This paper presents the first use of recrystallized

parylene as masking material for silicon chemical etch.

Recrystallized parylene was obtained by melting parylene C at

350°C for 2 hours. The masking ability of recrystallized parylene

was tested in HNA (hydrofluoric acid, nitric acid and acetic acid)

solution of various ratios, KOH (potassium hydroxide) solution

and TMAH (tetramethylammonium hydroxide) at different

temperatures and concentrations. It is found that interface

between parylene and the substrate can be attacked, which

results in undercuts. Otherwise, recrystallized parylene exhibited

good adhesion to silicon, complete protection of unexposed silicon

and silicon etching rates comparable to literature data

Kuo, Wen-Cheng

Lo, Hsi-wen

Tai, Yu-Chong

Yang, Yao-Joe

English

Caltech Authors - Main

Recrystallized parylene as a mask for silicon chemical etching

RECRYSTALLIZED PARYLENE AS A MASK 
FOR SILICON CHEMICAL ETCHING  
Hsi-wen Lo1, Wen-Cheng Kuo1,2, Yao-Joe Yang2, and Yu-Chong Tai1  
1Department of Electrical Engineering, California Institute of Technology, USA 
2Department of Mechanical Engineering, National Taiwan University, Taiwan 
 
Abstract—This paper presents the first use of recrystallized 
parylene as masking material for silicon chemical etch. 
Recrystallized parylene was obtained by melting parylene C at 
350? for 2 hours. The masking ability of recrystallized parylene 
was tested in HNA (hydrofluoric acid, nitric acid and acetic acid) 
solution of various ratios, KOH (potassium hydroxide) solution 
and TMAH (tetramethylammonium hydroxide) at different 
temperatures and concentrations. It is found that interface 
between parylene and the substrate can be attacked, which 
results in undercuts. Otherwise, recrystallized parylene exhibited 
good adhesion to silicon, complete protection of unexposed silicon 
and silicon etching rates comparable to literature data. 
Keywords- parylene; silicon wet etching; recrystallization;  
I.  INTRODUCTION 
Silicon wet etching processes are applied in fabrications of 
traditional MEMS structures, such as silicon cantilevers, silicon 
thin diaphragms, and other freestanding structures. Although 
inductively-coupled plasma etching, or deep reactive ion 
etching (DRIE) can achieve high aspect ratio structures, silicon 
wet etching provides relatively cheap, simple and easy 
processes and remain indispensable in MEMS fields.  
The common masks used in silicon wet etchings, such as 
TMAH and KOH are silicon oxides and silicon nitrides. Silicon 
oxides can be obtained by LPCVD, PECVD or thermal 
oxidation and silicon nitrides is obtained through LPCVD and 
PECVD. These deposition processes somehow add additional 
complexity and design challenges to the overall processes.  
Thermal oxidation and LPCVD processes require high 
temperature and thus cause stress problems in the deposited 
thin film while PECVD oxide and nitrides can be etched at a 
considerable rate by silicon wet etchants. Sometimes, using 
hard masks is simply impossible since deposition of hard 
masks will inevitably damage the existing microstructures.  
To address problems aforementioned, polymeric mask 
materials have been developed [1][2]. Polymeric masks emerge 
as easy and economical alternatives to hard masks and provide 
fair protection and pattern-transferring capabilities.    
Parylene, or poly(paraxylylene), has been widely used in 
many MEMS application. Microfluidic channels [3], 
electrospray inject nozzles [3], pressure sensors [4], 
implantable electrode arrays [5], and etc. have been 
demonstrated.  
Tooker and Tai first reported using recrystallized parylene 
C to improve the adhesion between parylene C and glass slides 
[6]. Parylene C was heated at 350? in nitrogen ambient and 
was melted and recrystallized. Recrystallized parylene C 
provides much stronger adhesion to glass and can still be 
etched by oxygen plasma [6]. This paper then explores the 
capability of recrystallized parylene C as the etching masks for 
silicon etching. 
II. EXPERIMENTAL DETAILS 
A. Sample Preparation 
All samples are prepared in the same way. It starts with 
prime silicon wafers, be it double-side polished or single-side 
polished. Silicon wafers are first cleaned isopropyl alcohol and 
acetone. Next, wafers are cleaned with standard piranha 
solution to fully remove organic contamination. Prior to 
parylene deposition, wafers are dipped in 5 wt % hydrofluoric 
acid to clean the surface and to remove silicon dioxides.  
