Comparison of PVD, PECVD and PEALD Ru-TaN films with high Ru concentration for direct Cu plating
This work is motivated by 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 in BEOL processing. Ru itself has insufficient barrier properties, even though a remarkable difference was found between PVD and PECVD or PEALD Ru-C films . Since Ta(N) is known as excellent Cu diffusion barrier, alloying of TaN had been considered a promising method to retard Cu diffusion in Ru films during the last years. Chun-Wei Chen et al. demonstrated a thermodynamic stability of higher than 600°C for PVD Ru 39- Ta 16N 19 films using analytical techniques like TEM and XPS depth profiling . In our recent study we investigated PVD Ru 65-Ta 17N 18 films on the basis of bias temperature stress (BTS) and triangular voltage sweep (TVS) after 600°C annealing, revealing an excellent Cu diffusion barrier performance . All Ru-TaN diffusion barrier layers discussed above were deposited on the SiO 2 layer by reactive magnetron cosputtering using Ru and Ta targets. The total gas flow consisted of a Ar / N 2 mixture. However, these potential Ru-TaN films were not directly plateable by means of Cu ECD since their Ru content was too low. Sung-Wook Kim et al. produced PEALD Ru 52-TaN 45 films and demonstrated a thermodynamic stability higher than 700 °C on the basis of TEM, XRD and sheet resistance measurements . S. Kumar et al. identified a PEALD Ru 92-TaNC 8 layer, i.e. with a very high Ru concentration, to be a sufficient Cu diffusion barrier according to their BTS / TVS measurements . Cu ECD and feature fill were conveniently demonstrated directly on the latter films, but no high temperature annealing and subsequent BTS, as well as oxygen diffusion barrier tests or Cu wetting tests were carried out on PEALD Ru-TaNC so far. However, T. Nogami et al. reported that neither PVD Ru-Ta nor their nitrides in form of RuTa(N) would act as a Cu diffusion barrier according to their TVS measurements . The Ru content in this study varied between 70 at.% and 90 at.%. In another study, N. Torazawa et al. investigated RuTa(N) films on the basis of SIMS profiles and also found that their RuTa(N) films were not thermodynamically stable . It should be noted, however, that these films were fabricated using an alloyed RuTa target and reactive sputtering in a Ar / N 2 plasma. Thus, two major questions arise from the literature so far. First, the possibility of plateable PVD Ru-TaN films that have a Ru concentration higher than 85 at.% and exhibit also excellent Cu diffusion barrier properties, good oxygen barrier properties and good Cu wetting properties. Second, the high temperature stability of plateable PEALD or PECVD Ru-TaNC films, as well as their oxygen barrier properties and Cu wetting behaviour. This work comprises a comparison of PECVD, PEALD and PVD Ru-TaN films with a focus on those films that exhibit a Ru concentration higher than 85-at.%. Fig. 1 plots XRD curves of several PEALD Ru-TaNC films. It was found that even a small amount of 4 at.% TaN inside Ru significantly decreases crystallization. For PECVD and PVD films a similar trend was observed (not shown here). Fig. 2 is a HRTEM cross-sectional picture of a Ru 90-TaNC 10 film. It still shows lattice fringes. Its corresponding HAADF STEM image reveals a continuously mixed phase that was also quantitatively found in this shape in the XPS depth profile for the Ru-signal (not shown here). Fig. 3 + 4 are TVS plots of different barrier samples after 350°C and 600 °C, respectively, and subsequent BTS. It was found for PEALD Ru-TaN that addition of 5 at.% of TaN into the Ru matrix produces an excellent Cu diffusion barrier, since no Cu peak was detected after 600 °C annealing and BTS of + 2 MV/cm, 1/2 hr, at 250 °C. Nanolaminate-like PECVD films needed somewhat higher TaN concentrations for an excellent behaviour. Investigations on PVD Ru-TaN are still ongoing. Table I summarizes some results from the O 2 barrier, Cu direct plating and Cu wetting tests. PVD Ru-TaN films showed good Cu wetting behaviour and better oxygen barrier properties than PEALD Ru-TaNC so far. Direct Cu plating occurred only for samples with Ru concentrations of more than 90 at.% for all investigated PVD, PECVD and PEALD Ru-TaN films.