Laser ablation

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When laser ablation , and laser vaporization mentioned, the removal of material is of a surface by bombardment with pulsed laser radiation , respectively. The high power density laser radiation used here leads to rapid heating and the formation of a plasma on the surface. This exceeding of the plasma threshold is used to distinguish it from laser desorption , which does not produce plasma.

Physical basics

The energy of the photons is first transferred to the electrons in the solid.

With femtosecond pulses, the electrons can reach a high temperature; Within a very short time (a few picoseconds), thermal oscillations of the atomic nuclei are excited with this energy. Until the temperature of the electrons coincides with the temperature of the atomic vibrations, the state is modeled on the two temperatures . two-temperature model . The high-energy electrons can break chemical bonds; with non-metals, such short laser pulses can also lead to a Coulomb explosion . This means that the electrons leave the solid and some of the remaining positive ions are ejected from the surface by Coulomb repulsion .

With laser pulses in the nanosecond range, the energy of the laser leads to heating of the surface (in the sense of thermal movements of the atoms) during the laser pulse. Since the heat conduction allows only a slow energy transport into the volume, the radiated energy is concentrated on a very thin layer (approx. 1 µm at 10 ns pulse length), so the surface reaches very high temperatures and the material evaporates suddenly. By ionisation (thermal, by laser light or electron impact) of the laser results in a high power density plasma of electrons and ions of the removed material; the ions can be accelerated to energies of over 100 eV .

The minimum power or energy density at which (for a given wavelength and pulse length) ablation is possible is called the ablation threshold . At energy densities above this threshold, the ablation rate increases sharply. With nanosecond pulses and high power densities, the plasma can become so dense that it absorbs a large part of the laser light and thus protects the surface from further heating. The ablation rate then only increases approximately linearly with the energy density.

In order to ensure sufficient absorption of the laser light, ultraviolet radiation is often used, especially in the case of nanosecond pulses. At these wavelengths, the reflectivity of metals is lower than that of visible light; light absorption by insulating materials is also generally higher.

Applications

Material processing

Laser ablation can be used for targeted ablation of materials, for example instead of mechanical engraving of hard materials or for drilling very small holes. Laser ablation can also be used to remove thin layers of various contaminants; the process is relatively gentle because only the top (µm thick) layer is heated strongly, the workpiece as a whole remains cold.

Coating method

The removed material can be used to coat another surface. This technique is called laser beam evaporation (English: Pulsed Laser Deposition).

Analytics

With the help of a finely focused laser beam, the smallest amounts of samples can be removed. The removed sample particles are flushed into a detector (e.g. ICP-MS ) by means of a gas flow (He, Ar, ...) and characterized there with regard to their element and / or isotopic composition. Is a subspecies of laser ablation (LA), the laser-induced plasma spectroscopy (English laser-induced breakdown spectroscopy , LIBS), directly from the not ablated sample particles, but the absorbed from them by laser excitation, and is then analyzed in the form of light energy emitted. Here, too, information about the elemental composition of the sample can be obtained by atom-specific emission of this light. The advantages here are the relatively high spatial resolution in the µm range, which is lost when the sample is completely broken down and the liquid phase is analyzed.

medicine

Laser ablation is also used to ablate tissue in medicine. In contrast to continuous laser radiation, when using pulsed lasers, the thermal load on the neighboring tissue can be kept low. For minimally invasive surgery , the laser beam can also be guided into the interior of the body via light guides .

restoration

Because different materials begin to ablate at different power densities, it is possible to selectively remove impurities or certain colors without damaging the layers below. This is used in restoration .

Individual evidence

  1. ↑ Material processing of semiconductors and nitride ceramics with ultra-short laser pulses. Online (accessed April 2, 2020)
  2. ↑ Analysis of plastic additives using laser ablation (accessed April 2, 2020)
  3. ↑ Ablation of dental restorative materials with an ultra-short pulse laser (UKPL). Online (accessed April 2, 2020)

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