Deposition of Boron Nitride Coatings by Reactive Rf Magnetron Sputtering: Correlation Between Boron and Nitrogen Content
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Mat. Res. Soc. Symp. Proc. Vol. 396 0 1996 Materials Research Society
Rf plasma sputtering, however, doesn't permit the active independent control of the growth parameters like energy and flux of bombarding species on the substrates, because of the strong dependence of the rf discharge parameters on the geometry of the process chamber. For this reason a preliminary measurement of the involved parameters and the study of the relationships between them is necessary [14] in any given deposition platform. About this Reinke et al. [5] presented an interesting overview on BN thin films deposition confirming once more the importance of the ion bombardment during the deposition and asserting that the parameters governing it (i.e. ion flux and energy) must be related to the boron and nitrogen deposition rates. In the light of these thoughts, the aim of this work is to deposit by means of rf magnetron sputtering thin films of boron nitride at controlled conditions of flux and energy of Ar+ ions coming from the plasma. The links between the plasma parameters and the composition and structural properties of films are investigated. EXPERIMENT The rf magnetron sputtering system used in this study consists of a planar 2 in. magnetron cathode connected to a RFX600 generator (13.56 MHz by Advanced Energy inc.) through an ATX600 Matching Unit. A second power supply of the same kind was connected to the planar thermoregulated sample-holder. This sample-holder was connected to the grounded chamber walls through a LC series circuit consisting of a large 4 turns stainless steel coil (20 cm spiral diameter) and a shielded variable air capacitor operated through a rotary vacuum feedthrough. The high plasma region between the target and the sample holder was surrounded by the excited coil with the effect of increasing the plasma density (mixed capacitive and inductively coupled plasma). The targets used for the depositions were 2 in. hexagonal BN (97.5% hot pressed powder, major contaminants Ca 1.5%) by Cerac Incorporated. In all the depositions the power supplied to the BN target was 200 W, corresponding to a target dc-self bias, Vdc, of about 1 kV (960-990 V) at the pressure of 1 Pa in Ar (99.99990%) gas. The target to substrate distance was 15 cm. This distance was chosen in order to test the possibility of obtaining hard BN films in experimental conditions that could permit a good homogeneity of the plasma parameters in front of the substrates with the consequent homogeneity of the deposited layer properties. If the depositions are carried out at very low target-to-substrate distances the plasma density and dc potential can vary considerably from place to place due to the presence of the strong magnetic field and to the temperature gradients near the target surface. Furthermore the thermalization of light elements is not reached and the impingement conditions are unknown making it very difficult to determine the ion flux, energy and direction. The temperature of the sample-holder has been fixed at the value of 300'C, which not is too h
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