Plasma substrate interaction effects on composition and chemical structure of reactively r.f. magnetron sputtered carbon nitride films

Abstract

During variable unbalanced r.f. magnetron sputtering of graphite in a 70/30 N 2/He atmosphere the neutral and ionic particle fluxes on the substrate were analyzed for various discharge conditions using energy-resolved mass spectrometry and probe measurements. These data were related to the chemical composition, bonding structure and Young's modulus of the deposited CN x films. The magnetron discharge was found to produce an unexpectedly high concentration of N atoms being the predominant nitrogen precursors for the carbon nitride formation. Depending on r.f. power density and discharge pressure, the arriving fluxes of N and C atoms range from 3×10 16 to 6×10 18 cm −2 s −1 and from 1×10 15 to about 8×10 15 cm −2 s −1, respectively, while the ionic fluxes of nitrogen and carbon were measured to be two orders of magnitude lower. The energy flux density of the ion bombardment varies from 5×10 −4 to 2×10 −1 W/cm 2 over the range of discharge conditions. The overall chemical composition of the CN x films is primarily governed by the flux ratio of N to C atoms. For low Φ N/Φ C ratios, N atoms are incorporated up to 25 at.% as substitutions of C atoms in aromatic clusters of a sp 2/sp 3 hybridized carbon matrix. With increasing arrival flux ratio the excess of N atoms is additionally bonded in linear CN structures terminated with H atoms when hydrogen is present in the discharge. An intensive for bombardment results in a reduced N concentration of the films due to an enhanced desorption of N atoms from the growing surface, mainly, however, in changing the film structure from graphite-like to amorphous. Caused by these structural changes, the Young's modulus of the CN x films decreases from about 110 to 55 GPa.