Retardation of Ta silicidation by bias sputtering in Cu/Ta/Si(111) thinfilms

Abstract

In this investigation, we have studied the effect of negativebias voltage on the properties of a sputtered Ta layer deposited at highAr pressure (13.3 Pa) which is used as a diffusion barrier in a Cu/Ta/Sistructure focusing on its silicidation process. According toRutherford backscattering spectrometry, the Cu(85 nm)/Ta(100 nm,unbiased)/Si(111) structure was found to be stable up to 300 °Cin a N2 environment for 30 min. At a temperatureof 450 °C, TaSi2 was formed at the Ta/Si(111)interface, and the Ta diffusion barrier completely failed. Byapplying various negative bias voltages ranging from 0 to -150 V,an optimum bias voltage of Vb = -50 V for the sputtered Ta layerwas found from scanning electron microscopy and four-point-proberesistivity measurements. In addition, the normalized ionflux (ni), defined as the ratio of Ar+ ion flux to Taflux, was determined to be 7.5 for optimum experimentalconditions. As a result, the biased Ta layer was used as adiffusion barrier between Cu and Si; it showed a low resistivityof 99 µΩ cm, a density of about 14.1 g cm-3 witha good surface morphology and a contribution of bcc-Taphase structure of about 65%. The Cu(50 nm)/Ta(50 nm, biased)/Si(111)structure was demonstrated to be thermally stable under perfectconditions up to 500 °C and TaSi2 formation in anN2 environment for 30 min was retarded up to 700 °C.