Gimballed Electrostatic Microactuators with Embedded Interconnects

A microgimbal with an integrated in-plane electrostatic microactuator was designed, fabricated, and tested. The microgimbal structure provided pitch and roll compliance between the microactuator and ground, and thus offered mechanical isolation from external disturbances, such as vibration and temperature variations. The gap-closing, electrostatic microactuator operated in the yaw direction, although translational actuators are possible. The co-planar, co-fabricated device included embedded electrical interconnects to the microactuator and the actuated load. A microgimbal is useful in mechanically isolating microelectromechanical systems (MEMS) which are required to operate in precision machinery, such as microactuators for hard disk drive applications required to operate in close proximity to the magnetic media.
A dual thin film molding process was developed to enable the fabrication of monolithic mechanical structures with embedded regions of conductivity and interconnects. A high-aspect-ratio composite structure was created from undoped polysilicon, low stress nitride and doped polysilicon, in a dual molding process. Electrical isolation was achieved with a combination of low stress nitride and undoped polycrystalline silicon. Various isolation geometries were investigated. Current leakages of less than 1 nA at 30 V were measured for 40 um long, 80 um tall isolation structures with less than 1200 um^2 cross sectional  area.
The molding process enables the construction of torsional springs with unique cross sectional designs by combining high-aspect-ratio beams with horizontal surface features. Cross sections such as T-bars, pi sections and channels were utilized in creating torsional springs with low torsional stiffnesses and high in- and out-of-plane bending stiffnesses. Experimental modal analysis was used to determine torsional stiffnesses as low as 0.13 uNm/deg with T-bar springs 45 um tall, 50 um wide and 100 um long. Springs of the same dimensions with solid rectangular cross sections have calculated torsional stiffnesses of at Ieast two orders of magnitude greater. Torsional stiffness models for open, thin walled sections which included nonlinear warping effects, were developed and used to predict the torsional natural frequencies of open, thin walled springs to within +/-20%.
The dual molding process was utilized in the fabrication of an integrated microgimbal/microactuator, a major component in a MEMS-based architecture for the disk drive arm. A gimballed rotary electrostatic microactuator with an in-plane stiffness of 0.21 uNm/deg, supported by a microgimbal providing pitch and roll compliances of 0.54 and 0.67 uNm/deg, respectively, was demonstrated. A maximum output torque of 18.3 pNm was sufficient to displace a 1.6 mg picoslider +.I .5 mrad. The demonstrated micromachined gimbal has torsional stiffnesses similar to macro gimbals with the added benefits of an integrated microactuator and embedded interconnects.
Lilac Muller
Albert P. Pisano
Publication date: 
May 31, 2005
Publication type: 
Ph.D. Dissertation
Muller, L. (2000). Gimballed Electrostatic Microactuators with Embedded Interconnects. United States: University of California, Berkeley.

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