Capacitive Drive and Detection of MEMS Resonant Motion: Electrical Integration of MEMS

Incorporation of microelectromechanical system (MEMS) oscillators into devices has been limited by the lack of an easily-integrated motional detection scheme. A capacitive detection scheme enabled by impedance matching techniques has been demonstrated for nanowires. However, no measurement had been demonstrated in micron-scale oscillators of interest in sensor and electrical filter applications. Through our work at the CNF, we have demonstrated entirely electrical drive and detection of micron-scale dome oscillators. Summary An easily-integrated readout mechanism would enable MEMS/NEMS oscillators to be incorporated to electrical devices, whose applications could include electrical filtering, ultra-sensitive inertial mass sensing [1], and any mechanical operations requiring electrical feedback. A purely electrical readout scheme, in place of the effectivebut-bulky intereferometric or magnetomotive techniques commonly employed, naturally lends itself to device integration. We are developing a technique that detects the fluctuations in capacitance between the oscillating MEMS/ NEMS structure and a charged gate in its vicinity, as demonstrated for nanowires [2]. The tiny resulting signals can be effectively isolated and amplified through the use of an impedance matching circuit to match the oscillator to conventional 50Ω-electronics (Figure 1). Ultimately this approach could be integrated into complementary metal oxide semiconductor (CMOS) processes and devices, allowing far wider application to MEMS/NEMS technology. The electrical readout technique is demonstrated using concave polysilicon domes. The single crystal silicon substrate and upper polysilicon device layer are insulated by a layer of thermal oxide. The oxide is removed by buffered oxide etch through photolithographically defined etch holes opened in the polysilicon in a reactive ion etch, resulting in a radially symmetric drum, dome, or bowl, depending on film stresses, whose upper and lower surfaces represent the plates of a capacitor (Figure 2). The dome oscillator and a neighboring gold bond pad are surrounded by a Figure 1: Impedance matching circuitry diagramed with a cross section of a dome oscillator. The gate capacitance originates from the area of the bond pad and oscillator. Figure 2: SEM of single device. A trench in the polysilicon electrically isolates the enclosed area from the rest of the wafer.