PARYLENE ETCHING TECHNIQUES FOR MICROFLUIDICS AND BIOMEMS
Ellis Meng1 and Yu-Chong Tai2
1Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA
2
Caltech Micromachining Laboratory, California Institute of Technology, Pasadena, CA, USA
ABSTRACT
Parylene C (poly(monochloro-p-xylylene)) is a memberof a unique family of thermoplastic, crystalline polymers.
Compared to other polymers, parylene films are
exceptionally conformal and chemically inert owing to its
vapor deposition polymerization (VDP) coating process.
These properties bring about many interesting
possibilities for MEMS, particularly in microfluidic andbioMEMS applications. Dry etching techniques arerequired to define fine features in parylene films. For thefirst time, selective parylene C removal using oxygenbased plasmas is characterized for plasma etching,reactive ion etching (RIE), and deep reactive ion etching(DRIE) based methods. The ability of these techniques toachieve high aspect ratio (HAR) structures desirable for
MEMS applications is also investigated.

1. INTRODUCTION
Parylene as a MEMS Material
Parylene is the generic name for a family of unique vapor
phase deposited polymers. It was originally discoveredby Michael Mojzesz Swarc in 1947 and commercialized
by Union Carbide Corporation in 1965 after thedevelopment of a practical deposition process by WilliamF. Gorham [1]. Although over 20 types of parylene havebeen developed, only three are commonly available (Fig.
1).

Figure 1: Commercially available types of parylene
Parylene has found wide acceptance in the
microelectronics and medical industries as a coating
material. Recently, much attention has been given toparylene by MEMS researchers in applications ranging
from microstructures to integrated microfluidic systems[2-6]. The need for biocompatible materials has driven
the popularity of polymers for MEMS. Among them,
parylene has particularly attractive properties including
low process temperature, low defect density,transparency, and chemical inertness.
In addition,
parylene technology can accommodate multi-layer
processing to produce complex structures and features(Fig. 2).

Figure 2:
Low-temperature, multi-layer parylene
processing technology for MEMS
Biocompatibility of Parylene
Parylene has attracted a great deal of interest due to its
biocompatibilityandbiostability.
Biologicalenvironments are extremely corrosive to most MEMS andmicroelectronic materials however it does not affectparylene as it cannot be degraded hydrolytically [7]. In
addition, parylene has low cytotoxicity which has lead toits application as a platform for neural growth [8, 9].

2. SELECTIVE PARYLENE REMOVAL
Chemical Removal
Parylene resists dissolution by solvents at temperatures
below its melting point. Although it is possible todissolve parylene in chloronaphthelene or benzolyl
benzoate at temperatures above 150 °C [10], thistechnique is incompatible with commonly usedlithographic processes. Lift-off is also problematic due tothe highly conformal nature of parylene films. Sabeti [11]reported the possibility of selectively depositing parylene
by creating heat using localized resistors. UV laser
ablation of parylene has also been demonstrated [12, 13].
However, the only practical method to pattern parylene isthrough plasma etching.
Plasma Removal
Previously, plasma [14, 15], reactive ion beam (RIBE)[16], and reactive ion [17, 18] etching of parylene inoxygen environments has been reported. Parylene film
thicknesses of only a few microns were etched and noattempt at HAR structures was made. Yeh [17] has
observed that etch selectivity between parylene andstandard photoresist masking materials is near unity due
to their organic nature.

Majid [18] reported that etching parylene by RIE
using low pressure and power with a substrate bias gave
rise to trenches with vertical profiles, however these same
conditions resulted in surface roughness of the etchedfeatures. Specifically, delicate and dense micrograss
structures were observed on the parylene surface. Ratier[16] noticed that RIBE gave better surface uniformity
than RIE however the etch rates are very low (~15-55
Å/m).
Zahn [19] reported achievable aspect ratios of 20:1
by employing Bosch-like switching chemistries for theetching of polymethylmethacrylate (PMMA). Standaert[20] demonstrated vertical sidewalls in parylene N using
an inductively coupled plasma source. Based on thesepromising results, a similar technique has been
demonstrated to etch parylene and photoresist.
Plasma Etching Mechanisms
The mechanisms that govern parylene etching processes
are not well understood. A possible mechanism foretching parylene N using a microwave plasma source wasreported [21]. The proposed etching mechanism involves
the opening of the benzene ring brought about by thefollowing sequence of reactions: (1) hydrogen abstraction
from the ethyl carbons between the benzene rings in thepolymer chain, (2) molecular or atomic oxygen adsorption
onto the exposed reactive site, (3) polymer degradation
through the formation of volatile carbon monoxide (foratomic oxygen adsorption) or dioxide (for molecularoxygen adsorption), and (4) radical site formation on thearomatic ring. The parylene C etching mechanism isbelieved to be similar to that for parylene N. Chemically,
their structures differ by the presence of a chlorine atomin parylene C which reduces the exposed carbon bondingsites by one. This may correspond to the observed
reduction in etch rate by approximately 17 % [22].

and profiled again to determine the etch rates for parylene
and photoresist.

