The polymerization of gases by electron-beam (e-beam) exposure was discovered accidentally. During experiments with electron tubes and electron-beam equipment, it was noted that thin films formed and clouded the inside of the tube. These films were identified as resulting from trace amounts of contaminant gases entering the chamber from silicone oil used in the vacuum pump. For years, these films were considered a nuisance,[58] but in 1954 it was suggested that e-beam polymerization could be useful as a new method for depositing ultrathin polymers,[59] and, in 1958, the significance of this technique in fabricating microelectronic circuits and devices was reported.[60]
In effecting polymerization, the electron beam generates free radicals, ion radicals, and other activated species from liquids or gases adsorbed on a surface. These activated species are short-lived and quickly combine to form long-chain solid polymers. In this respect, e-beam polymerization is similar to uv polymerization. Also, as with uv polymerization, gases or liquids used are those that contain double bonds (ethylenic groups). Thus tetrafluoroethylene has been polymerized with high-energy electrons. Under the same energy conditions, copolymers of tetrafluoroethylene with hexafluoropropene, polyvinylidene fluoride, and other ethylenic polymers have been prepared.[61] The continuous and rapid curing of styrene, polyesters, diallylphthalate, and vinyl carbazole monomers to form 1-mil-thick coatings using an e-beam source of 50 to 250 keV was reported by Davison.[62] Styrenated polyesters, polyvinylchloride plastisols, epoxy acrylics, and acrylic monomers have also been polymerized in films 0.5 to 125 mils thick using an e-beam source of 300 kV, 25 mA, and total output of 7.5
kW.[63]
The advantages of applying and curing coatings by e-beam exposure are the speed of curing and the fact that heat is not required, which obviates the possible degradation of heat-sensitive devices.[64] Thin films may also be deposited in selected areas without masking by programming and directing the beam. Very fine lines and areas may thus be defined.[65] However, in spite of these advantages, the e-beam process has found limited application because of the high initial equipment cost, subsequent high operating cost, shielding problems, and the sensitivity of many semiconductor devices to electron radiation. Depending on the electron energy, overheating and damage to the substrate can occur. Still another problem encountered not only with e-beam curing, but also with curing most ethylenic monomers by the free-radical mechanism, is the inhibition of cure that occurs in the presence of atmospheric oxygen.[63] Films often remain tacky and partially cured. Techniques to prevent this by excluding oxygen during the cure include evacuation, use of an inert gas ambient, or use of oxygen scavengers.