Ablation of Organic Polymers by Ultraviolet and Visible Radiation: A Theoretical Approach

There has been considerable interest recently in the use of laser radiation to remove material from a solid. Of particular interest is the observation that short pulses far-UV (e.g., 193 nm) radiation ablates organic polymers cleanly, i.e., the remaining sample exhibits a precisely defined pit (1-3). It is possible that the radiation could have melted or damaged the remaining sample. Indeed for longer wavelength radiation, e.g. visible light, damaged samples are generally observed (4). Obviously there are at least two distinct processes that can result in ablation of material when it is irradiated by laser light. In the following discussion we will refer to the phenomenon that cleanly etches the material as the photochemical process and the one that melts the sample as the thermal process. We have developed microscopic models for each of the two processes (5). In the photochemical model we assume one photon is directly responsible for dissociating one bond via a transition to an excited electronic state. In the thermal model we assume that the laser energy is absorbed into vibrational modes of the solid.