High Resolution Electron Beam Negative Resist with Very Narrow Molecular Weight Distributions

Abstract

Positive resists, for example, p o l y m e t h y l m e t h a c r ylate (PMMA) have been found to be the best materials for electron beam lithography. The 250A linewidths can be fabricated on 225A thick films of P d -A u alloy (1). A 400A linewidth has been demonstrated on thick silicon substrates (2). In contrast with such high resolution patterns of positive resist, negative resists have been considered as being lower resolution materials (3, 4). The resolution of negative resists is obviously limited by several factors such as low contrast and developer solvent-induced swelling.The molecular weight distribution of polymer has been shown recently to be an important factor for lithographic contrast (4-6). Lai and Shepherd examined polystyrenes as negative resists, demonstrating an improvement of the contrast with a n a r r o w e r molecular weight distribution (6). T h e y used polystyrenes obtaiaed from Goodyear Chemicals, Akron, Ohio, whose molecular weight distribution (Mw/Mn) lay from 1.07 to 2.8, as determined by gel permeation chromatography (GPC). Other polystyrenes from Pressure Chemical Company, have been examined by Felt and Stillwagon (5). Although recent papers have reported how contrast changes with uniformity for a series of aromatic homopolymers, the molecular weight distributions of the polymers examined before seem not to be n a r r o w enough to merit being called monodispersed polymers.It is both interesting and important to study how polymers with more tightly controlled M w / M n can improve the performance of negative resists. Fujimoto et al. have extensively studied the preparation of monodispersed polystyrenes (7), p o l y -~-m e t h y l s t yrenes (8), and other polymers (9, 10) with an extremely n a r r o w molecular weight distribution. They ~ Electrochemical Society Active Member.have succeeded in the synthesis of polymers with molecular weight distribution (Mw/Mn) less than 1.01. In this note, we wish to address an e x p e r i m e n t a l result obtained with polystyrenes having e x t r e m e l y n a r r o w molecular weight distributions. Figure 1 shows an example of sedimentation patterns of polystyrene having Mw --~ 1.00 • 105 obtained with a Spinko Model-E ultracentrifuge. Each pattern was taken every 16 rain after reaching m a x i m u m height (Fig. l a ) . The crucial results of the sedimentation patterns are that very sharp sedimentation boundaries are obtained and the heights of the patterns of d, e, and f in Fig. 1 are almost the same even when the sedimentation is continued. These results indicate that the polymer examined here has a very n a r r o w molecular weight distribution. GPC analysis of the sample was also carried out with a Toyo Soda Model-HLC-802 equipped with a low-angle light scattering photometer Model LS-8. Two T S K -G E L (GMH6000) columns, Toyo Soda Company, were used. A detail discussion of the GPC measurement has been published in a previous paper (11). The molecular weight of the polymer was determined by three different methods: light scattering, visco...

Effect of molecular weight distribution on e-beam exposure properties of polystyrene

“…In previous studies by us [4][5][6][7] and other research groups [10][11][12][13][14][15][16], in order to avoid any uncertainties, polystyrene with very narrow molecular weight distribution with PDI ≤ 1.06 has been utilized. Ku's paper published in 1969 also recommended polystyrene with narrow Mw distribution for EBL in order to avoid potential pinhole formation after development [17].…”

Anionic living polymerization of useful monomers that can provide intermolecular chemical links

“…Thus, PVS is easier to handle when carrying out a lithographic evaluation compared to ordinary cyclized polyisoprene negative resists [83,84].…”