Significance of plasma-photoresist interactions for atomic layer etching processes with extreme ultraviolet photoresist
Extreme ultraviolet (EUV) lithography has emerged as the next generational step in advancing the manufacturing of increasingly complex semiconductor devices. The commercial viability of this new lithographic technique requires compatible photoresist (PR) materials that satisfy both the lithographic and etch requirements of good feature resolution, chemical sensitivity, a low line edge roughness, and good critical dimension uniformity. Achieving the decreased feature pitches of modern processing nodes via EUV lithography places a limit on the available photoresist thickness for a pattern transfer process. Therefore, etch processes are required to maximize the etching selectivity of a hard mask material, such as SiO2, to an EUV photoresist. In this work, the authors evaluated the ability of an atomic layer etching (ALE) process to maximize the SiO2/EUV PR etching selectivity. Through the flexible parameter space available in an ALE process, the authors evaluated the etching behaviors as a function of the ALE parameters of ion energy, etch step length, fluorocarbon (FC) deposition thickness, and precursor gas type. The authors found that the interaction between the energetic argon ion bombardment and a deposited FC layer produces a modified surface layer on the PR material that can strongly control the PR etch rate and even produce an etch stop under some conditions. Under the same processing conditions, the etching behavior of SiO2 continues unimpeded, thus resulting in a high overall SiO2/PR etching selectivity. Secondary characterization using x-ray photoelectron spectroscopy and atomic force microscopy was used to support the conclusions derived from the ellipsometric modeling based on the surface chemistry evolution and determine the impact of the ALE process on the surface roughness of the EUV PR, respectively. Additionally, attenuated total reflection Fourier-transform infrared spectroscopy was used to track the impact on specific functional groups within the PR composition from both the argon ion bombardment and FC deposition components of the ALE process. The ALE-based PR etching concept established in this work serves as a foundation for both the understanding of the impacts of an ALE process on an EUV PR material and for future works, employing an ALE process for PR-based pattern transfer.