Development of silicon-on-glass electrostatic microactuators with a high aspect ratio and vertical configuration

Abstract

Actuators are the key components that produce the mechanical output and perform physical functions of particular systems. Electrostatic actuators are widely used in micro-electromechanical systems (MEMS) applications for their combining versatility and simple technology. In this thesis, an electrostatic microactuator fabricated on silicon-on-glass (SOG) is studied. The electrostatic microactuator utilizes a vertical configuration of comb-drive working plane to the mounting plane of driving components, which are different from the planar configuration of comb drives. The vertical configuration provides more space area for more comb drives to be arrayed, making low driving voltage possible. The electrostatic microactuators have high-aspect-ratio structure, high structural strength, and low cost. The proposed microactuator can be used in the hard disk drives (HDD) to meet their high-capacity and high-performance trend. The microactuator can be used as a dual-stage actuator in the head-positioning system to move the magnetic heads, while providing high speed and accuracy of the positioning. To fulfill the high-bandwidth and high-accuracy requirements of the electrostatic actuators, the flexures that support the microactuator should have a high stiffness and at the same time be flexible enough to have the desired displacement in the operational direction. The straight-beam, folded-beam, symmetric-quad and asymmetric-quad flexures were studied. The analytical and finite element analysis proved that the symmetric-quad flexure has the highest stiffness among the four flexures. Fabrication of the electrostatic microactuator was realized by using silicon-on-glass wafer process. The fabrication started with a 200um-thick n-type silicon wafer, and then bonded to a Pyrex glass substrate, and finally the comb structures were formed by high-anisotropy deep reactive ion etching (DRIE) technique. The fabrication processes II ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library of glass deep wet etching, wafer bonding and high-aspect-ratio DRIE have been developed. This SOG process is alternative to the SOI process for some applications but at a much lower cost. The prototypes of the microactuators were characterized. With a driving voltage of 40 V, the microactuators with straight flexures, folded flexures and asymmetric-quad flexures have the displacement of 0.94 um, 1.73 um and 0.26 um, respectively. They have the primary resonant frequency of 7.2 kHz, 5.89 kHz and 15.85 kHz, respectively. The characterization results were compared and discussed with the theoretical results. The fabrication process, especially the DRIE process, generated various geometrical tolerances of the comb drive actuators, basically including the profile tolerance and undercut tolerance. Studies showed that these fabrication influences have significant impact on the device performance, such as the electrostatic force, flexure stiffness, displacement and quality factor. The study also shows t...