Department of Mechanical and Aerospace EngineeringNEWS & EVENTS

Design, Fabrication and Packaging of Capacitive MEMS Microphone Using MEMS Foundry
Miss Wenshu SUI
Department of Mechanical and Aerospace Engineering, HKUST
Date  :  25 May 2017 (Thu)
Time  :  2:00 p.m.
Venue  :  Room 2571B, HKUST (2/F., Lift #27/28)


With the increasing opportunities offered by the consumer market, the microphone industry is more important than ever. In this work, the design parameters of capacitive MEMS microphone was analyzed based on a general one-dimensional (1D) model. The theoretical sensitivity of the capacitive microphone is shown to be a nonlinear function of diaphragm radius (a), thickness (h), back chamber length (L), the residual stress (σrs) and some other parameters. Based on the general 1D model, a critical diaphragm radius was found located at the inflection point of the sensitivity – radius scaling analysis curve, which reflects the minimal equivalent spring constant of the microphone. We then modified this 1D model by revising the diaphragm as a composited layer that is fitted to the PolyMUMPs process. The modified 1D model was applied to predict the critical design parameters of the capacitive MEMS microphone to be fabricated by the PolyMUMPs process, and the critical diaphragm radius of 300 μm is obtained. The agreement between experimental result and theoretical prediction verifies our general 1D model, and proves the critical value can be used to guide the design of capacitive microphone.

As parylene can be utilized in the MEMS microphone field such as the movable diaphragm and packaging material for its great dielectric properties and high coating uniformity, the properties of parylene should be further studied. In this work, a comparative study of the viscoelasticity of parylene C was presented by using Nanoindentation technology and Molecular Dynamics (MD) simulations. By applying different types of loadings on parylene C films at different temperatures and frequencies, the complex modulus, relaxation modulus and glass transition temperature (Tg) of the parylene C were obtained. The predicted Tg determined from the temperature-dependent density change in the MD model is consistent with the results in the measurements and previous works. Furthermore, with Time-Temperature Superposition Principle (TTSP), the master curve of parylene C were successfully determined on its creep and relaxation behaviour, for the first time, which is critical for the parylene reliability study of bio-MEMS devices.

(Supervisor: Prof. Yi-Kuen Lee)