The fabrication of high aspect ratio silicon trenches (critical dimension <100 nm, aspect ratio> 10:1) by dry etch processing has proven to be a challenge mainly due to limited etch selectivity of conventional hardmask materials to Si. Moreover, for future technology nodes the hardmask thickness will be limited by the thickness of the photoresist. This work focuses on a concept to enable the usage of very thin resist layers (<100 nm) for patterning of silicon trenches by the integration of an unconventional hardmask stack consisting of SiO2 and ZrO2. Deposition of such material films has been investigated, as well as e-beam lithography exposure and finally pattern transfer by dry etching. Using this hardmask stack and 100 nm thin resist, the fabrication of 35 nm wide trenches with an aspect ratio of similar to 20:1 is demonstrated revealing a very high selectivity (>100:1) of the ZrO2 layer to Si during the deep silicon etch. A silicon etch rate > 1.5 mu m/min was achieved. The ZrO2 layer itself provides the main selectivity improvements of the final hardmask stack. (C) 2011 Elsevier B.V. All rights reserved.