Photooxidation of σconjugated Si–Si network polymersR. R. Kunz, M. W. Horn, P. A. Bianconi, D. A. Smith, and C. A. Freed Citation: Journal of Vacuum Science & Technology A 9, 1447 (1991); doi: 10.1116/1.577643 View online: http://dx.doi.org/10.1116/1.577643 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/9/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Photo-oxidation effects of light-emitting porous Si J. Appl. Phys. 105, 113518 (2009); 10.1063/1.3140677 Dramatic enhancement of photo-oxidation stability of a conjugated polymer in blends with organic acceptor Appl. Phys. Lett. 92, 243311 (2008); 10.1063/1.2945801 Three-color polymeric light-emitting devices using selective photo-oxidation of multilayered conjugated polymers Appl. Phys. Lett. 90, 063505 (2007); 10.1063/1.2472179 A photo-oxidation mechanism for patterning and hologram formation in conjugated polymer/glass composites J. Appl. Phys. 88, 1236 (2000); 10.1063/1.373809 Effects of photo-oxidation on conjugated polymer films Appl. Phys. Lett. 71, 1483 (1997); 10.1063/1.119943

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Photo-oxidation of a-conjugated Si-Si network polymers R. R. Kunz and M. W. Horn Lincoln Laboratory. Massachusetts Institute of Technology. Lexington .. 'I1assachusetls02 173-9108

P. A. Bianconi, D. A. Smith, and C. A. Freed Department of Chemistry. The Pennsyh1ania State Unil'crsity. Unil'('I'sily Park. Pennsyll'Gllia 16802

(Received 15 October 1990; accepted 9 December 1990)

The ultraviolet-induced photooxidation ofpolyalkylsilyne network polymers has been examined. Infrared and x-ray photoelectron spectroscopies indicate a higher oxygen coordination about the Si atoms following photolysis than observed for the linear polysilanes. A photochemical pathway leading to cleavage of the alkyl groups from the Si backbone has been observed, the products of which then desorb as the respective alkanes or l-alkenes via a thermally activated process. Finally. the photooxidation rate at 193 nm of thin polyalkylsilyne films is photon limited at fluences less than 1-5 mJ/cm2 per pulse; whereas above this fluence, the photooxidation is oxygen limited. For the latter, the amount of oxygen chemically incorporated into the Si-Si network per laser pulse is dictated by the solubility level of oxidant in the film.

I. INTRODUCTION

In 1988, the synthesis of a new and very different class of a­

conjugated Si-Si backbone polymers was reported. I Unlike the linear polysilanes, these polymers, called polysilynes ( [SiR] n ), contain only one pendant alkyl group per silicon atom. Their structure, however, bears no similarity to poly a­cetylene, their carbon-backbone analog. Rather, polysilyne molecules exist as Si-Si O'-bonded networks2 and exhibit op­tical and photochemical properties which make them desir­able for use as photoresists in ultraviolet-based bilayer Iith­ographyJ and as precursors for thin-film optical components.4 In this paper we report on the deep-ultraviolet photooxidation chemistry of the polysilynes in light of pre­vious studies performed on the linear polysilanes,5.6 and on the kinetics of the photooxidation reaction.

II. EXPERIMENTAL

The polyalkylsilynes were synthesized by the ultrasonical­ly stimulated reductive condensation of trichloroalkylsi­lanes using a liquid Na/K alloy; this procedure has been described in detail elsewhere. 2 The polysilynes studied were R = n-butyI2, isobutyl7, cyclohexyl7, and copolymers con­taining the forementioned homopolymers. Following purifi­cation 2

, the samples were filtered through a 0.2-/Lm filter and spin-coated onto various substrates using either toluene, xy­lene, or trichloroethane as the solvent.

