Experimental investigation on low-temperature electrical resistivity of silver thin films

Abstract

Quantum computing has gained a lot of interest from researchers and industry due to its great potential to solve some complex problems in various fields. One of the biggest challenges is developing hardware suitable for the extremely low operation temperatures required by quantum computers. Specifically, the wiring material of a quantum computer must provide good electrical conductivity while keeping the thermal load to a minimum. Delft Circuits’ cryogenic flexible cable design made from metalized Polyimide (PI) with Silver (Ag) thin films as conductors offers a promising solution for quantum computer i/o systems. This thesis aims to investigate the effect of fabrication and design parameters on the electrical and thermal properties of silver thin films, utilizing the Residual Resistivity Ratio (RRR) as an indicator to assess the thermal load of cryogenic flexible cables. The RRR is calculated by taking the ratio of electrical resistivity measured at room temperature and extremely low temperatures of liquid He (4.2 K). The key parameters for the experimental investigation include oven heat treatment, lamination, silver purity, film thickness, aging and storage. The RRR behavior of samples with varying fabrication and design characteristics is explored through cryogenic measurements. Subsequently, the RRR results are analyzed and supported by material characterization via SEM and XRD. The results show that heat treatment results in higher RRR values due to grain growth and lower grain boundary density, leading to lower electrical resistivity at low temperatures. Heat treatment parameters including temperature, pressure, duration, ambiance, and cooling rate play a significant role in the resulting microstructure of the silver film, and consequently, in their RRR values. Furthermore, the study reveals that the lower purity level leads to decreased RRR values of the silver film due to higher electron scattering caused by impurities. A linear relationship is found between the film thickness and the RRR behavior of silver thin films. Lastly, aging and storage do not result in a significant change in the RRR values of heat-treated silver thin films. This thesis provides a deeper understanding on the influence of fabrication and design parameters on the low-temperature resistivity of silver thin films. It highlights the key role of these parameters in tailoring the microstructure of silver thin films to achieve desired material properties.