THIN-FILM IMAGING - PAST, PRESENT, PROGNOSIS

As the limitations of conventional optical lithography approach, poten tial extensions of a current technology are examined more closely. One of these extensions is to limit the photoresist thickness that is nee ded for recording the imaging information. Because the low etch resist ance of resist typically precludes the use solely of resists utilizing very thin film, a variety of alternatives have been explored. These r ange from elaborate trilayer schemes to relatively simple processes su ch as top-surface imaging (TSI) and a number of combinations thereof. In all of these systems, the aim is to limit the imaging resist thickn ess to a thin layer by confining the radiation near the surface of the resist. This improves process latitude (e.g., depth of focus, exposur e latitude) and also reduces reflective notching and thin-film interfe rence 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 pro cessing. In this paper, we refer to all of these types of approaches a s thin-film imaging (TFI) systems. We review TFI approaches from a his torical perspective, examine a number of the schemes that have been pr oposed, 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 applica tions at wavelengths at, or less than, 193 nm.