Xiaobang Shang

WR-3 Band Waveguides and Filters Fabricated Using SU8 Photoresist Micromachining Technology

This paper demonstrates a two-layer SU8 photoresist micromachining technology that has similar performance to conventionally machined metal. The technology is demonstrated in the WR-3 band (220-325 GHz). Three different WR-3 band circuits, namely a WR-3 band straight through waveguide, a bandpass filter and a dual-band filter are demonstrated. For the measurements, a conventionally precision machined metal block was used for the WR-3 band waveguide and the bandpass filter to achieve good calibration and accurate interconnection with standard waveguide flanges; whereas, for the dual-band filter, two back-to-back right-angle bends are added in order to achieve accurate, reliable waveguide interconnection without using the metal block. A measured average insertion loss of 0.03 dB/mm has been achieved for the 14.97 mm long straight through waveguide. This is comparable to the loss of around 0.02 dB/mm for a standard metal waveguide at this frequency. The fifth-order waveguide filter exhibits an 8% 3 dB bandwidth at a central frequency of around 300 GHz. The minimum passband insertion loss was measured to be around 1 dB and the return loss was better than 10 dB throughout the passband. The filter results showed a notable improvement over those obtained from the separate SU8 layer technique that was also used to make the same devices for comparison. To further demonstrate the advantages of the new two-layer SU8 micromachining technique, the dual-band filter included isolated regions in the waveguide channels that would have not been possible for micromachining using the previous separate single layer technique. The performance of the micromachined dual band filter was excellent in terms of very low insertion losses on both passbands.

Novel Multiplexer Topologies Based on All-Resonator Structures

This paper presents two novel multiplexer topologies based on all-resonator structures. Such all-resonator structures remove the need for conventional transmission-line-based splitting networks. The first topology is a diplexer with transmission zeros in the guard band, shared by both channels. These transmission zeros are generated by introducing a cross coupling in a quadruplet in resonators common to both channels. A twelfth-order diplexer with a pair of transmission zeros is presented here as an example. The second topology is a multiplexer with a bifurcate structure that limits the connections to any resonator to three or less, regardless of the number of output channels. A sixteenth-order four-channel multiplexer is presented as an example. Both topologies have been demonstrated at X-band using waveguide technology. Good agreements between measurements and simulations have been achieved.

A 3-D Printed Lightweight X-Band Waveguide Filter Based on Spherical Resonators

A fifth order X-band waveguide bandpass filter, based on spherical resonators, has been designed, and fabricated by 3-D printing. In comparison with rectangular waveguide, spherical resonators have a higher unloaded quality factor, but at the same time suffer from closer higher order modes. In this letter, a special topology has been proposed to relieve the impact of the first three higher order modes in the resonator and ultimately to achieve a good out-of-band rejection. Stereolithography based 3-D printing is used to build the filter structure from polymer and a 25 μm thick copper layer is deposited to the filter. The measurement result of the filter has an excellent agreement with the simulations. The filter is also considerably lighter than a similar metal filter.

A SU8 Micromachined WR-1.5 Band Waveguide Filter

A WR-1.5 band (500-750 GHz) waveguide 3rd order bandpass filter has been designed, fabricated, using SU8 photoresist technology, tested and presented. The filter is composed of three silver-coated SU8 layers, each of the same nominal thickness of 191 μm. This filter structure is based on three offset resonators. This novel structure is ideally suitable for the layered SU8 micromachining process. The filter exhibits a 53.7 GHz 3-dB bandwidth at a central frequency of 671 GHz. The median passband insertion loss is measured to be 0.65 dB and the return loss is better than 11 dB over the whole passband.

Measurements of micromachined submillimeter waveguide circuits

In this paper two calibrated measurement methods for submillimeter circuits are presented and micromachined waveguides operating between 220 and 325 GHz have been fabricated using thick SU-8 photoresist technology and tested. The first measurement method is achieved by employing a pair of micromachined embedded H-plane bends, which were specially designed to enable direct and accurate connection between the micromachined circuit and standard waveguide flanges. A 16 mm long WR-3 waveguide has been designed, fabricated and measured using this technology. The measured average insertion loss is 2.3 dB or 0.144 dB/mm over the frequency range of 220–321 GHz. The second measurement technology employs a conventionally machined metal block constructed with two separate pieces in which to mount the micromachined circuit. A choke flange has been adopted for eliminating the effect of air gap at the interfaces between the micromachined circuit and metal block. A 15 mm long WR-3 straight waveguide has been fabricated and tested. The measured insertion loss is between 1.4 dB and 3.2 dB corresponding to 0.093 and 0.213 dB/mm. A comparison between these two measurement technologies has been carried out and presented.

