Alois Schauer

Dielectric properties of anodized Al-Ta alloy films with high Al content☆

Abstract At 1 mA cm-2 the anodization voltage of sputtered Al-Ta alloy films with about 7 at.% Ta content rises linearly with time at a rate of 0.45 V s-1. The Al-Ta thickness-anodization voltage ratio and the oxide growth constant have been determined, for 7–14 at.% Ta in the films with 1% citric acid electrolyte, to be 1.03-0.94 nm V-1 and 1.42–1.5 nm V-1, respectively. The permittivity was found to range from 10.0–11.4, corresponding to a capacitance density of about 330 pF mm-2 for an anodization voltage of 200 V. The temperature coefficient of capacitance (t.c.c.) was 500 ppm K-1. The capacitance between 400 Hz and 1 MHz remained constant within ±1%. At 1 kHz the disspation factor tan δ was 0.6%. Nearly all leakage currents were between 0.1 and 0.5 nA using 5.6 nF test capacitors and 50 V as the test voltage. This corresponds to an insulation resistance R of 1011 Ω or more, or a time constant τ ( = RC) of approximately 1000 s. For the 50 V test the yield was about 90% and at least 80% of the capacitors had no short circuits after step stress tests up to 90 V. The current-voltage characteristics of capacitors made from Al-Ta (7 at.% Ta) films showed non-destructive breakdown voltages greater than 100 V. Only a slight deviation from symmetry was noticed whether the Al-Ta electrode was positive or negative, indicating that the capacitors were virtually non-polar. In some cases non-shorting breakdown was observed.

Cosputtered Aluminum-Rich AlTa Alloy Films

An R F sputtering system was employed with a target consisting of a 20cm diameter aluminum disk, about 1/4 of which was covered by a segment of tantalum. This target construction makes it possible to obtain on a single substrate a broad range of film compositions in one sputtering run. The result, concluded from X-ray analysis, is that only three phases are observed on a 10 cm by 10 cm Coming glass substrate. 1 ) On the aluminum side there is the fcc AI lattice with about 7 at% Ta dissolved in it-93 at% AI 7 at% Ta. 2) On the tantalum side there is a \beta -Ta lattice with about 20 at% AI dissolved in it--80 at% Ta 20 at% AI. 3)Between these two phases exists an amorphous phase--85 at% AI 15 at% Ta. The thickness profile over the substrate shows a deep well between the 80 at% Ta and the 15 at% Ta-phase. This profile indicates that there is a condensation gap for the composition from 15 to 80 at% tantalum. Resistivity and measurements of temperature coefficient of resistivity (TCR) yield values from 30 to 240 \mu \Omega -cm and from +500 to --125 ppm/deg K, respectively, depending on the position on the substrate, that is, depending on the phase present, The two aluminum-rich phases have, as far as we know, not previously been reported, neither in bulk nor in thin film form. We have observed most attractive features of these phases in their annealing and anodizing behavior.