With the rapid development of modern industrial technology, silicon carbide has broad application prospects owing to its excellent physical and chemical properties. Compared to conventional cutting methods, laser stealth dicing has the benefits of less debris with higher cutting accuracy. The research on the effect of laser parameters on surface ablation, edge chipping and cross-section roughness, and the development of new laser cutting techniques are of great practical significance to the development of silicon carbide cutting technology.
In this study a high power ultrafast laser processing system is used to cut out 300 μm thick SiC wafers with the diameter of 4 inch (1 inch=2.54 cm). Firstly, the effects of laser single pulse energy, feed distance, pulse repetition frequency, pulse width and scanning speed on cutting results are investigated using the control variables method. Based on the results of the multi-pulse mode (Fig.5(a)), the burst mode (Figs.5(b) and (c)) is used to reduce the edge chipping size and cross-section roughness. In burst mode, sub-pulses with the same pulse repetition rate as the output pulse sequence are selected from seed source pulses by adjusting the transistor-transistor logic (TTL)signal in the acousto-optic modulator (Fig.4). The surface ablation, edge chipping size and cross-section roughness are analyzed using laser confocal microscopy.
The effects of laser single pulse energy, feed distance, pulse repetition frequency, pulse width and scanning speed on cutting results under the multi-pulse mode are investigated. If the pulse energy is lower than 4 μJ, modified layers cannot be formed inside the SiC wafers, resulting in failure to separate the wafer (Fig.6). The feed distance has little effect on kerf width, however, the significant effects on edge chipping size and cross-section roughness are observed (Fig.7). A too low or too high pulse repetition frequency results in large kerf width, large edge chipping size and high cross-section roughness (Fig.8). Appropriately increasing pulse width can improve the quality of surface, edge and cross-section (Fig.9). Utilizing the appropriate scanning speed can reduce kerf width, edge chipping size and cross-sectional roughness (Fig.10). Based on these results, the burst mode is used to cut the wafers. It is confirmed that the cutting accuracy significantly improves under the burst mode (Fig.11). Because interaction time between the laser and material is too short in multi-pulse mode, the density of free electrons is too low and the internal material modification is insufficient, which affects the quality of the edge and cross-section. The burst mode extends the interaction time between the laser and material which induces a high density of free electrons and good internal crack continuity. Therefore, the edge chipping size and cross-sectional roughness are reduced.
This study investigates the effects of laser pulse energy, feed distance, pulse repetition frequency, pulse width and scanning speed on the top and bottom surfaces, edge chipping and cross-section of SiC wafers in multi-pulse mode using the control variable method. It is identified that the best cutting results are produced under the single pulse energy of 6 μJ, +5 μm feed distance, 100 kHz pulse repetition frequency, 10 ps pulse width, and 100 mm/s scanning speed. The kerf width on the top and bottom surfaces is 15.9 μm and 5.1 μm, respectively, the top and bottom surface edge chipping size is 7.8 μm and 2.1 μm, respectively, and the cross-section roughness is 3.1 μm. To further improve the edge size and cross-section morphology, the burst mode effect on the cutting results is investigated. It is confirmed that burst mode improves the continuity of modified cracks and reduces edge chipping size. When the number of sub-pulses is five, the kerf width on the top and bottom surfaces is 21.4 μm and 7.6 μm, respectively, and the minimum edge chipping size on the top and bottom surfaces is 1.2 μm and 1.0 μm, respectively, which is 85 and 52% less than those under the same cutting parameters in multi-pulse mode. Also at five sub-pulses, the lowest cross-section roughness is 2.3 μm, which is 26% less than that under the same cutting parameters in multi-pulse mode. This is because of the high density of free electrons generated in multi-pulse mode, which results in full and homogeneous material modification, thus reducing cross-section roughness. The burst mode increases dimensionalities of the laser stealth dicing parameters compared with multi-pulse laser stealth cutting and facilitates better cutting results.
