MEMS oscillators are making a serious challenge to quartz these days. We looked at Sand 9’s approach recently, but as I thumbed back through other recent announcements, I came back across one that, in retrospect, had some relevant bits to discuss.
Silicon Labs’ earlier announcement focused on the CMOS+MEMS aspect of their work. At the time, I didn’t see anything I could add to the discussion, so I let the announcement stand on its own. But in light of some of the issues I covered in Sand 9’s release, I thought there were some things to come back to on the Silicon Labs story – some of which weren’t immediately apparent in their release.
This relates to temperature compensation, which seems to be the number one concern with these devices. Yes, everyone tries to compensate with circuitry, but if you can minimize the raw temperature effects, then the compensation is easier.
We looked at the stack that Sand 9 built to do this – silicon and oxide having opposing temperature coefficients and therefore physically compensating for each other. Well, Silicon Labs does something similar but not identical.
They use SiGe as the active material for the resonator, but they back it with SiO2, which again opposes the temperature characteristics of the SiGe.
The other subtlety here relates to the CMOS processing aspect, although again, it seems to be two different ways of accomplishing the same thing. Sand 9 discussed how having the compensation ASIC in the same package was important so that the ASIC was experiencing the same temperature as the sensor it was compensating.
With the Silicon Labs approach, this happens as a direct result of combining MEMS and CMOS on the same die: The compensation circuitry isn’t just next to the sensor; it’s on the same die as the sensor. So again, it experiences the same temperatures as the sensor. It’s probably even closer, although at some point, if you start arguing about hot spots on the actual die, you could question whether mere monolithic integration guarantees better compensation. It depends on where things are on the die and how “hot” the circuits are. So it remains to be proven whether monolithic compensation is practically any more effective than a well-engineered die-by-die solution.
You can find more on Silicon Labs’ process here.