Manas Ranjan Sahu

before my lithium niobate (LN) appreciation thread, i gotta trash talk about it one more time: pyroelectric effect. as the name suggests, you get electric field from temperature change (but not vice versa as would be the case for a lot of other effects since here entropy is involved. The other direction is electrocaloric effect). This means you can do cool stuff with it, such as rapid heating of a chip to get 100 kV (x.com/jwt0625/status…), but it also means a lot of horrible unexpected stuff could happen if you don't know about it. when you bake your chip during fab on a hotplate, the electric field from the temperature change could fry the devices, or even fry and crack the whole substrate. People would add conductive coating or even wrap the chip in aluminum foil during processing to provide discharge paths. And of course it's dependent on crystal cut, and usually worse on z-cut. You'll find literature from the 1980s complaining about channel waveguide MZI drifting more on z-cut LN (doi.org/10.1063/1.96640). here is another example where electric field from a LN chip ESD-ed a device on *another* chip during cooldown to 4 K (x.com/jwt0625/status…). They have to add shielding electrodes to protect those devices and they would survive, and the IDTs on LN chip would still get fried. pyroelectric effect also bothered me a few times, and I was not even on z-cut. I had some x-cut thin film LN devices on silicon substrate, mostly released (i.e. floating) with a few anchor points, that suddenly started mysteriously collapsing when cooling down to 4 K. I ruled out CTE mismatch because that won't explain why it suddenly started happening, but it could be pyroelectric + electrostatic pull-in because I may have moved to a new wafer with better crystal quality and worse discharge. So I put extra aluminum electrodes everywhere to short different LN components to the silicon substrate, and they stopped the collapse..