Oxygen plasma induced hydrophilicity of ParyleneC thin films
Tatiana Trantidou*, Themistoklis Prodromakis, Chris Toumazou
Centre for Bio-Inspired Technology, Department of Electrical and Electronic
Engineering, Imperial College London, London SW7 2AZ, UK
Email: tatiana.trantidou09@imperial.ac.uk, Tel: +44 (0)20 7594 0840.
ABSTRACT
This paper investigates the surface modification of Parylene-C thin films under
oxygen plasma treatment. We specifically focus on the extent of hydrophilicity by
experimenting with varying process parameters, such as power intensity (50:400W)
and exposure time (1:20min). Correlations between treatment parameters, film
thickness, restoration of hydrophilicity and etching rates were experimentally
established. We also demonstrate the selective modification of Parylene-C films,
facilitating distinct hydrophilic and hydrophobic areas with µm-resolution that can be
exploited in self-alignment applications.
Keywords: Parylene-C, oxygen plasma, surface modification, hydrophobic,
hydrophilic.
1.
Introduction
Over the past few decades, poly(chloro-para-xylynene) (Parylene-C) has been
extensively used as a biocompatible encapsulant of implantable microdevices, such as
pacemakers [1], microelectrodes [2], probes [3], as well as chemical sensors [4,5].
The wide applicability of such encapsulating coatings is mostly attributed to its pinhole free nature above 100 nm, acting as an electrolyte-barrier layer, but also to its
hydrophobic surface, resulting in the retraction of electrolytes that come in contact.
Other studies have focused on Parylene’s stretchable properties, employing it as a
flexible substrate for implantable sensors for monitoring glaucoma patients’
intraocular pressure [6], as well as in stretchable implementations of chemical sensing
arrays [7]. More recently, Parylene-C has shown a considerable potential in
applications where the self-aligning of cells and proteins is essential [8,9]. In these
studies, patterned Parylene-C coatings have been used as soft masks, which can be
easily be peeled-off, to facilitate alignment.
Despite its many advantages, the naturally occurring hydrophobic surface could
prohibit the exploitation of its other remarkable properties. It is thus essential to study
parylene’s surface properties and exploit mechanisms for altering these that could
facilitate biomedical applications, such as providing substrates for proteins and cells.
Previous studies exploiting Parylene-C as a medium to pattern cells in particular
orientations employed UV irradiation to explore whether alterations on the surface
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chemistry facilitate the adhesion of neuronal cells on parylene stripes. It was shown
that UV exposure typically inhibits the absorption of serum proteins, such as albumin,
which act as binding sites for cells, and therefore hinders neuronal patterning
especially at higher doses [10]. In similar fashion, chemical modification of the
surface of Parylene-C films through acids [11] or plasma treatment [12-14] have
shown that the surface properties of such films can be effectively altered. Particularly
the latter method has found applications in cell culturing [15-19]. Although a
significant change in the hydrophilicity of treated Parylene-C surfaces has been
reported, the effect of different process parameters in the long-term has not been
thoroughly investigated yet.
This paper presents the effect of distinct parameters that facilitate the surface
modification of Parylene-C films using oxygen plasma treatment, with particular
emphasis on hydrophilicity. Section 2 describes the experimental methodology
ascertained throughout this study. Section 3 elaborates on the implications of distinct
processing parameters in a) the hydrophilic behaviour of Parylene-C over the long
term, b) the relationship between thickness and hydrophobicity, and c) the selective
hydrophobic/hydrophilic patterning via standard lithography. Finally, quantitative
relationships are provided that have been validated against experimental results.
2.
Experimental procedure
2.1
Deposition of Parylene-C film substrates
Microscope glass slides of standard dimensions (76mm x 26mm) were
thoroughly degreased in Acetone, Isopropanol (IPA) and Deionised (DI) water, and
dehydrated for 60 s at 90oC. Parylene C films were deposited by Chemical Vapour
Deposition, using a commercially available coater (PDS2010) by vaporising (150oC)
and then pyrolising Parylene-C dimer (690oC). Numerous runs were carried out,
facilitating five different thicknesses, and the samples were accordingly divided into
five groups; a) 50 nm (group 1), b) 1 µm (group 2), c) 5 µm (group 3), d) 7 µm (group
4), and e) 12 µm (group 5).
