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Materials Chemistry and Physics 130 (2011) 702– 707

Contents lists available at ScienceDirect

Materials Chemistry and Physics

jo u rn al hom epage : www.elsev ier .com/ locate /matchemphys

hemical changes in PMMA as a function of depth due to protoneam irradiation

.Z. Szilasi a,∗, R. Huszanka, D. Szikrab, T. Váczi c, I. Rajtaa, I. Nagyd

Institute of Nuclear Research of the Hungarian Academy of Sciences, H-4026 Debrecen Bem tér 18/c, H-4001 Debrecen, P.O. Box 51, HungaryUniversity of Debrecen, Institute of Nuclear Medicine, H-4032 Debrecen, Nagyerdei krt. 98, HungaryInstitute for Nanotechnology, Bay Zoltán Foundation for Applied Research, H-3515 Miskolc-Egyetemváros, P.O. Box 46, HungaryDepartment of Physical Chemistry, University of Debrecen, P.O. Box 7, H-4010 Debrecen, Hungary

r t i c l e i n f o

rticle history:eceived 19 January 2011eceived in revised form 22 June 2011ccepted 15 July 2011

eywords:

a b s t r a c t

In this work we determined depth profiles of the chemical change in PMMA irradiated with 2 MeV pro-tons by infrared spectroscopic and micro-Raman measurements. The measurements were carried out on10 �m thin stacked foil samples using an infrared spectrometer in universal attenuated total reflectance(UATR) and transmission modes; while the thick samples were analyzed with a confocal micro-Ramanspectrometer. The depth profiles of the changes formed due to the various delivered fluences were

olymerson implantationadiation damageaman spectroscopy and scattering

compared to each other.The measurements show the strong dependence of the degree of modification on the energy transfer

from the decelerating protons. Depth profiles reveal that at the fluences applied in this work the entireirradiated volume suffered some chemical modifications. In case of low-fluence samples the zone ofmaximal modification is restricted only to the Bragg peak, but with increasing fluences the region ofmaximal modification extends towards the sample surface.

. Introduction

Poly(methyl methacrylate) (PMMA) is a commonly used resistaterial in proton beam writing (PBW). This material is often used

o create 3D microstructures [1] with different wet chemical etch-ng methods [2,3] which are based on the chemical change of PMMA4]. With the PBW method, fabrication of certain microstructuresuch as integrated or buried waveguides without any developingethods is also possible [5,6]. The key feature in the operation of

he structures which do not need any further development afterrradiation is the physical and the chemical change of the materialroperties.

When energetic particles penetrate the medium and depositnergy, they create damage along the trajectory. As the energy ofhe charged particles decreases, the interaction cross-section withhe surrounding atoms increases, so immediately before the par-icles come to rest a peak occurs in the energy loss. This peak ishe Bragg peak, the curve of the energy loss is called the Braggurve. The shape of the curve is determined by the nature of energy

oss which has two modes: nuclear stopping and electronic stop-ing. Nuclear stopping arises from collisions between the incidentnergetic ions and target nuclei, causing atomic displacements.

∗ Corresponding author. Tel.: +36 52 509 200; fax: +36 52 416 181.E-mail address: szilasi@atomki.hu (S.Z. Szilasi).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.07.048

© 2011 Elsevier B.V. All rights reserved.

The electronic stopping is the result of the interaction betweenthe charged bombarding particles and target electrons, resultingthe formation of free radicals. At the majority of the path of anenergetic ion electronic stopping dominates causing the majorityof the occurred chemical changes; nuclear stopping prevails onlyat the end-of-range region. The resulting chemical change, whichis investigated in this work, occurs as the sum of these stoppinginteractions along the ion’s path.

