Modeling of UV pulsed-laser ablation of metallic targets

A model to describe the laser ablation of metallic targets is presented. It accounts for the main physical processes involved in the laser–solid–plasma interaction by considering the photon absorption and the ionization mechanisms that are active in the plasma, as well as the laser-produced plasma kinetics. The model is used to simulate the laser ablation of aluminum targets irradiated with a 6-ns UV laser pulse at 0.35 μm, and the results are compared with experimental findings. Calculations show that all the investigated plasma parameters strongly depend on the laser intensity until a roll-off is reached at irradiance ≥1.5 GW cm^-2. The satisfactorily good agreement between model predictions and experimental findings confirms that laser–plasma interaction processes and plasma kinetics play a relevant role during nanosecond laser ablation of metals in the laser intensity range of concern in this study. Received: 12 February 1999 / Accepted: 12 April 1999 / Published online: 7 July 1999     

Femtosecond laser ablation of silicon–modification thresholds and morphology

We investigated the initial modification and ablation of crystalline silicon with single and multiple Ti:sapphire laser pulses of 5 to 400 fs duration. In accordance with earlier established models, we found the phenomena amorphization, melting, re-crystallization, nucleated vaporization, and ablation to occur with increasing laser fluence down to the shortest pulse durations. We noticed new morphological features (bubbles) as well as familiar ones (ripples, columns). A nearly constant ablation threshold fluence on the order of 0.2 J/cm² for all pulse durations and multiple-pulse irradiation was observed. For a duration of ≈100 fs, significant incubation can be observed, whereas for 5 fs pulses, the ablation threshold does not depend on the pulse number within the experimental error. For micromachining of silicon, a pulse duration of less than 500 fs is not advantageous. Received: 4 December 2000 / Revised version: 29 March 2001 / Published online: 20 June 2001     

Ultrashort-pulse laser ablation of indium phosphide in air

Ablation of indium phosphide wafers in air was performed with low repetition rate ultrashort laser pulses (130 fs, 10 Hz) of 800 nm wavelength. The relationships between the dimensions of the craters and the ablation parameters were analyzed. The ablation threshold fluence depends on the number of pulses applied to the same spot. The single-pulse ablation threshold value was estimated to be φ_th(1)=0.16 J/cm². The dependence of the threshold fluence on the number of laser pulses indicates an incubation effect. Morphological and chemical changes of the ablated regions were characterized by means of scanning electron microscopy and Auger electron spectroscopy. Received: 30 May 2000 / Accepted: 31 May 2000 / Published online: 23 August 2000     

Wet bone ablation with mechanically Q-switched high-repetition-rate CO2 laser

2 laser using a miniature water spray is demonstrated. An ablation threshold of 1.4 J/cm², an optimal energy density of 9–10 J/cm², and a corresponding specific ablation energy of 25–30 J/mm³ are found for pig thighbone compacta at λ=9.57 μm and a beam waist diameter of 0.5 mm. The water spray alleviates tissue carbonization even at high laser pulse repetition rates and increases ablation efficiency. Received: 9 March 1998/Revised version: 6 July 1998     

Low-threshold field electron emission of Si micro-tip arrays produced by laser ablation

Low-threshold field electron emission (FEE) is reported for periodic arrays of micro-tips produced by laser ablation of Si wafers. The best samples show emission at threshold fields as low as 4–5 V/μm for n-type Si substrates and of 1–2 V/μm for p-doped Si substrates, as measured with a flat-screen technique. Auger electron spectroscopy and X-ray electron spectroscopy reveal island-like deviation of the SiO₂ stoichiometry on the tip surfaces, with lateral dimensions of less than 100 nm. Microscopic studies using a special field-emission STM show that the emission originates from well-conducting regions of sub-micron size. The experimental data suggest FEE from the tip arrays by a geometric field enhancement of both the individual micro-tip and the narrow conducting channels in the tip body. Received: 3 May 2002 / Accepted: 1 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/135-82-34, E-mail: shafeev@kapella.gpi.ru     

Energy distribution of ions produced by excimer-laser ablation of solid and molten targets

2 . The measurements reveal components with different charge-to-mass ratio and distinct components with the same charge/mass ratio. The most probable kinetic energy has values of several tens of eV for singly charged ions, and is larger by a factor exceeding 2 for doubly charged ions. The role of target material and state, solid or liquid, laser photon energy and fluence has been investigated and is discussed in comparison to the findings of previous investigations. An estimate of the electrostatic plasma potential in PLA, based on electron loss rate arguments, is presented to account for the high ion energies observed. Received: 9 March 1998 / Accepted: 27 November 1998 / Published online: 24 February 1999     

