Solving Transistor Contact Resistance Requires Materials Engineering Innovations
In my previous blog, I described how increasing contact resistance is becoming a major bottleneck to transistor power and performance.
Now I will explain why contact resistance has become more challenging to the foundry-logic roadmap, reaching a point where an entirely new approach to building transistor contacts is needed to scale to the 5nm node and beyond.
Previously, I explained how the cladding layers in tungsten (W) transistor contacts are becoming a major cause of contact resistance. The cladding is in the form of a titanium nitride (TiN) liner-barrier and a W nucleation layer. These layers serve a critical function of ensuring that the contact adheres well to surfaces and grows evenly, without defects—but they do a very poor job of conducting electrons.
To make an analogy, think about painting a wooden fence: you apply a coat of primer before applying a thin, first coat of paint and then a thick, final coat. Similarly, without the TiN barrier, the bulk W adheres poorly to the surfaces and simply flakes off. Without the W nucleation layer, the bulk W doesn’t grow.
Unfortunately, these “preparation layers” have become increasingly problematic because they do not scale. The result is that a higher percentage of the available volume goes to the liner-barrier while a dwindling percent goes to the W conductor. With each foundry-logic node, contact resistance increases, starving the finely engineered transistor and chip and preventing them from achieving their power and performance potential.
Another challenge to the traditional W fill process is that a seam forms, further increasing resistance and potentially impacting yield in high-volume production (see Figure 1).
Over the years, the industry has focused on lowering contact resistance primarily by reducing the volume of the liner-barrier layers and introducing new contact materials. For example, in 2016, Applied introduced a new film that could replace the TiN liner-barrier and W nucleation layers with one tungsten-carbon film, providing more volume for bulk W. We also offered a solution designed to suppress seam formation during bulk W fill. Two years later, Applied introduced a cobalt alternative to bulk W which further thinned the TiN liner and could be grown without a nucleation layer. While more effective, these solutions still relied on “primer” layers to reliably grow the metal conductors. However, as contact vias scale further with the introduction of EUV, the industry needs a bigger breakthrough.
The optimal solution would be to remove the cladding layers altogether and achieve a perfect bottom-up fill with no voids or seams. This has not been achievable with conventional chemical vapor deposition because it deposits material across the entire wafer surface, with poor control for sidewall adhesion and growth. The cladding layers are needed to ensure smooth growth and a complete gap fill.
In my next blog, I’ll describe a unique, Integrated Materials Solution which provides a materials engineering breakthrough aimed at solving the contact resistance bottleneck and enabling 2D scaling to continue well into the future.