To solve the problems of high surface roughness and large subsurface damage of heteroepitaxial GaN wafer surfaces, the work aims to achieve high-quality planar lapping of single-crystal GaN by optimizing the planar lapping process. Single-factor lapping experiments on the Ga face of single-crystal GaN were conducted with free abrasives to study the effects of abrasive type, abrasive particle size, lapping fluid flow rate, lapping speed, lapping pressure, lapping time, and lapping disk material on the lapping results. A Python-based image processing method was used to perform color recognition and image segmentation on the colored surface morphology maps of the processed surfaces to accurately identify the brittle-plastic domain removal area and study the effect of process parameters on the brittle-plastic domain removal area. The material removal rate and surface morphology of single-crystal GaN are affected by multiple factors. Diamond abrasives with a particle size of W3 are most suitable for GaN lapping, with the smallest surface roughness and brittle domain removal area ratio compared to other abrasives and particle sizes. As the lapping fluid flow rate increases, the material removal rate firstly increases and then decreases, while the surface roughness and brittle domain removal area ratio gradually decrease. The maximum material removal rate is 4.39 μm/min, the minimum Ra is 36 nm, and the minimum brittle domain removal area ratio is 12.88%. As the lapping speed increases, the material removal rate continuously increases, while the surface roughness and brittle domain removal area ratio firstly decrease and then increase. The maximum material removal rate is 7.78 μm/min, the minimum Ra is 44 nm, and the minimum brittle domain removal area is 17.60%. As the lapping pressure increases, the material removal rate continuously increases, while the surface roughness and brittle domain removal area ratio firstly decrease and then increase. The maximum material removal rate is 10.15 μm/min, the minimum Ra is 36 nm, and the minimum brittle domain removal area is 14.68%. As the lapping time increases, the surface roughness and brittle domain removal area ratio firstly gradually decrease and then stabilize. After 12 minutes of lapping on a cast iron disk, the Ra is 40 nm and the brittle domain removal area ratio is 13.33%. After 12 minutes of lapping on a synthetic copper disk, the Ra is 20 nm and the brittle domain removal area ratio is 7.66%. In the cast iron disk, the material removal modes are mainly two-body plastic removal, two-body brittle removal, and three-body brittle removal. In the synthetic copper disk, the material removal modes are mainly two-body plastic removal and three-body brittle removal. The synthetic copper disk is more suitable for single-crystal GaN lapping. Under the process parameters of 1wt.% W3 diamond abrasives, synthetic copper disk, lapping fluid flow rate of 8 mL/min, lapping pressure of 166 kPa, and rotation speed of 40 r/min after 12 minutes of lapping, the surface roughness decreases from Ra 190 nm to Ra 20 nm, the material removal rate is 3.61 μm/min, and the brittle domain removal area ratio is only 7.66%. The optimized process for single-crystal GaN free abrasive lapping can effectively reduce surface roughness, and the image segmentation recognition method can quickly and accurately identify the brittle-plastic domain removal area, which can be used to guide the optimization of process parameters.
Key words
single-crystal GaN /
planar lapping /
material removal /
surface roughness /
brittle domain removal
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Funding
The Natural Science Foundation of Jiangxi Province (20224BAB214054, 20242BAB25267, 20232BAB214057); National Natural Science Foundation of China (52405486); The Jiangxi Provincial Department of Education Science and Technology Research Project (GJJ2201116); The Graduate Innovation Special Fund Project of Nanchang Hangkong University (YC2024-009)
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