In this work, issues involved in the design resonant sensors in a surface- micromachined fabrication technology are investigated. The goal of this work is to describe the important considerations in each portion of the sensor design, including the resonator mechanical and electrical design, the design of the oscillation loop, and the design of the sensor structure.
The transducer used in these sensors is the double-ended tuning fork (DETF). This structure has a natural frequency that depends on the amount of axial force applied to it. By including the DETF structure in an oscillation loop, changes in the natural frequency, and therefore the axial force, can easily be detected. The output of the sensor is a change in frequency of the oscillator, a quantity that is easily measured using digital techniques.
Linear and nonlinear analyses are performed on the mechanical and electrical design of the DETF structure. These analyses form the basis of the design models and uncover some of the trade-offs faced by the designer. It is shown that choices made in the design of the DETF structure can have a very strong influence on the stability of the oscillator, and therefore on the noise floor of the sensor itself.
The oscillation loop is analyzed from the point of view of the circuit designer, with an emphasis on minimizing background noise. To this end, a number of circuit topologies are investigated and evaluated. These results are combined with the resonator analyses to fonn a noise model for the oscillation loop. It is shown that a number of sources of instability exist, both linear and nonlinear.
A number of possible sensor structures that utilize DETFs are presented, along with experimental results from prototype devices developed during this work. Results from a force sensor, a high-frequency oscillator, and two different kinds of accelerometer are discussed. The culmination of the prototype devices is an accelerometer with a minimum detectable signal of 89 pg. It is shown that the dominant source of instability in these devices is a nonlinear mixing effect that determines the frequency floor of the oscillator.
May 31, 1998
Roessig, T. W. (1998). Integrated MEMS Tuning Fork Oscillators for Sensor Applications. United States: University of California, Berkeley.