Low-loss Deposited Dielectrics for Superconducting Integrated Circuits

Abstract

Deposited dielectrics with low loss at millimeter-submillimeter (mm-submm) wavelengths are beneficial for the development of superconducting integrated circuits (ICs) for astronomy, such as filter banks, on-chip Fourier-transform spectrometers, and kinetic inductance parametric amplifiers. Although it is possible to fabricate microstrip lines using crystalline Si extracted from a silicon-on-insulator wafer by a flip-bonding process, deposited dielectrics al- low for simpler and more flexible chip designs and fabrication routes. In the ∼10–100 THz frequency range, dielectric losses are typically dominated by infrared absorption due to vibrational modes, whereas in the microwave fre- quency band (∼1–10 GHz) and at sub-Kelvin temperatures the dielectric loss is typically dominated by absorption due to two-level systems (TLSs). How- ever, the origin of the mm-submm (∼0.1–1 THz) loss in deposited dielectrics was unknown. In this dissertation we show that the mm-submm loss in de- posited dielectrics can be explained by the absorption tail of vibrational modes which are located above 10 THz. Furthermore, we found that hydrogenated amorphous silicon carbide (a-SiC:H) has a very low mm-submm loss tangent of 1.3×10−4 at 350 GHz, which makes it a promising low-loss deposited dielectric for mm-submm superconducting integrated circuits.

Based on a literature study we identified a-SiC:H and hydrogenated amor- phous silicon (a-Si:H) as potentially promising low-loss dielectrics. In order to find the origin of the mm-submm loss, and to define which materials we investi- gated in this PhD project, we characterized the dielectrics’ material properties at room temperature prior to performing the cryogenic loss measurements. We deposited a-SiC:H at a substrate temperature Tsub of 400◦C using plasma- enhanced chemical vapor deposition (PECVD), and we deposited the a-Si:H films at Tsub of 100◦C, 250◦C, and 350◦C. We characterized the films’ material properties using Fourier-transform infrared spectroscopy (FTIR), Raman spec- troscopy and ellipsometry. For the a-Si:H we determined the hydrogen content and the microstructure parameter from the FTIR data, the bond-angle disorder from the Raman data, and the void volume fraction from the ellipsometry data. For both the a-Si:H and the a-SiC:H we determined the band gap and optical refractive index from the ellipsometry data, and the infrared refractive index from the FTIR data. From the Raman spectra we observed that the a-SiC:H and the a-Si:H films were amorphous. Furthermore, we performed electron diffraction spectroscopy to determine the Si to C ratio of the a-SiC:H. For the a-Si:H we found that the all the material properties depend monotonically on Tsub. Additionally, we measured the cryogenic microwave loss of the a-Si:H films, but we found no correlation between the microwave loss and Tsub.

No cryogenic mm-submm and microwave loss data was available for a- SiC:H. We measured the low-power and cryogenic microwave loss of the a- SiC:H and found that the microwave loss tangent (tanδ ∼ 10−5) is compa- rable to the loss of a-Si:H. Furthermore, we measured the mm-submm loss in the range of 270–385 GHz using an on-chip Fabry-Pérot experiment. The observed mm-submm losss value of 1.2 × 10−4 at 350 GHz was significantly lower than what was reported for a-Si:H, which previously exhibited the low- est reported microwave and mm-subm wave loss values among the deposited dielectrics which are commonly used in superconducting ICs. Furthermore, we found that the mm-submm loss of the a-SiC:H increases monotonically with frequency. This was surprising in the framework of TLSs and led us to the hypothesis that another loss mechanism than TLSs might be dominant at mm- submm wavelengths. In addition to the low losses, the a-SiC:H was found to be beneficial thanks to its very low stress, lack of blisters, and the possibility to fabricate a membrane from the a-SiC:H on a c-Si wafer.

To study the origin of the frequency dependent mm-submm loss in the a- SiC:H, we extended the on-chip Fabry-Pérot experiment to the 270–600 GHz range by making use of a wideband leaky antenna. Additionally, we measured the complex dielectric constant of the a-SiC:H in the 3–100 THz range using Fourier-transform spectroscopy (FTS). We modeled the FTS data using the Maxwell-Helmholtz-Drude (MHD) dispersion model and obtained the complex dielectric constant in the 3-100 THz range. Finally, we modeled the combined on-chip loss data from the Fabry-Pérot experiments and the FTS data by fitting the MHD dispersion model in the frequency range of 0.27–100 THz. Our model demonstrates that the mm-submm loss in the a-SiC:H above 200 GHz can be explained by the absorption tail of vibrational modes which are located above 10 THz. These results pave the way for a thorough understanding of the mm-submm loss in deposited dielectrics.

The low losses of the a-SiC:H allow for integrated superconducting spec- trometers with a large frequency bandwidth and relatively high resolving pow- ers without sacrificing too much optical efficiency. This has led to the application of the a-SiC:H in the DESHIMA 2.0 filter bank, which has seen first light in 2023 at the ASTE telescope in the Atacama Desert.