A Low-Power, High Performance MEMS-based Switch Fabric

Abstract

An approach with the potential for building large low power high performance crossbar networks is presented. Thin film polysilicon MEMS devices are developed to provide crosspoints. These devices are vertically moving plates that serve as variable capacitors. Addressing of large arrays using 2n rather than n-squared lines despite no active circuitry on the MEMS chips is facilitated by bistable device operation. Derivations of equations for bistable device operation are presented. Low power operation is possible as the devices are electrostatically controlled and are stationary except during reconfiguration. Early devices are fabricated using the MUMPS process. The bistability and array addressability properties are demonstrated. The substrate effect on device operation is measured and modeled; methods for utilizing the substrate effect to tune device operation are presented. Later devices are fabricated using the SUMMiT process. Changes in the SUMMiT design rules to increase allowable vertical motion range are proposed and designs using them fabricated. S-parameter characteristics of devices in both 'on' and `off' states are measured. Addition of metallization after chip fabrication and release is necessary to lower the resistance of interconnect. A self masking method for applying this metallization allowing for decreased resistance at line crossings is proposed. This method is tested using each of sputtering and evaporation as the deposition technique for a gold and adhesion layer stack. Effectiveness of the method with each technique is evaluated. Chips suitable for providing high voltage control for large MEMS arrays are fabricated in a 2um feature size CMOS process. Architectures suitable for building large crossbars employing variable capacitor arrays are discussed. Optimization of hybrid CMOS/MEMS Clos arrays on the basis of criteria other than minimization of crosspoints is discussed. Array sizings to provide 192*192 and 256*256 crossbars are presented, and software examples for sizing and controlling Clos networks are provided. Evaluation of the suitability of the MEMS devices developed for use as digital or broadband crosspoints is evaluated, and potential future directions are proposed.