Oxygen-Based Reactive Ion Etching of Ruthenium Thin Film
Authors: Yoon Eric J , Su Jia Quan , Kuo Yue 
Published: November 2022

Copper (Cu) is a popular interconnect material in ICs and other microelectronics. As the technology node is reduced, the performance of copper interconnect lines becomes challenging due to the CMP limitation and the reliability concern [1,2]. It has been suggested that Ruthenium (Ru) can replace Cu in the sub 10 nm node for better conductivity and reliability [3]. An effective plasma etching method for Ru is needed to meet the demand of the industry [4,5].

O2 can be utilized in the plasma etch of Ru as it can form volatile RuO3 and RuO4 in the process [5-9]. However, the solid RuO2 can also be formed on the Ru surface, which decreases the overall etch rate of the film [9]. Gases, such as N2, Cl2, and CF4, can be added to the O2 plasma to enhance the removal of RuO2
[6,7]. In this study, authors investigated the RIE etch of Ru using the O2/Ar and O2/N2 feed gases.

The TiW (10nm)/Ru (280 nm) stack was sputter deposited on a silicon dioxide (SiO2) coated silicon (Si) wafer [10]. TiW served as a barrier layer similar to the Cu case [10]. The etching experiment was done in a parallel reactor system (PlasmaTherm 700) under the RIE mode with a constant flow rate of 25 sccm at various O2/Ar and O2/ N2 ratios, 450W to 600W power, and 50 mTorr and 70 mTorr pressure. The photoresist and Ru etch rates were determined from the thickness change before and after the etch.

Figure 1 (a) shows the Ru etch rate as a function of the Ar concentration in the Ar + O2 feed stream. As the increase of Ar concentration in the feed stream, the overall etch rate decreased, unlike in previous studies, where the addition of additive gases with an O2 mixture improved the etch rate of the thin films [5-7]. However, this phenomenon can be explained by changes in the ion bombardment energy and plasma phase chemistry, as shown in Fig. 1 (b) and (c).

Fig. 1 (b) shows the slight change of the cathode self-bias voltage -Vdc with the increase of the Ar concentration in the feed stream over a large range of plasma power. Since ion bombardment enhances the removal of the reaction product, the very small change of the -Vdc indicates that the change of the etch rate is very slightly contributed by the removal of the RuOx product.

Fig. 1(c) shows changes in two major radical components, i.e., O 777.5 nm and Ar 751 nm, with the increase of the Ar concentration in the feed stream. With the addition of Ar in the feed stream, the O radical concentration is reduced while the Ar radical concentration is increased. This result indicates that the Ru etch is more influenced by the O radical etchant concentration than the ion bombardment [5,7].

A minimum ion bombardment energy may still be needed for the etch process, which will be discussed in the presentation. In addition, the influence of N2 on the Ru etch rate will be discussed with respect to the plasma phase chemistry and ion bombardment energy.

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Figure 1