J. Dieleman

Gas flow dynamics in laser ablation deposition

The gas flow dynamics of laser ablation plumes is investigated experimentally and theoretically. Experimentally, angular‐resolved time‐of‐flight (ARTOF) measurements are performed on a model system (laser etching of copper in a chlorine environment). The TOF spectra obtained can be fitted by elliptical Maxwell–Boltzmann distributions on a stream velocity. Theoretically, an analytical model is constructed, based on the hydrodynamical problem of an expanding elliptical gas cloud. The model allows semiquantitative prediction of ARTOF distributions and angular intensity distributions. Observed trends in laser ablation deposition such as independence of the angular intensity distribution on mass of the atom and laser fluence, and dependence of the angular distribution on spot dimensions are explained.

Reduction of droplet emission and target roughening in laser ablation and deposition of metals

The droplet concentration in laser‐deposited metal, Si, and alloy thin films is studied. It is found for these materials that the number of droplets is strongly dependent on the laser fluence and is low at high laser fluences. This behavior is contradictory to what is usually observed for oxidic materials. It is also found that the amount and average size of the droplets correlate closely with the surface roughness. Rough surfaces generally emit more droplets. The target used in laser ablation and deposition experiments usually roughens during ablation. By intelligently varying the azimuthal angle of incidence, the roughening, and thereby the emission of droplets, can be greatly reduced.

Incongruent transfer in laser deposition of FeSiGaRu thin films

The laser ablation and deposition of FeSiGaRu is studied. The deposited thin films are analyzed with Auger electron spectroscopy and Rutherford backscattering spectrometry. It is found that the gallium and ruthenium content of the thin films is strongly dependent on the laser fluence. At high laser fluences (6 J/cm2) the thin films are depleted of gallium due to preferential sputtering of the gallium atoms from the thin film. Near the threshold fluence (1.9 J/cm2) the films contain an excess of gallium due to preferential evaporation of gallium from the target. The latter conclusions are based on time‐of‐flight studies of ablated atoms and ions and on measurements of the atoms that are sputtered from the substrate by the incoming flux.

Laser ablation deposition of TiN films

Excimer laser ablation of TiN ceramic in vacuum and of Ti metal in N2 gas environment is used to deposit thin TiN films on Si substrates. The dependence of thin film properties (stoichiometry, morphology, and crystallinity) on process parameters such as substrate temperature, background pressure, target quality, applied voltages, and laser fluence is investigated. It is shown that it is virtually impossible to grow stoichiometric TiN films by ablation of Ti in a N2 background atmosphere at a substrate temperature of 300 K. At a substrate temperature of 300 °C and a background pressure below 1 × 10−8 mbar, it is possible to grow polycrystalline TiN films with good stoichiometry from a TiN ceramic target. Comparison with other physical and chemical thin film deposition techniques shows that films of equivalent quality can be grown at a considerably lower substrate temperature (150 –250 °C lower) by this technique, a feature that opens interesting technological perspectives.

Angle‐resolved time‐of‐flight studies on ground‐state neutrals formed by near‐threshold excimer laser ablation of copper

The angle‐resolved velocity distributions of neutral copper atoms created by near‐threshold ultraviolet excimer laser ablation of polycrystalline copper foils are measured as a function of the polar desorption angle and the laser fluence. The obtained time‐of‐flight spectra can be fitted by elliptical Maxwell–Boltzmann distributions on a stream velocity. Integration of the spectra allows one to determine the kinetic energy of these atoms. This kinetic energy is hyperthermal (1 eV).

