MODIFICATION OF POLYMERIC MATERIAL SURFACES WITH PLASMAS^* - Page 3

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POLYMERIZATION AND DEPOSITION OF SURFACE COATINGS

Polymerization is the creation of very large molecules by the joining of many small, linkable, molecules called monomers. Classical monomers, as used in wet chemistry polymerization, have reactive structures such as double bonds that allow them to bond to one-another when the appropriate conditions are present. The double bond in methyl methacrylate provides the linking site for forming the useful plastic, polymethyl methacrylate (N = a large number of repeat units, e.g., 100,000) resulting in the reaction

Uv light, free radicals or energetic ions from the plasma, initiate the polymerization process. The monomer methyl methacrylate, when used as the feed gas, will begin the polymerization process by linking repeatedly, increasing its molecular weight many thousand fold. This plasma polymerization has been studied by Fourier Transform InfraRed Attenuated Total Reflectance (FTIR/ATR) spectroscopy in real time.⁵ The resulting polymer was directly grown onto a Ge crystal ATR optic element inside the plasma reaction chamber. The crystal was IR probed through a window in the chamber. A polymethyl methacrylate film was deposited at 65-W power, 0.2 Torr pressure and 30 sccm flow rate of monomer. Interestingly, the polymer as deposited continued to change its IR signature even after the plasma power was turned off. This is not uncommon in plasma induced reactions due to long-lived free radical species that continue to react.

In the above example, a known "polymerizible" monomer was reacted into a polymeric film.

Surprisingly however, plasma conditions can also create polymer films from materials that ordinarily do not form polymers by conventional wet chemistry techniques. Plasmas can fractionate feed gases that lack linkable sites into many new and reactive compounds that subsequently may polymerize. For instance, ethane (C₂H₆) in an rf plasma will deposit as a polyhydrocarbon that has a density approaching 1.6 g/cc.

The structure of plasma polymers can be varied by the use of co-reactants or the introduction of 0₂, N₂, or NH₃ into the reaction chamber during polymerization. This technique is commonly employed to incorporate specific atomic species into the resulting polymeric material that may be missing in the primary monomer. Ammonia or acrylonitrile are used as the co-reactants during the deposition of films from a methane plasma to incorporate nitrogen.⁶ Similarly, hydroxyl and carboxylic acid functionalities can be incorporated by plasma co-polymerizing acrylic acid⁶ or ally alcohol with the primary monomer to provide oxygen and hydrophilicy. ^7,8

Studies have developed correlations between the power input, the type of monomer feed gas used, and the gas flow rate to the density and type of active species in the plasma. These factors in turn determine the rate of deposition and the film structure.^9,1O Depending on the monomer used, deposition rates typically range between 5 and 100 nm/min, at 100 W rf power levels and monomer flow rates of a few sccm.¹¹ Benzene is observed to have a relatively high deposition rate¹³ even though it lacks a conventional polymerization linking site, and, thus, would not form a polymer under usual wet chemical means. The properties of materials polymerized in this manner can be very different from polymers obtained from these same materials via conventional, wet

chemical polymerization methods (if indeed, such polymers can even be made by wet chemical means). The physics and chemistry of plasma polymerization processes have been described in sufficient detail elsewhere for the interested reader.^10-13

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^* Chapter IV of Plasma Processing of Advanced Materials, edited by George A. Collins and Donald J. Rej, MRS Bulletin, August 1996