LIGA-micromachined tight microwave couplers

There are a significant number of microwave applications, including active antenna arrays, wireless communication systems, navigational applications, etc., where improvement of such qualities as manufacturing costs, size, weight, power consumption, etc. is still on the agenda of today’s RF design. In order to meet these requirements, new technologies must be actively involved in fabrication of RF components with improved characteristics. One of such fabrication technologies is called LIGA, used before primarily in fluidics, photonics, bioengineering, and micromechanics, and only recently receiving growing attention in RF component fabrication. One of the RF components suffering limitations in performance due to limitations in fabrication capabilities is the compact single metal layer (SML) coupled-line 3-dB coupler, also called “hybrid”, required in some applications thanks to its ability to divide power equally and electrically isolate the output from the input. In today’s practical edge-coupled SML coupler designs, the level of coupling is limited by the capabilities of the photolithographic process to print the coupled lines close enough for tight coupling and it is usually no tighter that 8 dB. A promising way to overcome this limitation is increasing the area of metallic interface of the coupled lines, thus increasing the mutual capacitance of the lines, and inherently the coupling between them. This should be preferably done with keeping the coupler compact with respect to the footprint area, which is attained by making taller conductors, i.e. employing the third dimension. In contrast with previously used RF component fabrication processes, LIGA is the technology that allows the designer to explore the third dimension and build tall conductors while being also able to use small features. When the two-dimensional edge-coupled SML couplers are extended into the three-dimensional structures, they rather become the side-coupled SML couplers. Tall-conductor coupled lines have been characterized in this work to reveal their dependence on their geometry and a 3-dB SML coupler with tall conductors has been developed and fabricated using LIGA at the Institute for Microstructure Technology (IMT), Karlsruhe, Germany. The simulation and measurement results demonstrate the potentially superior performance of LIGA couplers, and the promising capabilities of LIGA for fabrication of RF microstructures

High aspect ratio microstructure coupler

Couplers are one of the most frequently used passive devices in microwave circuitry. The main function of a coupler is to divide (or combine) a radio frequency signal into (from) two separate signals by a specific ratio and phase difference. With the need for smaller electronic devices, a reduction in the area of a distributed coupler would prove to be valuable. The purpose of this research is to develop, simulate, fabricate and test high aspect ratio microstructure couplers that are smaller in area than existing distributed couplers, and have comparable or better performance. One method used to reduce the area of a distributed coupler is to replace single or multiple transmission lines with lumped element equivalent circuits. One category of lumped elements that has not been extensively implemented is high aspect ratio lumped elements. High aspect ratio lumped elements fabricated with deep X-ray lithography are able to take advantage of using the vertical dimension, and reduce their planar area. In this thesis high aspect ratio lumped elements are used in the design of 3-dB microstructure couplers that show significant area reduction compared to equivalent distributed couplers. The designs of the microstructure couplers were based on the lumped element equivalent circuits of a 3-dB branch-line and a 3-dB rat-race distributed coupler. Simulations were performed to determine the lumped element values that would provide the largest 3-dB bandwidth while still maintaining close to ideal coupling and through values, return loss bandwidth, isolation bandwidth, and phase. These lumped element values were then implemented in the microstructure coupler designs as high aspect ratio microstructure lumped elements. 3-D electromagnetic simulations were performed which verified that the structures behaved electrically as couplers. The microstructure couplers were designed to be 220 µm tall nickel structures with capacitance gap widths of 6 µm. Fabrication of the microstructure couplers using deep X-ray lithography was performed by the microfabrication group at IMT/KIT in Karlsruhe, Germany. Before testing, detailed visual inspection and the etching of the structures was performed at the Canadian Light Source. A total of five microstructure couplers were tested. Four of the tested couplers were based on the 3-dB branch-line coupler, and the fifth coupler was based on the 3-dB rat-race coupler. The microstructure branch-line design that had the best overall results was fabricated on quartz glass substrate and had an operation frequency of 5.3 GHz. The 3-dB bandwidth of the coupler was measured to be better than 75.5% and extrapolated to be 95.0%. At the centre frequency the through and coupled values were -4.32 dB and -4.44 dB. The phase difference between the couplers output ports was designed to be 90.0° and was measured to be 95.8°. The ±5° phase bandwidth was measured to be 12.7% and the isolation bandwidth was 28.8%. The measured results from the other couplers were comparable to simulation results. The main advantage of the microstructure coupler designs over existing distributed couplers is that the microstructure couplers show a significant area reduction. The branch-line microstructure designs were at least 85% smaller in area than their distributed equivalent on quartz glass. The rat-race microstructure design showed an area reduction of 90% when compared to its distributed equivalent on quartz glass

