Ablation Threshold Fluence Research Articles

Mechanism of femtosecond laser nano-ablation for metals

Metals have three ablation threshold fluences (high,middle and low-threshold fluence, here called) forfemtosecond laser pulses. In order to investigatethe physics of metal ablation under an intenseoptical field, the ions emitted from a laserirradiatedcopper surface were studied by time-offlightenergy spectroscopy. The low laser fluenceat which ions are emitted, Fth,L is 0.028 J/cm2, andtwo higher emission thresholds were identified atfluences of Fth,M =0.195 J/cm2 and Fth,H =0.470J/cm2. The relation between the number of emittedions per pulse Ni and laser fluence F was in goodagreement with Ni ∝F4 for Fth,L - Fth,M, Ni ∝F3 forFth,M - Fth,H, and Ni ∝F2 for ≥ Fth,H. Thedependence of ion production on laser energyfluence is explained well by multiphotonabsorption and optical field ionization.For fluence levels near the middle to high ablationthreshold, the formation of grating structures onmetal surfaces has been observed. The interspacesof grating structures were shorter than the laserwavelength, and the interspaces depend on fluencefor Mo and W with a 160 fs laser pulse. Thisphenomenon is well explained by the parametricdecay model proposed by Sakabe et al.

Resonant effects in nonlinear photon absorption during femtosecond laser ablation of Nd-doped silicate glass

This study investigates the resonant effects in nonlinear photon absorption in femtosecond laser ablation of Nd-doped silicate glass (Nd:glass). During the femtosecond laser ablation process, the resonant ablation threshold fluence is decreased by up to 40% compared with that of ordinary ablation. However, it is found that the resonant effect is closely related with laser intensity, and lower laser intensities are required to achieve a significant enhancement. When the intensity is lower than 2.28×10(14) W/cm(2) at which multiphoton ionization dominates, resonant effect is enhanced by a factor of 1.4 to 4.4. When the intensity is higher than 2.28×10(14) W/cm(2), at which intensity tunnel ionization dominates, the resonant effect becomes weak and gradually fades away. It is shown that the resonant effect is still important for multiphoton ionization yet insignificant for tunnel ionization.

Identification of ultra-fast electronic and thermal processes during femtosecond laser ablation of Si

Ultra-fast electronic and thermal processes for the energy deposition mechanism during femtosecond laser ablation of Si have been identified by means of atomic force microscopy and Raman scattering techniques. For this purpose, Si targets were exposed with 800-nm, 25-fs Ti:sapphire laser pulses for different laser fluencies in air and under UHV (ultra high vacuum) conditions. Various nano- and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Ultra-fast electronic processes are dominant in the lower-fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower-fluence regime (0.06–0.5 J cm−2 single-shot irradiation under UHV condition and 0.25–2.5 J cm−2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J cm−2 multiple-shot irradiation), and a higher-fluence regime (2.5–3.5 J cm−2 multiple-shot irradiation). Around the ablation threshold fluence, most significant features identified at the Si surface are nanohillock-like structures. The appearance of these nanohillocks is regarded as typical features for fast electronic processes (correlated with existence of hot electrons) and is explained on the basis of Coulomb explosion. The growth of these typical features (nanohillocks) by femtosecond laser irradiation is an element of novelty. At moderate irradiation fluence, a ring-shaped ablation with larger bumps and periodic surface structures is observed and is considered as a footprint of ultra-fast melting. Further increase in the laser fluence, i.e. a higher-fluence regime, resulted in strong enhancement of the thermal process with the appearance of larger islands. The change in surface topography provides an innovative clue to differentiate between ultra-fast electronic processes, i.e. Coulomb explosion (sub-100 fs) at a lower-fluence regime and ultra-fast melting (hundreds of fs) at a moderate-fluence regime, and slow thermal processes (ps time scale) at a higher-fluence regime. These fast electronic and thermal processes are well correlated to structural and crystallographic alterations, inferred from Raman spectroscopy.

Influence of the ablation threshold fluence on laser-driven acceleration

Laser ablation threshold measurements has been carried out by the nanosecond-class Nd:YAG laser at LNS-INFN in Catania. Advanced targets, such as hydrogen-enriched silicon slabs and sub-micro structured polymeric samples, have been investigated. The estimated ablation fluences are correlated to recent experimental and theoretical results on high intensity laser driven ion acceleration. Characteristics of H-atoms/protons and heavier atoms/ions coming out from the bulk of the irradiated target or from surface contaminants have been determined by optical and time-of-flight spectroscopy as well as mass quadrupole spectrometry.

