Transport in refractory metals and their interaction with SiO₂: Comparison of tungsten and molybdenum

Abstract

We have investigated sputtered and electron-beam-evaporated thin films of tungsten and molybdenum deposited onto 100 Å SiO2. These two refractory metals have comparable room temperature resistivities (5-6 μΩ cm) and a work function at the midgap between n+ and p+ polycrystalline silicon. Therefore both can be considered for application as a gate electrode in submicron very large scale integration technology. We have probed their interaction with SiO2 using Auger spectroscopy, cross-sectional transmission electron microscopy (CTEM) and with current-voltage (I-V) characteristics of molybdenum and tungsten metal/oxide/ semiconductor (MOS) capacitors as a function of the annealing temperature. We have also measured electrical resistivity in the 4.2-300K temperature range to ascertain the differences and the similarities between the two. We find that, in terms of resistivity, the difference between the two metals is slight. Although at room temperature, ρMo is 10% higher than ρw, at liquid nitrogen, molybdenum is around 30% more conductive after a 900-1000°C anneal. The interface with SiO2 appears in CTEM to be very smooth for both. Auger depth profiling, however, reveals some interface interaction for molybdenum, but not tungsten. This is confirmed with leakage and breakdown measurements on an MOS capacitor structure 32 mm in diameter. After anneals at and above 900°C, molybdenum capacitors on 100 Å oxide are all shorted. The same annealing schedule produces a reasonable distribution of breakdown voltages for tungsten capacitors. © 1987.