Relative and absolute pressure sensor combined on chip
A shallow 220 and a deep 230 cavity are etched in a wafer 210 and a top wafer 510 is applied and thinned for forming first respectively second membranes 262, 272 over the shallow and deep cavities. Back grinding of the wafer 210 exposes deep cavity 230 but not the shallow cavity 220 to form a first sensor 260 for measuring an absolute pressure with respect to the evacuated shallow cavity 220 and a second sensor 270 for measuring a relative pressure applied across the membrane 272. A gas flow sensor housing for accommodating the combined sensor comprises first and second cavities 1460, 1470 exposed to each side of the relative pressure sensor for measuring pressure differences generated by a gas flow (fig 14).