Photoresist modifications by plasma vacuum ultraviolet radiation: The role of polymer structure and plasma chemistry

While vacuum ultraviolet (VUV) photon irradiation has been shown to significantly contribute to material modifications of polymers during plasma exposures, the impact of radiation-induced material alterations on roughness development during plasma processing has remained unclear. The authors have studied the interaction of the radiation of Ar and $C 4 F 8 / Ar$ plasma discharges with 193 and 248 nm advanced photoresists (PRs). Optical filters were used to vary the radiation exposure wavelength range in the ultraviolet (UV) and VUV emission spectra. This enables clarification of the respective roles of plasma photon radiation wavelength and PR polymer structure on the chemical and structural changes produced in the materials. Chemical changes in polymer composition at the film surface and in the material bulk were determined by vacuum transfer x-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. Morphological changes, film thickness reduction, and changes in surface and pattern morphology were characterized by ellipsometry, scanning electron microscopy, and atomic force microscopy. The exposure of methacrylate based 193 nm PR to photon radiation in the UV/VUV spectral range $( λ = 112 – 143 nm )$ leads to detachment and removal of oxygen containing polymer pendant groups to a depth of about 200 nm. This causes changes in the polymer structure by chain scission, significant film thickness reduction, and reduced pattern critical dimensions and line edge roughness. Chain-scission reactions and residual detached polymer pendant groups are expected to effectively soften layers of 193 nm PR. In contrast to 193 nm PR, styrene based 248 nm PR was found to be significantly more stable under plasma-produced irradiation due to the low oxygen content, low ester linkage concentration, and absence of lactone. Small thickness reduction, reduced oxygen loss, and cross-linking were observed in the surface region of 248 nm PR. Radiation-induced material modifications of both PR materials decreased with increasing photon wavelength in Ar discharges. Increasing modification of 193 nm PR was observed for increasing photon flux at higher wavelengths $( λ = 143 – 300 nm )$ by the emission characteristic of fluorocarbon containing plasmas. In $C 4 F 8 / Ar$ plasma, the authors observed strongly increased loss of oxygen at the film surface and in the $C  O  C$ and $C  O$ lactone bonds in the material bulk along with film thickness reduction compared to pure Ar discharges. These modifications are directly relevant to plasma processes used for pattern transfer, which often contain fluorocarbon species.