SOI RF-MEMS Based Variable Attenuator for Millimeter-Wave Applications

The most-attractive feature of microelectromechanical systems (MEMS) technology is that it enables the integration of a whole system on a single chip, leading to positive effects on the performance, reliability and cost. MEMS has made it possible to design IC-compatible radio frequency (RF) devices for wireless and satellite communication systems. Recently, with the advent of 5G, there is a huge market pull towards millimeter-wave devices. Variable attenuators are widely employed for adjusting signal levels in high frequency equipment. RF circuits such as automatic gain control amplifiers, broadband vector modulators, full duplex wireless systems, and radar systems are some of the primary applications of variable attenuators. This thesis describes the development of a millimeter-wave RF MEMS-based variable attenuator implemented by monolithically integrating Coplanar Waveguide (CPW) based hybrid couplers with lateral MEMS varactors on a Silicon–on–Insulator (SOI) substrate. The MEMS varactor features a Chevron type electrothermal actuator that controls the lateral movement of a thick plate, allowing precise change in the capacitive loading on a CPW line leading to a change in isolation between input and output. Electrothermal actuators have been employed in the design instead of electrostatic ones because they can generate relatively larger in-line deflection and force within a small footprint. They also provide the advantage of easy integration with other electrical micro-systems on the same chip, since their fabrication process is compatible with general IC fabrication processes. The development of an efficient and reliable actuator has played an important role in the performance of the proposed design of MEMS variable attenuator. A Thermoreflectance (TR) imaging system is used to acquire the surface temperature profiles of the electrothermal actuator employed in the design, so as to study the temperature distribution, displacement and failure analysis of the Chevron actuator. The 60 GHz variable attenuator was developed using a custom fabrication process on an SOI substrate with a device footprint of 3.8 mm x 3.1 mm. The fabrication process has a high yield due to the high-aspect-ratio single-crystal-silicon structures, which are free from warping, pre-deformation and sticking during the wet etching process. The SOI wafer used has a high resistivity (HR) silicon (Si) handle layer that provides an excellent substrate material for RF communication devices at microwave and millimeter wave frequencies. This low-cost fabrication process provides the flexibility to extend this module and implement more complex RF signal conditioning functions. It is thus an appealing candidate for realizing a wide range of reconfigurable RF devices. The measured RF performance of the 60 GHz variable attenuator shows that the device exhibits attenuation levels (|S21|) ranging from 10 dB to 25 dB over a bandwidth of 4 GHz and a return loss of better than 20 dB. The thesis also presents the design and implementation of a MEMS-based impedance tuner on a Silicon-On-Insulator (SOI) substrate. The tuner is comprised of four varactors monolithically integrated with CPW lines. Chevron actuators control the lateral motion of capacitive thick plates used as contactless lateral MEMS varactors, achieving a capacitance range of 0.19 pF to 0.8 pF. The improvement of the Smith chart coverage is achieved by proper choice of the electrical lengths of the CPW lines and precise control of the lateral motion of the capacitive plates. The measured results demonstrate good impedance matching coverage, with an insertion loss of 2.9 dB. The devices presented in this thesis provide repeatable and reliable operation due to their robust, thick-silicon structures. Therefore, they exhibit relatively low residual stress and are free from stiction and micro-welding problems

Reconfigurable Cryogenic Microwave Devices Using Low Temperature Superconducting rf-SQUIDs

In the physical implementation of modern superconducting quantum computing systems, microwaves play a significant role in the control and measurement of qubits. The advances towards the use of microwaves in the quantum signal processing regime have inspired research on a suite of superconducting radio frequency (RF) components such as phase shifters, beam splitters, circulators, isolators, Josephson parametric amplifiers, and kinetic inductance travelling wave amplifiers. Although many room-temperature microwave components are commercially available, but there is a need to develop cryogenic microwave components, considering the challenges involved in the successful delivery of room-temperature generated microwave signals to the qubit chip. For the furtherance of large-scale quantum systems, tremendous efforts are being made to integrate the microwave photon generation, modulation, and routing in the cryogenic conditions adjacent to the qubits. This has created an urgent demand for various superconducting RF components. In order to tune the phase of on-chip coherent microwave sources in cryogenic environment, a compact, low-loss, fast-tunable, wideband phase shifter is needed. Superconducting phase shifters are also employed in the chip-to-chip quantum network communication with microwave photons and have recently found applications in the future secure communication using far-field microwave quantum technologies. This thesis reports the development of radio frequency superconducting quantum interference devices (rf-SQUIDs) based analog phase shifters for such quantum applications. Each rf-SQUID is a superconducting loop shunted by a Josephson junction (JJ) and a long array of rf-SQUIDs is coupled with a low temperature superconductor (LTS) niobium (Nb) microwave transmission line (TL). The inductance of rf-SQUIDs is dependent on the flux threading the loop, which can be precisely controlled by applying a dc current or RF power or a combination of the two. Since the variable inductance of an array of rf-SQUIDs is tightly coupled the TL, they change the inductance of the TL leading to a phase shift. The issue with this design is that the phase shift is achieved at the expense of changing characteristic impedance of the TL. In order to address this issue, a phase shifter using a reflective-type topology is developed. It utilizes a superconducting hybrid coupler monolithically integrated with two tunable reflective loads. An array of rf-SQUIDs is used to achieve inductive tuning in the reflective loads, resulting in a broadband true time delay phase shift. The inductance tuning using rf-SQUIDs is also demonstrated in a superconducting microwave resonator which offers ultra-wide tuning range of 1.24 GHz at the fundamental resonance at 5.6 GHz. Superconducting tunable resonators have vast potential applications in microwave tunable filters, tunable couplers, tunable parametric amplifiers, SQUID multiplexers, and astrophysical detectors. This thesis reports first ever implementation of a power dependent cryogenic power limiter based on rf-SQUIDS. The objective is to develop a LTS power limiter which can provide protection against power levels above -15 dBm and can be monolithically integrated with other components on the superconducting chip. The output power increases linearly with the input power up to -15 dBm, and as the input power level increases beyond -15 dBm, the device offers an increasing attenuation to limit the output RF power to -15 dBm. Apart from the quantum measurement systems, cryogenic power limiters find applications in digital RF receivers. Such digital receivers are rapid single flux quantum (RSFQ) based, which cannot handle high power levels. When controlled by using dc current, this power limiter topology can be used as an analog variable attenuator as demonstrated in this thesis. Microwave circulators have found a prominent role in the qubit readout circuitry and are used in conjunction with Josephson parametric amplifiers. The scheme is based on the parametric modulation of three identical, strongly, and symmetrically coupled resonators. The devices are realized using MIT Lincoln Laboratory SFQ5ee eight-layer niobium-based process which provides a solid technology platform for building superconducting circuits