Mechanism Of Laser Ablation Research Articles

Ultraviolet Laser Patterning of Fluorine-Doped Tin Oxide with Different Radiation Directions

Selective laser patterning of thin films in a multilayered structure is an emerging technology for fabricating optoelectronics and microelectronics devices. As the application of ultraviolet (UV) lasers is more and more popular in electronics manufacturing, in this study, we compare UV laser patterning of fluorine-doped tin oxide (FTO) films from the front and back sides. We summarize a formula for calculating the focus offset in back-side ablation (BSA) and analyze systematically the relationships between the laser ablation crater size, the damage on glass substrate, and the laser ablation parameters. In addition, the laser ablation process and mechanisms of the BSA and the front-side ablation (FSA) are also elaborated in this paper.

Dynamics of nanosecond-laser-induced melting of tin in vacuum, air, and water

Tin with its low melting point and vapor pressure is a good model material to investigate laser ablation mechanisms under various ambient conditions. Here we measured the nanosecond-laser-induced damage thresholds of tin in vacuum, air, and water. The threshold fluence is found to be ~ 0.1 J/cm2 regardless of the environment unlike more refractory metals when threshold values in water are considerably higher than those in air. Analysis of the morphology and chemical composition of the irradiated surface as well as numerical simulations of tin laser heating demonstrate that the observed surface modification is due to melting but not oxidation. For the case of water environment, the conductive heat transfer to water is found to play only a minor role in tin heating and melting. The simulations show also that the formation of a water vapor layer near the tin surface occurs at a considerably higher fluence, above 0.15 J/cm2, and thus the surface damage is not affected by scattering of the incident laser light by the vapor–liquid interface, typical for more refractory metals. Peculiarities of laser ablation of low-melt materials in liquids and nanoparticle formation are discussed.

Investigation of dielectric layers laser ablation mechanism on n-PERT silicon solar cells for (Ni) plating process: Laser impact on surface morphology, composition, electrical properties and metallization quality

Laser contact opening is a critical step for solar cells manufacturing and needs to be optimized to achieve high efficiencies. In this paper, laser contact opening using a picosecond laser (wavelength 355 nm, pulse duration 10 ps) has been carried out on n-PERT precursors composed of a SiOx/SiOxNy stack on the rear polished side and a SiOxNy layer on the front textured side. By varying peak fluence from 0.130 J/cm2 to 2.159 J/cm2 and spot overlapping, ninety parameters combinations have been tested to open these dielectric layers. Surface morphology characterization, before and after laser ablation, has been realized using Confocal Laser Scanning Microscopy and Scanning Electron Microscopy. Bulk and surface compositions have also been investigated by Energy Dispersive Spectroscopy and X-ray Photoelectron Spectroscopy analysis, respectively. Results have shown the existence of four separated laser impacted areas on the polished side and a related ablation mechanism is suggested. Also, electrical characterization using four probe measurements and calibrated lifetime photoluminescence revealed that electrical properties of the silicon underlying increased when post laser annealing was performed associated with no spot overlapping. Then, nickel electroless deposition has been performed and first characterizations indicate adherence issues and inhomogeneous metallization. Characterization of metallized samples revealed that these observations were closely linked to the non-homogenous surface morphology and composition after laser ablation.

Laser ablation mechanism of silicon nitride with nanosecond and picosecond lasers

Silicon nitride (Si3N4) has exceptional mechanical, thermal and chemical properties and is therefore, advantageous for many applications. However, the employment of Si3N4 is impeded by the high finishing cost. Short and ultrashort pulse laser ablation are considered as a non-contact machining processes and are utilised in recent years to overcome constrains of conventional machining of Si3N4. However, there is still a lack of research on the effects of different laser parameters such as pulse duration on the mechanism of laser ablation. In this study, laser-material interaction of two different types of solid-state lasers with 10 ps and 100 ns pulse durations was investigated at the constant power of 50 W on a Si3N4 workpiece. The width of thermal damages and heat-affected zone (HAZ) is extremely reduced within laser input energy density (EL−Input) less than 40 J/mm2 for both types of laser. It is found that in case of the nanosecond laser ablation, molten droplets inside the laser cut prevent diffusion of the laser beam at EL−Input higher than 62.5 J/mm2. This effect could not be observed in ablation with the picosecond laser. Compared to the nanosecond laser, the picosecond laser is more efficient for ablating the workpiece material at laser input energy densities higher than 120 J/mm2.

