Preliminary Feasibility Study of Silicon on Insulator (SOI) Microphones
This work discusses the feasibility of fabricating capacitive microphones from SOI wafers. Many current designs of capacitive
microphones are fabricated by processing two individual wafers and then bonding them together afterwards. If successful, SOI wafers would offer the ability to make the microphones on one wafer and eliminate the possibility of alignment problems. The ultimate goal was to create functioning microphone membranes with different
geometries and mechanically test their deflection under acoustic actuation.
Several microphone designs were examined and discussed. The fabrication process of creating the capacitive microphones from SOI wafers is discussed in full detail, and the four main process steps include fabrication, observation, modeling, and dynamic characterization. The fabrication process was altered between trials in order to produce better results, such as changing the etching time, etching acid, and drying process. Once the microphones were fabricated, they were observed with an SEM machine to examine the surfaces and cross-sections. Four membrane
thicknesses were modeled in ANSYS and a nonlinear static analysis was performed to predict the deflection of the membrane and the natural frequencies were calculated. The microphones were also mechanically tested with two different methods, the Microvision system and a fotonic sensor, to measure the deflection of the fabricated membrane.
Overall, there were five square microphone sizes fabricated, which included 1/2", 1/4", 1/8", 1/16", and 1/32". They were fabricated on two different SOI wafers. The first wafer had a membrane thickness of 20 microns while the second wafer had a 4 micron thickness. The SEM images taken after each trial were used to determine the success of the fabrication process. The calculation of natural frequencies was used to indicate which membranes could be actuated. Some, mostly the smaller sizes, were proven to be
too stiff to allow for recordable deflection. The ANSYS results were used as a comparison to the tested results and provided information for which geometries were better suited under
different pressures. At higher pressures, the larger microphones are predicted to have a deflection larger than the gap between the
membrane and the backplate, so those pressures would not be suitable for typical sound pressure levels experienced for testing. It also again proved that the smaller microphones were too stiff to get a measurable deflection. The microphones that had the most promise were the 1/2" 20 micron membrane and the 1/4" 4 micron membrane. There were many inconsistencies in the mechanical test data which suggest that stiction is a problem. However, the use of a profilometer showed thin cracks in the membranes.
Advisor:Sung Kwon Cho; William W. Clark; Jeffrey Vipperman
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:09/30/2004