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The emission distribution characteristics of an evaporation source can be used to define the correct geometry in the vacuum chamber for the production of uniform-thickness coatings.We first measured the thickness of coatings on test pieces positioned at known radial distances on a single rotation flat rack in the vacuum evaporation chamber and used these data in a computer program which found the source emission function, in the form cos Q φ, which provided the best fit to the data. φ is the emission angle of the evaporant stream from the source, measured from the vertical. The known emission function was then used to determine the source offset and calotte curvature which produced the best thickness uniformity over the diameter.In one example, we found Q = 1.31 for Al 2 O 3 evaporated from an electron beam source. This enabled us to predict a chamber geometry which yielded coatings across a calotte of diameter 81 cm with a thickness variation of +/-0.3%. For Si0 2 Q = 1.70, but the uniformity was less excellent (+/-3%) because this material is difficult to evaporate controllably. The technique is a powerful one for anyone setting up his coating chamber to produce a large number of coated substrates.

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