Resist-polymer ablation by mid-infrared-free-electron laser

Laser ablation of poly(4-hydroxystyrene) (PHOST) film was studied using mid-Infrared free-electron laser (mid-IR FEL), of which irradiation wavelength was tuned to the vibrational absorption peaks of PHOST in mid-IR spectral region. A PHOST film was ablated by mid-IR FEL light to produce a clear hole in a PHOST film which suggested the photochemical ablation. As the threshold energy for silicon ablation was larger than that of PHOST, it was possible to ablate a PHOST film without any damage to a silicon substrate. The ablation threshold-energy ratio for PHOST to silicon was less than 0.2 and depended on the mid-FEL wavelength. The ablation threshold for a PHOST film depended also on the film thickness. The highly efficient ablation by mid-IR FEL was found to be due to the vibrational excitations of C-O stretching and C-O-H bending of PHOST.