Effect of Plasma Exposure on the Chemistry and Morphology of Aerosol‐Assisted, Plasma‐Deposited Coatings

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Effect of Plasma Exposure on the Chemistry
and Morphology of Aerosol-Assisted,
Plasma-Deposited Coatings
Barry Twomey,* Mahfujur Rahman, Gerry Byrne, Alan Hynes,
Lesley-Ann O’Hare, Liam O’Neill, Denis Dowling
Introduction
Silicon-containing coatings have found applications in the
packaging,[1–3]biomedical,[4]automotive[5]and microelec-
tronic[6]industries. Commonly, these coatings are depos-
ited using low-pressure plasma enhanced chemical vapor
deposition (PECVD) techniques,[1–3]using gaseous or vapor
precursors. The precursors most frequently discussed in
the literature are hexamethyldisiloxane (HMDSO), tetra-
ethoxysilane (TEOS), tetramethoxysilane (TMOS), tetra-
methyldisiloxane (TMDSO), and SiH4, reflecting the need
to use small molecules, easily converted to a gas or
vapor.[7–10]The requirement for the vacuum process
however limits the exploitation of this technology to
batch-reel processing and an increased system footprint.
As a result, there has been an increasing interest in the use
of atmospheric pressure plasmas for continuous proces-
sing.[11–13]Dielectric barrier discharges have been used to
depositsiloxanecoatingsatatmosphericpressure.[13–16]As
well as the use of small molecule siloxane precursors,
recent developments using atmospheric pressure glow
discharge have enabled low molecular weight cyclic
siloxane polymers, octamethylcyclotetrasiloxane (OMCTS
– D4) and tetramethylcyclotetrasiloxane (TMCTS – D3) to
Full Paper
B. Twomey, M. Rahman, G. Byrne, D. Dowling
School of Electrical, Electronic and Mechanical Engineering, UCD,
Belfield, Dublin 4, Ireland
Fax: þ353-1-716 1942; E-mail: barry.twomey@ucd.ie
A. Hynes, L.-A. O’Hare, L. O’Neill
Dow Corning Plasma Solutions, Owenacurra Business Park,
Midleton, Co. Cork, Ireland
This study reports on the effect on the morphology and chemistry of atmospheric pressure
plasma deposited nm-thick coatings (21?3 nm) as the level of exposure to the plasma is
systematically altered. Coatings were deposited by directly injecting hexamethyldisiloxane,
polydimethylsiloxane or tetramethyldisiloxane liquid precursors through a nebulizer into a
helium/oxygen atmospheric pressure plasma. An
increase in the level of the precursor was found to
beassociated with adecrease intheconcentration
of methyl functional groups in the coating and to
an increase of the Si–O crosslinking, as demon-
strated using surface energy and XPS analysis.
This resulted in an increase in the coating refrac-
tive index, and in a reduction of the number of
surface particulates, as well as of surface rough-
ness.
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be deposited to achieve siloxane coatings with tunable
chemistries.[17]In this study, silicon-containing coatings
were deposited in an atmospheric pressure plasma liquid
deposition system from three types of liquid siloxane
precursors with varying molecular weights, but with
similar molecular structures, as shown in Figure 1 -
poly(dimethylsiloxane) (PDMS) (1 cst), hexamethyldisilox-
ane (HMDSO) and tetramethyldisiloxane (TMDSO). The
objective of this investigation is to evaluate how both the
siloxane precursor chemistry and the level of exposure of
the precursor into the plasma influence the physical and
chemical properties of the deposited coatings.
Experimental Part
Atmospheric Pressure Plasma Treatment
ThecoatingsweredepositedusingtheDowCorning1SE-1100AP4
Workstation atmospheric pressure plasma system. A schematic of
the deposition system is shown in Figure 2. It comprises 2 vertical
plasma chambers arranged in conjunction with a dedicated web
handling system. The 300?320 mm2electrodes consist of a
conductive liquid housed in a dielectric perimeter and have a
5 mm electrode gap. Input powers of up to 2000 W can be applied
to the electrodes using a radio frequency (RF) power supply
(frequency ?20 kHz). Manual valved rotameters are used to
control gas flows. Helium and oxygen gas flow rates were
maintainedat 20l?min?1and0.1l?min?1,respectively.Asyringe
pump was used to supply a continuous flow of reactive fluids to 2
atomizerspositioned at the top of theplasmadepositionchamber.
