  A biased tunnel junction between a superconductor and a normal metal can cool
the latter electrode. Based on a recently developed cooler with high power and
superior performance, we have successfully integrated it with a dielectric
silicon nitride membrane, and cooled phonons from 305 mK down to 200 mK.
Without perforation and covered under a thin alumina layer, the membrane is
rigorously transformed into a cooling platform that is robust and versatile for
multiple practical purposes. We discuss our results and possibilities to
further improve the device.