The energy deposition in the material occurs discretely, not con-tinuously, because there is an energy width over which absorptioncan take place. The discrete energy levels play an important rolefor example in ionization when the electrons have to overcome apotential energy barrier to be released from the orbit; similarly,the atomic displacements also require threshold energy to movethe atom over a potential barrier and break the bonds. The elec-tronic excitation is restricted by quantized energy levels as well.During ion irradiation all of these processes take place in the irra-diated volume of the target material. The molecules that becomeexcited by absorbing energy from the penetrating ion can return tothe ground state through radiationless decay or undergo homolyticdissociation reactions and form free radicals. These radicals are veryreactive and able to cause numerous chemical reactions in organic

materials.

In PMMA due to low-fluence irradiation with hydrogen ions,chain scissioning prevails [7], making it to be a positive resist forlithography [8]. During low linear energy transfer (LET) radiation

S.Z. Szilasi et al. / Materials Chemistry a

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Fig. 1. Sketch of a PMMA sample prepared for Raman measurements.

he scission probability remains constant, but for high-LET irradia-ion it decreases with increasing LET. The critical LET for PMMAs a few keV �m−1 [9]. For the radiation sensitivity the quater-ary carbon atoms of the main chain are responsible [10]. Whenhe polymer main chain splits, functional groups brake away andolatile products, like H2, CH4, CO2 form [11]. The principal gaseousroducts are formed by the elimination reaction of the esterroup.

Since the energy transfer of the protons to the surroundingedium (stopping power) varies according to the Bragg curve, the

hemical structure of the irradiated polymer changes as a functionf depth also in a nonlinear way. At energetic ion irradiation thenergy transmission to the medium is higher as the ion slows down,he breakage of more chemical bonds are expected in the depth ofhe material. The highest chemical change is expected where thenergy loss reaches its maximum, at the Bragg peak.

The change of the chemical structure of PMMA due to ioniz-ng radiation [12] has already been investigated in earlier studies13,14] but there are no reports about the depth dependenthanges. The aim of this work was the investigation of the depthrofile of the chemical changes in poly(methyl methacrylate) dueo 2 MeV proton beam irradiation.

To analyze the composition of solid sample surfaces oftenecondary ion mass spectrometry (SIMS) and secondary neutralass spectrometry (SNMS) techniques are used, but due to their

ampling methods these are not appropriate for our purposes.o understand the process of chemical modification as a func-ion of depth, an infrared spectrometer in transmission and totaleflectance (UATR-FTIR) modes and a micro-Raman spectrometerere used. Since PMMA is a very versatile resist material, studying

he above mentioned processes can contribute to the better under-tanding of the connection between the changes of the physical (e.g.efractive index, hardness) and chemical properties due to particlerradiation. The results can also be important in the field of polymer

icromachining, e.g. for proton beam writing (PBW).

. Experimental

The different characterization techniques required different sample prepara-ions to be able to carry out the necessary measurements regarding to variousepths. For the transmission and total reflectance infrared spectroscopy measure-ents thin PMMA foils were created. Poly(methyl methacrylate) powder (moleculareight 350,000, Alfa Aesar GmbH & Co KG) was dissolved in toluene and spin-

oated on a glass substrate. The ∼10 �m thick layers (density 0.95 g cm−3) wereemoved from the substrate, ten such layers were stacked on each other and irradi-ted. The layers were separated after irradiation and observed one by one with IRpectroscopy.

For micro-Raman measurements small blocks of the size ca. 20 mm × 4 mm wereut from a 3 mm thick PMMA sheet (GS233, Röhm; density 1.19 g cm−3) and polishedn the cut surface. Two such blocks were attached with the original (uncut) surfacesn contact to avoid direct proton exposure on that side; the polished faces formed

flat surface. The irradiation was done from the direction of the polished side, theroton beam was centred on the line of attachment. Raman measurements were

arried out on the original PMMA surfaces after the separation of the attached blocksFig. 1).

All the irradiations in this work have been performed in the proton-induced X-ay emission (PIXE) chamber of the Institute of Nuclear Research of the Hungariancademy of Sciences (HAS–ATOMKI), Debrecen, Hungary [15]. The energy of the

nd Physics 130 (2011) 702– 707 703

incident protons was 2 MeV, the size of the proton beam was 5 mm in diameter,and it was made homogeneous using a 0.51 �m thin Ni foil. The target chamber isisolated allowing the measurement of absolute current and charge.

