argon plasma immersion ion implantation of polystyrene films

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www.elsevier.com/locate/nimb

Nuclear Instruments and Methods in Physics Research B 266 (2008) 1074–1084

NIMBBeam Interactions

with Materials & Atoms

Argon plasma immersion ion implantation of polystyrene films

A. Kondyurin a,*, B.K. Gan a, M.M.M. Bilek a, D.R. McKenzie a, K. Mizuno a, R. Wuhrer b

a Applied and Plasma Physics, School of Physics (A28), University of Sydney, New South Wales 2006, Australiab Microstructural Analysis Unit, University of Technology Sydney, P.O. Box 123, Broadway, NSW 2007, Australia

Received 22 January 2008; received in revised form 19 February 2008Available online 6 March 2008

Abstract

Plasma immersion ion implantation (PIII), using bias voltages of 5, 10, 15 and 20 kV in an argon plasma and fluences in the range of2 � 1014–2 � 1016 ions/cm2, was applied to 100 nm polystyrene films coated on silicon wafer substrates. The etching kinetics andstructural changes induced in the polystyrene films were investigated with ellipsometry, Raman and FTIR spectroscopies, optical andscanning electron microscopies, atomic force microscopy and contact angle measurements. Effects such as carbonisation, oxidationand cross-linking were observed and their dependence on the applied bias voltage is reported. Variations in the etching rate duringthe PIII process and its relationship to carbonisation of the modified surface layer are explored.� 2008 Elsevier B.V. All rights reserved.

PACS: 52.77.Dq; 68.35.Bm; 81.65.Cf; 81.05.Uw

Keywords: Plasma immersion ion implantation; Polystyrene; Etching; Surface modification; Thin film

1. Introduction

High energy ion irradiation is a method for inducingstructural changes in polymers in which is finding applica-tions in many areas, such asmodern biochip technologiesfor the creation of micro- and nano-structures [1], micro-electronics manufacturing [2], biomaterials [3,4] and spacematerials [5]. Ion beam treatments have also been appliedto polymer films to enhance cell adhesion [6,7].

The structural changes observed can be divided into twophases [8,9]. At low ion implantation fluences (1013–1014 ions/cm2) cross-linking occurs but the polymer struc-ture is otherwise preserved. At high fluence (1015–1017 ions/cm2) a high degree of carbonisation is observedwith loss of the characteristic polymer structure. Both theseprocesses are followed by oxidation of the ion beam mod-ified polymers upon exposure to atmospheric oxygen [10].The two processes have been investigated by various

0168-583X/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.nimb.2008.02.063

* Corresponding author.E-mail address: [email protected] (A. Kondyurin).

authors using different polymers. [11–18]. However, thereare still no physico-chemical models for structural transfor-mations of polymers as a function of parameters such asion fluence, energy, ion species, temperature, post-treat-ment environmental exposure and so on. The lack of amodel is related to the difficulties associated with obtainingexperimental observations of structural changes in the thinpolymer layers undergoing modification. Real-time mea-surements of structural changes during implantation aredifficult to obtain. We propose a semi-empirical modelbased on the structural analysis of the modified polymersafter ion implantation for understanding the structuraltransformations observed.

Earlier we reported investigations of polystyrene thinfilms on silicon substrates subjected to nitrogen and argonion implantation with 20 keV ions [19,20]. The structurechanges and etching kinetics of the polystyrene film weredetermined by spectral methods. In this paper, we reportthe structural investigation of polystyrene films ionimplanted using argon ions with four different energies inorder to investigate the effect of ion energy.

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A. Kondyurin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1074–1084 1075

2. Experimental methods

Polystyrene (PS) films of 100 nm nominal thickness wereprepared by spin coating onto (100) silicon substrates(square of 10 � 10 mm) at 2000 rpm using a SCS G3P-8Spincoater. The spin coating solution consisted of polysty-rene (Austrex 400 from Polystyrene Australia Pty. Ltd.)dissolved to a concentration of 10 g/l in toluene (SigmaAldrich, Australia, Product No. 34866, purity >99.9%).The solution’s concentration and spin coating rate wereselected to give a homogenous film thickness over the entiresilicon wafer. The presence of toluene in the spincoated PSfilms was monitored by FTIR spectroscopy.