The siloxane precursors were sourced from Sigma Aldrich with a
purity of at least 98% for each. Poly(ethylene terephthalate) (PET)
film of width 10 cm and 50 mm thickness, available from AB
Supplies Ltd. (UK), was passed through the electrodes using the
integrated web handling system. Each siloxane was continuously
nebulizedintotheplasmazoneasthePETfilmwaspassedthrough
the chamber at a speed of 1.5 m?min?1. At this speed, the
residence/deposition time of the plasma is 25 s per pass. Coatings
on2?2cmSiwaferswereachievedbymountingthewaferonthe
PET film as it was passed through the chamber. The liquid flow
rate and the number of times the PET was passed through the
system were varied to produce coatings with varied performance
characteristics.
Current and voltage are measured using a Pearson 6585 fast
current monitor and a North Star PVM-5 high voltage probe
respectively. Measurements are obtained at the output of the RF
power supply and collected on a Tektronix TDS 2024 oscilloscope.
Opticalemissionspectroscopy(OES)wascarriedoutusinganOcean
Optics USB4000 UV-VIS spectrometer. The fiber optic and collimat-
ing lens were mounted vertically above the
discharge. A minimum of 4 measurements
were carried out under each test regime.
Coating Characterization
Contact angle measurements were obtained
using the sessile drop technique using an OCA
20 video capture apparatus from Dataphysics
Instruments. Drop volumes of 1.5ml of the
following liquids were used to determine
surfaceenergy:deionized
methane and ethylene glycol. Surface energy
wasthendeterminedusingtheOWRK(Owens,
Wendt, Rabel and Kaelble) method.[18]In each
measurement,threedropsweredepositedover
thewidthofthesampleandtheircontactangle
was measured after settling on the surface for
10s.Thecoatingsurfaceenergywascalculated
fromtheaveragecontactanglevalueforeachof
theprobeliquids.Coatingsurfaceenergieswere
measured one week after deposition. This was
carried out to eliminate any surface energy
changes immediately after plasma treatment
duetohydrophobicrecoveryinthetimeperiod
after deposition.[19]
A CP-II (Veeco) atomic force microscope
(AFM) was employed to perform the top
surface morphology analysis and surface
water,diiodo-
B. Twomey et al.
Figure 1.Chemicalstructureof liquidsiloxaneprecursors(n¼1 for
1 cst PDMS).
Figure 2. Schematic of atmospheric pressure plasma liquid deposition process and
experimental setup.
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roughness measurement (Average roughness – Raand Root Mean
Square roughness - Rq) of the coating using a 100 micron scanner
and scan rate of 1 Hz. The AFM images were recorded in non-
contact mode with silicon (phosphorous doped) cantilevers
(nominal spring constant¼40N?m?1, tip radius <10 nm). Coat-
ing thickness and refractive index were measured using a
Woollam M2000 variable angle spectroscopic ellipsometer. The
Cauchymodelwasusedtodeterminethethicknessandtheoptical
constants of the coatings deposited on silicon wafers.[20]
Measurements were carried out at multiple angles of incidence
(60–75 varied by 58) to improve the measurement confidence
(?0.005 max error). X-ray photoelectron spectroscopy (XPS) was
performed using a Kratos Analytical Axis Ultra photoelectron
spectrometer. The instrument is equipped with a spherical mirror
analyzer(165mmmeanradiusHSA),anintegralautomaticcharge
neutralizer and a magnetic lens. A monochromated aluminium
(Al Ka) X-ray source was used to record spectra at normal
emission. All samples were stored at standard temperature and
pressure before analysis. Fourier transform infrared spectroscopy
(FTIR) measurements were carried out using a Bruker Vertex-70
system equipped with a LN-MCT detector cooled with liquid
nitrogenandaKBrbeamsplitter.Thesamplechamberwaspurged
byN2gasbeforethescanswereobtained.Spectrawerecollectedin
the range of 850–4000 cm?1using a spectral resolution of 4 cm?1.