Parylene C of different thickness is then deposited on 
silicon wafers via a room-temperature chemical vapor 
deposition (CVD) process. Thickness of parylene is controlled 
by the amount of dimmer weight loaded in the machine. All 
parylene dimmers are used as purchased from Specialty 
Coating Systems. No further purification is performed.  After 
parylene deposition, wafers with parylene are heated in a 
furnace filled with nitrogen. The temporal heating profile of the 
furnace is such that it ramps up to the hold temperature, 350oC 
at a rate of 10oC per minute and maintains the hold temperature, 
350oC, for 2 hours and finally cools down to room temperature 
naturally (the ramp down rate is less than 2 oC per minute. The 
hold temperature, 350oC, and 2 hours of hold time are selected 
so as to ensure parylene C is fully melted and recrystallized. 
After parylene C is recrystallized, etching windows are 
opened with conventional photolithography process and 
reactive ion etching (RIE) oxygen plasma etching. Here, it is 
worth note that recrystallized parylene C has a slower etching 
rate than generic parylene C, typically around 0.5 times of that 
of generic parylene C.  
After etching windows and patterns are formed, 
recrystallized parylene C coated silicon wafer samples are 
dipped into various silicon etching solutions 
B. Etching Solution Preparation 
Tetramethylammonium hydroxide (TMAH) solutions are 
prepared by diluting TMAH 25 wt % solution, purchased from 
Sigma Aldrich, with deionized (DI) water. Similarly, 
*Contact author: Hsi-wen Lo, Caltech Micromachining Group, 1200 E. 
California Blvd, Pasadena, CA, 91125, USA, email: lo@mems.caltech.edu. 
881978-1-4244-1908-1/08/$25.00 ©2008 IEEE.
Proceedings of the 3rd IEEE Int. Conf. on
 Nano/Micro Engineered and Molecular Systems
January 6-9, 2008, Sanya, China
Authorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on April 15,2010 at 16:18:10 UTC from IEEE Xplore.  Restrictions apply. 
Potassium hydroxide (KOH) solutions are prepared by diluted 
KOH solution 45 wt % solutions, purchased from Sigma 
Aldrich, with DI water.  Hydrofluoric acid - nitric acid – acetic 
acid (HNA) etching systems are prepared by mixing 49% 
hydrofluoric acid, 98% nitric acid and anhydrous acetic acid at 
different ratio.  
Reflux systems are used for TMAH and KOH etching 
processes. 
C. Measurement Methods and Instrument 
The undercut length is measured under optical microscopes 
and the etching depth is measured with KLA Tencor P15 
surface profilometer.  Scanning electron microscopy (SEM) is 
also used to further identify surface properties of recrystallized 
parylene and cross-sectional structures of silicon after etching.   
III. RESULTS AND DISCUSSION 
Recrystallized parylene C has polycrystalline structures. 
Different from generic parylene C, grain boundary can be 
easily and clearly observed under optical microscopes, 
especially when using optical microscopes with polarized light 
sources. Fig. 1 shows the pictures of generic and recrystallized 
parylene under polarized green microscopes and SEM pictures 
of recrystallized parylene surfaces.  
As can be seen from Fig. 1, grain boundaries appeared after 
baking generic parylene C at 350oC for 2 hours and crystallized 
did occur in the heating processes. That is why we call 
parylene C after the heat treatment “recrystallized parylene”.  
To assess whether recrystallized parylene C is a reliable 
silicon etching mask, several issues have to be investigated. 
Among these, crucial question such as how thick parylene 
should be to provide decent protection, what the undercut rate 
is, if any, and what is etching selectivity with respect to silicon 
will be addressed in this paper.  Several experiments are 
performed to answer these questions. 
A. Selectivity 
Typical silicon wet etchants are KOH, TMAH and HNA. 
There is no known chemical reaction between parylene and 
these silicon etchants, and we can surely assume parylene will 
not be damaged during etching processes. Later experiments 
confirmed this claim. The selectivity of silicon with respect to 
parylene is, therefore, infinity. That makes recrystallized 
parylene an excellent mask for silicon wet etching.   
B. Thickness 
Thickness of parylene reduces less than 5% after 
recrystallization. Since the change is small, thicknesses 
discussed in this paper hereafter are all the thickness of 
parylene before recrystallization. 
Three samples of silicon wafers are prepared to answer this 
 
