Femtosecond Pulsed Laser Ablation Research Articles

Probing the heat affected zone by chemical modifications in femtosecond pulse laser ablation of titanium nitride films in air

A new approach is presented to quantify the so-called “heat affected zone” (HAZ) during femtosecond laser pulse processing. Ablation of titanium nitride (TiN) thin films (∼3 μm thickness) by multiple femtosecond laser pulses (τ=130 fs, λ=800 nm) in air environment was studied by means of two different surface analytical methods both being sensitive to chemical alterations at the surface. Scanning Auger electron microscopy was applied for a visualization of the spatial distribution of specific elements (Ti, O) within the laser-modified areas. The chemical state of the irradiated surface was revealed by complementary x-ray photoelectron spectroscopy. Both methods were used for a depth-profiling chemical analysis (tracking the elements Ti, N, O, and C) using an Ar-ion beam for surface sputtering. In a narrow laser fluence range slightly below the ablation threshold of TiN significant superficial oxidation can be observed leading to the formation of substoichiometric TiO2−x. At fluences above the ablation threshold, an increased titanium concentration is observed within the entire ablation craters. Following upon sputter removal the elemental distribution into the depth of the nonablated material, the results allow an estimation of the heat-affected zone for femtosecond laser ablation in air environment. According to our analyses, the HAZ extends up to a few hundreds of nanometers into the nonablated material.

Characterization of the ablation of TeO2 crystals in air with femtosecond laser pulses

Femtosecond (fs) laser pulse ablation (pulse duration of 150 fs, wavelength of 775 nm, repetition rate of 1 kHz) of single-crystalline TeO2 surfaces was performed in air using the direct focusing technique. The lateral and vertical dimensions of laser ablated craters as well as the laser damage thresholds were evaluated for different pulse numbers applied to the same spot. The joint observation using optical microscopy, atomic force microscopy and scanning electron microscopy revealed the surface morphology of the ablated craters and also showed that the ablation threshold depends significantly on the number of laser pulses applied to the same spot due to incubation effects. The incubation effects change the absorption processes involved in fs-laser ablation of the transparent material from multiphoton absorption to a single-photon absorption. These results are discussed on the basis of recent models of the interaction of fs-laser pulses with dielectrics.

Effect of boron incorporation on the structure and electrical properties of diamond-like carbon films deposited by femtosecond and nanosecond pulsed laser ablation

The influence of the incorporation of boron in diamond-like carbon (DLC) films on the microstructure of the coatings has been investigated. The boron-containing DLC films (a-C:B) have been deposited by pulsed laser deposition (PLD) at room temperature in high vacuum conditions, by ablating graphite and boron targets either with a femtosecond pulsed laser (800nm, 150fs, fs-DLC) or with a nanosecond pulsed laser (248nm, 20ns, ns-DLC). Alternative ablation of the graphite and boron targets has been carried out to deposit the a-C:B films. The film structure and composition have been highlighted by coupling Field Emission Scanning Electron Microscopy, Electron Energy Loss Spectroscopy and High Resolution Transmission Electron Microscopy. Using the B K-edge, EELS characterization reveals the boron effect on the carbon bonding. Moreover, the plasmon energy reveals a tendency of graphitization associated to the boron doping. Pure boron particles have been characterized by HRTEM and reveal that those particles are amorphous or crystallized. The nanostructures of the boron-doped ns-DLC and the boron-doped fs-DLC are thus compared. In particular, the incorporation of boron in the DLC matrix is highlighted, depending on the laser used for deposition. Electrical measurements show that some of these films have potentialities to be used in low temperature thermometry, considering their conductivity and temperature coefficient of resistance (TCR) estimated within the temperature range 160–300K.

Mobility of Nanoparticles Generated by Femtosecond Laser Ablation in Liquids and Its Application to Surface Patterning

Metal nanoparticles have been generated in deionized water and acetone by femtosecond-pulsed laser ablation. The expansion velocity was measured in–process in dependence on pulse energy using particle tracking image velocimetry. Particle size distributions of silver were determined at periodic time intervals during the generation process, reaching stabilization level around 50 nm after 5·10 5 pulses. Electrophoretic mobility of laser-generated metal nanoparticles was studied by laser scattering velocimetry and the electrophoretic deposition of charged nanoparticles was used to generate nanoparticle-derived nanostructures on 3-dimensional microstructured surfaces.

