Study of the source-substrate geometry to achieve thickness uniformity of thin films

Abstract

Despite the various technical skills that are available, the thickness uniformity cannot be made better than ±1% on a large number of substrates. In addition to the theoretical limitations, one has practical difficulties such as insufficient mechanical stability of the rotating and static installations in the vacuum chambers. The planetary system has been advantageously used by manufacturers of vacuum coating plants to have a large figure of merit. Spherical work holders of proper geometry have been mounted (maximum 3) in such a manner that they form part of the optimum sphere and make planetary motion. According to Knudsen's cosine law, if the source is positioned at a vertical distance equal to the diameter of the sphere, then according to equation 1.16, uniform deposition occurs on the inside of the spherical dome and any non-uniformity in the emission characteristics is averaged by the planetary rotation. These systems are therefore popular in the production of optical coatings and chips. The advantage of a rotating spherical work holder for mounting small workpieces (25 mm dia) is as follows: It accommodates more substrates than a planar rotating work holder and if the coating area is to be increased, canted domes can be used.