Analysis of laser-exposed polysilynes was performed us­ing ultraviolet-visible spectrophotometry (UV-VIS), Four­ier-transform infrared spectroscopy (FTIR), x-ray photo­electron spectroscopy (XPS), Auger electron spectroscopy (AES), and laser desorption mass spectrometry (LDMS). The LDMS system used was housed in an ultrahigh vacuum (UHV) chamber (base pressure 3 X 10- x Torr) and used a differentially pumped quadrupole mass analyzer (base pres­sure 8 X 10- 9 Torr) equipped with electron ionization. The ionizing potential was 70 V.

III. RESULTS END DISCUSSION

A. Polysilyne properties

The polysilyne polymers [SiRL are isolated as yellow, moderately air- and light-sensitive powders with mean mo­lecular weights ranging from 8000 to 100 000 daltons, or degrees of polymerization, n, equal to approximately 300 to 1000 alkylsilicon units. X-ray powder diffraction analyses indicate amorphous, glasslike structures, whereas photolu­minescence studiesR indicate that their electronic properties resemble those of small, semiconducting clusters. These un­usual properties stem from their all-silicon a-bonded network backbone. These network materials represent structural as well as stoichiometric intermediates between linear polysilanes [SiR2 ] n and amorphous silicon, and their optical and electronic properties are similarly intermediate. For example, unlike the strong, discrete 0'--0'* transitions which linear polysilanes display in the near UV, polysilynes exhibit an intense UV absorption band-edge tailing down into the visible, with integrated absorptions per Si atom 3-10 times greater than those measured for linear polysilanes. The optical absorption is similar in shape and intensity, although somewhat blue-shifted, to that of amorphous silicon.'l The photoelectron spectrum in Fig. 1 shows the Si 2p binding energy for poly(cyclohexylsilyne) to be 99.62 eV. This value is slightly lower than the 99.88 eV reported for an aliphatic­substituted linear polysilane. 5 The photoelectron spectrum of crystalline silicon is shown for comparison.

B. Photooxidation chemistry

Infrared spectroscopy was performed on poly(n-butylsi­lyne) before and after photooxidation (Fig. 2). In the Si-H stretch region (2000-2300 cm - I), peaks appear at both 2091 cm - I and 2155 cm - I when irradiated at 254 nm [Fig. 2(b)}. When irradiated at 193 nm, peaks appear at 2155 cm I at lower doses [Fig. 2(c)], and at 2210 cm I (not

1447 J. Vac. Sci. Technol. A 9 (3), May/Jun 1991 0734-2101/91/031447-05$01.00 © 1991 American Vacuum Society 1447

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1448 Kunz et al.: Photo-oxidation of O'"conjugated 5i-5i network polymers 1448

(i) :: c ~

~ f! -:c ~ c -oJ W :; Z o a: ~ u W -oJ W o ~ o ::r: Q.

102 101

(b)

100 99 98 97

BINDING ENERGY (eV)

1'1< '. J (a) The Si lp x·ray photoelectron 'pe<:l rum of poly (cyclohexylsi· IYllel The peak is centered at 99.62eV. (h) A similar'pectrum of crystal· Ii Ill' ,ilicon ,h"wll for n'onlpari"lIl. The 2p, ,alld 2p, ' splitting is visihle for

I he crystalline Si.

shown) at higher doses ( > 1 J/cm:'), In contrast, photooxi­dation of linear polysilanes results in a single Si-H vibration at 2123 cm I (Ref. 6), It is known III that the Si-H stretching frequency varies with the sum electronegativity of the three other moieties bound to the Si because of changes in electron density on the central Si atom. Hence the Si-H stretching frequency can be used to determine the R ,R.,R \ combina­tion, provided some prior information is known (i.e., that there is a limited number of possible combinations),I' At 254 nm, the initial R,R~R, Si-H formed at 2091 cm I we calculate to be R I = Si, R c = Si, R, = 0. The peak at 2155 cm I is the most intense at both 254 and 193 nm, and we assign it as R I = Si, R ~ = alkyl, R \ = 0. The 221O-cm - I

peak that appears only at high-dose 193-nm exposures we assign as R J = Si, Rc = 0, R, = 0. We believe the peak at 2155 em I results from the same R IR2R.\ pairing as the peak at 2123 em I observed in the linear polysilanes. Based on UV absorbance data, we attribute the difference in frequencies between polysilynes and polysilanes to the more extensive a