Colossal laser ablation threshold of Ge crystal due to formation of GeO2 nanolayer: “Lid Effect” - Subsurface boiling mechanism

Colossally increased laser ablation threshold intensity Ith = 637.0 MW/cm2 of mechanically polished Ge crystal was found. It exceeds by factor of 100 that of chemically etched Ge single crystal (1 0 0), which is about 6.0 MW/cm2. This phenomenon is related to the formation of thin GeO2 layer during the action of laser pulse (wavelength λ = 1064 nm, pulse duration 6 ns, energy in a pulse W = 0.12 J, repetition rate of 10 Hz and beam diameter 2 mm) on the mechanically polished amorphous α-Ge. The increased threshold intensity is explained by the “lid effect”- a non-monotonous temperature distribution with maximum in the bulk of the crystal, where melting and even boiling of material takes place under a solid GeO2 nanolayer, preventing the ablation at a much lower intensity. Applying SEM and optical microscope methods, appearance of bubbles at the irradiated surface was observed. In experiments, the fundamental frequency of pulsed Nd:YAG laser in the regime of multi-pulse irradiation of the same surface spot was used. The thickness of GeO2 layer, estimated from SEM images and by the use of Casino simulation software, is up to 50 nm. According to our calculations of temperature field in GeO2/Ge structure, the maximum of temperature is located near the interface and Ge boiling temperature is reached under solid GeO2 layer at a sub-threshold laser intensity. The theoretical and experimental results are thus explained by the subsurface boiling mechanism of laser ablation in GeO2/Ge structure in the presence of solid GeO2 layer, acting as a “lid”. The possible applications of the investigation results are proposed.

Study on water-assisted laser ablation mechanism based on water layer characteristics

Water-assisted laser process machining process is a promising way to cut materials with less thermal damage. An investigation of different water layer characteristics of laser ablation in low-pressure water was studied. In this study, single-crystalline silicon was selected as a work sample that was grooved by using nanosecond-pulse laser ablation in two different water-assisted conditions. Finite Element (FE) modeling technique and optical transmission technique were employed to gain a better understanding of the water layer characteristics on underwater laser machining process. The groove geometry morphology was observed and analyzed. Besides, the influences of the water layer shape and thickness on the laser beam transmission were analyzed, and the impact of the flow behaviors on the groove geometries and the groove sidewall surface were also analyzed, such as the velocity and pressure of water flow. The experimental results and numerical qualitative analysis show that the main factors affecting the groove width are the water layer shape, the total pressure of the water layer, and the velocity of water flow. Below the threshold of the softening remove pressure, the water velocity plays a dominant role in the depth of the groove, that is, the groove depth decreases with the increase of the flow velocity. The water velocity and the total pressure of the water layer have essential effects on the surface morphology of the inner wall of the groove. This study helps to better understand the impact of water layer characteristics on water-assisted laser ablation and promote its potential application in the processing of hard and brittle materials.

Single- and multishot femtosecond laser ablation of silicon and silver in air and liquid environments: Plume dynamics and surface modification

The experimental study of the single-shot 1030-nm femtosecond laser ablation of bulk silicon and silver in air and liquid media (deionized water, isopropyl alcohol) is reported. The main laser ablation mechanisms were studied via analysis of the resulting microcrater surface morphology and elemental composition in single- and multi-shot (number of pulses N = 10, 100) regimes by scanning electron microscopy with a built-in module for energy-dispersive X-ray spectroscopy. It was concluded, that multi-shot ablation in liquids leads to the lesser extent of the processed surface oxidation, than in dry conditions. The monitoring of the ablative plumes dynamics was implemented with the use of time-resolved emission spectroscopy in the non-filamentation regime, corresponding to the formation of nanoparticles without fragmentation of the ablation products. It was demonstrated, that the lifetime of the plume emission in liquid is significantly shorter, than in air: 5 and 15 ns, correspondingly, for silicon, and 10 and 30 ns for silver.

