In situprobing of pulsed laser melting and laser-induced periodic surface structures formation by dynamic reflectivity

Abstract

The melting process and nanostructure formation induced by nanosecond and picosecond laser pulses on bulk silicon and copper thin film were studied by ex situ analysis and in situ real time reflectivity. Three different probing wavelengths (633, 473 and 326 nm) were used during the pump laser processing and were correlated to the beam parameters (pulse duration, laser fluence and number of laser shots) and copper thin film thickness. On a silicon surface using a KrF laser beam (27 ns, 1 Hz, 248 nm), the melting threshold was determined close to 700 mJ cm −2 and the melting duration increased from 10 to 130 ns as the fluence increased from 700 to 1750 mJ cm −2 . Nanostructures with a spatial period close to the laser wavelength were formed on both copper thin film and silicon substrate after nanosecond Nd:YAG laser (10 ns, 266 nm, 1 Hz) irradiation. In the picosecond regime, using an Nd:YAG laser (40 ps, 266 nm, 1 Hz), different nanostructures, from spikes to laser-induced periodic surface structures, were formed on 500 nm copper thin film and were analyzed with respect to the drop in dynamic reflectivity changes versus the number of laser shots.

Nano-Squares and Regular Lipss on Ysz Coating by Picosecond Uv Laser Beam: Thin Film Mediated and Direct Texturing

“…Thus, higher laser intensities are needed to work near the ablation threshold to induce LIPSS formation but simultaneously this would lead to enhanced damage formation in such fragile ceramic materials, and thus subsequently reduces the process window. To circumvent such limitations, it has been shown that the deposition of a thin metallic film on the substrate surface prior to laser structuring was beneficial either for lowering the threshold of LIPSS formation on dielectrics and semiconductors or to directly structuring the irradiated thin films by ultrashort beams [18][19][20][21][22][23][24][25][26][27]. For example, Liu et al [28] have achieved HSFL fine structures (period ~100 nm) on the surface of ZnO assisted by aluminum film after femtosecond laser processing at 800 nm and 120 fs.…”

On the Melting Thresholds of Semiconductors under Nanosecond Pulse Laser Irradiation

“…One of the basic parameters for laser material processing that involves surface patterning, modification, crystallization and ablation, is the melting (damage) threshold. This key parameter is usually determined in the experimental studies of laser material processing as a reference point for controlling the laser modification process and for testing theoretical models developed with the aim of better understanding the fundamental processes at laser-matter interaction [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. For detection of the phase change, several approaches are used, such as time-resolved reflectivity (TRR) [9,11,[19][20][21][22], acoustic [23] and electrical conductivity [24] measurements and pump-probe microscopy [25], or combined techniques as in [13], where time-of-flight velocity distributions together with the evaporation rate and reflectivity were measured and analyzed.…”

“…In other cases, a stable change of the measured value is detected, corresponding to the established molten phase on the irradiated surface at higher fluences. Thus, in TRR experiments, the obtained data on the reflectivity of the molten phase and melting threshold are strongly dependent on the sensitivity and time resolution of the 2 of 16 measurements [19] and on the wavelength of the probe laser beam [20]. For instance, a UV beam, which probes a thinner surface layer, results in a lower melting threshold of silicon as compared to a visible probe beam with a higher absorption depth under identical conditions [20].…”

“…Thus, in TRR experiments, the obtained data on the reflectivity of the molten phase and melting threshold are strongly dependent on the sensitivity and time resolution of the 2 of 16 measurements [19] and on the wavelength of the probe laser beam [20]. For instance, a UV beam, which probes a thinner surface layer, results in a lower melting threshold of silicon as compared to a visible probe beam with a higher absorption depth under identical conditions [20]. We believe, therefore, that the threshold values obtained with the TRR technique are generally overestimated if we assume the onset of melting as a threshold.…”

Sunflower effect: enhancement of laser-pulse-induced impulse in the beam incident direction due to surface undulation

“…This is well known as 'laser-induced periodic surface structure (LIPSS).' [12,13] LIPSS is observed on various materials, that is crystals [12,[14][15][16][17][18][19][20][21][22], metals [20][21][22][23][24][25] and polymers [8,26], with various laser wavelength, that is far-infrared [8,14], near-infrared [15,17,18,22,23,25,26], visible [16,19,[22][23][24]26] and ultraviolet [21,26], and with various pulse duration, that is of the order of micro-second [8,14], nano-second [15,[17][18][19][20][21][22][23][24], pico-second [16,…”