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Depositing a film via chemical vapor deposition results in superior conformity compared with other deposition techniques, primarily due to the unique chemical interactions between the surface and the reactive compounds. This technique requires a readily accessible surface and so, if the transport of the reactive species is impeded, irrespective of whether this depletion is caused by diffusion or convective flow, a homogeneous layer thickness cannot be achieved. This is often the case when applying films to the interiors of tubes, especially tubes with a dead-end, such that the inevitable loss of film-building components leads to a drop in thickness along the deposition length. The present work examined the deposition of the organic polymer poly-p-xylylene, using a reactor with dimensions that were large compared with the mean free path and tubes in which this factor (the Knudsen number) becomes unity, such that the deposition can be approximately described with the continuum model. A so-called temperature seesaw was employed to mitigate variations in layer thickness by generating an opposing temperature gradient. It was found that, under a vacuum of several tens of mTorr, the polymer could be deposited on the interior wall of a tube with an aspect ratio of at least 100 with an accuracy of ±7.5 %. The true ceiling temperature for the N derivate of this polymer was also determined to be 70 ± 2 °C. [ABSTRACT FROM AUTHOR]

The common industrial technique for the deposition of coatings of polyparylene on three-dimensional substrates is the so-called Gorham method, which makes use of thermally cracked, dimeric precursors that form a polymeric film on a cold substrate. Although this method is easily applied, it is still a challenge to coat surfaces with thin, homogeneous layers less than 1 μm in thickness since mass flow controllers cannot be applied. To overcome this deficiency, several methods have been developed. The authors present here a new technique which allows sudden starts and stops simply by variation of the chamber pressure with an inert gas. Moreover, deposition of reproducible, precise layers of polymeric parylenes requires knowledge of the vapor pressures of the dimeric precursors, the equilibrium for the dissociation into monomers, and the flow of dimers into the reactor. Two straightforward manometric methods are used to measure the vapor pressure, whereas the equilibrium is measured by mass spectrometry. The flow into the reactor is precisely determined under various conditions. Modeling of the equilibrium performed with gaussian simulation software at the ab-initio level yields unexpected, good agreement with the measured data. [ABSTRACT FROM AUTHOR]