Surface temperature measurements using pyrometry during excimer laser pulsed etching of silicon in a Cl2 environment

Abstract It appears from angular-resolved time-of-flight (TOF) studies, performed on the products desorbed during nanosecond pulsed excimer laser etching of silicon in a low pressure chlorine environment, that the mean energy of the desorbed particles is dependent on the chlorine surface coverage (θ). Monte-Carlo simulations of the desorption process have shown that all of the results of the TOF experiments can be explained by assuming that the maximum surface temperature Ts reached during the laser pulse depends on θ. For the laser fluence used, Ts varies from 1600 K at θ ≈ 0.01 up to 3500 K at full monolayer coverage. In this study we present time-resolved emission spectrometry of a silicon surface during laser irradiation, in order to elucidate the role of the surface temperature in the etching mechanism. The emission of the irradiated spot is measured as a function of laser fluence and chlorine coverage. The measured emission can be interpreted as thermal emission at the surface temperature. The temperature obtained from the emission measurements is in good agreement with that derived from the TOF experiments. The influence of the adsorbed chlorine on the surface temperature is discussed in the light of possible models.

A time-of-flight study on the nanosecond laser induced etching of Cu with Cl2 at 308 nm

Chemical etching of Cu is studied using Cl2 and a ns pulsed UV laser at 308 nm. At Cl2 pressures in the range of 10−6–10−4mbar and a laser fluence up to 0.82 J/cm2 the velocity distributions of the ejected species are determined. CuCl and Cu3Cl3 are the main products. The time-of-flight spectra of these particles can be fitted with Maxwell-Boltzmann distributions at high temperatures viz. 1750<T<6000 K. Starting with a clean Cu sample the system evolves to a steady state situation in which a considerable amount of Cl has diffused into the bulk. The chlorinated Cu layer has a pronounced influence on the coupling of the laser beam into the substrate, thereby determining the amount of particles desorbed and their time-of-flight distributions. A model is presented to explain the results.

Nanosecond excimer laser-enhanced chemical etching

Abstract The use of lasers in etching and deposition processes has been studied extensively, especially during the last decade. Progress in the understanding of the mechanisms of pulsed laser-enhanced chemical etching is largely of recent data. An overview of the mechanisms of laser-enhanced etching is given and discussed. Experimental results of the excimer laser-enhanced chemical etching of copper and silicon in a low-pressure chlorine environment are emphasized. In particular the measured detection angle and time-of-flight distributions are discussed. Calculations of the effects of post-desorption collisions between the etch products on the measurement of the kinetic energy of the etch products are presented and used to explain the experimental results. It is shown that both the laser-driven diffusion of chlorine into copper and the dependence of etch behaviour on surface coverage are effects which have to be taken into account in the description of the etch mechanism.

A simple formalism for the prediction of angular distributions in laser ablation deposition

Abstract An analytical model allowing the quantitative description of gas clouds generated by near-threshold laser ablation is described. The model is based on a continuum description of the expansion of an idealized laser-generated gas cloud. This analysis yields the time- and space-dependent gas density and the angular intensity distribution. Dependencies of the angular intensity distribution on experimental variables (laser fluence, atom mass and laser spot size) are determined. It is found that the laser spot dimensions are most important parameters in determining the angular distribution. For a circular spot, a simple formula is derived which allows quantitative prediction of angular distributions. These predictions are in good agreement with the angular distributions of ablation plumes reported in the literature. Finally, recommendations towards deposition practice are given.

Effects of post-desorption collisions on the energy distribution of SiCl molecules pulsed-laser desorbed from Cl-covered Si surfaces: Monte-Carlo simulations compared to experiments

Si surfaces covered with up to a monolayer of chlorine by exposure to a low chlorine pressure have been irradiated with nanosecond excimer-laser pulses at a fluence just large enough to melt the surface. Angle-resolved time-of-flight (TOF) distributions and surface temperatures have been measured as a function of chlorine dose between laser pulses. The TOF distributions can be fitted well by Maxwell-Boltzmann (MB) distributions for all coverages and at all desorption angles. With increasing coverage, the intensity and kinetic energy distributions become increasingly peaked along the surface normal. Monte-Carlo simulations of the effect of post-desorption collisions, occurring when many molecules are desorbed within a very short time, reproduce the experimental results quite well. It is shown that just a few collisions per molecule are sufficient to convert any initial desorption distribution into a MB one.