With the way things are headed, more and more functionality moves into a single chip each year. However, one thing that seems to remain completely untouched by this is MEMS devices like accelerometers and gyros.
Despite many device classes practically requiring accelerometers, integrating MEMS into chips seems astonishingly rare, except for a few expensive (and weak) outliers by ST and Bosch. I assume the reason is technical.
In particular I'm interested in the following questions:
- What makes them so rare?
- Do process differences have an impact on this?
- How do the components that do exist circumvent these problem(s)?
Best Answer
If you meant your question as "why do we not integrate them into a full SOC", then I am afraid I do not really answer your question below. Else:
In addition to the reasons already given here is that they not just require extra steps, but compromises on steps. In other words, your MEMS part will not be as good (or as cheap) when it is integrated with CMOS than it would be if it was done with a separate process. The CMOS would not be as good as a dedicated CMOS process either (for example, heating steps of your MEMS part would impact the doping profiles of your CMOS devices. Many cutting steps like plasma etching, DRIE, etc, use large fields and can cause charge to damage devices). However, it is done: take this example from the Melexis MLX90807/MLX90808 pressure sensors (source).
Because of this, it is often cheaper to just use different processes and connect in-package. Here is an example fro mCube (source). You can see two die in the top-left picture. According to the source, the top die is connected to the bottom one with through-silicon-vias.
An example where bond wires are used to interconnect multiple die (source):