Dual-mode NEMS self-oscillator for mass sensing

We report the first experimental demonstration of a heterodyne self-oscillator operating alternatively on the first and second flexural mode of a silicon NEMS resonator. This architecture features a downmixing scheme where the NEMS motion-induced piezoresitive signal at 25 MHz and 70 Mhz is shifted down to few tens of kHz thus reducing the bandwidth constraint on the electronics. In closed loop operation, the oscillator presents excellent frequency stability, identical to the one obtained in PLL operation. While monitoring successively the two modes of the oscillator, mass addition on the NEMS was simulated by electrostatically-induced frequency shifts. This self-oscillator scheme represents a compact and power saving architecture compatible with the readout of dense sensor arrays required in applications such as mass sensing.