Study On Novel Microwave And Terahertz Waveguide Components Based On 3-D Printing And Micromachining

High-performance and multi-functional devices,operating at microwave/millimeterwave to terahertz(THz)bands,are highly demanded in modern radio communication systems to adapt ever-increasing electromagnetic environments.Conventional microwave passive waveguide devices have advantages of a low RF loss and a high power handling capability over the ones based on planar microwave transmission lines(e.g.,microstrip and coplanar waveguide),and therefore waveguide devices play an important role in these systems particularly the RF front ends.On the other hand,such requirements in the multi-functionality for these waveguide devices result in an increase in their geometrical complexity.Conventionally,microwave waveguide devices are manufactured using a computer-numerical-control(CNC)milling technology,and devices with complex physical structures are usually split into several pieces to facilitate the milling.Moreover,the fabrication cost of the milling increases dramatically when the frequencies go up to THz band.This also demands a high accuracy in both the fabrication and assembly.Furthermore,these microwave waveguide components are usually made of metals such as copper and aluminum,suffering from a high weight of redundant structural materials,as opposed to the trends of developing miniaturized and lightweight communication systems for aerospace and aviation applications.An alternative option of fabrication technology benefiting from a much higher fabrication accuracy for 3-D structures and tiny feature sizes is micromachining.Such process can be carried out based on substrates such as silicon and photoresist(e.g.,SU-8)thick films,and is able to achieve a fabrication accuracy in an order of magnitude of submicron.The micromachining process has been widely applied to the fabrication of high-performance waveguide devices operating at millimeter-wave to THz frequencies.Emerging 3-D printing technology is becoming another effective method to fabricate waveguide devices.This technology enables an easier manufacturing of geometrically complex structures,and a wide variety of materials can be used to print.Nonmetallic materials(e.g.,photosensitive resins,plastics,nylons and ceramics)with a density much lower than that of metal can be used as the building materials,leading to a significant reduction(over 50%)of the weight of the device.This is of great significance to the development of lightweight radio communication systems for aerospace and satellite applications,showing advantages not only in a great reduction in the fabrication cost but also an enhanced design flexibility.This dissertation provides an in-depth investigation on(i)millimeter-wave passive waveguide devices that are fabricated using 3-D printing technology;and(ii)THz waveguide frequency conversion components that are fabricated using CNC milling technology and an SU-8 photoresist-based micromachining process.The waveguide frequency conversion components,involving frequency multipliers and mixers,are core RF modules for a 300-GHz communication system.Details about the study are described in the following sections.Section ?: The terminology “3-D printing” is not for a single fabrication process but the collection of multiple additive manufacturing(AM)technologies and some of them can be used for the fabrication of microwave devices.However,different 3-D printing technologies need to be involved in order to fabricate different types of devices or devices working at different frequencies.Based on the profound research of the existing 3-D printing technologies,the methods for the manufacturing of microwave / millimeter-wave devices by using 3-D printing have been studied and classified into four categories according to how the device is constructed.Among them,the most common method is to print the block of the device using non-metallic material and surface metallization can be applied later to create a conductive layer onto the printed building blocks.In this dissertation,this method is used to fabricate several cavity filters working from 10 GHz to 100 GHz.This includes two X-band bandpass filters based on novel high-quality-factor single-mode and dual-mode spherical cavity resonators(FBW 5% and 3%,respectively),two W-band bandpass filters based on a slotted rectangular waveguide and a compact onplate structure(FBW 11% and 4%).Most of the presented filters demonstrate good agreements between measured and simulated RF performances.The geometric structures for most of the filters are complicated and can hardly be made using conventional fabrication technologies.Benefited by the proposed novel structures as well as the high accuracy of the latest 3-D printing technology,the filters made by 3-D printing demonstrate state-of-art performances.Moreover,compared to their counterparts made from all metal(e.g.,copper),these 3-D printed devices exhibit a significant weight reduction of over 80% due to the use of nonmetallic printing materials,without any penalty of degrading their RF performances.Section ?: The filters in Section I are constructed layer by layer using nonconductive photosensitive resin and employing an ultraviolet laser for defining the shape(such process is called Stereolithography(SLA)).This is followed by a surface metallization process.However,many cured resins suffer from a poorer thermal handling capability as well as a degraded mechanical strength in comparison to conventional metals,which limits applications of such SLA printed devices under high-power and high-temperature operational scenarios.For example,some filters in Section I are fabricated using a resin based polymer Accura Xtreme with a suggested working temperature of no more than 50 °C.They are able to achieve desired filtering responses at room temperature,but applications in high-temperature environments are not possible.Thermal handling capability of resin-based devices can be enhanced by employing ceramic filled resins with working temperatures of usually over 140 °C(e.g.,Somos PerForm).In this dissertation,the X-band dual-mode BPF discussed in Section I is re-printed by using this ceramic filled resin.Thermal handling capability of these 3-D printed filters(printed by ordinary resin and ceramic filled resin,respectively)are experimentally characterized.The experiment quantitatively demonstrates that the ceramic filter is capable of operating at much higher temperatures and has a much lower center frequency shift(?f)with temperature than the one made of ordinary resin,while maintaining its light-weight advantage over the metallic one.However,due to the ceramic powder filled in the resin,the density of the material is inevitability increased by approximately 20%.Section ?: For some of the conventional active circuit designs(such as amplifiers,multipliers and mixers,etc.),impedance matching networks are integrated with the semiconductor devices(e.g.Schottky diodes,HEMT transistors)in planar transmission lines(e.g.microstrip,coplanar line),and this solution can be lossy especially at high frequencies such as millimeter-wave or THz frequency bands due to the low Q property of these planar transmission lines.In order to reduce the losses,a method that performs filtering as well as impedance matching simultaneously in low loss waveguide cavities is proposed in this dissertation.To do this,a novel N+2 coupling matrix incorporation impedance matching is proposed by using the circuit model for the existing N+2 coupling matrix and the source/load impedances of the filter described by this coupling matrix can be arbitrary complex numbers while the filtering function is still retained.With this novel coupling matrix,a filtering matching network based on rectangular waveguide resonators can be constructed and most of the impedance matching networks from the high loss planar circuits can be removed.Section ?:The impedance matching method proposed in Section III is utilized in the design of several Schottky diode based millimeter-wave and THz frequency multipliers and mixers in this dissertation.For these designs,due to the existence of waveguide resonators,the modeling and simulation of all these waveguide structures must be conducted using full-wave simulators such as CST or HFSS.On the other hand,the modeling and simulation for the non-linear semiconductor devices relies on some circuit simulators(such as ADS)with harmonic balance(HB)function.In order to accurately predict the performance of these circuits,the two simulators above(linear full wave and nonlinear HB)must be combined and hence two co-simulation methods are proposed in this dissertation.Among those two,a circuit simulator dominant with full wave simulator assist(by exporting SNP file sets)co-simulation method shows good feasibility as well as high accuracy and efficiency.Based on these aforementioned cosimulation methods,five millimeter-wave and THz frequency multipliers and mixers are designed,and these include a 90 GHz tripler,two 142.5GHz triplers,a 300 GHz subharmonic mixer as well as a 47.5-142.5-300 GHz integrated tripler/mixer.Due to the high Q characteristics of the waveguide resonators,the simulated RF performances of these millimeter-wave and THz circuits are good.The cavities of those frequency conversion circuits are fabricated using CNC milling and SU-8 micromachining.