Reliability of Complete Plasma Etched Ru/Cu Lines
Authors: Su Jia Quan , Kuo Yue 
Published: November 2022

Copper (Cu) is a widely used interconnect material in high density ICs and large area TFT LCDs. A barrier layer is required for the Cu line to prevent it from diffusion to the bottom dielectric and to improve the adhesion. When TaW or TaN is used as the barrier, the line resistance is increased and the reliability may be compromised [1-3]. Ru is a good barrier material for Cu because of its low resistivity scaling, high heat conductivity, low diffusivity in dielectric, and high oxidation resistance [4-7]. Ru. In this study, the reliability of plasma-etched Cu lines with Ru barrier layer is studied.
Ru/Cu films were sputter deposited on SiO2 coated Si <100> wafers and lithographically defined into 4-point patterns. The Cu film was etched with an O2 plasma etched process as described in ref. 8, i.e., exposing to O2 plasma at 40 sccm, 100 mTorr, and 600 W followed by dipping in a diluted HCl (HCl:H2O = 1:8) solution. The Ru barrier layer was etched with O2 plasma at 40 sccm, 50 mTorr, and 600W. Both plasma reactions were done in a parallel-plate plasma reactor (PlasmaTherm 700) operated under RIE mode at room temperature. For the comparison purpose, TiW/Cu lines were fabricated and tested. The reliability test was done by stressing the sample with a constant current, i.e., the electromigration (EM) method, or a constant voltage at room temperature.
Figures 1(a) and (b) are top views of a Ru/Cu line under the (a) bright and (b) dark fields, separately. The surface of the line is uniform and the line has a small edge roughness. Since few bright dots, i.e., light or particle scattering of light [9], are observed in Fig. 1(b), the Ru/Cu line is of good quality.
Figure 2 shows the resistance-time (R-t) curves of Ru/Cu and TiW/Cu lines stressed at a constant voltage of 8V. For both samples, the resistance remained nearly constant throughout the stress period except the drastic increase at the broken point. The line failure mechanism involves void formation, growth, merge, and interconnect [10]. The Ru/Cu line has a longer lifetime than the TiW/Cu line, which can be contributed to Ru’s large thermal conductivity, i.e., 117 W/(mK) vs. 5/ W/(mK) for TiW [11,12], and large resistance to void formation during electric stress [5]. Therefore, Ru is a potential barrier material for Cu in future technology nodes.
More detailed characterization of the plasma etched Ru/Cu lines and the geometry effect on the stress results will be presented and discussed.
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Figure 1