Transmission electron microscopy of gold-silicon interactions on the backside of silicon wafers

Abstract

The Au-Si interactions on the backside of 〈111〉 silicon wafers have been studied over the temperature range of 275–400 °C by transmission electron microscopy. The Au films annealed at temperatures below the Au-Si eutectic temperature (363 °C) all have similar structure—columnar Au grains of small size (∼450 Å). An orthorhombic gold silicide, Au3Si, forms at the Au/Si interface below the eutectic temperature due to the migration of Au into the silicon substrate. The Au3Si grains are of much larger size (∼2000 Å) than Au and contain twins. Annealing at 400 °C, which is above the eutectic temperature, results in a completely different structure. It consists of numerous small Si islands protruded and dispersed throughout a continuous, large grain (>2.8 μm), polycrystalline gold film. Gold silicide was not observed at 400 °C. Many Au grains are found to grow epitaxially on the silicon substrate. The silicon islands are also found to grow epitaxially on silicon substrate. The structure after 400 °C annealing is interpreted as the result of the eutectic reaction between the substrate Si and the overlaying Au film.