Viscoelastic deformation of borosilicate glass substrates induced by a laser-patterned silicon suboxide film

In general, a thin film deposited on a substrate surface exhibits a certain amount of stress that causes a deformation of the substrate. Usually, the deformation is elastic, however, under some circumstances, a non-recoverable deformation of the substrate can be observed. Recently, we demonstrated that a large non-recoverable deformation of fused silica substrates can be obtained if on one side a film of substoichiometric silicon oxide is deposited and the film-substrate system is annealed at a temperature in the glass transition range. By laser patterning of the film, specific surface shapes can be obtained. Here, we report on a similar approach for the viscoelastic deformation of plates of borosilicate glass (Schott N-BK7). The substrates are coated by a film of substoichiometric silicon oxide, which is patterned by application of a XeCl-excimer laser. We measured the surface shape of the non-coated side after different annealing durations. Deformation of the samples corresponds to a tensile film stress. But by application of a line pattern, toroid surface shapes with curvatures of different sign can be easily obtained. With further investigation and development, the process might be applied for production of freeform surfaces.