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Applied Surface Science 253 (2006) 1506–1511

The effect of fluorine-based plasma treatment on morphology and chemical

surface composition of biocompatible silicone elastomer

Dariusz Szmigiel a,*, Krzysztof Domanski a, Piotr Prokaryn a,Piotr Grabiec a, Janusz W. Sobczak b

a Institute of Electron Technology, Al. Lotnikow 32/46, 02-668 Warsaw, Polandb Institute of Physical Chemistry of Polish Academy of Science, 44/52 Kasprzaka, 01-224 Warsaw, Poland

Received 29 October 2005; received in revised form 9 January 2006; accepted 15 February 2006

Available online 31 March 2006

Abstract

X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) have been used to investigate the effect of reactive ion

etching (RIE) on poly(methylhydrogensiloxane-co-dimethylsiloxane) surface in fluorine-based plasmas. Polysiloxane layers supported on the

standard silicon wafers were etched using SF6 + O2 or CF4 + O2 plasmas. SEM studies show that the polysiloxane morphology depends on plasma

chemical composition strongly. Presence of a columnar layer likely covered with a fluorine rich compound was found on the elastomer surface after

the CF4 + O2 plasma exposure. After the SF6 + O2 or CF4 + O2 plasma treatment the polysiloxane surface enriches with fluorine or with fluorine

and aluminum, respectively. Different morphologies and surface chemical compositions of the silicone elastomer etched in both plasmas indicate

different etching mechanisms.

# 2006 Elsevier B.V. All rights reserved.

PACS: 52.77.Bn; 68.47.Mn; 79.60

Keywords: Biocompatible polysiloxane; Silicone elastomer; Dry etching; Fluorine-based plasma; XPS

1. Introduction

Polydimethylsiloxane elastomer (PDMS) has been used in

wide band of applications for example as fluid or adhesive in

automotive or electronic industries. Some of the siloxane

elastomers are also used as components of medical implants

due to their biocompatibility—they are stable and inactive

materials in a living body. Proper structures on the polymer

surface may be formed using wet etching or plasma etching

processes. The wet process is isotropic and the etch rate

decreases systematically over time [1].

Carbon–hydrogen polymers can be effectively etched in an

oxygen plasma. Then the chemical reaction is a simple

oxidation of the hydrocarbon: CxHyðsolidÞ þ O2ðgasÞ �!plasma

COðgasÞ þ H2OðgasÞ. Polysiloxane elastomers require slightly

different etch chemistry due to its Si–O backbone. Since

siloxane chain does not turn into volatile products in an oxygen

* Corresponding author. Tel.: +48 22 7165992x28; fax: +48 22 7165991.

E-mail address: darek@ch.pw.edu.pl (D. Szmigiel).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.02.027

plasma, such plasma can only be used as a kind of surface

activator. Earlier studies on dry etching of PDMS showed that

an increase in elastomer wettability after an oxygen plasma

exposure is only temporary—the surface restores its common

hydrophobic properties in time [2]. Moreover, the prolonged

oxygen plasma treatment converts PDMS into a cracked silica-

like material [3]. A systematic removal of the siloxane polymer

takes place when the fluorine-based plasma is used. Such

plasma is commonly applied in e.g. silicon oxide or silicon

nitride etching. Garra et al. [1] investigated the effect of the

CF4:O2 ratio on the etch rate of PDMS. A 3:1 ratio appeared to

be the most effective in their studies. The etch rate of the

elastomer at a pressure of 50 mTorr was ca. 20 mm/h

(�0.33 mm/min) and the process was found to be anisotropic.

Our recent results [4] indicate that the 3:1 ratio is also effective

for SF6:O2 mixture, when poly(methylhydrogensiloxane-co-

dimethylsiloxane)-based elastomer is treated. In this work the

effect of the CF4 + O2 and SF6 + O2 plasmas on morphology

and surface chemical composition of poly(methylhydrogensi-

loxane-co-dimethylsiloxane)-based elastomer has been inves-

tigated. Changes in polymer morphology and surface

D. Szmigiel et al. / Applied Surface Science 253 (2006) 1506–1511 1507

Fig. 1. SEM graph showing the elastomer surface after the CF4 + O2 plasma

exposure: (A) etched area; (B) etched and rinsed in de-ionized water area; (C)

aluminum mask area.