A model of femtosecond laser ablation of metal based on dual-phase-lag model

Femtosecond laser ablation possesses a variety of applications due to its better control, high power density, smaller heat-affected zone, minimal collateral material damage, lower ablation thresholds, and excellent mechanical properties. The non-Fourier effect in heat conduction becomes significant when the heating time becomes extremely small. In order to analyze the femtosecond laser ablation process, a hyperbolic heat conduction model is established based on the dual-phase-lag model. Taken into account in the model are the effect of heat source, laser heating of the target, the evaporation and phase explosion of the target material, the formation and expansion of the plasma plume, and interaction of the plasma plume with the incoming laser. Temperature-dependent optical and thermophysical properties are also considered in the model due to the fact that the properties of the target will change over a wide range in the femtosecond laser ablation process. The effects of the plasma shielding, the ratio of the two delay times, and laser fluence are discussed and the effectiveness of the model is verified by comparing the simulation results with the experimental results. The results show that the plasma shielding has a great influence on the femtosecond laser ablation process, especially when the laser fluence is high. The ratio between the two delay times (the ratio <i>B</i>) has a great influence on the temperature characteristic and ablation characteristic in the femtosecond laser ablation process. The augment of the ratio <i>B</i> will increase the degree of thermal diffusion, which will lower down the surface temperature and accelerate the ablation rate after the ablation has begun. The ablation mechanism of femtosecond laser ablation is dominated by phase explosion. The heat affected zone of femtosecond laser ablation is small, and the heat affected zone is less affected by laser fluence. The comparison between the simulation results and the experimental results in the literature shows that the model based on the dual-phase-lag model can effectively simulate the femtosecond laser ablation process.

Calculation of the temperature field of the material in the thermal model of laser ablation

The mechanisms of laser ablation of materials under the interaction of powerful pulsed laser radiation with a material coated with a thin film system were studied. One of the first models of such interaction of high-power laser radiation with matter was the thermal model. A onetemperature thermal model was used when considering the interaction of laser radiation with dielectrics, and a two-temperature model was used when considering the interaction of laser radiation with metals. When studying the processes of laser ablation within the framework of the thermal model, the method of moments with a special selection of the test function was chosen. Computer simulation was conducted to study the dependence of the surface temperature and the characteristic thermal length on the radiant flux. Variants with both constant and variable optophysical characteristics of the phenomenon were considered. Calculations were performed for different time ranges under different initial conditions. The results of the numerical experiment agree well with the data of other authors.

Simulation of laser ablation mechanism of silicon nitride by ultrashort pulse laser

A mathematical model is developed to find the depth of laser ablation and laser-material interaction in laser processing of Si3N4 (SL200-BG) by an ultrashort (picosecond) pulse laser. The model is verified with experiments which were carried out at different laser scan speeds from 1 mm/s to 100 mm/s. Experimental validation shows a model accuracy of 85%. Additionally, the results show that in laser intensities (IL) higher than 1.5 × 109 W/cm2, the laser-material interaction is “Multi Photon Ionization” (MPI) with no effects of thermal reaction while in lower values of IL, there are effects of thermal damages adjacent to the laser cut.

Optimizing the substrate-mediated laser ablation of biological tissues: Quest for the best substrate material

Laser micro-sampling is a promising tool for ambient pressure chemical imaging of biological material by mass spectrometry. Conventional approaches use laser energy coupling into the analyzed sample through electronic (UV) or vibrational (IR) channels. In a recent study [B. Fatou, M. Wisztorski, C. Focsa, M. Salzet, M. Ziskind, I. Fournier, Sci. Rep. 5:18135, 2015], we have demonstrated efficient micro-sampling of biological material through a substrate-mediate laser ablation (SMLA) mechanism using non-resonant visible radiation. Taking advantage of this effect, a large-scale proteomic analysis of micro-sampled tissue sections was performed, demonstrating the possible identification of about 400 proteins, including minor species, from an irradiated surface of 400 μm diameter. Fundamental investigations on biological standards revealed the role of the substrate optical properties in the ejection process. In the present work, we extend these studies by considering the substrate thermal properties. A systematic investigation on the sampling efficiency of homogeneous tissue sections placed on top of various selected substrates was performed. Two ejection mechanisms are clearly evidenced and interpreted in the frame of a simple model based on the substrate properties and the laser beam profile. Improvement of the ejection yield and spatial resolution is also discussed in this frame, in view of the practical application of this technique in the field of (bio)chemical mass spectrometry imaging.

Optimization of Analysis Conditions by the Method of Inductively Coupled Plasma Mass Spectrometry with Laser Sampling

The effect of laser radiation parameters on the sensitivity and relative sensitivity coefficients of element determination in the analysis of various materials by inductively coupled plasma mass spectrometry in combination with laser ablation is studied. It is found that, when the power density of laser radiation is increased above 2 × 1010 W/cm2 for the samples based on silicate glass, basalt glass, and polymetallic sulfides, the laser ablation mechanism changes from thermal to “phase explosion.” It is shown that the coefficients of relative sensitivity of determination of impurity elements with respect to the matrix elements change by no more than 3–5% when the power density varies in the range of 5 × 109–1.5 × 1011 W/cm2. For most impurity elements, the coefficients of the relative sensitivity of determination in different matrices differ by no more than 10–15%, but for a number of elements, a significant difference up to 1.5–2 times is observed. A comparative study of two ablation modes is carried out: when the laser beam is moved along the surface of the sample and during ablation at a “point.” It is shown that the sensitivity of determination of the elements in the first case is 2–3 times higher, while the coefficients of the relative sensitivity of determination are the same for both cases.

