Micromachined Gap Waveguide Devices

University dissertation from Chalmers University of Technology

Abstract: Gap waveguide technology is a new technology that is well suited for millimeter and submillimeter applications. Compared to conventional rectangular waveguides, this technology does not need any solid conductive walls to confine the wave by utilizing metamaterial surfaces such as "bed of nails" that creates an artificial magnetic conductive surface (AMC). These surfaces together with a parallel electrical conductive surface (less than l=4 apart) creates an electromagnetic stopband. By introducing a conductive ridge or groove in between the AMC, a path that allows the electromagnetic wave to propagate is presented. The wave is then confined by the AMC around it. This thesis presents gap waveguide devices with frequency ranges above 100 GHz. All devices have been micromachined and there is a focus on optimization techniques for this microfabrication. A ridge gap waveguide and a resonator for the frequency range 220-325 GHz are presented. The resonator has the purpose to evaluate the small losses at these frequencies. The initial measurements gave a loss of 0.049 dB/mm at 284 GHz which is better than a common microstripline and close to a rectangular waveguide. Imperfections when connecting to high frequency devices for measurements was noticed to be a huge problem. Therefore a pin-flange adapter was introduced. The adapter is placed between the measurement flange and the measured object. The pin-flange adapter has an AMC surface around the waveguide opening which suppresses any leakage due to gaps between it and the measured object. The pin-flange adapter with a 56 mm gap has the same performances as a standard connection with no gap and performs better than the standard connection when no screws are used to fixate the connection. Groove gap waveguides and ridge gap waveguides were fabricated for frequencies around 100 GHz by utilizing SOI wafers. The groove gap waveguide has waveguide opening dimension similar to a rectangular waveguide and therefore does not need any transition structure. The ridge gap waveguide is connected with a novel microstrip-to-ridge gap waveguide transition, thus creating a very small leakage gap. The thesis also compares silicon versus SU8 as base material when micromachining gap waveguides, specifically implemented on a ridge gap resonator for 220-325 GHz. SU8 has the benefit of a faster turnover than silicon and is a more robust and flexible material. Overall gap waveguide technology has the benefit of avoiding leakage from misalignment or gaps. The assembly becomes easier and there is less sensitivity. Micromachining enables the fabrication of high frequency gap waveguides with high precision.

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