होम Micromachines SF6 Optimized O2 Plasma Etching of Parylene C

SF6 Optimized O2 Plasma Etching of Parylene C

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9
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english
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Micromachines
DOI:
10.3390/mi9040162
Date:
April, 2018
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micromachines
Article

SF6 Optimized O2 Plasma Etching of Parylene C
Lingqian Zhang 1 , Yaoping Liu 1 , Zhihong Li 1,2 and Wei Wang 1,2, *
1
2

*

Institute of Microelectronics, Peking University, Beijing 100871, China; zlqpku@gmail.com (L.Z.);
yaopingliu@gmail.com (Y.L.); zhhli@pku.edu.cn (Z.L.)
National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Beijing 100871, China
Correspondence: w.wang@pku.edu.cn; Tel.: +86-10-6276-9183

Received: 3 February 2018; Accepted: 27 March 2018; Published: 2 April 2018




Abstract: Parylene C is a widely used polymer material in microfabrication because of its excellent
properties such as chemical inertness, biocompatibility and flexibility. It has been commonly adopted
as a structural material for a variety of microfluidics and bio-MEMS (micro-electro-mechanical system)
applications. However, it is still difficult to achieve a controllable Parylene C pattern, especially on
film thicker than a couple of micrometers. Here, we proposed an SF6 optimized O2 plasma etching
(SOOE) of Parylene C, with titanium as the etching mask. Without the SF6 , noticeable nanoforest
residuals were found on the O2 plasma etched Parylene C film, which was supposed to arise from
the micro-masking effect of the sputtered titanium metal mask. By introducing a 5-sccm SF6 flow, the
residuals were effectively removed during the O2 plasma etching. This optimized etching strategy
achieved a 10 µm-thick Parylene C etching with the feature size down to 2 µm. The advanced
SOOE recipes will further facilitate the controllable fabrication of Parylene C microstructures for
broader applications.
Keywords: Parylene C; SF6 ; O2 plasma etching

1. Introduction
Parylene C, or poly(monochloro-p-xylylene), is one of the most-used polymer materials in MEMS
(micro-electro-mechanical system) for its compatibility with microfabrication techniques and excellent
material properties. It is a chemically stable, USP (United States Pharmacopeia) Class VI biocompatible,
and flexible ma; terial that has been widely implemented in microfluidics and bioMEMS applications
such as microvalves [1], accelerometers [2], flexible electrodes [3,4], and neural probes [5–7]. To pattern
the Parylene C, different fabrication techniques have been proposed, such as wet etching using
chloronapthelene or benzoyl benzoate [8], dry etching based on O2 plasma [9–15], thermal imprinting
or micro-molding [16,17] and laser micromachining [18–20]. Among the existing fabrication strategies,
the dry etching technique is a relatively clean and effective method that is suitable for batch fabrication
of Parylene C microstructures. Therefore, the O2 plasma removal of Parylene C has been investigated
and widely used [9–15,21–25] (Figure 1). These studies presented the effects of process parameters
such as mask material, temperature, gas flow rate and power on the etching performance. However,
fabrication of controllable Parylene C patterns with small feature sizes for a thick film, i.e., with high
aspect ratios, still calls for optimized etching approaches. Various methods such as switching chemistry
plasma etching by deep reactive ion etching (DRIE) [26], O2 plasma etching with aluminum or nickel
hard mask [27,28] and O2 plasma etching with thick negative photoresist mask [28] have been reported.
Although these methods have achieved Parylene C microstructures for the specific devices, problems
such as low geometrical resolution limited by the thick photoresist mask and Parylene C residuals
during the metal masked etching are still not fully solved.
In this work, we attempted to address these problems by developing an SF6 optimized O2 plasma
etching (SOOE) strategy using titanium as the etching mask. The titanium hard mask can achieve
Micromachines 2018, 9, 162; doi:10.3390/mi9040162

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higher pattern accuracy and etching selectivity than the commonly used photoresist for deep Parylene
C etching, and the added SF6 can remove the residuals caused by the sputtering effects of titanium
mask during the O2 plasma etching. Compared with other reported etching methods, this SOOE
strategy has the merits of controllability for high aspect ratio Parylene C patterning without residuals.

Figure 1. Principle of the O2 plasma etching of Parylene C, showing the schematic of a reactive ion
etching system and representative chemical processes of Parylene C removal. During the Parylene C
etching, the polymer chain scission occurred with carbon dioxide as the etch product, then aromatic
ring was broken to form either aldehyde groups or carboxylic groups on the resultant chain [11,21,22].

