Abstract

Traditional optical lithography uses semitransparent single-layer resists where the exposing radiation produces a latent image throughout the thickness of the resist, and an isotropic liquid-based development step is used to create the desired resist profile. However, resolution of 0.25-μm or better, which is required for 256 Mbit DRAM chips and beyond, cannot be obtained with conventional photolithographic technology (250-400 nm wavelength). Shorter wavelengths are required, such as 193-nm (DUV-193), soft-X- ray (10-50 nm) or hard-X-ray (1-2 nm) radiation. Unfortunately, between the wavelengths of 10 and 220 nm few materials are semitransparent For example, at 193 nm novolac resins commonly used at longer wavelengths have an absorption depth of only 40 nm, and at the 13-nm wavelength it is 400 nm. New resist processes capable of accommodating a latent image confined to the near-surface region must therefore be developed. The lithographic process must now include an additional processing step, namely pattern transfer into the bulk of the resist. This step must be highly anisotropic so that the pattern in the surface layer is faithfully reproduced at the resist-substrate interface. These general considerations were outlined already in 1984 by Taylor et al.[1]. They apply also to other forms of strongly absorbed radiation such as ion beams. (For instance, a 30 keV Ga⁺ beam has a projected range of only 34 nm in photoresist.) Today, surface imaging processes have been demonstrated not only for soft X-ray projection (SXP) [2] and deep ultraviolet (DUV) lithographies [3], but are being considered for manufacturing processes at optical wavelengths as well [4]. In this paper we review some of the recent developments in the areas of surface imaging and multilayer resists, with emphasis on application to 193-nm lithography. It should be noted, however, that some of the reviewed resist concepts may be applicable to SXP lithography as well.

© 1992 Optical Society of America