Thin-film imaging: Past, present, prognosis

Abstract

As the limitations of conventional optical lithography approach, potential extensions of a current technology are examined more closely. One of these extensions is to limit the photoresist thickness that is needed for recording the imaging information. Because the low etch resistance of resist typically precludes the use solely of resists utilizing very thin film, a variety of alternatives have been explored. These range from elaborate trilayer schemes to relatively simple processes such as top-surface imaging (TSI) and a number of combinations thereof. In all of these systems, the aim is to limit the imaging resist thickness to a thin layer by confining the radiation near the surface of the resist. This improves process latitude (e.g., depth of focus, exposure latitude) and also reduces reflective notching and thin-film interference effects. The imaged pattern in the thin-film resist processed by TSI is then transferred by plasma etching into a thicker underlayer. This "stack" then serves as the resist mask for subsequent wafer processing. In this paper, we refer to all of these types of approaches as thin-film imaging (TFI) systems. We review TFI approaches from a historical perspective, examine a number of the schemes that have been proposed, and describe the various technical issues associated with the implementation of such systems. From this perspective, we suggest that TFI systems may find a role in manufacturing for lithographic applications at wavelengths at, or less than, 193 nm.