For terahertz systems, rectangular waveguide is an ideal low loss medium for interconnectivity and the construction of passive circuits. A drawback when manufacturing waveguides at submillimeter wavelengths is the demanding tolerances due to small dimensions. For example a WR-3 waveguide (operating between 220 and 325 GHz) has a cross-sectional dimension of just 864 by 432 µm, and higher frequency waveguides get proportionally smaller. An additional challenge is that if using waveguide for passive circuits such as filters, there are additional structures inside the waveguide which are significantly smaller than the waveguide itself. Traditionally, computer numerical control (CNC) milling has been used for waveguides, however at terahertz frequencies this is difficult to utilise. In this paper emerging technologies for terahertz waveguides are compared with conventional CNC solutions. The technologies include the photolithography based polymer etching of waveguides using SU-8 photoresist, and the laser machining of metal. Both have shown promise, and good quality terahertz passive components have been fabricated and measured.
Rectangular waveguides are fundamental structures for the transmission of signals at millimetre and submillimetre wavelengths. This paper describes the design and measured results for two rectangular waveguides based on layered SU-8 photoresist micromachining technology, with double-layer fabrication techniques to minimise the air gaps between layers. A brief description of the SU-8 photoresist micromachining procedure is given in the paper. One waveguide is demonstrated for the WR-3 band from 220 GHz to 325 GHz the other is for the WR-6 band 120 GHz to 170 GHz both are made of layered SU-8 with a 3 piece construction. Both waveguides have novel bends in order to connect to the measurement apparatus. The measured performance is presented and compared to conventional machined metal waveguide structures. The measured insertion loss for the SU-8 waveguides in both bands is better than 0.03 dB/mm.
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