With the rapid development of modern high-tech industries such as communication, optoelectronics, and artificial intelligence, there is an urgent need for semiconductors that can operate stably in extreme environments of high power, high temperature, and high frequency. Silicon carbide (SiC) semiconductors, featuring high thermal conductivity, excellent thermal stability, large breakdown voltage, high electron mobility, fast saturated drift velocity, and a wide bandgap, are an ideal third-generation semiconductor material. However, their brittleness and hardness bring great difficulties to the processing of SiC semiconductor materials.
In the existing SiC wafer processing technology, the polishing process is of vital importance. It is the final step that directly determines the surface quality of the SiC crystal. The target is to obtain a super-smooth surface without any subsurface damage. Currently, chemical-mechanical polishing (CMP) is a common method in the final polishing stage, but it suffers from problems like non-uniform material removal and potential subsurface damage. Magnetorheological polishing (MRF), a flexible polishing technology, has a low processing efficiency although it can avoid surface and subsurface damage to a certain extent.
The work aims to improve the efficiency and effectiveness of magnetorheological finishing (MRFC) for SiC processing. A Halbach array magnet made of neodymium-iron-boron permanent magnetic material is used in the polishing equipment. The simulation analysis with the electromagnetic field finite-element analysis software Ansys Maxwel finds that the region with a magnetic flux density B > 400 mT almost covers the entire area above the excitation device, and the excitation area reaches 18 000 mm2, meeting the requirements of magnetorheological polishing. The orthogonal experimental method is adopted to optimize process parameters such as workpiece speed, polishing plate speed, working gap, and excitation gap. Through the combination of the response function model and range analysis, the optimal process parameter combination is determined as a workpiece speed of 200 r/min, a polishing plate speed of 15 r/min, an excitation gap of 13 mm, and a working gap of 1 mm.
Single-factor experiments are carried out to study the effects of abrasive types, particle sizes, and mixed abrasives on the performance of SiC magnetorheological polishing. The results show that diamond abrasives have the best polishing effect. After 60 minutes of polishing, the surface roughness Ra reaches 1.6 nm, outperforming other abrasives such as aluminum oxide, silicon dioxide, and silicon carbide. Reducing the particle size of diamond abrasives can generally improve the polishing effect. However, when the particle size is less than 0.5 μm, the agglomeration effect of abrasives becomes significant, increasing the effective particle size and reducing the polishing performance. A composite abrasive polishing fluid with a 0.4 ratio of 1 μm and 0.2 μm diamond abrasives can achieve excellent polishing results, obtaining a super-smooth surface with a surface roughness Ra of 0.45 nm.
In conclusion, the application of the Halbach array-excited magnetorheological polishing method for SiC materials can achieve sub-nanometer surface roughness. This research provides a new approach and theoretical basis for the high-quality processing of SiC semiconductor materials, which is of great significance for promoting the development and application of SiC in related fields.
Key words
silicon carbide /
magnetorheological finishing /
Halbach array /
surface roughness /
diamond abrasives
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Funding
The National Natural Science Foundation of China (52130503, 52075160); Hunan Provincial Science and Technology Department (2021JC0005)
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