Femtosecond-laser wafer slicing of n-type 4 H-SiC in double-pulse MHz burst mode

Abstract This study employs a MHz burst-mode femtosecond laser to induce internal modification in 350-µm-thick n-type 4 H-SiC and to demonstrate wafer slicing using a single-/double-pulse ( N = 1–2) configuration under a fixed average power of 4.5 W and a burst repetition rate of 600 kHz (burst energy ≈ 7.5 µJ). Microstructure, surface topography, and residual stress were evaluated using scanning electron microscopy, laser-scanning confocal microscopy, and Raman spectroscopy. Single-pass line modification shows that N = 1–2 provide higher sub-pulse peak intensity, promoting lateral-crack formation that facilitates separation, whereas N = 3–5 drive deeper modification without lateral cracks. Raman measurements reveal a characteristic tension–compression residual-stress distribution, with tensile stress above and predominantly compressive stress below the lateral cracks; for example, at N = 2, the stresses are + 77.5 MPa (tension) and − 51.7 MPa (compression) at y = ± 4 μm. Full-area slicing experiments demonstrate that N = 2 shortens the step-like period and reduces the sliced-surface roughness from Sz = 47.8 μm to 32.2 μm, while maintaining a tensile strength of 3.4 MPa. Comparative Raman analysis of the two sliced surfaces further shows that laser-induced amorphous Si/C and lattice disorder accumulate mainly above the crack plane, leading to an asymmetry between the upper and lower halves of the specimen. These results establish a minimal N = 2 (double-pulse) MHz-burst configuration that enhances lateral-crack interconnection and lowers surface roughness.