Graduate Exam Abstract

Sam Wright

M.S. Final
January 20, 2016, 1:30 pm - 3:30 pm
Atlas Studio, Scott Bioengineering
Sub 1 fF Capacitive Sensor with PVT Compensation and Wide Dynamic Range

Abstract: High sensitivity capacitive sensors have been of interest in many fields of study including, but not limited to, electronics, chemistry, biology and medicine. A high accuracy and high precision sensor of this type would allow for faster, safer, and cheaper research solutions. The research presented targets an easy to use device focused on label free affinity sensing for biological target detections in a laboratory environment and in the field using a mobile platform. Many tests targeted towards pathogen detection, including polymerase chain reaction, require a great amount of time and attention to detail from a professional to produce accurate results. Even then, additional tests are often requested to verify the results. Tests such as fluorescence detection rely on the binding of fluorescent tags bound to targets of interest, and optically confirming the presence of light emission from said target. Introducing foreign bodies such as these can cause complications, and often result in expensive tests. Capacitive sensors are a relatively new means of detection with the introduction and widespread application of CMOS processes. Unfortunately, this approach has not been developed mature enough, and specifically, its reliability needs to be further improved. A compact, low power, high sensitivity capacitive sensor could be used with micro-arrays for multi-target detection in real time. Comprehensive designs would allow for easy-to-use and affordable solutions to the previously mentioned approaches. Devices of such nature implemented in a nomadic platform could then be used to detect pathogens in the food industry, water supplies, first-aid, and law enforcement. The capacitive sensor presented contains matched oscillators whose frequency is a direct function of the target capacitive value, designed in two separate processes with minor differences. Dual oscillators allow for a single level of calibration against a controlled oscillator to isolate minute changes. An additional stage of calibration can be carried out in real time to account for variations in temperature, and variability introduced during manufacturing. Combined, the dual stage calibration provides a detection sensitivity of less than 1 fF relative change by eliminating the effects of process variation, voltage, and temperature (PVT), produced in a matter of seconds. The first design, completed in 180 nm, takes advantage of temperature behaviors of integrated resistors to compensate for environmental changes. Active body biasing, and digital calibration account for active PVT correction. The second design contains a 6.35 pF dynamic range control oscillator capacitive adaptation can compensate for changes in the environment during testing to retain the target sensitivity. Digital calibration alone accounts for PVT compensation. Both designs have been implemented in silicon layout using commercial 180nm and 600nm CMOS processes with sub-1fF sensitivity. In addition to the integrated circuit, a mobile platform was developed to provide an easy to use device to perform similar tasks. The bench top sensor has the advantage of using sensitive test equipment, computers, and their peripherals. A mobile device can provide a hand held lab environment, mimicking the likes of an at-home blood sugar meter used by diabetics. Sensitivity, power, and size were the focus of such a device, in addition to ease of use that would be expected from any consumer product. Utilization of low power microcontrollers and off the shelf, high sensitivity capacitive sensors housed in a 3D printed case provide a foundation for a functional prototype capable of 1.5 fF sensitivity.

Adviser: Dr. Tom Chen
Co-Adviser: Dr. Edwin Chong
Non-ECE Member: Dr. Chuck Henry, Chemistry
Member 3: N/A
Addional Members: N/A

A PVT-Compensated Capacitive sensor with sub 1 fF Sensitivity: MWSCAS 2015

Program of Study: