Surface Morphology Evolution Induced by Multiple Femtosecond Laser Ablation on 4H-SiC Substrate and Its Application to CMP

4H silicon carbide (SiC) substrates were processed by multiple femtosecond (fs) laser ablation. Its surface morphologies changed dramatically at different scanning pitches, different scanning velocities, different repetition rates and near-threshold fluence. The evolution of surface morphologies and the uniformity of rippled structures were respectively investigated and discussed. The overlapped zones of adjacent pulses were focused in discussion. In the case of high repetition rate, periodic ripples with average spatial periodicities ranging from 145 nm to 196 nm were induced; whereas in the case of lower repetition rate, coarse ripples (roughly 440 nm) and fine ripples (about 117 nm) were simultaneously formed. Large amount of agglomeration was fabricated in the case of scanning pitch 0.05 μm and scanning velocity ≤ 5 mm/s. At near-threshold fluence, 4H-SiC surface was partly ablated. When the rippled structures induced in Transverse Irradiation Mode were further ablated in Cross Scanning Mode, horizontal ripples changed into vertical ripples. X-ray diffraction and Raman spectra were utilized for surface analysis. Confirmatory experiment was carried out, demonstrating that chemical mechanical polishing (CMP) process could be promoted by fs laser ablation.