VortrNge und Poster

An ESCA study on the X-ray induced changes in polymeric materials

Robert Chaney and Giinter Barth Surface Science Laboratories, Mountain View, CA 94043, USA

ESCA-Untersuchungen von Riintgenstrahlungs-induzierten ~,nderungen an Polymeren

Introduction

Electron Spectroscopy for Chemical Analysis (ESCA) is generally considered to be a non-destructive surface analyti- cal technique as compared to e.g., SIMS and Auger spec- troscopy. In fact, there have appeared only a few reports in the literature where soft X-ray induced spectral changes were noted. These studies have recently been reviewed by Storp [1]. Several of these studies have been carried out with non- monochromatic X-ray sources which typically expose a sample surface with a flux of high kinetic energy electrons and thermal radiation. As pointed out by Storp, it is not entirely clear whether or not the reported changes can be therefore considered as uniquely caused by X-ray photons. Wheeler and Pepper [2] have published a detailed beam damage study on polytetrafluoroethylene (Teflon) using a non-monochromatic X-ray source. The authors provided convincing evidence that the detected fluorine depletion of the surface is indeed caused by X-rays rather than electron bombardment. We wish to report ESCA experiments on Teflon and several other polymeric materials using a spectrometer with a monochromatic A1 Kc~ X-ray source. Due to the physical separation between source and sample an unambiguous differentiation between X-ray vs. electron induced damage can be obtained.

Experimental

The ESCA measurements were carried out with a Surface Science Laboratories SSX-100 spectrometer using a mono- chromatic A1 Ke X-ray excitation source and a beam size of 600 gm diameter. The instrument was outfitted with a Physical Electronics non-monochromatic A1 Ke X-ray source, Model 04-500. The distance between the source and sample was adjusted to approximately 5 cm. An aluminum window of 5 gm thickness was placed between source and sample. All spectral data were taken with the mono- chromatic excitation source and a charge neutralizer set at 3 eV. This resulted in considerable better spectral resolution as well as an improved signal to noise ratio for a given data acquisition time. Spectral data were collected within 2.6 min for each spectral region. Infrared data were measured on an

Offprint requests to: R. Chaney

IBM IR/85 FTIR spectrophotometer using a variable angle ATR attachment and a KRS-5 crystal. Absorption spectra of the X-ray exposed photochromic films (Kodak) were taken with a Nanospec/10 UV/VIS microscope. The polymeric materials were purchased from Polymer Science and measured in either powder form attached to doubly sided sticky tape or as films cast from chloroform solution onto aluminum foil. Polytetrafluoroethylene (Teflon) was purchased in tape form from Saunders. All Teflon samples were cleaned with Freon prior to analysis.

Results and discussion

A 4 cm 2 Teflon film of 0.5 mm thickness was exposed to monochromatic X-rays and spectral data taken over the C (1 s) and F (1 s) signal regions at approximately 2 h time intervals. The spectral changes observed for the C (1 s) re- gion after 5 and 12 h exposure time are given in Fig. 1. The variation of the F (1 s)/C (1 s) signal intensity ratios vs. time is shown in Fig. 2. As evident from Fig. l, the C (1 s) signal associated with the -CFz-funct ional group decreases with length of exposure time. Simultaneously spectral compo- nents on both sides of this signal appear and increase in intensity. The F (l s)/C (1 s) signal ratio is observed to de- crease linearly with exposure time. These observations are very similar to the results reported in ref. [2].

In order to differentiate between X-ray vs. eJectron in- duced effects the following sets of experiments were carried out. A virgin Teflon sample was exposed to an electron flux from the charge neutralizer for a period of 9 h. The kinetic energy of the electrons was adjusted to 20 eV. No spectral changes were observed after this time period. Secondly the time study was carried out with a) the flood gun left on during the duration of the experiment and b) the flood gun turned on only during the time of data acquisition (5.2 rain). The observed spectral changes were found to be identical for both operational modes. Perhaps the most convincing experimental evidence against an electron induced mecha- nism derives from the observation that the compositional changes of the Teflon surface are localized within the area exposed by the X-rays (ca. 0.75 mm2). Spectral data taken from an area adjacent to the one exposed to X-rays for 8 h were identical to those for a virgin surface. The area covered by the electrons from the flood gun is several square centi- meters.

