Analysis of the hole shape evolution in ultrashort pulse laser drilling

Ultrashort laser pulses are capable of generating micro structures with high precision and challenging quality for electronics, optics, medical and automotive applications. However, the realization of microstructures with high aspect-ratio, especially microdrillings, is still a demanding task. Deep drillings show the formation of bulges, a bending of the hole and multiple capillaries. A direct investigation of the drilling process in opaque materials, especially metals, is not feasible and different explanations for the hole shape formation have been developed. The specific contribution of the possible influences on the shape formation are not fully understood yet. In this work, an in-situ observation of drilling in an opaque material is realized for the first time. Silicon is used as a model system for laser drilling in semiconductors and metals. The influence of the processing parameters on the hole shape evolution is studied in detail, in particular for the pulse energy, pulse duration, repetition rate as well as ambient pressure. The reasons for the special drilling behavior are examined with focus on the particle deposition, the plasma interaction and the light propagation inside the hole capillary.