2.2
Oxygen plasma treatment of Parylene-C thin films
Oxygen plasma treatment was performed using an ultra high purity plasma etcher
(Nano UHP) at a working pressure of 0.8 mbar. Samples from group 3 were utilised
for three experimental sets: 1) plasma treatment for 1 min at varying power values
(50W-400W), 2) plasma treatment at 50W and 400W for various exposure time
intervals (1, 2, 3, 5 and 10 min), and 3) plasma treatment at 25W for 18 min. Samples
from groups 1, 2, 3 and 5 were treated at 50W for 1 min to investigate any
relationship between thickness and hydrophilicity.
2.3
Evaluation of surface hydrophilicity
Evaluation of the hydrophilicity of the surface of the Parylene-C coated glasses
was performed through static contact angle measurements. For consistency, a drop of
2
DI water of 10 µl volume was always employed and the affinity of the drop to the
surface was monitored via a low-cost USB microscope at a fixed position. Contact
angle was extrapolated via a dedicated image processing software (ImageJ, [20]).
Contact angle measurements were performed for each sample before treatment,
immediately after treatment and afterwards once every day for 7 consecutive days to
further investigate the restoration of parylene’s hydrophilic properties. Selective
surface modification was also demonstrated with the same set-up, utilising samples
that have been masked appropriately with photoresist. In addition, since oxygen
plasma is known to be useful for etching Parylene-C films [7], we have determined
the corresponding etching rates when present.
3.
Results and discussion
3.1 Power density
We first investigated the effect of varying the oxygen plasma power density on 5
µm thick coated samples for a timespan of 1 min. Static contact angles were measured
before and immediately after plasma treatment for samples treated at 50W, 100W,
150W, 200W, 250W, 300W, 350W, and 400W. The contact angle of untreated
Parylene-C was approximately 85o and gradually decreased to 16.8o after treatment at
50W, to 16.1o at 150W, to 14.4o at 200W, to 13.7o at 250W, to 11.6o at 300W, to 4.6o
at 350W, and to less than 4o at 400W. The corresponding drop affinity to the films
surface is illustrated in Figs. 1(a)-(f), while a summary of the contact angle
measurements is depicted on Fig. 1(g). As expected, a large power density has a more
profound effect on parylene’s hydrophobicity.
(Figures 1(a)-(f) about here)
3.2 Exposure time
In order to explore the effect of exposing Parylene-C films (5 µm thick) in
oxygen plasma, we maintained the power intensity fixed at 50W and 400W for a set
of exposure intervals ranging from 1 to 10 min. Static contact angles were measured
before and immediately after plasma and are progressively shown in Figs. 2(b)-(f)
respectively. Contact angle was initially 85o on untreated Parylene-C surface, and it
reduced after oxygen plasma treatment at 50W to approximately 10o for 1 min, to 12o
after 2 min, to 12.3o after 3 min, to 13.7o after 5 min, and to 16.8o after 10 min. These
measurements are summarised in Fig. 3(a), suggesting that hydrophilicity does not
improve with process time for treatment at low power intensities (50W). On the
contrary, the surface hydrophilicity of samples treated at higher power intensities
(400W) increased accordingly with treatment time, resulting in contact angles less
than 1o after treatment for 2 min (Fig. 3(b)).
(Figures 2(a)-(f) about here)
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(Figures 3(a) and 3(b) about here)
3.3 Restoration of hydrophilicity
Determining the restoration of parylene’s surface hydrophilicity is important for
long-term applications. We examined the hydrophilicity of plasma treated Parylene-C
films (5 µm thick) over a period of one week through two sets of experiments. The
first set included samples treated for a fixed duration of 1 min but with the power
intensity varying as shown in Fig. 4(a). The second set involved samples treated at a
constant power of 50W but for different time intervals (Fig. 4(b)). Results reveal that
after a week, saturation of parylene’s hydrophilicity occurs within the range of 40o60o for samples of both experimental sets. Beyond this point, there is no significant
increase in the contact angles. For 1 min treatment the lowest contact angles are
retained for samples treated at higher power (400W). On the other hand, increasing
the treatment duration at 50W does not appear to improve the long-term
hydrophilicity of parylene’s surface.
