POLYMERIZATION OF FLUOROCARBONS IN REACTIVE ION ETCHING PLASMAS

Polymerization reactions in radio frequency fluorocarbon plasmas of CF 4, C2F6, and C4F8 have been studied by electron attachment mass spectr ometry (EAMS). In these plasmas polymerization occurs readily and mole cules containing up to ten carbon atoms (the mass limit of the mass sp ectrometer) have been found. The densities of large polymers increase with increasing size of the parent gas. In a fluorine-rich environment like a CF4 plasma the detected polymers are mainly fully saturated wi th F (CnF2n+2). As the amount of fluorine in the parent gas decreases, also the degree of saturation of the polymers decreases, which is cle arly seen in C2F6 and C4F8 plasmas. The unsaturated polymers are more reactive, so they can stick more easily to surfaces and possibly creat e thick polymer films, which are often observed after discharge operat ion. The polymerization rate depends on the chemical activity of the p lasma, which can be easily enhanced by increasing the radio frequency power. The positive ions, extracted from the plasma, are generally som ewhat smaller than the neutral polymers and their fluorine content is lower. This is probably due to dissociation of neutrals during their i onization by plasma electrons and to ion collisions in the sheath regi on. Finally, we have shown that EAMS has considerable advantages in th e study of electronegative plasmas and polymerization processes in com parison with traditional mass spectrometry. Unlike the traditional mas s spectrometry, employing ionization by high energy electrons, EAMS mu ch better preserves the structure of high polymers, allowing us to det ect them as large negative ions. (C) 1998 American Vacuum Society.