MEMS microbridge vibration monitoring using spin-valve sensors

Spin-valve sensors were used to measure on chip, the vibration of microelectromechanical systems microbridges excited by dc + ac voltages. A spin-valve sensor (10 × 2 μm², MR = 6.5%) was placed 3 μm away and 2.6 μm below the central region of an a-Si:H/Al bridge, with a 1-μm air gap. This sensor detects the fringe field created by a Co₇₈Pt₂₂ micromagnet deposited on top of the bridge. The bridge movement was controlled by an applied voltage on the gate. An unforeseen capacitance coupling effect was found between the control gate and the spin-valve sensor, affecting all ac measurements. Once this coupling was isolated and taken into account, bridge oscillation at joint dc + ac excitation voltages was monitored. Bridge vibration amplitudes of several tens of angstroms peak to peak were detected for dc and ac gate voltages up to 30 V.