Ultra-thin Ta2O5dielectric film for high-speed bipolar memories

A new capacitor technology, with extremely thin (5.3-20 nm) Ta2O5film deposition and weak-spot oxidation, is developed to realize high capacitance and high reliability. The Ta2O5film was reactively sputtered, followed by weak-spot oxidation. The weak-spot oxidation is achieved by placing the Ta2O5film on Si in a high-temperature dry O2ambient. The oxidation significantly improves the time-dependent-dielectric-breakdown (TDDB) characteristics and reduces the defect density of Ta2O5capacitors without reducing the capacitance by selectively oxidizing the Si surface at weak spots where the Ta2O5is locally thin or missing. The technology is based on the new discovery that Ta2O5film less than 20 nm thick shows no reduction in dielectric breakdown strength after dry O2high-temperature annealing up to 1000° C. The Ta2O5(7.5-nm) capacitor with a capacitance of 8.5 fF/µm²is applied to a high-speed bipolar memory. This makes it possible to reduce the memory cell area to one-third that of a conventional bipolar memory. The memory provides high-speed operation; access time is less than 5 ns, and sufficient soft error immunity is provided.