Study on Lifetimes of Plasma Etched Ru Lines
Authors: Su Jia Quan , Kuo Yue 
Published: July 2022

Copper (Cu) is a widely used interconnect material in ULSICs, large area TFT LCDs, and other electronics. However, as the dimensions shrinks, electronic scattering at grain boundaries and surfaces causes the large increase of line resistance and the RC delay [1]. Also, the barrier layer occupies a large fraction of the interconnect line, which is an issue in conductance and reliability [2]. An alternative interconnect material for Cu is necessary. Ruthenium (Ru) is a possible material for this purpose because of its low resistivity, high resistant to oxidation, good heat dissipation, low diffusion and good adhesion to dielectrics [3–6]. Furthermore, it was reported that Ru has a low resistivity scaling [7], which suggested that it can be more conductive than Cu lines in advanced technology nodes. In this study, the reliability of the plasma etched Ru line is studied using the electromigration (EM) stress method [8].

The Ru film was sputter deposited on top of the SiO2 coated layer Si <100> wafer and covered with a sputter deposited SiO2 layer as a hard mask layer. After the sample was lithographically defined into a 4-point test pattern, the SiO2 hard mask layer was etched at CF4 10 sccm, 60 mTorr, and 600 W. Then, the Ru layer was etched under the condition of O2 40 sccm, 50 mTorr, and 600W. All etches were carried out in a parallel-plate plasma reactor (PlasmaTherm 700) under the RIE mode at room temperature. The remaining SiO2 hard mask on top of the Ru film was removed by dipping in a dilute HF solution. The electromigration (EM) test was done at room temperature on a probe station (Signatone S-1160). The EM stress was conducted under the constant current density (J) condition. Separately, in order to facilitate the breakdown of the Ru line, the sample was stresses at the constant voltage (V) with a power supply (Agilent E3645A).

Figure 1(a) and (b) show the top views of an electric stressed Ru line under the (a) bright and (b) dark fields of an optical microscope. The surface of the line is smooth and has a small edge roughness. In the bright field mode, few voids, which are voids generated in the line, are barely visible. However, in the dark field, the light is diverged into a cone. Light is scattered through the voids, which shows as bright dots [9]. Figure 1(c) shows a broken line that was generated under a large constant voltage. The color change near the center of the line is the broken spot, which indicates the high local temperature in the process.

Figure 2 shows the resistance-time (R-t) curves of a plasma-etched TiW/Cu line stressed at J = 6.00 105 A/cm2 and a Ru line stressed at J = 1.25 106 A/cm2. The Cu line was broken before 10,000 sec. However, the Ru line was not broken after 12,000 sec and the resistance remained the same through the stress period. Therefore, the Ru line is much resistant to the EM stress than the Cu line even under a larger current density condition. Although voids were observed in the stressed Ru line, they did not cause the line broken, i.e., through a series of voids growth, merge, and connection steps [10,11]. Although no barrier layer was used for the Ru line, no line peel off was observed before and after the stress. This can be another advantage over the Cu line.

More detailed discussion on the plasma etching of the Ru line and the EM phenomenon will be presented.

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Figure 1