Ultrashort pulse laser ablation of silicon: an MD simulation study

Received: 30 July 1997/Accepted: 5 August 1997     

Polarization dependence in the UV laser desorption of NO from NiO(100)

- and -polarized radiation. When the yield is related to the incident photon flux, desorption cross sections of (1.9±0.3)×10^-17 cm² and (3.3±0.5)×10^-17 cm² for - and -polarized light, respectively, are deduced. In order to assess the importance of substrate excitation the optical constants of NiO have been determined for the photon energy employed. When the desorption yield is then related to the photon flux absorbed in the NiO film, much larger cross sections are obtained, and an even larger effectiveness of -polarized light. For a direct optical excitation of charge transfer states the implications for the symmetry character of the states involved are discussed. It is concluded that excitations to A^′′ states are preferred. Received: 21 October 1998     

Chemical, morphological and accumulation phenomena in ultrashort-pulse laser ablation of TiN in air

Ultrashort-pulse laser ablation (τ=130 fs, λ=800 nm, repetition rate 2–20 Hz) of titanium nitride was investigated for laser fluences between 0.3 and 4.5 J/cm² using the direct focusing technique in air. The influence of the laser pulse number and the peak fluence was investigated by means of several surface analytical techniques (optical microscopy, dynamic friction atomic force microscopy, scanning Auger electron microscopy and small-spot electron spectroscopy for chemical analysis). The correlation of the results about optical, physical and chemical properties of the irradiated areas allows us to propose a simple oxidation model, which explains different observed phenomena associated with surface damage such as mound formation and crater widening and clarifies the incubation behavior reported earlier for this material. Received: 8 May 2000 / Accepted: 9 May 2000 / Published online: 13 September 2000     

Polarization effects in ultrashort-pulse laser drilling

A strong influence of the polarization of the laser radiation on the geometry of laser-machined microdrillings has been observed for ultrashort pulses. For drillings with a certain aspect ratio, reflections at the hole walls take place, leading to a non-uniform intensity distribution deep inside the formed hole. Experimental and theoretical results on this subject are discussed. It is shown that a rotation of the polarization during the drilling process (“polarization trepanning”) significantly improves the quality of the produced holes. Received: 21 August 1998 / Accepted: 25 November 1998 / Published online: 17 March 1999     

Holographic interferometry for the structural diagnostics of UV laser ablation of polymer substrates

Holographic interferometry is examined for its potential as a diagnostic tool of the structural modifications effected in laser-processing applications. The interferometric ‘comparison’ of the holographic images of the sample recorded before and after irradiation enables the full-field spatially resolved detection of the induced structural modifications. The potential of the method is illustrated in the ablation of polymer (polymethylmethacrylate and polystyrene) films with nanosecond pulses at 193 and 248 nm. The detailed characterization and quantitative monitoring of the growth of the induced modifications as a function of laser-material parameters is attained. A most novel result is the observation of delocalised structural modifications at distances relatively far away (d≈2 cm) from the irradiated area. Received: 15 August 2001 / Accepted: 16 August 2001 / Published online: 2 October 2001     

Excimer laser polymer ablation: twenty years on

Research and development in excimer laser polymer ablation has been actively pursued for some twenty years, driven by interest in the basic science as well as by numerous applications that have emerged for this high-resolution technique for material removal. This paper reviews some of the basic mechanistic aspects of the UV laser–polymer interaction as a prelude to dealing with practical matters related to polymer processing by ablation. Applications in micro-machining and potential areas for future research are briefly covered. Received: 7 October 2002 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +44-1482/465-606, E-mail: p.e.dyer@hull.ac.uk     

Rate dependence of short-pulse laser ablation of metals in air and vacuum

Ablation rates of aluminum and stainless steel are studied as a function of fluence, hole depth, pulse duration and ambient pressure (air vs vacuum). We find a weak rate dependence on pulse duration from 150 fs to 500 ps, and a strong rate dependence on hole depth due to surface roughness. Machining in air plays an important role in deep holes, but has a weaker influence on initial surface ablation rates. Oxidation greatly reduces drilling rates for deep holes in aluminum. Received: 26 December 2001 / Accepted: 9 July 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/422-5537, E-mail: stuart3@llnl.gov     

Dynamics of the plume formation and parameters of the ejected clusters in short-pulse laser ablation