Millimeter Wave Traveling Wave Tubes for the 21st Century

Traveling wave tubes are rapidly evolving to provide unprecedented power level in comparison to solid state devices in the millimeter waves region of the spectrum (80 – 300 GHz) thus enabling a wide range of applications. Wireless communications, imaging, security, plasma diagnostics, healthcare and many others will gain substantial features if high power in the millimeter wave region would be available from compact sources. The development of fabrication technologies is proving crucial for introducing new topologies and structures for millimeter wave vacuum electronic devices, compatible with the dimensions dictated by the short wavelength that poses substantial challenges due to tight tolerances and surface quality. This review paper will provide an overview of the principles, evolution and state of the art of one of the most widely utilized vacuum electronic devices, the traveling wave tube (TWT). The wide band, high gain features of TWTs make those devices the most promising solutions for high power at millimeter waves and THz frequencies

Wafer-To-Wafer Alignment Method

A silicon alignment pin is used to align successive layer of component made in semiconductor chips and/or metallic components to make easier the assembly of devices having a layered structure. The pin is made as a compressible structure which can be squeezed to reduce its outer diameter, have one end fit into a corresponding alignment pocket or cavity defined in a layer of material to be assembled into a layered structure, and then allowed to expand to produce an interference fit with the cavity. The other end can then be inserted into a corresponding cavity defined in a surface of a second layer of material that mates with the first layer. The two layers are in registry when the pin is mated to both. Multiple layers can be assembled to create a multilayer structure. Examples of such devices are presented

AN INVESTIGATION INTO QUASI-TUNABLE RF PASSIVE CIRCUIT DESIGN

Modern wireless electronic circuit design is continually challenged by the needs to reduce circuit size, and to also function reliably with lower power levels. To that end, two aspects of RFIC circuit design and technology have gained great interest, i.e. RF MEMS switching technology, and RF MEMS passive component development. MEMS (Micro-Electromechanical Systems) technology, originally developed for the defense industry, has been in development since the 1970s, and today enjoys wide range of utilization, from the defense industry to the automotive industry. Spiral inductors used in RFIC circuits, e.g. silicon technology, are ubiquitous in wireless RFIC applications. The tradeoff with low cost fabrication processes are inductors with very low quality factors which greatly affect the losses in RF passive circuits, and hence their performance. Research in the area of RF MEMS inductors has shown promise for components with significantly higher Q, and hence has the potential for wide range of benefits in both tunable and non-tunable applications. Electronic design environments such as Agilent ADS provide automated tools for generating passive circuits, e.g. band-pass filters, based on a specified desired frequency response and circuit topology. However, they typically do not incorporate component Q, which can greatly affect the actual circuit’s performance, into the results of their suggested designs. With this in mind, the development of a systematic approach to predict the relationship between passive circuit component’s Q and its S-parameters can be of great benefit to the RF electronic circuit designer, especially in the area of wireless passive circuits. The first of part of this work develops an analytical approach, using mesh-current analysis to derive the relationship between inductor Q, and the S-parameters of a generalized passive RF circuit. For the analysis, the S-parameters of a 90ᵒ Lumped Element Hybrid coupler are derived in terms of even mode and odd mode coupler responses using mathematical functions that relate the S-parameters of each circuit to their associated even mode Q, and odd mode Q factors The results of this research demonstrate that work can still be done in the area of circuit analysis to extend the capability of common passive circuit design tools to include the effects of component Q on the design results, e.g. filter design tools which commonly utilize simple LC circuits as building blocks for more complicated filters. The second part of this work investigates the performance of different RF switching technologies, i.e. MEMS Switching vs. RF PIN Diode, to a 2-3 GHz quasi-tunable RFIC 90ᵒ Lumped Element Hybrid Coupler design utilizing high Q three-dimensional air-core solenoidal MEMS inductors, and IPD Capacitors. The results of this investigation demonstrated the following: The concept of a tunable RFIC Lumped Element Hybrid coupler in the 2-3 GHz range is feasible, and if implement with high Q inductors, comparable to that of off-the-shelf 90° Hybrid Couplers in terms of return loss and isolation performance, but in a much small area, ~ one fiftieth of the surface area at 2 GHz. RF PIN Diodes at low current levels can be sufficient when only the phase imbalance of the coupler is critical. If either magnitude loss or magnitude balance is critical, then RF MEMS switching may provide a better alternative. RF PIN Diode forward bias resistance approaches that of DC contact switch resistance at higher current levels, e.g. 60 mA, and hence their power consumption becomes the main issue in determining the technology best suited for this application. The concept of a ground switched tapped capacitor bank was developed to maximize the switched capacitor Q. This approach optimized the coupler performance compared to a signal switched design. In the third part of this work, a selectable dual-band 630 MHz and 900 MHz PCB lumped element hybrid coupler is designed, fabricated, and measured. The inductors and capacitors are fabricated with only printed conductors and metal patches respectively on a four-layer PCB. The S-parameters of the measured results and simulations correlated extremely well after adjustment of the substrate dielectric thicknesses used for the simulation of the capacitors. This work demonstrates that lumped element passive components can be cheaply fabricated in PCB technology that are useful in the frequency range of 600 MHZ to 1300 MHZ, partially covering the GSM and LTE bands, that can be used in quasi-tunable wireless PCB applications, e.g. base stations, while also reducing circuit size in place of commonly found in microstrip distributed circuits

