Physical vapor deposition on cylindrical substrates

It is well established that low pressure physical vapor deposition processes such as thermal evaporation and the many variants of sputtering utilize nearly collisionless vapor transport to a substrate. This results in line-of-sight deposition. The deposition of uniform coatings on complex shapes using these approaches therefore requires substrate rotation or a multiple evaporation source strategy. In many cases, the line-of-sight requirement precludes the use of these processes entirely. Recently, developed rarefied gas jet based deposition processes, however, operate at much higher pressures where many gas phase collisions occur. Vapor scattering from a laminar flow that propagates around a non-planar substrate provides opportunities for non-line-of-sight deposition. Experiments indicate that the coating thickness around the circumference of a stationary, non-rotated fiber placed perpendicular to the axis of a gas jet containing aluminum atoms is sensitively dependent upon the jetʹs Mach number and the chamber pressure near the substrate. By employing gas jets having low Mach numbers (<0.1), highly uniform coatings of aluminum on cylindrical fibers have been achieved without fiber rotation. Direct simulation Monte Carlo (DSMC) simulations have been used to understand the fundamental phenomena involved and to identify the role of the process conditions on the coatingʹs uniformity.