Micromachined Circuits and Devices

In this Chapter an overview of the radio frequency microelectromechanical systems is given. Focus is oriented towards discussion on different micromachined passive circuits that includes transmission line, varactor, inductor, switch, and switch-based circuits followed by resonators. Fabrication of RF micromachined devices on different technology platforms are also discussed in this Chapter. Applications of RF micromachined devices and components are also discussed with respect to the super-heterodyne radio receiver and electronically steerable antenna for radar application. Finally, this Chapter discusses organisation of the book.

In this Chapter an overview on different classes of passive components that can be realized using radio frequency microelectromechanical systems (RF MEMS) technology is provided. Focus is oriented toward the performance description of RF passive components in micromachining technology, compared to their standard semiconductor counterparts. It includes, micromachined coplanar waveguide transmission line, varactors, inductors, power divider, coupler, and impedance matching tuners. Conductor Backed Coplanar Waveguide (CBCPW) lines on Si micromachined substrate are in use for more than a decade and it is given prime emphasis in this Chapter.

This Chapter presents behavioral analysis of ohmic contact micromachined switches. In this Chapter, electrostatically actuated micromachined switch configurations that can be easily integrated in uniplanar circuits are presented. The design procedure and fabrication process of micromachined switch topologies that can control the propagating modes of multimodal uniplanar structures (based on a coplanar waveguide (CPW)) are be described in detail. Generalized electrical (multimodal) and mechanical models will be presented and applied to the switch design and simulation. The switch-simulated results are compared to measurements, confirming the expected performances. Mostly cantilever types of switches are chosen in this study and analysis procedure is described with lumped model representation. Different types of ohmic-contact switches are discussed in this Chapter with extensive set of measurement stages. It includes switch profile analysis where surface profile and initial deformation are measured using Optical Profilometer, mechanical resonance frequency measurement and characterization using Laser Doppler Vibrometer (LDV), characterization of switch actuation and release voltages using LCR meter under a DC probe station, switching and release time measurements utilizing a proposed set up, linearity and temperature stability analysis and characterizations within a laboratory environments, S-parameter measurements using RF probe station with a Vector Network Analyzer (VNA) followed by the power handling capability and its analysis within a laboratory environments. Switch performances are discussed against each measurement stages. Functionalities of ohmic contact broadband switches are discussed with behavioral analysis. Comparison between simulated versus all measured responses are discussed up to a reasonable extent.

This Chapter presents design, development, and characterization of broadband (1–30 GHz) microelectromechanical systems (MEMS) based electrostatically driven vertical and lateral switching networks. Initially, single switch performances are optimized, and the same switch is used to develop different switching networks starting from single-pole-double-thru (SPDT) to single-pole-fourteen-thru (SP14T) using vertically actuated beams. The vertically actuated switch is designed using three springs to gain the mechanical stability. Later, laterally driven single MEMS switches are designed, fabricated, and tested. Lateral switch is designed with a mechanical springs and stopper to improve the stability without compromising the electromagnetic performances. The lateral switch doesn’t use any dielectric layers. The operation principle of the lateral switch is described in detail. Both switch types (vertical and lateral) are electrostatically driven and implemented on coplanar waveguide transmission line. Switch performances is given importance over a broadband spectrum. The S-parameter performances of all switching networks are described in detail with design guidelines. All experimental results are validated with a circuit analysis and full-wave EM simulation.

This Chapter presents micromachined resonator and its applications in Radio Frequency (RF) circuits. A wide range of topics and prior work related to MEMS resonators are covered to provide readers with a better understanding up to a reasonable extent. Initially, operating principles of the MEMS resonators and their properties are discussed. Primary modes of the resonators are discussed that includes flexural modes, bulk modes, shear modes and torsional modes. It then discusses three different transduction techniques like capacitive, piezoelectric and piezoresistive. Different challenges behind the manufacturing of MEMS resonators are also discussed. Finally, emerging applications of MEMS resonators, namely their use in timing, sensing and radio-frequency systems are described.