In the case of the stacked PMMA foils a 6.25 × 1013 ions cm−2 (100 nC mm−2)fluence was delivered with 1 nA beam current. For the micro-Raman measurementsthree samples were prepared and irradiated with the following proton flu-ences: 1.25 × 1014 ions cm−2 (200 nC mm−2), 3.13 × 1014 ions cm−2 (500 nC mm−2)and 6.25 × 1014 ions cm−2 (1000 nC mm−2). The beam current was 2 nA. The abovefluences were chosen to avoid cross-linking that occurs at higher fluences [16]and the lower fluences play an important role in ion beam lithography e.g. atPBW.

During the irradiations PIXE spectra were collected by a Canberra Si(Li) detec-tor and the ATOMKI NZ-881 digital signal processor. On the spectra there were nocharacteristic X-ray lines showing that the PMMA samples were free from impuri-ties. Charging was eliminated by using a carbon filament electron source. The shapeof the background spectra showed that there was no significant charging of thesamples.

For IR spectroscopy two different spectrometer setups were used to study thePMMA foils. A Perkin-Elmer Spectrum One infrared spectrometer was used eitherin transmission or attenuated total reflectance (ATR) mode using a DTGS detector(4 cm−1 resolution) with the spectral range of 650–4000 cm−1. In these measure-ments 4 scans were applied and the data were collected and analyzed using theSpectrum ES 5.0 software. The transmission measurements give information aboutthe average chemical composition of the entire thickness of each polymer film,because the infrared beam passes through the whole sample. The proton-irradiatedarea was 5 mm in diameter, but the sample holder for these measurements restrictedthe IR-illuminated area to a 3 mm diameter circle at the centre of the irradiated spot.This way the illumination of the unirradiated parts of the foils was avoided.

In ATR mode the sample is placed onto the surface of a diamond prism andthe infrared radiation is collected after total reflection from the diamond–sampleinterface, therefore only a thin surface layer of each film was sampled with thismethod.

Confocal micro-Raman spectroscopy was used due to its inherently better spa-tial resolution compared to IR methods. These measurements were done withan Ntegra Spectra (NT-MDT) edge-filter-based dispersive Raman spectrometerattached to an Olympus IX71 microscope. The 473 nm emission of a 50 mWfrequency-doubled Nd:YAG laser was used for excitation; the laser power wasreduced so that the sample received not more than 2 mW. A 100× (N.A. 0.9) objec-tive was used to focus the laser beam onto the sample and collect Raman-scatteredlight. A 520 mm focal length spectrometer with a 600 grooves mm−1 grating deliv-ered Raman light onto a thermo-electrically cooled CCD detector. The exposure timewas 3 s per step for line scans and 300 s for point analyses. A confocal pinhole, act-ing also as the entrance slit of the spectrometer, of 100 �m was used for line scansand 25 �m for point analyses. The resulting spectral resolution was 8–9 cm−1 forline scans and better than 4 cm−1 for point analyses. Raman shifts were calibratedusing the elastic scattering from the laser (Rayleigh line) and a polystyrene standard;wavenumber accuracy was better than 1 cm−1.

Raman line scans were recorded with a piezo sample stage; each depth profileshown is an average of four line scans. To avoid artefacts related to surface uneven-ness, the beam was focussed 3 �m below the surface at the irradiated edge of eachsample. As a consequence, lateral resolution was worse than the 1 �m diameter ofthe focal spot: a weaker, lower-resolution signal from the light cone is superposedon the high-resolution data from the focus. To collect all data, an oversampling stepwidth of 0.5 �m was used. Since line scans were recorded parallel to the irradiationdirection, the depth resolution (i.e., in the microscopic z dimension) in this case isirrelevant.

Proton trajectories and proton–matter interaction distributions were calculatedwith the SRIM code [17].

