High-Q Millimeter Wave RF Filters and Multiplexers

Abstract

For a long period of time, millimeter waves (mm-Wave) were considered unsuitable for wireless data transmission due to high attention while propagating in the atmosphere. Over the past few years, due to the vigorous developments of multiple-in-multiple-out (MIMO) antenna technology and semiconductor technology, it is now feasible to have reliable wireless data transmissions using mm-Wave. Traditionally, mobile communication networks operate in the frequency spectrum under 6 GHz. In order to meet the ever-increasing demand for high communication data rate and high-quality multi-media services, the current fifth generation (5G) and the emerging 6G mobile communication systems will start to utilize the mm-Wave spectrum due to its bandwidth advantages, which in turn translates into a high data transmission rate. Millimeter-wave technology is also widely used in radar, imaging, medical therapy, and sensing applications. For those reasons, over the past few years, the interest in mm-Wave spectrum has significantly increased. RF filters are essential components in any communication systems to provide frequency selectivity. As the operating frequency of communication systems is extending to the mm-Wave spectrum, the conductor loss, the dielectric loss, and the radiation loss increase rapidly, which makes it challenging to develop high-Q mm-Wave filters. Three-dimensional (3D) waveguide filter structures exhibit excellent RF performance at mm-Wave frequencies and have been widely employed in high-performance RF systems. Nevertheless, as the operating frequency increases to mm-Wave frequency, the physical sizes of the waveguide filters become miniature in size impeding the use of post-fabricated tuning elements to compensate for the manufacturing tolerances of the traditional machining technologies. The silicon-micromachining technology has the potential to develop very accurate miniature 3D filters. This thesis focuses on the development of high-Q ultra-wideband mm-Wave planar filters using multilayer superconductor technology and 3D filter structures using silicon micromachining technology, making use of recent advances in deep reactive ions etching (DRIE) techniques. This thesis first introduces a new technique for filter design and tuning using the phase of the input impedance (PII) as the design parameter. This novel method is applicable to both narrow and wideband filters. Compared with conventional filter design and tuning methods, this approach requires less computation time and provides a clear step-by-step procedure for identifying the proper inter-resonator coupling and the resonant frequencies of the resonators. In practice, the physical realization of the filter always has a non-ideal I/O port, which can introduce an unexpected unknown transmission line between the physical reference plane and the port of the corresponding inverter in the circuit model. In this thesis, the PII response is used to determine the equivalent electrical length of this unknown transmission line. The validity of the proposed technique is demonstrated through the design of a wideband planar filter with a fractional bandwidth of 72%, the tuning of filters with transmission zeros and the design of a wideband diplexer. The multilayer superconductor technology allows to realize high-Q planar structures with highly miniature physical dimensions. The superconductor digital receivers can directly digitalize RF signals up to very high frequencies, eliminating the need to use mixers and oscillators to convert the RF signals to lower frequencies. This thesis demonstrates the feasibility of an ultra-wide band superconductor mm-Wave continuous triplexer that can be integrated with superconductor analog to digital converter (ADC) on a single niobium chip. A wideband high-Q mm-Wave highly miniature niobium-based superconductor multiplexer realized on an 8-layer niobium process has been developed, fabricated, and tested covering the frequency range 20 GHz - 80 GHz. In addition to monolithic integration of the superconductor multiplexer with the superconductor ADC, the thesis also demonstrates the feasibility of mounting the triplexer chip on a multi-chip-module (MCM) substrate using flip-chip technology interfaced with 1 mm mm-Wave connectors. This thesis also demonstrates using a unique behavior of spiral inductors designed intentionally to have a large parasitic capacitance in the realization of a tunable band reject filter. It is shown that, regardless of the operating frequency, the conductivity of the metal strips forming the inductor has a significant impact on how the spiral inductor behaves as an inductor or a capacitor. The concept is used to demonstrate a band reject filter made from a multilayer niobium circuit operating at 4 Kelvin. Such band reject filters are needed in the front-end of superconductor digital receivers to eliminate interference. Micromachining fabrication processes provide much higher manufacturing accuracy than traditional CNC machining technologies. Moreover, the DRIE silicon micromachining process is more economical for mass production and makes it possible to produce highly accurate 3D waveguide structures. This thesis presents filter designs composing of highly miniature silicon-micromachined ridge waveguide resonators. The proposed filter designs provide highly compact physical size with reasonable high Q values. An ultra-high-Q mm-Wave cavity filter employing a silicon-micromachined barrel-shape cavities operating in TE011 mode has been developed, fabricated and tested. The barrel-shape is proposed to realize a high-Q cavity, while circumventing the spurious issues of the degenerate TM modes that exist in traditional cylindrical-shape cavities. The filter was realized on silicon using DRIE techniques.