Figure 1. Parylene surfaces: Polarized green optical microscopic picture 
before recrystallization (a), and after recrystallization (b). (c) is the SEM 
pictures of recrystallized parylene. Grain diameters in (b) are around 200m.
(a) (b) 
(c) 
(a)  
(b)  
(c)  
Figure 2. Recrystallized parylene samples after 9 hours in 58 oC KOH 
(a) 1 m,  (b) 2.5 m, and (c) 5 m. Serious undercuts occur in (a). 
882
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question. The thicknesses of these three samples are 1m, 
2.5m, and 5m. These samples are dipped in 20% KOH 
solutions at 58 oC for 9 hours. Fig. 2 shows the pictures of 
these three samples after etching. 
As shown in Fig. 2, 1-m recrystallized parylene renders 
serious undercuts and poor pattern transferring ability while 5-
m recrystallized parylene gives satisfactory results both for 
undercuts and pattern transferring. It follows that the thicker 
parylene is, the better etch-masking ability it provides. 
However, time and effort to pattern recrystallized parylene 
increases as the parylene thickness increase and the etching rate 
of recystallized parylene is, for the same oxygen plasma 
etching conditions, half as much as that of generic parylene. In 
determining proper parylene thickness, one has to balance 
masking ability and the time to patterning it. 5-m 
recrystallized parylene was chosen in the experiments 
determining undercut rates.  
C. Single-Side Polished  and Double-Side Polished Wafers 
In the previous experiment determining the proper 
thickness of recrystallized parylene, single-side polished (SSP) 
wafers were used. Serious delimination of recrystallized 
parylene and therefore undesired etching and damages always 
occurred on the backside even when recrystallized parylene is 
thick enough to provide protections on the front side. Based on 
this fact, double-side polished (DSP) wafers are used for 
following experiment. 
D. Undercuts 
The most important two specifications of a wet-etching 
mask are selectivity and undercut rates.  For recrystallized 
parylene as silicon etching masks, only undercut rates have to 
be addressed, since the silicon wet etchants do not react with 
recrystallized parylene.  Mask pattern designers have to take 
into consideration the undercut rates of recrystallized parylene 
when they choose recrystallized parylene as the etching masks.  
Here the undercut rate is defined by the ratio of the 
undercut length divided by total etching time. The undercut 
length is the distance from the start of parylene delamination to 
the edge of the etching openings, as shown in Fig. 3.     
TABLE II. TMAH ETCHING SYSTEMS 
TMAH  
Wt% 
Temperature  
(?) 
Silicon etching rate 
(?m/hr) 
Undercut rate
 (?m/hr) 
10 70 19.995 15 
10 80 33.36 30.5 
10 90 36.19 44 
15 70 17.942 15.5 
15 80 29.81 55 
15 90 43.31 50 
20 70 15.73 13 
20 80 23.354 32 
20 90 38.204 67.05 
25 70 11.798 13 
25 80 22.053 41 
25 90 40.929 65
TABLE III. HNA ETCHING SYSTEMS 
Weight Percentages (%) 
HF HNO3 HCH3COOH 
Silicon etching rate
(?m/min) 
Undercut rate
(?m/min) 
10 60 30 2.5 2.5 
10 70 20 4.3 3.4 
10 80 10 2.65 2 
20 50 30 7.2 6.67 
20 60 20 6.5 6 
20 70 10 7.2 5.4 
30 30 40 18.9 20.3 
30 40 30 9 11.3 
30 50 20 11.7 9.5 
30 60 10 10.316 9.16 
40 20 40 30.6 51.7 
40 30 30 54.3 107 
40 40 20 35.7 33.3 
50 10 40 22.5 21.7 
50 20 30 59.4 113.3 
50 30 20 76 132.3 
50 40 10 43.76 95.8 
60 10 30 29.85 27.5 
60 20 20 48.5 56.25 
60 30 10 50.625 75 
70 10 20 36.6 32.22 
70 20 10 61.3 53.93 
TABLE I. KOH ETCHING SYSTEMS 
KOH  
Wt% 
Temperature  
(?) 
Silicon etching rate
 (?m/hr) 
Undercut rate
 (?m/hr) 
20 50 9.35 25 
20 60 17.8 55 
20 70 30 141 
30 50 10.5 25 
