7.3.5 Ion beam deposition
PVD still has some limitations, however. It must operate in a gaseous atmosphere (to provide the plasma) so is not well suited to the long-throw, directional line-of-sight mode accessible to low-pressure evaporation. Also, if magnets behind the target are used to generate a magnetic field for magnetron sputtering the technique cannot be easily used to deposit magnetic metals such as nickel and cobalt, owing to the influence from the magnetic field on the deposition process.
In an alternative technique, sputtering is achieved not by drawing ions from a plasma, but by firing them onto the target from a separate ‘ion gun’. This broad ion beam deposition technique does not require magnetic fields and can operate at higher levels of vacuum to achieve more directional deposition of material. However, the flux from an ion gun is relatively low, so the technique is slower than magnetron sputtering and it does not lend itself to the use of a platen bias voltage. As the equipment is yet more intricate and expensive, this technique is restricted to specialist applications such as optical coatings and magnetic thin films.