In-Vitro Near-Field Ablation of Biological Samples in the Mid-IR Wavelength Region

As a first step towards the chemical analysis of biological samples with sub-micron resolution, we report our experiments on the sub-cellular ablation of biological samples in their native environment. This has the potential to combine the near-field IR ablation with mass spectrometry, thus facilitating the study of the spatial distribution of proteins in cellular samples, at sub-cellular length scales. We report the ablation of hard and soft materials: cellular acetate cover slips in water and myoblast cell samples in growth media, with spot sizes as small as 1.5um under 3um wavelength radiation. The ablation threshold and fluence in these processes have been measured. We found that there is a dramatic increase in the ablation threshold fluences when we go from far-field to the near-field region. We will also report on the difference in the ablation mechanism in air and water medium. This approach has the potential to identify the protein expressed in cells in a relatively non-destructive manner.

Porcine cortical bone ablation by ultrashort pulsed laser irradiation

Ultrashort pulsed lasers in bone ablation show promise for many orthopedic applications. To minimize collateral tissue damage and control the ablation process, the ablation threshold fluence must be well characterized. Using an amplified femtosecond laser (170 fs, 800 nm, 1 kHz), the ablation threshold on unaltered porcine cortical bone was measured using the D(2) method at multiple incident pulse numbers ranging from 25 to 1000 pulses per spot. The lowered threshold at greater pulse numbers indicated an incubation effect. Using a power law model, the incubation coefficient of unaltered porcine cortical bone was found to be 0.89 ± 0.03. Through extrapolation, the single-pulse ablation threshold was found to be 3.29 ± 0.14 J/cm(2).

Surface analysis correlated with the Raman measurements of a femtosecond laser irradiated Ca F2

Ultra fast electronic and thermal processes for energy deposition mechanism during femtosecond laser ablation of CaF2 have been identified by means of Atomic Force Microscopy (AFM) and Raman scattering technique. For this purpose, a single crystal CaF2 (111) was exposed with 800nm, 25fs Ti: Sapphire laser pulses at different laser fluences both in air and under UHV condition. Various nano and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Around the ablation threshold fluence, most significant features identified at the CaF2 surface are nanohillock like structures. These nanohillocks are typical features related to the fast electronic processes and are explainable on the basis of Coulomb explosion. At moderate irradiation fluence, bump formation is considered to be due to ultrafast melting. Further increase in the laser fluence resulted into strong enhancement of the thermal process with the appearance of larger humps and craters. These fast electronic and thermal processes are well correlated with the structural and crystallographic alterations inferred from Raman spectroscopy analysis. The nanohillocks appearing at a lower fluence are due to calcium colloid formation (aggregates of metal clusters). At higher fluences and dozes, the compressive as well as tensile stresses along with the presence of calcium carbonate are associated to diffusion, transformation and aggregation of defects which are typical features of thermal processes leading to the growth of larger humps.

Nanosecond laser ablation processes in aluminum-doped zinc-oxide for photovoltaic devices

Aiming to a future use in thin film solar modules, the processing of aluminum doped zinc oxide thin films with good optoelectronic properties with a nanosecond-pulsed ultraviolet laser has been studied. The ablation threshold fluence of the films has been determined and associated with the material properties. The ablation process has been optimized and grooves with good properties for photovoltaic devices have been obtained. The morphology of the ablated surfaces has been observed by confocal microscopy and its structure has been characterized by Raman spectroscopy. The influence of ablation parameters like focus distance, pulse energy and repetition frequency in the groove morphology has been studied with special attention to the thermal effects on the material structure.

