Characterization of arsenic-rich waste slurries generated during GaAs wafer lapping and polishing

The toxicology of gallium arsenide is well established; it is classified by the state of California as a known carcinogen. Consequently, environmental aspects of GaAs wafer manufacture are coming under greater scrutiny, with the cost of waste disposal becoming an economic issue for fabs operating under this jurisdiction. It is estimated that 85% of a GaAs boule is lost during manufacturing and device packaging, which usually ends up land filled as hazardous waste. This percentage is likely to increase as final wafer thickness is reduced to improve thermal dissipation. GaAs wafer backthinning and polishing generates waste slurries that are contaminated by arsenic and must be disposed of as hazardous waste. Although GaAs is largely insoluble in H2O, it is readily oxidized to soluble oxides and hydroxides, especially during chemo-mechanical polishing. Further, the valency state of the arsenic species determines the toxicity of effluent. Waste slurries from three sources were studied by ICP-MS and ICP-OES analysis to determine the amount of arsenic in the supernate and the form of the arsenic species. This data was related to mechanical lapping processes, such as the size distribution of particles in the slurry, and to the oxidation chemistry of the polishing processes. The analytical results provide guidance as to the most effective strategy to minimize the environmental impact of slurries produced during wafer thinning and polishing.