High Pulse Repetition Frequency Micro Hole Drilling of Silicon Using Ultrashort Pulse Laser Radiation

Laser processing of silicon, as the most important material for microelectromechanical systems (MEMS), is commonly known and accepted as an effective manufacturing method to fabricate complex and precise micro components. Especially for the fabrication of micro hole grids, the recently developed high pulse repetition frequency (PRF) laser micro drilling technology is a valuable alternative to standard processing techniques. This paper discusses laser percussion drilling of silicon using ultrashort laser pulses at high average powers with the aim to drill through a material thickness of 100 µm. The main process influencing parameters, such as PRF and laser fluence were varied in order to identify optimal parameter settings for high quality and high throughput micro hole drilling. Furthermore, the effect of additional laser pulses, irradiated after drilling through, onto the micro hole geometry was examined. It will be shown that with increasing PRF and thus, lower laser fluence as a result of constant average laser power of the used laser system, the drilling time decreases significantly. This might be due to thermal accumulation induced by the applied highly-repetitive pulses, leading to a higher substrate temperature which, in turn, lowers the energy needed for melting and evaporation. Surprisingly, this effect results in higher quality of the micro hole geometry without considerable debris at both, hole entrance and exit.

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