30 60 26.5 56.3 
30 70 49 230 
40 50 9 27.5 
40 60 25 122.5 
40 70 33 110 
Figure 3. Undercut length defintion. For this picture, the undercut legnth 
is 125m. 
Undercut Length 
883
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Table I, II, and III list silicon undercut rates of 
recrystallized parylene for KOH solutions of concentrations 
from 20% to 40% and temperatures from 50oC to 70 oC, 
TMAH solutions of concentrations from 10% to 25% and 
temperatures from 70 oC to 90 oC, and HNA etching systems of 
various composition ratios. All samples use 5m recrystallized 
parylene as the mask.  
Through comparison of undercut rates and silicon etch rates 
for TMAH and KOH, we fond that undercuts result from 
etchant attacking interfaces between parylene and silicon. 
Undercut rate should be smaller than silicon etching rate for 
TMAH and KOH since these two etching systems are 
anisotropic. Unlike hard masks materials such as thermal oxide, 
adhesion between parylene and silicon is less chemical bonding 
than mechanical adhesion. Although recrystallization process 
improves parylene adhesion to silicon, the adhesion is still 
within Van Der Waals  force range. To further improve the 
masking ability of recrystallized parylene C requires 
strengthening the adhesion between parylene and silicon. 
However, we haven’t, so far, found any other way as effective 
as recrystallization of parylene.  While undercuts are inevitable, 
designers can take undercut rates into consideration when they 
design and determine mask patterns.  
After etching, the recrystallized parylene can be removed 
by oxygen plasma. Fig. 4 shows the SEM pictures of the 
silicon sample before and after removing recrystallized 
parylene.  
IV. CONCLUSION 
We demonstrated using recrystallized parylene as the mask for 
silicon wet etching. Parylene C is recrystallized after heating 
at 350 oC for 2 hours in nitrogen ambient. The thickness 
required to provide decent protection and undercut rates for 
KOH, TMAH and HNA etching systems  are determined. 
After etching, recrystallized parylene can be removed with 
oxygen plasma. Recrystallized parylene provides a simple and 
economical alternative to conventional hard mask materials 
for silicon wet etching.     
ACKNOWLEDGMENT 
The authors thank all members from Caltech 
Micromachining Laboratory for their assistance on design, 
fabrication and testing.  
REFERENCES 
[1] K. Ruben, T. Flaim, and C. Li, “Polymer Protective Coatings for MEMS 
Wet-Etch Processes” Proc. of SPIE, vol 5342, 2004.  
[2] G. Ganavese, S. L. Marasso, M. Quaglio, M. Cocuzza, C. Ricciardi, and 
C. F. Pirri, “Polymer mask protection for alternative KOH silicon wet 
etching” J. Micromechanics and Microengineering, 17(2007) pp 1387-
1393.  
[3] Q. He, Y. Miao, T. Lee and Y. C. Tai, “An Integrated Microchip With 
Conventionally-Packed Liquid Chromotography Column and 
Electrospray Ionization Nozzle for LC-ESI-MS Applications”, Proc. Of 
MSB’05, 2005 
[4] P.J. Chen, D. Rodger, M. Humayun and Y. C. Tai, “Spiral-Tube 
Parylene Intraocular Pressure Sensor”, Proc of MEMS’05, 2005 
[5] D. C. Rodger, W. Li, H. Ameri, A. Ray, J. D. Weiland, M.S. Humayun 
and Y. C. Tai, “Flexible Parylene-Based Microelectrode Technology for 
Intraocular Retinal Prostheses”, Proc of IEEE-NEMS’06, 2006  
[6] A. Tooker,  Ph. D. Thesis, California Institute of Technology, 2007 
[7] B. Schwartz and H. Robbins, “Chemical Etching of Silicon”, J. 
Eletrochem. Soc, Vol 123, No 12, 1976 
[8] H. Seidel, L. Csepregi, A. Heuberger and H. Baumgartel, “The 
Mechanism Anisotropic Silicon Etching and Its Relevance for 
Micromachining”, Proc. of Transducer’87, 1987 
 
