Surface quality evaluation of single crystal 4H-SiC wafer machined by hybrid laser-waterjet: Comparing with laser machining

Abstract

Single crystal silicon carbide (SiC), as a 3rd generation semiconductor material, has a wide application range and development prospect, especially in some key fields such as defense and aerospace industries. It's a typical difficult-to-machine and hard-brittle material due to its high hardness and brittleness. A hybrid laser-waterjet micromachining technology was employed to implement near damage-free and high-efficient micromachining of single crystal SiC. The workpiece material is heated by laser to a temperature near but below its melting point. The material is under an extremely “softened” but still solid status. A waterjet is applied off-axially to expel the heating-softened material and cool the material to eliminate thermal damage. It combines the advantages of laser processing with those of waterjet cutting. Moreover, it's a green machining approach, which is pollution-free and recyclable. In the present study, the surface quality of single crystal 4H-SiC machined by the hybrid laser-waterjet is evaluated and compared with that machined by laser. Among the wafers respectively machined by laser, under-water laser, waterjet-assisted laser and the hybrid laser-waterjet, the one machined by the hybrid laser-waterjet is with the best surface quality. Its cut boundaries between the machined area and the unmachined area are clear, the cut edges are straight, also the cut sides are clean. On the contrary, there is severe thermal damage along the cut edges and cut sides of laser machining. The microgrooves obtained from the hybrid laser-waterjet micromachining are with V-shaped cross-sections and the transition between the cut edge and the unmachined surface is flat and smooth. The cross-section profile of the microgroove machined by laser is M-shaped and there are obvious humps along the two sides of the microgroove. The microgroove bottom is almost at the same height with the unmachined surface. Besides, there are HAZs with a width of 50–100 µm and scaly recast layers on the sides of the cut machined by laser. In contrast, there is no recast layer and HAZ on the cut side of the hybrid laser-waterjet micromachining. The texture of the cut sidewall is clear and with the typical features of the material removed under the plastic mode. The EDS results show that the oxidation behavior happens during both the hybrid laser-waterjet micromachining and laser machining. However, the content of silicon dioxide generated in the hybrid laser-waterjet micromachining process (the oxygen content is about 5%) is obviously lower than that generated in laser machining (the oxygen content is more than 30%).