A Monolithic Surface Micromachined Half-Coaxial Transmission Line Filter

In this paper, a novel monolithic surface micromachined half-coaxial transmission line filter was designed, fabricated and measured. The band pass filter presented here has a unique ground structure compared to the other research groups -the suspended ground plane is 100 ptm over the center conductor. The high Q
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results from this large gap and an additional reduction of loss is obtained by using quartz substrates. The filter is a 3-pole, 500 fractional bandwidth, bandpass filter centered at 31.75 GHz, consisting of three capacitively coupled resonators composed of half coaxial transmission lines, which are connected to input and output transitions designed to interface with external CPW probes for measurement. The spacing between resonators and the input and output coupling to the filter were calculated from a low pass filter prototype. The fabricated filter has a length of 13 mm and width of 1 mm. A 100-ptm-thick sacrificial layer was made by JSR THB-15IN photoresist. Suspended Au ground plane was supported at the substrate by electroplating process. The pass band return and insertion loss were -10.07 dB at 31.1 GHz and -2.83 dB at 32.0 GHz, respectively. In order to extract total losses of the proposed half coaxial transmission line, we fabricated and measured single resonators. A maximum Q
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value of 153 was obtained and these Q values showed the potential of this filter structure, because much higher air gap can be obtained with the same process, resulting in further increase of Q
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. Measured loss from the transition was around -0.