2. Materials and Methods
2.1. Parylene C Preparation
A 4-inch Si wafer (100, p-type) was prepared as the deposition substrate. The Parylene C
used in this study was chemical vapor deposited with a commercial coating system (PDS 2010,
Specialty Coating Systems Inc., Indianapolis, IN, USA). The deposition process consisted of three
main steps. First, the powder-like Parylene C dimer was vaporized at approximately 175 ◦ C under
vacuum. The evaporated dimer was then pyrolyzed to radical Parylene C monomers at 690 ◦ C. Finally,
the monomer vapor entered the room temperature deposition chamber, where it polymerized onto
all the exposed surfaces. The deposition pressure was 21 mTorr. 10 µm-thick Parylene C film was
prepared with the loaded 16 g dimer.
2.2. Mask for Etching
Figure 2a,b schematically showed the fabrication process of the thick photoresist mask and
titanium hard mask on Parylene C. AZ9260 (AZ Electronic Materials, Branchburg, NJ, USA) was used as
the thick photoresist etching mask. After priming the Parylene C substrate with hexamethyldisilazane
(HMDS), 12 µm of AZ9260 was spin coated by SSE Spin Coater with 600 rpm for 4 s followed by
1500 rpm for 60 s. After baking at 98 ◦ C for 15 min, exposure for 200 s, and development for 1 min,
the patterned photoresist was prepared on the Parylene C film for the following O2 plasma etching.
Titanium was used as the metal hard mask for its small surface stress, low cost and good adhesion with
Parylene C. 3000 Å titanium was sputtered on the Parylene C film by Research s-Gun II (Sputtered Films,
Santa Barbara, CA, USA). The titanium pattern was then generated by a regular photolithography using
a positive photoresist (RZJ-304, Ruihong Electronic Chemical Company, Suzhou, China), following
with a wet etching step by 10% HF for 40 s and a photoresist removal step by acetone.

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As shown in Figure 2c, two kinds of the mask patterns were used in this work. The mask pattern 1,
including rectangular lines with line width and spacing ranging from 4 to 100 µm, was applied to the
pure O2 plasma etching. The mask pattern 2, including hexagonal arrays with side length and spacing
ranging from 4 to 100 µm, was used for the SF6 optimized O2 plasma etching. For the titanium mask,
due to the undercut of 0.6–0.9 µm caused by the wet etching process, the corresponding feature sizes
of the patterns changed by approximately 1 µm after the fabrication, as listed in Figure 2c. The etching
loading (exposed area of the wafer relative to the total wafer area) was approximately 31% for the
4-inch wafer.

Figure 2. The fabrication process of the etching masks. (a) Fabrication of 12 µm AZ4620 photoresist
mask; (b) Fabrication of 3000 Å titanium mask; (c) Schematic of the mask patterns and the corresponding
dimensions. Mask pattern 1 was used for the etching by pure O2 plasma and mask pattern 2 was used
for the SF6 optimized O2 plasma etching.

2.3. Dry Etching Conditions
Parylene C etching was performed by a reactive ion etching (RIE) system (ME-6A, Chinese
Academy of Sciences, Beiijng, China) in this study. Test samples were etched under varied process
conditions including etching power ranging from 150 W to 350 W, different O2 gas flow rates (50–65 sccm)
and SF6 gas flow rates (0–8 sccm). Etches were composed of repeated etching cycles, and the etching
time of each cycle was set as 5 min to avoid the thermal effects and nonuniformity caused by the
long-term etching. This was because sustained ion bombardment at longer etching time would result
in a higher temperature on the wafer surface, which may lead to stress problems for the mask, loss of
etching anisotropy and more etching nonuniformity.
2.4. Etching Performance Measurements
To characterize the etching performance, etching depth and film thicknesses were measured using
a profilometer (AS-500, KLA-Tencor, Milpitas, CA, USA) and a thin film-thickness measurement
system (ST-2000, K-MAC, Daejeon, Korea). As shown in Figure 3, etching parameters such as
Parylene C etching rate, mask etching rate, etching selectivity and uniformity were extracted from
the measurements. Measurements were made over 4-inch silicon wafers at five fixed points across
the sample for statistical analysis. Morphologies of the etched Parylene C were observed using
a high-resolution scanning electron microscope (FEI Quanta 200 FEG, FEI Company, Hillsboro, OR, USA).
To determine the elements on the etched surface and residuals, analysis on SEM-EDX experiments
were performed.

