Relationship between deprotection and film thickness loss during plasma etching of positive tone chemically amplified resists

Positive tone chemically amplified (CA) resists have demonstrated the sensi tivity, contrast, and resolution necessary to print state-of-the-art subwav elength features using 248 nm and more recently 193 nm lithography. These m aterials are also being considered for printing sub-100 nm features with 15 7 nm and next-generation lithography technologies such as extreme ultraviol et and electron beam projection lithography. The basis for solubility diffe rential and image formation in these resists is the acid catalyzed deprotec tion of labile protecting groups of an inherently base soluble polymer, The deprotection is effected by the photochemical generation of strong acid du ring the exposure process. Such acid-catalyzed deprotection reactions can a lso occur in unexposed resist areas when etched in a plasma. This can be du e to UV exposure, high-energy ion bombardment. elevated substrate temperatu res, or interaction of the resist surface with plasma species to form acidi c moieties. Deprotection has been associated with resist mass loss and film shrinkage during plasma etching, leaving inadequate masking material for t he entire etch step. In this article. we report the film thickness loss of several unexposed CA resists as a function of etch time in a variety of pla smas and correlate these data with film composition, monitored by Fourier t ransform infrared spectroscopy. These results are compared with theoretical predictions based on generally accepted deprotection mechanisms. Our findi ngs indicate that the "acidic'" nature of certain plasmas such as Cl-2/O-2 can result in deprotection in the resist film. even in the absence of a pho toacid generator. Additionally, the data suggest that the nature of the res ist polymer and, in turn, the identity of the deprotection products directl y influence resist mass loss and etch rate linearity, both of which can be controlled by careful selection of resist materials. (C) 2001 American Vacu um Society.