Design, Modelling and Control of IRST Capacitive MEMS Microphone

Condenser MEMS microphones are becoming a promising technology to substitute the current standard microphones, and modelling such systems has become very important for designing a condenser microphone fulfilling the given constrains. In this dissertation a deep analysis of capacitive MEMS microphone has been presented coming up with a complete model which is able to fit the experimental data of the microphone sensitivity. Furthermore, a simple noise model, able to fit the experimental data, has been developed considering the well-know Brownian noise and the more subtle 1/f component, usually neglected. With such models, it is possible to have a reliable estimation of the microphone SNR. Many characterizations have been performed on the produced samples and different problems of the manufacturing process have been highlighted, gaining a deeper understanding on the structure of the microphone and on the production process. Finally, to reply to the more and more demanding constraints, two applications of control law have been applied: a force feedback and a controller to tune the resonant frequency of the microphone. This last application shows how a controller can make the system more flexible and reduce the problem of some defects on the production. The force feedback is a technique already used in MEMS systems, such as gyroscopes and accelerometers, where it has shown to be able to improve the performance of the systems. In the presented configuration, a force feedback has been implemented in a digital readout interface, realizing the so-called electromechanical sigma delta converter. Its stability has been evaluated and the improvements have been verified experimentally: due to the extra filtering action of the embedded MEMS system inside the converter loop, the A-weighted in-band noise has been reduced from -63dBA to -73dBA.