On the Trade-Off Between Quality Factor and Tuning Ratio in Tunable High-Frequency Capacitors

A benchmark of tunable and switchable devices at microwave frequencies is presented on the basis of physical limitations to show their potential for reconfigurable cellular applications. Performance limitations are outlined for each given technology focusing on the quality factor (Q) and tuning ratio (eta) as figures of merit. The state of the art in terms of these figures of merit of several tunable and switchable technologies is visualized and discussed. If the performance of these criteria is not met, the application will not be feasible. The quality factor can typically be traded off for tuning ratio. The benchmark of tunable capacitor technologies shows that transistor-switched capacitors, varactor diodes, and ferroelectric varactors perform well at 2 GHz for tuning ratios below 3, with an advantage for GaAs varactor diodes. Planar microelectromechanical capacitive switches have the potential to outperform all other technologies at tuning ratios higher than 8. Capacitors based on tunable dielectrics have the highest miniaturization potential, whereas semiconductor devices benefit from the existing manufacturing infrastructure

Reconfigurable Impedance Matching Networks Based on RF-MEMS and CMOS-MEMS Technologies

Reconfigurable impedance matching networks are an integral part of multiband radio-frequency (RF) transceivers. They are used to compensate for the input/output impedance variations between the different blocks caused by switching the frequency band of operation or by adjusting the output power level. Various tuning techniques have been developed to construct tunable impedance matching networks employing solid-state p-i-n diodes and varactors. At millimeter-wave frequencies, the increased loss due to the low quality factor of the solid-state devices becomes an important issue. Another drawback of the solid-state tuning elements is the increased nonlinearity and noise at higher RF power levels. The objective of the research described in this thesis is to investigate the feasibility of using RF microelectromechanical systems (RF-MEMS) technology to develop reconfigurable impedance matching networks. Different types of tunable impedance matching networks with improved impedance tuning range, power handling capability, and lower insertion loss have been developed. Another objective is to investigate the realization of a fully integrated one-chip solution by integrating MEMS devices in standard processes used for RF integrated circuits (RFICs). A new CMOS-MEMS post-processing technique has been developed that allows the integration of tunable RF MEMS devices with vertical actuation within a CMOS chip. Various types of CMOS-MEMS components used as tuning elements in reconfigurable RF transceivers have been developed. These include tunable parallel-plate capacitors that outperform the available CMOS solid-state varactors in terms of quality factor and linearity. A tunable microwave band-pass filter has been demonstrated by employing the proposed RF MEMS tunable capacitors. For the first time, CMOS-MEMS capacitive type switches for microwave and millimeter-wave applications have been developed using TSMC 0.35-µm CMOS process employing the proposed CMOS-MEMS integration technique. The switch demonstrates an excellent RF performance from 10-20 GHz. Novel MEMS-based reconfigurable impedance matching networks integrated in standard CMOS technologies are also presented. An 8-bit reconfigurable impedance matching network based on the distributed MEMS transmission line (DMTL) concept operating at 13-24 GHz is presented. The network is implemented using standard 0.35-µm CMOS technology and employs a novel suspended slow-wave structure on a silicon substrate. To our knowledge, this is the first implementation of a DMTL tunable MEMS impedance matching network using a standard CMOS technology. A reconfigurable amplifier chip for WLAN applications operating at 5.2 GHz is also designed and implemented. The amplifier achieves maximum power gain under variable load and source impedance conditions by using the integrated RF-MEMS impedance matching networks. This is the first single-chip implementation of a reconfigurable amplifier using high-Q MEMS impedance matching networks. The monolithic CMOS implementation of the proposed RF MEMS impedance matching networks enables the development of future low-cost single-chip RF multiband transceivers with improved performance and functionality