An inductively coupled radio-frequency (13.56 MHz)plasma was used as the source for plasma immersion ionimplantation (PIII). The base pressure of the vacuumchamber was 10�6 Torr. The pressure of argon duringimplantation was 3.3 � 10�4 Torr. The plasma power was100 W with reverse power of 12 W when matched. Acceler-ation of ions from the plasma was achieved by the applica-tion of high voltage (5–20 kV) bias pulses of 20 ls durationto the sample holder at a frequency of 50 Hz.

The silicon substrates coated with polystyrene film weremounted on a metal substrate holder. Ion implantationoccurred through a metal grid which was electrically con-nected to the holder and held parallel to the sample, 5 cmin front of it. The samples were treated for durations of10–3200 s, corresponding to implantation ion fluences of0.25 � 1015–20 � 1015 ions/cm2.

Ion fluence estimates were obtained from the number ofhigh voltage pulses multiplied by the fluence correspondingto one pulse. The fluence corresponding to one high voltagepulse (with bias 20 kV) was estimated in previous experi-ments on polyethylene films treated using the same PIIIprocess. The fluence in these experiments was calibratedby comparing the UV transmission spectra of the treatedpolyethylene films and with data obtained in previous PIIIand ion beam treatments with 20 keV ions and known flu-ences. The fluences (F) for biases (U) lower than 20 kVwere calculated using the following Child–Langmuir equa-tion [21,22]:

I � U 3=2; andF

F 20

¼ U 3=2

U 3=220

; ð1Þ

where I is current, F20 is the fluence for a bias ofU20 = 20 kV. In our case, 1 s PIII treatment time givesion fluences corresponding to applied biases as following:2.5 � 1013 ions/cm2 for 20 kV, 1.7 � 1013 ions/cm2 for15 kV, 0.89 � 1013 ions/cm2 for 10 kV and 0.31 � 1013

ions/cm2 for 5 kV. For all PIII treatments, satellite poly-ethylene films with softening transition temperatures of70 �C were placed on the sample holder together with thepolystyrene samples and used as controls to detect occur-rences of overheating.

For surface morphology studies, the ion implanted poly-styrene films were washed by soaking the silicon wafer sub-strates in pure toluene for 10 min and drying in open air.

The thicknesses and optical constants of the spun poly-mer films, before and after exposure to the PIII treatment,were determined using a Woollam M2000V spectroscopicellipsometer. Ellipsometric data was collected for fourangles of incidence: 60, 65, 70 and 75�. In the case of theuntreated film, a model consisting of a transparent Cauchylayer on top of the silicon substrate was sufficient toachieve a good fit to the data. When fitting the data col-lected for PIII treated samples, a model with a transparentCauchy layer was attempted initially. If a good fit was notobtained for all four angles of incidence then an absorptionfactor was added to the Cauchy layer in the model.

Transmission FTIR spectra were recorded before andafter plasma treatment using a Digilab FTS 7000 FTIRspectrometer. Difference spectra between those taken fromthe silicon wafers with and without the spun PS film werecalculated. The results of normal mode analysis of the PSmacromolecular vibrations were used to interpret the spec-tra. The optical density of spectral lines associated withparticular bond vibrations were used to quantify structuralchanges in the PS films.

Micro-Raman spectra (k = 532.14 nm) were obtained inthe backscattering mode using a diffraction double mono-chromator spectrometer HR800, Jobin Yvon, LabRamSystem 010. The spectral resolution was 4 cm�1 and thenumber of scans and integration time were varied to ensuresufficient signal-to-noise ratio. An optical microscope(Olympus BX40) with a �100 objective was used to focusthe laser beam and collect the scattered light. LabRam soft-ware was used to analyse the spectra.

Optical micrographs were obtained using a Carl ZeissJena microscope fitted with a digital video camera. Themorphology of the PS surface was examined using aLEO FEG scanning electron microscope (SEM) operatingat 2 kV. Atomic force microscopy (AFM) images were col-lected using a PicoSPM instrument in tapping mode. Anal-ysis of the AFM images was performed using the WS�Msoftware (version 3, Nanotec Electronica S.L. Spain).

The TRIM-95 and SRIM-2003 codes [23,24] were usedto predict the depth of Argon implantation into the PSfilms and the associated distribution of defects. The resultsof the calculation are presented in Fig. 1.

3. Results

The spun untreated polystyrene coating appears darkblue in colour due to the interference between light reflectedfrom the polystyrene surface and that reflected from the sil-icon–polystyrene interface. After treatment, the colourchanges according to the treatment time: for short treat-ment times the colour is brown and for long treatments itbecomes light gray. The colour on both the untreated andthe treated samples is uniform over the whole sample sur-face, indicating a uniform modification of the surface.

