Abstract
A novel technology for manufacturing thin silicon diaphragm structures is presented. Controllability of thin silicon diaphragm is one of the most important issues in fabricating silicon micromechanical sensors whose sensitivity depends on the diaphragm thickness. This can be accomplished by epitaxial lateral overgrowth (ELO) of single crystal silicon on a patterned layer of masking material, typically Si02, combined with crystallographic etching of which etching rate depends on the crystal plane. With recent improvement of EL0 material, good quality of lOμm thick, 200μm x lOOOμm single crystal silicon was obtained with its thickness being precisely controlled by growth rate (≤ lμ m/min.). The junction leakage of the p-n junction diodes fabricated on merged EL0 silicon indicated the material quality is comparable to the substrate silicon. Using this technology, a bridge-type piezoresistive accelerometer with four beams and one proof mass was fabricated successfully. Its sensitivity and resonant frequency were comparable to the accelerometers made by other methods. They were analyzed by comparing the experimental results to a simple analytical solution as well as ANSYS stress simulator using a finite element: methods. The experimental results showed a potential application of the new technology to silicon sensor fabrication but some further refinement is remaining. Free-standing single crystal cantilever beams were fabricated using, MELO and RIE, of over lOOOμm long and 5μm by 10μm in cross section. These beams were very short, straight, indicating little residual stress. Wide, short beams were fabricated using EL0 which were also free standing. Special treatment of MELO indicated that diodes and bipolar transistors fabricated on top of the oxide stripes showed nearly ideal characteristics, hence the quality of the MELO was improved. With MELO of thicker than 5μm, no voids were observed. Test structures significantly with all surface micromachining, were designed for further development of silicon membranes.
Date of this Version
January 1993