Wavelength Dependence of Poly(p-hydroxystyrene) Ablation by Mid-Infrared Free-Electron Laser

Ablation of polymer thin films for resists was studied using a mid-Infrared (IR) Free-Electron Laser (FEL). Irradiation fluence of the IR light in the mid-IR region (5.6 to 8.0 μm) to a thin film of poly(p-hydroxystyrene) (PHOST) on a silicon wafer increased to cause PHOST ablation. The ablated spot size was smaller than the irradiated area calculated by wave optics, because the ablation is a multiphoton absorption process. The ablation threshold energy was determined using the irradiation area calculated by the wave optics. The threshold energy depended upon the film thickness of PHOST and irradiation wavelength of FEL. The threshold energy of thin PHOST film with 61 nm increased at the wavelength of vibrational modes of phenyl ring. It associates the durability of phenyl ring to photon decomposition.