Influence of femtolaser notch sharpening technique in the determination of essential work of fracture (EWF) parameters

The fracture behaviour of a 0.5 mm thick ethylene-propylene block copolymer is analyzed using the essential work of fracture method, using DENT-type specimens. The influence of three experimental parameters in the technique is evaluated: the effect of the notch sharpening technique, the use of a videoextensometer for monitoring the deformations and the measurement of ligament lengths before or after fracture tests were carried on. The results showed that the femtosecond pulsed laser ablation technique (femtolaser) produced sharp notches with no plastic deformation ahead of the notch tip, which yield smaller specific essential work of fracture ( w e) values than in the cases where the notches were sharpened with razor blades. The use of a videoextensometer has allowed removing the viscoelastic energy from the plastic work, with lower values of βw p. The measurement of the ligament lengths before or after the test did not affect the results.

Optical properties of nickel-incorporated amorphous carbon film deposited by femtosecond pulsed laser ablation

The present study investigates the optical properties of Ni-incorporated amorphous carbon (a-C:Ni) films covering a wide range of Ni concentration from doping to alloys. The films are deposited by sequential femtosecond pulsed laser ablation of graphite and Ni targets. The optical properties are analyzed by means of the Maxwell-Garnett theory with separated metal and carbon phases. This analysis permits to investigate the change of the carbon phase with Ni concentration. It is shown that the microstructure of the carbon phase is affected by the Ni incorporation even at low Ni concentrations (1 at.%) where a local reorganization of the carbon matrix occurs affecting the aromatic clusters. At higher Ni concentrations the films become more disordered.

Electrical properties of boron-doped diamond-like carbon thin films deposited by femtosecond pulsed laser ablation

We report on electrical measurements and structural characterization performed on boron-doped diamond-like carbon thin films deposited by femtosecond pulsed laser deposition. The resistance has been measured between 77 and 300 K using four probe technique on platinum contacts for different boron doping. Different behaviours of the resistance versus temperature have been evidenced between pure DLC and boron-doped DLC. The a-C:B thin film resistances exhibit Mott variable range hopping signature with temperature. Potential applications of DLC thin films to highly sensitive resistive thermometry is going to be discussed.

Structural and electrical characterization of boron-containing diamond-like carbon films deposited by femtosecond pulsed laser ablation

The present study investigates the influence of the incorporation of boron in Diamond-Like Carbon (DLC) films deposited by femtosecond laser ablation, on the structure and electrical properties of the coatings within the temperature range 70–300 K. Doping with boron has been performed by ablating alternatively graphite and boron targets. The film structure and composition have been highlighted by coupling Atomic Force Microscopy (AFM), Scanning Electron Microscopy equipped with a field emission gun (SEM-FEG) and High Resolution Transmission Electron Microscopy (HRTEM). Boron dilution ranges between 2 and 8% and appears as nanometer size clusters embedded in the DLC matrix. Typical resistivity values are 100 W cm for pure a-C films, down to few W cm for a-C:B films at room temperature. The resistance decreases exponentially when the temperature increases in the range 70–300 K. The results are discussed considering the classical model of hopping conduction in thin films. Some coatings show temperature coefficients of resistance (TCR) as high as 3.85%. TCRs decrease when the doping increases. Such high values of TCR may have interests in the use of these films as thermometer elements in micro and nanodevices.

Fabrication of microelectrodes deeply embedded in LiNbO 3 using a femtosecond laser

We present a novel technique to fabricate deeply embedded microelectrodes in LiNbO 3 using femtosecond pulsed laser ablation and selective electroless plating. The fabrication process mainly consists of four steps, which are (1) micromachining of microgrooves on the surface of LiNbO 3 by femtosecond laser ablation; (2) formation of AgNO 3 films on substrates; (3) scanning the femtosecond laser beam in the fabricated microgrooves for modification of the inner surfaces; and (4) electroless copper plating. The void-free electroless copper plating is obtained with appropriate cross section of microgrooves and uniform initiation of the autocatalytic deposition on the inner surface of grooves. The dimension and shape of the microelectrodes could be accurately controlled by changing the conditions of femtosecond laser ablation, which in turn can control the distribution of electric field inside LiNbO 3 crystal for various applications, opening up a new approach to fabricate three-dimensional integrated electro-optic devices.

