Effects of Proton Radiation Damage on the Conductance and Temperature Coefficient of Resistance of Reactively Sputtered, Discontinuous Tantalum Thin Film Resistors

Abstract

<p> Tantalum thin film resistors have been reactively sputtered in oxygen and nitrogen simultaneously. The films studied had resistivities ranging from 400μΩ-cm to 3 x 10^4 μΩ-cm. The corresponding TCR values ranged from -50 ppm/°C to -2,000 ppm/°C. Conductance-temperature measurements show that electrical conduction in discontinuous films of metallic islands (typically 100 A°) largely surrounded by regions of Ta2O5 (typically 50 A°) may be due to a tunneling mechanism of negative TCR operating concurrently with a metallic mechanism of positive TCR via interconnected metallic islands.</p> <p> Irradiation of these discontinuous films by 150 keV protons produces a conductance increase which is attributed to an enhanced tunneling mechanism via electronic defect levels in the inter-island oxide regions. During irradiation of these films at 30°K, the conductance change increases and approaches apparent saturation. This nonlinearity is attributed to a combination of spontaneous recombination and close-pair thermal annealing. The number of unstable sites surrounding each defect is found to be ≥ 4. Thermal recovery of the conductance proceeds in two main stages: Stage A (34°K to 150°K) is attributed to close-pair or correlated recombination; Stage B (150°K to 300°K) is attributed to uncorrelated migration of defects to gap-island interfaces, as is indicated by the greatly reduced Stage B annealing which is observed for continuous, polycrystalline films of Ta2O5, having a typical grain size of 1,500 A°. Negative annealing stages (characterized by a conductance increase) indicate a metallic conduction process via connected metallic islands.</p> <p> For 286°K irradiation of discontinuous films, the conductance initially increases with fluence in a nonlinear fashion until a threshold fluence is reached, at which point the conductance decreases with fluence. The nonlinearity of the conductance increase is attributed to trapping of mobile radiation-produced defects at gap-island interfaces during irradiation. The subsequent conductance decrease is attributed to a shift in the Fermi level, and thus the height of the tunneling barrier, as the result of the formation of unequal concentrations of stable radiation-produced donor and aeceptor defects since unequal concentrations of these defects can be expected to annihilate at the gap-island interface. The absence of this conductance decrease in continuous polycrystalline films is consistent with this model, since the absence of gap-island interfaces is expected to result in equal concentrations of stable donor and acceptor levels being produced.</p> <p> The observed negative increase in TCR with fluence is attributed to an increase in the proportion of the tunneling mechanism of negative TCR (as the result of radiation-produced defects in the inter-island oxide regions) relative to the proportion of the metallic conduction mechanism of positive TCR. The difference between the TCR recovery after irradiation at 30°K (little recovery between 150°K and 300°K) and the conductance recovery (about 50 percent of the recovery occurs between 150°K and 300°K) is attributed to the expected greater influence of metallic recovery on the annealing of the film TCR relative to the annealing of the film conductance.</p>