Deposited Dielectrics with Low Cryogenic Microwave Loss and Their Room Temperature Properties

Abstract

Superconducting resonators used in mm/sub-mm (MMW) astronomy would greatly benefit from deposited dielectrics with low dielectric loss. The excess loss in deposited dielectrics is mainly due to two-level systems (TLS), and there is no consensus on their microscopic origin. To study the relation between hydrogenated amorphous silicon’s (a-Si:H) microwave (MW) loss at 120 mK and its void volume fraction, hydrogen content, microstructure parameter, bond-angle disorder, and infrared (IR) refractive index, we deposited films at substrate temperatures of 100°C, 250°C and 350°C using plasma-enhanced chemical vapor deposition (PECVD). We measured the room temperature properties of the films using Fourier-transform infrared spectroscopy, Raman spectroscopy and ellipsometry. All room temperature properties except the IR refractive index decrease monotonically with increasing substrate temperature. The IR refractive index approaches the refractive index of crystalline silicon (c-Si) when increasing the substrate temperature to 350 °C. We measured the dielectric losses using superconducting coplanar waveguide resonators. Interestingly, we do not see a correlation of the room temperature results with the MW losses. All films have an excellent 120 mK MW loss tangent below 1e−5 at −50 dBm internal resonator power. More research on the loss tangents is recommended, for example using microstrip lines or lumped element parallel plate capacitors. The low dielectric losses make these films promising for application in MW kinetic inductance detectors and on-chip filters. These promising results could lead to the application of the dielectrics in the integrated superconducting spectrometer DESHIMA 2.0.