Conductivity and dielectric properties of silicon nitride thin films prepared by RF magnetron sputtering using nitrogen gas

Silicon nitride is an important material in very-large-scale integration fabrication and processing. Recent work on films prepared by radio frequency magnetron sputtering using nitrogen gas have shown that the relative permittivity is typically 6.3 and that aluminium forms an ohmic contact to this material. Under direct current (DC) bias the films exhibited space-charge-limited conductivity with a bulk trap density of the order of 2 X 10(24) m(-3). In the present work alternating current electrical measurements were made on identical samples as a function of frequency and temperature. Conductivity appeared to be by hopping at lower temperatures, giving way to a free-band conduction process with activation energy of typically 0.44 eV at higher temperatures. Over a limited range of frequency and temperature the model of Elliott was applicable, and yielded a value of 2.87 X 10(23) m(-3) for the density of localised states, in reasonable agreement with our estimate of the trap density from DC measurements. As in the DC measurements capacitance followed a geometric relationship with relative permittivity 6.3, and showed a moderate decrease with increasing frequency and an increase with increasing temperature, tending towards a constant value at high frequencies and low temperatures. The loss tangent showed a minimum in its frequency dependence, which appeared to shift to higher frequencies with increasing temperature. The measurements are consistent with the model of Goswami and Goswami for samples having ohmic contacts, and are typical of results obtained on other insulating thin film structures. (C) 2002 Elsevier Science B.V. All rights reserved.