F2 laser ablation of silicone rubber

The ablation of silicone rubber (polydimethysiloxane, [SiO(CH3)2]n) by 157-nm F2 laser irradiation has been studied. The ablation threshold fluence (Fth) was approximately 140 mJ/cm2. The irradiated surface swelled and was modified to SiO2 for laser fluences below Fth. No carbon contamination was observed in the SiO2. Above Fth, the surface of etched silicone rubber was structurally well defined and precisely patterned. The chemical composition of the etched surface demonstrated little change. When the silicone rubber was irradiated at laser fluences of nearly Fth, conical structures were formed on the surface. It is supposed that residual carbon photodissociated by laser irradiation initiated cone formation by locally shifting Fth to a higher value. In situ mass spectrometric analysis of gaseous products evolved during F2 laser irradiation of silicone rubber showed that CH3 and CH4 components dominated for laser fluences below Fth, and CO component dominated for laser fluences above Fth. The high photon energy (7.9 eV) of an F2 laser could photodissociate preferentially weak bonds (Si-C) of silicone for lower laser fluence and almost all bonds (Si-C, C-H, Si-O) of silicone for higher laser fluence.