Early Stages of Parylene VDP on Substrates which are Impermeable to Monomer
This class of substrates includes glass plates, silicon wafers, and metal components, including conductors and metallic components and fixtures on a printed wiring board.
Parylene VDP on an impermeable substrate passes through a sequence four growth stages or regimes. The time and thicknesses mentioned below are based on a typical deposition using the exemplary parameters in Table I of the 1978 paper. They of course can vary widely with deposition conditions.
1. The MONOLAYER REGIME, which starts out with the physical adsorption of monomer at a coverage in the range 0.1-1.0%. Even at such low coverage, the effective concentration of monomer in the monolayer is much greater than that in the gas phase from whence it came, greatly favoring especially the initiation chemistry. After initiation events occur, propagation to high molecular weight polymer chains proceeds quickly. The polymer strands on the substrate surface have very little organization, but are held to the surface and ultimately to one another by vanderWaals dispersion forces. The disorder includes the crossing of chains over one another well before the average thickness reaches one monolayer (~5 Angstroms). Hence, the continuity of the deposit is not achieved until the average thickness reaches at least a few monolayers (perhaps 10-20 Angstroms). The average thickness of one monolayer is passed about one minute after the start of exposure to monomer gas.
2. The HOMOGENEOUS REGIME, in which the film is thin enough and growth slow enough that monomer diffusion is unrestricted and consequently monomer concentration (M) is effectively uniform throughout, as is the concentration of reactive free radical polymer chain ends (P). Under these conditions, the rate of film growth increases exponentially, which is to say that film growth rate is proportional to film thickness (L).
This regime holds until the thickness is about 100 Angstroms, or about 4-5 minutes after the initial exposure to monomer gas.
3. The INTERMEDIATE REGIME, in which the flux of monomer through the growth interface to supply the growing film is large enough and the film is sufficiently thick that free radical polymer chain end groups located deeper in the film begin not to have the same availability of monomer as those nearer the surface. The result is that the growth rate acceleration experienced in the homogeneous regime is gradually throttled back to the steady growth rate of the steady state regime. In another five minutes or so, the film thickness is 3000-5000 Angstroms, and the characteristics of the steady state regime achieved. Interestingly, without the moderating effect of diffusion, the growth rate would continue to increase without limit. It is during this Intermediate Regime that the deposit becomes sufficiently thick to exhibit the rainbow effect.
4. The STEADY STATE REGIME is much as described in the 1978 model.