Comparison of acid generating efficiencies in 248 and 193 nm photoresists

Photoacid generation is a critical step in the application of chemically amplified (CA) resist technology. During the key exposure step, a catalytic amount of a strong Bronsted acid is released within these resists. The photoacid is subsequently used in a post-exposure bake step to catalytically react with the resist polymer. In the case of a positive tone resist, an acid sensitive polymer is deprotected to render the exposed areas soluble in dilute aqueous base thereby allowing for pattern development. As the semiconductor industry begins to focus on developing 193 nm photoresists for production, it is important to identify and understand differences between prototype 193 nm CA resists and current state of the art 248 nm production worthy photoresists. The major difference between 193 and 248 nm photoresists is the exposure wavelength, which is reduced to achieve higher resolution based on the Rayleigh equation, However, this change in wavelength has several ramifications: Firstly, the tried, tested and true phenolic polymers used in DUV resists are too absorbent to be used for 193 nm application and had to be replaced by low absorbing, non-aromatic systems. Second, since even these new platforms are still more absorbing at 193 nm than the phenolic matrices are at 248 nm, the PAG loading had to be lowered significantly in order to keep the overall absorbance of the resist down. This paper describes the results of our systematic studies on understanding the reasons for observed differences in photoacid generating efficiency between 193 and 248 nm chemically amplified resist systems. First, the wavelength effect is studied by comparing the relative acid generating efficiency of onium type PAGs in a prototype 193 nm and a DUV photoresist at both 193 nm and 248 nm exposure. Second, the photoacid generating efficiency for these PAGs at 248 nm is compared in both phenolic and non-phenolic based photoresists to probe resist polymer matrix effects. Third, these experiments were repeated while varying the PAG loading in order to probe whether there is an effect of PAG loading on acid generation efficiency. Lastly, by performing all of these studies on two different onium PAG classes (iodonium and sulfonium salts), the impact of the PAG chromophore on acid generation efficiency in both sensitized and unsensitized environments was probed. In all these studies, the C-parameter method is used to determine the quantum yield of photoacid generation. First, the exposure wavelength was found to play, a significant role in determining the acid generation efficiency of both PAGS, namely efficiency significantly decreases when switching exposure wavelength from 248 to 193 nm. Second, it was also found that the change in the resist matrix polymer has a profound impact on the manner in which acid is generated: The phenolic matrix enables sensitized acid generation via electron transfer from the matrix to the PAG, whereas in the acrylate polymer only direct acid generation is observed. Due to the different oxidation potential of iodonium and sulfonium PAGs, the matrix effect impacts the photoacid generation efficiency of the two PAGs very differently. This is apparent in the observed change when going from the phenolic to the methacrylate matrix. Lastly, the presence of the sensitized channel is also responsible for the observed impact of PAG loading in the phenolic polymer, which is largely absent in the acrylate matrix.