Multi-pulse Laser Ablation Research Articles

Multipulse feedback in self-organized ripples formation upon femtosecond laser ablation from silicon

The influence of positive feedback on self-organized nanostructure (ripples) formation is investigated for multipulse femtosecond laser ablation from silicon surface. We find an increase of the modified surface area and of complexity and feature size with accumulated dose, confirming the previously postulated feedback effect of dose accumulation. More interestingly, a variation of temporal pulse-to-pulse separation, at constant total incident irradiation dose, strongly affects the structure formation. Though the feedback becomes weaker with increasing time intervals between successive pulses, pulses do not act independently even for separations of up to one second. To account for this observation, a model of perturbation decay and outdiffusion from the excited volume is suggested and compared to the experimental results. Inspection by surface sensitive microscopy (AFM, SEM) and conventional and high-resolution transmission electron microscopy reveal complex structural modification upon the laser interaction: even well outside the irradiated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. This is confirmed by photoluminescence studies on the band–band and the dislocation-related D1-line.

Numerical and experimental depth profile analyses of coated and attached layers by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is applied for depth profile analysis of different thicknesses of copper foils attached on steel and aluminum substrates. In order to account interfacial effects, depth profile analysis of copper coated on steel is also carried out. Experiments are done at ambient air and at two different wavelengths of 266 and 1064 nm of a Nd:YAG laser with pulse durations of 5 ns. A three-dimensional model of multi-pulse laser ablation is introduced on the base of normal evaporation mechanism and the simulation results are compared with the experiments. A normalized concentration ( C N) is introduced for determination of interface position and results are compared with the usually used normalized intensity ( I N). The effect of coating thickness on average ablation rate and resolution of depth profiling are examined. There is a correlation coefficient higher than 0.95 between the model and experimental depth profiles based on the C N method. Depth profile analysis on the base of C N method shows a better depth resolution in comparison with I N method .Increase in the layer thickness, leads to a decrease in the ablation rate.

Experimental study of infrared nanosecond laser ablation of silicon: The multi-pulse enhancement effect

Experimental study has been performed on nanosecond (ns) laser ablation of silicon at 1064 nm, through which a so-called “multi-pulse enhancement effect” has been revealed, which has been rarely reported in literature. The major features of this effect are: (1) for multi-pulse laser ablation at the same spatial location, the ablation efficiency increases as the pulse number increases and the pulse-to-pulse temporal distance decreases; (2) for multi-pulse ablation performed sequentially at a group of locations, the ablation quality and efficiency starting from the second location can be significantly enhanced if the distance between adjacent locations is sufficiently small. Further study is needed to confirm and understand the underlying physical mechanism for the multi-pulse enhancement effect, which can be utilized to significantly improve the quality and efficiency of laser silicon micromachining using the low-cost and low-energy-consumption infrared ns lasers. This may decrease the cost and energy consumption of many relevant areas, such as the solar cell industry.

INFRARED NANOSECOND LASER ABLATION OF SILICON: THE SPATIAL MULTI-PULSE ENHANCEMENT EFFECT AND ITS DEPENDENCE ON LASER PULSE DURATION – TECHNICAL COMMUNICATION

An experimental study has been performed for laser ablation of silicon at 1064 nm with variable pulse durations from 50 nanoseconds to 200 nanoseconds. A spatial multi-pulse enhancement effect has been revealed, which has been rarely reported in literature. The specific feature of this effect is that for multi-pulse laser ablation of silicon performed sequentially at a group of locations, the ablation efficiency and quality starting from the second location can be enhanced if the distance between adjacent locations is sufficiently small, and the ablation efficiency enhancement becomes more obvious as the distance decreases or the laser pulse duration increases. Further study is needed to understand the underlying physical mechanism. This effect, if well understood, can be utilized to significantly improve the quality and efficiency of infrared nanosecond laser silicon ablation. This will widen the practical applications of the low-cost and low-energy-consumption infrared nanosecond lasers and hence significantly decrease the manufacturing cost and energy consumption in many relevant areas.

