CVD process (Gorham method): The three-dimensional dimeric species (di-p-xylylene, lhs)—which contains two ethylene bridges in each p-position—is cracked to form the monomer (middle) (p-xylylene radical), which forms polymeric chains of poly-p-xylylene (rhs) (Ref. 25 ). This reaction can occur either in the gas (volume polymerization) or on the cold surface (surface polymerization).
Termination of the polymerization reaction through the addition of the diradical O2 to a radical position of the monomeric species. At the beginning of the process, the number density of molecular oxygen is higher than that of the monomer in a vacuum which is generated by rotary vane pumps.
(Color online) Determination of the time lag between the temperature of the evaporator and the sample holder both empty and full of solid diparylene. During the heating ramp, the vapor pressure of the dimeric precursor increases in a nonreproducible manner.
Device to measure the vapor pressure by a dynamic method.
(Color online) Evaporation of diparylene-N. Heating (solid) and cooling (broken) occur without any measurable hysteresis. The pressure measured by the Pirani valve is higher by a constant factor of almost 4 compared to the Baratron gauge.
Sketch of the CVD reactor.
(Color online) Different methods for pressure measurement yield different absolute values but almost the same slope (evaporation energy), shown here for diparylene-N. Pirani: upper line, Baratron: lower line.
(Color online) (a) Vapor pressure curves of diparylene-F (powder), diparylene-N (powder), and diparylene-C (powder and melted block). (b) The heats of evaporation for these diparylenes are calculated from the slopes of the ln p lines.
Energy of evaporation is extracted from mass spectrometry, shown here for parylene-C.
(Color online) (a) Peaks (Gaussian bell curves) at 103 and 104 amu, respectively, vs the temperature of the cracker. (b) The heat of dissociation can be evaluated to approximately 1.68 eV.
(Color online) By comparison of the masses for oxygen (32 amu) and the monomeric species of the unsubstituted monomer (104 amu), it is proven that oxygen is trapped by the monomer. This can occur only at a cracker temperature of at least 450 °C (sigmoid fit).
(Color online) Pumping speed of argon and diparylene.
(Color online) Deposition rate (DR), layer thickness (d), pressure (p), and flow rate (f) during the heating ramp of the evaporator without (a) and with (b) supporting pressure of an inert gas (Ar).
Measured density ρ of parylenes, this work (1st column) and literature (Refs. 26 and 27 ).
Enthalpy of evaporation ΔH v of the various derivates of parylene as obtained in this work and by Refs. 28 and 29 . Degree of dispersion: flakes if not specified.
Energy of dissociation for the various derivates of parylene as obtained by a simulation and by reference NIST.
Pressure dependent mass flow of parylene-N at Tvap 130 °C and Tcracker 700 °C.
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