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Figure 3. Schematic of the etching measurements.

3. Results and Discussions
3.1. Reactive Ion Etching of Parylene C by Pure O2 Plasma
Firstly, the reactive ion etching of Parylene C by pure O2 plasma was performed for optimizing the
etching parameters of the etching system. The etching measurements including etching rate, selectivity,
and uniformity were investigated under different etching parameters such as mask, gas flow and power.
As summarized in Table 1, it was clear that the Parylene C etching rate increased from 218.4 nm/min
to 435.4 nm/min with the increment of the plasma power (from 150 W to 350 W) and oxygen flow
rate (from 50 sccm to 65 sccm). Using titanium as the etching mask, we obtained 56.4 nm/min
and 74.9 nm/min slower Parylene C etching rates compared with the photoresist mask under the
conditions of 350 W, 60 sccm and 250 W, 60 sccm. This may be attributed to the micro-masking
effect of the residuals during the etching. Neither the etching selectivity nor the uniformity showed
a clear correlation with the flow rate and etching power. The photoresist was etched off at a similar
rate to Parylene C, while the titanium maintained a high etching selectivity over 100 under pure O2
plasma etching. All of the etching recipes achieved a good etching uniformity ranging from 1% to 3.7%
(less than 5%).
Table 1. Summary of the etching recipes and measurements for reactive ion etching of Parylene C by
pure O2 plasma.
Parameters
Mask
Type

Measurements

O2 Flow
(sccm)

Power
(W)

Parylene C Etching
Rate (nm/min)

Mask Etching
Rate (nm/min)

Selectivity

Uniformity

Photoresist

60
60
60
50
55
65

150
250
350
350
350
350

218.4 ± 5.9 1
334.2 ± 7.1
414.7 ± 4.7
359.9 ± 5.5
381.0 ± 12.5
435.4 ± 4.2

199.4 ± 17.1
321.1 ± 34.9
426.9 ± 24.8
366.7 ± 61.8
392.1 ± 14.9
445.2 ± 14.2

1.10 ± 0.09
1.05 ± 0.11
0.97 ± 0.05
1.00 ± 0.17
0.97 ± 0.05
0.98 ± 0.04

3.7%
2.6%
1.4%
2.1%
1.7%
1.0%

Titanium

60
60

350
250

358.3 ± 3.6
259.3 ± 5.9

<20
<20

>100
>100

1.0%
2.8%

1

Data represent mean ± standard deviation (S.D.), n = 5, the same as below.

Then, continuous etching of the 10 µm-thick Parylene C films were performed. It was found that
although increasing the power and gas flow rate could achieve a fast etching rate, it can also generate
heavier thermal loads and even result in a crumpled mask. Temperature tests were performed by the
surface temperature indicating strips (THERMAX, TMC Hallcrest, Connah’s Quay, UK) as a reference
for the highest temperature during the etching. The results showed that the surface temperature
was up to 149 ◦ C under continuous etching of 350 W power, 60 sccm O2 flow, while for the 250 W
power, 60 sccm O2 group, the highest surface temperature was 121 ◦ C. Within the range of the above

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parameters, 250 W power, 60 sccm O2 flow showed the best etching performances during continuous
etching of 6 cycles. The etching rates of Parylene C during continuous 250 W, 60 sccm O2 plasma
etching were shown in Figure 4.

Figure 4. Etching rates of Parylene C during continuous pure O2 plasma etching using photoresist
and titanium masks (n = 5, error bars represent SD). For the photoresist mask, Parylene C films were
etched to the silicon substrate after 6 cycles; for the titanium mask, nanoforest structures appeared on
the etched Parylene C surface after 3 cycles.

Under the 12 µm-thick AZ9260 photoresist mask, 10 µm Parylene C films were successfully etched
off till the silicon substrate was exposed, as shown in Figure 5. Limited by the etching selectivity,
the photoresist mask had to be thick enough, which increased the process difficulty and reduced the
pattern resolution. It was not easy to achieve a steep sidewall for the thick photoresist (thickness larger
than 10 µm), which would lead to an unavoidable mask width loss and the dimension variation
between photoresist mask and Parylene C. The calculated total mask width loss was approximately
7 µm for the Parylene C etching.

