Sensitive detection of nanomechanical motion using piezoresistive signal downmixing

We have developed a method of measuring rf-range resonance properties of nanoelectromechanical systems (NEMS) with integrated piezoresistive strain detectors serving as signal downmixers. The technique takes advantage of the high strain sensitivity of semiconductor-based piezoresistors, while overcoming the problem of rf signal attenuation due to a high source impedance. Our technique also greatly reduces the effect of the cross-talk between the detector and actuator circuits. We achieve thermomechanical noise detection of cantilever resonance modes up to $71\mathrm{MHz}$ at room temperature, demonstrating that downmixed piezoresistive signal detection is a viable high-sensitivity method of displacement detection in high-frequency NEMS.