Microwave properties of microstrip line circuits used for Josephson voltage standard arrays at 70 GHz

The microwave properties of microstrip lines used for Josephson voltage standard circuits at frequencies from 70 GHz to 75 GHz were investigated for temperatures of 4.2 K. The work focuses on the microwave losses and permittivity of a SiO2 layer fabricated by PECVD. Experimental results confirmed by microwave modelling show that parasitic substrate modes are excited in the silicon wafer. These modes dominate the losses for thick wafers. The substrate modes can induce an inhomogeneous microwave distribution in the Josephson junction array, thereby, causing a poor operation of the voltage standard circuits. Test circuits in dedicated microstrip structures allow the differentiation between the properties of the dielectric and of the substrate. The permittivity of the SiO2 layer and the Si wafer were determined as r = 4.4±0.3 and 11±2, respectively. The dielectric losses of SiO2 are below α < 0.07 dB/cm and for the superconducting niobium a surface resistance of RS ≈ 0.7 m was found. If the test circuits are fabricated on the same wafer as the Josephson voltage standard arrays, this method will enable an effective process control.