Synthesis of Ag-deionized water nanofluids using multi-beam laser ablation in liquids

Multi-pulse laser ablation of silver in deionized water was studied. The laser beams were arranged in a cross-beam configuration. In our experiments, two single-mode, Q-switched Nd-Yag lasers operating at 1064 nm, pulse duration of 5.5 ns and 10 Hz rep rate were used. The laser fluence of the second beam was 0.265 J/cm 2 for all tests. Two levels of the laser fluences were used for the ablating beam: 0.09 and 0.265 J/cm 2 (11,014 and 33,042 J/cm 2 at the focal point, respectively). The silver target was at 50 mm from the cell window and 10 mm deep. The second beam was aligned parallelly with the silver target and focused at 2 mm in front of the focal point of the ablating beam. For all cases, the delay time between the ablating beam and the cross-beam was 40 μs. In general, the ablated particles were almost all spherical. For fluence of 0.09 J/cm 2 and single-beam approach, the mean particle size was about 29 nm. The majority of the particles, however, were in 19–35 nm range and there were some big ones as large as 50–60 nm in size. For double-beam approach, the particles were smaller with the average size of about 18 nm and the majority of the particles were in 9–21 nm range with few big one as large as 40 nm. For the beam fluence of 0.265 J/cm 2 and single-beam configuration, the particle sizes were smaller, the mean particles size was about 18 nm and the majority of the particles were in the range of 10–22 nm with some big one as large as 40 nm. For double-beam approach, the mean particle size was larger (24.2 nm) and the majority of the particle were distributed from 14 to 35 nm with some big particles can be found with sizes as big as 70 nm. Preliminary measurements of the thermal conductivity and viscosity of the produced samples showed that the thermal conductivity increased about 3–5% and the viscosity increased 3.7% above the base fluid viscosity even with the particle volume concentration as low as 0.01%.

Synthesis of functionally graded bioactive glass-apatite multistructures on Ti substrates by pulsed laser deposition

Functionally graded glass-apatite multistructures were synthesized by pulsed laser deposition on Ti substrates. We used sintered targets of hydroxyapatite Ca 10(PO 4) 6(OH) 2, or bioglasses in the system SiO 2–Na 2O–K 2O–CaO–MgO–P 2O 5 with SiO 2 content of either 57 wt.% (6P57) or 61 wt.% (6P61). A UV KrF* ( λ = 248 nm, τ > 7 ns) excimer laser source was used for the multipulse laser ablation of the targets. The hydroxyapatite thin films were obtained in H 2O vapors, while the bioglass layers were deposited in O 2. Thin films of 6P61 were deposited in direct contact with Ti, because Ti and this glass have similar thermal expansion behaviors, which ensure good bioglass adhesion to the substrate. This glass, however, is not bioactive, so yet more depositions of 6P57 bioglass and/or hydroxyapatite thin films were performed. All structures with hydroxyapatite overcoating were post-treated in a flux of water vapors. The obtained multistructures were characterized by various techniques. X-ray investigations of the coatings found small amounts of crystalline hydroxyapatite in the outer layers. The scanning electron microscopy analyses revealed homogeneous coatings with good adhesion to the Ti substrate. Our studies showed that the multistructures we had obtained were compatible with further use in biomimetic metallic implants with glass-apatite coating applications.

Change in absorptance of metals following multi-pulse femtosecond laser ablation

Absorptance change of copper caused by surface modification following multi-pulse femtosecond laser ablation is studied as a function of number of applied laser pulses at various laser fluence. We show that laser-induced surface modification enhances the absorptance due to nano-, micro-, macro-structures and re-deposition of nanoparticles produced by the ablation. At sufficiently high fluence and with a large number of applied pulses, the absorptance of copper following ablation can reach a value of about 85%.

Investigation on Laser Micro Ablation of Steel Using Short and Ultrashort IR Multipulses

Investigations on laser micro ablation of steel have been made using multipulses instead of single pulses. The aim is to improve the efficiency of laser micro ablation by increasing the ablation rate and quality at the same time. For the improvement of the multipulse laser ablation a flash-lamp pumped Nd:YAG laser with 20 to 100 ns pulse duration is used which can supply multipulses with a specially triggered pockels cell at 10 Hz. A mayor increase of ablation by distribution of energy on several pulses is observed. Further investigations have been made with a commercial ps laser system with a pulse duration of 12 ps at a repetition rate of 100 kHz. The used bursts consisted of up to four pulses with interpulse separations Δt between 20 ns and 100 μs. A cut-off frequency νcut-off up to which the double pulse ablation equals single pulse ablation is defined by plasma enhanced absorption of the second laser pulse. For the comparison of multipulse and single pulse ablation the geometry and the roughness of the irradiated area have been detected by white light interference microscopy.

