The microreed, an ultra-small passive MEMS magnetic proximity sensor designed for portable applications

A passive magnetostatic sensing MEMS device, similar to a reed switch, having a packaged volume of 2 mm(3) is described. It is capable of sensing the field of a 1 mm(3) SmCo magnet at a distance of more than 2 mm. The contact force between the microformed rhodium contact surfaces lies between 10 and 30 muN, and contact resistances as low as 2 Omega are measured. No contact sticking is observed. The device operation life extends to at least 200'000 cycles when packaged at wafer level with epoxy sealing. The device operation life increases to 100x10(6) cycles when measured on bare wafers in argon, showing that these MEMS sensors are intrinsically reliable in spite of the very low contact forces.