The etching mechanism of titanium polycide in a mixture of SF6 and O2

Abstract

Reactive ion etching of titanium disilicide films on top of undoped polycrystalline silicon has been investigated in order to determine the etching mechanism of the silicide in a fluorine plasma. Mixtures of SF6 and O2 and of CF4 and O2 were used. Vertical and lateral etch rates have been determined as a function of pressure and the amount of oxygen addition. The density of fluorine atoms and positive ions in the plasma has been determined from optical emission spectroscopy and electric probe measurements, respectively. From these results, the fluxes of fluorine to and away from the sample and the ion flux towards the sample have been calculated. With x-ray photoelectron and Auger electron spectroscopy, the surface constitution after etching has been studied. The TiSi2 layer is etched anisotropically, independent of pressure and the addition of oxygen. A relation between the etch rate and the density of fluorine atoms in the plasma is not found, in contrast with results obtained elsewhere [K. C. Cadien, S. Sivaram, and C. D. Reintsema, J. Vac. Sci. Technol. A 4, 739 (1986)]. From our results it is shown that the etch rate of TiSi2 in a low-pressure fluorine plasma is not limited by the supply of reactive fluorine to the surface. It is limited by ion-induced desorption or ion-induced formation of volatile titanium fluoride compounds. The magnitude of the observed vertical and lateral etch rate of silicon are described by means of the reaction probability of fluorine at the silicon surface and the density of fluorine atoms in the plasma. Anisotropic etching of the polysilicon layer in a SF6/He/O2 plasma occurs when more than 10% oxygen is added. The effect of oxygen on etch rates and anisotropy is due to a surface reaction and not to gas-phase reactions. It is argued that an oxyfluoride layer is formed on the silicon surface that hinders the formation of saturated silicon fluoride species.