Static and dynamic characterization of pull-in protected CMOS compatible poly-SiGe grating light valves

Abstract

Grating Light Valve (GLV) display pixels are reflection type diffraction gratings consisting of electrostatically movable coplanar microbeams. Once actuated, the alternate movable beams deflect downwards which produces controlled diffraction of light creating bright and dark pixels in a display system. GLV displays provide a huge improvement in contrast ratio and resolution over other MOEMS devices. At the same time, compared to hybrid integration, post processing of MEMS monolithically on top of CMOS can lead to increased functionality, performance and reliability. Poly-SiGe structural layers can be deposited at low temperature (~450°C), allowing to retain the performance of underlying CMOS electronics though possessing the desired material properties for MEMS. Hence the aim of this work is to fabricate CMOS compatible poly-SiGe GLVs and to study their static and dynamic behavior. A novel process flow was developed regarding the deposition of thin poly-SiGe structures which is well within the maximum thermal range to retain the full functionality of the underlying CMOS circuitry. A contrast of over 1500:1 was obtained showing excellent optical response of the devices. The effect of squeeze film damping in determining the dynamic response of the GLVs is thoroughly investigated. Influence of variation in dimensional parameters on the settling time of the structures is discussed in detail. A minimum settling time of 2 µs was achieved for our devices. We also showed the analog gray scale nature of the GLVs. In addition, we also use the technique of mechanical stoppers to avoid accidental destruction of the devices because of the pull-in phenomenon.

A 2D MEMS grating based CMOS compatible poly-SiGe variable optical attenuator

“…The actuation voltage is therefore always maintained well below the pull-in voltage, saving it from deterioration. The detailed background theory regarding the function of these movable gratings can be found elsewhere [15].…”

“…Thickness values of the different layers and the separation among them was later confirmed by SEM cross-sectional measurements of the functional devices. A detailed description regarding the step by step fabrication of the devices can be found in our earlier report [15].…”

“…By fitting the resonance frequency of fabricated fixed-fixed beams of different lengths to simulation results, a Young's modulus of 120 GPa and a tensile stress of 20 MPa were derived for the SiGe structural layer [15]. The inherent tensile stress within the poly-SiGe layer was helpful in avoiding the initial bending of the suspended membrane and to keep it flat.…”