Microstructural Evolution of Annealed Ruthenium–Nitrogen Films

Abstract

Microstructure evolution of Ru layer deposited in N 2 gas has been investigated and compared with that deposited in Ar gas for Cu diffusion barrier application. Deposition of Ru in N 2 results in high N dissolution, seemingly uniformly distributed in the Ru layer. The film microstructure is a mixture of nanocrystals and amorphous Ru instead of the columnar structure observed when deposited in argon. Annealing at T Ͼ 250°C causes N out-diffusion and crystallization of the amorphous Ru. Voids were observed at Cu/Ru interface in Ru deposited in N 2 when annealed at T Ն 275°C as a consequence of N out-diffusion.The possibility of Ru performing as a diffusion barrier and a seed layer for direct deposition of Cu stimulated interest in Ru thin films. 1,2 Its low solubility in Cu 3 is an advantage because the impurity effect on Cu resistivity is reduced. Ru is also compatible with the established chemical vapor deposition and atomic layer deposition technologies 4 unlike the conventional Ta/TaN barriers. Therefore, the interest in Ru is consistent with the current trend in research to move away from the physical-vapor-deposition-based TaN/Ta/Cu technology where conformality is a real challenge. Despite these merits, Chan et al. 1 reported that a 15 nm Ru layer in Cu/Ru/Si system failed when annealed at 450°C for 10 min, warranting in-depth studies of its barrier properties. In our previous study, 5 we showed that the Ru barrier performance significantly improves when the film is sputtered in a N 2 atmosphere as compared to an Ar atmosphere. The high level of dissolved N in Ru was suspected to be the reason for this improvement, but no direct evidence could then be reported. Subsequent work in analytical electron microscopy has given clear evidence for N dissolution in Ru when sputtered in N 2 atmosphere. This paper reports these findings, including evidence for cavity/void formation at Cu/Ru interface during N out-diffusion from the Ru films upon annealing.A ϳ12 to 17 nm thick Ru layer ͑99.95% purity͒ was sputter deposited onto p-Si͑111͒ and p-Si͑111͒ with SiO 2 cover in Ar or N 2 atmosphere as described in our previous paper. 5 Cu ͑99.99% pure͒ was then sputtered onto the Ru layer under the same sputtering conditions, but with only Ar as the sputtering gas, to give the following sets of samples: . In this nomenclature, Ru-N refers to Ru film deposited in N 2 atmosphere, while Ru refers to that deposited in Ar. The samples were then annealed for 30 min at progressively increasing temperatures up to 900°C in a high-vacuum ambient of 10 −5 Torr.Cross-sectioned samples were examined in an FEI Tecnai F30 ͑300 kV͒ transmission electron microscope ͑TEM͒ equipped with electron energy loss spectroscopy ͑EELS͒ and energy dispersed X-ray spectroscopy ͑EDX͒ detectors for elemental analysis. The TEM samples were prepared conventionally using a mechanical grinder and polisher, dimpling, and ion milling. Figure 1 shows a series of high-resolution TEM micrographs of the Ru and Ru-N layers in the as-deposited state and annealed at 275, ...

Characteristics of Plasma-Enhanced Atomic Layer Deposited RuSiN as a Diffusion Barrier against Cu

“…In the XRD patterns of RuSiN film with the number of SiN x unit cycles of 9 (total numbers of SiN x sub-cycles: 225), it was almost impossible to detect the specific peaks from HCP-Ru, suggesting that a RuSiN film was almost amorphized. It was also reported that by incorporating Ta, N, and TaN into Ru, the polycrystalline columnar grain growth of Ru was destroyed, its grain size was reduced as compared to that of Ru, and the microstructure was changed to nanocrystallite embedded structure in an amorphous matrix, 14 amorphous with some isolated nanocrystals of the order of ∼2 to 3 nm, 23 and amorphous. 15 Though the reason for it is not clear yet, it was suggested that a high level of internal stain due to the incorporation of impurity atoms could promote nanocrystalline structure with shorter range ordering, or even an amorphous structure, in the deposited film.…”

Diffusion Barriers

“…Further, it has almost half of Ta resistivity and (even very thin) Ru films can act as a diffusion barrier enabling direct plating of copper [196]. Although N addition to Ru leads to a mixed nanocrystalline-amorphous microstructure, N out-diffusion is observed already at low annealing temperatures (Figure 6.9) [200]. However, thin Ru films are not very effective as a Cu diffusion barrier.…”

Robust ultra-thin RuMo alloy film as a seedless Cu diffusion barrier

“…However, pure Ru film exhibits a columnar grain structure; the vertical grain boundaries provide Cu with the high diffusion paths, which lead to early failure of the Ru barrier [11]. Recently, the improvement of Ru barrier-performance against Cu has been widely studied [12][13][14]. Adding extra elements, such as N, P, and Ta as impurities in the lattice matrix of Ru film is a way to enhance Cu diffusion barrier properties [12,14,15].…”

“…Recently, the improvement of Ru barrier-performance against Cu has been widely studied [12][13][14]. Adding extra elements, such as N, P, and Ta as impurities in the lattice matrix of Ru film is a way to enhance Cu diffusion barrier properties [12,14,15]. Damayanti et al indicated that the dissolved N in Ru film can modify crystalline Ru film trending to an amorphous microstructure [12].…”

“…Adding extra elements, such as N, P, and Ta as impurities in the lattice matrix of Ru film is a way to enhance Cu diffusion barrier properties [12,14,15]. Damayanti et al indicated that the dissolved N in Ru film can modify crystalline Ru film trending to an amorphous microstructure [12]. The elements added into Ru film could not only be stuffed at the grain boundaries but also alter the microstructure or amorphize the Ru film.…”