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

The toxicology of gallium arsenide (GaAs) 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 up to 93% of a GaAs boule is lost during manufacturing and device packaging, which ends up land filled or incinerated 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 voltammetric analysis to determine the amount of arsenic in the supernate. 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.