A micromachined pressure sensor

Abstract

Capacitive Micromachined Ultrasonic Transducer (CMUT) is a microelectromechanical device that is basically formed by a moving top electrode, a stable bottom electrode and a gap in between. In spite of its this simple mass-spring construction, CMUT is a nonlinear device and its working principles have been formulated. According to these studies, the top electrode can be set in motion by the applied pressure on it and by depending on the amount of that pressure, the resonant frequency of the CMUT can be altered. Therefore, it is possible to use CMUT to obtain a pressure sensor. In this respect, what we have to do is keep tracking of its resonant frequency to deduce the pressure. The most e ective way of doing it, on the other hand, is using an oscillator circuit which also provides us the capability of tracking the resonant frequency in real time. Also, to design an integrated circuit that works with the CMUT, the best way is utilizing a Colpitts oscillator. In this thesis, we design a pressure sensor with CMUT based Colpitts oscillator. In order to achieve our design, rst of all, we examine the small signal equivalent circuit model of an uncollapsed mode CMUT and investigate the related analytical equations that models the behavior of it. To simplify the equations, we liken the small signal equivalent circuit model to a crystal oscillator by making necessary transformations. After that, we investigate the \feedback system approach" and \negative resistance concept" methods that help us to analyze the oscillator circuits; and we determine the Colpitts oscillator circuit as the oscillator circuit part of our device. We evaluate the CMUT based Colpitts oscillator circuit and derive the limitations on the circuit parameters for achieving a power e cient device. In addition to that, we discuss the dc biasing of the oscillator circuit that does not cause any loading e ect on the oscillator circuit and design a ring oscillator and a charge pump circuit which help us to obtain bias voltage on the CMUT. Finally, we calculate the sensitivity (in Hz/Pa) and the temperature sensitivity of a CMUT in addition to the Quality factor of our circuit; and by being based on these calculations, we obtain the optimum CMUT parameters for the best available sensitivity and conclude the design. At the end, we design a CMUT based Colpitts oscillator that works as a pressure sensor which measures pressure between zero atm and one atm with sensitivity of 14.6 Hz/Pa at 1 atm. The selected CMUT parameters, on the other hand, for the radius of the CMUT cell, the gap height of the CMUT cell, the thickness of the insulator layer and the thickness of the top plate are 44 m, 100 nm, 100 nm and 3 m respectively. The Quality factor of the circuit is 5 and the inherent Quality factor of the CMUT is 432.