Radiation Physics and Chemistry 60 (2001) 461–466

Radiation effects on branching polysilanes

Kensaku Maedaa, Shu Sekia, Seiichi Tagawaa,*, Hiromi Shibatab

a Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, JapanbResearch Center for Nuclear Science and Technology, The University of Tokyo, 2-22 Shirakata-Shirane, Tokai, Ibaraki 319-1106, Japan

Abstract

Crosslinking and main chain scission reactions were investigated in the present study for g-rays and ion beams inlinear and branching polysilanes which have silicon branchings in the main chain at 5–32 at.%. The crosslinking

reactions become predominant for the irradiation with an increase in the branching density. However, the efficiency ofcrosslinking reactions in linear and branching polysilanes are almost consistent in case of the irradiation to ion beams.This is due to the differences in distribution of deposited energy by g-rays and ion beams, and suggests that the size ofthe chemical track is responsible for the gelation behavior of the polymers. The trace of formed gel was also concluded

not to give the accurate G values of crosslinking and main chain scission reactions in the polymers for high LETradiations such as ion beams. # 2001 Published by Elsevier Science Ltd.

1. Introduction

Polysilane derivatives have been extensively studiedbecause of their potential use as polymer materials forbi-layer resist processes. Since the report on the high

efficient reactions of main chain scission upon exposureto UV light, several groups reported reaction mechan-isms of photodecomposition or photovolatilization ofsubstituted polysilanes. It was reported by Trefonas

et al. (1983) that films of high molecular weight poly(n-hexylmethylsilane) showed a UV absorption spectralshift and molecular weight reduction upon exposure to

UV light (the wavelength was 313 nm). Zeigler et al.(1985) and Miller et al. (1989) reported that photo-volatilization was caused by excimer laser irradiation

(248–306 nm) for alkyl substituted polysilanes. Based ontheir results, polysilanes have been investigated aspotential positive photoresist materials because of these

results of UV photolysis. Polysilane derivatives and theirreactions induced by electron beam (EB) irradiation arealso of great interest. Miller et al. (1988) reported thatpatterns could be made on alkyl substituted polysilanes

using EB with high sensitivity as a positive resist. Taylor

et al. (1988) studied polysilane copolymers as EB resistmaterials. However, the polymers showed low sensitivity

as positive resist materials and low contrast for EB.They concluded that high vacuum conditions increasedthe efficiency of crosslinking, leading to low sensitivity

and contrast. The polymers were also confirmed to showpositive-type resist properties for ionizing radiations:X-rays and g-rays, however the efficiency of main chainscission reaction was very low for the radiations in

comparison with that for UV light. Recently wereported the predominant reactive intermediates in mainchain scission and crosslinking of polysilanes for

ionizing radiations (Seki et al., 1997a, b), and suggestedthat the reaction was controlled by the density of thereactive intermediates and by the structure of the Si

backbone.The EPR spectroscopy and the product analysis

reveal that Si based neutral radicals: silyl radicals

play a significant role in the crosslinking reactionsof polysilanes (Seki et al., 1998). The yield of theradicals strongly depends on the backbone structure ofpolysilanes, especially on the number of Si branching

sites. The present paper describes mechanisms of g-raysand ion beams induced reactions in polysilanes with avariety of backbone conformation. The role of reactive

intermediates is discussed on the analysis of radiolysis*Corresponding author.

E-mail address: tagawa@sanken.osaka-u.ac.jp (S. Tagawa).

0969-806X/01/$ - see front matter # 2001 Published by Elsevier Science Ltd.

PII: S 0 9 6 9 - 8 0 6 X ( 0 0 ) 0 0 4 1 7 - 5

products and structures of crosslinking points in thepolymers.

2. Experimental

2.1. General

Poly(methylphenylsilane) (PMPS) was synthesized bythe conventional sodium condensation (Kipping Reac-tion) method from the methylphenyldichlorosilanemonomer. PMPS with Si-branchings was synthesized

by the same procedure with a monomer mixture ofmethylphenyldichlorosilane and p-tolyltrichlorosilane.The ratio of the monomers was changed from 5 to

32wt.%. Fig. 1 displays the structure of the obtainedpolymers in this study. All chlorosilanes were doublydistilled products from Shin-Etsu Chemical Co. Ltd.

Polymerization reactions were carried out in an Aratmosphere, in 0.1 dm3 of dry toluene which wasrefluxed with sodium during 10 h and distilled before

use. The monomer was added into the reaction vesseland mixed with sodium dispersion during 12 h. Thesodium micro-dispersion in toluene was purchased fromAcros Co. LTD. PMPS and defect-containing PMPS

solutions were precipitated in iso-propylalcohol (IPA)after passing through a 0.45mm PTFE filter to roughlyeliminate NaCl, and precipitates were dried under

vacuum. The toluene solutions of these polymers weretransferred into separatory funnel, washed with water toeliminate the remaining NaCl, and precipitated twice

with toluene-IPA and tetrahydrofran(THF)-methanol.PMPS and PMPS with Si-branchings showed goodsolubility for toluene, THF, 2-methyltetrahydrofran(MTHF), chloroform and dichloromethane. The

amounts of residual Cl atoms were confirmed to be lessthan 0.1% in all polysilanes by elemental analysis.The Si based defect density (D) was confirmed from

the ratio of the 1H contents in p-tolyl and methyl groupsdetermined by a JEOL EX-270 NMR spectrometer at270MHz. A 29Si NMR spectra were also recorded using

a JEOL EX-600 NMR spectrometer at 120MHz. Themolecular weight distributions in all the polymers weremeasured with a Shimadzu C-R3A gel permeationchromatography (GPC) system with polystyrene cali-

bration standards. Glass transition temperatures weremeasured with a Perkin-Elmer DSC-7 system. Thecharacteristics of all the polymers are summarized in

Table 1.

2.2. Irradation

The polymers were weighed at �1 g into break-sealtubes, which were subsequently evacuated down to10�6 Torr. The tubes containing the polymers were kept

under high vacuum for 1 h to complete removal ofoxygen and sealed off. The irradiation was carried out atRT with 60Co g-ray source at ISIR, Osaka University.

The yield of the radiolysis products was measured byusing a Shimadzu GC-14B gas chromatograph and aShimadzu GC-17A-QP5000 gas mass spectrometer.

The PMPS was dissolved in toluene and spin-coartedon Si wafers at ca. 1.0 mm thick. These films wereirradiated in a vacuum chamber (51� 10�6 Torr) withelectron beams from JEOL JMT-300 and 2MeV He+

ion beams from a Van de Graff accelerator at theResearch Center for Nuclear Science and Technology,the University of Tokyo. The irradiated part of the film,

where gel was generated, was insoluble in toluene. Asurface profiler measured the thickness of remainingfilms after development. The normalized thickness was

defined as the ratio of the thickness after irradiation tothat before.

3. Results and discussion

The efficiency of 60Co g-ray induced reactions had

been reported in typical polysilanes, showing very highefficiency of main chain scission. The crosslinkingreactions were negligible, and were not discussed in

detail. Fig. 2 shows the changes in the molecular weightof PMPS and PSi(D=x)s. The distribution apparently

Fig. 1. Structures of polysilanes. x denotes the ratio of

branching Si atoms.

Table 1

Characteristics of polysilane derivatives

Entry Feed ratio Da Mwb Mw/Mnb Tg(K)c

PMPS 0 0 2.4� 104 2.3 379

PSi (D=0.05) 0.05 0.05 2.2� 104 2.9 385

PSi (D=0.15) 0.15 0.15 1.9� 104 3.7 387

PSi (D=0.32) 0.35 0.32 1.1� 104 2.6 392

aD, Si based defect density per total Si units.bMw andMn, weight and number average molecular weight.cTg, glass transition temperature.

K. Maeda et al. / Radiation Physics and Chemistry 60 (2001) 461–466462

shifts towards small molecular weight region in PMPS,suggesting the predominant reaction as main chain

scission. The efficiency of the scission reaction isestimated as G(s)=0.30 (G-value of main chain scission:number of reactions per absorbed 100 eV) by the trace of

molecular weight. It is considerably lower than apreviously reported value observed in the solution ofPMPS. Neutral silyl radicals (�RR0Si . ) and silylenes(RR0Si . . ) were already reported as the predominant

reactive intermediates in the main chain scission reac-tions at solution phase. The recombination of the radical

species plays a significant role in the decrease in theefficiency of main chain scission at solid phase. Wealready reported that the stability of neutral radicals

strongly depended on the structure of Si backbone inpolysilanes (Seki et al., 1998). The EPR signals of thesilyl radicals were observed in highly branched poly-silanes even at RT despite of the extinction of the signals

