The cost of precious metals, be it gold or platinum or even silver, makes them, well, precious. Thermal evaporation provides an efficient approach to making great thin films with the minimum amount of precious material. But you need to do some simple calculations.
In a recent TechInfo inquiry one of our thin film deposition experts, Rob Belan, described the process to calculate the precise amount of material needed to make a thin film by thermal evaporation. One of the great features of thermal evaporation is the fact that films can be made at the lowest possible pressure the system can achieve—here is no need for a process gas. Typical evaporation systems are designed to reach pressures in the 10-8 Torr region when they are clean, pristine, and not contaminated by poor vacuum practice or back streamed hydrocarbon pump oil vapors.
Figure 1: Basic set-up for thermal evaporation
To calculate the volume of material required to make a film of a specific thickness you need to know the distance between the source material (in blue above) and the substrate (in green) in centimeters. This is called the throw distance. Use the throw distance as the radius of a hemisphere and determine the surface area of that hemisphere in square centimeters. For a throw distance of 20 centimeters the surface area of the hemisphere is 2,513 square centimeters. Now assume that you want to make a film of a certain thickness that completely covers the hemisphere. For example, consider a 1 micron thick film, which converts to 0.0001 centimeters. That film will have a volume of 0.2513 cubic centimeters.
Now consider the material to be deposited. If the material is gold, for example, it has a theoretical density of 19.32 grams/cc, with a melting temperature of 1,064oC and a Z-ratio of 0.381. In order to achieve an equilibrium vapor pressure of 10-4 Torr, gold needs to be heated to 1,132oC. (Make sure your power supply and boat combination can get this hot, or hotter.)
Figure 2: Detail of boats, shields, and shutters in a typical thermal evaporation system
Using the theoretical density of gold, with a throw distance of 20 centimeters and a goal of making a 1 micron thick film, 0.2513 cubic centimeters x 19.32 g/cc or 4.85 grams of gold is required. The complete calculation is detailed below.
Figure 3: Material requirement calculator for thermal evaporation
This handy calculator is also available on line at https://www.lesker.com/materials-calculators.cfm. Many of the intrinsic properties of metals, oxides, and other materials, as they relate to thin film deposition, can be found in our Deposition Materials Chart pages located at https://www.lesker.com/newweb/deposition_materials/materialdepositionchart.cfm?pgid=0.
Category: Deposition Materials
Sub-Category: Evaporation Materials