In addition, we conducted a third set of experiments to explore whether high
power-short time treatment is more efficient in the long-term when compared against
low power-long time treatment. In this set we compared the long-term hydrophilic
behaviour of two samples treated at 25W for 18 min and at 400W for 1 min
respectively (Fig. 4(c)). Fig. 5 shows the adhesion of water drops on the two
Parylene-C surfaces before treatment, immediately after treatment, the first day, the
third day and the seventh day respectively. It is interesting to note that although the
immediate reactivity of Parylene-C surfaces is more apparent when treated with a
high-power intensity (400W), in the long-term this obtains indistinguishable
hydrophilicity (~40o) with surfaces that have been treated with low-power (25W) but
received longer exposure (18 min), as illustrated in Fig. 4(c).
(Figures 4(a)-(c) about here)
(Figures 5(a)-(e) and 6(a)-(e) about here)
The longevity of the oxygen plasma induced hydrophilicity is further
demonstrated in Fig. 7, where water drop affinity is compared for 1 µm thick
Parylene-C films, before treatment (~80o) and 40 days after treatment (~48.8o).
Evidently, Parylene-C tends to restore its hydrophobic nature to a certain extent,
however the water drop affinity is nearly doubled even after 40 days.
(Figures 7(a)-(b) about here)
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We defined the restoration ratio at day i (ri) as the percentage of the difference in
contact angles between untreated films (CAinitial) and films treated i days ago (CAi) as
follows:
ri=(CAinitial - CAi)/ CAinitial ·100%.
(1)
Table 1 summarises the restoration ratios for various treatment parameters. The
restoration ratio at day 7 of treated parylene films ranges between approximately
40%-50% for all samples. It appears that the best long-term results are achieved either
by mild treatment for longer period or by intense treatment for a short period.
(Table 1 about here)
3.4 Film thickness
The water drop affinity experiment was performed on untreated Parylene-C
coated surfaces of different thicknesses (50 nm, 1 µm, 5 µm, and 12 µm) to observe
any meaningful relationship between the films thickness and hydrophobicity. Contact
angle measurements are summarised in Fig. 8(a), supporting that the thicker the
Parylene-C film the more hydrophobic the surface tends to be. These conclusions are
in agreement with previous studies exploring the correlation between thickness and
hydrophobicity of Parylene-C [21]. Subsequently, all samples were treated with
oxygen plasma at 50W for 1 min. Fig. 8(b) depicts the corresponding contact angle
measurements after treatment. It appears that the hydrophilicity of Parylene-C films is
inversely proportional to the thickness of the film, and that larger power or treatment
time is required for attaining a comparable behaviour.
(Figures 8(a)-(b) about here)
The thickness of each sample was also measured before and after plasma
treatment to investigate the etching rate of Parylene-C in relation with the plasma
process parameters (Table 2). Thickness was measured using a step profiler as the
average of three measurements across a step. Treatment at 50W resulted in significant
etching of the surface only after exposure for more than 5 min. Results presented in
Table 2 suggest that the etching rate increases with power intensity.
(Table 2 about here)
3.5 Selective hydrophilic patterning
Samples coated with 1 and 7 μm thick parylene films have undergone standard
lithography to investigate the selective modification of the polymer’s surface
hydrophobicity. Hexamethyldisilazane (HMDS) was spin-coated on the Parylene-C
coated glasses, succeeded by a 1.4 µm thick positive photoresist. The samples were
then soft baked on a hotplate at 90oC for 60 s, selectively exposed to UV light for 90
s, and developed to remove the exposed photoresist. Before inserted into the plasma
etcher, samples were prebaked on a hotplate at 110oC for 60 s for hardening the
protective photoresist mask. After treatment, the masking photoresist was removed to
5
conduct the evaluation experiments. DI water was rinsed on the surface of the samples
to demonstrate selective patterning.
Selective hydrophilic patterning was achieved in two ways. In the first case, 1 μm
thick parylene films were treated at 400W for 15 min. The exposed Parylene-C areas
were completely etched to reveal the hydrophilic glass substrate underneath. Fig. 9(a)
demonstrates the self-containing of water in the hydrophilic wells. In the second case,
7 μm thick Parylene-C films received oxygen plasma treatment at 400W for 5 min.
Selective hydrophilic modification of the film surface was accomplished without
significant etching. Fig. 9(b) depicts DI water confined on the hydrophilic stripes.
(Figures 9(a)-(b) about here)
4.
Conclusion
Oxygen plasma treatment of Parylene-C thin films results in the formation of
hydrophilic functional groups near the surface. In this work, we investigated the effect
of different plasma process parameters on altering the surface properties of ParyleneC coated substrates, and evaluated effectiveness of treatment in terms of film
thickness, restoration of the surface hydrophilicity and selective patterning, using
static contact angle measurements.