 
 
 
 
 
 
 
 
 
 
Figure 4.  SEM pictures before (top) and after (bottom) removing 
recrystallized parylene 
884
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null Hsi-wen Lo

null Wen-Cheng Kuo

null Yao-Joe Yang

null Yu-Chong Tai

Crossref

This paper presents the first use of recrystallized parylene as masking material for silicon chemical etch. Recrystallized parylene was obtained by melting parylene C at 350degC for 2 hours. The masking ability of recrystallized parylene was tested in HNA (hydrofluoric acid, nitric acid and acetic acid) solution of various ratios, KOH (potassium hydroxide) solution and TMAH (tetramethylammonium hydroxide) at different temperatures and concentrations. It is found that interface between parylene and the substrate can be attacked, which results in undercuts. Otherwise, recrystallized parylene exhibited good adhesion to silicon, complete protection of unexposed silicon and silicon etching rates comparable to literature data

RECRYSTALLIZED PARYLENE AS A MASK 
FOR SILICON CHEMICAL ETCHING  
Hsi-wen Lo1, Wen-Cheng Kuo1,2, Yao-Joe Yang2, and Yu-Chong Tai1  
1Department of Electrical Engineering, California Institute of Technology, USA 
2Department of Mechanical Engineering, National Taiwan University, Taiwan 
 