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2000 2050

2091 em 1 2155 em 1

2100 2150 2200

WAVENUMBER (em·1 )

Xl

Xl

XO.3

2250

Fi(,. 2. (a) The FTIR 'pectrulll of unirradiated poIY(I/·hutybilyne). (hi The FTIR spectrum ofpoly( II-hutylsilyncl exposed to a deep·UV Hg lamp (principal line al 254 nm) for.10 s. (c) The FTIR spectrum ofpoIY(II­hUlylsilyne) e~posed to J9.1 nm at a dose of 500 mJ/cm '. The spectral region shown contains the Si-H stretching hands.

conjugation, remaining in the partially photolyzed linear po­Iysilanes. The peak at 2091 cm - I has no analog in the linear polysilanes because it corresponds to a photolysis product that still retains a Si-(Si R H)-Si linkage. This product can­not arise from cleavage of a linear Si-Si chain, The peak at 2210 cm - I appears only when irradiated at very short (193-nm) wavelengths because of the greater extent of network degradation that shorter wavelengths provide,4 and has no analog in the linear polysilanes. A proposed structure is Si­-SiH0 2 • XPS studies (Fig. 3) also confirm the presence of highly oxygen-coordinated Si, as the Si 2p binding energy of 103.12 eV observed for poly(cyclohexylsilyne) photooxi­dized at 100 J/cm" is typical for tetrahedrally bound Si04 2

(Ref. 12). In addition, a 50-fold decrease in absorptivity at 193 nm is observed for the polyalkylsilynes exposed to I J/em 2 at 193 nm. This is again in contrast to the linear poly­silanes, where such bleaching at wavelengths less than 250 nm is not observed.

o ..J W >., z .!:: 0" a:::> 1->­u iii w~ ..J .-w€ 0< 1--o J: Q.

106 102 100 98 96

BINDING ENERGY (eV)

FI(L .1. The x·ray photoeicclron 'pectru of hoth unexposed and expo,ed poly( cycJohexylsilyne). The exposure was performed at 19.1 nm, at a dose of JO() J/cm'. The exposed sample ha, a peak centered at Im.12 eV.

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1449 Kunz et al.: Photo-oxidation of I1'"conjugated Si-Si network polymers 1449

The Si-H bond formation during photolysis of linear po­lysilanes has been attributed to hydrogen abstraction from pendant alkyl groups" following the photogeneration of the silyl radical intermediate. In the polyalkylsilynes, the forma­tion ofSi-H is accompanied by a decrease in the asymmetric and symmetric CH, stretches at 2974 and 2886 cm ',re­spectively, relative to the asymmetric and symmetric CH, stretches at 2941 and 2S75 cm -- '. This is apparent in Fig. 4, where the exposure was performed at 193 nm and 500 mJ/cm l total dose. The relative decrease of intensity in the CH, stretching modes indicates that the terminal methyl group may be the primary source for the H-atom abstrac­tion.

LDMS was used to examine the composition of species volatilized during laser exposure. LDMS of poly (n-butylsi­Iyne) and poly (cyclohexylsilyne) yielded cracking patterns consisting almost entirely of alkyl fragments. However, sig­nificant alkyl desorption from poly (cyclohexylsilyne) using 193-nm light was only detected at fluences > 5 mJ/cm" per pulse, where a significant temperature rise (100-200 eC) is estimated, whereas desorption products were measured at fluences as low as 2 mJ/cm 1 per pulse for poly(n-butylsi­lyne). At 2 mJ/cm 2 per pulse the temperature transient is not expected to be more than 40-80 ec above room tempera­ture. Mass spectra for poly (cyclohexylsilyne) are shown in Fig. 5 for three exposure conditions, and for comparison the mass spectra of cyclohexene and cyclohexane are also shown.1.1 From these spectra, it is apparent that the most abundant peaks for both cyclohexane (rnle = 56, C4 H K

t ;

