Study On The High Efficiency And Low Damage Processing Technology Of CdZnTe Wafer

CdZnTe wafer is the most promising material for the fabrication of the room temperature radiation detector; it is also widely used as the perfect substrates to grow epitaxial layers of HgCdTe. The machined surface and subsurface damages of CdZnTe wafer can result in decreasing the resolution of CdZnTe detector and increasing the defects of epitaxial layers; therefore, the ultrasmooth and damage-free CdZnTe wafer is indispensable. Currently, there is no systematical study on the mechanical behavior, damage mechanism of machining and ultra precision machining of CdZnTe wafer, which restricts the application of CdZnTe wafer. In this paper, based on the study on the mechanical behavior of CdZnTe crystal, the damage mechanism in existing process is analyzed, then, the new high-efficiency and low-damage process of CdZnTe wafer is studied, which has great theoretical significance and applied value to promote the process technique and application level of CdZnTe wafer.The main research contents and conclusions are as follows:The.mechanical behavior of CdZnTe wafer is investigated by nanoindentation test and nanoscratch test. The test results show that, the hardness values on different planes are all less than 2 GPa, and the elastic modulus values are less than 60 GPa. The maximum diversity of average hardness on different planes is 35%, and the hardness diversity of 40% is found at different direction on the same plane, which adequately approves that the CdZnTe crystal has anisotropic natures. It is discovered by the nanoscratch test that, the frictional coefficient is smooth, there is plastic deformation on the surface when the load is small, while when the load is large, the frictional coefficient is fluctuant and there is micro fracture on the surface. During nanoscratch test, the frictional coefficient is small when scratching along [110] direction on (110) plane and [112] direction on (111) plane. It is difficult to remove the material when machining along the direction with large elastic deformation, which provides theoretical instruction for the process of CdZnTe crystal.The surface quality and subsurface damages are detected and analyzed for the existing process of CdZnTe wafer (lapping with loose abrasive, mechanical polishing and etching), and the material removal mechanism and damage generation mechanism of machined surface are explained. The results show that, the material removal mechanism is synthetical mechanism of two-body abrasion and three-body abrasion during lapping CdZnTe wafer with loose abrasives. There are pits and embedded abrasives on the lapped surface, The surface roughness is high, and the subsurface damage is serious, which needs much time to remove the surface damage layer in the later machining. The material is scratched and removed by the abrasives during mechanical polishing CdZnTe wafer; there are still scratches on the polished CdZnTe wafer surface, and cracks and polycrystal layer, et al. on the subsurface. There are also pits and scratches on the local places of CdZnTe wafer after etching, and the surface quality could not satisfy the request of material of detector and substrates.Aiming at the problem of pits and embedded abrasives on the lapped CdZnTe surface, the new process of the lapping and grinding with fixed abrasive is presented to machine CdZnTe wafer. When lapping and grinding the CdZnTe wafer with fixed abrasives, the material removal mechanism is two-body abrasion mechanism of micro cutting role on the surface; there are no pits and embedded abrasives on the machined surface. The CdZnTe subsurface damage depth during lapping with fixed abrasives is 1/3 time than that of loose abrasives. The CdZnTe subsurface damage depth during ultraprecision grinding is much less than that of lapping process, which is less than 100 nm. The small roughness value could be obtained when grinding along<110> direction on (110) plane and<112> direction on (111) plane.Aiming at the soft and brittle character of CdZnTe crystal, the chemical mechanical polishing process is presented to replace of mechanical polishing process, the chemical mechanical polishing process is studied and the polishing slurry is developed. There are no pits and embedded abrasives on the polished surface; the high surface quality is obtained with the surface roughness Ra of less than 1 nm, the subsurface damage of 2 nm amorphous layer.It needs 6 minutes to remove 51μm sawed damage layer when using ultraprecision grinding process to machine CdZnTe wafer, and 3 minutes to remove ground damage layer when chemical mechanical polishing CdZnTe wafer, the efficiency of new process is improved by 3 times than that of existing process.