Elsevier

Microelectronic Engineering

Volume 92, April 2012, Pages 71-75
Microelectronic Engineering

Electrical Evaluation of Ru–W(-N), Ru–Ta(-N) and Ru–Mn films as Cu diffusion barriers

https://doi.org/10.1016/j.mee.2011.03.165Get rights and content

Abstract

Co-sputtered Ru–Ta(N), Ru–W(N) and Ru–Mn composites are investigated in terms of their barrier properties against Cu diffusion. A wide range of stoichiometries is analyzed with regard to crystallization, barrier properties, resistivity, Cu adhesion and direct Cu plating behaviour. All films were annealed at 350 °C and 600 °C in forming gas for 1h and subsequently stressed at elevated temperatures and electrical fields (BTS, 250 °C, 2 MV/cm, 30 min). The leakage current was monitored during BTS to observe increased leakage due to Cu diffusion. The Cu ions that eventually have passed the barrier and drifted into the dielectric of the MIS test structure were detected and quantified using the triangular voltage sweep method. The addition of 10% W or Ta into a Ru film already leads to a highly improved barrier performance against Cu diffusion, comparable to TaN, as long as the temperatures involved are kept below 350 °C. Outstanding barriers were identified after 600 °C annealing and subsequent BTS, among them Ru50W50, Ru50Ta50 and Ru95Mn5. However, only Ru90Ta10 and Ru95Mn5 offer an excellent Cu adhesion and the possibility of direct Cu plating.

Introduction

The quest for a material system that can simultaneously act as a Cu diffusion barrier and seed layer for the galvanic fill of vias and trenches gives rise to the promising noble metal ruthenium as interlayer between the dielectric and the Cu in BEOL processing. It is broadly accepted, however, that Ru itself has poor barrier properties due to its poly-crystalline structure even at moderate temperatures [1]. Ta(N) and W(N) are known as excellent Cu diffusion barriers. Straightforward, alloying of W, Ta and/or N is considered a promising method to retard Cu diffusion in Ru films. Damayanti et al. [2] found that the thermodynamic stability of Ru–N alloys is given only up to ∼270 °C. Nogami et al. [3] reported that neither Ru–Ta nor their nitrides in form of Ru–Ta–N would act as a Cu diffusion barrier according to their TVS measurements. The Ru content in this study varied between 70% and 90%. On the contrary, other authors reported that Ru–Ta and Ru–Ta–N do possess good barrier properties. For instance, Chun-Wei Chen et al. [4] demonstrated the thermodynamic stability >600 °C of Ru39–Ta16–N19 films using analytical techniques like TEM and XPS depth profiling. However, no electrical fields were applied in their study. Mori et al. [5] investigated RuTa(N) films on the basis of TDDB measurements. Their RuTa(N) splits exhibited a somewhat smaller lifetime than TaN/Ta, but it was not clear whether CMP problems or Cu diffusion were responsible for that behaviour. Kumar et al. [12] identified a PEALD Ru–Ta(N)C layer with a Ru:Ta ratio of 12 to be a sufficient Cu diffusion barrier according to TVS measurements. In the context of Ru–W alloys, no PVD studies are published yet, but Greenslit et al. [6] found that a PEALD grown Ru-WCN mixed phase with a Ru:W ratio of 11:1 can be employed as a directly platable Cu diffusion barrier solution, also according to TVS measurements after barrier stressing at 150–250 °C.

Earlier works regarding Cu–Mn films showed that manganese segregates and reacts at the interface to the dielectric with silicon and oxygen upon annealing, and thus a self-formed barrier film of high conformity and only a few nm in thickness [7], [8] is produced. For Cu–Mn (<8%) Koike et al. [7] found that a 3 nm barrier film is formed after 30 min annealing at 350 °C, according to a TEM study. TDDB measurements at 100 °C by Tokei et al. [9] indicated a similar Cu diffusion barrier lifetime for Mn–Si–O as the standard Ta/TaN stack.

There is, however, a concern that Cu is in direct contact to a surrounding dielectric before annealing in the Cu–Mn approach and thus before the barrier formation occurs. Furthermore, a recent study revealed that the Cu–Mn approach is not likely to enable scaling of damascene structures as far as the terminal effect during Cu electroplating [10] plays a significant role. The resistance of a Cu–Mn seed layer is too high to initiate Cu plating over the whole wafer instantly when the voltage is applied, and the Cu–Mn seed layer might be etched and damaged in consequence.

Ru as a more noble metal probably does not suffer from etching due to the terminal effect. The binary phase diagram of Ru–Mn does not show any intermetallic phases and the solubility of manganese is nearly zero. It is imaginable therefore that Mn segregates also from Ru to the dielectric interface and forms a MnSiO-like layer, though is does not penetrate Cu at all. It is interesting, thus, to clarify whether there is an improved barrier performance for Ru–Mn (5–15%) alloys.

The scope of this paper was to anneal (350–600 °C) and to ultimately stress (+2 MV/cm, 250 °C, 30 min) PVD grown RuW(N), RuTa(N) and RuMn alloys as Cu diffusion barriers and to detect Cu ions in a surrounding dielectric using the triangular voltage sweep (TVS) method.

Section snippets

Experimental

Ru–Ta(-N), Ru–WRe6(-N) and Ru–Mn alloys of different stoichiometries were produced via PVD co-sputtering of Ru, Ta, WRe6 and Mn targets, respectively. The incorporation of nitrogen was achieved by adding a partial flow of N2 to the Ar plasma. Film compositions (100 nm film thickness) were determined by means of X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering (RBS), (see Table 1). In order to get an indication whether an amorphous structure is obtained or crystallization

Results

Fig. 1 shows the XRD graphs of the investigated RuTa(N) films. In comparison to pure Ru, a remarkable decrease of crystallization can be achieved if about 10 at.% of Ta are incorporated into the film. A further decrease of crystallization or even a complete amorphization of the RuTa(N) composite occurs if the Ta or TaN content is about 50 at.%. It should be noted that stoichiometric TaN, which is an excellent diffusion barrier, is crystalline as well.

Fig. 2 shows the leakage current of the MIS

Conclusion

Co-sputtered Ru–Ta(N), Ru–W(N) and Ru–Mn composites were fabricated in order to improve the barrier properties of a Ru-based film against Cu diffusion. A wide range of stoichiometries were produced to investigate the impact on crystallization, barrier properties, resistivity, Cu adhesion and direct Cu plating behaviour. All films were annealed at 350 and 600 °C in forming gas for 1 h and subsequently stressed at elevated temperatures and electrical fields (BTS, 250 °C, 2 MV/cm, 30 min). The leakage

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