  We have designed and characterized micro-electro-mechanical systems (MEMS)
for applications at low temperatures. The mechanical resonators were fabricated
using a surface micromachining process. The devices consist of a pair of
parallel plates with a well defined gap. The top plate can be actuated for
shear motion relative to the bottom fixed plate through a set of comb-drive
electrodes. Details on the operation and fabrication of the devices are
discussed. The geometry was chosen to study the transport properties of the
fluid entrained in the gap. An atomic force microscopy (AFM) study was
performed in order to characterize the surface. A full characterization of
their resonance properties in air and at room temperature was conducted as a
function of pressure, from 10 mTorr to 760 Torr, ranging from a highly rarefied
gas to a hydrodynamic regime. We demonstrate the operation of our resonator at
low temperatures immersed in superfluid 4He and in the normal and superfluid
states of 3He down to 0.3 mK. These MEMS oscillators show potential for use in
a wide range of low temperature experiments, in particular, to probe novel
phenomena in quantum fluids.
