J. MATER. CHEM., 1991, 1(4), 673-675 Variation with Composition of the Intrinsic Sensitivity of Halogen-substituted Styrene Copolymers to Electron-beam Radiation Philip C. Miller Tate* and Richard G. Jones Centre for Materials Research, Chemical Laboratory, University of Kent at Canterbury, Canterbury, Kent CT2 7NH, UK In the polymer literature (Handbook of Polymer Science and Technology, ed. N. P. Cheremisinoff, Marcel Dekker, New York, 1989, vol. 1, p. 307) a simple equation has been derived to estimate the lithographic sensitivity of a copolymer of known composition and molecular weight from the constituent homopolymer values. Sensitivity has often been equated with the reactivity parameter DgMwwhere 0, is the gel dose and Mwthe weight-average molecular weight of the polymer.This report demonstrates that the use of the reciprocal reactivity (DgMw)-' leads to a clearer interpretation of the lithographic results and that furthermore, the conclusions drawn from earlier analyses neglect the consequences of co-operative action between differing monomer units within copolymers . Keywords: Lithography; Electron-beam resist; Halogenated polystyrene; Copolymer In an article entitled Radiation-Induced Reactions of Poly-styrene Derioatiues within the Handbook of Polymer Science and Technology,' the authors apply a formula derived on the basis of Charlesby's theory2 to define a theoretical relationship between copolymer composition and sensitivity to electron- beam radiation for polystyrene sensitized with halogen-con- taining functional groups.The form of the equation is as follows: where xMand xNare the mole fractions of monomer units M and N in the copolymer, DgMN, DgMand DgN are the doses corresponding to the onset of gelation of the copolymer and the homopolymers of M and N, respectively, and MwMN, MwM and MwNare the weight-average molecular weights of the copolymer and homopolymers, respectively. It is suggested that the sensitivity of a copolymer may be estimated at any composition by employing this equation with foreknowledge of the sensitivities and molecular weights of the homopoly- mers, and a graphical presentation of experimental data is provided to support the analysis. Although an apparently reasonable fit of data to the theory is shown, their choice of ordinate confuses the issue.DgMNMwMN is plotted on a decreas- ing logarithmic scale, presumably in order to place points for high sensitivity materials at the top of the plot. This leaves the theoretical fit depicted as a curve whose relationship to eqn. (1) is by no means clear to the reader. It would appear far more appropriate to plot the sensitivity parameter as (DgMNMwMN)-', since from eqn. (I), this would present the theoretical fit as a straight line joining the homopolymer points, and any deviation from this line would be immediately apparent. In previous publication^,^ we have dubbed this parameter the intrinsic sensitivity of a negative-working elec- tron-beam resist, since it increases in value with increasing resist sensitivity.This avoids the confusion of citing decreasing gel dose as a measure of increasing sensitivity, and also corrects for the effect of molecular weight variation; hence its value for a particular polymer depends only on the polymer microstructure and is intrinsic to that structure. As a param- eter characteristic of sensitivity, it can be further improved upon by converting the incident lithographic dose to an absorbed radiation dose, or better still by expressing it in terms of the radiation chemical yields for cross-linking and chain scission, G, and G,. It should also be noted that large errors are possible when evaluating the intrinsic sensitivity, since the estimation of both the gel point and the molecular weight of a resist are potentially subject to significant error, particularly when comparing data obtained from different sources.Results and Discussion The data from the above-mentioned article are re-plotted in Fig. 1 as a linear correlation. Despite the scatter in the data, it is now clear that the experimental results do not fit the theoretical prediction of behaviour very