Abstract—This paper presents the first use of recrystallized 
parylene as masking material for silicon chemical etch. 
Recrystallized parylene was obtained by melting parylene C at 
350  for 2 hours. The masking ability of recrystallized parylene 
was tested in HNA (hydrofluoric acid, nitric acid and acetic acid) 
solution of various ratios, KOH (potassium hydroxide) solution 
and TMAH (tetramethylammonium hydroxide) at different 
temperatures and concentrations. It is found that interface 
between parylene and the substrate can be attacked, which 
results in undercuts. Otherwise, recrystallized parylene exhibited 
good adhesion to silicon, complete protection of unexposed silicon 
and silicon etching rates comparable to literature data. 
Keywords- parylene; silicon wet etching; recrystallization;  
I.  INTRODUCTION 
Silicon wet etching processes are applied in fabrications of 
traditional MEMS structures, such as silicon cantilevers, silicon 
thin diaphragms, and other freestanding structures. Although 
inductively-coupled plasma etching, or deep reactive ion 
etching (DRIE) can achieve high aspect ratio structures, silicon 
wet etching provides relatively cheap, simple and easy 
processes and remain indispensable in MEMS fields.  
The common masks used in silicon wet etchings, such as 
TMAH and KOH are silicon oxides and silicon nitrides. Silicon 
oxides can be obtained by LPCVD, PECVD or thermal 
oxidation and silicon nitrides is obtained through LPCVD and 
PECVD. These deposition processes somehow add additional 
complexity and design challenges to the overall processes.  
Thermal oxidation and LPCVD processes require high 
temperature and thus cause stress problems in the deposited 
thin film while PECVD oxide and nitrides can be etched at a 
considerable rate by silicon wet etchants. Sometimes, using 
hard masks is simply impossible since deposition of hard 
masks will inevitably damage the existing microstructures.  
To address problems aforementioned, polymeric mask 
materials have been developed [1][2]. Polymeric masks emerge 
as easy and economical alternatives to hard masks and provide 
fair protection and pattern-transferring capabilities.    
Parylene, or poly(paraxylylene), has been widely used in 
many MEMS application. Microfluidic channels [3], 
electrospray inject nozzles [3], pressure sensors [4], 
implantable electrode arrays [5], and etc. have been 
demonstrated.  
Tooker and Tai first reported using recrystallized parylene 
C to improve the adhesion between parylene C and glass slides 
[6]. Parylene C was heated at 350  in nitrogen ambient and 
was melted and recrystallized. Recrystallized parylene C 
provides much stronger adhesion to glass and can still be 
etched by oxygen plasma [6]. This paper then explores the 
capability of recrystallized parylene C as the etching masks for 
silicon etching. 
II. EXPERIMENTAL DETAILS 
A. Sample Preparation 
All samples are prepared in the same way. It starts with 
prime silicon wafers, be it double-side polished or single-side 
polished. Silicon wafers are first cleaned isopropyl alcohol and 
acetone. Next, wafers are cleaned with standard piranha 
solution to fully remove organic contamination. Prior to 
parylene deposition, wafers are dipped in 5 wt % hydrofluoric 
acid to clean the surface and to remove silicon dioxides.  
Parylene C of different thickness is then deposited on 
silicon wafers via a room-temperature chemical vapor 
deposition (CVD) process. Thickness of parylene is controlled 
by the amount of dimmer weight loaded in the machine. All 
parylene dimmers are used as purchased from Specialty 
Coating Systems. No further purification is performed.  After 
parylene deposition, wafers with parylene are heated in a 
furnace filled with nitrogen. The temporal heating profile of the 
furnace is such that it ramps up to the hold temperature, 350oC 
at a rate of 10oC per minute and maintains the hold temperature, 
350oC, for 2 hours and finally cools down to room temperature 
naturally (the ramp down rate is less than 2 oC per minute. The 
hold temperature, 350oC, and 2 hours of hold time are selected 
so as to ensure parylene C is fully melted and recrystallized. 
After parylene C is recrystallized, etching windows are 
opened with conventional photolithography process and 
reactive ion etching (RIE) oxygen plasma etching. Here, it is 
worth note that recrystallized parylene C has a slower etching 
rate than generic parylene C, typically around 0.5 times of that 
of generic parylene C.  
After etching windows and patterns are formed, 
recrystallized parylene C coated silicon wafer samples are 
dipped into various silicon etching solutions 
B. Etching Solution Preparation 
Tetramethylammonium hydroxide (TMAH) solutions are 
prepared by diluting TMAH 25 wt % solution, purchased from 
Sigma Aldrich, with deionized (DI) water. Similarly, 
*Contact author: Hsi-wen Lo, Caltech Micromachining Group, 1200 E. 
California Blvd, Pasadena, CA, 91125, USA, email: lo@mems.caltech.edu. 
881978-1-4244-1908-1/08/$25.00 ©2008 IEEE.
Proceedings of the 3rd IEEE Int. Conf. on
 Nano/Micro Engineered and Molecular Systems
January 6-9, 2008, Sanya, China
Potassium hydroxide (KOH) solutions are prepared by diluted 
KOH solution 45 wt % solutions, purchased from Sigma 
Aldrich, with DI water.  Hydrofluoric acid - nitric acid – acetic 
acid (HNA) etching systems are prepared by mixing 49% 
hydrofluoric acid, 98% nitric acid and anhydrous acetic acid at 
different ratio.  
Reflux systems are used for TMAH and KOH etching 
processes. 
C. Measurement Methods and Instrument 
The undercut length is measured under optical microscopes 
and the etching depth is measured with KLA Tencor P15 
surface profilometer.  Scanning electron microscopy (SEM) is 
also used to further identify surface properties of recrystallized 
parylene and cross-sectional structures of silicon after etching.   
III. RESULTS AND DISCUSSION 
Recrystallized parylene C has polycrystalline structures. 
Different from generic parylene C, grain boundary can be 
easily and clearly observed under optical microscopes, 
especially when using optical microscopes with polarized light 
sources. Fig. 1 shows the pictures of generic and recrystallized 
parylene under polarized green microscopes and SEM pictures 
of recrystallized parylene surfaces.  
As can be seen from Fig. 1, grain boundaries appeared after 
baking generic parylene C at 350oC for 2 hours and crystallized 
did occur in the heating processes. That is why we call 
parylene C after the heat treatment “recrystallized parylene”.  
To assess whether recrystallized parylene C is a reliable 
silicon etching mask, several issues have to be investigated. 
Among these, crucial question such as how thick parylene 
should be to provide decent protection, what the undercut rate 
is, if any, and what is etching selectivity with respect to silicon 
will be addressed in this paper.  Several experiments are 
performed to answer these questions. 
A. Selectivity 
Typical silicon wet etchants are KOH, TMAH and HNA. 
There is no known chemical reaction between parylene and 
these silicon etchants, and we can surely assume parylene will 
not be damaged during etching processes. Later experiments 
confirmed this claim. The selectivity of silicon with respect to 
parylene is, therefore, infinity. That makes recrystallized 
parylene an excellent mask for silicon wet etching.   
B. Thickness 
Thickness of parylene reduces less than 5% after 
recrystallization. Since the change is small, thicknesses 
discussed in this paper hereafter are all the thickness of 
parylene before recrystallization. 
Three samples of silicon wafers are prepared to answer this 
 
