Evidence for phase-explosion and generation of large particles during high power nanosecond laser ablation of silicon

Abstract

The craters resulting from high-irradiance (1×109–1×1011 W/cm2) single-pulse laser ablation of single-crystal silicon show a dramatic increase in volume at a threshold irradiance of 2.2×1010 W/CM2. Time-resolved shadowgraph images show ejection of large particulates from the sample above this threshold irradiance, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of a superheated layer near the critical state. Considering the transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency), the calculated thickness of the superheated layer at a delay time of 200–300 ns agreed with the measured crater depths. This agreement suggests that induced transparency promotes the formation of a deep superheated layer, and explosive boiling within this layer leads to particulate ejection from the sample.

Molecular-dynamics study of ablation of solids under femtosecond laser pulses

“…For high nucleation rates, the liquid can rapidly decompose into an equilibrium mixture of liquid droplets and monomers; in this case one speaks of phase explosion. This process is widely considered to be a common ablation process in the ultrashort pulse regime [8,[10][11][12][25][26][27]. However, a clear evidence of its occurrence still lacks.…”

Response to “Comment on ‘Evidence for phase explosion and generation of large particles during high power nanosecond laser ablation of silicon’ ” [Appl. Phys. Lett. 79, 442 (2001)]

“…6 were reported for a Ni sample and the ablation depth per pulse was only about 0.05-0.1 m, which is two orders of magnitude smaller than that in Ref. 1. Studies in Refs.…”

“…The phase explosion phenomena described in Ref. 1 is not a mere observation of droplets alone, but instead represents collective phenomena including all of the features previously mentioned. The actual processes for droplet formation in each of the studies are likely different, regardless that they were all called volume boiling.…”

“…1 are briefly summarized as: ͑i͒ nonlinear increase of the crater depth by three-four fold across the threshold irradiance of ϳ22 GW/cm 2 , ͑ii͒ crater depths ranging up to 20 m per single pulse ablation, ͑iii͒ local peak and valleys at the bottom of the craters for irradiances above the threshold, ͑iv͒ resolidified droplets scattered around the crater, ͑v͒ ejection of particulates or droplets of approximately 10-30 m in diameter, and ͑vi͒ delay of mass ejection by about 300-400 ns from the beginning of the ablation pulse. 2 The sample was pure silicon of about 1 mm thickness.…”

Pulse energy packing effects on material transport during laser processing of $$<1|1|1>$$ < 1 | 1 | 1 > silicon

“…The obvious solution has been to shift the interaction towards a vapour-driven process thus minimising the molten while at the same time minimising the vapour recondensate which is carried away from the crater by the vapour plume; a mechanism which is attributed to the peak pulse power. There is a wealth of literature examining laser ablation in relation to laser-induced plumes [1][2][3][4][5][6] or material ejection mechanisms [7][8][9][10]. Similarly, variation in pulse duration has been studied and applied to minimise these artefacts by reducing the interaction time.…”