Research methods - Sputtering

Single crystalline as well as amorphous materials can be grown by the use of magnetron sputtering and/or Molecular Beam Epitaxy (MBE).

 

Phases that do not exist naturally can be made in the lab

The use of sputtering growth makes it possible to fabricate thin films with physical properties that are significantly different from those in bulk. The deposition of layers can be made with an accuracy at the sub-Angstrom level through alternating opening of computer controlled shutters. One necessary prerequisite is an extremely pure environment, i.e. ultra-high vacuum conditions with background pressures below 1e-9 mbar, reducing the level of contamination to an acceptable level in the samples.

 

Sputtering is a discharge process where atoms of noble gases, e.g. Argon, are ionized by an electric field (usually DC for metal deposition). They are accelerated towards a cathode which contains the target material to be deposited. The ion bombardment breaks the bonds between the atoms and the target atoms are ejected. Some of these atoms reach the substrate where they are adsorbed. The amount of ejected material, and consequently the thickness of the film, can be controlled. In magnetron sputtering a magnetic field (loop) is used to increase the probability of ionization close to the target. Magnetron sputtering therefore operates at relatively low sputter gas pressures, where the cleaner conditions allow the production of films of very high purity and quality. The deposition rate is also increased compared to sputtering without a magnetic field.

 

The choice of pressure and acceleration voltage between the target material and the growing sample, it is possible to alter the kinetic energy of the impinging atoms. Increasing the pressure of the sputter (inert-) gas allows the thermalization of both neutrals and ionic species arriving at the sample. By adjusting the voltage between the sample and the plasma region, ions can be accelerated as well as retarded, giving vide range of adjustment for altering the growth of samples.

 

 

By simultaneous co-deposition from several targets one can produce alloys where the composition is largely determined by the electric power on each target. Compounds such as oxides can also be deposited, either using a target of the desired composition or by reactive sputtering, where a reactive gas (Oxygen in the case of oxide deposition) is used in the sputtering gas. When an oxide target is used, the field has to be oscillating at a radio frequency (RF) to avoid electric charging of the target.

 

One of the biggest obstacles of the sputtering technique is the limited possibility of in-situ treatment of the target materials. This limitation excludes the use of sputtering techniques for growing pure actinides and lanthanides, due to the unavoidable presence of hydrogen.

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