Figure 5. Surface morphology of the patterned photoresist and the Parylene C after O2 plasma etching.
(a) Patterned 12 µm-thick AZ9260 photoresist on the Parylene C film; (b) Etched Parylene C structure
after photoresist removal, with inset showing the sidewall, scale bar = 10 µm.

For the titanium mask, after three cycles of etching, the nanoforest structures appeared on the
etched surface of Parylene C and prevented further etching. Even after several subsequent cycles of O2
plasma etching, the nanoforest still existed. The surface morphology of the etched Parylene C structure
is shown in Figure 6. Tests showed that the probe of the profilometer could scratch the nanoforest off
the etched surface. The nanoforest structures were inferred as residuals during etching, which was also
reported in Parylene C etching with aluminum or nickel metal masks [27,28], the photoresist etching
with different substrates [29,30] and the polymeric etching with different reactor wall materials [31–33].
To further investigate the origins of the observed Parylene C nanoforests, we performed SEM-EDX
experiments to characterize the element distribution on the patterned titanium area, top and bottom
area of Parylene C etching surface, and silicon substrate after peeling the Parylene C film off. As shown
in Figure 7, the element analysis showed an unexpected titanium on the etched Parylene C area, which

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would not appear unless the titanium mask was sputtered by the O2 plasma. It can be inferred that
the nanoforest was attributed to the micro-masking effect caused by the titanium mask sputtering.
This inference was also supported by the reported formation process of nanotexturing by plasma
etching, that the sputtering of reactor wall material would create micro-masking for different kinds
of nanostructures [31–33]. The existence of sputtered wall metal material on the etched surface was
verified by the X-ray photoelectron spectroscopy (XPS) analysis in Ref. [33], showing the cause of the
micro-masking formation which led to the development of nanotexture. Similarly, in this work, the
titanium mask was supposed to be sputtered under the O2 plasma bombardment, and then deposited
on the Parylene C etched surface as the micro-mask for the nanoforest formation. After the formation
of nanoforest structures on the etched Parylene C area, the etching rate dropped dramatically due to
its morphology. As a result, the nanostructures on the Parylene C finally prevented the plasma etching
from proceeding normally.

Figure 6. Surface morphology of Parylene C after pure O2 plasma etching using titanium mask.
(a) Structure with Parylene C nanoforest after three cycles of 5 min etching; (b) Nanoforest scratched
by the probe,:the Parylene C was etched after six cycles of 5 min pure O2 plasma etching.

Figure 7. SEM-EDX results of the Parylene C etching structure on different focused area. (a) Patterned
titanium area; (b) Top of the Parylene C forest; (c) Bottom of the Parylene C forest; (d) Silicon substrate
after peeling the Parylene C off.

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3.2. SF6 Optimized O2 Plasma Etching (SOOE)
To prevent the formation of the nanoforest structures, we added a small flow rate of fluorine-based
gas, SF6 , into the O2 plasma etching of Parylene C in this work. The SF6 could be dissociated and
excited to fluorine free radicals during the process, which turned the metallic compound into metal
fluoride. Therefore, during the O2 plasma etching, the SF6 could simultaneously remove the titanium
or titanium oxide micro-masks on the etched surface and keep the etching proceeding. Because the
fluorine free radicals also react with the metal mask, the SF6 gas flow should be controlled within
a proper range to keep a high selectivity. We performed the measurements for SF6 optimized O2
plasma etching (SOOE) under 250 W, 60 sccm O2 with SF6 flow rate from 5 to 8 sccm, as summarized
in Table 2. The etching rates of Parylene C were basically the same as approximately 350 nm/min with
uniformity of less than 5% for the SOOE groups, while the etching selectivity showed a significant
reduction from 40.4 to 27.1 when increasing the SF6 flow from 5 sccm to 8 sccm. Compared with the
titanium masked etching without the SF6 , the etching rates of the SOOE groups showed an obvious
increment, implying micro-mask removal by SF6 . Continuous etching was also performed under
250 W, 60 sccm O2 with SF6 flow rate from 5 to 8 sccm, as shown in Figure 8. The reduced selectivity
under 8 sccm SF6 resulted in the dissipation of metal mask after continuous etching.
Table 2. Summary of the etching recipes and measurements for SF6 optimized O2 plasma etching (SOOE).
Parameters
Mask
Type
Titanium

Measurements

O2 Flow
(sccm)

SF6 Flow
(sccm)

Power
(W)

Parylene C Etching
Rate (nm/min)

Mask Etching
Rate (nm/min)

Selectivity

Uniformity

60
60
60

0
5
8

250
250
250

259.3 ± 5.9 1
352.4 ± 11.4
349.6 ± 3.2

<2
8.7 ± 0.5
13.0 ± 0.7

>100
40.4 ± 1.1
27.1 ± 0.8

2.8%
3.9%
1.1%

1

Data represent mean ± standard deviation (S.D.), n = 5, the same as below.

