Ultrafast pulse laser inscription and surface quality characterization of micro-structured silicon wafer

Abstract

• Established an efficient methodology based on Ultrafast laser inscription principle for fabricating higher quality microchannels. • The proposed methodology will be an alternative technique for the usual industrial practice to attain higher ablation depth. • For the first time, ALF over the crystalline coating of Si3N4 on silicon was analysed and reported to be predominant on structural integrity. • The occurrence of the bulging phenomenon on coated silicon surface at low repetition rates were reported. • The surface temperature was predicted which justified the enhancement in surface integrity processed at a specific range of lasing parameters. We report the applicability of the ultrafast pulse laser inscription technique to achieve high ablation depth on uncoated silicon wafer despite its higher surface reflectivity. The proposed methodology of this research work can be an alternative approach for the usual industrial practice of coating silicon surface with highly reflective materials for increasing the absorption phenomenon. To unveil the potential of the proposed methodology, a comparative study was carried out by fabricating microchannels of higher depth on uncoated and coated silicon wafer by varying repetition rate from 10 kHz to 500 kHz at a constant pulse energy of 18 μJ. The formation of ablation depth, ablation width and amorphous layer thickness was taken as the standard for evaluating the effectiveness of the proposed methodology. The experimental results revealed the formation of a higher ablation depth of 6.6 μm and an amorphous layer thickness of 0.039 μm for uncoated silicon material. Whereas, in the case of coated silicon material the ablation depth was found to be 3.199 μm with an amorphous layer thickness of 0.101 μm. This justified the applicability of the ultrafast pulse laser inscription technique for achieving quality microchannels having higher depth on silicon material without any surface coating. The underlying mechanism for the improved performance is due to the low temporal separation (μs) property of ultrafast lasers which results in negligible heat diffusion into the bulk material, thereby minimizing the collateral thermal damages. This was further proved based on an analytical model by evaluating the surface temperature at various repetition rates. The experimental and analytical results from the present work will be highly beneficial for the electronic industry, where the laser micro structuring of MEMS components made of silicon material is highly challenging due to its reflective property.