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Spot size measurements of a 260 fs, 1030 nm focused spatially Gaussian pulsed laser beam were performed on a Silicon surface with native and thermally grown SiO 2 layers using a widely known method of evaluating laser beam energy dependent damage area. Single pulse laser induced damage thresholds of both samples were also measured. Modification of the thermally grown SiO 2 layer was analyzed in detail using optical, confocal and scanning electron microscopy. Restrictions in Gaussian beam spot size measurements on the samples with transparent coatings and several observable thresholds are discussed.