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

• Published in: Journal of Photopolymer Science and Technology Vol. 15; no. 3; pp. 453 - 464
• Main Authors: Cameron, James F., Pohlers, Gerd, Suzuki, Yasuhiro, Chan, Nicholas
• Format: Journal Article
• Language: English
• Published: The Society of Photopolymer Science and Technology(SPST) 2002
• Subjects: acid generating efficiency; C-parameter; deep UV and 193nm chemically amplified resists; P-parameter; photoacid generator; quantum yield
• DOI: 10.2494/photopolymer.15.453
• ISSN: 0914-9244

The advent of 193nm lithography has stimulated the search for novel photocid generators (PAGs) with increased transparency at 193nm. This need for more transparency stems from the use of phenyl groups in the classic 248nm PACs, such as triphenylsulfonium salts. Unfortunately for 193nm resist development, the phenyl group is highly absorbing at that wavelength, thereby severely restricting the PAG formulation space. It has not 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 for 248nm phenolic systems, i.e. the light absorbed by the polymer does not contribute to acid generation. Furthermore, the photoacid generation efficiency of sulfonium PACs drops considerably when going from 248nm to 193nm. Thus, the ability to quantify the photoefficiency of a new PAG quickly becomes key to successful new 193nm PAG development. While there are many ways to determine the quantum yield of photoacid generation Φ. of PACs, they are usually 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, Φ to characterize the photoefficiency of a PAG. Results for a number of sulfonium PAGs at both 248 and 193nm are determined this way, and the observed trends are discussed. In the second part of this work we investigate how lithographic performance is impacted by those two parameters. A transparency/efficiency matrix of nine PAGs is evaluated in a standard 193nm resist formulation. The results indicate that the PAGs with a combination of high efficiency and high transparency yield the best lithographic results, underlining the importance of efficiency and transparency as key parametesr in 193nm PAG design.