MEMS and Microstructures in Aerospace Applications - Robert Osiander et al (Eds) Part 7 ppt

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Osiander / MEMS and microstructures in Aerospace applications DK3181_c007 Final Proof page 147 1.9.2005 12:04pm Microtechnologies for Science Instrumentation Applications 147 © 2006 by Taylor & Francis Group, LLC 8 Microelectromechanical Systems for Spacecraft Communications Bradley Gilbert Boone and Samara Firebaugh CONTENTS 8.1 Introduction 150 8.2 MEMS RF Switches for Spacecraft Communications Systems 150 8.2.1 MEMS Switch Design and Fabrication 151 8.2.1.1 Switch Configuration 151 8.2.1.2 Contacting Modes 153 8.2.1.3 Actuation Mechanism 154 8.2.1.4 Geometric Design 155 8.2.1.5 Fabrication Methods and Materials 155 8.2.2 RF MEMS Switch Performance and Reliability 156 8.2.2.1 Figures of Merit 156 8.2.2.2 Example Performance 157 8.2.2.3 Failure Modes 157 8.3 MEMS RF Phase Shifters 158 8.3.1 Switched-Line Phase Shifters 158 8.3.2 Loaded-Line Phase Shifters 159 8.3.3 Reflection Phase Shifters 159 8.4 Other RF MEMS Devices 161 8.5 RF MEMS in Antenna Designs 161 8.5.1 Electrically Steered Antennas 161 8.5.2 Fractal Antennas 162 8.6 MEMS Mirrors for Free-Space Optical Communication 163 8.6.1 Fabrication Issues 164 8.6.2 Performance Requirements 166 8.6.3 Performance Testing for Optical Beamsteering 168 8.7 Applications of MEMS to Spacecraft Optical Communications 169 8.7.1 Optical Beam Steering 169 8.7.2 Recent Progress 173 8.8 Conclusion 176 References 176 Osiander / MEMS and microstructures in Aerospace applications DK3181_c008 Final Proof page 149 1.9.2005 12:05pm 149 © 2006 by Taylor & Francis Group, LLC configuration, the conducting bar sits between the signal line and ground. The on state is when the conducting bar is up, so that the signal can pass unimpeded. Researchers have pursued switches in series configurations 15,16,26–30 and shunt configurations. 10,11,17,18 In series-configured switches, the insertion loss is deter- mined by the impedance of the switch in its closed state, which in turn depends on the intimacy of the contact achieved by the switch. The isolation is set by the capacitance between the conducting bar and the signal line in the off state. Series switches can be implemented with both microstrip and coplanar waveguide transmission lines. 15,31–33 Figure 8.2 shows a series switch developed at RSC. In a shunt switch, the insertion loss is the result of any impedance mismatch that occurs because of the unactuated mechanical structure (with careful calculations, the unactuated switch can be sized to match the characteristic impedance of the line), and the isolation depends on ratio between the capacitance in the ‘‘down’’ state and the capacitance in the ‘‘up’’ state. Shunt switches are only easily imple- mented with coplanar waveguide transmission lines. 10,17 Figure 8.3 shows a scan- ning electron micrograph of a shunt switch. The impedance ofa capacitor decreases with frequency. Therefore, theisolation of a series switch diminishes with frequency, while in a shunt switch that relies on a V in V in V out V out V control V control Series Configuration Shunt Configuration FIGURE 8.1 Different configurations for microwave switches. Cross section through bridge Biased - ON Unbiased - OFF Spring Drive capacitor RF lineRF line Anchor Contact shunt FIGURE 8.2 Structure and operation of a MEM series switch developed by the Rockwell Science Center. (Courtesy of the Rockwell Science Center and from Mihailovich, R. E., et al.) Osiander / MEMS and microstructures in Aerospace applications DK3181_c008 Final Proof page 152 1.9.2005 12:05pm 152 MEMS and Microstructures in Aerospace Applications © 2006 by Taylor & Francis Group, LLC element, easily implemented in microstrip, which separates the input signal into two signals that are 908 out of phase. The two switches are tied together. If the switches are closed, the signal is reflected back into the quadrature hybrid, where the two reflected waves will add constructively at one port and destructively at another port. If the switches are open, a total phase shift of Df will be added to the signal. If the switches are perfectly matched and lossless, and the quadrature hybrid is lossless, these phase shifters should have little insertion loss. Like the switched-line phase shifter, several bits with a binary sequence of phase delays can be combined for digital phase control. In a MEMS implementation of a reflection phase shifter, MEMS switches control the reflection stub length. There are fewer MEM reflection phase shifters FIGURE 8.5 Photograph of a 2-bit switched-line phase shifter developed by the University of Michigan and Rockwell Scientific. (Courtesy of Rockwell Scientific Company.) Signal ou t Signal in FIGURE 8.6 Schematic of a loaded-line phase shifter. Varying the capacitance alters the phase shift between the input and output. Osiander / MEMS and microstructures in Aerospace applications DK3181_c008 Final Proof page 160 1.9.2005 12:05pm 160 MEMS and Microstructures in Aerospace Applications © 2006 by Taylor & Francis Group, LLC deposition by low-pressure chemical vapor deposition (LPCVD), followed by patterning and etching, to create the desired structures on the silicon substrate. Significant progress has been made in manufacturing commercial-quality mirrors using these methods. Stress-free optical thin film surfaces are critical for optical networking as well as free-space beamsteering applications, but film stress is difficult to control in the fabrication process. It can vary dramatically with a relatively small change in