High performance resonators for mm-wave ICs

A detailed comparison of the use of signal elevated and all-elevated CPW to obtain high performance resonators (matching networks) in the frequency range 140-320 GHz is given for the first time. We investigate the effect of elevation height and number and size of the posts which are used to elevate the CPW traces above the substrate. The number and size of the supporting posts is found to have a significant effect on resonator performance and, in general, both should be minimized for optimum performance. This is due to the fact that, as a greater proportion of the line is in physical contact with the substrate, the effective permittivity increases, the characteristic impedance decreases and the line becomes less like an ideal line suspended in air. One measured result shows that the addition of one extra post can increase the loss factor of a resonator by 0.2. Increasing the elevation height from 6 to 13 μm can reduce the impedance bandwidth by around half. At 13 μm elevation the performance of the two types of elevated CPW are comparable. However, the fabrication of the signal elevated CPW is less time consuming and the final line dimensions are less subject to variation.