Electron Beam Evaporation Technique Vs. Sputtering

Electron beam evaporation (E-Beam evaporation) and sputtering both can be classified into Physical Vapor Deposition (PVD) category.PVD covers a number of deposition technologies in which material is released from a source and transferred to the substrate. The two most important technologies are evaporation and sputtering.

The choice of deposition method (i.e. evaporation or sputtering) may in many cases be arbitrary, and may depend more on what technology is available for the specific material at the time. In VLSI fabrication, sputtering technology is widely-used for accomplishing thin films.

Evaporation
In evaporation, the substrate is placed inside a vacuum chamber, in which a target (source) material to be deposited is also located. The source material is then heated to the point where it starts to boil and evaporate. This process requires a high vacuum (10^-6 to 10^-7 Torr range)to allow the molecules to evaporate freely in the chamber, and they subsequently condense on all surfaces. This principle is the same for all evaporation technologies, only the method used to heat (evaporate) the source material differs. In E-Beam evaporation, a high kinetic energy beam of electrons is directed at the material for evaporation. Upon impact, the high kinetic energy is converted into thermal energy, heating up and evaporating the target material, on the premise that the heat produced exceeds the heat lost during the process. The rate of mass removal from the source material as a result of such evaporation increases with vapor pressure, which in turn increases with the applied heat. Vapor pressure greater than 1.5 Pa is needed in order to achieve deposition rates which are high enough for manufacturing purposes.

Sputtering
Sputtering is a technology in which the material is released from the source at much lower temperature than evaporation. The substrate is placed in a vacuum chamber with the source material, named a target, and an inert gas (such as argon) is introduced at low pressure. A gas plasma is struck using an RF power source, causing the gas to become ionized. The ions are accelerated towards the surface of the target, causing atoms of the source material to break off from the target in vapor form and condense on all surfaces including the substrate. As for evaporation, the basic principle of sputtering is the same for all sputtering technologies. The differences typically relate to the manor in which the ion bombardment of the target is realized.

Advantages offered by evaporation for PVD
1) high film deposition rates;
2) less substrate surface damage from impinging atoms as the film is being formed, unlike sputtering that induces more damage because it involves high-energy particles; 3) excellent purity of the film because of the high vacuum condition used by evaporation;
4) less tendency for unintentional substrate heating.



Disadvantages of using evaporation for PVD
1) more difficult control of film composition than sputtering;
2) absence of capability to do in situ cleaning of substrate surfaces, which is possible in sputter deposition systems;
3) step coverage is more difficult to improve by evaporation than by sputtering;
4) x-ray damage caused by electron beam evaporation can occur.