Fig. 2. The elastomer surface etched in the CF4 + O2 plasma and rinsed in de-

ionized water—zoomed view.

contamination with plasma constituents may be essential,

especially when the plasma treated polymers are considered to

be used in medical application.

2. Experimental

2.1. Sample preparation

Samples were prepared using Nusil Technology MED-6215

two-part silicone elastomer based on dimethylsiloxane–

methylhydrogen siloxane copolymer (�90%). Two liquid

components of MED-6215 were mixed in a 10:1 ratio and

subsequently the mixture was de-aired in a desiccator. The

polymer layers ca. 20 mm thick were obtained by spin coating

of the liquid polymer at 9000 rpm for 30 s onto the standard 400

silicon wafers and then dried at 100 8C. Thicker polysiloxane

layers (40–140 mm) were prepared by repeating the above

procedure a proper number of times. The elastomer surface was

sputter-deposited with an aluminum layer of �1 mm thick. The

mask pattern was transferred using conventional photolito-

graphy and subsequent wet etching.

2.2. Polysiloxane etching and surface characterization

studies

Dry etching was carried out in an Oxford Plasma Technology

mP80 RIE reactor. The plasma etcher was fed with CF4:O2 = 3:1

or with SF6:O2 = 3:1 gas mixtures; the total flow rate was held

constant at 100 sccm. The etching processes were performed at

the pressure of 50 mTorr, at a constant source power of 270 W.

Specimens were etched for 30 or 60 min typically.

Detailed inspections of the etched surfaces including etch

progress and morphology were performed using a Leo 1530

Gemini scanning electron microscope. Prior to SEM examina-

tion a thin gold or chromium layer (�100 A) was sputter-

deposited onto the polymer in order to decrease surface charging.

The X-ray photoelectron spectroscopy measurements were

performed in an ESCALAB-210 spectrometer with Mg Ka

(1253.6 eV) irradiation source. The pressure in the spectrometer

chamber was ca. 5 � 10�9 mbar. The measurements were

preceded by cleaning sample surfaces with acetone in order to

remove the weakly bonded post-etching remnants. The curve

fitting was performed using the AVANTAGE software provided

by Thermo Electron, which describes each component of the

complex envelope as a Gaussian–Lorentzian sum function; a

constant 0.3 G/L ratio was used. The background was fitted using

a non-linear model function proportional to the integral of

elastically scattered electrons (Shirley background). The spectra

were shifted to the binding energy of C 1s peak at 284.38 eV. This

binding energy (BE) value was taken from Briggs for C–Si [5].

3. Results

3.1. Surface morphology and etch rate

The morphology of polysiloxane underwent the specific

changes after both plasma treatments. Fig. 1 shows, as an

example, the SEM picture of the polysiloxane surface etched in

the CF4 + O2 plasma. Three different parts of the surface are

indicated: (A) etched, (B) etched and rinsed in de-ionized (DI)

water, and (C) covered with aluminum mask. It can be noticed

that only very little material was removed by the plasma

treatment and a specific columnar structure was formed on the

surface. The columnar layer is not observed in the region that

was immersed in water (B area). The unveiled surface is very

rough (see also Fig. 2) in comparison to the originally smooth

polysiloxane surface. A closer SEM inspection (see Fig. 3(a)

and (b)) reveals that the columns are significantly wider at the

top than at the bottom. This fragile connection between the

columns and a compact polymer surface destroys easily while

immersing in water. The columnar layer is not observed for the

polysiloxane etched in the SF6 + O2 plasma. However, as it can

be seen in Fig. 4(a) and (b) the sample morphology in this case

D. Szmigiel et al. / Applied Surface Science 253 (2006) 1506–15111508

Fig. 3. Zoomed SEM side view of the (a) columnar layer, (b) upper part of the

columnar layer observed after the CF4 + O2 plasma treatment.

