Elemental distribution in fluorinated amorphous carbon thin films

Abstract

Focused ion beam-secondary ion mass spectrometry (FIB-SIMS) with 20 nm spatial resolution has been used to analyze amorphous fluorinated carbon thin films, deposited by plasma assisted chemical vapor deposition (PACVD), at micro-to nano-scale. Mass spectra and ion imaging of film surface were acquired and the presence and distribution of contaminants were investigated. Surface images show the secondary ion distribution for F Ϫ , CH Ϫ , CF Ϫ . A change in size and topology of fluorine-rich areas is correlated with film hardness and with microstructure transition from diamond-like to polymer-like, as indicated by infrared and Raman spectroscopies. Based on the surface distributions of CF Ϫ and CH Ϫ and on the vibrational spectroscopy results, a mechanism of fluorine substitution for hydrogen and an attempt to explain the film structure and microstructure is proposed. . The physical properties of the ϪCF x groups, which are a film constituent, permit a considerable reduction in friction and a substantial improvement in wear resistance, and confer a hydrophobic behavior to the film surface. Electronic applications of such a material are also foreseen because of the low value of the dielectric constant k [5][6][7][8][9][10][11][12][13][14].Several material properties (structural, chemical, mechanical, electrical, optical, etc.) have been studied as functions of process parameters [4, 12, 14 -20], but a direct visualization of the surface element distribution is still lacking. We obtained a chemical surface image to reveal elements distributions, including contaminants and non-uniform elemental concentration. Such data can be used in conjunction with the other surface analysis results to gain a complete understanding of the film chemistry and structure.We analyzed two samples, representative of a set of deposited films, synthesized by PACVD, starting from C 2 H 2 and CF 4 , with different CF 4 content in the plasma atmosphere, namely 33 and 71.4% of the total gas influx; the films have different micro-structure and hardness, as deduced from previous analyses [19,20]. In particular, vibrational spectroscopies [19] enabled determination of the amorphous nature of the films, the fraction of sp 2 and sp 3 coordinated carbon atoms in the network and the presence of specific groups, specifically ϪCF x . The features of Raman spectra were also correlated with film hardness, which was measured by micro-indentation [20]. Infrared and Raman spectroscopies clearly showed the formation of a partially hydrogenated fluorocarbon film, evolving from diamond-like to polymer-like behavior as the fluorine content increases in the gas mixture.Focused ion beam secondary ions mass spectrometry (FIB-SIMS) is a unique technique for the visualization of the surface element distribution with high sensitivity and high resolution. It consists of a finely focused primary ion beam that scans and erodes a selected area on the sample surface. During the sputtering process, neutrals and secondary ions of the elements from the surface are ejected. The...

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