3. Results and discussion

3.1. Attenuated total reflectance infrared spectroscopy(ATR-FTIR) measurements

The 2 MeV protons cannot penetrate through the ten layers of10 �m thick polymer films because the maximum range is approx-imately 84 �m in this material (density of the PMMA prepared thisway is ∼0.95 g cm−3, see Fig. 3). The Bragg peak (the place of themaximum energy loss) is at about 80 �m depth.

The IR spectra of the irradiated foils representing differentdepths are shown in Fig. 2 in various colours. The characteris-tic peaks of the functional groups were identified and listed in

Table 1.

The breakage of chemical bonds, the free radicals formation andunsaturation result in a decrease of peak heights in the IR spectra. Todetermine the percental change of the peak heights in the spectra,

704 S.Z. Szilasi et al. / Materials Chemistry and Physics 130 (2011) 702– 707

Fig. 2. The ATR-FTIR spectra of the initial and irradiated PMMA samples at different depzoomed in. (For interpretation of the references to color in this figure legend, the reader i

Table 1Peak assignments for the FTIR measurements.

IR absorption (cm−1) Band assignment

3000–2800 C–H stretching1724 nonconjugated C=O stretching1483 C–CH3 planar deformation1449 CH2 bend., C–CH3 out-of-plane def.1386 CH2 twisting or wagging1260–1000 C–O–C stretching1238 C–O–C asymmetric stretching1144 C–O–C symmetric stretching1063 skeletal rock. of the polymer backbone988 C–O–CH3 rocking965 C–CH3 rocking

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because a higher-wavelength (lower wavenumber) IR beam pene-trates deeper into the foil, and interacts with the modified polymer

749 and 841 CH2 rocking1900–2300 absorption in the diamond head

he heights of the peaks from the irradiated part of the sample wereeasured and compared to those from the unirradiated areas. All

he peak heights decrease upon irradiation, which confirms thathain scissioning occurs at the applied ion fluence and energies.

he percental change of the peak heights are plotted in Fig. 3 as aunction of depth.

Fig. 3 shows significant decrease in peak heights of the above-entioned functional groups as a function of depth. As ions

ig. 3. Depth dependence of the percental change of some IR peak heights. Theontinuous black line is the simulated Bragg curve of 2 MeV protons in the PMMAoil samples.

ths represented with different colours. The peak of the CH2 stretching vibration iss referred to the web version of the article.)

penetrate deeper in the material they lose energy. When the energyof an ion decreases, the energy transferred to the medium (LET)increases and this leads to the breakage of more chemical bonds andhigher level of degradation of the polymer. In case of the samplesthat were irradiated with 6.25 × 1013 ions cm−2 (100 nC mm−2)fluence the region of the maximal change is localized at about70–80 �m depth from the surface. This is in good agreement withthe position of the Bragg peak.

During irradiation the refractive indices of the samples alsochange, which have an effect on the infrared measurements. Therefractive index of the unirradiated PMMA foils is approximately1.51 [18] and the maximal refractive index change in the samplesis 0.012 [19], then the path length of the IR beam is altered by only2.6%, causing the same amount of variation of the peak heights. Theobserved decrease in peak heights is much higher than what couldbe explained by the effect of refractive index change. The extentof change is higher at lower wavenumbers, which indicates thatthe chemical change took place inside the foil, not at the surface,

on a longer path length.

Fig. 4. The thickness change/shrinkage of the PMMA foils after irradiation as afunction of depth.

S.Z. Szilasi et al. / Materials Chemistry and Physics 130 (2011) 702– 707 705

Fo

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Fig. 6. Raman spectra of unirradiated and irradiated PMMA. The intensity of thetopmost spectrum (unirradiated PMMA) was reduced to 25%. The spectra of the

sity and show increasing broadening with higher doses, as bestseen on the composite band at around 2948 cm−1 (Fig. 6). Mostof the smaller Raman bands become nearly obscured in the noise

Table 2Peak assignment for the Raman spectrum of unirradiated PMMA shown in Fig. 7(peak identification is based on [20]).

