A 3-by-3 400-MHz LC microsensor array for wireless capacitive biosensing

Abstract

Biosensing has been developed rapidly for a number of biomedical applications, including neural monitoring, protein detection, bacteria detection and blood glucose monitoring. Among the branches of bio-sensing, capacitive sensing is a promising technique and utilized widely, since it is rapid, simple, label free and inexpensive. Compared with traditional capacitance sensing front end, capacitance to frequency (CFC) based frontend can be utilized to measure the capacitance change based on the frequency shift in the output. LC voltage-controlled oscillator (VCO) based CFC benefits from low power, low cost, high sensitivity, embedded wireless capability and miniaturized characterization, and it can be based on passive or active approaches. The limitation of cutting edge LC VCO based system is they are single channel. This thesis presents the design of a 3-by-3 400-MHz LC VCO based microsensor array for measuring bacteria concentration and related bio-analyte which behave capacitively in that frequency range. This would allow for high-resolution wireless capacitive sensing in a power-efficient way. The wireless capacitive microsensor array has been designed in a standard 0.18 um CMOS process. The sensor array is comprised of 3-by-3 pixels, and it is implemented by customized interdigitated capacitors. Each pixel has two customized 10.5 um * 10.5 um capacitors with 100fF capacitance value. A class-B VCO is implemented with 300fF buried metal-oxide metal (MOM) capacitor and 300nH off-chip inductor. The class-B oscillator converts the capacitance change in the sensor array into frequency shift and transmits the signal through wireless inductance link. The central frequency varies within ±3.3 MHz, with less than 1.4% RMS phase noise in process corners. By post-layout extraction, the circuit has 417 kHz/fF responsivity, 99.8% R^2 value and 1.5% RMS phase noise. Each pixel occupies and area of 30 um * 25 um area. In total, the whole front-end core circuit takes 140 um* 160 um chip area. This project contributes to further development of the multichannel LC VCO based sensing system, which could be utilized as a multi-channel low-power capacitance sensing technique in a long-term vision with combined detection and wireless transmission in the same front-end.