Laser-generated plasmas in length scales relevant for thin film growth and processing: simulation and experiment

Abstract

In pulsed laser deposition, thin film growth is mediated by a laser-generated plasma, whose properties are critical for controlling the film microstructure. The advent of 2D materials has renewed the interest in how this ablation plasma can be used to manipulate the growth and processing of atomically thin systems. For such purpose, a quantitative understanding of the density, charge state, and kinetic energy of plasma constituents is needed at the location where they contribute to materials processes. Here we study laser-induced plasmas over expansion distances of several centimeters from the ablation target, which is the relevant length scale for materials growth and modification. The study is enabled by a fast implementation of a laser ablation/plasma expansion model using an adaptive Cartesian mesh solver. Simulation outcomes for KrF excimer laser ablation of Cu are compared with Langmuir probe and optical emission spectroscopy measurements. Simulation predictions for the plasma-shielding threshold, the ionization state of species in the plasma, and the kinetic energy of ions, are in good correspondence with experimental data. For laser fluences of 1-4 J/cm 2 , the plume is dominated by Cu 0 , with small concentrations of Cu + and electrons at the expansion front. Higher laser fluences (e.g., 7 J/cm 2 ) lead to a Cu + -rich plasma, with a fully ionized leading edge where Cu 2+ is the dominant species. In both regimes, simulations indicate the presence of a low-density, high-temperature plasma expansion front with a high degree of ionization that may play a significant role in doping, annealing, and kineticallydriven phase transformations in 2D materials.

Research on the mechanism of particle deposit effects and process optimization of nanosecond pulsed laser truing and dressing of materials

“…Furthermore, the velocities of particles throughout the entire mass are close to the state of dynamic equilibrium aer collisions with each other. This situation conforms to the characteristics of the Maxwell velocity distribution, 6 and the relation is as follows: 13 nðr; tÞ z exp À r 2 4L R ðtÞ 2…”

“…However, plasma expansion can be regarded as a whole gas mass, and the study of plasma expansion characteristics can be carried out. S. B. Harris et al 6 proposed an expansion model for laser ablation plasma and studied the length scale of laser-induced material growth and modification of the expansion distance. Z. Lin et al 7 used the finite-difference time-domain (FDTD) method to study the propagation properties of Gaussian beams in expanded plasma.…”

Experimentally constrained multidimensional simulation of laser-generated plasmas and its application to UV nanosecond ablation of Se and Te

“…The effective model couples laser-induced surface evaporation with fluid plasma expansion [27,2,8]. The pertinent equations are expressed in 2D-axisymmetric coordinates.…”

“…For constraining considerations, it is instructive to convert the time axis used for n(t) into an energy axis defined by the transformation ε = m i (d/t) 2 /2, where m i is the mass of the ionic species comprising the plasma. The quantity ε is an approximate representation of the kinetic energy of such ions [8].…”

“…Detailed analysis of this domain is valuable in applications dominated by near-target processes, such as micromachining [5], laser-induced breakdown spectroscopy (LIBS) [6], and laser sintering/additive manufacturing [7]. Simulations up to several centimeters in length and times >10 µs, have received much less attention but are useful to guide materials synthesis by pulsed laser deposition (PLD), in which substrates are placed several centimeters away from ablation targets [8].…”