well, since the majority of the data points of all three systems, namely poly(styrene-co-chloromethylstyrene),poly(styrene-co-chloro-styrene) and poly(styrene-co-iodostyrene) fall above their pre- dicted lines. In fact, this is to be expected, since the contributions to resist sensitivity in terms of cross-linking mechanisms can be divided into three groups: those arising from processes inherent to one or other homopolymer, which will become more or less prevalent according to the mole fraction of the appropriate monomer unit in the copolymer, plus an additional contribution from cross-linking mechanisms which arise from chemical interactions between both co-monomer units in the copolymer.It is clear that eqn. (1) takes into account only the 'homopolymer' processes, and yet we -0 , 0 I5l =---0+= 0.0 0.25 0.50 0.75 1.0 substituted styrene in copolymer (mole fraction) Fig. 1 Intrinsic sensitivity uersus copolymer composition (data from ref. 1). 0, Poly(styrene-co-chloromethylstyrene);A, poly(styrene-co-iodostyrene); 0,poly(styrene-co-chlorostyrene) have observed that the ‘copolymer’ processes can significantly enhance the sensitivity of the resist.Indeed, the published mechanisms of cross-linking in resists sensitized with chloro- methylstyrene4 centre on the cleavage of the carbon-chlorine bond on irradiation to produce a benzylic radical and a chlorine atom, followed by a-hydrogen or pendant methyl hydrogen atom abstraction from an adjacent styrene or methylstyrene unit by the chlorine atom to yield, overall, two potential cross-linking sites on the chain; this emphasizes the co-operation of the co-monomer units. Only in the absence of such co-operation would the behaviour predicted by eqn. (1) be observed in practice; in fact, the equation treats a statistical copolymer as though it were an ideal blend of the equivalent mole fractions of its constituent homopolymers, and in most cases this assumption is invalid. In the particular case of poly(methy1styrene-co-chloromethylstyrene), there is observed a significant increase in sensitivity for only 5% chloromethylstyrene content and thereafter the improvement is much less pronounced as the composition approaches that of the chloromethylstyrene homopolymer.Such co-operative effects involving the two monomer units are pronounced in copolymer systems containing chlorostyr- ene and our investigations into the performance of resists based on poly(styrene-stat-p-chlorostyrene)yield very different results from those reported in ref. 1. For the purposes of comparison, the variation of intrinsic sensitivity with compo- sition that we have observed, and that of Tanigaki and Tateishi,’ are presented in Fig.2. Our observation is that the synergy arising from the combination of the co-monomers is so great that the intrinsic sensitivities of all of the copolymers are greater than either of the two homopolymers. This phenomenon has also been reported for the same system by Whipps’ who observed a peak in sensitivity for copolymers of similar molecular weight, at a copolymer composition of between 20 and 30% chlorostyrene. We have also reported the same phenomenon in both poly(o-methy1styrene-stat-p-chlorostyrene) and poly(p-methylstyrene-stat-p-chlorostyr-ene)6 and have offered an explanation in terms of a cross- linking mechanism that correlates with the distribution of the two types of monomer unit in the copolymer.The discrepancy between our value of the intrinsic sensitivity of p-chlorostyrene homopolymer and the value given by Tateishi and Tanigaki is large (almost a factor of 2) but this is not unusual when comparing lithographic values in the literature, particularly when dealing with gel dose, which is notoriously difficult to estimate accurately. Our estimates of gel dose are determined by extrapolation from data plotted in the manner described by Charlesby and Pinner,7 a technique which is superior to estimation from lithographic contrast curves. In the case of poly(styrene-co-iodostyrene), Tateishi and Tanigaki refer to a discrepancy in the fit of experimental data 0 0 0 0 0q I 0.0 0.25 0.50 0.75 1-0 chlorostyrene in copolymer (mole fraction) Fig.2 Intrinsic sensitivity uersus copolymer composition for poly(styrene-co-chlorostyrene):0 Data from ref. 1; 0 authors’ data J. MATER. CHEM., 1991, VOL. 1 12 - k3 r ‘do - r I 0 -8-E 6 N 6- % 0 0 A A 0 7 4- z g h i QA 07 I Fig. 3 Intrinsic sensitivity versus copolymer composition for poly- (chlorostyrene-stat-chloromethylstyrene). 