Vacuum Deposition of Parylene Films: Influence of Process Factors and Baffling on Film-Thickness Distribution

Deposition in vacuum of poly(paraxylylene)-PPX or "Parylene"-and derivative polymers from dimer that have been ionized and vaporized is a process used for over 35 years to form protective and insulating coatings on electronic and other devices and to generate extremely thin, pinhole-free films. Coating thickness has been asserted to be highly uniform, but with scant data in the literature. We investigated thickness distribution in a series of coating runs in each of which 60 or 80 small aluminum disks were positioned throughout the coating chamber. The processing factors tested were rate of coating, disk location (radius and altitude), flow-distribution devices, and whether the rack holding the disks was stationary or rotated. In the initial sequence of five runs, 320 film thicknesses were determined. Some findings include: (1) film thickness was heaviest at chamber midheight, diminishing symmetrically toward top and bottom by as much as 25%, and, less strongly, inward from wall to axis. (2) Disk orientation-whether vertical, hoizontal, or at 45°-did not affect film thickness. (3) Rotation of the rack eliminated an inlet-to-outlet downslope of thickness. (4) Dividing the chamber with a midplane separator, together with special inlet and takeoff distributors, reduced the total range of thickness from 40% to 25% of the average. A thickness variation of less than± 10% is possible by positioning objects in a limited vertical range and while preventing shadowing.