Enhanced absorptance of gold following multipulse femtosecond laser ablation

In contrast to the common belief for femtosecond laser ablation that the thermal energy remaining in the ablated sample should be negligible, we recently found that a significant amount of residual thermal energy is deposited in metal samples following multishot femtosecond laser ablation. This suggests that there might be a significant enhancement in laser light absorption following ablation. To understand the physical mechanisms of laser energy absorption, we perform a direct measurement of the change in absorptance of gold due to structural modification following multishot femtosecond laser ablation. We show that besides the known mechanisms of absorption enhancement via microstructuring and macrostructuring, there is also a significant absorption enhancement due to nanostructuring. It is found that nanostructuring alone can enhance the absorptance by a factor of about three. The physical mechanism of the total enhanced absorption is due to a combined effect of nanostructural, microstructural, and macrostructural surface modifications induced by femtosecond laser ablation. Virtually, at a sufficiently high fluence and with a large number of applied pulses, the absorptance of gold surface can reach a value close to 100%.

Effect of the pulse duration on graphitisation of diamond during laser ablation

Processes of graphitisation of laser-irradiated polycrystalline diamond surface exposed to multipulse irradiation are studied experimentally. The thickness of the laser-modified layer as a function of the laser-pulse duration ranging from 100 fs to 1.5 μs and the effect of the radiation wavelength on this thickness are studied. It is shown that the diamond graphitisation during multipulse laser ablation is a thermally stimulated process. The dependences of the diamond-ablation rates on the radiation energy density under the action of laser pulses of various durations are presented.

Direct observation of enhanced residual thermal energy coupling to solids in femtosecond laser ablation

We perform direct measurement of the thermal energy remaining in the bulk of Cu, Mg, Au, and Si samples following multi-pulse femtosecond laser ablation. In contrast to the previous belief that the thermal energy remaining in the ablated sample is negligible using femtosecond pulses, we show a significant amount of residual thermal energy deposited in various materials. In fact, with a sufficiently large number of pulses at high fluence, virtually all the incident laser energy can be retained in the sample. Several possible mechanisms are investigated for their role in residual heating, including laser-induced surface modification, exothermic chemical processes, and pressure effects.

Deep drilling of metals by femtosecond laser pulses

Results of recent investigations on deep drilling of metals by femtosecond laser pulses are reported. At high laser fluences, well above the ablation threshold, femtosecond lasers can drill deep, high-quality holes in metals without any post-processing or special gas environment. It is shown that for high-quality drilling of metals, the following processes are important: (1) laser-induced optical breakdown of air containing metal vapor and small metal particles (debris) generated by multi-pulse femtosecond laser ablation, (2) transformation of laser pulses into light filaments, and (3) low-fluence finishing.

Single- and multi-pulse femtosecond laser ablation of optical filter materials

Ablation experiments employing Ti:sapphire laser pulses with durations from 30 to 340 fs (centre wavelength 800 nm, repetition rate 1 kHz) were performed in air. Absorbing filters (Schott BG18 and BG36) served as targets. The direct focusing technique was used under single- and multi-pulse irradiation conditions. Ablation threshold fluences were determined from a semi-logarithmic plot of the ablation crater diameter versus laser fluence. The threshold fluence decreases for a shorter pulse duration and an increasing number of pulses. The multi-pulse ablation threshold fluences are similar to those of undoped glass material (∼1 J cm −2). That means that the multi-pulse ablation threshold is independent on the doping level of the filters. For more than 100 pulses per spot and all pulse durations applied, the threshold fluence is practically constant. This leads to technically relevant ablation threshold values.

Defect capture under rapid solidification of the melt induced by the action of femtosecond laser pulses and formation of periodic surface structures on a semiconductor surface

A theory of defect-strain instability with formation of periodic surface relief in semiconductors irradiated by ultra-short (τp=10-13 s) powerful laser pulses is developed. The period and time of formation of surface relief are calculated. Regimes of multi-pulse laser ablation leading to formation of either a smooth surface or arrays of surface-relief spikes are pointed out and corresponding experimental results are interpreted from the viewpoint of the developed theory.

