Plasma-Activated Ion Beam Reactive Sputtering of NbN Thin Films
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PLASMA-ACTIVATED ION BEAM REACTIVE SPUTTERING OF NbN THIN FILMS. D.J. LICHTENWALNER*,**, ALFREDO C. ANDERSON*, and D.A. RUDMAN** *MIT Lincoln Laboratory, Lexington MA 02173 **MIT Department of Materials Science and Engineering, Cambridge MA 02139 ABSTRACT In order to better study and control the processes occurring in a reactive sputtering situation, a unique deposition method has been used in which the Ar-ion-beam sputtering of an elemental Nb target is combined with an auxiliary Ar/N2 plasma at the substrate. The ion source allows independent control of the sputtering parameters (ion flux, energy). The magnetically enhanced triode plasma provides a source of ionized and excited nitrogen at the film surface, and allows independent control of the substrate plasma parameters. A conductancelimiting enclosure surrounds the substrate, resulting in a pressure differential of nearly an order of magnitude between the substrate and target regions. This enables us to separate the substrate and target reaction regimes at low nitrogen flows. NbN has been investigated because of its technological importance and the fact that it is representative of transition metal nitrides. With the substrate plasma off and N2 provided at the substrate, the cubic superconducting NbN (8 phase) is produced even at low N2 flows, when the target is in the unreacted, metallic state. Upon increasing the N2 flow, the nitrogen content of the films abruptly increases as the target reaction proceeds. The addition of the substrate plasma results in the nonsuperconducting hexagonal 8' phase, which to our knowledge has not previously been produced as a singlephase thin film. The electrical properties of the 8' phase are reported. This work is sponsored by the Office of Naval Research and the Air Force Office of Scientific Research. INTRODUCTION Reactive sputtering has proven to be an effective method for producing high superconducting transition temperature (T c) NbN thin films (1-5], as well as for producing other transition metal nitrides and carbides [6,7]. It is generally accepted that, when using nitrogen flow as the independent parameter, the Tc will have a maximum at a given flow, then slowly decrease at higher flows. The Tc also generally has a maximum for a certain total pressure and sputtering power. Since the noble and reactive gas pressures, current, and voltage are interdependent in typical systems, it is difficult to determine the role of these parameters in the optimization of film properties. Because the nitrogen pressure at the target and substrate may be nearly equal during film growth and the sticking coefficient of nitrogen on a clean Nb suface is about 0.5 (8], the role each reaction plays in determining final film properties has not yet been clarified. In most studies of reactively sputtered NbN, the highest T films were obtained when the target had nitrided (seen by a drop in deposition rate), but it is not clear from those studies if this was a geometrical consequence of target-to-substrate distance or a fundamentally necessary par
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