Characterisation of laser ablation of silicon using a Gaussian wavefront and computer generated wavefront reconstruction

Abstract

The work described in this paper characterises the laser ablation of wafer grade silicon, using both a Gaussian beam and the reconstructed wavefront of a femtosecond laser. The reconstructed wavefront was produced by transmission through a computer generated hologram (CGH). The laser used was a chirped pulsed amplification (CPA), Ti:sapphire system, operating at a centre wavelength of 775nm, and with a average pulse duration of 150fs. The dependence of the size of the ablated region on the number of pulses, used over a range of fluences, enabled the ablation threshold as a function of the number of pulses to be determined, for different beam profiles.With the Gaussian profile of the femtosecond laser, it was observed that the ablation threshold for silicon changes with the number of pulses used. The ablation threshold for a single laser pulse was determined as 0.45Jcm−2 as compared to 0.18Jcm−2 for 20 laser pulses. This behaviour can be attributed to the incubation parameter for silicon, which was estimated to be 0.7. In high resolution scanning electron microscope (SEM) images of the silicon surface, it was possible to observe the progression of the ablated area, within the Gaussian profile of the laser, from surface features such as circular ripples, machined with 2 laser pulses at the centre of the beam, to larger diameter laser holes, machined with 20 laser pulses.To improve the lateral precision of the ablated region over a range of pulses and fluences, a CGH was designed and constructed to transform the Gaussian profile of the laser beam into a more uniform fluence distribution. The reconstructed wavefronts produced from different CGH transmission structures were evaluated and the most successful was used to demonstrate improved laser machining of silicon. It was observed that the reconstructed beam was not Gaussian and that the ablated holes converged to a constant diameter with higher pulse fluences and pulse numbers.