Compositional variations of sputter deposited Ti/W barrier layers on substrates with pronounced surface topography

Abstract

Sputter deposited Ti/W barrier layers have been found to be Ti deficient with respect to the target composition, which is attributed to the preferential resputtering of Ti from the deposited films by energetic neutrals or ions from the discharge. On the other hand, the sputtering yield of most materials is known to be strongly dependent on the angle of incidence of the bombarding species. Due to this angular dependence the resputtering rate of Ti/W films will also be a function of the local orientation of the surface. The substrates of the IC circuits onto which the Ti/W barrier layers are deposited normally possess a pronounced surface topography. The purpose of this work is to study the deviation of the expected Ti/W concentrations at sloped surfaces as a function of the energy of the bombarding species as well as the atom to ion flux arrival ratio. It is shown that the Ti concentration in the films as a result of the preferential sputtering does exhibit substantial concentration variations across sloped surfaces at constant discharge parameters. The experiments are done in an Ar rf discharge, using a Ti/W target with 50 at.% Ti and 50 at.% W. The films have been analysed by Rutherford Backscattering Spectrometry (RBS) and energy-dispersive X-ray Spectroscopy (EDS). The experimental results are discussed and compared with dynamic simulations using the Transport of Ions in Matter (TRIM) code, which indicate that the loss of Ti in the deposited films is primarily due to ballistic preferential sputtering effects.