Polymerization of fluorocarbons in reactive ion etching plasmas

Abstract

Polymerization reactions in radio frequency fluorocarbon plasmas of CF4, C2F6, and C4F8 have been studied by electron attachment mass spectrometry (EAMS). In these plasmas polymerization occurs readily and molecules containing up to ten carbon atoms (the mass limit of the mass spectrometer) 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 with F (CnF2n+2). As the amount of fluorine in the parent gas decreases, also the degree of saturation of the polymers decreases, which is clearly seen in C2F6 and C4F8 plasmas. The unsaturated polymers are more reactive, so they can stick more easily to surfaces and possibly create thick polymer films, which are often observed after discharge operation. The polymerization rate depends on the chemical activity of the plasma, which can be easily enhanced by increasing the radio frequency power. The positive ions, extracted from the plasma, are generally somewhat smaller than the neutral polymers and their fluorine content is lower. This is probably due to dissociation of neutrals during their ionization by plasma electrons and to ion collisions in the sheath region. Finally, we have shown that EAMS has considerable advantages in the study of electronegative plasmas and polymerization processes in comparison with traditional mass spectrometry. Unlike the traditional mass spectrometry, employing ionization by high energy electrons, EAMS much better preserves the structure of high polymers, allowing us to detect them as large negative ions.