ADVANCED MEMS SPATIAL LIGHT MODULATOR FOR COMMUNICATIONS, IMAGING, AND TARGETING

Advanced SLM arrays for coherent communications, imaging and targeting require a superset of specs including high-pixel- count arrays of flat mirrors with piston, tip and tilt analog degrees of motion, large displacements and fast response time. The combination of all these characteristics makes the MEMS design and process implementation a formidable challenge. Furthermore, the fabrication and integration with driving electronics has to be scalable to provide upgradeable systems within the same platform. More specifically, these mirror arrays are expected to be scalable up to 1 million pixels. Considering MEMS fabrication with 8- inch wafers and an active array area of the order of 15cm results in mirror sizes not exceeding 120x120 mm2. In order to be scalable, all driving electronics necessary to drive all three degrees of freedom of the mirror have to fit within that footprint. Current custom High-Voltage-Transistor process can provide compact driving electronics on this area with maximum breakdown voltages in the order of 70-100V. This puts a severe limitation on the maximum mechanical power that can be achieved in simple, parallel-plate actuators, especially when trying to achieve large range, fast tip-tilt motion. The design solution to this limitation is to explore the third dimension: deep, dense, in-plane comb-drive actuators. Our basic design is presented in Fig.1. It consists of four dual, rotational, in-plane comb drives, each attached to an arm that rotates out of plane. A flexible joint at the end of each arm attaches to the mirror, allowing tip-tilt-piston motion. Typical dimensions for the most relevant polysilicon structural layers are: combs 6-12mm tall with 0.5mm wide fingers, spring / arm layer thickness of 1-2mm with 0.5mm/0.5mm minimum lines and spaces, and a 1-2mm thick mirror. This design has been implemented for 642 and 2562 array sizes, in a 10-mask process using 248nm stepper optical lithography. This structure is monolithically fabricated on a wafer with Through-Wafer-Vias (TWV) and a polysilicon routing layer to provide electrical connection to the driving electronics chip. Fig. 2 shows a SEM picture of the finished device where the mirror has been removed to expose the underlying actuators. A critical step in the fabrication is the silicon oxide filling and planarization of the high-aspect-ratio combs, achieved by a controlled etch-back and re-deposition step. Metalized mirror front topography shows peak-to-peak amplitude of 50nm, with RMS roughness of 8nm including the strut print-through and less than 2nm without it. Results presented here were obtained in a hard-wired system, with an extra wiring layer replacing the TWVs and grouping the mirrors in clusters of up to 64 elements. Fig. 3 shows tip-tilt range of 3 to 4 degrees and piston motion of the order of 3mm, all under 65V. Small-signal resonance curves show damping-limited response with roll-offs at around 10-15KHz at atmospheric pressure, and natural frequencies at around 20KHz. In this platform, simulations show that we can achieve the full spec of 10ms response time by increasing the spring stiffness (spring layer ~2mm) and increasing the actuator comb force by fabricating deeper combs of 12mm, both well within our fabrication capabilities. Note that current systems are limited to about 32x32 arrays with piston-only motion at about 10KHz [1]. We have presented an integration-ready, scalable SLM mirror array. This work demonstrates a degree of complexity never achieved before in terms of number of degrees of freedom per chip, speed, motion range and optical quality combined.