In this study, the wetting angle was measured 2 daysafter treatment. The wettability of the polystyrene surfacewas found to increase due to the PIII treatment, as shown

0

10

20

30

40

50

60

70

80

90

100

50 10 15 20

Fluence, *1015 ions/cm2

Wat

er w

etti

ng

an

gle

, deg

rees

Fig. 2. Wetting angle of water on PIII treated polystyrene films as afunction of ion fluence for 20 keV (circles), 10 keV (squares) and 5 keV(triangles) ion energies.

0

0.05

0.1

0.15

0.2

0.25

0 20 40 60 80 100 120

Depth, nm

Car

bo

nva

can

cy, v

ac/A

/ion

Sili

con

subs

trat

e

Polystyrene film

20 keV

15 keV

10 keV

5 keV

Fig. 1. Distribution of carbon vacancies in a polystyrene film on a siliconsubstrate caused by the implantation of Argon ions with different energiescalculated using TRIM.

1076 A. Kondyurin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1074–1084

in Fig. 2. The water wetting angle decreased from 90� (forthe untreated film) to 60� for the treated films. The fluencehas a minor influence on the wetting angle. At high fluence(2 � 1015 ions/cm2), the wetting angle increases slightly to63�. The wetting angle does not depend on the ionimplanted energy.

The surface morphology of the spincoated polystyrenefilm on the silicon substrate corresponds to the morphol-ogy of the silicon substrate itself. The polystyrene surfacemorphology, as observed in optical micrographs, scanningelectron micrographs and AFM images, did not changeafter PIII for any of the fluences and ion energies investi-gated. The root mean square (RMS) roughness for thepolystyrene surface initially and after PIII is comparable

Table 1Roughness (RMS by AFM image) of polystyrene surface on silicon wafer aft

PIII fluence, �1015 ions/cm2 0 1.5 5RMS on 2 lm scale, nm 0.25 0.22 0.2RMS on 10 lm scale, nm 0.45 0.44 0.5

to the RMS roughness of the silicon wafer (approximately0.25 nm measured in a 2 � 2 lm AFM image) (Table 1).

The Raman spectrum of the bulk polystyrene prior toion implantation shows lines at 1605, 1586, 1452, 1201,1184, 1157, 1033, 1003, 797 and 623 cm�1, correspondingto vibrations of the polystyrene macromolecules (Fig. 3).The Raman spectrum of the 100 nm untreated polystyrenefilm on a silicon substrate is similar to the bulk polystyrenespectrum only it has lower intensity peaks. Besides thepolystyrene lines, lines associated with silicon vibrationsare observed at 979–943 (overlaid peaks), 826, 672 and622 cm�1.

In contrast, the Raman spectrum of a polystyrene film,PIII treated with fluence of 2 � 1016 ions/cm2 and 20 kVpulse bias, shows intense, wide peaks at 1543 and1384 cm�1. The lines of the polystyrene vibrational modesare no longer observed. The 1543 cm�1 peak correspondsto the E2g vibration mode and 1384 cm�1 peak correspondsto the Ag vibrational mode of a graphitic ring. Followingthe model of Raman spectra for carbon structures [25],the observed spectrum corresponds to glassy carbon.According to the position of the G-peak at 1546 cm�1

and the ID/IG ratio of 0.65, the carbonised structure con-tains nanocrystalline graphitic clusters of about 1 nm insize with sp3 hybridized of carbon atoms on the edges ofthe clusters.

The optical properties as determined by ellipsometrychange due to the PIII treatment. As the PIII fluenceincreases the films become optically denser (Fig. 4). Theirrefractive index increases from 1.6 (untreated) to 2.0–2.2(for films after 20 kV PIII treatment with a fluence of2 � 1016 ions/cm2). Such a high refractive index is typicalof a dense carbon structure. The untreated polystyrene filmdoes not show dispersion of the refractive index, while thePIII treated film does. At low fluence, the refractive indexdecreases with wavelength and the dispersion is positiveover the visible wavelength range. At high PIII fluences,the refractive index shows anomalous dispersion (increas-ing with wavelength for short wavelengths up to 530–580 nm) and normal dispersion at long wavelengths. Thisindicates that after high fluence PIII treatment the filmbecomes absorbing at short wavelengths as shown by theextinction coefficient as a function of wavelength (Fig. 5).

