We report on a high-performance laser milling process and system for the fabrication of large arbitrarily shaped geometrical structures in silicon. A custom designed multi-step approach, combining high speed nanosecond laser ablation (roughing) with high precision ultrashort pulsed ablation (finishing) and inline 3D-measurement allows for the realization of large-volume cavities of up to and exceeding 20 mm3 with virtually freely selectable geometry at very high ablation rates Rabl > 0.1 mm3/s while maintaining high surface quality (Sa < 0.5 μm) on the laser processed areas. During the roughing step, nanosecond laser ablation creates the coarse structure of the target geometry. Subsequently, a white-light interferometer obtains a 3D image of the raw shape, enabling a purpose developed algorithm to compute an individual finishing laser pattern by comparing actual and target geometry. The following ultrashort-pulsed laser finishing step creates the final geometry by precisely removing surplus material according to the computed pattern. Depending on the absolute ablation volume and the precision requirements, several finishing steps are conducted successively to generate smooth functional surfaces. The achievable structure quality crucially depends on the perfect alignment of the measured 3D data and the applied laser ablation pattern. Thus, a high precision machine platform connects laser process and measurement modules by an automated handling system. Camera based alignment systems provide long-time repeatable positioning accuracies < 5 μm, which allow for reliable high-volume wafer processing.
The flexibility of new laser sources and process-monitoring enables new possibilities in laser-based production technology, for instance the combination of different laser processes with many adjustable parameters. The fusion of domain knowledge and probabilistic models in the form of hybrid models allows an efficient optimization of these processes with machine learning. This can be a key technology to realize self-learning laser-based universal machines in the future. The article discusses some examples where algorithm-based optimization, partly supported by hybrid models, can already greatly reduce the effort required to find suitable process parameters.
Scanning ultra-short pulse laser ablation is a very flexible technology that can be used for the subtractive manufacturing of complex three-dimensional structures with precision requirements on micrometer level. In our studies, inadvertent periodic deviations at the bottom of ablated cavities in silicon were observed after laser ablation with ultra-short laser pulses. We introduce the hypothesis of an interdependency between the ablation process and ultrasonic resonant acoustic waves, also known as standing waves, forming in the air within the ablated volume. Using basic acoustic wave equations, the corresponding periodicity of the deviations at the bottom surface of the cavities is described with good agreement to our experimental data.
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