Grass formation in GaAs backside via etching with Cl-2 based chemistry was studied using the inductively coupled plasma (ICP) system. A parametric approach adjusting rf bias power, TCP source power, pressure, and gas ratio was performed to observe the effect of etching condition on grass formation. Scanning electron microscopy was used to investigate the surface morphology as well as the cross section of the via hole. Vertical grass was observed at the bottom of the via hole at certain unique processing and etching conditions. The grass growth is attributed to the surface defects on GaAs induced during a backside thinning combined with weak physical ion bombardment in the etch condition. We found that the grass formation can be prevented by pre-etching with Ar bombardment on the surface