Elastic and viscoelastic deformation of glass plates by excimer laser induced stresses and patterned silicon suboxide films
von Clemens Martin Beckmann
Datum der mündl. Prüfung:2021-09-16
Betreuer:Dr. Jürgen Ihlemann
Gutachter:Prof. Dr. Alexander Apl. Egner
Gutachter:Prof. Dr. Hans Christian Hofsäss
EnglischA mechanically stressed film can cause a bending of the underlying substrate. Usually, the substrate deforms elastically, but in special cases a plastic or viscous deformation can be observed. Such deformations are mostly unwanted. However, methods have been developed by several people to specifically deform plate-like substrates via the generation of plane stress components inside a near-surface region, and by this to correct errors in the surface topography of thin optical components (figure correction). It was also proposed to make use of the viscous deformation of a glass substrate due to a stressed film for forming of optical components. Excimer lasers are applied in precise structuring of glass surfaces and patterning of dielectric films on glass substrates. In this thesis, I report on the deformations and stresses in two systems related to the application of an ArF excimer laser (193 nm, 20 ns). The first system consists of sheets of the borosilicate glass Schott D263M. When irradiated, a large tensile stress is generated in a surface-near region. From the deformation of the samples, I measured the integrated stress in dependence on the irradiation parameters, its long-term temporal evolution and the stress distribution in direction normal to the surface. I show that antibiaxial plane-stress components can be generated by irradiation with a line pattern. I performed a proof of principle for an application of the laser induced stresses in figure correction. The results also demonstrate the relevance of long-term thermal stresses in excimer laser ablation of glasses. The second system consists of continuous or laser patterned thin films of a substoichiometric silicon oxide on plate-like substrates of vitreous silica. Annealing of the samples at a temperature close to the glass transition causes a large viscous deformation of the substrates, which is determined by the film pattern. I measured the deformation for different film patterns and for different atmospheres during annealing. By comparison to the results of a finite element simulation and analytic calculations, I show that the deformation can be understood by analogy to the case of an elastic substrate and stresses due to the oxidation of the film. The results pave the way for an application for forming of plates by a patterned film.
Keywords: glass; D263; borosilicate; fused silica; vitreous silica; suprasil; laser; excimer; ablation; rear-side ablation; backside ablation; film stress; thermal stress; temperature gradient mechanism; laser forming; elastic deformation; viscoelastic deformation; viscous flow; substoichiometric silicon oxide; silicon suboxide; silicon-rich silicon oxide; oxidation; figure correction; stress shaping; reflective optics; finite element