When a laser interacts with a solid surface, a variety of processes can occur. We are mainly interested in the interaction of pulsed UV lasers (in first instance with nanosecond pulse duration, although femtosecond lasers attract our interest as well) with a solid surface, in first instance a metal (e.g., copper). The typical laser irradiance of interest to us ranges from 107 - 1010 W/cm2.
When such a laser interacts with a copper surface, the laser energy will be transformed into heat. The temperature of the solid material will increase, leading to melting and evaporation of the solid material.
The evaporated material (vapour atoms) will expand. Depending on the applications, this can happen in vacuum (or very low pressure), or in a background gas (helium, argon, air,…), e.g., at 1 atm. In the latter case, there will be interactions between the vapour plume and the background gas, yielding confinement of the plume (compared to the vacuum case), whereas the background gas is pushed further away from the solid target. Because the temperature in the vapour plume can rise to high values (10,000 K and higher), a plasma will be formed. Hence, the vapour plume does not only consist of atoms, but also of electrons and ions.
Consequently, when the laser is focused on the solid target, it can partly be absorbed in the plasma, by inverse Bremsstrahlung, so that the laser energy reaching the target is reduced. This is called plasma shielding.
Besides atoms, electrons and ions, the material plume also consists of particles, with dimensions ranging from nm till µm. The smallest particles (~ nm size) are probably formed in the expanding vapour plume, by condensation of vapour atoms. The larger particles (~ µm size) are expected to be created by direct ejection from the solid target. Several mechanisms might be possible, depending on the kind of material. For organic materials (such as polymers) or geological materials (e.g., silicates), photomechanical fracture (laser induced stress) is expected to be important for ejection of solid particulates. For metals, however, mechanical fragmentation is expected to be less important, and particles are assumed to be formed by liquid (large droplet) ejection (liquid splashing). Moreover, at very high laser irradiance (above 1010 W/cm2), explosive boiling of the target material beneath the surface layer, and mass ejection of large particulates, might occur.
The interaction of a laser with a target, also generally termed as “laser ablation”, is used for a growing number of applications, such as pulsed laser deposition, nanoparticle manufacturing, micromachining, surgery, as well as chemical analysis. Several analytical techniques make use of the mechanism of laser-solid interaction, in different regimes of laser irradiance, including, among others, matrix assisted laser desorption ionisation (MALDI), laser microprobe mass spectrometry (LMMS), laser induced breakdown spectrometry (LIBS), and it is also finding great interest as a solid sample introduction method for inductively coupled plasma mass spectrometry and optical emission spectrometry (LA-ICP-MS and LA-ICP-OES).
This crater, formed by laser ablation, shows that not only evaporation plays a role, but that liquid splashing of molten material is also important (picture courtesy of Prof. Dr. D. Bleiner).
In spite of the many applications, the exact mechanisms of laser ablation (e.g., thermal vs. mechanical effects, mechanisms of particle formation,…) are not yet fully understood.
In spite of the many applications, the exact mechanisms of laser ablation (e.g., thermal vs. mechanical effects, mechanisms of particle formation,…) are not yet fully understood. We have tried to obtain this better insight by numerical modelling of the various mechanisms occurring during and after laser ablation, in order to improve the applications. This includes:
- Heating, melting and evaporation of the solid surface by a heat transport equation;
- Expansion of the evaporated material plume: by Navier-Stokes equations;
- Formation of a plasma in the material plume: by Saha equations.
This picture shows an optical micrograph of a laser lithography on steel, made by a pulsed Nd-YAG laser using a flat top laser profile with a 50 nm diameter, with a pulse length of 6 ns and a 10 Hz repetition rate; deposited energy is 1 mJ/pulse.
More information about this kind of modelling, and the various mechanisms playing a role in laser ablation, can be found in the following paper: A. Bogaerts, Z. Chen, R. Gijbels and A. Vertes; Laser ablation for analytical sampling: what can we learn from modeling? Spectrochim. Acta Part B, 58, 1867-1893 (2003). An updated version of the model, with a more complete description of the mechanisms, can be found in: D. Autrique, G. Clair, D. L’Hermite, V. Alexiades, A. Bogaerts and B. Rethfeld; The role of mass removal mechanisms in the onset of ns-laser induced plasma formation. J. Appl. Phys., 114, 023301 (2013) [Copyright (2013) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. Following article appeared in Journal of Applied Physics and may be found at: http://jap.aip.org/resource/1/japiau/v114/i2/p023301_s1.]