Top Surface Imaging at 157 nm

Top surface imaging (TSI) has had an interesting history. This process showed great promise in the late 1980’s and several attempts were made to introduce it to full-scale manufacturing. Unfortunately, defect density problems limited the process and it fell from favor. TSI emerged again as an important part of the EUV and 193 nm strategies in the early stages of those programs because it offered a solution to the high opacity of common resist materials at both wavelengths. A flurry of research in both areas identified the seemingly insurmountable problem of line edge roughness, which caused the process to be dropped from both programs. Recent developments in TSI have demonstrated the ability to print high-resolution, highaspect ratio images at 193 nm with less line edge roughness than typical single layer resist systems. This has largely been due to the development of polymers specifically tailored for this end use. The optimum materials must be moderately transparent and have high Tg’s in the silylated state. The 157 nm program has much in common with the early stages of the 193 nm program. The optical density of even 193 nm resist materials at 157 nm is far too high to allow their use in single layer applications. The less stringent optical density requirements of TSI make it a potentially viable imaging scheme for use at 157 nm. Various TSI materials, including the traditional poly (t-BOC-hydroxystyrene), as well as novel aliphatic cyclic polymers bearing bis-trifluoromethyl carbinol substituents, have been investigated for use at 157 nm, and smooth highresolution images have been generated.