Submillimetre Rectangular Waveguides based on SU-8 photoresist micromachining technology David Glynn, Tianhao He, Jeff Powell, Yingtao Tian, Xiaobang Shang and Michael J. Lancaster School of Electronic, Electrical and System Engineering The University of Birmingham. Abstract—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. Keywords—waveguide; SU-8; micromachining INTRODUCTION The desire for ever higher frequency communication systems has made implementing even the simplest of components difficult as terahertz frequencies are approached. However, due to micromachining technologies, rectangular waveguide transmission lines and passive circuits are now accessible for frequencies at submillimetre wavelengths. Such micromachined circuits are also ideal conduits for integrating active components and with novel methods of matching, efficient high frequency systems can be constructed. One such circuit considered when initiating this research is the frequency multiplier. Multipliers are used as it is difficult to generate a local oscillator frequency high enough to mix with a high frequency target signal. Frequency multipliers typically utilise a nonlinear harmonic response to distort a lower frequency into a required harmonic multiple. The frequency spans of the two waveguides in this paper were chosen to coincide with an input and doubled output frequency range for such a multiplying circuit. Care was taken to design the waveguides using just five layers of SU-8 photo resist, so that the waveguides orientations would allow an input circuit of 150 GHz to connect to a future frequency doubler design, and the resultant 300 GHz frequency would utilise the second waveguide. Ninety degree E- plane or and H-plane bends were constructed to connect to test equipment, or potentially to other stages in a communication system. I. WR-3 WAVEGUIDE DESIGN The rectangular waveguide designed here is a standard WR-3 band waveguide with a cross-section size of a = 864 μm by b = 432 μm. The length of the waveguide is 15.95 mm to match previous waveguide circuits [1] and a novel bend is used to attach the waveguide to the measurement network analyser. A device based on SU-8 pieces is easy to fabricate and is mechanically robust. It has a low insertion loss and good transmission coefficient for the specified operating frequency band. In this rectangular waveguide design, specifications for the transmission coefficient S 21 are better than -20 dB for the whole WR-3 band. The WR-3 waveguide is built with 5 layers of SU-8 photoresist with each layer having a thickness of 288 μm. Three layers form the total thickness of 864 μm for the WR-3 waveguide internal dimensions with additional two layers for the top and bottom of the guide and to feed to the waveguide bends connecting the waveguide to the measurement apparatus as illustrated in Fig 1. The design of the waveguide bend is based on a similar previous structure in [1] and it is modified and configured to meet the dimension requirements of WR-3 waveguide. The waveguide bend is specified to be an H-plane. Fig. 1. 3D EM model for the WR-3 waveguide with bends. Here the inside of the metal waveguide is shown in blue and the layers are clearly seen. II. WR-6 WAVEGUIDE DESIGN The 150 GHz waveguide dimensions are derived from the WR-6 waveguide specification (a=1650 μm, b=830 μm), which covers the frequency range 110 GHz to 170 GHz. WR-6 was chosen for its centre frequency of 150GHz, which would be useful in a 300 GHz communication system as part of the local oscillator multiplying circuit. However, because we are to