PMN–PT (lead magnesium niobate–lead titanate) piezoelectric material micromachining by excimer laser ablation and dry etching (DRIE)

In the attempt to find the appropriate micromanufacturing technology of PMN–PT (lead magnesium niobate–lead titanate) piezo material, whose MEMS-related applications look promising, two methods are investigated: excimer laser ablation (using KrF gas) and inductively coupled plasma (ICP) dry etching, also known as DRIE (using Ar/C4F8 gases). The paper quantitatively reports the optimal parameters for PMN–PT micromachining and compares both methods. KrF excimer laser ablation threshold fluence is of 14J/cm2, the etching rate may reach 100μm/min (50nm/pulse) but the ablated surfaces are cone-shaped restricting thus the method to cutting/drilling 2D shapes. Excimer ablation method is found to be more convenient for rapid prototyping with less precise features requirements (>10μm). On the other hand DRIE method on PMN–PT is time-consuming, requires a hard mask deposition but is cleaner and more precise. Dry etching maximum recorded speed is of 0.19μm/min. The nickel hard mask selectivity varies around 8:1. In exchange this method provides flatter surfaces with no etched material redeposition.

Damage and ablation thresholds of fused-silica in femtosecond regime

We present an experimental and numerical study of the damage and ablation thresholds at the surface of a dielectric material, e.g., fused silica, using short pulses ranging from 7 to 300 fs. The relevant numerical criteria of damage and ablation thresholds are proposed consistently with experimental observations of the laser irradiated zone. These criteria are based on lattice thermal melting and electronic cohesion temperature, respectively. The importance of the three major absorption channels (multi-photon absorption, tunnel effect, and impact ionization) is investigated as a function of pulse duration (7-300 fs). Although the relative importance of the impact ionization process increases with the pulse duration, our results show that it plays a role even at short pulse duration (<50 fs). For few optical cycle pulses (7 fs), it is also shown that both damage and ablation fluence thresholds tend to coincide due to the sharp increase of the free electron density. This electron-driven ablation regime is of primary interest for thermal-free laser-matter interaction and therefore for the development of high quality micromachining processes.

Single Femtosecond Laser Pulse Irradiation of Silicon on Different Crystallographic Facet Planes

Experiment on ablation of silicon wafer on different crystallographic facet planes by single laser pulse irradiation was carried out with a femtosecond pulsed laser operating at a wavelength of 780 nm and a pulse width of 160 fs. The quality and morphology of the laser ablated silicon surface were evaluated by atomic force microscopy. The ablation threshold fluences on different crystallographic facet planes were obtained through the relationship between the squared diameter of the craters and pulse energy. The effects of different crystallographic facet planes of silicon wafer on the process of femtosecond laser ablation of silicon wafer were studied.

An Alternate Treatment of the Vapor-Plasma Transition

In this paper, we address the problem of predicting p(I), the variation of surface ablation pressure vs. incident pulsed laser intensity I, from the onset of ablation through the transition to its mediation by laser-induced plasma in vacuum. Despite its simplicity, the recently published approach of Sinko and Phipps [1] to this problem describes momentum coupling for many laser-target interactions quite well, for one material at a single wavelength where the ablation fluence threshold is clearly defined. Alternatively, if vapor pressure vs. temperature data p(T) is available for a material, e.g., using the SESAME tables, a different model can be used. In addition to the p(T) data, this model only requires knowledge of basic parameters for the material, such as specific heat, thermal conductivity, optical absorptivity, atomic weight, and its ionization state energies and their partition functions. Since all these parameters, except for optical absorptivity, are independent of laser wavelength, it is possib...

ArF laser surface modification of polyethersulfone film: Effect of laser fluence in improving surface biocompatibility

ArF laser treatment of polyethersulfone (PES) films was performed to improve biocompatibility of surfaces. For this purpose, the threshold fluence for laser ablation of PES was obtained from experimental measurements and then samples were irradiated at 2 separate ranges of fluences, i.e. below and above the ablation threshold. In order to investigate the physico-chemical changes, the modified surfaces were characterized by attenuated total reflectance (ATR) infrared spectroscopy and contact-angle measurements. The biocompatibility of the treated samples in comparison to those untreated was examined in vitro using a platelet adhesion test. The number of adhered platelets was obtained using the lactate dehydrogenase (LDH) method. For surfaces irradiated below the ablation threshold, a high reduction in the number of the adhered platelets was observed; while this number increased in samples treated at the fluence above the ablation threshold. The change in platelet adhesion was attributed to the change in chemistry and roughness of the irradiated surfaces.

