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High performance micro scanners for miniature laser projection displays

: Hsu, S.-T.
: Janschek, K.; Lakner, H.

Dresden: TUDpress, 2009, XVI, 125 S.
Zugl.: Dresden, TU, Diss., 2009
ISBN: 978-3-941298-27-9
Fraunhofer IPMS ()

A high performance micro scanner is a critical element to enable efficient beamsteering in miniature laser projection displays. Such scanner requires a large mirror size, large scan angles, and a high frequency to ensure good optical resolutions. However, these conditions also induce large dynamic deformations on the mirror plate, resulting in optical aberrations. This dissertation describes a new design of two-dimensional micro scanner with good dynamic atness. The design targeted to achieve a resolution analogue to VGA video requirements (640 pixels x 480 pixels) with a modulation transfer function (MTF) value of 0.5. Another portion of the dissertation is dedicated to the fracture strength investigations of single crystal silicon torsion springs. The fracture strength knowledge will contribute to the design of a reliable micro scanner under high frequency, large angle actuation conditions.
The demonstrated two-dimensional micro scanner has a high horizontal scanning frequency of 30.84 kHz and a low vertical scanning frequency of 335 Hz. It has two special features to enhance the mirror plate atness: 1. the backside islands to increase rigidity and 2. the multiple-joint torsion springs to evenly distribute forces onto the mirror plate. A novel DRIE-TMAH combination process was developed to fabricate the backside islands with an average height of 86 um and > 96% within-wafer uniformity. When operated with a mechanical scan angle of 4.8°, the 1 mm diameter mirror plate had a root-mean-square dynamic deformation of 13.86 nm. This translates to a 25 nm deformation at +- 10°, which is less than 1/20 wavelength of the 635 nm light source. The MTF of the scanner was calculated based on the deformations and proved the scanner's ability to achieve a VGA resolution with 0.5 MTF.
Another part of the dissertation documents the findings of the fractures strength experiments performed with single crystal silicon torsion springs. Three types of specimens with different sizes and stress distributions were fabricated for the experiments. The specimens were actuated to different angles to determine the stress levels at which the springs fracture.
Fitting the fractures stress data with a two-parameter Weibull distribution function, the characteristic stresses of the three specimen types were found to be 3.10 GPa, 2.51 GPa, and 4.76 GPa. Among them, the design with the highest stress concentration and lowest surface roughness exhibits the highest fracture strength. This indicates a high stress concentration to be desirable for torsion spring designs. A normalized characteristic stress was implemented to attempt the fracture strength prediction of a new device. However, the analyses demonstrated such predictions to be unreliable if the surface condition of the new device is different from that of the known and tested specimens.