Sublimation behavior of SiO2 from low- and high-index silicon surfaces

Abstract

The authors have used atomic force microscopy to investigate the sublimation behavior of 100-nm-thick oxide layers from the low-index Si(001) and Si(111) surfaces, as well as the stable, high-index Si(113) and Si(5 5 12) surfaces. Similar to previous thin-film (<50nm) sublimation studies, high vacuum annealing results in the formation of circular voids that grow laterally with annealing time (1–6min, 1150–1350°C). The depth of these voids is ∼200nm, or twice the thickness of the original oxide film, which is consistent with the thermal decomposition of SiO2 to form volatile SiO. There are subtle morphological differences, however, between the voids formed on the different surface orientations. Line profiles of the bottom Si surfaces inside the voids indicate flat terraces for the (111) and (113) orientations and sloped conic sections for the (001) and (5 5 12) orientations, indicating that the latter surfaces are less stable with respect to step formation during oxide sublimation. At the centers of voids, features such as deep pits (5μm diameter, 200–700nm depth) or vertical structures (1μm diameter, up to 700nm height) are present. Deep depressions are also observed at vertical sidewalls that occur during sequential anneals or on a patterned surface. After the thick oxide is completely desorbed, an undulating surface morphology occurs on all surface orientations due to the coalescence of the void sidewalls.