PIII treatment at low fluence does not cause absorbancein the polystyrene film. The absorbance appears only at flu-encies higher than 2 � 1015 ions/cm2. The extinction coeffi-cient is wavelength dependent such that a higher extinctioncoefficient is observed at shorter wavelengths. Followingprevious interpretation of the absorbance spectra of poly-mers after ion beam implantation [26], the absorbanceand refractive index changes correspond to the introduc-

er Ar+ PIII with 20 keV energy of ions

20 0.5 (washed in toluene)4 0.25 30.29 0.35 30.2

50

100

150

200

1600 1400 1200 1000 800 600

Wavenumber, cm -1

Ram

anin

ten

sity

, a.u

.

a

b

c G

D

Silicon lines

Fig. 3. Micro-Raman spectra from (a) bulk polystyrene, (b) 100 nm spincoated polystyrene on a silicon substrate, and (c) a polystyrene coating PIIItreated with 20 keV Ar+ ions for 400 s to a fluence of 1016 ions/cm2.

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

400 500 600 700 800 900 1000

Wavelength, nm

Ind

ex o

f ref

ract

ion

Fig. 4. Refractive indices of polystyrene films on silicon substrates, PIIItreated with 20 kV pulse bias. Treatment times (from bottom to top) are 0,10 s, 40 s, 80 s, 400 s and 800 s and corresponding ion fluences (frombottom to top) are 0, 2.5 � 1014, 1015, 2 � 1015, 1016, 2 � 1016 ions/cm2.

0.0

0.1

0.2

0.3

0.4

0.5

400 500 600 700 800 900 1000

Wavelength, nm

Ext

inct

ion

coef

ficie

nt

Fig. 5. The extinction coefficient of polystyrene films on silicon substratestreated with PIII at 20 kV bias. Treatment times (from bottom to top) are0, 80 s, 400 s and 800 s and corresponding fluences (from bottom to top)are: 0, 2 � 1015, 1016, 2 � 1016 ions/cm2.

1.6

1.7

1.8

1.9

2

2.1

2.2

0 500 1000 1500 2000 2500 3000 3500

Time, sec

Ref

ract

ive

ind

ex a

t 50

0 n

m

Fig. 6. Refractive indices as a function of treatment time of polystyrenefilms treated with PIII at applied bias’ of 20 kV (rhombi), 15 kV (squares),10 kV (triangles), 5 kV (circles) and plasma treated with 0 bias (stars).


tion of electron energy levels with lower energy comparedto the initial polystyrene film. They are due to unsaturatedcarbon–carbon bonds like polyene and aromatic conju-gated structures. The red shift of the absorbance is causedby the growth of conjugated aromatic and polyene struc-

tures, where delocalisation of valence electrons takes place.The absorbance at short wavelengths corresponds to smallconjugated unsaturated structures and the absorbance atlong wavelengths corresponds to large conjugated unsatu-rated structures [27]. The stronger absorbance at shortwavelengths indicates that the concentration of small con-jugated structures in PIII treated polystyrene is higher thanthat of the large conjugated structures.

The refractive index of the treated polymer depends onthe energy of the ions implanted. Fig. 6 shows the refractiveindex at 500 nm of the polystyrene films after PIII with arange of applied biases. The refractive index increases withPIII treatment time and with applied bias. At long treat-ment times the refractive index saturates. The saturationlevel is higher for higher applied biases and correspond-ingly higher ion energy. A similar dependence is observedfor the extinction coefficient of the polystyrene film(Fig. 7). The extinction coefficient increases with PIII treat-ment time and with ion energy and also saturates at highlevels of both parameters.

The increase in the refractive index and extinction coef-ficient for increasing energy PIII treatments is explained by

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 500 1000 1500 2000 2500

Time, sec

Ext

inct

ion

co

effi

cien

t at

500

nm

Fig. 7. Extinction coefficient as a function of treatment time of polysty-rene films treated with PIII using applied bias’ of 20 kV (rhombi), 15 kV(squares), 10 kV (triangles), 5 kV (circles) and by plasma without bias(stars).

0

0.03

0.06

0.09

0.12

0.15

0 5 10 15 20

Fluence, 1015 ios/cm2

Etc

h r

ate,

nm

/sec

Fig. 9. Etch rate as a function of ion fluence of polystyrene films treatedwith PIII in argon with applied bias of 20 kV (rhombi), 15 kV (squares),10 kV (triangles) and 5 kV (circles).


an increasing concentration of unsaturated conjugatedstructures in modified polystyrene film as the ion energyof the treatment increases. The low energy ions do not pen-etrate as deeply into the polystyrene film and therefore themodified region makes a smaller contribution to the mea-sured optical properties.

