MEMS technology is providing new ways to generate reliable frequencies that conventionally require bulky LC tanks and crystals. Granted, it’s early days (as other monolithic ideas are commercialized), but research proceeds apace, with bulk acoustic wave (BAW) technology now being added to the use of actual mechanical moving parts as candidates for commercialization.
The challenge with an approach requiring a moving part can be summed up in one word: release. While release is required for most MEMS, it’s always extra work to do, and avoiding it is tempting. The alternative to a moving mass is the use of waves transported in a solid, which is the BAW approach. The simplest such device involves two reflectors, top and bottom, but that involves back- and front-side etching.
So-called Bragg reflectors* eliminate the need for so-called “free surface” reflectors by using an sequence of two materials with different acoustic velocities. You typically have them alternating at quarter-wavelength distances, and, if you have enough layers, it acts like a reflector. This can be used at the bottom, for instance, to eliminate the need for all the backside work to get a “real” reflector in there. This is built using alternating thin films in a stack.
In that configuration, the waves travel vertically; there have also been attempts to do this laterally, some of which have challenges and some of which still require release. But a paper at IEDM takes a slightly different approach, using deep-trench capacitors to create the Bragg reflectors and the drive and sense elements.
The good news is that the spacing of the trenches can establish the frequency – that is, lithography provides flexible target frequency design (as opposed to having to rely on a deposited film thickness or etch depth). However, quality is somewhat traded off for manufacturability in that the spacing doesn’t necessarily follow the ideal quarter-wavelength target.
The other piece of good news is, of course, that the manufacturing steps are common for creating shallow-trench isolation (STI) on ICs. (I know, there’s the obvious question: make up your mind, is it deep trench or shallow trench? I guess that, by capacitor standards, it’s a deep trench; by isolation standards, it’s a shallow trench.)
Despite this tradeoff, the researchers claimed that their 3.3-GHz resonator, built on an IBM 32-nm SOI technology, approached the performance of similar suspended-mass resonators. If you have the IEDM proceedings, you can find the details in paper # 15.1.
*If you’re unfamiliar with Acoustic Bragg Reflectors, as I was, and want to Google it, be aware… most useful information appears to be locked behind the infamous pay walls. There were some bits and pieces I could salvage, but apparently such knowledge isn’t for us, the hoi polloi…