The transmission spectra for the coated NaCl disks were obtained
by the overlay of 64 scans to increase signal to noise ratio. The
NaCl IR cards were obtained from Apollo Scientific Ltd. and had a
15 mm aperture. NaCl substrates were used to minimize substrate
interference during the measurement of nanometer thick SiOx
coatings. The IR cards were mounted on the PET film during
deposition using double sided tape.
Results and Discussion
Influence of Plasma Exposure on Coating Properties
The effect of increasing plasma exposure on deposited
coating properties was evaluated as follows: Coatings
were deposited as the PET substrate passes through the
plasma system at an applied plasma power of 1000 W. In
orderto buildupcoating thickness,the polymeris passeda
number of times through the deposition chamber. During
deposition, the level of plasma exposure per unit of
monomer was varied by altering the flow rate of the liquid
precursor and the number of passes through the chamb-
er.[21]Low levels of plasma exposure were generated using
high precursor flow rates (50 ml?min?1) and four coating
passes. High plasma exposure was obtained by reducing
the precursor flow rate to 17 ml?min?1and by increasing
the number of passes to 20. Intermediate levels of
exposure were generated using 32.5 ml?min?1and 8
passes. The number of passes was set to ensure that all
coatings produced a similar coating thickness. Irrespective
of the liquid precursor used, the average coating thickness
ofall thedeposited coatings was21nm?3 nm, suggesting
that the variations in precursor chemistry are not
significantfactorswhenitcomes toalteringthedeposition
rate. This is not completely unexpected, as all of the
precursor molecules are based on similar methyl siloxane
chemical structures, as shown in Figure 1.
It is anticipated that increasing the plasma exposure
will cause an increase in the level of crosslinking and
coating oxidation. According to the model of vacuum
plasma polymerization developed by Yasuda, in order to
maintain the same level of plasma polymerization at
decreasing precursor volumes (Va>Vb), the input dis-
charge power (W) also has to decrease (Wa/Va¼Wb/Vb).[22]
In order to demonstrate if this effect applies at atmo-
spheric pressure, theplasma inputpower wasmeasured at
increasing precursorflow ratesforcoatings deposited from
HMDSO. The average plasma input power was calculated
from Equation 1 over one period of the sine wave using
measured current and voltage values.[23]
W ¼ F
ZtþT
t
IðtÞVðtÞdt(1)
Where F is frequency, T¼1/F (period), V is voltage and I is
current. The effect of increasing precursor flow rate on
plasma input power is shown in Figure 3. For increased
precursorflowrates(50ml?min?1)theplasmainputpower
was reduced by up to ?15% from that of the plasma
operating with no precursor flow.
Optical emission spectroscopy was used to examine the
relationship between input power and precursor flow
ratesonplasmaintensity.Atypicalspectrumforahelium/
oxygen plasma in the spectral range of 200 to 880 nm
is shown in Figure 4. The changes in plasma intensity were
obtained by measuring the integrated area under the
emission spectrum after correcting for background light. It
was observed that the drop in plasma input power,
measured using current/voltage data, with increased
precursor flow rates also correlated with a decrease in
Effect of Plasma Exposure on the Chemistry and Morphology of...
Figure 3. Plot of average plasma input power and integrated
optical emission intensity at increasing precursor flow rates.