4. RESULTS AND DISCUSSION
Plasma etching results exhibit increasing etch rate forincreasing power (Fig. 3). However, the etch rate
decreases for increasing pressure. Generally, the etch rateincreases with flow rate (flow limited regime), reaches amaximum, then decreases for higher flow rates. Thisdecrease is attributed to the pumping speed whereby theactive species is removed before it can react [23].
Similar trends are observed for RIE and DRIE (Fig.s4-5). The effects of adding argon to the oxygen plasma
and using switched chemistries in DRIE mode were also
examined (Fig.s 6-7). The addition of Ar is thought tocontribute to the etch rate through physical sputtering.
Here, however, it reduces the etch rate. The effect of flow
limited etching is apparent for both etch step time settings
at low flow. The highest etch rates are obtained foroxygen flows of 100 sccm in RIE mode whichcorresponds to the maximum flow rate limit of the etching

3. EXPERIMENTAL METHODS

Figure 3: Etch rates of parylene and AZ 4620 for varying
power and pressure by plasma etching

Silicon test coupons measuring 20 mm × 10 mm weremanually separated from a wafer coated with 10 µm of
parylene and masked by 14 µm of AZ 4620 photoresist.
Coupons were etched using plasma (Technics PEII-A),
RIE (The Semi Group RIE System 1000 TP/CC), andDRIE (PlasmaTherm SLR-770B) modes under varying
conditions. Etch rates of parylene and the photoresist
masking layer were obtained for varying pressure, gasflow, power, and etching chemistries. In addition,
programmed switching chemistries were examined forDRIE mode. Each etching cycle, or loop, consists of twosteps: (1) deposition of C4F8 and (2) O2 etching, where thefluoropolymer layer protects feature sidewalls from lateraletching. Sputtered amorphous silicon (0.5µm) and oxide(0.3µm) were also examined as possible masking
materials. After etching, step heights were measured
using a surface profile (Tencor AlphaStep 200). The
photoresist was stripped from half of the etched sample

Figure 4: Etch rates of parylene and AZ 4620 for varying
power and pressure in oxygen plasma by RIE

perform poorly in DRIE switched chemistry etching due
to poor adhesion and etching, respectively. Others have
tried spin-on glass and nitride masks with some success[20].

Figure 5: Etch rates of parylene and AZ 4620 for varying
power and pressure in oxygen plasma by DRIE

Table 1: Maximum observed etch rates and respective
etching conditions for parylene and AZ 4620 for various
etch methods
Parylene
AZ 4620Plasma.19 µm/min .29 µm/min(200 mT, 400 W)
RIE.56 µm/min .67 µm/min(100 sccm, 200 mT, 400 W)
DRIE.77 µm/min .94 µm/min(60 sccm, 23 mT, 800 W)

Figure 6: Etch rates of parylene and AZ 4620 for varying
power and pressure in oxygen and argon plasma by DRIE

Profiles of features etched using plasma, RIE, andDRIE techniques are compared in Fig. 8. Plasma etchedsamples exhibited significant lateral etching and tapered
profiles whereas DRIE switched chemistry etchingproduces nearly vertical sidewalls. Preliminary results
indicate that aspect ratios of at least 2:1 can be achievedfor etching parylene with both RIE and DRIE.
Traditional oxygen plasma etching techniques yield atbest a ratio of 1:1. With DRIE it is also possible toproduce tapered, vertical, or reentrant sidewall profiles,
which suggests interesting possibilities for using etchedparylene features for molding applications. Further
improvement of etched aspect ratio is expected with theapplication of a substrate bias in RIE and DRIE modes[18, 20]. It has also been reported that parylene etching
can be enhanced by the addition of CF4 [18].

Figure 7: Etch rates of parylene and AZ 4620 for
switched chemistry etching by DRIE
apparatus. It is possible that etch rates obtained using
DRIE may be higher than that of RIE as the data does not
subtract the etch time required to remove the deposited
fluorocarbon sidewall protection layer. Recipes andresults for maximum parylene and photoresist etch rates
are summarized in Table 1. Etch selectivity remains adrawback of using photoresist masks for all methods
examined. Sputtered α-silicon and oxide masks also

Figure 8: Etched parylene profiles of line features forplasma etching (a, b); RIE (c); DRIE oxygen only (d);DRIE oxygen and argon (e); and DRIE switchedchemistry etching (f)5. CONCLUSION
The etching of parylene C has been examined for plasma,RIE, and DRIE modes. Masking materials, etchingchemistries, and process parameters (power, pressure, andgas flow) were varied to ascertain the effect on the etchrate and resultant feature profiles. Initial results arepromising for the development of a process to produce
HAR structures in parylene by using RIE and DRIE.
Future studies involving the application of a substrate bias
and different etching species are expected to further
improve upon the results presented here.

6. ACKNOWLEDGMENTS
This work was supported in part by the Engineering
Research Centers Program of the NSF under Award
Number EEC-9402726. We would like to thank Mr.
Trevor Roper and Mr. Damien Rodger for assistance infabrication and Mr. Tuan Hoang for help withproofreading.

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