Currently, chemical mechanical polishing (CMP) rather plasma etching is the most popular production process for Cu lines [1,2]. This is due to the low volatility of the copper halide, e.g., CuCl_x or CuBr_x, at room temperature [3,4]. A plasma-based Cu etch process was invented by the authors’ group [5-7]. This process has been successfully used in fabricating ICs and TFT LCDs [8,9]. The Cu layer was converted into a CuCl_x or CuBr_x layer in a parallel-plate plasma reaction, which was then stripped off with a HCl solution. The Cu conversion rate can be very high, e.g., 140 nm/min at room temperature. The reliability of the Cu line has been studied using the electromigration (EM) stress method [10,11]. The TiW capping layer effect on the Cu lifetime was reported [11]. In this study, the molybdenum (Mo) capping layer effect on the Cu failure process is studied.

Mo/Cu and Mo/Cu/Mo stacks were deposited on the Corning glass and etched into a 4-point pattern for the EM test. All films were sputter deposited. The Mo capping and barrier layers were reactive ion etched in a PlasmaTherm 700C system at CF₄/O₂ 10/10 sccm, 60 mTorr, 600W for 2 minutes. The Cu to CuCl_x conversion reaction was done in the same RIE reaction at HCl/CF₄ 20/5 sccm, 70mTorr, 600 W for 2 minutes. The CuCl_x layer was dissolved in a H₂O:HCl (8:1 v/v) solution for 1 min. The Cu samples were stressed at room temperature at fixed current densities (J). Changes of line color and shape were recorded.

Figure 1 shows the top view of a 30 µm wide Mo/Cu/Mo line after J = 0.6 × 10⁶ A/cm² stress for 10 cycles. The line broken spot occurred at the cathode or anode pad which was not observed with the TiW capped Cu line. This is because the mechanical property of the Mo film is weaker than the TiW film is [12,13]. When the probe needle was applied to the pad, a high stress was generated locally, which initiated the failure process.

Figure 2(a) shows the resistance-time (R-t) curve and temperature-time (T-t) curve of a 30 μm Mo/Cu/Mo line stressed at J = 0.4 × 10⁶ A/cm². The resistance gradually increased with the increase of the stress time, and the resistance increased dramatically near the line broken time. The temperature of the Cu line was calculated using the JEDEC standard guidelines of JESD33-B [14]. Since the temperature increase is caused from the Joule heating effect, the T-t curve follows the same trend as that of the R-t curve. Figure 2(b) shows the top view of the Mo/Cu/Mo line at different stress time at J = 0.4 × 10⁶ A/cm². The original line had the sage green color. It changed to the aquamarine color with the increase of the stress time. A white region appeared before the line was broken, which became dark upon the line breakage. The color change is related to the change of the local temperature.

The EM failure phenomena of the Mo/Cu and Mo/Cu/Mo lines were compared. The result will be discussed from their R-t and T-t curves as well as the color changes. The capping layer material and properties are important to the EM lifetime of the Cu line.

Authors acknowledge the financial support of this work through NSF CMMI project 1633580.

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