3 A BRIEF EXPLANATION OF THE DEPOSITION PROCESS

THE FOLLOWING IS A BRIEF DISCUSSION of the mechanics and chemistry of the deposition process. This mechanism remains the same regardless of the nature of the material being treated.

The parylene polymers are deposited directly from a gas phase. No liquid phase has ever been isolated and no solvents are involved. Polymerization must take place in a “medium vacuum” (4–10 microns mercury).

Materials to be coated are placed in the chamber at the right of the illustration (Fig. 1). Parylene dimer is placed in the vaporizer zone at the left. The system is closed and evacuated till the proper vacuum is reached. The temperature of the vaporization zone is gradually raised to around 160�C. The parylene dimer then begins to sublime forming a gas. As the pressure increases in this zone, the gas moves downstream into the cracking or pyrolysis furnace which is maintained at 690�C. As the parylene dimer passes through this heated zone the molecule is split to form two identical reactive monomers. These reactive molecules stream out of the furnace zone into the roomtemperature deposition chamber. By this time the parylene molecules have very high energies and as such will bounce around the chamber many times before finally polymerizing on the surface to be coated. This is the reason for the great penetrating power of parylene. No liquid phase has ever been isolated, and the substrate temperature never rises more than a few degrees above ambient. Additional components of this system are the mechanical vacuum pump and associated protective cold traps.

Fig. 1. Schematic diagram of the parylene process.