Air-clad InGaAsP/InGaAs quantum well waveguides have been fabricated by inductively coupled plasma (ICP) etching using hydrogen silsesquioxane (HSQ) as an etch mask. First, HSQ has been studied for its contrast and resolution in electron beam lithography by varying e-beam exposure conditions and developer concentrations. Second, its etch resistance has been investigated in a chlorine-based ICP along with UVN30, a commercially Available negative tone e-beam resist. Then, the optimum conditions for the exposure and development of HSQ for a chlorine-based ICP etching of InP-based material have been explored in terms of etch resistance. InGaAsP/InGaAs quantum well material was patterned with HSQ by electron beam lithography. The waveguide was formed by the Cl-2/BCl3 ICP etching-of InGaAsP/InGaAs quantum well material and subsequent HCl-based wet etching of InAlAs sacrificial layer. The optical properties of the released waveguides were investigated and the initial optical measurements show low waveguide loss. (c) 2006 American Vacuum Society.