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MEMSdesign3 - JOURNAL OF SYSTEMS VOL 7 NO 2 JUNE 1998 141...

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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 7, NO. 2, JUNE 1998 141 Design and Fabrication of an Angular Microactuator for Magnetic Disk Drives David A. Horsley, Michael B. Cohn, Angad Singh, Roberto Horowitz, and Albert P. Pisano Abstract— Angular electrostatic microactuators suitable for use in a two-stage servo system for magnetic disk drives have been fabricated from molded chemical-vapor-deposited (CVD) polysilicon using the HexSil process. A 2.6-mm-diameter device has been shown to be capable of positioning the read/write elements of a 30% picoslider over a 1- m range, with a predicted bandwidth of 2 kHz. The structures are formed by depositing polysilicon via CVD into deep trenches etched into a silicon mold wafer. Upon release, the actuators are assembled onto a target wafer using a solder bond. The solder-bonding process will provide easy integration of mechanical structures with integrated circuits, allowing separate optimization of the circuit and structure fabrication processes. An advantage of HexSil is that once the mold wafer has undergone the initial plasma etching, it may be reused for subsequent polysilicon depositions, amortizing the cost of the deep-trench etching over many structural runs and thereby significantly reducing the cost of finished actuators. Furthermore, 100- m-high structures may be made from a 3- m deposition of polysilicon, increasing overall fabrication speed. [256] Index Terms— Disk drive, electrostatic actuator, HexSil, micro- fabrication, polysilicon, servo control. I. I NTRODUCTION T HE AREAL density of magnetic disk drives is increasing at an annual rate of 60% and is expected to reach 10 Gb/in by the turn of the century [1]. Until recently, these increases have been achieved by a combination of increased track density—which is the number of tracks per inch along the radius of the disk—and an increase in linear bit density—which is the number of bits written along one inch of a track. Increases in bit density require either more sensitive head elements or reduced flying height. Tribological problems posed by contact and near-contact recording have been delayed by the introduction of magnetoresistive (MR) heads, which allow higher flying heights due to their greater sensitivity compared to inductive elements. However, given that current bit densities are approximately ten times greater than track densities, significant areal density increases can be achieved by reducing the track width without reducing flying height or resorting to giant magnetoresistive (GMR) heads. To achieve the goal of 10 Gb/in , the data track density is expected to reach 25 000 tracks per inch, resulting in tracks which are 1 m wide and an allowable servo tracking accuracy of 100 nm at bandwidths of 2 kHz or greater.
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