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the optical emission intensity, as seen in Figure 3. As
illustratedinFigure4,theemissionintensityisobservedto
decrease upon addition of the precursor. The decrease in
emission intensity is primarily around the N2and Nþ
peaks between 300 and 435 nm. The occurrence of Nþ
moderate pressures is due to excess helium.[24]The
reaction is commonly associated with Penning ionization
of the nitrogen molecules due to helium metastables.[25]
The decrease in emission intensity is therefore most likely
due to consumption of the helium metastables caused by
plasma quenching due to theincreased reactionassociated
with the increasing precursor volumes as flow rates
increase. This decrease in ionization rate will reduce the
production of free electrons,[26]and in turn, reduce the
possible dissociation of the siloxane precursors through
the breaking of the Si–O, Si–H or Si–C bonds.[27]Based on
this result and on Yasuda’s ‘power/precursor volume’
relationship,[22]it is anticipated that increasing precursor
flow rates would result in a decrease in the level of
precursor plasma polymerization and oxidation during
deposition. In order to verify if decreased levels of plasma
oxidation are associated with increased precursor flow
rates, surface energy measurements were carried out. As
illustrated in Figure 5, as the plasma exposure of the
siloxane precursor to the plasma is increased (from 4
passes to 20 passes), the surface energy is found to
2
2at
increase. It is primarily the polar component that was
found to increase with increasing plasma exposure, while
the dispersive remains almost constant at 24–28 mJ?m?2.
Due to the increased polarity of the Si–O bond over the
Si–C and Si–H bonds, an increase polar surface energy
indicates an increase in the oxygen content of the
deposited coatings as a result of the removal of methyl
and hydrogen functionalities.[21]It was observed that
TMDSO deposited coatings exhibited the highest surface
energy values of the three siloxanes examined for each of
the coating deposition conditions. This may be a result of
the increased reactivity of the Si–H bond in the TMDSO
molecule due to its bond energy. The bond energies of
the different siloxane components are as follows:[28]Si–O,
88kcal?mol?1;Si–CH3,74kcal?mol?1;Si–H,70kcal?mol?1.
The increased surface energy of TMDSO indicates the
preferential breaking of the weaker Si–H bond. This is
followed by reaction of the Si–CH3 bond, resulting in
similar surface energy increases to HMDSO and PDMS at
higher plasma exposure rates.
To investigate this reaction mechanism, XPS analysis
was carried out 2 weeks after deposition on coatings
deposited from HMDSO. After a 2 week period, surface
energy is typically stable, exhibiting changes of <5%.[21]
The elemental composition and siloxy chemistry of the
coatings were examined. As illustrated in Table 1, with
B. Twomey et al.
Figure 4. Optical emission spectrum of a helium/oxygen atmos-
pheric pressure plasma with no precursor (bottom) and HMDSO
at 50 ml?min?1(top).
Figure 5. Effect of increasing plasma exposure on coating surface
energy (-) and the polar surface energy component (–). 4 pass at
50 ml?min?1; 8 pass at 32.5 ml?min?1; 20 pass at 17 ml?min?1.
Table 1. Change in coating elemental composition with increasing plasma exposure for the HMDSO coatings.
Passes Elemental compositionSiloxy chemistry
%%
OC SiMDTQ
4 55 24 22012 24 64
8 6115 240130 87
20 697 2403790
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increased levels of plasma exposure, the oxygen content of
the coating increases with a corresponding decrease in
carbon concentration. This correlates well with surface
energy measurements, as the oxygen concentration
increase causes an increase in the polar surface energy.
This result would be expected to reflect a change in the
siloxy chemistry of the coatings as a result of ablation of
methyl groups, followed by increased crosslinking and
oxidation. To examine this in detail, curve fitting of
the silicon (Si 2p) core level was carried out, as seen in
Figure 6. This method has been developed to provide
unambiguous assignment of the siloxy chemistry of the
coating and is described in detail else-
where.[29,30]When describing coating
siloxy chemistry, a simplified notation
is used to represent the number of oxy-
gen atoms attached to the silicon:[28–30]
M[(CH3)3SiO1/2], D[(CH3)2SiO2/2], T [(CH3)
SiO3/2] and Q [SiO4/2]. As summarized in
Table 1, the siloxy chemistry is in good
agreement with the elemental composi-
tion and the surface energy measure-
ments: as the oxygen concentration
increases, the percentage of more highly
oxidized siloxy species also increases. A
Q-type chemistry level of up to 90% was
obtained for a coating deposited by
passing the PET film 20 times through
the plasma chamber, with a HMDSO
flow rate of 17 ml?min?1. As the siloxane
approaches almost complete conversion
to SiO2, differences in precursor chem-
istry become less relevant resulting
in similar surface energy of coatings
deposited under these conditions for each of the pre-
cursors, as seen in Figure 5.
