Ultra-thin Ta2O5dielectric film for high-speed bipolar memories

A new capacitor technology, with extremely thin (5.3-20 nm) Ta
2
O
5
film deposition and weak-spot oxidation, is developed to realize high capacitance and high reliability. The Ta
2
O
5
film was reactively sputtered, followed by weak-spot oxidation. The weak-spot oxidation is achieved by placing the Ta
2
O
5
film on Si in a high-temperature dry O
2
ambient. The oxidation significantly improves the time-dependent-dielectric-breakdown (TDDB) characteristics and reduces the defect density of Ta
2
O
5
capacitors without reducing the capacitance by selectively oxidizing the Si surface at weak spots where the Ta
2
O
5
is locally thin or missing. The technology is based on the new discovery that Ta
2
O
5
film less than 20 nm thick shows no reduction in dielectric breakdown strength after dry O
2
high-temperature annealing up to 1000° C. The Ta
2
O
5
(7.5-nm) capacitor with a capacitance of 8.5 fF/µm
²
is 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.