In order to bring low-power epitaxy-based gallium antimonide (Gash) electronics and electro-optics to market, high-quality Gash substrates with smooth surfaces and no surface damage are required. Here, a novel final polishing technique, gas cluster ion beam (GCIB) processing, is shown to improve the surface finish of chemical-mechanical polished (CMP) 50 mm (1 0 0) Gash wafers by etching and smoothing CMP surface atoms through the sub-surface damage. For the first time, a fluorine-based gas cluster ion beam is reported for GCIB surface etching and smoothing of Gash material. For the selected processing sequence, the surface roughness of a high-quality, 0.70 nm RMS GaSb wafer was reduced to 0.18 nm RMS without any observed changes in the full-widths at half-maximum (FWHM) of the (4 0 0) and (I 1 1) X-ray peaks of 14 and 20 arcsec, respectively. Results indicate that the GCIB process did not contribute to wafer surface or sub-surface polish damage. In a second case, a GCIB etch removed 200 nm of material from a non-optimal CMP (10 0) Gash surface and reduced the full-width at half-maximum (I 1 1) X-ray peak from 76 to 52 arcsec in conjunction with a surface roughness decrease from 0.70 to 0.35 nm RMS. The data suggests that GCIB processing appears to be promising as a final Gash wafer polish with an etch rate compatible for large scale manufacturing. (C) 2003 Elsevier B.V. All rights reserved.