Raman shift (cm−1) Band assignment

301 C–C skeletal def. of CC4

364 C–C skeletal def. of CC4

483 C–C skeletal def. of CC4

558 O–C=O deformation599 O–C=O deformation732 O–C=O def. coupled with methyl rocking810 symmetric CC4 stretching965 main chain C–C stretching986 main chain C–C stretching

1120 C–O str. Coupled with methyl rocking1156 C–O str. Coupled with methyl rocking1183 C–C degenerate stretching of CC4

1237 C–C degenerate stretching of CC4

1387 CH3 symmetric bending1449 CH2 deformation1480 CH3 asymmetric bending

ig. 5. The percental change of some typical peaks due to irradiation as a functionf depth.

.2. Transmission measurements

During transmission mode measurements interference peaksppear in the spectrum, which enables the determination of theoil thicknesses using Eq. (1):

= n

2(v1 − v2)(1)

here d is the layer thickness, �1 and �2 are the wavenumbers ofwo distant minima and n is the number of the maxima betweenhem. In case of each sample, spectra were recorded at the cen-re and outside the irradiated spot and the layer thicknesses wereetermined. The percental change of the foil thicknesses, i.e. thehrinkage as a function of depth is plotted in Fig. 4.

These measurements show that the samples became thinneruring the irradiation, shrinkage occurred. The position of the high-st change in the thickness of the samples corresponds to theosition of the Bragg peak. The error of the thickness determination

s about 2%.In the irradiated and background transmission spectra the

eights of several characteristic peaks were measured and the per-ental change due to irradiation was determined in case of each foil.he results are plotted in Fig. 5 as a function of depth. The highesthanges, according to the expectations, occurred around the Braggeak.

The changes in peak heights are higher than the post-irradiationhange in the thickness of the samples. This can be attributed tohemical changes, which decreases the concentration of all func-ional groups. The ATR measurements show greater changes in peakeights, which can be explained by the fact that ATR samples onlyhe top 0.3–1.9 �m of the foil, depending on the wavelength; whilen transmission mode the whole cross-section is sampled, and pre-umably the chemical structure changes takes place in a thin layernside the 8th foil, close to its surface.

No new band appeared in the spectra after irradiation, whichenotes that no IR active new functional groups are formed fromhe decomposing polymer. This precludes the detailed examinationf the chemical reaction.

The infrared (IR) measurements showed the change of the

mount of the functional groups at discrete depths in PMMA. Theeights of all characteristic peaks corresponding to these func-ional groups decrease upon irradiation, which confirms that chaincissioning occurs at the applied ion fluence and energies. The

irradiated samples were measured at the Bragg peak. The decrease of peak intensi-ties and the increase of background intensity are clearly seen. The spectra are offsetfor clarity.

depth profile of the chemical change corresponds to the depth pro-file of the energy loss of the bombarding particles, the maximalchange occurs at the place where the energy loss is maximal, atthe Bragg-peak. The transmission measurements showed the depthdependence of the shrinkage as well.

3.3. Micro-Raman measurements

Point measurements on irradiated PMMA samples were placedat the maximum LET, the position of which was determined fromline scans along irradiation depth profiles (see below). The Ramanspectra of unirradiated and irradiated PMMA samples are comparedin Fig. 6; peak assignments are listed in Table 2. One prominentspectral change upon irradiation is the increase of the backgroundintensity with increasing dose. Generally, Raman bands lose inten-

1725 C=O stretching2840 CH2 symmetric stretching2948 CH3 symmetric stretching2997 CH3 symmetric stretching of the –O–CH3

706 S.Z. Szilasi et al. / Materials Chemistry a

Fig. 7. Surface plot of a Raman line scan (250 spectra) along the irradiation direction(1.25 × 1014 ions cm−2 fluence, 200 nC mm−2). The dark band on the left is outsidethe PMMA block. The pronounced rise in background intensity below 70 �m corre-sponds to the Bragg peak. On the right is unirradiated PMMA.