Femtosecond pulsed laser ablation dynamics and ablation morphology of nickel based superalloy CMSX-4

Pump-probe shadowgraphic imaging of single pulse femtosecond laser ablation was performed to investigate the dynamics of material removal during femtosecond laser machining of the intermetallic superalloy CMSX-4. Time-resolved shadowgraphic images were collected, which showed the presence of an expanding shock wave in the air in front of the target, following the onset of laser ablation. The dimensions of the shock wave were measured as a function of time (0–10.3ns), following the onset of ablation. The energy release associated with the observed shock wave and the pressure at the shock wave front versus time as a function of incident laser fluence (1.27–62.8J∕cm2) were inferred from the shock dynamics. The measured shock wave dynamics and inferred shock energy release are discussed in light of the evolving ablation morphology and ablated crater depth as a function of incident laser fluence.

Synthesis of monoclinic KGd(WO4)2 nanocrystals by two preparation methods

The monoclinic crystalline phase of KGd(WO4)2 is a well-known solid-state laser host. The present article describes how monoclinic nanocrystals of KGd(WO4)2 are synthesized by two different methodologies: the sol–gel modified Pechini method and femtosecond pulsed laser ablation of bulk material in deionized water. Transmission Electron Microscopy images show KGdW nanocrystals between 15 and 100 nm for both methods. The nanocrystals are studied by X-ray powder diffraction (XRD) in order to identify the crystalline phase and to refine the unit cell parameters. For the sol–gel modified Pechini method, differential thermal analyses and XRD techniques were also used to optimize the method of preparation.

Femtosecond-pulsed laser micromachining of a 4H–SiC wafer for MEMS pressure sensor diaphragms and via holes

The challenging issues in conventional microfabrication of SiC pressure sensor diaphragms from bulk wafers are low etch rates, thicker (>40 µm) diaphragms, low spatial resolutions, rough surfaces and substantial contamination. In via hole drilling of SiC, the critical concern is the low drilling speed (nm per minute). In this work, femtosecond (fs)-pulsed laser ablation was conducted to overcome some of these deficiencies. Circular diaphragms (0.5 to 1 mm) by trepanning mode and via holes (30–50 µm) by percussion drilling mode were micromachined in 250 µm thick 4H–SiC single crystals using an 800 nm wavelength, 120 fs, 1 mJ Ti:sapphire laser. Pulse energy, number of pulses and scan rate were varied to obtain a high etch rate and high quality features. Results showed that the etch rates are 2–10 µm per pulse, diaphragm thicknesses are 20–200 µm, surface roughness is 1–2 µm Ra and via hole drilling speeds are up to 25 µm per second. The etch depth control was well within ± 1%. High aspect ratio features with excellent spatial resolutions were obtained due to the absence of thermal damages such as a recast layer and contamination. Thus, femtosecond-pulsed laser ablation by virtue of its unique characteristics such as multiphoton ionization and the absence of lattice heating offers high speed, precision and accuracy in micromachining 4H–SiC wafers.

Propagation of a femtosecond pulsed laser ablation plume into a background atmosphere

We investigate the effects of ambient gas on the expansion dynamics of laser plume produced during femtosecond laser ablation of a metallic target. We studied experimentally plume propagation for ambient air pressure ranging from 10−6to50mbar, observing that the atomic and nanoparticles plume components experience different effects. We interpret these results with a simplified model of the plume front propagation, which is able to fully reproduce the main experimental features. Our results allow us to estimate quantitatively the nanoparticles plume content (up to about 80%), and can help identifying optimal conditions of nanoparticles deposition for thin films production.

One-step deposition of diamond-like carbon films containing self-assembled metallic nanoparticles, by femtosecond pulsed laser ablation

Femtosecond pulsed laser ablation has been used for the deposition of various thin films by co-ablating pure graphite and pure metallic targets (nickel or tantalum) under vacuum conditions with an amplified Ti:sapphire laser working at 800 nm, with about 1.5 mJ per pulse at a repetition rate of 1 kHz. The plasma plume created during the femtosecond pulsed laser deposition process has been investigated, by fast imaging and spectroscopic characterizations, depending on the ablated target and deposition conditions. Particular attention has been paid to the correlation between the film nanostructure and the plasma plume properties, in terms of ejected species and energies of impinging species on the substrate. The tantalum clusters in the tantalum-doped carbon films appear under two crystalline phases: a stable phase (α-Ta) corresponding to that of the target, and a metastable phase (β-Ta), already observed at high temperatures. The presence of this metastable phase (β-Ta) is correlated to the temperature of the clusters measured from the plasma plume investigation.