Crystalline structure of very hard tungsten carbide thin films obtained by reactive pulsed laser deposition

Very hard (Hv≅26 GPa), uniform and adherent tungsten carbide thin films were obtained by multipulse excimer laser ablation of tungsten targets in methane at (5×10−2–1) Pa pressure. The films were deposited on substrates placed parallel to the target at a distance of 70 mm. Increases in the gas pressure result in an increase of the carbon content in the deposited films accompanied by a decrease of the mean WC crystallite size in the deposited layers. The microhardness of the films shows a nonlinear evolution with the methane fill pressure with a maximum in excess of 26 GPa at p=1×10−1 Pa.

Optical studies of carbon nitride thin films deposited by reactive pulsed laser ablation of a graphite target in low pressure ammonia

We report the characteristics revealed through optical investigations (microscopic studies, optical and IR transmission) of the thin films deposited by multipulse UV reactive laser ablation of a graphite target in 10 2 Pa ammonia. We observe that the films deposited at room temperature contain a mixture of carbon bonded to nitrogen in various configurations including the triple one and have a large optical band gap. On the other hand, the films deposited at 320°C have a lower content in nitrogen. We present evidence of C–N bonds with lower charge transfer. The layers are non-uniform and have a significantly narrower optical band gap. We consider that this difference is due to the desorption by heating of the highly reactive CN and CNH radicals.

Surface instability of multipulse laser ablation on a metallic target

Large scale wavelike patterns are observed on an aluminum surface after it is ablated by a series of KrF laser pulses (248 nm, 40 ns, 5 J/cm2). These surface structures have a wavelength on the order of 30 μm, much longer than the laser wavelength. We postulate that these wave patterns are caused by the Kelvin–Helmholtz instability at the interface between the molten aluminum and the plasma plume. A parametric study is given in terms of the molten layer’s thickness and of the spatial extent and kinetic energy density in the laser-produced plasma plume. Also included is an estimate of the cumulative growth in a multipulse laser ablation experiment. These estimates indicate that the Kelvin–Helmholtz instability is a viable mechanism for the formation of the large scale structures.

Synthesis of tungsten carbide thin films by reactive pulsed laser deposition

We performed reactive pulsed laser deposition of tungsten carbide thin films by multipulse UV laser ablation of W targets in low pressure (5×10 −3–1 Pa) CH 4. The films were deposited onto Si wafers or glass substrates placed parallel with the target at a separation distance of 70 mm. The thin films were uniform and adherent to the substrate even though the deposition was conducted with collectors at room temperature. We reached deposition rates in the region of 0.15 Å pulse −1.

Pulsed laser deposition of silicon nitride thin films by laser ablation of a Si target in low pressure ammonia

We report the deposition of Si-N films by multipulse excimer laser (λ = 308 nm, τFWHM = 30 ns) ablation of Si wafers placed in a slow flow of NH3 in the pressure range (1 μbar-1 mbar). The films are deposited on to a Si collector placed parallel to the Si target. We succeeded in depositing pure amorphous Si3N4 films at a pressure of 1 mbar of NH3. The deposition rate reached a maximum value of 0.2–0.3 nm per pulse. At lower pressures, the deposited films consist of a fine mixture of three amorphous phases (amorphous stoichiometric silicon nitride, amorphous non-stoichiometric silicon nitride and amorphous silicon). The amorphous silicon is prevalent in films deposited at a pressure of several to several tens of μbars. Droplets of polycrystalline α-Si are sometimes visible on the film surface. The experimental evidence, is analysed with a view to elucidating the participation in the chemical synthesis of the three main stages of the process: the substance expulsion from the target by laser ablation, the transition through the gas of the expulsed substance and it's final impact on the collector. We conclude that silicon nitride is mostly synthesized during the impact on the collector of the flow of the ablated substance.

Laser ablation in a reactive atmosphere: application to the synthesis and deposition performance of titanium carbide thin films

We report the synthesis and deposition of Ti carbide thin lay- ers by multipulse excimer laser ablation of Ti targets in CH4 at low am- bient pressure (in the microbar range). The layers deposited on single- crystalline Si wafers are characterized by optical microscopy, scanning electron microscopy, x-ray diffraction, photoelectron spectroscopy, and spectroscopic ellipsometry. We obtained deposition rates in the range 0.2 to 0.3 A/pulse for a target-collector separation distance of 12.5 mm. The deposition parameters were found to be in good agreement with the predictions of the theoretical model based on the assumption of the adiabatic expansion of the plasma in the ambient gas. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: laser ablation; titanium carbide; thin films; spectroscopic ellipsom- etry.