Laser vapor evaporation

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Pulsed laser , and laser vaporization or laser deposition called ( English pulsed laser deposition is, PLD), a method of physical vapor deposition (PVD) and is closely related with the thermal evaporation . This is understood to mean the deposition of layers by laser ablation . For this purpose, both the layer material to be deposited ( target ) and the base on which the layer is to be deposited ( substrate ) are placed in a vacuum container ( recipient ).

Procedure

The material of the target is illuminated in a vacuum chamber with high intensity pulsed laser radiation (≈ 10 MW / cm 2 ) and thereby evaporated. The evaporation process of the target material takes place via the absorption of the energy of the laser beam by the material to be evaporated. Above a certain (sufficient) amount of energy, a plasma forms on the target and atoms are released from the target. When high process gas pressures (> 1 mbar) are used, the material vapor can be condensed into clusters (groups of atoms) in the gas phase . This material vapor moves through the vacuum chamber away from the target to the substrate and condenses there to form a thin layer . For the production of crystalline layers, the substrate is additionally heated to allow diffusion processes and thus the rearrangement of the atoms. In this way, other particles can also be incorporated into the crystal, either to produce more complex materials or to generate doping .

Particularly good results are achieved with UV lasers (e.g. XeCl or KrF excimer lasers ), since their radiation has a high photon energy which is absorbed by a large number of materials because it is above the plasma frequency . Other pulsed lasers for PLD are transversely excited CO 2 lasers , Q-switched Nd: YAG lasers and increasingly also pulsed femtosecond lasers . The pulse length is typically in the range of 10-50 ns with a repetition frequency of a few Hertz .

Advantages and disadvantages

Advantages:

  • One advantage of the method over other deposition methods is that the number of laser pulses can be used to precisely determine the amount that can be deposited on the substrate. So that the same amount of material is always removed from the target, the target is shifted a little bit after each laser pulse, otherwise the laser always hits the same spot.
  • Another advantage is that even complicated (stoichiometric) compositions of elements can be transferred exactly, with other methods the composition of the elements is often changed during transfer, so that the chemical compounds on the substrate are not exactly the same as on the target.
  • Simple production of multilayered layers (engl. Multilayers )
  • Simultaneous production of high quality layers of different material classes such as ceramics , metals, semiconductors and some polymers

Disadvantage:

  • slower deposition than with other PVD processes such as electron beam evaporation
  • Droplet formation on the substrate possible
  • Clusters are often undesirable
  • no large areas possible in contrast to sputtering
  • comparatively expensive

application areas

The PLD method is used in material science to novel materials with many components, in particular metastable structures such as amorphous diamond-like carbon (engl. Diamond-like carbon DLC), ceramics (for example, the high temperature superconductors of yttrium-barium-copper short YBaCuO) or to produce special ferromagnetic functional layers ( AMR , GMR or GMI layers).

literature

  • Gerhard Kienel: vacuum coating . Springer, Berlin et al. 1997, ISBN 3-540-62274-8 , pp. 80-84 (laser ablation section).