Figure 8. Measurements of the etching by the SOOE process with titanium as the etching mask (n = 5,
error bars represent SD). (a) Etching rates of Parylene C during continuous SOOE; (b) Etching selectivity
of Parylene C to mask using 250 W, 60 sccm O2 plasma and SF6 .

With SOOE etching parameters of 250 W, 60 sccm O2 and 5 sccm SF6 , 10 µm Parylene C films
were successfully etched off till the silicon substrate was exposed. Both the Parylene C hexagon line
patterns with width of 2 µm (corresponding photomask line width of 4 µm) and hexagon pore patterns
with side length of 5 µm (corresponding photomask side length of 4 µm) were controllably etched
with no obvious residuals, as shown in Figure 9. The etching also showed a relatively steep sidewall of
85◦ with Parylene C lateral etching rate of 27 nm/min.

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Figure 9. SEM of the etched Parylene C structures with the SOOE process. (a,b) Structures with 2 µm
hexagon line patterns; (a) top view; (b) oblique view after tilting the structures with 45◦ ; (c,d) Structures
with 5 µm hexagon pore patterns; (c) top view; (d) oblique view after tilting the structures with 45◦ ,
with inset showing the zoomed bottom area, scale bar = 1 µm.

The comparison of Parylene C etching performance using different metal masks is briefly shown
by Table 3. It is clear from the table that, without the SF6 , the etching using aluminum, nickel or
titanium masks faced the same situation: noticeable residuals were created in the openings. By
introducing a 5-sccm SF6 flow in this work, the residuals were effectively removed during the O2
plasma etching, and the 10 µm-thick Parylene C etching with feature size down to 2 µm was achieved.
It was also worth mentioning that the titanium, which was commonly used as a metal adhesion layer
on the Parylene C in device fabrication, could achieve better adhesion than the aluminum or nickel
masks and could be easily wet etched by 10% HF in room temperature, which would facilitate the
mask patterning with smaller feature sizes on the Parylene C.
Table 3. The comparison of Parylene C etching performance using different metal masks.
Mask Type

Etching Method

Minimum
Feature Size

Parylene C
Thickness

Aspect
Ratio

Residuals

Aluminum
(Ref. [27])

O2 plasma, ICP
(inductively
coupled plasma)

6 µm

10–55 µm

9:1

Unavoidable residuals on the substrate
(>1 µm in width & height)

Nickel
(Ref. [28])

O2 plasma,
ICP-RIE

50 µm

23 µm

1:2

Residuals found in the opening & prevented the
carved Parylene pieces from proper peeling

Titanium
(this work)

O2 plasma, RIE

2 µm

4 µm
(not etched to
substrate)

2:1

Nanoforest residuals appeared on the surface &
prevented the etching from proceeding normally

SF6 added (SOOE)

2 µm

10 µm

5:1

Residual free etching was achieved

4. Conclusions
In summary, this work developed an SF6 optimized O2 plasma etching (SOOE) of Parylene C.
This method overcame the challenges existing in the pure O2 plasma etching of thick Parylene C
film, i.e., low geometrical resolution when using photoresist as the etching mask and the nanoforest
residuals when using metal as the etching mask. The SF6 effectively removed the nanoforest residuals

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caused by sputtered metal micromask, without an increase of the fabrication complexity. The results
showed an excellent 10 µm Paryene C etching under the recipe of 250 W, 60 sccm O2 and 5 sccm SF6 ,
with line width down to 2 µm. The developed SOOE process will further facilitate the controllable
fabrication of Parylene C microstructures for a variety of applications in microfluidic, bio-sensing or
implantable devices.
Acknowledgments: This work was financially supported by the Advanced Research Program of the Ministry of
Education (Grant No. 6141A02033604) and the Beijing Natural Science Foundation (Grant No. 4172028 and L172005).
Author Contributions: Wei Wang and Zhihong Li conceived and supervised the experiments; Lingqian Zhang
and Yaoping Liu performed the experiments and analyzed the data; Lingqian Zhang wrote the paper; Wei Wang
revised the paper with comments from all authors.
Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.

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