High throughput, precision laser milling for arbitrary 3D structures in silicon

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 mm(3) with virtually freely selectable geometry at very high ablation rates R-abl > 0.1 mm(3)/s while maintaining high surface quality (S-a < 0.5 mu m) 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 mu m, which allow for reliable high-volume wafer processing.