Optimization of Deposition Uniformity for Large-aperture Nif Substrates in a Planetary Rotation System

LLE Review, Volume 94 67 Introduction Large substrates for precision optical applications require the accurate, uniform deposition of multilayer thin-film coatings. Typically, this results in the use of electron-beam evaporation in a “box-coater” configuration, utilizing either simple or planetary rotation of the substrates. Simple rotation of the substrate minimizes the size of the necessary coating chamber but generally results in films with nonuniformity of 2% or greater,1 an unacceptable level for the precise requirements of the National Ignition Facility (NIF). Typical planetary rotation systems, containing four to five individual substrate holders, or planets, tend to produce relatively uniform coatings but would necessarily be quite large in order to process optics of a significant size. To process large optics for the NIF, a counterrotating planetary geometry was developed and implemented in a 72-in. electron-beam deposition system.2 Although this rotation system utilizes planetary motion to reduce the effect of deposition fluctuations, the large optic sizes relative to the overall chamber size and geometry result in coatings with a significant degree of nonuniformity if regions of the vapor plume are not masked.