the number of atoms, and hence, the film’s chemical composition. As a consequence, it is difficult to make polysilicon mirrors very flat, particularly if they need to be relatively large (~few millimeters). After a surface is initially deposited and all the supporting layers are removed, it may not remain flat. Even thin gold over-coatings can cause substantial deformation of an uncoated plate. Bulk micromachining is used to form MEMS microstructures by either wet or dry anisotropic etching. In this case silicon on insulator (SOI) wafers are useful, especially in separating moving parts from the bulk silicon structure, and this was determined early. When a plate-type structure is freed in the fabrication process, a mirror can be produced on either side, with that surface in contact with the oxide often being superior in terms of scattering properties. The availability of both sides allows the deposition of perfectly stress-balanced gold reflection layers for en- hanced reflectivity, which makes manufacturing easier and more predictable. Leading candidates for optical switches and cross-connects are free-space micromirror switch arrays, and a scheme to do this using conventional scanning mirrors was first proposed as early as 1982. 80 Arrays of collimators are positioned such that light from each collimator is directed toward a dual-gimbaled mirror. The first mirror reflects the beam toward a corresponding mirror in the opposing array. The latter mirrors adjust their angles to send their respective beams to each receiving fiber. Light from each fiber can only be directed toward its corre- sponding mirror at a given instant. Likewise, the receiving mirror can only send light to its associated fiber, but both mirror arrays can be virtually infinitesimally adjusted, so that any mirror that receives a beam can send it to any of the opposing mirrors, thereby making fully free connections. The supporting parts of each mirror, such as the hinges and drive structures, are kept small to maximize mirror area fill factors. For low-loss transmission the mirrors must be very flat, with flatness better than one fifth the operating wavelength. Mirrors with gold coatings can have reflectiv- ities over 98%, and mirror arrays can be several square millimeters in size, with square or rectangular aspect ratios. Fiber-to-fiber losses through the cross-connect can be as low as 0.7 dB, and mirrors have been exercised over 60 billion cycles without any failures. Cross-coupling between the various channels also turns out to be negligible because even a small amount of angular offset between the input and output mirrors will cause a significant displacement of the inappropriate beam at a given output fiber entrance. Small-scale cross-connects with fewer optical switches have switching times as low as 50 ms or less, although larger N Â N switches, configured into 2-D crossbar arrays, have switching times on the order of 500 m s. Osiander / MEMS and microstructures in Aerospace applications DK3181_c008 Final Proof page 165 1.9.2005 12:05pm Microelectromechanical Systems for Spacecraft Communications 165 © 2006 by Taylor & Francis Group, LLC The two most basic requirements, FOR and angular accuracy, depend upon the required link range and terminal separation on the ground, as illustrated in Figure 8.11. For instance, for optical communication terminals down-linking to earth from GEO, beam widths on the order of 5 to 10 mrad are desired to support the link with reasonable laser transmitter powers (at hundreds of milliwatts), but their steered angular coverage will be limited to angles set by the dynamic limits of the MEMS mirrors and the optical transmitter beam expander design (assuming coarse steering via spacecraft attitude control). The laser beam reflecting from a given micromirror, however, must be significantly expanded to set the desired output (diffraction-limited) beamwidth to meet link margin requirements through the optical ‘‘antenna gain.’’ The mirrors need to be physically steered to a greater angle than the output optical beam, given by the beam expansion ratio. For example, a beam expansion ratio of 250 increases the transmitter beam waist (which is assumed to be 0.5 mm at the micromirror) up to 12.5 cm, which yields a diffraction-limited beamwidth of approximately 8 mrad. Assuming that the micromirrors peak steering range is 420 mrad (+ 128) before beam expansion, then the peak-to-peak output optical beam steering range would be approximately FOR Coverage footprint Ground terminals Multiple or sequential beam positions GEO S/C Beam jitter Beamwidth FIGURE 8.11 GEO-to-ground scenario for applicability of MEMS micromirrors to multi- channel optical communications. The same terminal could support intersatellite links. Osiander / MEMS and microstructures in Aerospace applications DK3181_c008 Final Proof page 167 1.9.2005 12:05pm Microelectromechanical Systems for Spacecraft Communications 167 © 2006 by Taylor & Francis Group, LLC . M.D. et al. , Single-crystal silicon triple-torsional micro-oscillators for use in magnetic resonance force microscopy, Proceedings of SPIE 4559, 24, 2001. Osiander / MEMS and microstructures in Aerospace. measurement device, thermal noise and spin relaxation, Proceed- ings of SPIE 5 276 , 250, 2004. Osiander / MEMS and microstructures in Aerospace applications DK3181_c0 07 Final Proof page 1 47 1.9.2005 12:04pm Microtechnologies. Center and from Mihailovich, R. E., et al. ) Osiander / MEMS and microstructures in Aerospace applications DK3181_c008 Final Proof page 152 1.9.2005 12:05pm 152 MEMS and Microstructures in Aerospace