30years of electroless plating for semiconductor and polymer micro-systems

The barrier layer in Cu technology is essential to prevent Cu from diffusing into the dielectric layer at high temperatures; therefore, it must have a high stability and good adhesion to both Cu and the dielectric layer. In the past three decades, tantalum/tantalum nitride (Ta/TaN) has been widely used as an inter-layer to separate the dielectric layer and the Cu. However, to fulfill the demand for continuous down-scaling of the Cu technology node, traditional materials and technical processes are being challenged. Direct electrochemical deposition of Cu on top of Ta/TaN is not realistic, due to its high resistivity. Therefore, pre-deposition of a Cu seed layer by physical vapor deposition (PVD) or chemical vapor deposition (CVD) is necessary, but the non-uniformity of the Cu seed layer has a devastating effect on the defect-free fill of modern sub-20 or even sub-10 nm Cu technology nodes. New Cu diffusion barrier materials having ultra-thin size, high resistivity and stability are needed for the successful super-fill of trenches at the nanometer scale. In this review, we briefly summarize recent advances in the development of Cu diffusion-proof materials, including metals, metal alloys, self-assembled molecular layers (SAMs), two-dimensional (2D) materials and high-entropy alloys (HEAs). Also, challenges are highlighted and future research directions are suggested.

Recent Advances in Barrier Layer of Cu Interconnects

The evolution of copper-based interconnects requires the realization of an ultrathin diffusion barrier layer between the Cu interconnect and insulating layers. The present work reports the use of atomically thin layer graphene as a diffusion barrier to Cu metallization. The diffusion barrier performance is investigated by varying the grain size and thickness of the graphene layer; single-layer graphene of average grain size 2 ± 1 μm (denoted small-grain SLG), single-layer graphene of average grain size 10 ± 2 μm (denoted large-grain SLG), and multi-layer graphene (MLG) of thickness 5-10 nm. The thermal stability of these barriers is investigated after annealing Cu/small-grain SLG/Si, Cu/large-grain SLG/Si, and Cu/MLG/Si stacks at different temperatures ranging from 500 to 900 °C. X-ray diffraction, transmission electron microscopy, and time-of-flight secondary ion mass spectroscopy analyses confirm that the small-grain SLG barrier is stable after annealing up to 700 °C and that the large-grain SLG and MLG barriers are stable after annealing at 900 °C for 30 min under a mixed Ar and H2 gas atmosphere. The time-dependent dielectric breakdown (TDDB) test is used to evaluate graphene as a Cu diffusion barrier under real device operating conditions, revealing that both large-grain SLG and MLG have excellent barrier performance, while small-grain SLG fails quickly. Notably, the large-grain SLG acts as a better diffusion barrier than the thicker MLG in the TDDB test, indicating that the grain boundary density of a graphene diffusion barrier is more important than its thickness. The near-zero-thickness SLG serves as a promising Cu diffusion barrier for advanced metallization.

Graphene as an atomically thin barrier to Cu diffusion into Si.

We demonstrate the thinnest ever reported Cu diffusion barrier, a 1-nm-thick graphene tri-layer. X-ray diffraction patterns and Raman spectra show that the graphene is thermally stable at up to 750 °C against Cu diffusion. Transmission electron microscopy images show that there was no inter-diffusion in the Cu/graphene/Si structure. Raman analyses indicate that the graphene may have degraded into a nanocrystalline structure at 750 °C. At 800 °C, the perfect carbon structure was damaged, and thus the barrier failed. The results of this study suggest that graphene could be the ultimate Cu interconnect diffusion barrier.

1-nm-thick graphene tri-layer as the ultimate copper diffusion barrier

https://iopscience.iop.org/article/10.1149/2.0281901jes/pdf

Review—Ruthenium as Diffusion Barrier Layer in Electronic Interconnects: Current Literature with a Focus on Electrochemical Deposition Methods

Phosphorous doped Ru film for advanced Cu diffusion barriers

Ultrathin Cr added Ru film as a seedless Cu diffusion barrier for advanced Cu interconnects

Self-forming AlOx layer as Cu diffusion barrier on porous low-k film

The copper diffusion barrier properties of a 3 nm self-forming InOx layer on a porous ultralow-k (p-ULK) film have been investigated. A 5 at. % In doped Cu film was directly deposited onto porous low-k films by co-sputtering, followed by annealing at various temperatures. Transmission electron microscopy (TEM) images showed that a 3 nm layer was self-formed at the interface between Cu–In and p-ULK films after annealing at 400 °C for 1 h. An EDS line scan on the region near this interface showed obvious accumulation of In at the interface. X-ray photoelectron spectroscopy (XPS) analyses indicated that the self-formed interfacial layer was InOx. The self-forming InOx layer prevented Cu agglomeration on the p-ULK film surface. The XPS atomic depth profiles showed that the self-formed InOx barrier was thermally stable against Cu diffusion to at least 500 °C for 5 h. The sheet resistance of the post 500 °C annealed Cu–In film was comparable to that of a pure Cu film. The Cu–In self-forming barrier approach may be a viable candidate for Cu/p-ULK interconnects.

A 3 nm Self-Forming InOx Diffusion Barrier for Advanced Cu/Porous Low-k Interconnects

The failure mode and Cu barrier properties of a 5 nm thick boron and carbon added Ru (Ru―B―C) film deposited on Si substrate have been investigated. Results from X-ray diffraction (XRD) and Fourier-transformed electron diffraction patterns indicate that the Ru―B―C film is amorphous up to 700°C. Unlike pure Ru film, the Ru in the Ru―B―C film recrystallized at 750°C instead of reacting with Si at the interface to form Ru 2 Si 3 . The sheet resistance and XRD results show that the 5 nm Ru―B―C barrier is thermally stable up to 750°C, whereas the 5 and 10 nm Ru are only stable below 550 and 600°C, respectively.

5 nm Amorphous Boron and Carbon Added Ru Film as a Highly Reliable Cu Diffusion Barrier

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