Parylene - Parylenes As Molecular Layers

Parylenes As Molecular Layers

The classic molecular layer chemistries are self-assembled monolayers (SAMs). SAMs are long-chain alkyl chains, which interact with surfaces based on sulfur-metal interaction (alkylthiolates) or a sol-gel type reaction with a hydroxylated oxide surface (trichlorosilyl alkyls or trialkoxy alkyls). However, unless the gold or oxide surface is carefully treated and the alkyl chain is long, these SAMs form disordered monolayers, which do not pack well. This lack of packing causes issues in, for example, stiction in MEMS devices.

The observation that parylenes could form ordered molecular layers (MLs) came with contact angle measurements, where MLs thicker than 10 Å had an equilibrium contact angle of 80 degrees (same as bulk parylene N) but those thinner had a reduced contact angle. This was also confirmed with electrical measurements (bias-temperature stress measurements) using metal-insulator-semiconductor capacitors (MISCAPs). In short, parylene N and AF-4 (those parylenes with no functional groups) are pin-hole free at ~10 Å. This results because the parylene repeat units have a phenyl ring, and because of the high electronic polarizability of the phenyl ring, adjacent repeat units order themselves in the XY-plane. As a result of this interaction parylene MLs are surface independent, except for transition metals where they don't deposit unto. This finding of parylenes as molecular layers is very powerful for industrial applications because of the robustness of the process and that the MLs are deposited at room temperature.