The above experiments were repeated using a non-mono- chromatic X-ray source located in close proximity to the sample surface. The observed spectral changes were found

Fresenius Z Anal Chem (1987) 329:143-146 �9 Springer-Verlag 1987

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Fig. 1. C (1 s) spectra of Teflon taken at different times of exposure with a monochromatic X-ray source; a 0 h, b 5 h, c 12 h

u \

u _

8 u

7

6

5

i i i i i i i i p

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Time {h)

Fig. 2. Variation of the F (t s)/C (1 s) signal intensity ratios for Teflon exposed to monochromatic ([~) and non-monochromatic (O) X-rays

to be very similar to those obtained with the monochromatic source. The F (1 s)/C (i s) signal ratio decreases again approximately linearly with time (Fig. 2). Because of the large area irradiated by the X-rays the damage was found to be uniform over the entire Teflon surface. A comparison of the data given in Fig. 2 reveals that the F (1 s)/C (1 s) signal ratio decreases faster for the non-monochromatic as compared to the monochromatic source, a linear fit to the data point yields a slope of - 0.21 + 0.02 and - 0.16 + 0.02, respectively. This difference could result from a higher X- ray flux density or additional thermal and electron damage caused by the non-monochromatic source. In order to obtain comparative values for the X-ray flux densities of both sources a radiochromic film [3] was exposed for 5 min. The optical density of the induced blue discoloration was determined with a UV/VIS microscope. The absorbance at 560 nm was measured to be 0.55 and 0.60 for the mono- chromatic and non-monochromatic sources, respectively. These values in conjunction with ESCA measurement of the Au (4t7/2) signal intensities taken from a 0.03 mm 2 gold spot on silicon, lead to the conclusion that the Brems- strahlung contribution of the non-monochromatic X-ray source is approximately 37% of its total flux density.

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Fig. 3. Curve deconvolution of the C (1 s) spectrum of Teflon ex- posed for 24 h to monochromatic X-rays

In summary the above experimental evidence leads to the quite conclusive result that the observed spectral changes are caused by X-ray induced chemical degradation of the Teflon surface.

Identification of decomposition products

The C (1 s) spectrum of a Teflon sample exposed for 24 h to monochomatic X-rays (Fig. 3) was curve-fitted with a non- linear least squares fitting routine assuming a 80% Gaussian/20% Lorentzian lineshape function. An optimum fit was obtained with a minimum of six individual spectral components. The binding energies were referenced to the lowest binding energy component assumed to be located at 284.6 eV. With the available reference data for fluorinated polymeric materials [4] the following chemical binding state assignments are made:

Binding energy Chemical binding state (eV) assignment 284.6 C - C 286.0 C - C F 2 287.3 C F - C 288.9 C F - C F 2 291.2 CF2 - CFz 292.9 - C F 3

Additional information was obtained from infrared spectroscopic data. The spectra were taken in the attenuated total reflection (ATR) mode using a KRS-5 crystal. The spectrum of the damaged sample (Fig. 4) showed in addition to the principal bands of virgin Teflon at 1199 and 1143 cm-1 the following vibrational components:

1718, 1670 cm -1 C = C F ( C = C stretch) 1342 cm- 1 - CFs (C - F stretch)

985 c m - ~ ? 730 cm- 1 ?

Reaction mechanism

A description of the reaction mechanism leading to the ob- served decomposition products is quite hypothetical in nature. It is well known that the predominant de-excitation mechanism for low Z number elements with a vacancy in the 1 s core level proceeds via the Auger process [5]. The final state of this transition is an atom with two vacancies in the p electron shell, i.e., the valence orbital. It is therefore

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Fig. 4. F T I R / A T R spect ra t aken f r o m virgin Teflon sample (a) and one exposed to monochromatic X-rays for 24 h (b)

quite plausible to assume that the initial reaction involves a fragmention of C - F , C - H or C - C bond leading to a radical or ionic intermediate. For instance, a radical inter- mediate formed by elimination of fluorine could undergo further elimination of fluorine to form - C = C F - C F 2 - or react with an adjacent molecule under cross-linking. Pecu- liar is the observation of - C F 3 functional groups as a reaction product. This could be explained by assuming a re- arrangement process taking place after a fragmentation of a C - C bond:

- - C F 2 - - C F 2 ~ - C F - CF3 .

A similar mechanism seems to occur in electron inducted fragmentation of fluorinated organic materials [6]. The principal fragment in the mass spectrum of perfluoro- cyclohexane is CFa with a mass of 69 m/e.

Depth of X-ray induced damage

The infrared spectra given in Fig. 4 indicate that even after 24 h of exposure, the principal spectral pattern resembles closely the one for virgin Teflon. The ATR technique has a sampling depth of 1 - 2 pm which leads us to conclude that the induced damage is confined to a depth considerable less than this value.

