14.5: Comparisons and Complications
- Page ID
As seen above, the evaporated atoms will have relatively low energies which correspond mainly to their thermal energies. This is typically between 0.1-0.3 eV.
However, sputtered atoms can have a much larger range of energies, from 5-50eV. This large range can have an effect on the uniformity of the film.
The effect of the energy of the incoming (source) species will be discussed in the next section.
Alloys and Compounds
If an alloy source is used, then the different components will have different vapour pressures and so the vapour fluxes will not be equal. One way of avoiding this is to use co-evaporation from multiple charges, but this rapidly becomes difficult to control and maintain accurately. Compounds may be evaporated directly, but the high temperatures encourage dissociation.
Sputter deposition is much more flexible as using multiple targets, alloy or compound targets is more feasible.
Another effective method for compound film growth is the use of Laser Ablation (also known as Pulsed Laser Deposition). In this scenario, the vapour flux is created by firing high energy, focused laser pulses onto the surface of the target. This produces a plume of ablated target material. The laser typically has a wavelength of 200-300nm, and the pulse lasts between 6-12ns. The most efficient plume production occurs when the laser strikes the target surface at 45 degrees.
The speed of the heating process means that all the components evaporate simultaneously – avoiding fractionation.
Laser Ablation simply requires a vacuum chamber, a support for the target, and a window for the laser. Unfortunately, the laser can be quite expensive and difficult to scale up to industrial applications.