Design of a mems membrane based on fresnel zone plate pattern for a fiber optic mems microphone

Date

2023-9-08

Author

Cömert, Cemil

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Fiber optic MEMS microphones have several advantages compared with other types of microphones. In this thesis work, MEMS membranes, CMUTs, are designed based on patterns of Fresnel Zone Plates (FZP). The main objective of the thesis is to focus the incoming light above the membrane by utilizing the diffraction characteristic of FZPs on reflected lights. Optical simulations are done with the Finite-Difference-Time-Domain (FDTD) method. Optical simulation results on unit Fresnel patterns show greater electric field intensity concerning the membranes without patterns for a specific focal range and at different levels. While the focal length changes between 8 – 60 μm to the unit pattern, the increasing ratio in the electric field intensity changes between approximately %2 - %40 in magnitude vise. The diffracted lights by the proposed double-rings and single-hole – single-ring unit patterns form plateau shaped focus characteristics with the changing focal length thanks to multiple foci. On the other hand, single-hole unit pattern forms at most two peaks shaped focus characteristics, and the electric field intensity level decreases when the length is chosen smaller or greater than the focal length. As a result, two different focus characteristics are achieved with the proposed designs. The microfabrication masks are drawn in the light of these results for the PolyMUMPs process. Mechanical simulations are done with Finite-Element-Method (FEM). There are two types of membranes for proposed designs whose resonance frequencies are 135 kHz and 295 kHz respectively for thin (1.5 μm) and thick (3.5 μm). Both cover the audible range, which is less than 20 kHz. While the thick one has a more flat response, the thin one has higher responsivity. Thin membranes can be used in applications where sensitivity is prioritized, and thick membranes can be used in applications where flat response is prioritized. Designs that can operate in the audible frequency range are achieved. The 𝑅𝑠 and 𝐶𝑠 model parameters are simulated for both DC and frequency-dependent cases. This thesis proposes optically improved designs for fiber optic MEMS microphone membranes.

Subject Keywords

MEMS membrane, Diffraction, Fresnel Zone Plate, CMUT

URI

https://hdl.handle.net/11511/105356

Collections

Graduate School of Natural and Applied Sciences, Thesis