The dynamics of the early stages of the ablation plume formation and the mechanisms of cluster ejection are investigated in large-scale molecular dynamics simulations. The cluster composition of the ablation plume has a strong dependence on the irradiation conditions and is defined by the interplay of a number of processes during the ablation plume evolution. At sufficiently high laser fluences, the phase explosion of the overheated material leads to the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets, gas-phase molecules, and small clusters. The ejection of the largest droplets is attributed to the hydrodynamic motion in the vicinity of the melted surface, especially active in the regime of stress confinement. Spatially resolved analysis of the dynamics of the plume formation reveals the effect of segregation of the clusters of different sizes in the expanding plume. A relatively low density of small/medium clusters is observed in the region adjacent to the surface, where large clusters are being formed. Medium-size clusters dominate in the middle of the plume and only small clusters and monomers are observed near the front of the expanding plume. Despite being ejected from deeper under the surface, the larger clusters in the plume have substantially higher internal temperatures as compared to the smaller clusters. The cluster-size distributions can be relatively well described by a power law Y(N)∼N^-τ with exponents different for small, up to ∼15 molecules, and large clusters. The decay is much slower in the high-mass region of the distribution. Received: 13 October 2001 / Accepted: 18 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-434/982-5660, E-mail: lz2n@virginia.edu     

Pulsed laser ablation of solids: transition from normal vaporization to phase explosion

We present experimental data on mass removal during 1064-nm pulsed laser ablation of graphite, niobium and YBa₂Cu₃O_7-δ superconductor. Evidence for the transition from normal vaporization to phase explosion has been obtained for these materials, showing a dramatic increase in the ablation rate at the threshold fluences of 22, 15 and 17.5 J/cm², respectively. A numerical model is used to evaluate the ablation rate and temperature distribution within the targets under near-threshold ablation conditions. The results are analyzed from the viewpoint of the vaporized matter approaching the critical point with increasing laser fluence. A possible means of the estimating the thermodynamic critical temperature from the data for nanosecond laser ablation is discussed. It is suggested that the critical temperature of refractory metals is higher than that estimated with the traditional methods due to plasma effects. An analogy with the boiling crisis (the transition from nucleate to film boiling) is drawn to explain the formation of ablation craters with spallated edges. Received: 18 May 2000 / Accepted: 14 July 2000 / Published online: 22 November 2000     

Early stages of laser graphitization of diamond

A mechanism for photographitization of a free diamond surface is proposed. The quantum-kinetic rate of this process is determined. The graphitization rate is close to zero if the activation energy of the graphitization process is taken as being equal to the binding energy of a carbon atom with the surface (i.e. equal to the sublimation energy of a carbon atom). On the contrary, if the activation energy is close to the energy of C–C bonds, the graphitization process may occur at a noticeable rate and be observed under ‘relatively smooth’ experimental conditions. The temperature rise leads to a considerable increase in the graphitization rates. Preliminary experimental data on the low-rate laser ablation of diamond are presented to support the proposed model of photographitization. An early stage of laser-induced graphitization in the bulk of diamond is also considered. It is found that the nucleation of a ‘tiny graphite drop’ is possible in the bulk of the diamond inside the focal area of a laser beam; the ‘graphite drop’ growth causing the appearance of mechanical stresses in the surrounding regions. The maximum size of the graphite drop is determined, which, when exceeded, leads to mechanical damage of the sample and to a change in the mechanism of laser graphitization. An evident mechanical criterion for laser-induced damage of diamond is proposed. Received: 2 October 2002 / Accepted: 5 October 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. E-mail: stvn@stankin.ru     

Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

Dynamic ablation studies of sub-micron gratings on metals with sub-picosecond time resolution

Received: 27 July 1998/Accepted: 28 July 1998     

Formation of conical microstructures upon laser evaporation of solids

The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (∼10⁴) at laser fluence of 1–2 J/cm² below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 10⁵ Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20–30 μm. It is suggested that the spatial period of the structures (10–20 μm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40–70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti. Received: 12 February 2000 / Accepted: 28 March 2000 / Published online: 20 June 2001     

Micro-humps formed in excimer laser ablation of polyimide using mask projection system

In this paper we report the results of an investigation into surface deformation caused by thermal effects during excimer laser ablation of polyimide. Obvious surface deformation around hole entrances was observed during the experiment. The surface topology and cross section of the ablated holes were analyzed using topography measurement tool and scanning electron microscopy. It was shown that a micro-hump of 17 to 150 nm in height and 1 to 3 μm in width was formed above the level of the unablated surface. The deformed surface showed rough and color-changed characteristics. An optical diffraction model was employed to explain the cause of this kind of deformation. It was found that the ablating and heating by a near- and under-threshold laser beam became a thermal effect in polyimide material ablation, which was contributed to by a diffraction effect of the optical projection system. Received: 9 October 2001 / Accepted: 17 October 2001 / Published online: 23 January 2002