To keep pace with Moore's Law and enable greater interconnect scaling, the semiconductor ecosystem is continually experimenting with variations of materials. Tungsten has been the interconnect metal of choice for nearly a quarter century in NAND, DRAM, and logic/foundry middle-of-line applications. But scaling requirements are now pushing the boundaries of what is possible with tungsten. A variety of metals are being considered, but one — molybdenum — is an especially promising candidate. At the atomic dimensions required for advanced chipmaking, moly is emerging as the most suitable material to replace tungsten, creating a major inflection in the industry.
There are a number of factors to consider in the selection of a metal, and resistivity is among the most important — that is, how resistive a metal is to conduct electrical currents. A common driver for adoption of moly is its thin film resistivity. Thin-film deposition is required in device scaling of metal contacts, vias and lines. All metal films, when they are deposited as thin films, show increased resistivity compared to their known bulk (or thick film) values.
For some metals this is more pronounced because the mean-free path of their charge carriers is too long. A mean-free path similar to the dimensions of the feature to be filled is more desirable than a longer one. And that makes moly a better solution for advanced devices than tungsten, cobalt, copper and many other candidates.
Kaihan is CVP and GM, ALD/CVD metals