Optical Modulators Based on Microfabricated Tilting Flaps Operating at low Actuation Voltages

In this thesis the concept, modelling, design, fabrication process and characterization of novel optical modulators are presented. The devices consist of flaps exhibiting a large tilt with electrostatic actuation at low voltages. The applications comprise, but are not limited to, a monochromatic reflective display, a transmissive shutter array and switchable diffraction-gratings. Nowadays the LCD-technology for reflective displays which is the most widely used technology exhibits a low contrast and reflectivity. Different technologies have been developed or commercialised, but still do not reach the quality of printed paper. It is demonstrated that the technology developed in this thesis is very competitive. If it is applied as reflective display it could reach an overall reflectivity of 45% in the white state, a contrast ratio of 50, a 100 dpi resolution, a fast switching time of 0.5ms and an actuation voltage below 40V; leading to low power consumption. When the device is employed as a transmissive shutter array a 40% fill-factor of the modulated area is obtained. The modulators consist of flaps with 250 µm width and 50 µm height. They are in vertical position at rest and in particular after fabrication. The flaps in array-configuration are suspended by a torsion beam and are electrostatically tilted to almost horizontal position. For applications as a reflective display, light gets absorbed on an underlying black-layer as the flaps are in vertical rest state, giving a black pixel. At tilted position incoming irradiation is reflected and the pixel is perceived as white. In the application as transmissive shutter array light passes through as the flaps are in rest-state and light is blocked in the actuated-state. A model of the device is presented which permits us to properly design the dimensions, knowing the scaling laws. A novel bulk microfabrication process was developed that employs micro-moulding for obtaining flaps and torsion beams made out of poly-silicon. Thin high aspect-ratio trenches are refilled by poly-silicon followed by dry etching of the surrounding material. With this process a large variety of flap shapes are fabricated, such as a wave-, lens- , comb- and grid-form. For electromechanical and optical characterization special setups and methodologies were conceived. Typically 60° tilt angles for 60V actuation voltages were measured. Depending on the geometry there was a hysteresis in the actuation trajectory and some flaps exhibited tilt angles close to 90°. The actuation voltage ranged from 35 to 85V. The resonance frequency was at 700 Hz in the rest state and at 2.3kHz in the actuated state. The poly-silicon flap surface had a reflectivity of 37% and an absorbance of 85 to 96%, depending on the wavelength. When the configuration was used as a reflective display a maximal contras ratio of 145, with typical values between 30 and 40, was measured on the active modulation area. The use of a blazed-grating shaped flap as switchable diffraction-grating was also demonstrated. Besides the concept of vertical flaps, a study is presented for the design of surface micromachined horizontal flaps actuated electrostatically to vertical position towards a transparent electrode.