Microwave dielectric properties of anisotropic materials at cryogenic temperatures

The permittivity and dielectric loss tangent of cross-linked polystyrene (Rexolite), polytetrafluoroethylene (Teflon), and single-crystal quartz were measured at microwave frequencies and at temperatures of 77 K and 300 K using a dielectric resonator technique. Dielectric loss tangents as low as 7×10^-6 at 77 K were determined by applying high-temperature superconducting (HTS) films as the endplates of the dielectric resonator. Two permittivity tensor components for uniaxially anisotropic crystalline quartz were measured. Although the permittivities at 77 K changed very little from their room temperature values at 300 K, large changes in dielectric losses were observed. The decreased losses of these microelectronic substrates can markedly improve the performance of many microwave devices at cryogenic temperatures