Physical Vapor Deposition (PVD) by  Evaporation


Physical Vapor Deposition (PVD) is a process by which a thin film of material is deposited on a substrate according to the following sequence of steps:  1)  the material to be deposited is converted into vapor by physical means; 2) the vapor is transported across a region of low pressure from its source to the substrate; and 3)  the vapor undergoes condensation on the substrate to form the thin film. 


In VLSI fabrication, the most widely-used method of accomplishing PVD of thin films is by sputtering.  However, there is a second method of PVD also used in semiconductor fabrication, albeit to a lesser extent.  This is PVD by evaporation.


In PVD by sputtering, the material to be deposited as a film is converted into vapor by bombarding the source material with high-energy particles or ions.  In PVD by evaporation, the conversion into vapor phase is achieved by applying heat to the source material, causing it to undergo evaporation.  This is done in a high-vacuum environment, so that the vaporized atoms or molecules will be transported to the substrate with minimal collision interference from other gas atoms or molecules.   


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.


In the semiconductor industry, PVD by evaporation has been used primarily in the deposition of aluminum (Al) and other metallic films on the wafer.


Figure 1. Examples of PVD Evaporation Systems


The advantages offered by evaporation for PVD are:  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.


The disadvantages of using evaporation for PVD are: 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; and 4) x-ray damage caused by electron beam evaporation can occur. 


There are several ways by which heating is achieved in PVD by evaporation.  The simplest (and one that has many disadvantages) is to employ resistive heating, wherein a wire of low vapor pressure metal such as tungsten is used to support strips of the material to be evaporated.  The wire is then resistively heated, so that the metal to be deposited melts first and evaporates.


In electron 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.


Evaporation can also be achieved by heating the source material with RF energy.  This technique employs an RF induction heating coil that surrounds a crucible containing the source.  This method of evaporation is known as inductive heating evaporation.


See Also:  Thin Films MetallizationPVD by SputteringCVD




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