Exciting the Low Permittivity Dielectric Resonator Antenna Using Tall Microstrip Line Feeding Structure and Applications

The development of wireless communications increases the challenges on antenna performance to improve the capability of the whole system. New fabrication technologies are emerging that not only can improve the performance of components but also provide more options for materials and geometries. One of the advanced technologies, referred to as deep X-ray lithography (XRL), can improve the performance of RF components while providing interesting opportunities for fabrication. Since this fabrication technology enables the objects of high aspect ratio (tall) structure with high accuracy, it offers RF/microwave components some unique advantages, such as higher coupling energy and compacted size. The research presented in that thesis investigates the properties of deep XRL fabricated tall microstrip transmission line and describes some important features such as characteristic impedance, attenuation, and electromagnetic field distribution. Furthermore, since most of traditional feeding structure cannot supply enough coupling energy to excite the low permittivity DRA element (εr≤10), three novel feeding schemes composed by tall microstrip line on exciting dielectric resonator antennas (DRA) with low permittivity are proposed and analyzed in this research. Both simulation and experimental measured results exhibit excellent performance. Additionally, a new simulation approach to realize Dolph-Chebyshev linear series-fed DRA arrays by using the advantages of tall microstrip line feeding structure is proposed. By using a novel T shape feeding scheme, the array exhibits wide band operation due to the low permittivity (εr=5) DRA elements and good radiation pattern due to the novel feeding structure. The tall metal transmission line feed structure and the polymer-based DRA elements could be fabricated in a common process by the deep XRL technology. This thesis firstly illustrates properties and knowledge for both DRA element and the tall transmission line. Then the three novel feeding schemes by using the tall transmission line on exciting the low permittivity DRA are proposed and one of the feeding structures, side coupling feeding, is analyzed through the simulation and experiments. Finally, the T shape feeding structure is applied into low permittivity linear DRA array design work. A novel method on designing the Dolph-Chebyshev array is proposed making the design work more efficient

Polymer-Based Micromachining for Scalable and Cost-Effective Fabrication of Gap Waveguide Devices Beyond 100 GHz

The terahertz (THz) frequency bands have gained attention over the past few years due to the growing number of applications in fields like communication, healthcare, imaging, and spectroscopy. Above 100 GHz transmission line losses become dominating, and waveguides are typically used for transmission. As the operating frequency approaches higher frequencies, the dimensions of the waveguide-based components continue to decrease. This makes the traditional machine-based (computer numerical control, CNC) fabrication method increasingly challenging in terms of time, cost, and volume production. Micromachining has the potential of addressing the manufacturing issues of THz waveguide components. However, the current microfabrication techniques either suffer from technological immaturity, are time-consuming, or lack sufficient cost-efficiency. A straightforward, fast, and low-cost fabrication method that can offer batch fabrication of waveguide components operating at THz frequency range is needed to address the requirements.A gap waveguide is a planar waveguide technology which does not suffer from the dielectric loss of planar waveguides, and which does not require any electrical connections between the metal walls. It therefore offers competitive loss performance together with providing several benefits in terms of assembly and integration of active components. This thesis demonstrates the realization of gap waveguide components operating above 100 GHz, in a low-cost and time-efficient way employing the development of new polymer-based fabrication methods.A template-based injection molding process has been designed to realize a high gain antenna operating at D band (110 - 170 GHz). The injection molding of OSTEMER is an uncomplicated and fast device fabrication method. In the proposed method, the time-consuming and complicated parts need to be fabricated only once and can later be reused.A dry film photoresist-based method is also presented for the fabrication of waveguide components operating above 100 GHz. Dry film photoresist offers rapid fabrication of waveguide components without using complex and advanced machinery. For the integration of active circuits and passive waveguides section a straightforward solution has been demonstrated. By utilizing dry film photoresist, a periodic metal pin array has been fabricated and incorporated in a waveguide to microstrip transition that can be an effective and low-cost way of integrating MMIC of arbitrary size to waveguide blocks

