Plasma based dry etching is a key process widely used in micro-fabrication today. In this article, we look at the challenges involved in the anisotropic etching of buried SiO2 layers at the bottom of high aspect ratio structures on SOI wa-fers. We present our etch results that show the limitations of using a process with radio frequency (RF) substrate bias. This is followed by results obtained with a newly developed dielectric etch process based on a pulsed low frequency (LF) bias which makes it possible to etch through even rela-tively thick buried oxide layers. Finally we present an application in which this newly developed process was used.