69, C,H<)i ; 84, C .. H,i ) and cyclohexene (rnle = 54, C4 H,,+ ; 67, C,H7 ' ; 82, C6 H ,i») appear in all the laser desorption products of poly (cyclohexylsilyne). In fact, except for the peaks at rnle = 43 and rnle = 44, the desorption products appear to be a combination of both cyclohexane and cyclo­hexene, and do not vary significantly with peak ftuence or

3050 3000 2950 2900 2850 2800

WAVENUMBER (cm-1 )

FIG. 4. (a) The FTIR spectrum of unexposed poly( II-butylsilyne). (b) The FTIR spectrum of poly( n-butylsilyne) exposed to 500 mJ/cm' at 19J nm. The spectral region shown contains the CH, and CH, stretching modes.

J. Vac. Sci. Technol. A, Vol. 9, No.3, May/Jun 1991

100

50

50

50

50

(a) 0.40 W

(b) 0.75 W

40 45 50 55 60 65 70 75 80 85

m/e RATIO

FIG. 5. (a )-( c) LDMS obtained from irradiating poly (cyclohexylsilyne) at 193 nm (a) at 8 mJ/cm' per pulse and 50 Hz, (b) at 15 mJ/cm' and 50 Hz, and (c) at 10 mJ/cm' and 200 Hz. (d) Cracking pattern of cyclohexene. (e) Cracking pattern of cyciohexane.

average power over the range examined. In contrast, pyroly­sis mass spectral of polysilynes yields only the respective 1-alkenes, with a threshold temperature for decomposition of 300 ec. The ablation threshold of this material was measured to be 30 mJ/cm", well above the ftuences examined in Figs. S(a)-S(c).

In general, the alkane formation can be described by a homolytic Si-C bond cleavage followed by H-abstraction by the alkyl radical during its migration to the surface. Alkene formation, at least in the linear polysilanes, is believed to require a silylene intermediate!4 formed by either a radical disproportionation of the photodissociated Si backbone!' or by a thermally activated I, I-elimination 16; alkene formation then proceeds via silylene insertion into the f3 C-H bond, the result being abstraction of hydrogen from the desorbing al­kyl radical. The laser-induced desorption spectra of poly (cy­clohexylsilyne) are interpreted as being the result of Si-C bond cleavage followed by the thermally-activated desorp­tion of the cyclohexane/ene. The small differences in the observed mass spectra (Fig. 5) can be attributed to the tem­perature dependence of the desorption step, as both cyc1o­hexane and cycIohexene have boiling points around 80 ec (Ref. 17) and would hence require elevated temperatures for desorption to occur (at 1 atm pressure). Whether direct homolytic photoscission occurs or some secondary radical rearrangement results in the Si-C scission is not known, al-

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1450 Kunz et al.: Photo-oxidation of ~conjugated Si-Si network polymers 1450

though alkyl loss during irradiation at 254 nm suggests the latter, since most Si-alkyl honds are transparent at that wavelength. Ix

The net mass change in films ofhoth poly(cyclohexyl-co­II-hutylsilyne) and poly (cyclohexylsilyne) was monitored using a quartz-crystal microbalance (QCM). Figure 6(a) shows the mass change (a frequency decrease corresponds to a mass increase) versus time for poly (cyclohexylsilyne) ex­posed at 0.05, 0.25. and 1.5 mJ/cm' per pulse at 193 nm and 5 Hz repetition rale. In each case, the net mass change de­pends only upon the total exposure dose and is independent of fluence (the example of 375 mJ/cm~ clearly shows this

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-200

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025 mJ em' pulse'

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TIME (5)

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" , o 25 mJ em' pulse

400 600 800 1000

TIME (5)

