The glow-discharge method for depositing and polymerizing thin films, also known as the electric- or silent-discharge method, entails striking an electrical discharge between two electrodes in the presence of a monomer gas. A potential of several hundred to several thousand volts is employed. Under dc conditions, the substrate to be coated is made the anode or is attached in some manner to the anode. In ac discharge, the substrate is placed between the anode and the cathode. A third variation of glow-discharge uses radio-frequency-excited molecules and does not require the use of electrodes. This method is referred to as electrodeless discharge or plasma polymerization. The need for low-dielectric-constant thin films as interlayer insulation for high-speed circuits has made plasma polymerization an area of intense study.
In plasma polymerization, a monomer is excited by radio frequency or microwave energy forming active species, primarily free radicals and ions. These activated species combine to form thin polymer films on a surface. The physical and electrical properties of the resulting polymers depend on the energy frequency,[66] the monomer gas pressure,[67] and the reactor design,[68] as well as the monomer itself and the surface onto which the coating is deposited. Some surfaces lend themselves well to the formation of strong chemical bonds, while others are rather passive resulting in weak adhesion. The energy used can vary from low radio frequencies (13 MHz) to high microwave frequencies (2.5 GHz).[69] A typical pressure is 100 millitorr, although higher pressures have been studied.[67]
To polymerize, the monomers must possess active groups such as double bonds, oxygen, fluorine, amines, or combinations of these. Mixtures of two or more monomers may also be used to create new classes of polymers having unique properties.[70] A large number of monomers have been polymerized by glow discharge, producing films from several hundred to 10,000 angstroms thick. Bradley and Hammes,[71] for example, reported the formation of films from over 40 different monomers and provided considerable data on their electrical properties at various temperatures. However, no data were given on the exact characterization of the polymer films. Thirty different monomers were polymerized by radio-frequency plasma by Smolinsky and Heiss[72] as potential thin-film capacitor dielectrics. Of these, the best films were derived from dimethylpolysiloxane, triethylsilane, diethylvinylsilane, vinyltrimethylsilane, heptene-2, cyclohexene, styrene, valeronitrile, and 2,5-dimethyl-2,4-hexadiene. Films were defined in terms of their electrical and physical properties; but again, as with other investigations, no structural or compositional characterizations were given. The glow-discharge technique has also been used to produce thin polymeric films from halogenated olefins, including fluorinated deriva-tives.[73]–[74] These films, found suitable as dielectric coatings, were formed by the application of a potential of several hundred volts between elec-trodes.[75]
Finally, a series of hydrocarbon and fluorocarbon films was prepared by the electrodeless discharge method, with isobutylene, ethylene, acetylene, diacetylene, toluene, trifluoroethylene, and tetrafluoroethylene used as starting materials.[76] Films formed from ethylene, acetylene, and diacetylene possessed structures analogous to highly cross-linked polyethylene. Films derived from toluene showed structures (using infrared spec-troscopy) similar to polystyrene, while films derived from the fluorinated monomers were assumed to be Teflon-like in structure.
Key advantages of the glow-discharge or RF plasma methods are the simultaneous deposition and polymerization and the complete coating coverage that they afford. The large spread in the angle of incidence with which the activated gas molecules strike the surface allows the polymers to form within and behind all crevices and to cover all surface imperfections, resulting in an integral coating.[77] Other advantages include: choice of a wide variety of monomer starting materials, low substrate temperature (the part may be kept even at room temperature), and no necessity for a high vacuum. On the negative side, the properties of plasma-polymerized thin films have been difficult to reproduce because of the large number of variables influencing their formation. Often free radicals remain in the thin films causing instability and degradation.[78]
An effective use of plasma has been to induce surface changes in already cured polymers. Exposure of a polymer to plasma can alter surface properties such as wettability, adhesion, surface resistivity, and the friction coefficient.[79] Polar or non-polar groups can also be grafted onto a polymer surface in the presence of a plasma rendering the surface either hydrophilic or hydrophobic. Fluorocarbon plasmas generated from carbon tetrafluoride or hexafluoroethane, for example, have been used to produce a Teflon-like surface on an organic polymer, rendering it hydrophobic.[80] Other functional groups such as hydroxyl or amino groups can also be grafted to a polymer surface by using allyl alcohol or allylamine plasmas, respectively, rendering the surface hydrophilic.