Fig. 4. The elastomer surface: (a) non-etched (after aluminum mask

removal)—(A) area, etched in the SF6 + O2 plasma—(B) area; (b) zoomed

view of the etched surface.

is different in comparison to that after the CF4 + O2 plasma

treatment and flushing (see Fig. 2).

The etch rate is usually calculated by dividing the etched

depth by the etching time. The etch rates calculated for samples

non-rinsed and rinsed in DI water are compared in Fig. 5. A big

difference obtained for both as etched samples is due to the

presence of a columnar structure on the surface etched in the

CF4 + O2 plasma. However, if the material decrement arising

from flushing the surface is considered, the etch rate in the

CF4 + O2 plasma becomes only slightly lower than that in the

SF6 + O2 plasma.

3.2. Chemical composition of the surface

The XPS analysis indicates the compositional changes in

poly(methylhydrogensiloxane-co-dimethylsiloxane) surface

after the CF4 + O2 and SF6 + O2 plasma etching. The fresh

material (MED) contains silicon, oxygen and carbon. The

analysis performed for etched samples reveals the presence of

additional elements—fluorine, nitrogen and aluminum in the

specimen etched in the CF4 + O2 plasma (MED-CF4) and

fluorine in the specimen etched in the SF6 + O2 plasma (MED-

SF6). The high-resolution spectra obtained in the range of Si

2p, C 1s, O 1s, F 1s and Al 2p are described below to clarify the

chemical changes on the surface.

D. Szmigiel et al. / Applied Surface Science 253 (2006) 1506–1511 1509

Fig. 6. Silicon 2p photoemission spectra of the polysiloxane samples: (a) fresh,

(b) etched in the CF4 + O2 plasma, and (c) etched in the SF6 + O2 plasma. Si 2p

peak is due to photoelectrons from C–Si–O; Si 2p A peak arises from SiO2 or SiOx.

Fig. 5. Rates of the polysiloxane etching in the CF4:O2 and SF6:O2 plasmas at

50 mTorr pressure and 270 W power; * denotes the etch rate estimated after the

additional procedure of flushing in de-ionized water.

Fig. 6 shows the spectral regions corresponding to Si 2p

taken from MED surface: (a) fresh, (b) exposed for 30 min to

the CF4 + O2 plasma, (c) exposed for 30 min to the SF6 + O2

plasma. The spectra obtained for all the specimens are similar.

The results of the deconvolution procedure indicate the

presence of two different silicon chemical states at ca. 102

and 103.3 eV. The former energy value relates to Si bonded in

the polymer chain (C–Si–O), the latter to a composition of SiO2

(or SiOx). Comparison of the areas under the fitted curves leads

to the conclusion that the ratio of C–Si–O:SiO2, which is equal

to 4:1 in the fresh sample decreases into 2.5:1 after the

CF4 + O2 plasma treatment and slightly increases into 5:1 after

etching in the SF6 + O2 plasma.

The results of XPS analysis in the range of C 1s are

presented in Fig. 7. A single peak of C 1s detected for all the

samples indicates carbon bonded in CH3 groups. However, a

visible peak asymmetry, which is the most pronounced in case

of MED-CF4 specimen, could arise from the traces of oxidized

carbon atoms; deconvoluted curves in the region of 286–289 eV

Fig. 7. Carbon 1s photoemission spectra of the polysiloxane samples: (a) fresh,

(b) etched in the CF4 + O2 plasma, and (c) etched in the SF6 + O2 plasma. C 1s

peak arises from C–Si–O; C 1s A peak might be ascribed to C–OH; C 1s B and C 1s

C peaks might be due to photoelectrons from C O–O* and C O, respectively.

D. Szmigiel et al. / Applied Surface Science 253 (2006) 1506–15111510

Table 1

Elemental composition of the elastomer samples: fresh (MED), etched in

CF4:O2 = 3:1 plasma (MED-CF4), and in SF6:O2 = 3:1 plasma (MED-SF6)

as determined by XPS method

Sample Si 2p O 1s C 1s F 1s Al 2p N 1s

MED 27.2 27.1 45.6 – – –

MED-CF4 22.5 24.4 43.8 6.2 2.7 0.4

MED-SF6 30.0 26.0 42.1 1.9 – –

The values are expressed in atomic percent (at.%).

correspond to C bonded in the C–OH, C O and O C–O*

species.

