Impact of ground via placement in on-wafer contact pad design up to 325 GHz

The design of RF contact pads is a challenging task, especially when operating in the high millimeter-wave frequency range up to 325 GHz. The pads need to fulfill low loss, low distortion, probe ability, and packagability. Compared to the dimensions of the circuit components, the contact pads are rather large because the pad miniaturization is limited by the available probing and packaging technologies. This large spatial extent makes the contact pads prone to the excitation of parasitic modes, which deteriorate the overall circuit performance. This paper presents a comprehensive study on these parasitic modes and acquires analytical and numerical solutions to estimate the critical frequencies. For verification, two different contact pad structures were realized using an indium gallium arsenide (InGaAs) monolithic microwave integrated circuit process, with a substrate thickness of 50 mm. The test structures were characterized up to 325 GHz in an on-wafer measurement setup. Despite the good agreement between measurement and simulation, detailed investigations of fabrication and measurement tolerances were carried out to estimate the robustness of the electrical performance. The optimized via configuration achieves resonance-free behavior of the pad over the full measurement frequency range with less than 1.8-dB loss per probe-to-pad contact at 325 GHz.