Simulation and experimental investigations of thermal degradation of polystyrene under femtosecond laser ablation

In the present work, the underlying mechanisms of femtosecond laser ablation of atactic polystyrene with a molecular weight of 10,000 g/mol are elucidated by large-scale molecular dynamics simulations, with an emphasis on the mechanisms of thermal degradation of polystyrene. In addition, experimental investigations of thermal degradation of polystyrene chips formed in femtosecond laser ablation, as well as pristine polystyrene, are conducted under a heating environment at 500 °C with Py-GC/MS analysis. Simulation results indicate that chain entanglement is predominant to facilitate thermal degradation of polystyrene chain, which is closely accompanied with single-chain excitation. The main degradation mechanisms of polystyrene revealed by simulations include β-scission of carbon backbone, dissociation of pendant group, breaking of π bond of pendant group and evaporation of individual atoms, which qualitatively agree well with experimental results. It is also found that the laser energy has a significant influence on the degradation of polystyrene under femtosecond laser ablation.

Impact of Liquid Medium on Laser Ablation Mechanism: Surface Heating and Cooling

Stainless steel target was ablated under both of Hexane and water to examine the effect of liquid on the ablation mechanism in terms of surface heating and cooling. Experimentally, a narrow, a smooth and a deep ablation is observed under water. In addition, the pit due to the collapse of cavitation bubble under water is insignificant as compared with Hexane. The focused beam on the target under liquid was estimated from measuring the crater’s diameter from the optical image. The difference in focused beam sizes is attributed mainly to the refractive index of the liquid. Heat transfer equation is solved to estimate the temperature profile on the solid surface for a single pulse. By considering the estimated beam size under liquid, the solution of heat transfer equation suggests for the first time rapid heating of the target under water, but doesn’t confirm the rapid cooling rate under Hexane. The smooth ablation in water is attributed to the more confined plasma.

Single pulse femtosecond laser ablation of silicon – a comparison between experimental and simulated two-dimensional ablation profiles

Abstract Ultrashort laser pulses are widely used for the precise structuring of semiconductors like silicon (Si). We present here, for the first time, a comparative study of experimentally obtained and numerically simulated two-dimensional ablation profiles based on parameters of commercially relevant and widely used near-infrared and diode pumped femtosecond lasers. Single pulse laser ablation was studied at a center wavelength of 1040 nm and pulse duration of 380 fs (FWHM) in an irradiating fluence regime from 1 J/cm2 to 10 J/cm2. Process thresholds for material transport and removal were determined. Three regimes, scaling with the fluence, could be identified: low and middle fluence regimes and a hydrodynamic motion regime. By comparing the simulated and experimental ablation profiles, two conclusions can be drawn: At 2 J/cm2, the isothermal profile of 3800 K is in excellent agreement with the observed two-dimensional ablation. Thus exceeding a temperature of 3800 K can be accepted as a simplified ablation condition at that fluence. Furthermore, we observed a distinct deviation of the experimental from the simulated ablation profiles for irradiated fluences above 4 J/cm2. This points to hydrodynamic motion as an important contributing mechanism for laser ablation at higher fluences.

On the mechanism of pulsed laser ablation of phthalocyanine nanoparticles in an aqueous medium

Laser ablation of phthalocyanine nanoparticles has potential for cancer treatment. The ablation is accompanied by the formation of microbubbles and the sublimation of nanoparticles. This was investigated in a liquid medium simulating tissue using optical-acoustic and spectral-luminescent methods. The thresholds for the appearance of microbubbles have been determined as a function of nanoparticle size. For the minimal size particles (80 nm) this threshold is equal to about 20–25 mJ cm−2 and for the maximal size particles (230 nm) this threshold is equal to about 7 mJ cm−2. It was estimated that the particle temperature at which bubbles arise is near 145 °С.