Figure 1. Parylene surfaces: Polarized green optical microscopic picture 
before recrystallization (a), and after recrystallization (b). (c) is the SEM 
pictures of recrystallized parylene. Grain diameters in (b) are around 200m.
(a) (b) 
(c) 
(a)  
(b)  
(c)  
Figure 2. Recrystallized parylene samples after 9 hours in 58 oC KOH 
(a) 1 m,  (b) 2.5 m, and (c) 5 m. Serious undercuts occur in (a). 
882
question. The thicknesses of these three samples are 1m, 
2.5m, and 5m. These samples are dipped in 20% KOH 
solutions at 58 oC for 9 hours. Fig. 2 shows the pictures of 
these three samples after etching. 
As shown in Fig. 2, 1-m recrystallized parylene renders 
serious undercuts and poor pattern transferring ability while 5-
m recrystallized parylene gives satisfactory results both for 
undercuts and pattern transferring. It follows that the thicker 
parylene is, the better etch-masking ability it provides. 
However, time and effort to pattern recrystallized parylene 
increases as the parylene thickness increase and the etching rate 
of recystallized parylene is, for the same oxygen plasma 
etching conditions, half as much as that of generic parylene. In 
determining proper parylene thickness, one has to balance 
masking ability and the time to patterning it. 5-m 
recrystallized parylene was chosen in the experiments 
determining undercut rates.  
C. Single-Side Polished  and Double-Side Polished Wafers 
In the previous experiment determining the proper 
thickness of recrystallized parylene, single-side polished (SSP) 
wafers were used. Serious delimination of recrystallized 
parylene and therefore undesired etching and damages always 
occurred on the backside even when recrystallized parylene is 
thick enough to provide protections on the front side. Based on 
this fact, double-side polished (DSP) wafers are used for 
following experiment. 
D. Undercuts 
The most important two specifications of a wet-etching 
mask are selectivity and undercut rates.  For recrystallized 
parylene as silicon etching masks, only undercut rates have to 
be addressed, since the silicon wet etchants do not react with 
recrystallized parylene.  Mask pattern designers have to take 
into consideration the undercut rates of recrystallized parylene 
when they choose recrystallized parylene as the etching masks.  
Here the undercut rate is defined by the ratio of the 
undercut length divided by total etching time. The undercut 
length is the distance from the start of parylene delamination to 
the edge of the etching openings, as shown in Fig. 3.     
TABLE II. TMAH ETCHING SYSTEMS 
TMAH  
Wt% 
Temperature  
( ) 
Silicon etching rate 
( m/hr) 
Undercut rate
 ( m/hr) 
10 70 19.995 15 
10 80 33.36 30.5 
10 90 36.19 44 
15 70 17.942 15.5 
15 80 29.81 55 
15 90 43.31 50 
20 70 15.73 13 
20 80 23.354 32 
20 90 38.204 67.05 
25 70 11.798 13 
25 80 22.053 41 
25 90 40.929 65
TABLE III. HNA ETCHING SYSTEMS 
Weight Percentages (%) 
HF HNO3 HCH3COOH 
Silicon etching rate
( m/min) 
Undercut rate
( m/min) 
10 60 30 2.5 2.5 
10 70 20 4.3 3.4 
10 80 10 2.65 2 
20 50 30 7.2 6.67 
20 60 20 6.5 6 
20 70 10 7.2 5.4 
30 30 40 18.9 20.3 
30 40 30 9 11.3 
30 50 20 11.7 9.5 
30 60 10 10.316 9.16 
40 20 40 30.6 51.7 
40 30 30 54.3 107 
40 40 20 35.7 33.3 
50 10 40 22.5 21.7 
50 20 30 59.4 113.3 
50 30 20 76 132.3 
50 40 10 43.76 95.8 
60 10 30 29.85 27.5 
60 20 20 48.5 56.25 
60 30 10 50.625 75 
70 10 20 36.6 32.22 
70 20 10 61.3 53.93 
TABLE I. KOH ETCHING SYSTEMS 
KOH  
Wt% 
Temperature  
( ) 
Silicon etching rate
 ( m/hr) 
Undercut rate
 ( m/hr) 
20 50 9.35 25 
20 60 17.8 55 
20 70 30 141 
30 50 10.5 25 
30 60 26.5 56.3 
30 70 49 230 
40 50 9 27.5 
40 60 25 122.5 
40 70 33 110 
Figure 3. Undercut length defintion. For this picture, the undercut legnth 
is 125m. 
Undercut Length 
883
Table I, II, and III list silicon undercut rates of 
recrystallized parylene for KOH solutions of concentrations 
from 20% to 40% and temperatures from 50oC to 70 oC, 
TMAH solutions of concentrations from 10% to 25% and 
temperatures from 70 oC to 90 oC, and HNA etching systems of 
