Laser helical drilling of silicon wafers with ns to fs pulses: Scanning electron microscopy and transmission electron microscopy characterization of drilled through-holes

Abstract

Electron microscopic methods (scanning electron microscopy and transmission electron microscopy) are used to characterize the precision and quality of microthrough-holes produced in 0.4 mm-thick silicon wafers by applying the pulsed laser helical drilling technique. The primary aim of the present work is to investigate how the mechanisms of material removal and redeposition change when the pulse width is systematically varied from nanosecond (ns) to femtosecond (fs) range (8 ns to 160 fs). Under the chosen processing conditions (Ti:sapphire laser, pulse energy: 0.3 mJ, beam spot diameter: 40 μm, resulting fluence: 24 J/cm2, processing time: 120 s) optimal drilling results, i.e., smooth holes being free of recast and free of thermally and mechanically driven structural damage, are achievable by using laser pulses with a width of 10 ps. On the contrary, drilling with ns pulses is associated with thick melt redepositions, high thermal load, and formation of microcracks, while processing with fs pulses suffers from detrimental mechanical effects causing defect generation, material degradation, and microroughness of the hole wall.