A 3-axis precision MEMS stage for tunable Fabry-Perot interferometer applications

Date

2023-4-24

Author

Mert, Sedat

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This thesis reports a 3-axis precision MEMS Stage for tunable Fabry-Perot interferometer (FPI) applications. The MEMS Stage is developed within the scope of an alcohol detection system project in which FPI is planned to be used as a tunable optical filter. Within the scope of this thesis, design, modeling, fabrication, characterization and control of the MEMS Stage are studied. The MEMS Stage was approached electromechanically and realization of optical specifications is out of scope. The main objective of the MEMS Stage is to precisely control the gap between two 3.5 mm diameter Bragg mirrors in a range of 5 – 6.25 μm which corresponds to wavelength range of 2000 – 2500 nm. To provide optical transmission, the Bragg layers (SiO2 & A-Si) are deposited onto substrate and Silicon-On-Insulator SOI wafers made from undoped silicon (Si) by utilizing Float Zone (FZ) method. The Bragg mirrors are initially brought together by wafer bonding and the positioning of the moving mirror is provided by drive and sense electrodes made from gold making use of electrostatic effect. From the approaches studied in this thesis, using double folded beam springs to support the moving plate in out-of-plane direction, using multiple drive and sense electrodes to control the tilt angle of the plate, and attaching the moving mirror with actuated plate via additional springs to lower the mirror curvature, were not encountered in the literature of Fabry-Perot Filters (FPF). 2 different versions have been studied for the MEMS Stage design. To simulate the performance of the MEMS Stage, an electromechanical model is constructed both analytically and by using finite element model (FEM). The analytical model has been useful for taking quick design decisions at the beginning and building the control system at the end. The FEM has been useful for verifying the analytical model and obtaining the parameters which could not be obtained by the analytical model because of using lumped elements. The process flow was not a previously proven one, it was developed and optimized for the MEMS Stage. Some steps like deposition of Bragg layers requires trials beforehand. Some processes like bulk etching of Si and wafer bonding are prone to problems. In addition, alternative techniques were tested for some steps like SiO2 etch. Throughout the fabrication optimization procedure of the MEMS Stage, 5 substrate and SOI wafer couples, and just as many dummy wafers were spent. Eventually, drivable devices were produced and some characterization tests were carried out. When 9 V of DC Bias was applied to all 4 of the drive electrodes, 1.8 μm of plate displacement was obtained and that is more than enough to cover the target tuning range. A control system was also designed by referencing analytical model to position the MEMS Stage in the desired way and maintain it. All of the design, fabrication, and characterization activities mentioned in this thesis were conducted at METU MEMS Center.

Subject Keywords

Fabry-Perot interferometer, Fabry-Perot filter, Tunable optical filter, MEMS stage, MEMS positioner

URI

https://hdl.handle.net/11511/103508

Collections

Graduate School of Natural and Applied Sciences, Thesis