A radio frequency micromachined based phase shifter is one of the key components in a modern electronically steerable phased array for satellite communication, radar systems and high precision instrumentation. The phase shifter controls the signal phase to steer the direction of the beam. MEMS technology provides a superior performance in terms of low loss, low power consumption and excellent linearity compared to other technologies. MEMS based digital phase shifters provide a large phase shift and low sensitivity to electrical noise with a high tuning ratio compared to analog versions. This Chapter describes different types of TTD phase shifters utilizing MEMS switches and MEMS varactors. A gold-based surface micromachining process is used to develop different kinds of MEMS phase shifters on alumina (ε_r = 9.8) substrates. All phase shifters are implemented using coplanar waveguide (CPW) transmission lines and actuated by electrostatic actuations. These include the analog type Distributed MEMS Transmission Line (DMTL) phase shifter using push–pull actuation, a 5-bit DMTL phase shifter using MEMS bridge and a fixed capacitor, 5-bit switched line phase shifter using DC-contact MEMS switches and a 2-bit & 5-bit phase shifter using MEMS SP4T and SPDT switches. This Chapter includes details on the design, development, and characterization of MEMS phase shifters. Furthermore, all experimental results are validated with a circuit analysis and full-wave EM simulation.

This Chapter presents radio frequency (RF) micro-electromechanical system (MEMS) based compact, high power and reliable tunable bandpass filter for millimetre wave RF front end. The design is fabricated on 635 µm alumina substrate using surface micromachining process. All functional building blocks of the filter are fabricated and tested independently to ensure the optimum filter performances. Total area of the filter is only 2.3 mm² including bias lines and pads. The proposed filter can be tuned to any frequency between 27–29 GHz, and it demonstrates insertion loss of 1.92 dB and matching of 23.4 dB at 28 GHz with fractional bandwidth of 6.7%. Filter also gives bandwidth tuning of 4.4 GHz with acceptable loss. A complete set of MEMS based tunable bandpass filter design guidelines are given for the readers.

When radio frequency microelectromechanical system (RF MEMS) based switches appeared more than 25 years ago, micromechanics has attracted huge attention for enabling near-ideal microwave devices. Since then, MEMS switches and circuits went through different development stages and are currently proving themselves commercially viable. Micromachining technology has the potential to become an enabling technology for microwave, millimetre wave to even sub-millimetre wave systems. The reliability of MEMS devices is one of the prime challenges to meet present state-of-the-art performances due to occurrence of multiple failure modes after few cycles of operations. The reliability becomes more critical when multiple MEMS switches operate simultaneously like in a higher order switching networks, multi-bit digital phase shifters or in a tunable filters. This Chapter presents detailed experimental studies on these devices considering life cycles up to 1 billion cycles of operation. A complete design guideline for reliable MEMS switches, phase shifter and filters are given for readers.

This Chapter presents different types of micromachined antennas at mm-wave frequencies. The design and fabrication of a Ka-band inset-fed microstrip patch antenna on a suspended 2 µm thick silicon-dioxide membrane using silicon bulk-micromachining technique have been discussed. The complete antenna structure is realized using post - CMOS and post-MMIC compatible metal dielectric deposition techniques. The parametric study to obtain the optimized inset length is also presented. Three different patch antennas integrated with micromachined SPDT and SP9T switches are presented at 60 GHz for different wireless applications at ISM band of 60 GHz. The utilization of SPDT switch in front end transmit/receive module is discussed with experimental investigation. The SP9T based antenna is a preliminary prototype for the proof of concept of antenna sectoring at 60 GHz for ISM applications. A 77 GHz micromachined polarization reconfigurable antenna is discussed, and it has capability to generate all polarization states. This antenna illustrates a measured return loss of >20 dB, −10 dB bandwidth of 10% and 14.3% in LP and CP cases respectively, at 77 GHz. A minimum best value of 1.1 dB axial ratio is accomplished at 77 GHz in circularly polarized state from this antenna. Next design and development of active micromachined antenna is presented. Finally, this Chapter concludes with design guideline for designing and developing micromachined cavity antenna including antenna arrays.

In this Chapter new techniques that enable broader bandwidths and operation at higher frequencies in conventional micromachined switches are presented. Focus is on the use of defective ground structure (DGS) and use of secondary switches to improve MEMS switch performance. Detailed performance analysis of metamaterial inspired MEMS switches are then discussed for application in millimetre wave frequency bands up to about 170 GHz. In addition, new design techniques to improve reliability by taking advantage of Casimir force to reduce stiction is reported. Layout and results of metamaterial inspired capacitive as well as resistive contact micro machined MEMS switches for application in modern electronic circuits and 5G/6G communication systems are also presented.

The Chapter presents future scope in radio frequency (RF) micromachined devices in the THz frequency range. The Chapter is broadly divided into two parts, first part focusses on different metamaterial based micromachined devices at THz frequency range. Major emphasis is given on the micromachined frequency selective surfaces and absorbers. The second part focusses on the micromachined waveguide-based switches, phase shifters and conductor backed coplanar waveguide to rectangular waveguide transition in the THz frequency range. Different state-of-the-art designs are discussed in this Chapter with recent references.