FTIR measurements were also carried out on HMDSO
coatings deposited on NaCl IR discs. Measurements were
made one week after deposition. As observed in Figure 7,
the dominant Si–CH3peaks of the HMDSO monomer at
840, 1260 and 2960 cm?1[31]were almost completely
eliminated in the plasma deposited coatings. A small peak
at 1260 cm?1is seen in coatings deposited at the lowest
plasma exposure (4 pass) indicating the retention of some
methyl functionalities. Coatings deposited at the highest
plasma exposure (20pass) exhibit a sharp peak at
Effect of Plasma Exposure on the Chemistry and Morphology of...
Figure 6. Curve fit of the silicon (Si 2P) core level for HMDSO deposited coating on PET at increasing plasma exposure rates: 4 pass at
50 ml?min?1(left) and 20 pass at 17 ml?min?1(right).
Figure 7. FTIR spectra of the HMDSO monomer and of the HMDSO coatings deposited at
increasing plasma exposure rates (4 to 20 pass).
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1060 cm?1which is in good agreement with the 90%
Q-type siloxy chemistry obtained from XPS analysis.
Si–OH characteristics were observed in both coatings at
930 cm?1,[32]although a lower intensity was seen for
coatings deposited at higher plasma exposures. A more
prominentbroadpeakat3400cm?1associatedwithwater
adsorption[11]was also observed for coatings deposited at
lower plasma exposures. The full width at half maximum
(FWHM) of the peak between 980 and 1240 cm?1was
examined for both coatings. An increase in the FWHM
indicates decreased coating homogeneity,[33]and a broad
shoulder may be attributed to coating porosity.[31]The
FWHM for coatings deposited at the lowest plasma
exposure is almost double that of coatings deposited at
the highest plasma exposure. Increased porosity and non-
homogeneity may result in a decrease in coating density.
The peaks observed at 2340 cm?1are attributed to CO2in
the chamber during measurement.[34]
Ellipsometry measurements were carried out to deter-
mine coating thickness and the effect of varying siloxy
chemistry on coating density. As detailed above, the
average coating thickness of the deposited coatings was
21?3 nm. The associated average coating growth rates
were 0.21, 0.11 and 0.04 nm?s?1for coatings deposited at
4, 8 and 20 passes through the plasma chamber. The total
precursor volumes deposited these passes were 83, 108
and 142ml respectively. This indicates that coatings
deposited at increasing plasma exposure exhibit lower
growth rates and require a higher total volume of
precursor to achieve comparable thickness values. This
is most likely due to the increased coating densification
caused by increased levels of crosslinking and oxidation.
The coating density was quantitatively investigated by
measuring coating refractive index (n) at the 632.8 nm
wavelength using the Cauchy model.[20]The optical
properties of the coatings were determined using multiple
angle measurements (658, 708
and 758) over a wavelength
range of 193 to 1 690 nm. As
the precursor plasma exposure
increases, the refractive index
increases from 1.31 to 1.46. An
increase in coating refractive
index is commonly associated
with an increase in coating
density and to a decrease in
the organic fraction of the coat-
ing.[35,36]The relatively low n
value of 1.31 may be due to the
increased surface roughness of
the coating,[20]or coating non-
homogeneities or porosity indi-
cated by the FTIR results. In
conclusion, the surface energy,
XPS, FTIR and ellipsometry results confirmed that with
increasedexposureoftheprecursortotheplasmathereisa
corresponding increase in precursor fragmentation, lead-
ing to the deposition of a denser, more crosslinked and
oxidized coating.
Coating Morphology
AFM analysis was used to examine the changes in surface
morphology and appearance of the deposited coatings at
increasing plasma exposure for each of the siloxanes.
