MEMS elements are delicate. They sit there in their little cavities, expecting to operate in some sort of controlled environment – perhaps a particular gas or pressure (or lack of it). And if they’re collocated with CMOS circuitry, then they need to be protected from any further processing steps. In other words, they need to be sealed off from the rest of the world. And wafer bonding is a common way to do that: bring another wafer (perhaps with etched features) face-to-face with the working wafer and get them to bond.
Covalent molecular bonds are the strongest; if you bring two silicon wafers together, for example, the ideal is to have the silicon atoms at the surface of each wafer bond covalently with their counterparts on the other wafer so that the whole thing starts to look like a continuous crystal. That’s the ideal.
Doing this isn’t trivial, of course, since the surfaces are likely to have imperfections and contaminants. So surface preparation has been an important part of the wafer bonding process. It has also involved intermediaries like water that establish a preliminary bond; an anneal then precipitates the reactions that result in the appropriate covalent bonds and out-diffusion of any extraneous elements.
Initially, high temperatures were required for the annealing. But, of course, anything over 450 °C won’t sit well with any CMOS that might be in place, so various surface preparation techniques have been devised to get the anneal temps down below that threshold.
But even these temperatures can be an issue for bonding unlike materials, or for wafers that have unlike materials in the stack, where stresses can result from differing rates of thermal expansion during the anneal process.
EVG has recently announced a new way of preparing the surface so that covalent bonding occurs immediately, at room temperature. To be clear, they have announced that they have this new process; they haven’t announced what it is; they’re still being coy on that. This eliminates the annealing step completely, and therefore the thermal expansion issue as well.
Equipment using this new technique should ship sometime this year. You can find out more in their release.