Nanoscale patterning of Si/SiGe heterostructures byelectron-beam lithography and selective wet-chemicaletching

Abstract

We present a low-damage fabrication technique forlateral sub-100 nm patterning of Si (13.5 nm)/Si0.76Ge0.24(74 nm)/Si(001) heterostructures, which isbased on successive selective wet-chemical etching. An oxidelayer of 1-2 nm thickness on the Si top layer, which wasformed well below the growth temperature of theheterostructure, was used as a sacrificial layer for transferringthe resist pattern. Electron-beam lithography was done at 15 kVwith a scanning electron microscope equipped with afield-emitter source. Transfer of the resist pattern is done byselective wet-chemical etching of the (i) oxide, (ii) Si and(iii) SiGe layers. The selectivity of the anisotropic Sietchant (25% w/w aqueous solution of tetramethyl ammoniumhydroxide (TMAH) at 70 °C) is 20:1 for SiGe andbetter than 4200:1 for SiO2. The etch rate andselectivity of the SiGe etchant (buffered hydrofluoric acid,hydrogen peroxide and acetic acid) both depend on the waitingtime between mixing and use.Due to the high anisotropy of TMAH the minimum width of groovesetched into the Si layer along the [110] direction of about30 nm is mainly determined by the dimension of the oxide mask.Further transfer into the SiGe layer results in an undercut ofthe Si edge of about 70% of the etch depth, if the depth isless than the SiGe layer thickness. The profile of the SiGegrooves reveals a weak etching anisotropy. This patterningtechnique was applied to fabricate a geometric constriction inthe epitaxial layers realized as a broken groove with the gapranging from 15 to 300 nm width. Sufficiently long etching ofthe SiGe leads to complete underetching of the Si layer inthe gap. The resulting suspended Si bridge is mechanicallystable for 20 nm width and up to 360 nm length.