Nanosecond laser ablation of graphite: A thermal model based simulation

Results on nanosecond pulsed laser irradiation and ablation of graphite are presented. Theoretical simulation based on a thermal model describing heat-transport and vaporization from a graphite target has been employed to calculate mass ablation rate per laser pulse. Attenuation of the incident laser beam in the generated vapor plume has been incorporated in terms of two coefficients, a and b, that serve as the only fitting parameters for our simulation model. Comparison between experimentally measured data and calculated mass ablation rate per pulse confirmed that the laser ablation mechanism was largely normal vaporization, in the incident laser fluence range of 10–25 J/cm2. Calculated maximum temperature reached by graphite target surface on laser irradiation and its dependence on average laser fluence enabled us to assess the possibility of the onset of explosive boiling in the target. A good agreement between model calculations and experimental results on the ablation rate for laser fluence below ∼30...

Numerical model and experimental demonstration of high precision ablation of pulse CO2 laser

To reveal the physical mechanism of laser ablation and establish the prediction model for figuring the surface of fused silica, a multi-physical transient numerical model coupled with heat transfer and fluid flow was developed under pulsed CO2 laser irradiation. The model employed various heat transfer and hydrodynamic boundary and thermomechanical properties for assisting the understanding of the contributions of Marangoni convention, gravitational force, vaporization recoil pressure, and capillary force in the process of laser ablation and better prediction of laser processing. Simulation results indicated that the vaporization recoil pressure dominated the formation of the final ablation profile. The ablation depth increased exponentially with pulse duration and linearly with laser energy after homogenous evaporation. The model was validated by experimental data of pulse CO2 laser ablation of fused silica. To further investigate laser beam figuring, local ablation by varying the overlap rate and laser energy was conducted, achieving down to 4 nm homogenous ablation depth.

Laser ablation mechanism of contamination on surface of single crystal iron

The laser induced damage in high-power laser system has received much attention in the area of laser engineering. Optical components with contaminants, which are installed in the final optical assembly (FOA), can be severely damaged under the action of extremely high laser energy. So the ultra-high cleanliness inside the high-energy laser system is required for both optical and mechanical components. Research shows that a large part of the metal particulate contaminants inside the device come from the mechanical components. The metal particulate contaminants are produced when mechanical structure surface is damaged under the irradiation of stray light. However the research about the cleanliness inside the device is mostly concentrated on the surfaces of optical components currently. The laser ablation of the mechanical components absorbing contaminants is studied little, so it is quite important to investigate the ablation mechanism of mechanical components under laser irradiation. Due to the presence of contaminants on the surfaces of mechanical components, laser ablation of monocrystalline iron absorbing contaminants is investigated by using molecular dynamics simulation. The ablation process of iron material under laser irradiation is presented. The influences of loading mode and energy density of laser as well as contamination on the surface are analyzed in the ablation process of monocrystalline iron. The results indicate that the surface atoms of monocrystalline iron show different motion states under the violent collision of contaminants atoms after laser loading. Ablated iron can be divided into ablation zone, melting zone and crystal zone according to the variation of the temperature and mass density of the atoms in each region of the ablated material. The atoms in each region show macroscopic characteristics of gaseous, liquid and solid atoms respectively. Iron is damaged more easily when laser energy is instantaneously loaded. Contaminants on the surface of iron can be removed, and iron cannot be damaged when laser energy density is below 0.0064 J/cm<sup>2</sup>. The result of the analysis shows that the presence of contaminants makes the ablation of iron easier. Different energy loading modes affect the heat transfer mode directly. Monocrystalline iron materials are more likely to be damaged in the mode of adiabatic laser ablation in the case of short laser pulse. Thermal effect can be thought as a dominant factor for the ablation in the case of long laser pulse. The research results of this paper are helpful for providing the theoretical basis for improving the cleanliness of high-power laser system.

Studies on Photo Chemical Machining and Laser Lithography Processes

Photo Chemical Machining (PCM) is one of the well known non-conventional machining techniques. This technique is similar to those employed for the production of printed boards, silicon integrated circuits, microchannels on metals, wide range of precision metal parts, decorative items, etc. Excimer laser ablation has been widely used in the fabrication of miniature polymeric parts viz. Micro-lens arrays, textured surface on the polymers, microchannels, which find potential applications in various industries. In projection lithography by excimer laser a thin metal mask is used which is generally made by PCM. This metal mask is made larger and the laser lithography is done through reduction optics to increase the resolution. If the metal mask contains lines or features of varying width, then it becomes difficult to apply same etch factor correction in the CAD artwork of the phototool. This work present a systematic study of the errors introduced in metal mask manufacturing at various stages of PCM process. An attempt has also been made to experimentally estimate the depth caused by the laser ablation at two different fluence levels and also understand the mechanism of laser ablation of polymers.