  We report on the realization of ultrasensitive absolute pressure sensors
based on silicon nitride membrane sandwiches. These sandwiches consist in a
pair of highly-pretensioned, ultrathin (50 nm), large area (0.25 mm2) films,
suspended parallel to each other and forming an ultrashort (500 nm), open
cavity. The compression of a gas in this cavity leads to a strong squeeze film
force, resulting in an increase in the membrane mechanical resonance
frequencies which is directly proportional to the absolute gas pressure. These
sandwiches show a record high responsitivity of >300 Hz/Pa in terms of squeeze
film-induced frequency shift, which, combined with high quality factor
mechanical resonances (Q>10^6), allows for bringing the sensitivity of absolute
squeeze film pressure sensors down to the sub-millipascal level.
