Polymerization in Fluorocarbon Plasmas

Polymerization processes in low-pressure plasmas used for industrial surface processing are surveyed. Special attention is given to fluorocarbon plasmas, because of their importance in reactive ion etching of semiconductor elements. Diagnostic methods for the detection of large molecules are described, including mass spectrometry, infrared absorption and laser techniques. By means of a new mass spectrometric method, large amounts of polymeric species have been detected in radio-frequency fluorocarbon plasmas. Mechanisms of macromolecule formation under low-pressure conditions are discussed, and energetically favorable negative ion-assisted polymerization channels are proposed. Several fluorocarbon gases are studied: CF4, C2F6, C4F8 and C5F8. Polymerization efficiency increases with increasing size of the parent gas molecule, and with its decreasing fluorine to carbon ratio. Gas phase polymerization has important consequences for plasma chemistry and surface processing. It is strongly related to fluorocarbon film deposition on the surface. The presence of the film determines the etching performance of the plasma, introduces new surface reactions for radicals and positive ions, and can result in dust particle formation