Is it only because of diffusion problems associated with Cu interconnect or we would need a contact material even if we replace Cu with some other material?
Dear Dr. Nayab Shiraz,
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.
For more details, please see the source, “How New Materials Can Solve Contact Resistance” by Mike Chudzik (2017) at: http://blog.appliedmaterials.com/how-new-materials-can-solve-contact-resistance
Best regards, Pierluigi Traverso.
Dear Nayab Shiraz;
The gap within the structure (tungsten plug) is sandwiched between a liner and barrier material. “The barrier is CVD or ALD TiN (titanium nitride),” Applied’s Bakke said. “The liner/nucleation material is a doped ALD W (tungsten).” At 16nm/14nm node, the critical dimension (CD) of the contact is approximately 25nm. At10nm node, the CDs of the contact are expected to range from 10nm to 15nm. To solve this problem, chipmakers may need to re-engineer the contact at 10nm and less. The idea is to increase the volume of tungsten in the contact without creating unwanted seams in the structure. This could be achieved by making the liner and barrier layers thinner, by increasing the tungsten conductor volume. That won’t work, however. In the MOL contact, the liner/barrier materials have reached their fundamental thickness limits. So to solve that problem, Applied Materials has devised a new plasma-enhanced chemical vapor deposition (PECVD) process, which lowers contact resistance by up to 90%. The process enables a metal-organic tungsten film, which is capable of replacing the barrier and nucleation layers. In effect, the film performs the functions of both. “The low resistivity film allows for greater volume of tungsten,” Bakke said.
more details in the following reference;
https://semiengineering.com/next-device-challenge-contact-resistance/
You can view this link
http://blog.appliedmaterials.com/contact-resistance-and-its-role-limiting-transistor-performance
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