FI( .. 6, QeM data ootained for irradiating polysilynes in I atm air. For each tl·ace. the fiuence is indicated Oil the Figure and the la,er repetition rate was :; ~V, A decrease in frequency corresponds to an increase in mass, The polymers irradiated were (a) poly (cyclohexylsilync) and (0) poly( II-OU­

tyl-dyne)

J. Vac. Sci. Technol. A, Vol. 9, No.3, May/Jun 1991

behavior). Little evidence suggests any competing pro­cesses, such as alkyl loss, at these fluences where the tem­perature rise is negligible. However, Fig. 6( b) shows poly (cycIohexyl [34% l-co-n-butyl [66%] silyne) under identical exposure conditions, and in this case the mass up­take decreases with increasing fluence per pulse. This result can only be explained by a competing process involving a photoactivated mass loss. Possibly the butane/ene products readily desorb at room temperature, as n-butane and I-bu­tene both have boiling points around O°C (Ref. 17).

C. Photooxidation mechanism for 193-nm pulsed exposure

Our FTIR, UV -VIS, and XPS studies demonstrate that the amount of photooxidation per pulse is oxygen limited even at modest fluences (1-5 mJ/cm~ per pulse), where it is proportional to the equilibrium solubility of oxygen atoms in the polymer (the chemical composition of the dissolved oxi­dant is unknown, but O~ or H~O seem most likely). This effect is attributed to the high UV absorptivity and quantum efficiency of the Si-Si bond cleavage. Only at high average laser intensity, where oxygen consumption is so great that the equilibrium solubility level cannot be maintained, the oxidation rate becomes limited by the net indiffusion rate of the oxidant. The diffusion coefficient of the oxidizing species was estimated by the measurement of the bleaching time at 193 nm for various laser repetition rates and average intensi­ties. This we found to be roughly 10- 1°-10 II cm2/s for both oxidized poly(n-butylsilyne) and oxidized poly (cyclo­hexylsilyne). From the initial slope of Fig. 6 (a) (using a measured 20-nm penetration depth for the laser pulse into the unbleached film) we determine that ~ 5 X 1020 oxygen atoms/cm' are incorporated per pulse. Increasing the fluence above 1.5 mJ/cm 2 does not raise the value signifi­cantly. It is noted that the value of 5 X 1020 is significantly higher than solubility limits for gases in organic polymers, which typically range from IO IK to 1020 molecules/cm' at STP (Ref. 19). The oxidant diffusion length during the laser pulse is extremely short, and oxidant indiffusion during the laser pulse can be neglected.

IV. CONCLUSION

The polyalkylsilynes have been shown to have different photooxidation products from the linear polysilanes, the pri­mary difference being higher oxygen coordination around the silicon atoms. This observation is commensurate with the proposed three-dimensional Si-Si network. A photoche­mical pathway leading to cleavage of the alkyl groups from the Si backbone has been observed, the products of which thermally desorb as the respective alkanes or I-alkenes. We observe the photooxidation at 193 nm to occur via a photon­limited process only at fluences less than 1-5 mJ/cm 2 per pulse. Above this fluence, the photooxidation is oxygen lim­ited. For the latter, the amount of oxygen chemically incor­porated into the Si-Si network per laser pulse is dictated by the solubility level of oxidant in the film for low-intensity exposures, whereas at high laser intensities (> 200-300 m W /cm') the rate is dictated by the indiffusion rate of oxi­dant. From these results, it is apparent that both the alkyl

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1451 Kunz et al.: Photo-oxidation of u-conjugated Si-Si network polymers 1451

concentration and the oxygen concentration can be con­trolled by careful choice of peak f1uence, total dose, and pulse repetition rate when the photooxidation is performed with an ArF laser.

ACKNOWLEDGMENTS

We thank R. W. Otten and B. E. Maxwell for expert tech­nical assistance and M. C. Finn for the Auger analysis. In addition, we thank M. Rothschild, D. C. Shaver, and D. J. Ehrlich for useful discussions. This work was sponsored by the Defense Advanced Research Project Agency.

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