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 contributions 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 and ambient pressure. The reasons for the special drilling behavior are examined.Apart from parameter-specific dependencies, the drilling process is characterized by three phases: (1) excavation oft the initial capillary, (2) deep drilling with statistical deformation of the hole, (3) stop of forward drilling. The pulse energy is the fundamental process parameter, determining the maximum achievable hole depth largely independent of the fluence. Debris can obstruct the hole capillary and restrain the pulse propagation with a significant influence on the process duration and the achievable depth. The light propagation inside the hole channel has been identified as the main influence on the hole shape formation. Deviations from the ideal hole geometry are induced by beam deflections on hole wall imperfections. In conclusion, a fundamental improvement of quality and precision in ultrashort pulse drilling requires a control of the light propagation inside the hole.