Deep micro hole drilling in a silicon substrate using multi-bursts of nanosecond UV laser pulses

This research work investigated drilling deep micro holes in semiconductor substrates with nanosecond laser ablation. The high pressure generated from the recoil of the ablated material and the expanding cloud of vapour and plasma leads to deformations in shape, including a barrel-shaped cross-section profile, bending of the hole channel and misalignment of the centre of the exit hole. It has been observed that deformations occur at all levels of pulse energy. To avoid the deformations and achieve smooth sidewalls, drilling with a multi-burst pulse train is proposed. First, a burst of pulses with low energy punches the substrate, producing a through hole smaller than the desired dimension. Then, a second burst of pulses with higher energy widens the exit hole and achieves holes with straight but tapered sidewalls. Since the through hole produced by the first burst provides an exit and expansion space for ablation plasma and vapour, the second burst will remove material without causing deformations. With this method we achieved 20 µm holes with nearly straight sidewalls in a 250 µm thick silicon substrate.