Real-time measurements of plasma photoresist modifications: The role of plasma vacuum ultraviolet radiation and ions

Plasma-induced roughness development of photoresist (PR) can be due to synergistic interactions of surface modifications introduced by ions, bulk material modifications by ultraviolet (UV)/vacuum ultraviolet (VUV) radiation, and increased temperature. While previous work identified the individual contributions of energetic ions and UV/VUV radiation, the dynamics of the concurrent modifications remained unclear. The authors studied the interactions of plasma radiation and ions with 193 nm PR and 248 nm PR in Ar plasma by in situ ellipsometry. Ellipsometry provides valuable information on changes in film thickness and material optical properties in real-time during plasma processing. MgF2, sapphire, and glass optical filters were used to reduce the plasma-material interactions to the radiation component of a selected wavelength range in the UV/VUV spectral region. Ar plasma radiation and its transmission through different optical filters were characterized by VUV spectroscopy. This characterization allowed for the identification of the relative contribution of specific wavelength ranges to the overall material modification in direct plasmas. Exposure of 193 nm PR to plasma UV/VUV radiation led to film thickness reduction and an increase in the refractive index ((N) over tilde) which was enhanced for shorter wavelengths. For direct plasma exposure at low maximum ion energy (E-ion <= 25 eV), radiation effects dominated and bulk material modifications saturated after approximate to 30 s. By comparison, for 248 nm PR film thickness reduction and increase of (N) over tilde were modest and were only seen for UV/VUV exposure with wavelength lower than 142 nm. Both material surfaces remained relatively smooth during 60 s exposure to plasma radiation (0.3-0.6 nm RMS roughness) independent of radiation wavelengths used. For direct Ar plasma exposure involving energetic ion bombardment (E-ion <= 125 eV), a graphitic surface layer was established within 3-5 s. During this time period approximate to 30% of the saturation UV/VUV modification in the material bulk of 193 nm PR were reached, which corresponds to a 60 nm thick soft layer underneath the ion crust. For 248 nm PR less than 20% of the bulk material modification that were established in 60 s were reached during formation of the ion-crust. This corresponds to a approximate to 4 nm thick hard layer underneath the ion-modified surface. This shows that for 193 nm PR, bulk material modifications by plasma UV/VUV radiation were fast and contributed significantly to the overall material modification in the first seconds of plasma exposure, whereas UV/VUV modifications of 248 nm PR were slow and small. This material specific difference in the dynamics of material modification was in good agreement with 193 nm PR developing much higher surface roughness (approximate to 6.0 nm) than 248 nm PR (approximate to 1.6 nm) during 60 s exposure to plasma exposures involving both UV/VUV radiation and energetic ion bombardment. (C) 2012 American Vacuum Society. [http://dx.doi.org/10.1116/1.3697752]