Fig. 8 shows that the thickness of the polystyrene film,as determined by ellipsometry, decreases with treatmenttime due to etching. In plasma without PIII bias the filmis etched at a constant rate of 0.0366 nm/s. The thicknessof the polystyrene film in plasma when PIII bias isapplied shows a more complicated behaviour. Fig. 9shows the etching rate calculated as the time derivativeof the thickness. For all biases the film is etched moststrongly at the beginning of the PIII treatment. At biasof 5 keV the rate of etching is initially equal to that seenin plasma without bias while it is higher with higher biasvalues increasing with bias. With fluence the etching ratedecreases for all biases. The rate of decrease of the etch-ing rate with fluence is higher for higher applied biases.The etching rates for all applied biases decrease to a levelof approximately 0.0156 nm/s at fluences of around

0

10

20

30

40

50

60

70

80

90

100

0 1000 2000 3000

Time, sec

Th

ickn

ess,

nm

Fig. 8. The thickness as a function of treatment time of polystyrene filmstreated with argon PIII using applied bias’ of 20 kV (rhombi), 15 kV(squares), 10 kV (triangles), 5 kV (circles), and no applied bias (crosses).

20 � 1016 ions/cm2. This etching rate is lower than theconstant etching rate observed in plasma without bias(0.0366 nm/s).

The FTIR transmission spectrum obtained from theuntreated spincoated polystyrene films corresponds to thatobtained from bulk polystyrene but with lower intensitypeaks due to the thickness being only 100 nm. As shownin Fig. 10, the spectrum shows lines due to aromatic ringvibrations at 3081, 3060, 3026, 1602 and 1493 cm�1 anddue to the aliphatic backbone of the polystyrene macro-molecule at 2922, 2851 and 1452 cm�1. The intensities ofthe lines decrease after plasma treatment, even in the casewhen no bias is applied to the substrate, due to decreasesin thickness caused by etching.

Fig. 11 shows FTIR transmission spectra of a poly-styrene film after PIII treatment using a 10 keV bias. Inaddition to the decrease in intensity observed in thecharacteristic polystyrene peaks, a new broad band isobserved in the 1750–1600 cm�1 region. This band has acomplex shape arising from overlapping lines attributableto m(C@O) stretch vibrations in carbonyl, carboxyl, alde-hyde and ester groups. The appearance of these new linesis due to the oxidation of the PIII treated polystyrene filmin atmosphere and they are similar for all applied biases.This band does not occur after long treatment times(>300 s for 10 keV bias). Atmospheric oxidation aftertreatment also creates a weaker broad band in of the3600–3400 cm�1 spectral region. This band is attributedto m(O–H) stretch vibrations in hydroxyl, carboxyl andhydroperoxide groups.

Fig. 12 shows the intensity of the 2922 cm�1 line attrib-uted to m(C–H) stretch vibrations in –CH2– groups of themacromolecule backbone. The intensity decreases withtreatment time at a rate which depends on the bias appliedduring the PIII treatment. The decrease in intensity of thisline is due to both the thinning of the film due to etchingand the structural transformations of the polystyrenemacromolecules due to ion bombardment. To analyse thestructural transformations we must eliminate the effect ofthickness reductions. This was achieved by normalising

0

.002

.004

.006

.008

3500 3000 2500 2000 1500

Wavenumber, cm-1

Ab

sorb

ance

, a.u

.

Fig. 10. FTIR transmission spectra of a polystyrene film on a silicon substrate after argon plasma treatment without applied bias. The spectrum from thesilicon wafer is subtracted. The treatment time (from bottom to top) is: 0, 60, 300, 600, 1200 and 1800 s. Arrays show lines of polystyrene aromatic ringvibrations.

.002

.006

.01

.014

3500 3000 2500 2000 1500

Wavenumber, cm-1

Ab

sorb

ance

, a.u

.

ν(OH) ν(C=O)

ν(C=C)

Fig. 11. FTIR transmission spectra of a polystyrene film on a silicon substrate after argon PIII treatment using a 10 keV bias. The spectrum from thesilicon wafer is subtracted. The treatment time (from bottom to top) is 0, 112 s, 224 s, 1120 s, 2240 s and corresponding ion fluence (from bottom to top) is:0, 1015, 2 � 1015, 1016, 2 � 1016 ions/cm2. Arrays show lines of new structures after PIII treatment.


the intensity of the lines by the thickness of the film asdetermined by ellipsometry.