various composition ratios. All samples use 5m recrystallized 
parylene as the mask.  
Through comparison of undercut rates and silicon etch rates 
for TMAH and KOH, we fond that undercuts result from 
etchant attacking interfaces between parylene and silicon. 
Undercut rate should be smaller than silicon etching rate for 
TMAH and KOH since these two etching systems are 
anisotropic. Unlike hard masks materials such as thermal oxide, 
adhesion between parylene and silicon is less chemical bonding 
than mechanical adhesion. Although recrystallization process 
improves parylene adhesion to silicon, the adhesion is still 
within Van Der Waals  force range. To further improve the 
masking ability of recrystallized parylene C requires 
strengthening the adhesion between parylene and silicon. 
However, we haven’t, so far, found any other way as effective 
as recrystallization of parylene.  While undercuts are inevitable, 
designers can take undercut rates into consideration when they 
design and determine mask patterns.  
After etching, the recrystallized parylene can be removed 
by oxygen plasma. Fig. 4 shows the SEM pictures of the 
silicon sample before and after removing recrystallized 
parylene.  
IV. CONCLUSION 
We demonstrated using recrystallized parylene as the mask for 
silicon wet etching. Parylene C is recrystallized after heating 
at 350 oC for 2 hours in nitrogen ambient. The thickness 
required to provide decent protection and undercut rates for 
KOH, TMAH and HNA etching systems  are determined. 
After etching, recrystallized parylene can be removed with 
oxygen plasma. Recrystallized parylene provides a simple and 
economical alternative to conventional hard mask materials 
for silicon wet etching.     
ACKNOWLEDGMENT 
The authors thank all members from Caltech 
Micromachining Laboratory for their assistance on design, 
fabrication and testing.  
REFERENCES 
[1] K. Ruben, T. Flaim, and C. Li, “Polymer Protective Coatings for MEMS 
Wet-Etch Processes” Proc. of SPIE, vol 5342, 2004.  
[2] G. Ganavese, S. L. Marasso, M. Quaglio, M. Cocuzza, C. Ricciardi, and 
C. F. Pirri, “Polymer mask protection for alternative KOH silicon wet 
etching” J. Micromechanics and Microengineering, 17(2007) pp 1387-
1393.  
[3] Q. He, Y. Miao, T. Lee and Y. C. Tai, “An Integrated Microchip With 
Conventionally-Packed Liquid Chromotography Column and 
Electrospray Ionization Nozzle for LC-ESI-MS Applications”, Proc. Of 
MSB’05, 2005 
[4] P.J. Chen, D. Rodger, M. Humayun and Y. C. Tai, “Spiral-Tube 
Parylene Intraocular Pressure Sensor”, Proc of MEMS’05, 2005 
[5] D. C. Rodger, W. Li, H. Ameri, A. Ray, J. D. Weiland, M.S. Humayun 
and Y. C. Tai, “Flexible Parylene-Based Microelectrode Technology for 
Intraocular Retinal Prostheses”, Proc of IEEE-NEMS’06, 2006  
[6] A. Tooker,  Ph. D. Thesis, California Institute of Technology, 2007 
[7] B. Schwartz and H. Robbins, “Chemical Etching of Silicon”, J. 
Eletrochem. Soc, Vol 123, No 12, 1976 
[8] H. Seidel, L. Csepregi, A. Heuberger and H. Baumgartel, “The 
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Micromachining”, Proc. of Transducer’87, 1987 
 
 
 
 
 
 
 
 
 
 
 
Figure 4.  SEM pictures before (top) and after (bottom) removing 
recrystallized parylene 
884


An Integrated Microchip With Conventionally-Packed Liquid Chromotography Column and Electrospray Ionization Nozzle for LC-ESI-MS Applications”,

Chemical Etching of Silicon”,

Flexible Parylene-Based Microelectrode Technology for Intraocular Retinal Prostheses”, Proc of IEEE-NEMS’06,

Polymer mask protection for alternative KOH silicon wet etching”

Spiral-Tube Parylene Intraocular Pressure Sensor”, Proc of MEMS’05,

The Mechanism Anisotropic Silicon Etching and Its Relevance for Micromachining”,

https://authors.library.caltech.edu/19148/1/Lo2008p86092008_3Rd_Ieee_International_Conference_On_NanoMicro_Engineered_And_Molecular_Systems_Vols_1-3.pdf

Recrystallized parylene as a mask for silicon chemical etching

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