Parylene

The major drawback of poly(p-xylylene) is that it reverts to a monomer when thin films are heated above ca. 400°C and it cracks when the films are annealed at 300-350°C in nitrogen. During module assembly the chip-joining (soldering) 

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The process takes place in two stages that must be physically separate but temporally adjacent. Figure 1 presents a schematic of a typical parylene deposition process, also indicating the approximate operating conditions. 

Fig. 1. Parylene deposition apparatus. To convert Pa to torr, multiply by 0.0075.

The widely used Parylene C owes its popularity ptincipaHy to the room temperature volatiUty of its monomer. The Parylene C monomer, chloro-A-xylylene, has become the de facto performance standard. By comparison, the Parylene N monomer, A"xylylene itself, is too volatile and would perform better ia a sub-ambient temperature deposition system. The Parylene D monomer, dichloro-A-xyljlene [85586-88-5] is too heavy, and causes distribution problems ia larger deposition systems. 

In contrast to the extreme reactivity of the monomeric PX (1) generated from it, the dimer DPX (3) feedstock for the parylene process is an exceptionally stable compound. Because of their chemical inertness, dimers in general do not exhibit shelf-life limitations. Although a variety of substituted dimers are known in the Hterature, at present only three are commercially available DPXN, DPXC, and DPXD, which give rise to Parylene N, Parylene C, and Parylene D, respectively. 

The stmcture of DPXN was determined in 1953 from x-ray diffraction studies (22). There is considerable strain energy in the buckled aromatic rings and distorted bond angles. The strain has been experimentally quantified at 130 kj/mol (31 kcal/mol) by careful determination of the formation enthalpy through heat of combustion measurements (23). The release of this strain energy is doubtiess the principal reason for success in the particularly convenient preparation of monomer in the parylene process. 

The linear polymer of PX, poly(p-xylylene) (PPX) (2), is formed as a VDP coating in the parylene process. The energetics of the polymerization set it apart from all other known polymerizations and enable it to proceed as a vapor deposition polymerization. 

The enthalpy of polymerization of unannealed (57% crystalline) Parylene N, as it is deposited, starting with Hquid monomer, is... 

Nxylylene system, substituents affect it only to a minor extent. AH parylenes are expected to have a similar molar enthalpy of polymerization. An experimental value for the heat of polymerization of Parylene C has appeared. Using the gas evolution from the Hquid nitrogen cold trap to measure thermal input from the polymer, and taking advantage of a peculiarity of Parylene C at — 196°C to polymerize abmptiy, perhaps owing to the arrival of a free radical, a = —152 8 kJ/mol (—36.4 2.0 kcal/mol) at — 196°C was reported (25). The correction from — 196°C to room temperature is... 

The enthalpy hberated on the VDP of parylene is real and in an adiabatic situation causes a rise in temperature of the coated substrate. For Parylene C, 229.1 kj/mol (54.7 cal/mol) corresponds to 1654 J/g (395 cal/g) whereas its specific heat at 25°C is only 1.00 J/(g-K) [0.239 cal/(g-K)] (33). In most practical situations, however, the mass of parylene deposited is dwarfed by the substrate mass, and the heat of polymeriza tion is dissipated within the coated substrate over the time required to deposit the coating with minimal actual temperature rise. 

Table 4. Typical Engineering Properties of Commercial Parylenes...

Property Parylene N Parylene C Parylene D ASTM method... 

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