The O 1s binding energy has been found to be the same for

fresh and etched materials. The analysis shows the Si–O–Si

backbone as a predominant form of oxygen. Traces of O C–O*

groups are also evident in all examined samples.

The surface of etched specimens enriches with fluorine.

Most likely a single F 1s peak at ca. 687.5 eVappears due to the

fluorination of methyl groups and might be assigned to C–F2 or

C–F3. In case of MED-CF4 sample the contribution of Al–F3 to

F 1s peak cannot be excluded because of the presence of Al 2p

peak at 77.8 eV.

The results of a quantitative chemical analysis are compared

in Table 1. The initial composition of the elastomer

Si:O:C = 1.0:1.0:1.7 changes into 1:1.1:2.0 and 1.0:0.9:1.4

for the specimens etched in the CF4 + O2 and SF6 + O2

plasmas, respectively. The silicon surface content of 27.2 at.%

in the fresh sample decreases to 22.5 at.% after the CF4 + O2

plasma treatment and increases to the value of 30.0 at.% in the

sample exposed to the SF6 + O2 plasma. The investigated

specimens do not differ considerably in the oxygen surface

content. Comparing to the fresh material the amount of oxygen

is a bit smaller in both etched samples. The carbon surface

content in MED sample changes slightly from the value of 45.6

to 43.8 at.% and to 42.1 at.% after etching in the CF4 + O2 and

SF6 + O2 plasmas, respectively.

The fluorine contribution to the surface chemical composi-

tion is about three times higher for MED-CF4 than for MED-

SF6 specimen. The XPS studies indicate the presence of traces

(<0.5 at.%) of nitrogen on the MED-CF4 surface. Additionally,

about 3 at.% of aluminum was detected on the surface of this

sample. The surface of the specimen etched in sulphur

hexafluoride plasma is aluminum free. While the presence of

Si, O, C, F and Al on the plasma treated surfaces is understood,

the presence of N is quite unexpected. However, it could

possibly be explained by the nitrogen adsorption on the plasma

activated polymer surface while airing of the reaction chamber

with nitrogen gas.

4. Discussion

As was mentioned earlier polysiloxanes are susceptible to

etching under halogen plasma environments. Fluorine contain-

ing species are mainly required to break the Si–O backbone.

One might expect that the species participate in breaking the

Si–C bonds present in the system. Oxygen addition to the gas

mixture enhances the generation of active etchants and might

ease the removal of methyl groups (CH3) from the surface. It

was found that 20–30% oxygen content in the CF4 + O2 or

SF6 + O2 mixtures leads to the highest etch rates [1,4]. At such

an oxygen content the highest F atom concentration has been

reported for the CF4 + O2 plasma [6]. The correlation between

the highest F atom concentration and the highest etch rate

suggests that F atoms are the most important active species

during polysiloxane etching [4]. That is why a higher etch rate

observed in the SF6 + O2 plasma could be ascribed to higher F

atom concentration in comparison to the CF4 + O2 plasma.

The etching mechanism seems not to be the same in both

plasmas, which is indicated by different morphology and

chemical composition of the surface. A slightly higher SiO2

amount in MED-CF4 sample in comparison to the fresh

material and significantly higher comparing to MED-SF6

specimen suggest that SiO2 (SiOx) byproduct is being removed

from the elastomer surface more effectively in the SF6 + O2

plasma.

It is suspected that in case of the CF4 + O2 plasma the treated

surface is partially covered with a CxFy film. The film can

slightly suppress the etch rate but can also promote the

formation of irregular columns (Fig. 3). The presence of well-

fixed post-etching fluorocarbon polymer may explain higher

fluorine concentration onto the MED-CF4 surface. Otherwise,

the fluorine content in both specimens (MED-CF4 and MED-

SF6) should be the same as a consequence of fluorination of

methyl groups present on the fresh surface.