Laser ablation of silicon dioxide on silicon using femtosecond near infrared laser pulses

Abstract Applying ultrashort laser pulses at wavelengths of 0.8 μm and 1.03 μm, the selective ablation of thin (∼100 nm) SiO2 layers from silicon wafers has been investigated. In particular, the effects of different pulse durations down to a minimum value of 50 fs, and single– as well as multi-pulse irradiation have been studied. Selective removal of the dielectric layer without any visible damage of the opened Si wafer was only possible with single pulse ablation. The threshold fluence for such complete ablation of the dielectric layer increases with increasing pulse duration. Irradiating two or more pulses on the same spot, significantly corrupted ablation craters are produced. The physical ablation mechanisms will be discussed with respect to the observed dependences on the laser pulse duration as well as on the number of laser pulses per spot.

Direct-write femtosecond laser ablation and DNA combing and imprinting for fabrication of a micro/nanofluidic device on an ethylene glycol dimethacrylate polymer

Arrays of microwells connected by nanoscale channels with sizes on the order of 10 nm can be created in an ethylene glycol dimethacrylate (EGMDA) polymer using the DNA combing and imprinting technique. Larger micro-scale channels which lead into the microwell/nanochannel arrays are needed to allow the arrays to be externally filled with desired reagents, molecules and cells. In this work, direct-write femtosecond laser ablation was employed as a post process to fabricate these microscale filling channels. Single pulse and multiple pulses overlap ablation was first conducted on an EGMDA polymer using a focused femtosecond laser beam. Scanning electron microscopy was employed to measure the ablated channel width. Single pulse ablation threshold fluence and incubation coefficient were found and were used to predict microchannel width. Finally, femtosecond laser ablation was used to fabricate filling channels on microwell/nanochannel arrays. Fluorescent flow testing was performed to verify fluid connectivity between the laser-ablated filling channels and the microwell/nanochannel array.

Role of near-field enhancement in plasmonic laser nanoablation using gold nanorods on a silicon substrate

We present experimental results for the plasmonic laser ablation of silicon with nanoscale features as small as 22 x 66 nm using single near-infrared, femtosecond laser pulses incident on gold nanorods. Near the ablation threshold, these features are photo-imprints of gold nanorod particles positioned on the surface of the silicon and have feature sizes similar to the nanorods. The single rod-shaped ablation pattern matches the enhancement patterns of the Poynting vector magnitude on the surface of silicon, implying that the ablation is a result of the plasmonic enhancement of the incident electromagnetic waves in the near-field of the particles. Interestingly, the ablation pattern is different from the two separated holes at the ends of the nanorod, as would be expected from the electric field--|E|(2) enhancement pattern. We measured the plasmonic ablation threshold fluence to be almost two orders of magnitude less than the femtosecond laser ablation threshold of silica, present in the thin native oxide layer on the surface of silicon. This value also agrees with the enhancement of the Poynting vector of a nanorod on silicon as calculated with electromagnetic simulations. We thus conclude that plasmonic ablation with plasmonic nanoparticles depends directly on the polarization and the value of the near-field enhancement of the Poynting vector and not the square of the electric field as previously suggested.

Atmospheric-pressure Molecular Imaging of Biological Tissues and Biofilms by LAESI Mass Spectrometry

Ambient ionization methods in mass spectrometry allow analytical investigations to be performed directly on a tissue or biofilm under native-like experimental conditions. Laser ablation electrospray ionization (LAESI) is one such development and is particularly well-suited for the investigation of water-containing specimens. LAESI utilizes a mid-infrared laser beam (2.94 μm wavelength) to excite the water molecules of the sample. When the ablation fluence threshold is exceeded, the sample material is expelled in the form of particulate matter and these projectiles travel to tens of millimeters above the sample surface. In LAESI, this ablation plume is intercepted by highly charged droplets to capture a fraction of the ejected sample material and convert its chemical constituents into gas-phase ions. A mass spectrometer equipped with an atmospheric-pressure ion source interface is employed to analyze and record the composition of the released ions originating from the probed area (pixel) of the sample. A systematic interrogation over an array of pixels opens a way for molecular imaging in the microprobe analysis mode. A unique aspect of LAESI mass spectrometric imaging is depth profiling that, in combination with lateral imaging, enables three-dimensional (3D) molecular imaging. With current lateral and depth resolutions of ~100 μm and ~40 μm, respectively, LAESI mass spectrometric imaging helps to explore the molecular structure of biological tissues. Herein, we review the major elements of a LAESI system and provide guidelines for a successful imaging experiment.

