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IEEE Transactions on Applied Superconductivity

Fabrication Process for RSFQ/Qubit Systems

Leif Grönberg, Juha Hassel, P. Helistö, Markku Ylilammi

Published Jun 1, 2007• 14 citations
Publisher

Abstract

We have developed a Nb/Al/AlOx/Nb trilayer process aimed towards integration of RSFQ and qubit circuits. The emphasis is to provide a process enabling RSFQ operation at sub-100 mK temperatures required for qubit operation. In this paper we describe the main properties of the process. We also present experimental characterization data and in particular introduce a method of determining the critical current density by wafer level room temperature measurements.

DOI
10.1109/tasc.2007.897721

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Submicrometer Nb/Al−AlOx/Nb{\rm Nb}/{\rm Al}{-}{\rm AlO}_{\rm x}/{\rm Nb}Nb/Al−AlOx​/Nb Integrated Circuit Fabrication Process for Quantum Computing Applications
We have developed a low Jc(100-1000 A/cm2) submicrometer Nb integrated circuit fabrication process for SQUID-based quantum computing applications. The baseline process consists of 7 masking steps including Pd-Au resistor, Nb/Al-AlOx/Nb trilayer, two Nb wiring layers and two sputtered SiO2dielectric layers. We have also fabricated wafers with an Nb ground plane. Using deep-UV lithography, inductively coupled plasma etch tools, and self-aligned lift-off for device definition, we routinely achieve micrometer lines and spaces with 400 nm minimum junction dimensions. Room temperature testing is used to select wafers in process and junction annealing has been calibrated for trimming current density. We will describe the process which has been used to produce circuits with over 100 junctions.
We have developed a fabrication technology for superconducting integrated circuits with Nb-based Josephson junctions. The standard fabrication process with 10 mask levels includes 3 Nb superconducting layers and a Mo resistor layer. The influence of deposition parameters on film stress, electrical properties, and surface roughness were studied systematically. High quality Nb, Al, Mo, and SiO2films were successfully deposited for the subsequent fabrication of circuits. The circuit fabrication started with the fabrication of Mo resistors with a target sheet resistanceRshof 2 Ω, followed by the deposition of Nb/Al-AlOx/Nb trilayer Josephson-junction. The target critical current densityJcwas set at 6 kA/cm2. Small-scale circuits such as our standard library cells have been successfully fabricated and tested, confirming the capability of our fabrication technology for superconducting integrated circuits.
We present the latest development of an Nb/Al-AlOx/Nb Josephson junction fabrication process for superconducting integrated circuits at SIMIT. Our fabrication process has four niobium superconducting layers and one Ti/Pd resistor layer with 1.0 Ω/square sheet resistor for junction shunting and circuit biasing. Critical current density Jcof the junctions is controlled from 30 A/cm2to 10 kA/cm2by exposing aluminum layer to pure or mixed O2atmosphere. Correlation between measured Jcand exposure conditions has been established. Average quality factor (Rsg/Rn) of the junctions in our process is larger than 20 with subgap leakage factor (Vm) beyond 30 mV. Critical current uniformity is obtained according to the switching current (Is) distribution of a 1000-JJ array. Measured standard deviation sigma of Isis less than 4% over the 100-mm wafer. Plasma resonance in a large Josephson junction is used to measure specific capacitance (Cs) of the junction. Sheet inductances of superconducting niobium strip-lines are extracted by a SQUID diagnose circuit and the measured results show good agreement with our design values.
Josephson tunnel junctions represent a key element in superconducting electronics and quantum circuits. For many years, shadow evaporation by means of Dolan-type bridges has been the state-of-the-art for deep sub- micrometer sized structures. Increasing demand in the number of Josephson junctions, e.g. in qubit circuits and travelling wave parametric amplifiers, requests for a wafer-scale fabrication process with precise control of junction parameters and have led to an advanced lift-off technique called Manhattan-type junction technology in recent years. Herein, we report on the development of a 100 mm wafer-scale fabrication technology for deep sub-micrometer sized Al Josephson junctions with linear dimensions down to 180 nm. The critical currentICof the junctions ranges from about 10 to 120 nA scaling with their linear dimensions. Low temperature transport measurements as well as room-temperature characterization has been used forICand process homogeneity determination of series arrays of up to 50 Josephson junctions. We discuss technology parameters such as yield, on-chip and on-wafer reproducibility of the junction's critical currents as well as main process limitations. Moreover, we present experimental results on the characterization of first transmon-type qubits fabricated using this technology.
The development of large-scale quantum processors benefits from superconducting qubits that can operate at elevated temperatures and be fabricated with scalable, foundry-compatible processes. Atomic layer deposition (ALD) is increasingly being adopted as an industrial standard for thin-film growth, particularly in applications requiring precise control over layer thickness and composition. Here, we report superconducting qubits based on NbN/AlN/NbN trilayers deposited entirely by ALD. By varying the number of ALD cycles used to form the AlN barrier, we achieve Josephson tunneling through barriers of different thicknesses, with critical current density spanning seven orders of magnitude, demonstrating the uniformity and versatility of the process. Owing to the high critical temperature of NbN, transmon qubits based on these all-nitride trilayers exhibit microsecond-scale relaxation times, even at temperatures above 300 mK. These results establish ALD as a viable low-temperature deposition technique for superconducting quantum circuits and position all-nitride ALD qubits as a promising platform for operation at elevated temperatures.