Chlorine adsorption on electron beam irradiated GaAs photo-oxides: mechanism of in situ EB lithography

GaAs surface photo-oxides formed by visible-light-enhanced oxidation have recently come to be used as in situ electron beam (EB) lithography masks for CI2 gas etching. We investigated the adsorption properties of chlorine on EB-irradiated GaAs photo-oxides using X-ray photoelectron spectroscopy (XPS). The photo-oxidized surface was irradiated by an electron beam of 1.5 keV energy. The electron dosage was in the range of 7.1 × 1016−2.1 × 1018 electrons/cm2. After EB irradiation, chlorine was dosed up to 20 langmuie using an AgCl electrochemical cell. We found that chlorine readily adsorbs on EB-irradiated photo-oxides, whereas little chlorine adsorbs on non-EB irradiated photo-oxides. XPS observations revealed that EB-irradiation not only removes some of the oxygen on the surface, but also induces the reduction of As5+ and As3+ oxides to less oxidized As suboxides and/or elemental As. Ga oxides were observed to increase as well. Chlorine exposure resulted in a preferential removal of the EB-induced As suboxides and/or elemental As, as well as As in the GaAs substrate. We attribute this to the formation and simultaneous desorption of AsC1x (x=1−3), and conclude that this ‘etching’ of the surface As oxide is the key to the patterning of a photo-oxide mask.