MEMS piston mirror arrays with force-balanced single spring

Applications like computer generated holography or wavefront shaping call for spatial light modulators (SLMs) with millions of phase-shifting pixels of only a few micrometers size which are precisely addressable in an analogue way at frame rates of many kHz. While liquid crystal on silicon (LCoS) devices are commonly used, they can't offer very high frame rates and their polarization effect is often considered a drawback. On the other hand, MEMS devices with phaseshifting micro mirrors are not readily available but have a high potential to fulfill the requirements. It is quite challenging to reach the desired deflection range with the limited available addressing voltages. For symmetry usually at least two springs are squeezed into the tight space the small pixels offer. This paper presents an innovative way to get a good sensitivity by using a single hinge per pixel that therefore can be made especially weak. It might be astonishing but there actually are various ways to design electrostatic actuators that deliver pure piston movement with only one spring. We discuss the basic concepts of such force-balanced single spring actuators and give a variety of examples and guidelines to tune a basic pixel layout for pure piston motion. Simulations show the tilt-free response and the stability against tilting torques that might be the result of possible imperfections in manufacturing, like lithographical overlay errors or stress gradients. We also show the possibility to dynamically balance the pixel for a pure piston first eigenmode.