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A Current-Controlled PCB Integrated MEMS Tilt Mirror
Keywords: MEMS, Printed Circuit Board, Packaging
Micromachined tilt mirrors are widely used in many applications, including projection displays [1], optical signal routing [2] and optical sensor systems [3]. Most MEMS tilt mirrors utilize voltage-controlled electrostatic actuators, such as parallel plate actuators or combdrive actuators, to adjust the tilt angle of a rigid, planar reflective surface. Although voltage controlled actuators are inherently low power devices, they often require high voltages compared to the breakdown voltage of modern transistors, which can be problematic for implementing the drive circuitry. A new concept for a MEMS tilt mirror has been developed that utilizes a current-controlled actuator to adjust the tilt angle of the rigid, planar reflective surface. The bulk micromachined Si device consists of a rigid planar platform that is connected to a surrounding rigid frame by a dual torsional suspension system such that the platform can tilt with respect to the frame. The rigid planar platform can serve as the reflective surface or a separate reflective structure, such as a discrete micro-mirror, can be attached to it. A miniature neodymium rare earth magnet is integrated onto the planar platform. The MEMS device is then attached to a printed circuit board (PCB) [4] near a current carrying Cu trace and aligned so that the magnetic field produced by the DC current will interact with the magnetic field from the permanent magnet to produce a torque on the rigid platform in the direction of rotation of the torsional springs. For a given DC current, the reflective platform tilts until the magnetically generated torque is balanced by the counter torque produced by the torsional suspension system. The tilt angle of the platform is linearly proportional to the DC current in the Cu trace on the PCB, where a positive current yields a positive tilt and a negative current yields a corresponding negative tilt. A prototype MEMS device has been fabricated and successfully tested. For a current ranging from -5A to +5A, the angular range of motion achieved a nearly linear tilt with a range exceeding -5.3 degrees to +5.3 degrees with an R-squared value of 0.9969. Several assembly issues had to be solved in order to realize the device. The strong neodymium magnet had to be handled with tooling only made of non-ferromagnetic materials. Also, temperatures normally associated with attaching electronic components onto a PCB had the potential to damage the permanent magnet, so a low temperature assembly process was developed. Additionally, the MEMS device must be kept a safe distance away from any ferromagnetic or magnetic objects to avoid damaging the device or inadvertently tilting the reflective platform. The fabrication and assembly process developed for this device could be utilized with other MEMS devices that use permanent magnets [5].
Robert N. Dean, Associate Professor
Auburn University
Auburn, AL

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