According to the Bouger–Lambert–Beer law, the lineabsorbance (D) is given by

D ¼ e � h � C;

where e is extinction coefficient, h is thickness of the poly-styrene film and C is the concentration of the functionalgroups responsible for the vibration associated with theline in question. Because e is a constant for any given line,the ratio D/h is proportional to the concentration of the

functional groups responsible for the vibration. We there-fore used this ratio to analyse structural changes in the filmdue to PIII treatment.

The polystyrene macromolecule has two main structuralcomponents: an aliphatic backbone and aromatic siderings. Fig. 13 indicates changes in the aromatic rings astracked by (a) the 1493 cm�1 line associated with the defor-mation aromatic ring vibrations and (b) the 3026 cm�1 lineassociated with the m(C–H) stretch vibrations of the aro-matic rings. Fig. 14 indicates the changes in the aliphaticbackbone as tracked by (a) the 2922 cm�1 m(C–H) stretch

0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000Time, sec

D(2

922)

, a.u

.

Fig. 12. Absorbance of the m(CH2) = 2922 cm�1 line in the FTIR spectraof PIII treated polystyrene films as a function of treatment time. AppliedPIII biases were: 20 kV (rhombi), 15 kV (squares), 10 kV (triangles), 5 kV(circles), no applied bias (stars).

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20

Fluence 1015 ions/cm2

D(1

452)

/thic

knes

s,a.

u.

Fig. 14b. Absorbance of the d(C–H) = 1452 cm�1 line as a function ofPIII fluence. Applied PIII biases are: 20 kV (rhombi), 15 kV (squares),10 kV (triangles), and 5 kV (circles).

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25

Fluence, 1015 ions/cm2

D(3

026)

/thic

knes

s,a.

u.

Fig. 13b. Absorbance of the m(C–H) = 3026 cm�1 line as a function ofPIII fluence. Applied PIII biases were: 20 kV (rhombi), 15 kV (squares),10 kV (triangles), and 5 kV (circles).

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20


D(2

922)

/th

ickn

ess,

a.u

.

Fig. 14a. Absorbance of the m(CH2) = 2922 cm�1 line as a function of PIIIfluence. Applied PIII biases are: 20 kV (rhombi), 15 kV (squares), 10 kV(triangles), and 5 kV (circles).

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25


D(1

493)

/thic

knes

s,a.

u.

Fig. 13a. Absorbance of the m(aromatic ring) = 1493 cm�1 line as afunction of PIII ion fluence. Applied PIII biases were: 20 kV (rhombi),15 kV (squares), 10 kV (triangles), and 5 kV (circles).


vibrations and (b) the 1452 cm�1 line associated with d(C–H)deformation vibrations in –CH2– groups. The intensity

changes with both treatment time and corresponding ionfluence are presented for all the biases studied.

The normalized intensity of these lines after plasmatreatment without bias is not shown because no changeswere detected.

The normalized intensity of the lines associated witharomatic ring vibrations decreases with treatment timeand fluence of PIII for all biases indicating decreasing con-centration of aromatic rings in the modified films. The rateof reduction of the aromatic ring concentration increaseswith ion energy. For the highest bias, 20 kV, the aromaticrings disappeared at high fluence, while for lower biases(5–15 kV) the residual concentration of the aromatic ringswas between 40% and 60% of the concentration in theuntreated films.

The aliphatic structures were found to be more resistantto ion damage. Even at highest fluence with 20 kV biasapplied, the residual concentration of hydrocarbon groupsis about 30–40% of the initial concentration, while for lowbias the residual concentration is about 80%.


The creation of aliphatic structures as a result of radia-tion damage to the polystyrene macromolecules is alsolikely to contribute to the relatively high concentration ofaliphatic groups found in the PIII treated films. A polysty-rene macromolecule monomer has 8 carbon atoms ofwhich 6 form an aromatic ring. If one of the carbon atomsin the ring is displaced due to a collision with an implantedhigh energy ion, the aromatic ring breaks reducing theFTIR signal from aromatic vibrations. The broken ringcan remain jointed to the polymer backbone and aftersome reactions with neighbouring groups it can becomea stable aliphatic side chain. Thus, destruction of thearomatic rings can lead to the formation of additionalaliphatic structures. Therefore, the intensity of lines associ-ated with aliphatic group depends on two processes: thefirst being ion damage to the aliphatic fragments of thepolystyrene macromolecules and the second being the for-mation of additional aliphatic structures as a result of iondamage to aromatic rings.