Fig. 8. Depth profiles of irradiated PMMA samples. Profiles A, B and D were scaled to setA: profile constructed from the CC4 symmetric stretching at 810 cm−1. B: profile construcfrom the radiation-induced band at 1635 cm−1. The intensity increase at shallow depth offrom the sample with 3.13 × 1014 ions cm−2 fluence. The narrow drop in intensities at ca.

nd Physics 130 (2011) 702– 707

of the intense background at the highest dose. Exceptions are thedouble band at 1449 and 1480 cm−1 (CH2 deformation and CH3asymmetric bending, respectively) and the band at 1725 cm−1 (C=Ostretching). The strong composite band peaking at 2948 cm−1 aswell is not wiped out by the doses applied in this study. One addi-tional Raman band is observed in all irradiated samples. This peakat 1635 cm−1 is strongly asymmetric (most likely a composite band,whose assignment is uncertain).

Raman line scans, running from unirradiated PMMA through theirradiated profile to outside the sample volume were also recorded.Fig. 7 shows an entire data set (200 nC mm−2 sample) withoutbackground subtraction, demonstrating the spectral changes as afunction of depth from the irradiated surface. The sample surfaceis visible as a sharp rise in background and peak intensities. Theunirradiated PMMA volume is seen on the right side with sharp,well-defined peaks and low background. The irradiated volume hasa slightly increased background. The Bragg peak (depth of maxi-mum LET) appears as a pronounced rise in the background intensityand a drop of peak intensities at the same time.

Raman depth profiles, i.e. intensity changes across the irradi-ation profile are shown in Fig. 8. The profiles were constructedusing different Raman bands after background subtraction. Thecomparison of band intensities (peak areas) and the quantitativedetermination of radiation-induced damage were not directly pos-

sible due to a number of reasons, including the unevenness of thesample surface, the resulting lack of strict control on the excitationvolume, and hence on confocality, the lack of reference intensitiesetc. Therefore, profiles were scaled to the signal intensities from

unirradiated peak intensities to 100% and no Raman signal in air to 0% (see text).ted from the C–C main chain stretching at around 975 cm−1. C: profile constructed

the 6.25 × 1014 ions cm−2 curve is an artefact. D: comparison of five depth profiles24 �m is an analytical artefact.

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S.Z. Szilasi et al. / Materials Chem

olumes with a known structural state. One such state is full struc-ural order, found in unirradiated PMMA; this was selected to be00% Raman band intensity. The other state is zero PMMA signal,hosen to be in the section of the line scans outside the samples inir; this is 0% intensity. Note that these normalised intensity valuesan be approximated as the concentration of the chemical specieshose vibrational signal was used to construct the profile. Scaling

hese two normalised points also allowed the comparison of dif-erent samples or depth profiles constructed using different Ramanands.

In Fig. 8 it is visible that the entire irradiated volume of PMMAamples is modified. A common feature of all plots in Fig. 8 is atrong dependence of Raman intensities (i.e., of the rate of modifi-ation) on the energy transfer (LET) from the decelerating protons.eaks corresponding to those found in unirradiated PMMA (seeig. 6) were found to lose intensity with increasing LET (Fig. 8A:tretching at 810 cm−1; Fig. 8B: main chain C–C stretching, a doubleeak at 975 cm−1). Intensity loss reaches its maximum in the sam-le irradiated with 6.25 × 1014 ions cm−2 (1000 nC mm−2) protonuence: the zone of (nearly) maximal modification is not restrictedo the Bragg peak, but instead it is extended, with a slightly decreas-ng degree, to the entire irradiated volume (Fig. 8A and B).

The radiation-induced broad peak at 1635 cm−1 was found toncrease with increasing LET and fluence (Fig. 8C; cf. Fig. 3); theseepth profiles closely resemble a Bragg curve. The peculiar sharpeak in the depth profile near the surface of the 1000 nC mm−2

ample (Fig. 8C) is a result of an increased concentration of theonds giving rise to the vibration at 1635 cm−1; this may be anrtefact related to altered chemistry on the irradiated surface.

