Characteristics of high-aspect ratio nanosecond laser ablation of silicon at 355nm wavelength

We report on pulsed laser ablation for dicing/scribing monocrystalline silicon with a frequency tripled DPSS Q-switched nanosecond laser (355nm, AVIA, Coherent). Using a Galvanometer scanhead, a laser beam (repetition rate 30kHz, 143µJ per pulse maximum) was focused down to a 30µm diameter spot on silicon wafer and scanned in single-line mode. The trenches were machined by applying different number of laser scan passes and different laser powers. The cross-section and the side-wall of the trenches were examined using optical and scanning electron microscopes. The laser machining throughput was evaluated by measuring the trench depth and the results show as the trench deepens with repetitive scans the effective ablation rate decreased and maximum depth of 500µm was achieved (aspect ratio 17 approximately). The debris generation and transport is known to be critical in optimising throughput of laser ablative processes. Study of the debris reveals that instead of being removed from the trench part of the debris generated from laser ablation re-deposits in the trench and on the side-walls and forms a recast layer. The formation of such features narrow the open space inside the trenches, which made laser beam delivery and debris transport more difficult.