flows. Ann Rev 
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Proceedings, The 16th IEEE International Conference on Micro Electro Mechanical Systems 
(MEMS'03), Kyoto, Japan, pp. 602-605. 
Shih CY, Chen Y, Xie J, He Q and Tai YC (2005), On-chip temperature gradient liquid 
chromotography, Proceedings, The Sixteenth IEEE International Conference on Micro Electro 
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Tai YC (2003), "Parylene MEMS: material, technology and applications," Proceedings of the 20th 
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29 


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Parylene BioMEMS

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