LIGA cavity resonators and filters for microwave and millimetre-wave applications

High performance microwave cavities for various circuits in the front-end of transceivers such as filters, diplexers, and oscillators have conventionally been built with rectangular or cylindrical metallic waveguides, which typically have low loss, high quality (Q) factor, and higher power handling capability. However such waveguide cavity based circuits made by traditional metal machining techniques tend to be costly, particularly for complex multiple cavity based circuits, and not well suited to high volume commercial applications and integration with planar microwave integrated circuits. As commercial transceiver applications progress toward higher microwave and millimetre-wave frequencies, the use of waveguide based circuits for compact, highly integrated transceivers is becoming feasible, along with an increasing need for cost effective batch fabrication processes for realizing complex metallic cavity circuits without sacrificing structural quality and performance. It is expected that significant advancements in both microwave performance and integration will be achieved through the development of novel technologies for realizing vertically oriented three-dimensional (3-D) structures.Although improvement has been made on increasing the resonator Q factor by exploiting silicon micromachining and low-temperature cofired ceramics (LTCC) techniques, there are some drawbacks inherent to silicon cavity micromachining and LTCC technology, including non-vertical sidewalls, depth limitations, and surface roughness for the silicon resonator, and dielectric and radiation loss for LTCC resonator.Polymer-based fabrication is a promising alternative to silicon etching and LTCC technologies for the batch fabrication of ultra-deep microwave cavity structures. In particular, deep X-ray lithography (XRL), as part of the LIGA process, is a microfabrication technology for precisely structuring polymers, and is increasingly being applied to RF/microwave microstructures. In addition to precise patterning capabilities, deep XRL is able to structure ultra-deep cavities due to the penetration ability of hard X-rays. Cavities of several millimetres are possible in a single lithographic exposure, and with excellent sidewall quality, including verticality near 90 degrees and surface roughness on the order of tens of nanometres. These structured polymers are subsequently used as electroforming templates for fabricating metal structures with correspondingly good sidewall quality.This thesis investigates the possibility of realizing high-Q cavity resonators and filters at microwave frequencies using the LIGA microfabrication process. Finite element method (FEM) electromagnetic simulation results based on the cavity models representing different fabrication conditions show that smooth LIGA cavity structures result in promising Q improvement over silicon and LTCC structures. And the potential advantages of LIGA resonators are more dramatic with cavity height and increasing operating frequency. Deep polymer cavity structures (1.8 mm) fabricated using deep XRL demonstrate excellent sidewall verticality in the PMMA structure, with only slight shrinkage at the top surface of 8.5 2.5 mm in either lateral dimensions. This corresponds to sidewalls with verticality between 89.82o and 89.9o. The structure polymers are subsequently used as templates for metal electroforming to produce cavity resonators. The performance of the resonator is measured in a planar environment. A RT/duroid6010 soft substrate patterned with coupling structures forms the sixth side, and thus completes the cavity. Despite the rather crude test assembly for the sixth side made by clamping, the measured resonator has a high unloaded Q of 2122.2 85 at the resonant frequency of 24 GHz, indicating that LIGA cavities are especially promising for high performance applications. The relatively simple, single-step lithographic exposure also facilitates extension to more structurally complicated waveguide and multiple cavity-based circuits. This research work also proposes a high performance ``split-post' 3-pole cylindrical post coupled Chebyshev bandpass filter suitable for LIGA fabrication. In addition to potentially batch fabricating such a filter lithographically by exposing the entire waveguide depth in a single exposure, the filter structures composed of three cavities with metallic multi-post coupling would be extremely difficult to fabricate using traditional machining techniques, due to the extremely fine post structure and high vertical aspect ratio required. However, these types of structures could be ideal for LIGA fabrication, which offers sub-micron features, aspect ratios of 100:1 or higher, resist thicknesses of up to 3 mm, and almost vertical and optically smooth sidewalls. Also, representative LIGA sidewall roughness is used to predict very low loss and high performance, suggesting that complicated structures with multiple resonator circuits and high internal components with high aspect ratios are possible