A Lightweight 3-D Printed

A 3-D printed fourth-order cavity bandpass filter (BPF) centered at 10 GHz and with a 3% fractional bandwidth is presented in this letter. The BPF was designed using two high-Q spherical dual-mode cavity resonators, and was fabricated using a stereolithography-based 3-D printing technique. Compared to dual-mode filters constructed by square or cylindrical resonators, the use of spherical resonator gives a wider spurious-free region. In order to fully exhibit the light weight advantage of additive manufacturing, the redundant material outside of the filter was removed. In addition, rectangular apertures were added through the cavities and waveguide walls without interrupting the surface current distributions, which further yields reduced filter weight as well as easier electroplating. Measured results of the BPF exhibit an excellent agreement with simulations.

X

Abstract. A microfabricated hollow air-filled waveguide with two back-to-back bends operating in the WR-3 frequency range of 220 to 325 GHz has been successfully fabricated through a multilayered ultrathick SU8 process and has ultralow transmission loss of 0.028 to 0.03  dB/mm in a performance test. This is the first SU8 constructed hollow waveguide operating at WR-3 frequencies having transmission loss comparable with the state-of-the-art computer numerical control (CNC) machined metal circuits. The multilayered SU8 processing technique has a great advantage of reducing the transmission loss by decreasing the probability of air gaps between SU8 layers. The excellent device performance is also attributed to the precisely controlled layer thickness and ultimately low surface roughness of evaporated silver which only contributes a small part of the microwave signal attenuation.

-Band Bandpass Filter Based on Spherical Dual-Mode Resonators

Making accurate and reliable measurements at WR-3 band is challenging. This is made more so when a T/R-T based test configuration is used. This paper compared the calibrations and measurements between a T/R-T and a full T/R-T/R configuration. The shortfall of load matching in the T/R-T configuration has been identified and described using signal flow analyses. The effects of both the device performance and the load match on the measurement uncertainty have been studied. The insertion of an attenuator at the T-module test port has been used to overcome the poor load matching, and significant improvements have been achieved on the measurements of micromachined waveguide devices.

Fabrication of multilayered SU8 structure for terahertz waveguide with ultralow transmission loss

Abstract. This paper provides a systematic review of the technical issues of SU8 fabrication for millimeter-wave and terahertz components based on research carried out at the University of Birmingham in the past decade. A design-for-manufacturability approach is followed. The flexibility of the SU8 process enables many device structures. Challenges and problems during fabrication will be discussed and demonstrated with examples. The measurement of the devices is also a significant challenge when the critical dimensions of the device shrink and special testing fixtures are needed in some cases. Finally, a brief overview of the issues discussed above is given for future guidance.

Measurements of micromachined waveguide devices at WR-3 band using a T/R-T module based network analyzer

The advances in terahertz technology drive the needs for the design and manufacture of waveguide devices that integrate complex 3D miniaturised components with meso and micro scale functional features and structures. Therefore, in parallel with the development of such terahertz devices it is necessary to design and validate new manufacturing platforms for their batch production. Especially, with the frequency increase the dimensions of the waveguide functional structures decrease and they are in the micro range from 200 μm to 50 μm with tight requirements for accuracy and surface integrity as they determine the devices’ performance. In this context, this paper presents a novel manufacturing route for scale up production of terahertz components, which integrates CNC milling and laser micro-machining. A solution to overcome the resulting tapering of the laser ablated volumes while achieving a high accuracy and surface integrity of the machined structures is proposed in this research. In addition, an approach for two-side processing of waveguide structures within one laser machining setup is described that employs a higher precision alignment procedure. The capabilities of the proposed manufacturing route are demonstrated on a terahertz waveguide component that is functionally tested to assess the effects of the achieved dimensional accuracy and surface integrity on its performance. The results show that the proposed manufacturing solution can be a very promising alternative for the scale up production of terahertz components.