Vapor Collection Efficiency in Any Deposition Process
For physical vapor deposition (PVD), one of the most important gauges of finesse is vapor collection, i.e. the percentage of material vapor collected by the substrates or substrate carriers.

Quantifying the vapor collection isn’t always easy. Some commercially available modeling tools, such as V-Grade 5S series software (Tin Model LLC), can report the vapor collection for common coating arrangements.

What about not-so-common arrangements? Here, we introduce a technique for computing the vapor collection for any arrangements. The method makes use of computed thickness values in unit of “true thickness”, i.e.

thickness per unit quantity of source material vaporized.

In V-Grade 5S series software, true thickness unit is expressed in nanometer per gram of source material vaporized. Note that true thickness is applicable to both stationary and moving substrates, for which averaging is carried out in the calculation.
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For demonstration, we consider coating of a 12-sided drum surface with three circular and planar magnetron targets arranged vertically, as shown in the figure above. Each flat surface of the rotating drum measures 16 cm wide and 50 cm tall for substrate mounting. (This is a low-cost setup that can achieve good uniformity over a large area without correction masks.)

For quantifying the vapor collection we place equally spaced sampling points over one of the 12 sides. Next, we compute the thickness values at all the points, averaging over the rotation of the drum. The resultant values are in units of true thickness, i.e. nanometer per gram of source material (the sputtering target) vaporized. These thickness values, along with the spacing of the sampling points and the material density, allow us to determine that the weight of the material deposited on a flat side is 0.036 gram for each gram of source material vaporized. The total weight captured by all 12 sides is:

12 x 0.036 = 0.429 gram.

This is to say that for each gram of the source material vaporized (sputtered), 0.429 gram lands on the substrate-mounting surfaces. Therefore, the vapor collection is 42.9%.