Quantum Limited Amplifier: Design, Fabrication and Characterization of a Superconducting NbTiN Kinetic Inductance Travelling Wave Parametric Amplifier

Abstract

The aim of this project was to design, fabricate and characterize a kinetic inductance travelling wave parametric amplifier (KI-TWPA) for cryogenic temperatures (10 mk) by adapting the design of Malnou et.al PRX Quantum (2020) [1] for a sapphire substrate of 430 μm. By utilizing the properties of periodic structures in a transmission line, the dispersive properties of the device could be modified, enabling phase matching of the signal and pump frequencies. The pattern was designed on Klayout, simulated in Sonnet[2] and fabricated in Chalmers MC2 cleanroom by sputtering of NbTiN, Electron beam lithography and Reactive ion etching. Additionally, a cryogenic microwave packaging was designed and fabricated. The packaging consists of a PCB designed in KiCad and a copper box designed in Autodesk Inventor. The PCB consists of two copper layers with Rogers RO4350B dielectric material in between the layers. The transmission line showed good matching in the frequency region of importance. When designing the copper box, the dominant cavity and chip resonant frequencies were engineered to exist outside the band of operation. The microwave properties of the PCB and copper box were simulated using COMSOL[3]. The superconductive device was measured at cryogenic temperatures and important device parameters were characterised. In particular, the kinetic inductance of the sample was characterised as well as the critical current of the device. The order of non-linearity was found to be I∗ = 7.33 - 8.26 mA and the critical current Ic = 2.2 mA. Through measurements of the transmission, the passband and stopband characteristics of the periodic structure were obtained. Thus, acquiring key parameters for the device. Due to fabrication issues, only one of the samples with a shorter amplification path could be measured. Therefore, signal gain could not be observed on the measured device since the gain is heavily dependant on the length of the transmission line.