Preparation of low resistivity Cu-1 at. %Cr thin films by magnetron sputtering

Abstract

Copper is an attractive metal for very large scale integrated interconnections because of its low bulk resistivity of 1.7 μΩ cm. Alloying of the copper may be required to improve the mechanical stability, adhesion, electremigration lifetime, and resistance to corrosion. However, the addition of chromium to copper increases its bulk resistivity at an initial rate of 4 μΩ cm per atomic percent at 0°C. We have investigated the effects of sputtering parameters and post-deposition annealing on the resistivity of Cu-1 at. %Cr thin films deposited by magnetron sputtering on thermally oxidized silicon wafers. Resistivities as low as 2.3 μΩ cm have been obtained. The resistivities of as-deposited films decreased with increasing thickness, from 10 to 2.3 μΩ cm, corresponding to film thicknesses of 100 to 1200 nm, respectively. The thicker films were heated in situ by increasing the number of passes the substrate makes in front of the target in the side-scan sputtering system. Ex situ resistivity versus temperature analysis, measured in purified He up to 600 °C on the deposited films, reveals that there are two drops in resistivity; at about 200 °C and 400 °C. Transmission electron microscopy shows the onset of grain growth at 125 °C-200 °C and the development of a bimodal grain size distribution at 400 °C-500 °C. The final resistivities of the films ranged from 2.3 to 4.5 μΩ cm. In general, the final resistivity of the in situ heated Cu-1 at. %Cr films is lower than that of the ex situ heated films. This suggests that Cr surface diffusion during heated deposition is more effective in lowering the resistivity than Cr bulk diffusion during post-deposition annealing. This model is supported by Auger electron spectroscopy, which reveals surface segregation of Cr during annealing. Heating during deposition allows low resistivity (<2.5 μΩ cm) films to be achieved at a low substrate temperature (200 °C-250 °C), whereas post-deposition annealing requires about 600 °C to achieve the same values. © 1992, American Vacuum Society. All rights reserved.