Characterization and lithographic performance of silsesquioxane 193 nm bilayer resists

Abstract

Positive 193 nm bilayer resists based on polysilsesquioxane are described. Fluoroalcohol is employed as an acid group instead of carboxylic acid because of its more attractive dissolution properties. Polymers were carefully analyzed by 19F, 13T. and 29Si NMR to determine composition and to quantify residual acetyl, silanol, and Q/T. In an attempt to better understand the dissolution behavior of exposed resist films, the silsesquioxane resist polymers were partially and fully deprotected in solution with acid and their dissolution kinetics investigated by using a quartz crystal microbalance (QCM). The exposed areas of the silsesquioxane resists can have a very fast dissolution rate (Rmax) of >20.000 A/sec (or even > 100,000 A/sec). Heating the fully deprotected model polymers to 150°C did not reduce the dissolution rate much, suggesting thermal condensation of silanol end groups is insignificant. Model deprotected polymers containing triphenylsulfonium nonaflate were exposed to 254 nm radiation, baked, and subjected to QCM measurements in order to probe whether or not acid-catalyzed silanol condensation would reduce the dissolution rate. A combination of high dose and high temperature bake resulted in significant reduction of the dissolution rate in the silsesquioxane polymer containing a small trifluoroalcohol. However, the dissolution behavior of the polymer bearing a bulky norbornene hexafluoroalcohol was unaffected by exposure and bake. Chemical and development contrast curves were generated by using a thermal gradient hotplate and bake temperature effects investigated. A postexposure bake temperature effect was quite small. The process window of the silsesquioxane bilayer resists was comparable to that of a high performance commercial 193 nm single layer resist for both isolated and nested contact hole imaging, Superior performance of our silsesquioxane resist was demonstrated in patterned etch in comparison with a cycloolefin-maleic anhydride bilayer resist. © 2005 TAPJ.