Effect of surface roughness on femtosecond laser ablation of 4H-SiC substrates

Abstract

Ablation threshold is an important concept in the study of femtosecond laser micro- and nano-machining. In this paper, the ablation experiments of three kinds of surface roughness 4H-SiC substrates irradiated by femtosecond laser were carried out. The feature thresholds were systematically measured for three surface roughness SiC substrates and found in the modification and annealing regions ranging from coincidence (R_a=0.5 nm) to a clear demarcation (R_a=5.5 nm), eventually being difficult to identify the presence of the former (R_a=89 nm). Under multi-pulse laser irradiation, oriented ripple structures were generated in the annealing region, where deep subwavelength ripples (about 110 nm, Λ ≈ 0.2λ) can be generated above substrates with surface roughness higher than 5.5 nm. We investigated the effect of surface roughness on the ablation morphology, ablation threshold, and periodic structures of femtosecond laser ablation of 4H-SiC substrates, while the ablation threshold was tended to decrease and stabilize with the increase of pulse number N⩾500.

摘要

飞秒激光加工材料中烧蚀阈值对微纳加工有极其重要的指导意义。本文利用飞秒激光辐照三种表面粗糙度4H-SiC 晶片进行烧蚀实验。研究了表面粗糙度对飞秒激光加工4H-SiC 烧蚀形貌、阈值及其周期性条纹的影响。实验系统地测定了三种表面粗糙度SiC 晶片单脉冲及多脉冲烧蚀阈值。分析了单脉冲激光作用下，不同表面粗糙度晶片随脉冲能量变化的烧蚀形貌。在多脉冲激光作用下，退火区产生了定向条纹结构，在SiC 衬底R_a⩾5.5 nm，其表面产生深亚波长波纹(约110 nm，Λ≈0.2λ)。而烧蚀 阈值随着脉冲数N⩾500的增加而降低，并最终趋于稳定。