Nanoscale patterning of Si/SiGe heterostructures by electron-beam lithography and selective wet-chemical etching

We present a low-damage fabrication technique for lateral sub-100 nm patter ning of Si (13.5 nm)/Si0.76Ge0.24 (74 nm)/Si(001) heterostructures, which i s based on successive selective wet-chemical etching. An oxide layer of 1-2 nm thickness on the Si top layer, which was formed well below the growth t emperature of the heterostructure, was used as a sacrificial layer for tran sferring the resist pattern. Electron-beam lithography was done at 15 kV wi th a scanning electron microscope equipped with a field-emitter source. Tra nsfer of the resist pattern is done by selective wet-chemical etching of th e (i) oxide, (ii) Si and (iii) SiGe layers. The selectivity of the anisotro pic Si etchant (25% w/w aqueous solution of tetramethyl ammonium hydroxide (TMAH) at 70 degrees C) is 20 : 1 for SiGe and better than 4200 : 1 for SiO 2. The etch rate and selectivity of the SiGe etchant (buffered hydrofluoric acid, hydrogen peroxide and acetic acid) both depend on the waiting time b etween mixing and use. Due to the high anisotropy of TMAH the minimum width of grooves etched into the Si layer along the [110] direction of about 30 nm is mainly determined by the dimension of the oxide mask. Further transfer into the SiGe layer r esults in an undercut of the Si edge of about 70% of the etch depth, if the depth is less than the Sice layer thickness. The profile of the SiGe groov es reveals a weak etching anisotropy. This patterning technique was applied to fabricate a geometric constriction in the epitaxial layers realized as a broken groove with the gap ranging from 15 to 300 nm width. Sufficiently long etching of the SiGe leads to complete underetching of the Si layer in the gap. The resulting suspended Si bridge is mechanically stable for 20 nm width and up to 360 nm length.