Hydrophilicity of Parylene-C films increased with power intensity for fixed
exposure time. On the other hand, treatment for longer time at a constant power
improves the surface hydrophilicity only at higher power values (400W). Parylene-C
tends to restore its hydrophobic nature after oxygen plasma treatment over the longterm; however its hydrophilicity saturates after a week to approximately 40%-50% of
its initial state. This observation suggests that plasma treated parylene can still be
classified as hydrophilic in the long-term. Complementary to this observation, it was
proved that high power-short time treatment has immediate results in the parylene’s
hydrophilicity than low power-long time treatment. Nevertheless, in the long term
both cases demonstrate the same behaviour.
Through water drop experiments, we have determined that a correlation exists
between thickness of Parylene-C films and their surface hydrophobicity. As
anticipated, hydrophobicity increases with thickness. Plasma treatment under the
same conditions revealed that in order to achieve a similar surface affinity on
Parylene-C films of varying thicknesses, larger power intensity or exposure time are
needed for thicker films.
Plasma treatment etching effect on Parylene-C surface was also examined.
Results showed that etching rate increases in proportion to the power intensity,
particularly for values above 50W. Finally, selective surface modification of parylene
has been demonstrated by creating hydrophobic and hydrophilic areas via standard
lithography. Based on this study, oxygen plasma can be exploited to optimise
Parylene-C surface modification to render the material application-specific. The
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impact of this study could be potentially translated in applications that require selfalignment of biological medium.
Acknowledgments
The authors would like to acknowledge the financial support of the A.G. Leventis
Foundation.
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FIGURE CAPTIONS
Fig. 1. 10 µl water affinity drops on Parylene-C surface: (a) untreated, (b) treated at 50W for 1 min, (c) treated at
150W for 1 min, (d) treated at 200W for 1 min, (e) treated at 300W for 1 min, (f) treated at 400W for 1 min. g)
Contact angle measurements as a function of power immediately after oxygen plasma treatment (1 min) of 5 µm
thick Parylene-C coated surfaces.
Fig. 2. 10 µl water drops affinity on Parylene-C surface: (a) untreated, (b) treated at 50W for 1 min, (c) treated at
50W for 2 min, (d) treated at 50W for 3 min, (e) treated at 50W for 5 min, and (f) treated at 50W for 10 min.
Fig. 3. Contact angle measurements as a function of exposure time immediately after oxygen plasma treatment of
5 µm thick Parylene-C coated surfaces, with the power intensity being fixed at (a) 50W and (b) 400W.
Fig. 4. Contact angle measurements as a function of longevity after oxygen plasma treatment of 5 µm thick
Parylene-C films: (a) restoration of hydrophilicity for various plasma power values (1 min treatment time), (b)
restoration of hydrophilicity for various plasma treatment time parameters (50W treatment power), and (c)
comparison between high power-short time (400W-1min) and low power-long time (25W-18min) treatment. -1
and 0 points at the time axis correspond to initial state of Parylene-C, and immediately after oxygen plasma
treatment.
Fig. 5. 10 µl water drops affinity on Parylene-C surface treated with oxygen plasma treatment at 25W for 18 min:
(a) untreated, (b) immediately after treatment, (c) 1 day after treatment, (d) 3 days after treatment, and (e) 7 days
after treatment.
Fig. 6. 10 µl water drops affinity on Parylene-C surface treated with oxygen plasma treatment at 400W for 1 min:
(a) untreated, (b) immediately after treatment, (c) 1 day after treatment, (d) 3 days after treatment, and (e) 7 days
after treatment.
Fig. 7. Water drops formed on Parylene-C surface of 1 µm thickness: (a) untreated Parylene-C surface, and (b)
Parylene-C surface treated with oxygen plasma at 50W for 1 min after 40 days.
Fig. 8. Contact angle measurements as a function of thickness of Parylene-C coated surfaces: (a) untreated
parylene films, and (b) parylene films treated at 50W for 1 min.
Fig. 9. Microscope images of DI water rinsed on selectively treated Parylene-C surfaces: (a) 1 µm thick Parylene-C
film treated at 400W for 5 min to create hydrophilic wells of 400 µm diameter, (b) 7 µm thick Parylene-C film
treated at 400W for 15 min to create sequential hydrophilic-hydrophobic stripes of 10 µm width.
8