Effect of beam profile on nanosecond laser drilling of 4H-SIC

Abstract

Laser processing has a great advantage of drilling of various materials due to its extremely high processing speed, particularly in the case that the deep drilling depth is required. For this reason, the through substrate via of semiconductors is now considered as one of the main target areas of laser processing. In this study, the numerical model, in which beam propagation is considered, is used to investigate the effect of beam profile on thermal ablation of 4H-SiC compared with experimental results. Considering the implementation of Bessel beam, near-infrared wavelength that was selected as 4H-SiC has good transparency to those wavelengths at room temperature. The main absorption mechanism was free carrier absorption, which indicates significant temperature dependence. The authors found that threshold fluence is dependent on the spot size of the beam due to heat conduction during several nanoseconds. In other words, resolution of the nanosecond laser ablation is limited no matter how small the spot size of the beam is. Also, carbonization induced by low fluence under the lattice melting temperature led to enlargement of the drilled crater. Our experimental results showed that Gaussian beam is a more efficient tool for deep drilling than Bessel beam because propagation of Bessel beam wavefronts is disturbed by opaque solid materials. Therefore, although the beam width of our Bessel beam was critically narrow (1.5 μm), a crater with high aspect ratio was not obtained. As a consequence, this study gives experimental and simple numerical analysis on the mechanism of the nanosecond laser drilling process of 4H-SiC.