in PMPS above 100K. This suggests that the recombi-nation reactions efficiently occur in the polysilanes withSi-branchings, leading to higher yield of crosslinking

instead of main chain scission. An increase in themolecular weight is clearly observed in g-ray irradiatedPSi(D=0.15) and PSi(D=0.32) as shown in Fig. 2(b)

and (c). The crosslinking reactions are accelerated by theinducement of Si-branchings, and PSi(D=0.32) indi-cates a monotone increase despite that the small fraction

of molecular fragment is observed for the irradiation toPSi(D=0.15). The G values of crosslinking are calcu-lated as G(x)=0.09, 0.22, 0.37 and 0.49 forPSi(D=0.05), PSi(D=0.15), PSi(D=0.25), and

PSi(D=0.32), respectively, in contrast to G(x)=0.05for PMPS. Fig. 3 plots the G values as a function ofD, indicating the continuous decrease in G(s) with

increasing D and the abrupt increase of G(x) at D>0.05.The overall tendency of the reactions turns from mainchain scission to crosslinking at the branching ratio:

D=0.05.A Kipping reaction of dichlorosilane with alkali metal

has been often the choice for the polymerization ofpolysilane derivatives. It has been suggested that they

contain small amount of structural defects, especiallySi-branchings in their main chains when the polymer isobtained by this method. Several groups reported

empirical relationship between the ratio of Si-branchingsand physical properties such as the intensity of broad

Fig. 2. 60Co g-rays induced changes in the molecular weight

distribution of PMPS (a), PSi (D=0.15) (b), and PSi (D=0.32)

(c). Solid and dashed lines indicate their initial distribution and

that after 3.5MGy irradiation, respectively.

Fig. 3. Dependence of G-values of crosslinking: G(x) and main

chain scission: G(s) on the defect density.

K. Maeda et al. / Radiation Physics and Chemistry 60 (2001) 461–466 463

photoluminescence at 400–600 nm (Fujiki, 1992), thevalues of hole drift mobility (van Walree et al., 1996),

and the transition energy of charged radicals (Seki et al.,1999a, b). The empirical relationship gave the potentialamount of Si-branchings in the linear polysilanes by

each measurement of a property, and it was estimated asD=0.001–0.05 in PMPS which was prepared byKipping method without using trichlorosilane. Thepresent result suggests that the yield of crosslinking

reaction in PMPS can be controlled by the introductionof Si-branchings, and its sensitivity as positive resist willbe improved by the careful elimination of Si-branchings

from the main chain. PMPS based polysilanes simulta-neously behave as negative resist materials with thepresence of Si-branchings over D=0.05.

A careful study of the radiolysis products of a series ofpolysilanes shows that there are more cleavage at phenylgroup than at methyl group in PMPS irradiated by

g-rays and there are more possibilities of cleavage ofchain side in branching PMPS as compared to linearPMPS in Table 2. In addition, we can see that thecleavage did not take place selectively at the branch

points by comparison of the ratio of benzene andtoluene at the moment of the synthesis with the ratio ofthe G values of the radiolysis products in Table 3. Silyl

radicals have great stability in comparison with carbon-centered alkyl radicals and radicals of the cleavageproducts are not stabilized by Si=Si conjugation as the

reaction mechanism in the irradiated polysilanes. There-fore, the density of Si . contributing to the crosslinking

reactions is so increased that the crosslinking reactionsbecame predominant compared to scission reactions

with branching density increasing by the irradiation ofg-rays. Fig. 4 shows the 29Si-NMR spectra ofPSi(D=0.32) before and after g-ray irradiation. Fig. 3

shows that the number of the tertiary Si increases morethan that of the secondary Si, therefore the crosslinkingreactions are mainly formed at Si unit and not at C unit.On the basis of the negative resist features of

polysilanes with Si-branchings, their sensitivity ismeasured for 20 keV electron beams as shown inFig. 5. The electron beam gives clear negative tones of

the thin films of PMPS and PMPS with Si-branchings.Although the sensitivity is considerably low in PMPS incontrast to that in PMPS with Si-branchings, PMPS

behaves as a negative resist for the electron beam, andforms polymer gel after the irradiation. Radiationinduced reactions in poly(di-n-hexylsilane) (PDHS) was

carefully investigated by us (Seki et al., 1999a,b). Thevalue of LET: 10 eV/nm was the threshold where thepolymer showed the conversion from positive tonegative resist materials. The values of LET are

calculated as 0.20 and 0.018 eV/nm for 20 keV electronsand 60Co g-rays, respectively. This suggests thatthe conversion threshold in PMPS locates around

