SynopsisThermal effects accompanying t h e vacuum deposition of poly-para-xylene (Parylene N ) at different temperatures have been studied by following the changes in the temperature of the substrate. Similarly to the case of polychloro-pun-xylylene (Parylene C), two distinct exothermic effects were observed; one discrete, resulting in sharp exothermic spikes and the other continuous, resulting in the shift of the baseline. The spike effect, attributed to the solid-state polymerization of pam-xylylene, is observed at the low-temperature range, the upper limit of which moves higher for higher deposition rates. The shift of a baseline as a function of deposition temperature exhibits two maxima, one independent of deposition rate and the second moving toward higher temperatures (that is, toward the first maximum) for higher deposition rates. First maximum falls at about -72°C and is attributed to the melting point of pam-xylylene crystals. X-ray diffraction studies of polymer samples have shown that the existence of the second maximum is always followed by the appearance of a n additional crystalline phase in the respective range of deposition temperatures. When the deposition rate is high enough, the second maximum merges with the first one, or virtually disappears. Under such conditions the new crystalline phase is no more detectable. It is postulated that the evolution of the additional amount of heat resulting in the appearance of the second maximum is due to the cyclization reaction and the formation of cyclic oligomers. This reaction very likely requires a particular spatial arrangement of monomer molecules and, therefore, it is suggested to take place in certain domains of the crystalline lattice. GAZICKI ET AL.ful in all technologies requiring high-quality coatings. The very advantageous combination of these two factors, that is, the ease and the conformity of the deposition with the excellent properties of the coatings, has been a key to the success of Parylene technology. This success is evident in at least two modern high-technology applications, namely, in microelectronics as protective coatings for integrated circuitry6q8 and in neurology as insulating coatings for implantable neural microelectrodes. l0-l2 Very recently, the Parylene deposition process has also been reported to successfully meet the requirements for another high-technology application, high-quality coatings for laser-fusion targets. l3As often is the case, the successful application of Parylene technology has preceded a detailed understanding of the deposition mechanism. Although a significant amount of information concerning the chemistry of the process is available, l.14-16 the physicochemical aspects of the deposition are not yet fully understood. Particularly, the influence of factors such as the temperature of deposition or the rate of dimer sublimation has not been investigated thoroughly. An attempt has, therefore, been undertaken in this series to study the influence of these factors on the mechanism of deposition, and particula...