Skin-Effect Self-Heating in Air-Suspended RF MEMS Transmission-Line Structures

Air-suspension of transmission-line structures using microelectromechanical systems (MEMS) technology provides the effective means to suppress substrate losses for radio-frequency (RF) signals. However, heating of these lines augmented by skin effects can be a major concern for RF MEMS reliability. To understand this phenomenon, a thermal energy transport model is developed in a simple analytical form. The model accounts for skin effects that cause Joule heating to be localized near the surface of the RF transmission line. Here, the model is validated through experimental data by measuring the temperature rise in an air-suspended MEMS coplanar waveguide (CPW). For this measurement, a new experimental methodology is also developed allowing direct current (dc) electrical resistance thermometry to be adopted in an RF setup. The modeling and experimental work presented in this paper allow us to provide design rules for preventing thermal and structural failures unique to the RF operation of suspended MEMS transmission-line components. For example, increasing the thickness from 1 to 3 mum for a typical transmission line design enhances power handling from 5 to 125 W at 20 GHz, 3.3 to 80 W at 50 GHz, and 2.3 to 56 W at 100 GHz (a 25-fold increase in RF power handling)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87277/4/Saitou15.pd

Compact, Wideband, Low-dispersion, Multi-bit MEMS Phase Shifters

Low-dispersion phase shifters are key components for electrically large phased-array radar and communication systems. Unlike true-time-delay phase shifters with linear dispersion, low-dispersion phase shifters can be designed by switching between right-handed (low-pass) and left-handed (high-pass) states to achieve a constant phase shift over a wide bandwidth. However, the implementation of low-dispersion phase shifters with MEMS switches has been challenging. The designs to date suffer from either high insertion loss or high dispersion. Most important, they all occupy a large area and use a large number of MEMS switches, which negatively impact the yield and reliability, especially in view of the relatively immature RF MEMS technology.This dissertation studies design, implementation, characterization and modeling of novel metamaterial-based low-dispersion multi-bit phase shifters that use single-pole-single-throw MEMS capacitive switches to switch between right-handed and left-handed states for a specified phase shift. Three-dimensional finite-element electromagnetic simulation was used to design the basic unit cells. Each phase shifter unit cell is based on a coplanar slow-wave structure with defected ground and uses two MEMS switches in series and parallel configurations. In this dissertation, for the first time, we enhanced the maximum achievable phase shift of metamaterial-based MEMS phase shifter unit cell from 45° to 180°.Thanks to our novel 180° unit cell design, for the first time, the number of required MEMS switches for multi-bit phase shifter was reduced to two times of bits count such that a 3-bit phase shifter requires only six MEMS switches. For 2-bit and 3-bit phase shifters fabricated on a 600-µm-thick sapphire substrate, a relatively flat phase shift was obtained across the band of 21.5‒24.5 GHz with a root-mean-square phase error of less than 14°. Across the same frequency band, presented 2-bit and 3-bit phase shifters have less than 2.7 dB and 3.4 dB insertion loss, respectively.Accurate modeling and electromagnetic simulations were performed to characterize the insertion loss of the presented phase shifters. The loss is mainly due to replacing gold for copper during fabrication as well as having lossy substrate. Furthermore, there is extra mismatch loss associated with the non-flat membrane as well as radiation loss. This can be further reduced by optimizing the MEMS switch and the coplanar waveguide. The present design principle appears to be sound and can lead to phase shifters with high performance, yield and reliability with low cost for electrically large phased-array antennas

Nanodevices for Microwave and Millimeter Wave Applications

The microwave and millimeter wave frequency range is nowadays widely exploited in a large variety of fields including (wireless) communications, security, radar, spectroscopy, but also astronomy and biomedical, to name a few. This Special Issue focuses on the interaction between the nanoscale dimensions and centimeter to millimeter wavelengths. This interaction has been proven to be efficient for the design and fabrication of devices showing enhanced performance. Novel contributions are welcome in the field of devices based on nanoscaled geometries and materials. Applications cover, but not are limited to, electronics, sensors, signal processing, imaging and metrology, all exploiting nanoscale/nanotechnology at microwave and millimeter waves. Contributions can take the form of short communications, regular or review papers