Hui Qiao

Study of polishing of HgCdTe wafers

Wafer thinning is a key process in the fabrication of bulk HgCdTe photoconductive detectors. The currently used method in SITP is chemo-mechanical polishing combined with a following bromine-ethanol etching, which leaves a rough and non-stoichiometric composition surface. The thickness of damaged layer caused by chemo-mechanical polishing might be several hundred angstroms, so the process of bromine-ethanol etching after the chemo-mechanical polishing which could be used to remove the damages must last sufficient time, which would produce a Te-rich surface because of different bond energy between Te-Cd and Te-Hg. In this paper, bromine-methanol polishing method was used to reduce the surface damage of the HgCdTe wafer instead of bromine-ethanol etching, and it turned out that bromine-methanol polishing could obviously reduce the damage depth and effectively produce a stoichiometric HgCdTe surface comparing with bromine etching treatment. The related physical and chemical mechanism of bromine-methanol polishing are analyzed and experiment results are presented in the paper. The AFM test showed that the HgCdTe surface treated with bromine-methanol polishing had a roughness of about only 10nm, while the roughness for bromine etching was about 30nm . In this work, the surface removing rate of bromine-methanol polishing and minority carrier lifetime of HgCdTe wafer treated with bromine etching and bromine-methanol polishing were also studied. The result shows that bromine-methanol polishing can improve the minority carrier lifetime significantly. Through a series of experiments we come to a conclusion that the optimized bromine concentration was 1:260 and the polishing time was one minute.