Design, fabrication and characterization of a femto-farad capacitive sensor for pico-liter liquid monitoring

This paper presents the design, fabrication and characterization of a femto-farad capacitive sensor, intended for pico-liter liquid measurements in microfluidic channels. The sensor has vertical silicon electrodes integrated into a through-wafer channel to measure the liquid level variations in the channel. An equivalent electrical model is developed to analyze the sensor's behavior. A six-mask, IC-compatible process is developed to fabricate the device. The optimization of critical process steps is discussed. The fabricated channel is 40 μm in diameter and 50 μm deep, corresponding to a total liquid volume of 63 pL. The sensor capacitance varies from 1.5 fF (empty channel) to 13.1 fF (channel filled with water). To reliably detect such small capacitance changes, a low-noise measurement system, based on a lock-in amplifier, is implemented. The measured sensitivity of the system is 14.1 mV/fF, and the capacitance detection limit as determined by thermal noise is 0.057 aF/Hz1/2.