0,poly(o-chlorostyrene-stat-chlorometh ylst yrene); A thylstyrene); 0,poly(p-chlorostyrene-stat-chloromethylstyrene) to their theoretical curve at high iodostyrene contents, and furthermore assert that this discrepancy is greater for the iodostyrene system than for the chlorostyrene system ‘due to the difference in hydrogen abstraction strength between a chlorine radical and an iodine radical’.They also point out that eqn. (1) does not take such considerations into account. When the data are plotted as in Fig. 1, it becomes clear that the major deviation from the theory is in fact at low iodostyr- ene content, but it is not clear as to whether this can be attributed to the susceptibility of hydrogen atoms to abstrac- tion by halogen atoms, since there is no obvious reason why this should vary with composition in the manner shown. In our explanation of the lithographic behaviour of the poly(me- thylstyrene-co-chlorostyrene)systems, the cross-linking mech- anism in the copolymer does not involve the generation of a halogen atom, but rather depends on the formation of an intramolecular exciplex between differing adjacent units on the chain.Again, Whipps has reported a peak in sensitivity of copolymers of styrene and iodostyrene corresponding to ca. 35% iodostyrene content,’ which is not very different from that suggested by the data presented in Fig. 1. We are aware of only one family of copolymers for which eqn. (1) does apparently hold true, namely copolymers of chlorostyrene and chloromethylstyrene where the chloro sub- stituent can be in the ortho, meta or para position and the chloromethylstyrene monomer is vinyl benzyl chloride (VBC), a 2: 1 mixture of the meta and para isomers. In all three systems there is a linear increase in intrinsic sensitivity with increasing VBC content (see Fig.3) and, furthermore, esti- mation of the radiation chemical yields demonstrates a con- comitant linear increase in both G, and G,. The reason for the apparent absence of ‘copolymer’ cross-linking processes in these materials is not yet fully explained. In conclusion, analysis of experimentally determined intrin- sic sensitivities of copolymer resists is useful in designing resists, but particularly so in systems where a maximum in the sensitivity plot may determine an optimal composition for a resist of given M,. However, it is our experience that simple analyses based on homopolymer contributions should be treated with considerable scepticism since generally they do not provide results which accord with current knowledge or understanding.Funding for the work described here has been provided by Plessey Research Caswell (now part of GEC Marconi) and SERC. The authors wish to express their gratitude to Dr. David Brambley and the UK Advanced Lithography Research Initiative for help and encouragement. J. MATER. CHEM., 1991, VOL. 1 675 References 5 P. W. Whipps, Proc. Microcircuit Eng. '79 Conf. (Microstructure Fabrication), Elsevier Science Publishers B.V., London, 1980, 1 K. Tanigaki and K. Tateishi, Handbook of Polymer Science and p. 118. Technology, ed. N. P. Cheremisinoff, Marcel Dekker, New York, 6 R. G. Jones, P. C. Miller Tate and D. R. Brambley, J. Mater. 1989, vol. 1, p. 307. Chew., 1991, 401. 2 K. Tanigaki, Y. Ohnishi and S. Fujiwara, ACS Symp. Ser. 242, 7 A. Charlesby and S. H. Pinner, Proc. R. SOC. London, A, 1959, ACS, Washington DC, 1984, p. 177. 249, 367. 3 D. R. Brambley, R. G. Jones, Y. Matsubayashi and P. Miller 8 P. W. Whipps, Society of Plastics Engineers Con5 Photopolymers: Tate, J. Vuc. Sci. Technol. B, 1990, 8(6), 1412. Principles, Processes and Materials, Ellenville N.Y., 1985. 4 See, for example, Y. Tabata, S. Tagawa and M. Washio, ACS Symp. Ser. 266, ACS, Washington DC, 1984, p. 151. Paper 1 lOl7201; Received 21st March, 1991