What Is Thermal Evaporation?
Over the past few years we have provided a series of BLOG articles discussing various aspects of Physical Vapor Deposition (PVD) as they relate specifically to Sputter Deposition. However, sputtering is only one subset of PVD processing. There are many other forms of thin film deposition that fall under the general category of PVD. For example, films can be produced by evaporation, sublimation, ion deposition, atomic laser deposition, laser ablation, etc. – just to name a few of the alternative processes whereby resultant films can be formed. It may make sense to briefly describe some of the basic principles involved that are associated with these various processes.
Evaporation, or more specifically, Resistance Thermal Evaporation, involves the heating of a material within a vacuum environment. This source material, which is to be evaporated, is heated to a temperature above which the vapor pressure of the material exceeds that of the vacuum environment in which it is contained (more on this later). In the simplest form, the equipment that is required to produce an evaporated thin film is actually quite minimal. First, some type of vacuum system is necessary to provide a suitable operating environment, similar to that of any PVD operation. Within the vacuum chamber it is necessary to provide a source of electrical energy, usually from a remote power supply being transferred through to the chamber with standard electrical feedthroughs to buss bar connections. Between these buss bars a source of electrical resistance is attached. This source may vary in composition based on the specific material being evaporated. Typically a highly resistance wire or sheet material shaped like a “boat” (usually a refractory metal) is attached between the two electrodes to enable heat to be generated as power is supplied to the buss bars. By applying a current to the electrodes, the source material heats up and the vapor pressure increases. Once the vapor pressure of the material exceeds that of background environment the vaporized molecules travel between the source material and the substrate where they condense.
Each element has a specific vapor pressure at any given temperature and background pressure. In general, the higher the temperature (power to the resistance source) the higher the deposition rate of the evaporation material. Charts of vapor pressures as a function pressure and temperature for various elements are readily available in the literature and can easily be reviewed through the internet. Unlike sputtering, which ejects single atoms or clusters at a time, thermal evaporation into a vapor stream can be quite robust, thus enabling quite high deposition rates with short run times. Resistance evaporation is best suited for elemental materials which, by definition, have a single melting point at a given pressure and allowing for homogeneous resultant films. Alloys, with the exception of eutectic compositions, will not typically have a single melting point but will emit a vapor stream consisting of various amounts of each constituent non-uniformly as the temperature of the material rises above the liquidus. This will result in non-homogeneous compositions in the resultant films.