ZeptoFarad capacitance detection with a miniaturized CMOS current front-end for nanoscale sensors

The experimental detection of capacitance variations with a resolution as low as few zeptoFarads (10⁻²¹ F) is presented. This is achieved by means of a CMOS ultra-low-noise and wide-bandwidth current sensing circuit, coupled to a lock-in amplifier to perform capacitance and conductance measurements in a frequency range from DC to 1 MHz. The adoption of an integrated implementation, based on an original circuital topology, provides miniaturization and performance improvement. The mm-sized chip can be easily integrated in extremely compact sensing setups. Resolution limits are analyzed in detail and experimentally investigated by means of a mechanical fixture that converts micrometric linear displacement into sub-aF capacitance steps. The experimental results match the theoretical expectation down to a resolution of 5 zF_rms (6 V at 100 kHz, with a 100 ms time constant). The achieved current resolution of 15 fA_rms (at ∼ms time scale) and the tracking of 40 zF capacitance steps demonstrate how the proposed read-out circuit can serve as a versatile tool for the development of nanosensors.