Transparency versus Efficiency: Important Considerations in the Design of Photoacid Generators for ArF Lithography

摘要

The advent of 193nm lithography has stimulated the search for novel photoacid generators (PAGs) with increased transparency at 193 nm. This need for more transparency stems from the use of phenyl groups in the classic 248 nm PAGs, such as triphenylsulfonium salts. Unfortunately for 193 nm resist development, the phenyl group is highly absorbing at that wavelength, thereby severely restricting the PAG formulation space. It hasnot yet been pointed out, however, that photoacid generation efficiency has also become more important for 193nm systems. The reason is that 193 polymers cannot sensitize the PAG, as is the case of 248 nm phenolic systems, i.e.the light absorbed by the polymer does not contribute to acid generation. Furthermore, the photoacid generation efficiency of sulfonium PAGs drops considerably when going from 248 nm to 193 nm. Thus, the ability to quantify the photoefficiency of a new PAG quickly become skey to successful new 193 nmn PAG development. While there are many ways to determine the quantym yield of photoacid generation (PHI. of PAGs, they are usaully both time and labor-intensive. Therefore, in the first part of this paper, we describe the P-parameter as a fast, simple and more practical way than the quantum yield, PHI to characterize the photoefficiency of a PAG. Results for a number of sulfonium PAGs at both 248 and 193 nm are determined this way, and the observed trends are discussed. In the second part of this work we investigate how lithographic performance is impaced by thos etwo parameters. A transparency/efficiency matrix of nine PAGs is evaluated in a sandard 193 nm resist formulation. The results indicate that the PAGs with a combination of high efficiency nd high transparency yield the best lithographic results, underlining the importance of efficiency and transparency as key parametrs in 193 nm PAG design.