A bulk micromachined tiltable mirror array digital variable optical attenuator

We present in this paper the simulation, fabrication, assembly, and some mechanical behavior of an electrostatically-actuated MEMS variable optical attenuator (VOA). It consists of two main parts; the device chip and the electrode chip. The device chip is fabricated with a 2-masks process from a SOI wafer, where mirror arrays are located on the front-side device layer with aligned openings on the back-side handle layer. We also examine Ta₂O₅ as a protection for the entire wafer and a mask for the handle layer where the exposed silicon is etched by KOH. The mirror array contains 16 equal-length gold-coated silicon platforms that span an area of 1500x1500 μm². The mirrors, which widths vary from 40 μm to 250 μm, are each suspended by two silicon torsion beams, where the widths are successfully reduced to as narrow as 1 μm by means of a HF vapor phase etcher (VPE). Current profilometric measurements were all performed on previously fabricated devices with 3 μm beams. Measured tilt angles, longitudinal bending, and resonance frequencies agree well with simulations. A reasonable yield can be achieved as the fragile structures survive handling as well as dicing shocks. Two suspension configurations were compared and investigated. The electrode, also fabricated with a 2-masks process, is a quartz glass piece that contains patterned aluminum and SU8 columns as spacers for electrostatic actuation. The two parts are aligned and assembled chip-wise. The device is designed to be capable of producing 16-bit=65536 levels of linear attenuation when a sequence of driving voltage of about 100 V is applied to corresponding electrodes.