Atmospheric-pressure Molecular Imaging of Biological Tissues and Biofilms by LAESI Mass Spectrometry

Ambient ionization methods in mass spectrometry allow analytical investigations to be performed directly on a tissue or biofilm under native-like experimental conditions. Laser ablation electrospray ionization (LAESI) is one such development and is particularly well-suited for the investigation of water-containing specimens. LAESI utilizes a mid-infrared laser beam (2.94 μm wavelength) to excite the water molecules of the sample. When the ablation fluence threshold is exceeded, the sample material is expelled in the form of particulate matter and these projectiles travel to tens of millimeters above the sample surface. In LAESI, this ablation plume is intercepted by highly charged droplets to capture a fraction of the ejected sample material and convert its chemical constituents into gas-phase ions. A mass spectrometer equipped with an atmospheric-pressure ion source interface is employed to analyze and record the composition of the released ions originating from the probed area (pixel) of the sample. A systematic interrogation over an array of pixels opens a way for molecular imaging in the microprobe analysis mode. A unique aspect of LAESI mass spectrometric imaging is depth profiling that, in combination with lateral imaging, enables three-dimensional (3D) molecular imaging. With current lateral and depth resolutions of ~100 μm and ~40 μm, respectively, LAESI mass spectrometric imaging helps to explore the molecular structure of biological tissues. Herein, we review the major elements of a LAESI system and provide guidelines for a successful imaging experiment.

Analysis of silicon via hole drilling for wafer level chip stacking by UV laser

For stacking wafers/dies, through-silicon-vias (TSVs) need to be created for electrical connection of each wafer/die, which enables better electrical characteristics and less footprints. And for via hole processing, chemical methods such as DRIE (Deep Reactive Ion Etching) are mostly used. These methods suffer the problems of slow processing speed, being environment-unfriendly and damage on the existing electric circuits due to high process temperature. Furthermore, masks are also needed. To find an alternative to the methods, researches on the laser drilling of via holes on silicon wafer are being conducted. This paper investigates the silicon via hole drilling process using laser beam. The percussion drilling method is used for this investigation. It is also examined how the laser parameters- laser power, pulse frequency, the number of laser pulses and the diameter of laser beam- have an influence on the drilling depth, the hole diameter and the quality of via holes. From these results, laser drilling process is optimized. The via hole made by UV laser on the crystal silicon wafer is 100µm deep, has the diameter of 27.2µm on the top, 12.9µm at the bottom. These diameters deviate from the target values by 2.8µm and 0.4µm respectively. These values correspond to the deviation from the target taper angle of the via hole by less than 1°. The processing speed of the laser via hole drilling is 114mm/sec, therefore, etching process can be replaced by this method, if the number of via holes on a wafer is smaller than 470,588. The ablation threshold fluence of silicon is also determined by a FEM model and is verified by experiment.

Pulse laser ablation at water–air interface

We studied a new pulse laser ablation phenomenon on a liquid surface layer, which is caused by the difference between the refractive indices of the two materials involved. The present study was motivated by our previous study, which showed that laser ablation can occur at the interface between a transparent material and a gas or liquid medium when the laser pulse is focused through the transparent material. In this case, the ablation threshold fluence is reduced remarkably. In the present study, experiments were conducted in water and air in order to confirm this phenomenon for a combination of two fluid media with different refractive indices. This phenomenon was observed in detail by pulse laser shadowgraphy. A high-resolution film was used to record the phenomenon with a Nd:YAG pulse laser with 10-ns duration as a light source. The laser ablation phenomenon on the liquid surface layer caused by a focused Nd:YAG laser pulse with 1064-nm wavelength was found to be followed by the splashing of the liquid surface, inducing a liquid jet with many ligaments. The liquid jet extension velocity was around 1000 m/s in a typical case. The liquid jet decelerated drastically due to rapid atomization at the tips of the ligaments. The liquid jet phenomenon was found to depend on the pulse laser parameters such as the laser fluence on the liquid surface, laser energy, and laser beam pattern. The threshold laser fluence for the generation of a liquid jet was 20 J/cm2. By increasing the incident laser energy with a fixed laser fluence, the laser focused area increased, which eventually led to an increase in the size of the plasma column. The larger the laser energy, the larger the jet size and the longer the temporal behavior. The laser beam pattern was found to have significant effects on the liquid jet’s velocity, shape, and history.