One problem faced by designers utilizing polysilicon based surface micromaching processes is the poor conductivity of polysilicon. Process factors preclude inclusion of metal layers in these processes before the final polysilicon layer is annealed. Adding metal after anneal but before release restricts the metal to only the top layer of the design, making it much less useful for interconnect, and restricting reflective surfaces to the top layer. We present techniques for adding metal after release which avoid some of the usual pitfalls. Application areas for which these techniques could prove useful include RF, Microwave, Optical MEMS, and MEMS devices used in high-speed digital communications. Creating a multilayer metal interconnect is enabled by utilizing a self-masking approach to avoid shorting, and applying e-beam evaporation from a variety of angles. Using this approach, even lower level polysilicon lines can be metallized. Results using two deposition angle recipes on test structures and devices fabricated in a thin film MEMS process are presented.
With the recent surge in popularity of RF and Microwave MEMS many different device topologies are being explored. Some devices provide large changes in capacitance, but lack the ability to provide a linear range of capacitance values between the minimum and maximum values of the device. We present a device design for a low-loss rotating MEMS tunable capacitor that once programmed to the required value consumes no power. This device design is transformed from gear structures currently designed in the SUMMiT process with modifications made so that the device may be used as a varactor. Modifications include alterations of physical structure, drive mechanism for programming capacitance value, and additional post processing steps needed to provide low-loss at RF and Microwave frequencies. Many different device structures are possible each with performance, potential reliability, and potential yield trade offs that must be considered. Post processing is required to add metal to provide sufficiently low loss for high quality components. Since device planarity is critical for operation, a novel post-process metal deposition technique for providing low stress metal was concieved. Additional modifications to compensate for polysilicon warpage are considered for future investigation. Simulation results based on high frequency full wave analysis software show a highly linear tuning range and a capacitance ratio approaching 6 to 1. A model is extracted from the scattering parameters provided by HFSS and then various device sizes and topologies are compared.
We describe a programmable capacitor technology under development at NCSU and its potential application in building programmable interconnect devices useful for system level connectivity functions, phased array beam steering, and RF switching. Crossbars are made from arrays of electrostatically controlled bistable MEMS-based capacitors. These new devices allow faster signaling and consume less power than BiCMOS crossbars. They also allow critical RF components to be shrinked in size substantially. We describe the essential elements of these arrays and present results obtained so far.
This paper presents some results from phase-1 research into developing a beam steerer based on micro-mechanical diffractive elements. The position of these elements is electrostatically controlled, to allow dynamic programming of a 2D phase function. Feasibility prototypes were constructed in the MUMPs polysilicon surface micromachine process.
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