Electrical conduction processes in silicon nitride thin films prepared by r.f. magnetron sputtering using nitrogen gas

Silicon nitride (Si3N4) is an important VLSI material owing to its high res istivity and breakdown strength and its use in surface encapsulation during ion implantation and annealing. Previous work has focused on films prepare d by low-pressure and plasma-enhanced chemical vapour deposition (LPCVD and PECVD), but in the present work the DC electrical properties of films prep ared by r.f. magnetron sputtering were investigated. Al-Si3N4-Al sandwich s tructures were fabricated from a Si3N4 target at a discharge power of 100 W using N-2 as the sputtering gas at a pressure of approximately 0.5 Pa. Cap acitance was independent of voltage, indicating the absence of a Schottky b arrier at the Al/Si3N4 interface. Measurements of the capacitance as a func tion of inverse dielectric thickness implied a relative permittivity value of 6.3. However, although films prepared using PECVD exhibited Poole-Frenke l conductivity and tunnelling at higher voltages, the present sputtered fil ms showed space-charge-limited conductivity (SCLC). Conductivity was domina ted by an exponential distribution of trap levels, as indicated by a power- law dependence of current density J on applied voltage V, with a typical ex ponent value of 3.2. Measurements of J as a function of temperature confirm ed the appearance of SCLC and indicated that the bulk trap density was of t he order of 2 x 10(24)m(-3) as observed in LPCVD and PECVD films, with the appearance of hopping conductivity at low temperatures. (C) 1999 Elsevier S cience S.A. All rights reserved.