The depth profiles constructed from several PMMA Ramanands of the 500 nC mm−2 sample are compared in Fig. 8D. The10 cm−1 band behaves very similarly to the 975 cm−1 band (CC4tretching and main chain C–C stretching, respectively); the sames true for samples with other irradiation fluences as well (nothown). A slight difference between the behaviour of the abovewo bands and the bands at around 1449 cm−1 (CH2 deformationnd CH3 asymmetric bending), 1725 cm−1 (C=O stretching) and atround 2948 cm−1 (CH2 and CH3 stretching) is observed: in theow-LET section of the CHx and C=O profiles there is a plateau at ca.6–93% normalised intensity, which is a 13–24% smaller decreasehan that found for the 810 cm−1 and 975 cm−1 bands. The differ-nce is thought to arise from new H–C bonds forming more easilyhan single C–C bonds.

Focussing beneath the surface and using a high numerical aper-ure (0.9) objective resulted in the polymer–air interface appearings a steep sigmoidal jump in intensity of the depth profiles. Theo-etically the interface should be located at the point of inversionut owing to irregularities (distortion due to shrinking, bubblesormed during degassing etc.) the exact position could not be estab-ished with a precision better than ±1 �m. However, the position of

inimum or maximum change, corresponding to the Bragg peak,an be clearly determined. Proton trajectory lengths were deter-ined to be ca. 64–66 �m for the 1.25 × 1014 ions cm−2 fluence

200 nC mm−2), 65–68 �m for the 3.13 × 1014 ions cm−2 fluence500 nC mm−2) and 68–70 �m for the 6.25 × 1014 ions cm−2 fluence1000 nC mm−2). The samples used for micro-Raman measure-

ents had a higher density (1.19 g cm−3, vs. 0.95 g cm−3 of theR samples), therefore the penetration depth of the 2 MeV pro-ons was less in these samples compared to the thin layers usedn IR experiments. SRIM calculations showed that the Bragg peaks at about 64 �m, which is in excellent agreement with exper-mental data for low fluences. The trajectory lengths appear to

ncrease with increasing fluence, which can be explained withhe structural changes of PMMA such as outgasing, micro-bubbleormation and a resulting decrease in density occurring duringrradiation.

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nd Physics 130 (2011) 702– 707 707

4. Conclusions

In this work we investigated the depth profile of the chemicalchanges in PMMA due to 2 MeV proton irradiation. The IR resultsshowed that the amount of all of the functional groups decreasesas a function of depth, corresponding to the energy loss of the inci-dent protons. At the low LET part of the ions trajectory (close tothe surface) less change occurs. The region of the maximal changeis localized at about 70–80 �m depth from the surface, which isin good agreement with the position of the place of the maxi-mal energy loss. In the IR spectra no new bands appeared at theapplied fluence indicating that only chain scissioning prevails dur-ing irradiation. The surface ATR technique was complemented withtransmission measurements that confirmed the ATR results andrevealed the depth dependence of the shrinkage as well.

Micro-Raman measurements showed the strong dependenceof the degree of modification on the energy transfer from thedecelerating protons. Depth profiles reveal that at the fluencesapplied in this work the entire irradiated volume suffered somechemical modifications, but in the case of low-fluence samplesthe zone of maximal modification (i.e., the saturation of radiation-induced chemical changes) is restricted only to the Bragg peak.As the fluence increases, the region of maximal modificationalso extends towards the sample surface, reaching it at about6.25 × 1014 ions cm−2 (1000 nC mm−2) delivered fluence. The qual-ity of the occurred chemical processes does not change, but theyield of chain scissioning changes as a function of depth, followingthe energy dissipation (the Bragg curve) of decelerating protons.The degree of chemical alteration is limited by the saturation levelof radiation-induced chemical changes, i.e. where certain bonds areall destroyed, or broken and newly formed bonds are in equilibrium.

C–C bonds were found to be slightly more radiation-sensitivethan C–H and C=O bonds. Under the fluences applied in this study,a radiation-induced peak was found at 1635 cm−1 that increaseswith increasing LET.

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