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

In this investigation, we have studied the effect of negative bias voltage on the properties of a sputtered Ta layer deposited at high Ar pressure (13.3 Pa) which is used as a diffusion barrier in a Cu/Ta/Si structure focusing on its silicidation process. According to Rutherford backscattering spectrometry, the Cu(85 nm)/Ta(100 nm, unbiased)/Si(111) structure was found to be stable up to 300 degreesC in a N-2 environment for 30 min. At a temperature of 450 degreesC, TaSi2 was formed at the Ta/Si(111) interface, and the Ta diffusion barrier completely failed. By applying various negative bias voltages ranging from 0 to -150 V, an optimum bias voltage of V-b = -50 V for the sputtered Ta layer was found from scanning electron microscopy and four-point-probe resistivity measurements. In addition, the normalized ion flux (n(i)), defined as the ratio of Ar+ ion flux to Ta flux, was determined to be 7.5 for optimum experimental conditions. As a result, the biased Ta layer was used as a diffusion barrier between Cu and Si; it showed a low resistivity of 99 mu Omega cm, a density of about 14.1 g cm(-3) with a good surface morphology and a contribution of bcc-Ta phase structure of about 65%. The Cu(50 nm)/Ta(50 nm, biased)/Si(111) structure was demonstrated to be thermally stable under perfect conditions up to 500 degreesC and TaSi2 formation in an N-2 environment for 30 min was retarded up to 700 degreesC.