�0.1 eV/nm which is two orders of magnitude smallerthan that in PDHS. Product analysis of g-ray irradiatedPMPS indicates high yield of benzene (G(benzene)=

4.2� 10�2) that is formed by the dissociation reaction ofphenyl groups from Si skeleton, in spite of the lower

Table 3

Relative Ratio of G-valuesa of g-radiolysis products formed by side chain scission reactions

Entry Initial ratio Radiolysis products

PSi (D=0.15) 85 (5.6) 15 (1) 80 (4.0) 20 (1)

PSi (D=0.32) 68 (2.0) 32 (1) 65 (1.9) 35 (1)

anumber of reactions per 100 eV absorbed energy.

Table 2

G-valuesa of side chain scission reactions in polysilanes for 60Co g-rays irradiation

Entry H2 CH3

PMPS No detect 5.6� 10�3 4.2� 10�2 }

PSi (D=0.15) 1.2� 10�3 8.4� 10�3 3.4� 10�1 4.8� 10�2

PSi (D=0.32) 1.1� 10�2 1.1� 10�2 5.6� 10�1 2.9� 10�1

anumber of reactions per 100 eV absorbed energy.

K. Maeda et al. / Radiation Physics and Chemistry 60 (2001) 461–466464

yield of hexane (G(hexane)=5.0� 10�3) in PDHS. Thehigh yield of silyl radicals produced by the dissociationreaction may play a significant role in the low threshold

in PMPS. The sensitivity of polysilanes is drasticallyimproved by the presence of Si-branching as shown inFig. 3. PSi(D=0.32) shows one order of magnitude

higher sensitivity than that in PMPS. PSi(D=0.32) giveshigh yield of side chain dissociation reaction: G(ben-g-ray irradiation, thus electron beams produce twokinds of silyl radicals: –RSi . – and –RR0Si . at high

density, and form T-type and H-type crosslinking pointsas illustrated in Scheme 1. It is concluded that hyper-branched polysilanes have the potentials as negative

resist materials, and the branching ratio will be the keyto control their sensitivity for the radiations.The gelation curve in linear PMPS irradiated by

2MeV He+ is almost consistent with these in branching

PMPS as shown in Fig. 6. Thus, the radiation effects inlinear and branching PMPS irradiated by ion beams

behaved differently as compared with that by g-rays.Seki et al. (1999a, b) have reported that the crosslinkingreactions were mainly promoted by side chain disso-ciated silyl radicals and the predominant reaction was

determined by the radical concentration in the iontracks. The density of the reactive intermediates controlsthe crosslinking reaction in PMPS, which is supported

by the presence of a LET threshold (ca. 10 eV/nm) toobtain a polymer gel. It indicates that the size of

Fig. 4. 29Si-NMR spectra of PMPS before and after irradia-

tion.

Fig. 5. Sensitivity of PMPS (dark circles) and PSi(D=0.32)

(open circles) for 20 keV electron beams.

Fig. 6. The gelation behavior of PMPS (dark circles),

PSi(D=0.15) (squares), and PSi(D=0.32) (open circles) irra-

diated by 2MeV He+.

K. Maeda et al. / Radiation Physics and Chemistry 60 (2001) 461–466 465

chemical track is responsible for the gel fraction.Therefore, the gelation curves in linear and branching

PMPS irradiated by 2MeV He+ is almost consistent,despite that the larger number of the crosslinking pointsis introduced within an ion track in branching PMPS

than that in linear PMPS.

4. Conclusion

The side-chain dissociation reactions are abruptly

accelerated by the presence of Si branching, giving thehighly concentrated radicals that are formed in themiddle of Si chains (–RSi . –). The efficiency of the

recombination reactions overcomes that of the mainchain scission reactions in the polysilanes containinglarge number of branching sites (Branching ratio>0.05), leading to the conversion from positive to

negative resist with an increase in the branching. TheLET threshold of the inversion was observed even in thelinear PMPS by using electron and ion beams, suggest-

ing that the size of chemical track is responsible for theformation of polymer gels for high LET radiations.

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Scheme 1. Schematic diagram of formation of crosslinking points in PMPS-based polysilane.

K. Maeda et al. / Radiation Physics and Chemistry 60 (2001) 461–466466