Correlation between properties of direct current magnetron sputtered thin niobium nitride films and plasma parameters

Superconducting thin films of NbN on SiO2 are prepared at ambient temperature by direct current magnetron sputtering with varying external deposition parameters, i.e. pressure, target-substrate distance, and power. Internal deposition parameters such as the plasma properties are determined with both a Langmuir probe and a calorimetric probe providing insight regarding the energy flux, the electron temperature, the plasma potential, and the ion current density of the plasma. Grazing incident x-ray diffraction and atomic force microcopy measurements reveal that the thin films have cubic polycrystalline structures with grain sizes of up to 51 angstrom, which define the superconducting behavior. Structural and electrical properties are related to the plasma parameters, showing a high correlation between the ion current density and the superconducting properties of the films. The results attest to the potential of plasma characterization as a powerful tool to optimize the deposition of thin NbN films and to predict their film properties.