Pump-probe imaging of femtosecond pulsed laser ablation of silicon with thermally grown oxide films

Femtosecond pulsed laser ablation of silicon substrates with thin thermally grown oxide films (20–1200 nm) was studied using pump-probe microscopy techniques. Images from both the front and side of the ablation event produced at a laser fluence of 1.3 J/cm2 were obtained, and results from the two imaging geometries were compared yielding the optical properties of the ablated material. Ablation dynamics were studied over the time scale from 0 to 10.35 ns following the onset of ablation, and ablated material velocities ranging from 200±20 to −3010±360 m/s were determined depending on the thermal oxide film thickness.

Metal absorptivity in femtosecond pulsed laser ablation

In this paper, the effect of the absorptivity of metal on femtosecond pulsed laser ablation is investigated. The formulas for the absorptivity depending on target temperature are derived from Maxwell Equations and the Lambert-Beer’s law. Based on this, a new two-temperature model is proposed to describe the femtosecond pulsed laser ablation with metal. Then, using Au as an example, a finite difference method is employed to simulate the space-dependent and time-dependent absorptivity and the target temperature. The temperature evolution of our model is compared with the result obtained form the heat conduction model taking the absorptivity as constant. It is shown that the absorptivity plays an important role in the femtosecond pulsed laser ablation. The results of this paper are helpful in choosing the best technical parameters in femtosecond pulsed laser ablation.

Role and importance of nanoparticles in femtosecond pulsed laser ablation deposition of Al–Cu–Fe quasicrystal

In this Letter, the secondary plume produced by femtosecond laser ablation of an Al–Cu–Fe quasicrystal has been studied. The data evidence the presence of hot nanoparticles produced several microseconds after the laser shot. The cooling mechanisms of these particles have been analysed together with the evolution of their composition. The results indicate a clear relation between the final composition of the particles and the high temperature equilibrium vapour pressures of the different elements. The elemental analysis of the cross-section of films deposited by femtosecond laser ablation states the importance of nanoparticles in the film growth.

Microstructuring using femtosecond pulsed laser ablation

A self-induced mechanism leading to the formation of periodic microstructures has been observed during femtosecond pulsed laser ablation with a high repetition laser source. The bottom of an ablated area shows two different kinds of microstructures. A periodic ripple or a chaotic columnar structure emerges. It has been observed that the second morphology grows out of the first one dependent mainly on the amount of fired laser pulses. This transition and the influence of several parameters on it are described. Both structures have several variations in morphology. Regarding the possible applications of such structures, this is of special interest. The presented experiments give a first glimpse of the possible microstructures emerging after ultrafast laser machining. Concerning the physical background of the found phenomena, it has been observed that a liquid phase is present in the ablation process. The formation of ripples may be closely linked to that fact. Existing models on ripple formation could not be linked to the ripple spacing found in the experiments.

Parametric study on femtosecond laser pulse ablation of Au films

Ablation process of 1 kHz rate femtosecond lasers (pulse duration 148 fs, wavelength 775 nm) with Au films on silica substrates has been systemically studied. The single-pulse threshold can be obtained directly. For the multiple pulses the ablation threshold varies with the number of pulses applied to the surface due to the incubation effect. From the plot of accumulated laser fluence N × ϕ th( N) and the number of laser pulses N, incubation coefficient of Au film can be obtained ( s = 0.765). As the pulse energy is increased, the single pulse ablation rate is increasing following two ablation logarithmic regimes, which can be explained by previous research.

Hopping current density in amorphous carbon/crystalline silicon heterojunctions

High density amorphous carbon (a-C) films deposited by femtosecond pulsed laser ablation were incorporated in p-type c-Si/a-C/Al devices. Rectifying I–V characteristics were measured versus temperature T (200–300K). At low fields, the conductivity behaves as Ln(σ/σ∘∘)=−(T0/T)1/4, corresponding to 3D hopping transport. As the electric field F increases from 104 to 105Vcm−1, the decrease of both apparent slope T01/4(F) from 230 to 100K1/4 and prefactor σ∘∘(F) from 1016 to 104Scm−1 is quantitatively similar for forward and reverse bias, with however a shift ΔV of 0.28eV attributed to the heterojunction built-in potential. The observed strong decrease in the apparent prefactor σ∘∘(F) is consistent with recent modeling of field-enhanced hopping transport in exponential band tails.