You may recall that there are various ways to approach CMOS-compatible MEMS. The one that yields the smallest die area is the CMOS-first process, where the CMOS circuits are built and then the MEMS layers are added afterwards. Done properly, it means the MEMS portions can be built right on top of the circuitry – this is, of course, what provides the space savings.
Imec has been doing a lot of work in this space, and in our earlier article, we pointed to their preference for poly-SiGe as a material for proof masses and such. But they had only tried building structures over older aluminum metallization layers.
More aggressive processes use copper metal, and so imec did a demonstration project to prove this out. Specifically, they built a piezoresistive pressure sensor over its readout circuitry, which used copper interconnect.
Bottom line: it worked. This presumably demonstrates that CMOS-first MEMS can smoothly transition over the aluminum-to-copper change at advanced nodes.
More detail in their release…