Present and former studies on dry etching of polydimethyl-

siloxane demonstrate the strong roughness of polysiloxane

surface. Garra et al. [1] reported that the surface roughness

might be caused by the random deposition of the aluminum

mask on the polymer surface due to its sputtering and

backscattering by the plasma. Aluminum detected on the

surface of the sample etched in the CF4 + O2 plasma (see

Table 1) and a systematic destruction of the aluminum mask in

the freon-based plasma observed recently [4] confirm the above

concept. Nevertheless, it should be noted that the surface

roughness is also evident post the SF6 + O2 plasma treatment

although no aluminum traces have been found by the XPS

analysis. The lack of aluminum seems to be in accord with the

stability of aluminum mask that was reported earlier [4].

Fluorine-based plasma etching may be considered as a

convenient way of releasing microdevices ‘‘sunk’’ in poly-

siloxane matrix or shaping the proper microforms in the

polymer bulk. It is widely known that the wettability of a

surface depends on its roughness and chemistry [7,8].

Modification of the morphology and surface contamination

observed in the present study may affect the wettability of the

originally biocompatible material. As was shown by Sterret

et al. [9] the contact angle of polyurethane elastomers may be

modified by the O2, CH4, CF4 and C2F6 plasmas. The contact

angle has been found to increase in the CH4 and CxFy plasmas

and decrease in the CF4 + O2 plasma. A future topic of interest

will be contact angle analysis and advanced bio-tests that will

verify the usefulness of such a plasma-modified polymer in

biomedical applications.

D. Szmigiel et al. / Applied Surface Science 253 (2006) 1506–1511 1511

5. Conclusions

The present study has illustrated the effect of fluorine-based

plasmas on morphology and chemical surface composition of

biocompatible silicone elastomer based on dimethylsiloxane–

methylhydrogen siloxane copolymer. Some aspects of etching

mechanisms were also discussed.

Gas mixtures of CF4:O2 = 3:1 or SF6:O2 = 3:1 have been

used for dry etching tests. SEM studies revealed significant

changes in polysiloxane morphology after the treatment in

both plasmas. In particular, a brittle columnar structure was

found on the elastomer surface etched in the CF4 + O2 plasma.

The XPS analysis showed that the polysiloxane surface

composition is affected by the plasma chemistry. Both plasma

treatments lead to fluorine contamination, which is three times

higher in case of the CF4 + O2 plasma. Some amount of

aluminum found on the polysiloxane surface etched in freon-

based plasma indicates sputtering and backscattering of the

aluminum mask. The surface etched in the SF6 + O2 plasma

was aluminum free. The observed changes in chemistry and

morphology of the polysiloxane surface are expected to affect

the wettability. Wettability measurements will be the subject of

future studies.

Acknowledgements

This research has been done in the frame of the Healthy

Aims Project supported by the European Commission Project

IST-2002-1-001837. D. Szmigiel is grateful to the Foundation

for Polish Science for financial support.

References

[1] J. Garra, T. Long, J. Curie, T. Schneider, R. White, M. Paranjape, J. Vac. Sci.

Technol. A 20 (2002) 975.

[2] K.L. Mittal (Ed.), Polymer Surface Modyfication: Relevance to Adhesion,

Utrecht, Netherlands, 1996, p. 33.

[3] K.L. Mittal (Ed.), Polymer Surface Modyfication: Relevance to Adhesion,

Utrecht, Netherlands, 1996, p. 3.

[4] D. Szmigiel, K. Domanski, P. Prokaryn, P. Grabiec, Proceedings of the 31st

International Conference on Micro- and Nano-Engineering 2005, Vienna,

Austria, Microelectron. Eng. (2006), in press.

[5] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers: the

Scienta ESCA300 Database, 1992.

[6] C.J. Mogab, A.C. Adams, D.L. Flamm, J. Appl. Phys. 49 (1978) 3796.

[7] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 998.

[8] A.B.D. Cassie, S. Baxter, Trans. Faraday Soc. 40 (1946) 546.

[9] T.L. Sterret, R. Sachdeva, P. Jerabek, J. Mater. Sci. Mater. Med. 3 (1992)

402.