Particulates with diameters of between 50 and 300 nm
were observed in the coatings. As previously observed,[32]
the size and number of coating particulates increased with
increasing the precursor flow rate, as seen in Figure 8. This
may be due to the increased gas phase reactions[32,37]
associated with an increase in the partial pressure of the
precursor within the plasma chamber. Coatings deposited
at higher precursor flow rates show larger particulates
present due to agglomeration caused by this excessive
reaction. The surface roughness (Raand Rq) of the coatings
deposited on Si wafer substrates were examined as the
reduced roughness of the wafer helped highlight small
height deviations in the coatings.
Roughness was examined in two stages. Firstly, the
surface roughness of a 5?5 mm2area of each sample was
measured.Atleast3measurementsweremadepersample
withtheaverageroughnessvaluespresented inTable2.As
shown in this table, as the plasma exposure increased,
both the Raand Rqvalues decreased. A significant factor
influencing the surface roughness is the number of
particulates deposited along with the coatings, as illu-
strated in Figure 8. In order to get a clearer measure of the
micro-roughness of the deposited coatings, a second stage
of the analysis was undertaken. This involved isolating an
area that was relatively free of particulates and carrying
B. Twomey et al.
Figure 8. AFM topography analysis of polymeric (left) and SiOx(right) coatings deposited from
TMDSO.
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out a high resolution (0.5?0.5 mm2) roughness measure-
ment on each sample. A minimum of five measurements
were made for each coating and their average value
presented in Table 2. It was again observed that,
independent of the siloxane precursor type and particu-
lates, as the level of plasma exposure increased, the micro-
roughness of the deposited coating decreased. This is most
likely due to the increase in coating densification at higher
levels of plasma exposure leading to a more homogenous
surface topography. Of the three precursors studied, the
roughness of the HMDSO and PDMS coatings were found
to be similar, while that of TMDSO was slightly lower. A
factor influencing this lower roughness is likely to be the
rateofreactionsintheplasma.Thiswould befacilitatedby
the presence of the Si–H group in TMDSO,[17,38]as
indicated by the surface energy results.
Conclusion
A range of polymeric and SiOx(silica) type coatings were
obtained from PDMS, HMDSO and TMDSO siloxane
precursors. The deposition rates obtained for these three
precursors were found to be largely independent of
precursor type, but were significantly influenced by the
level of plasma exposure. Higher plasma exposure led to
lower deposition rates and to a conversion from polymeric
to a more inorganic coating chemistry. Coating surface
energy and XPS results, in conjunction with average input
power and optical emission analysis, confirmed that for
increased levels of precursor plasma exposure, increased
rates of plasma polymerization occur. This is most likely
due tothe increased levelsof activeplasma species present
relative to the volume of siloxane precursor. The increased
surface energy values of TMDSO over PDMS and HMDSO
under comparable deposition conditions indicated the
preferential breaking of weaker Si–H molecular bonds. As
siloxy-chemistry approaches that of SiO2, the small
differences between molecules become less important
with the surface energy values converging for each of the
precursors. Increased plasma exposure leads to increased
coating crosslinking, and to an increase in the coating
refractive index. It was also observed that the more
inorganic coatings deposited at increasing plasma expo-
sure contain decreased amounts of coating particulates
and exhibit lower micro-roughness.
Received: March 5, 2008; Revised: June 12, 2008; Accepted:July 11,
2008; DOI: 10.1002/ppap.200800048
Keywords: aerosol; atmospheric pressure plasmas; organosilox-
ane precursors; surface morphology
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Table 2. Surface roughness (Raand Rq) values of HMDSO, PDMS and TMDSO coated Si deposited at increasing plasma exposure from 4 pass
to 20 pass.
Passes Surface Roughness, Ra
nm
HMDSOPDMSTMDSO
5T5 mm2
0.5T0.5 mm2
5T5 mm2
0.5T0.5 mm2
5T5 mm2
0.5T0.5 mm2
413.6 0.6313.60.537.1 0.46
89.70.48 4.50.46 5.40.46
20 2.00.401.1
Surface Roughness, Rq
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nm
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819.3 0.6114.30.5914.80.58
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Plasma Process. Polym. 2008, 5, 000–000
? 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Early View Publication; these are NOT the final page numbers, use DOI for citation !!
DOI: 10.1002/ppap.200800048
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