High-bandwidth piezoresistive force probes with integrated thermal actuation

We present high-speed force probes with on-chip actuation and sensing for the measurement of pN-scale forces at the microsecond timescale. We achieve a high resonant frequency in water (1-100 kHz) with requisite low spring constants (0.3-40 pN nm(-1)) and low integrated force noise (1-100 pN) by targeting probe dimensions on the order of 300 nm thick, 1-2 mu m wide and 30-200 mu m long. Forces are measured using silicon piezoresistors, while the probes are actuated thermally with an aluminum unimorph and silicon heater. The piezoresistive sensors are designed using the open-source numerical optimization code that incorporates constraints on operating temperature. Parylene passivation enables operation in ionic media and we demonstrate simultaneous actuation and sensing. The improved design and fabrication techniques that we describe enable a 10-20-fold improvement in force resolution or measurement bandwidth over prior piezoresistive cantilevers of comparable thickness.