We discuss the generation of high-density and high-temperature plasmas by focusing high peak power laser radiation onto a solid target. Emphasis will be put on the process of laser ablation and on its basic, physical mechanisms. A survey will be given of the main experimental techniques, namely optical emission and absorption spectroscopy, mass spectrometry, time-of-flight and charge collection measurements, devised to characterize laser-produced plasmas. The fundamental theoretical and numerical approaches developed to analyse laser-target interaction, plasma formation, as well as its expansion will also be reviewed, and their predictions compared with the experimental findings. Although the main emphasis of the review will be on metal target ablation, reference and comparison to results on multicomponent targets will also be frequently given.

https://iopscience.iop.org/article/10.1088/0953-4075/32/14/201/pdf

Characterization of laser-ablation plasmas

Comments on explosive mechanisms of laser sputtering

The optical absorption coefficient of silicon for photon energies between 1.65 and 4.77 eV (750– 260 nm) has been determined at elevated temperatures (up to 700 °C) using polarization modulation ellipsometry. For photon energies below ∼3 eV (∼410 nm), the absorption coefficient increases exponentially with temperature from room 24° C to 700 °C, increasing by a factor of approximately 5 over that temperature range. The threshold for direct band‐gap absorption moves to lower energies with increasing temperature.

Optical absorption of silicon between 1.6 and 4.7 eV at elevated temperatures

An investigation of ultrashort pulsed laser--induced surface modification due to conditions that result in a superheated melted liquid layer and material evaporation are considered. To describe the surface modification occurring after cooling and resolidification of the melted layer and understand the underlying physical fundamental mechanisms, a unified model is presented to account for crater and subwavelength ripple formation based on a synergy of electron excitation and capillary wave solidification. The proposed theoretical framework aims to address the laser--material interaction in subablation conditions and thus the minimal mass removal in combination with a hydrodynamics-based scenario of the crater creation and ripple formation following surface irradiation with single and multiple pulses, respectively. The development of the periodic structures is attributed to the interference of the incident wave with a surface plasmon wave. Details of the surface morphology attained are elaborated as a function of the imposed conditions, and results are tested against experimental data.

Dynamics of ripple formation on silicon surfaces by ultrashort laser pulses in subablation conditions

We have theoretically and experimentally analyzed the laser‐induced evaporation process for deposition of superconducting thin films from bulk targets. The spatial thickness variations have been found to be significantly different from a conventional thermal deposition process. Unlike a cos θ thickness variation expected from a thermal evaporation process, the laser evaporation process is characterized by a forward‐directed deposit with a sharp variation in its thickness as a function of distance from the center of the deposit. We have studied in detail the interactions of nanosecond excimer laser pulses with bulk YBa2Cu3O7 targets leading to evaporation, plasma formation, and subsequent deposition of thin films. A theoretical model for simulating the pulsed laser evaporation (PLE) process has been developed. This model considers an anisotropic three‐dimensional expansion of the laser‐generated plasma, initially at high temperature and pressure. The forward‐directed nature of laser deposition has been found to result from anisotropic expansion velocities of the plasma edges arising due to the density gradients in the gaseous plasma.The physical process of the laser ablation technique for deposition of thin films can be classified into three separate interaction regimes: (i) interaction of the laser beam with the bulk target, (ii) plasma formation and initial isothermal expansion, and (iii) adiabatic expansion leading to deposition of thin films. The first two regimes occur during the time interval of the laser pulse, while the last regime initiates after the laser pulse terminates. Under PLE conditions, the evaporation of the target is assumed to be thermal in nature, while the plasma expansion dynamics is nonthermal as a result of interaction of the laser beam with the evaporated material. The expansion velocities of the plasma edges are related to the initial dimensions and temperature of the plasma, and the atomic weight of the respective species present in it. Preliminary calculations have been carried out on spatial thickness variations as a function of various parameters in PLE deposited thin films. The effects of the various beam and substrate parameters including energy density and substrate‐target distance affecting the nature of deposition of superconducting thin films have been theoretically examined. Experimental results have been obtained from thin films deposited on silicon substrates by XeCl pulsed excimer laser (λ=308 nm, τ=45×10−9 s) irradiation. The spatial thickness and compositional variations in thin films have been determined using Rutherford backscattering technique and the results compared with the theoretical calculations.

Theoretical model for deposition of superconducting thin films using pulsed laser evaporation technique

Pulses of radiation from ruby and Nd:YAG $Q$-switched lasers have been used recently to anneal the lattice damage caused by ion implantation of semiconductors. Other similar applications include the laser-induced diffusion of thin dopant films deposited on the surface of samples, recrystallization of doped amorphous films deposited on single-crystal substrates, and the removal of precipitates present after conventional high-temperature dopant diffusion. All of these processes can be understood in terms of models and calculations based on macroscopic diffusion equations for heat and mass transport, cast in a finite-difference form to allow for the temperature and spatial dependences of the thermal conductivity, absorption coefficient, reflectivity, and other quantities. Results of calculations on silicon with the models show that the near-surface region of a sample can melt and stay molten for times of the order of 100 nsec during which dopant diffusion in the liquid state and nonequilibrium segregation during ultrarapid recrystallization are sufficient to explain the major features of the experimental results. In this paper, a description of the model used in our heat-transport calculations is given. Results of the modeling are illustrated by a variety of calculations which should be of particular interest to experimentalists working with pulsed-laser annealing. These results include, e.g., the effects of pulse duration, shape, and energy density, the effects of assumptions made about the latent heat of amorphous silicon, the effects of substrate heating, the role played by the absorption coefficient in determining melt-front penetration, and the duration of surface melting.

Macroscopic theory of pulsed-laser annealing. I. Thermal transport and melting

The radiation from $Q$ -switched ruby and Nd:YAG (yttrium aluminum garnet) lasers can anneal the lattice damage produced by ion implantation of semiconductors. In the first paper of this series, we described the models and methods we have been using for heat-transport calculations during pulsed-laser annealing and gave numerous illustrations of the type of results which are obtained. In this paper, we discuss dopant-diffusion calculations in detail, with particular emphasis on the incorporation of segregation effects into the modeling. From the forms of the experimental dopant profiles, it is established that pulsed-laser annealing is a nonequilibrium process, but in this paper the interface segregation coefficient is treated as an adjustable parameter and no attempt is made to justify the values obtained. Approximate analytical and finite-difference solutions to the diffusion equation are discussed and compared. It is argued here that the excellent fit between theory and experiment which is obtained is a strong indication of the basic validity of the melting model of pulsed-laser annealing.

G. E. Giles

Papers

Macroscopic theory of pulsed-laser annealing. I. Thermal transport and melting

Macroscopic theory of pulsed-laser annealing. II. Dopant diffusion and segregation