Electron‐beam lithography of chlorinated polystyrenes with narrow molecular weight distributions

Abstract

Polystyrenes with narrow molecular weight distributions have been chlorinated with only slight broadening of molecular weight distributions to yield materials that are five to six times more sensitive to electron‐beam irradiation. The chlorinated polystyrenes are useful negative resists for electron‐beam lithography. At molecular weights of 3‐4 × 105g/mole, their threshold sensitivities to 20 keV electrons are 1‐2μC/cm2. Their lithographic contrasts are ≥ 1.5; in fact, > 2.0 for the narrowest molecular weight distributions. They compare favorably in these regards to polystyrenes of similar molecular weight characteristics. Lithographic relief images that are 80% of the original thickness of the polymer film can be formed at three to four times the threshold dose. The degree of chlorination ranged from 0.65‐0.76 Cl/monomer unit. The infrared spectra of the chlorinated materials resemble the spectrum for poly(4‐chlorostyrene). Elemental analysis indicated, however that some addition of chlorine had occurred. More extensive chlorination or chlorination of the higher molecular weight starting materials broadened the molecular weight distributions. The chlorinated material, a kind of poly(4‐chlorostyrene‐costyrene), is less sensitive than poly(4‐chlorostyrenc) prepared by addition polymerization by almost a factor of four.

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

“…It has been pointed out that the parameter, 8, is relatively more sensitive to heterogeneity index than the parameter, ~, for heterogeneity indexes smaller than 2 (5). Felt and Stillwagon have reported a 8-value of about 3.3 for polystyrene having Mw _--1.6 • 105 with Mw/Mn ----1.12 (5). The value obtained here is obviously less than 3.3.…”

“…4. Feit and Stillwagon reported a similar characteristic curve for a polystyrene having Mw/Mn ~ 1.06 (5). Although sharply rising curves have been reported by Lai and Shepherd (6), even for a polystyrene having a broad distribution, they seem to be due to inaccurate determinations of the gel point.…”

“…This strongly suggests that the final resolution of the negative resist presented here is almost the same as that of positive resists. Although the importance of narrow molecular weight distribution, as estimated by the heterogeneity index (Mw/ Mn), has been examined previously (5,6), an important result of the present paper is that a negative resist with very narrow molecular weight distribution (Mw/Mn < 1.01) has very much superior capabilities for resolution in the submicron region and has almost the same resolution as positive resists. Such a general result can be extended to the high molecular weight polystyrenes that will be more sensitive than the polystyrene examined here and to other polymers.…”

On the Crosslinking Reaction in Negative Electron Resists Based on Polystyrene and Poly(α-methylstyrene)

“…Halogen groups were found to be particularly suitable to obtain both high resolution and sensitivity. Polychlorostyrene (PCSt), 2 chloromethylated PSt (CMS), 3 and iodinated PSt (IPS) 4 are representative negative electron resists developed so far for direct wafer processing.…”

Resolution Limit of Negative Electron Resist Exposed on a Thin Film Substrate

“…Negative resists based on polystyrene (PSt) structure (10)(11)(12)(13), however, have excellent d r y -e t c hing durabilities, thereby eliminating one of the d r a wbacks of using a thinner resist. Negative resists based on polystyrene (PSt) structure (10)(11)(12)(13), however, have excellent d r y -e t c hing durabilities, thereby eliminating one of the d r a wbacks of using a thinner resist.…”