Fig. 15 shows how the intensity of carbonyl group linevaries with PIII treatment time and correspondingly withthe ion fluence. According to Bouger–Lambert–Beer lawthe intensity change of with fluence of PIII treatment isinterpreted as the concentration variation of oxygen-con-taining groups in polystyrene modified film. Therefore,the concentration of oxygen-containing groups, which iscorrelated with the intensity of the m(C@O) line, increasesfor ion fluence up to 1015 ions/cm2. For higher PIII treat-ment time and fluence it decreases. Such a maximum isobserved for all applied high voltage biases investigated.The maximal concentration of oxygen-containing groupsdepends on the applied bias with the highest concentrationobserved for the highest applied bias (20 kV). Similardependences of the line intensity are observed for m(O–H)vibrations.

We studied the level of cross-linking in the polystyrenefilms after PIII treatment by exposing the films to toluene.The untreated spincoated polystyrene films dissolved andwere washed away after a few seconds in toluene whilefilms PIII treated with high fluence could not be dissolved.

0

0.0005

0.001

0.0015

0.002

0.0025

0 5 10 15 20


D(1

710)

, a.u

.

Fig. 15. Absorbance of the m(C@O) = 1710 cm�1 line as a function of PIIIfluence. Applied PIII biases are: 20 kV (rhombi), 10 kV (triangles), and5 kV (circles).

Films treated with intermediate fluence swelled in tolueneand changed colour as the interference condition for lightreflected from the interfaces changed. The FTIR transmis-sion spectra, including carbonyl group lines and intensityof the polystyrene vibration lines, show no effects of expo-sure to solvent once the solvent is evaporated and the driedfilm regains its pre-swelling colour. The optical constantsand thickness of the films after swelling in toluene and dry-ing return to pre-swelling values. These results indicate thatthe films are completely crosslinked after PIII such thateach polystyrene macromolecule has as minimum onecrosslink with neighbouring macromolecules.

The surface morphology of some of the treated filmschanges after swelling in toluene and drying. Opticalmicrographs, AFM images and scanning electron micro-graphs show the appearance of a wrinkled structure onthe surface after swelling in toluene and drying (Figs. 16–18). The optical micrographs shown in Fig. 16 show thatthe surface morphology depends on the implanted ion flu-ence. At high fluence (2 � 1016 ions/cm2) the surfacemorphology does not change after swelling in toluene. Atintermediate fluences (about 1015 ions/cm2) there are areas,where the surface remains smooth as well as regions, wherethe film is wrinkled. At low fluence (about 1014 ions/cm2)the surface becomes highly wrinkled. With increasing flu-ence the wrinkled areas decrease and smooth areasincrease. We did not observe any order or orientation ofthe wrinkles, which could be caused by flows of the solu-tion during spincoating. AFM measurements (Fig. 17)show that the height of some wrinkles reaches 60 nm,which is close to the thickness of the initial polystyrene film(100 nm). The SEM image of a fractured edge (Fig. 18)shows that the wrinkles are not associated with peeling ofthe polystyrene film from the silicon substrate. Since theFTIR spectra do not show any additional lines after swell-ing in toluene, there are no any additional compounds toelevate the wrinkles. The wrinkles must therefore beformed by a rearrangement of the crosslinked polystyrenemacromolecules during the exposure to toluene. At low flu-ence the crosslink density is low and the polystyrene mac-romolecules have enough freedom to move in the toluenesolution. At high fluence, the crosslink density is highenough to stop the movement so the film is stable in solu-tion. The morphology of the films treated with intermedi-ate fluences shows that these films are crosslinkedinhomogeneously with some parts being highly crosslinkedand therefore remaining smooth while other regions have alower density of crosslinks and wrinkle after exposure tothe toluene solution.

4. Discussion

The structural changes we observed in polystyrene filmssubjected to plasma immersion ion implantation are causedby collisions between the high energy ions and atoms orelectrons in the polystyrene macromolecules. Due to thecollisions, the energy of the penetrating ions is transferred

Fig. 16. Optical micrographs of polystyrene films on silicon substrates after 20 kV PIII treatment with argon ions, washed in toluene and subsequentlydried. The ion fluence is noted in ions/cm2 and the treatment time in seconds. The size of microphotographs is 1 � 1.2 mm.