ESCA spectra taken over the C (1 s) signal region were taken from the specimen at 18, 38 and 78 ~ take-off angles (Fig. 5). A change in intensity distribution between the various spectral components is clearly observed. The relative abundance of the decomposition products is highest for the data taken at shallow sampling depth (i.e., 18 ~ take-off angle). This is also consistent with the F (1 s)/C (1 s) signal intensity ratios which were observed to increase from 4.25 at 18 ~ to 5.18 at 78 ~ take-off angle. Thus, the induced damage is non-uniform within the sampling depth of the ESCA technique of ca. 50 -100 A.

Investigation of other polymers

The following polymers were investigated under similar ex- perimental conditions as described above for Teflon:

a

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Z

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296 276

Binding Energy (eV)

Fig. 5. C (1 s) spectra taken from a Teflon surface exposed to mono- chromatic X-rays for 24 h using take-off angles (angle between surface and analyser axis) of 18 ~ (a), 38 ~ (b) and 78 ~ (c)

Table 1. Rate of elemental composition changes with length of X- ray exposure time for various polymers

Polymer d (hetero atom (l s)/C(1 s))/dt" era b erM c

Non- Monochromatic monochromatic

Teflon -0.21 _+0.02 -0.16 +0.02 F (1 s) 4.43 3.25 PMMA - 0.003_+0.002 O (t s) 2.93 0.70 PVC -0.029_+0.002 -0.028_+0.005 Cl(2p) 2.29 0.71 PAN -0.003_+0.001 -0.004_+0.001 N(1 s) 1.80 0.68 PET - -0.003_+0.001 O (1 s) 2.93 0.99

a Values were de te rmined via a' s t ra igh t line fit to the exper imenta l da ta po in t s b Pho to ion iza t ion cross-sect ions o f h e t e r o a t o m A t aken f ro m R. N. King , Univers i ty o f U t a h ~ a~a is def ined as the s u m o f f a era +fB" erB + .-. w h e r e f i s the a tomic fract ion o f e lements A, B, . . . . The con t r ibu t ion o f H is negligible due to its low cross sect ion o f a = 0.0002

polymethylmethacrylate (PMMA), polyacrylonitrile (PAN), polyvinylchloride (PVC) and polyethyleneterephthalate (PET). Detectable damage was observed for PAN and PVC, but little or no spectral changes occurred for PMMA and PET. The results expressed in terms of the rate of change of the hetero atom (1 s)/C (1 s) ratios with respect to time are given in Table 1. It is interesting to compare these data with a value defined as "molar photoelectric cross section" aM (see Table 1). This value should be proportional to the absorption coefficient of X-rays for a particular polymeric material. Due to the high photoelectric cross section for the F (1 s) core level, the aM value is approximately four times higher for Teflon as compared to the other polymers investi- gated in this study. This proposed correlation certainly needs further experimental verification. One might speculate, how- ever, that polyvinylidenefluoride ( - - C H 2 - C F 2 - - , aM =

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1.79) should show less X-ray damage as compared to Teflon and polyvinylidenechloride ( - CH2 - CC12 --, O" M : 1 . 0 7 )

to be more susceptible to X-rays as compared to PVC.

Conclusions

The results of the present study confirm earlier results pre- sented by Wheeler and Pepper [2] that soft X-rays introduce compositional changes on Teflon. After prolonged exposure, the near surface region is found to be depleted of fluorine. Other non-fluorine containing polymers were found to undergo considerably less beam damage at equal exposure times. It is suggested that the rate of damage for a given X- ray flux density is proportional to the total photoelectron cross section.

The results indicate that more severe X-ray induced changes can be expected in photoelectron studies using higher flux densities than available with current instruments.

Acknowledgements. We wish to thank Dr.'s Robert Linder and Charles Bryson from Surface Science Laboratories for many

stimulating discussions and suggestions during the course of this work. We especially appreciate the help of Evan Snyder from Nanometrics, Inc. for performing the absorption measurements and providing us with access to the UV/VIS microscope.

References

1. Storp S (1985) Spectrochim Acta 40B:745 2. Wheeler DR, Pepper SV (1982) J Vac Sci Technol 20:226 3. Humpherys KC, Kantz AD (1977) Radiat Phys Chem 9:737 4. Clark DT, Kilcast D, Feast WJ, Musgrave WKR (1973) J Polym

Sci Polym Chem I1:389 5. Siegbahn K (1967) ESCA; Atomic, molecular and solid state

structure by means of electron spectroscopy. Almquist & Wiksells, Uppsala, p 35

6. Budzikiewicz H, Djerassi C, Williams DJ (1967) Mass spectrometry of organic compounds. Holden Day, p 441

Received April 18, 1987

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