Dielectric properties of AlN x thin films prepared by RF magnetron sputtering of Al using a N 2/Ar sputtering gas mixture

Abstract

Aluminium nitride is a low conductivity material which has applications as a dielectric and passivation layer in microelectronics. Normally, this material is prepared using chemical vapour deposition or molecular beam epitaxy, but following earlier work in which it has been demonstrated that silicon nitride may be successfully prepared by RF magnetron sputtering, we have investigated the electrical properties of aluminium nitride prepared by this technique. Al–AlN x –Al sandwich structure films were prepared by using a pure Al target with a sputtering gas mixture of N 2/Ar in the ratio 3:7 (partial pressures 0.40 Pa nitrogen and 0.93 Pa argon) using an RF discharge power of 180 W. Initial measurements indicated that the relative permittivity was approximately 9.1 and that the capacitance was independent of applied DC voltage, demonstrating that the contacts did not form Schottky barriers. The AC conductivity σ was observed to follow an expression σ= Aω s , where A is a constant, ω is the angular frequency and s is an index. Values of s were found to be in the range 0.62–1.31, increasing with increasing frequency in the range 100 Hz–20 kHz and decreasing with increasing temperature in the range 161–373 K. This type of behaviour was consistent with a carrier-hopping process having a density of localised states value N∼6×10 27 m −3. Low-temperature activation energies were in the range 0.001–0.008 eV, further indicating the presence of a carrier hopping process. The capacitance showed a decrease with increasing frequency and an increase with increasing temperature, tending towards a constant value at higher frequencies and lower temperatures. Measurements of loss tangent as functions of frequency and temperature showed a minimum in its frequency dependence, which appeared to shift to higher frequencies with increasing temperature. These measurements are in accordance with the model of Goswami and Goswami for samples with ohmic contacts, and are representative of results obtained on other insulating films, including nitrides.