Fig. 17. AFM image of a polystyrene film on a silicon wafer after 20 kVPIII with a low ion fluence of 0.5 � 1015 ions/cm2, and subsequent swellingin toluene and drying.


to atom, electron and phonon excitations, resulting in thebreaking of chemical bonds and the formation of free rad-icals. As a result, the polystyrene macromolecules becomefragmented and atoms, ions and molecular fragments arereleased from the film. Some of the fragments react witheach other to form new structures and with residual macro-

molecules to form crosslinks. Hydrogen is released and thefilm becomes carbon-rich. The carbon atoms become orga-nized in graphite-like and diamond-like structures. Whenthe treated sample is exposed to air, the active free radicalsreact with oxygen and after a chain of radical transforma-tions the oxygen-containing groups appear as stable struc-tures in treated film.

These structural transformation processes depend onthe fluence and the energy of the implanted ions. At lowfluence, the concentration of fragmented macromoleculesand molecular fragments is low. The etching rate is highand increases with the ion energy. The carbon fraction intreated film increases rapidly with fluence as does the con-centration of active free radicals and hence the concentra-tion of oxygen-containing groups upon exposure toatmosphere. The density of crosslinks is low and polysty-rene macromolecules have the freedom to change confor-mation or move under the influence of solvent.

At high fluence, the carbonisation of the film becomessaturated as the polystyrene macromolecules become heav-ily fragmented in the layer corresponding to the ion pene-tration depth. The optical constants approach those ofgraphite and the etching rate decreases towards that forgraphite. The concentration of free radicals decreases mak-ing the film less reactive in oxygen. The concentration ofoxygen-containing groups therefore decreases. The densityof crosslinks is high and the film becomes insoluble.

Fig. 18. SEM micrographs of polystyrene films on silicon substrates after washing in toluene: (a) PIII 20 � 1015 ions/cm2 ion fluence at 20 keV, (b) PIII0.75 � 1015 ions/cm2 at 20 keV, (c) and (d) PIII 1 � 1015 ions/cm2 at 5 keV.


The transition between the low and high fluence regimesoccurs at about 1015 ions/cm2 for all applied biases (i.e. ionenergies) investigated. Therefore, the structure transforma-tions occur at about the same rates in the modified layer,and only the thickness of the modified layer depends onthe energy of the ions. For 20 kV bias, the penetration depthof the ions is roughly equal to the thickness of the film athigh fluence. As a result, the remaining film does notcontain aromatic structures and the content of hydrogen-containing groups is minimal. For energies lower than20 keV, the thickness of the film is greater, than the ion pen-etration depth, so the bottom layer of the film containsresidual polystyrene macromolecules. The optical constantswhich are averaged over the film structure do not thereforereach values corresponding to graphite structures.

The wrinkles observed at low fluence are associated withthe formation of strongly crosslinked and densified regionsformed on the surface of the film while the underlayer isminimally altered from the initial structure. This resultsin a swelled free flowing underlayer with a skin on the sur-face after the introduction of solvent. The formation ofwrinkles indicates that the interfacial force between theskin and underlying material is lower than the forcesbetween two parallel layers of skin. The system energybeing minimised by the wrinkling could originate fromminimisation of surface energy, formation of chemicalbonds or the effects of Van der Waals forces between thedenser skin outweighing those between the skin and theunderlying material, or in fact a combination of these. Thisinterpretation is strongly supported by the morphology of

the wrinkles, which is closed and high as opposed torounded. The mobility afforded by the presence of solventallows the wrinkles to form. At very high fluences thismobility is inhibited by cross-linking in the underlayer.

5. Conclusions

Polystyrene films spincoated on silicon substrates weretreated by argon plasma immersion ion implantation witha range of ion energies. The effects of carbonisation, oxida-tion and cross-linking were observed after implantation.Two regimes of ion implantation fluence are observed: atlow fluence, the effects of cross-linking and oxidation havea dominant influence on the polystyrene structure; at highfluence, the effect of carbonisation is dominant until itresembles the final structure of carbon like material atthe highest fluence of 2 � 1016 ion/cm2. These structuretransformations occur with the same rate for all appliedenergies of ions, but in different layers in dependence onion penetration depth. The degree of etching in the polysty-rene films is determined by the degree of carbonised surfacelayer of the treated polystyrene layer.

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