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How New Materials Can Solve Contact Resistance

By Mike Chudzik

Most connected computing devices today use 3D FinFET transistors enabled in part by technology innovations from Applied Materials. The challenging transition to FinFETs was essential to continue Moore’s Law scaling. But as usual, when one problem is resolved others loom. One of the most serious of these is increased contact resistance. I briefly touched on this in a previous blog.

So why is the contact so critical and challenging? It is one of the smallest structures in the device that connects close proximity transistors to each other and to the upper levels of copper interconnect wiring, as shown in figure 1. The industry has spent years boosting transistor performance by changing materials and architectures. Interconnect wiring has also been optimized to support the transistor. Squeezed in between, the contact controls the flow of signals from the transistors and interconnects to the external world. If the conductivity of this electrical connection isn’t good, it will bottleneck the flow of current and limit performance.

That’s why mapping out the future for the contact is paramount. Pretty much every chipmaker is working aggressively to alleviate this issue. They understand if it’s not resolved then it won’t matter what else is done with the device to try and boost performance.

To mitigate resistance, alternative liner/barrier technologies (more on this in the blog by my colleague Jonathan Bakke) and moving to multiple levels of thinner contacts at 10nm are being implemented. This progression in design changes is illustrated in figure 2. But even with these innovations, scalability at 7nm and beyond will be extremely challenging.
For more than 25 years the main electrical conducting material in the contact has been tungsten (W). But with scaling, there is less volume left in the contact for W, making it increasingly difficult to get the current through. A promising option moving forward for the contact is to use a new material. Researchers are experimenting with cobalt (Co) to replace W.
Co offers major advantages as a contact replacement material to increase the volume of the contacts as they are scaled. It doesn’t require the thicker barrier layers like W does, which opens up more room for the contact (fill) material. Plus, it’s not just the deposition step for the bulk fill involved – there is annealing as well. Co has a higher thermal budget making it possible to anneal, which provides a superior, less granular fill with no seam and thus lowers overall resistance and improves yield.
Applied is focused on enabling customers to reduce contact resistance. While replacing W with Co is a crucial transition, there’s much more involved to make a robust contact. In a future blog, I’ll take a look at all the steps and technologies involved to make a really low-resistivity 7nm contact.

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