Leakage currents and conduction mechanisms of Ta 2O 5 layers on Si obtained by RF sputtering

Abstract

The leakage current and the conduction mechanisms in Ta 2O 5 layers on silicon with thickness in the range of 20–80 nm, obtained by reactive sputtering of Ta in an Ar/O 2 mixture have been investigated. Some dielectric and electrical properties, important for the application of the layers as storage capacitors in high-density dynamic random access memories (DRAM) and as a gate dielectric in metal oxide silicon transistors (MOSTs) are also considered. The results show that for as-deposited layers the leakage currents are higher for samples obtained at higher deposition temperature. The effect of postdeposition oxygen annealing depends on the thickness of Ta 2O 5 layers. For thicker layers (40 nm), the leakage current after annealing increases and the effect is stronger for layers deposited at T s=493 K. It has been established that for thinner oxides (25 nm) the annealing strongly improves the leakage currents (the density of leakage current is 10 −7 A/cm 2 at applied fields of about 1 MV/cm, which is low enough to satisfy the demands of 64 and 256 Mbit DRAM). For as-deposited samples the conduction mechanism is Poole Frenkel. After annealing depending on the field strength, different types of conduction mechanisms occur: for electric fields in the range 0.8–1.3 MV/cm, the conduction mechanism is dominated by electrode limited Schottky emission and for higher fields (>1.5 MV/cm) it is bulk limited Poole Frenkel emission. The results are discussed in terms of bulk traps in the initial Ta 2O 5 and their modification after oxygen annealing.