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Chinese Physics B

Bridge-free fabrication process for Al/AlO x /Al Josephson junctions

Ke Zhang, Mengmeng Li, Qiang Liu, Haifeng Yu, Yang Yu

Published Jun 1, 2017• 15 citations
Publisher

Abstract

We fabricate different-sized Al/AlO x /Al Josephson junctions by using a simple bridge-free technique, in which only single-layer E-beam resist polymethyl methacrylate (PMMA) is exposed at low accelerate voltage (below 30 kV) and the size of junction can be varied in a large range. Compared with the bridge technique, this fabrication process is very robust because it can avoid collapsing the bridge during fabrication. This makes the bridge-free technique more popular to meet different requirements for Josephson junction devices especially for superconducting quantum bits.

DOI
10.1088/1674-1056/26/7/078501

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Progettazione e sviluppo di un processo bridge-free per la realizzazione di giunzioni Josephson per applicazioni di quantum computing

GIOVANNI GENNARI

The present thesis focuses on the laboratory fabrication of Al/AlOₓ/Al Josephson junctions using a bridge-free shadow evaporation technique, developed within the broader context of superconducting technologies for quantum computing. The interest in such devices arises from the central role played by the Josephson junction in superconducting qubits: unlike a purely harmonic LC oscillator, characterized by equally spaced energy levels, the presence of the junction introduces a nonlinearity that leads to an anharmonic energy spectrum. This allows for the selective addressing of the fundamental transition, which is a necessary condition for implementing a circuit-based qubit. In this framework, the quality of the AlOₓ tunnel barrier and of the metal interfaces is of crucial importance, as it directly affects not only the static electrical parameters of the junction but also dielectric losses and, more generally, the performance of the device in quantum applications. 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This approach results in a more robust masking scheme compared to suspended-bridge processes, albeit at the cost of increased sensitivity to the precise definition and reproducibility of the undercut. The experimental activity was structured in several stages. First, the positive resists PMMA and MMA, used to form the bilayer lithographic stack, were characterized in order to determine their response as a function of electron-beam exposure dose and development conditions. A calibration mask consisting of an array of square patterns exposed at varying doses was employed to extract contrast curves and identify suitable exposure parameters. The development depth was quantitatively measured using atomic force microscopy (AFM), enabling a direct correlation between exposure dose and resulting resist profile. 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This required the design of a new sample holder and a complete recalibration of the quartz crystal microbalance, as the altered geometry significantly affected the relationship between the nominal thickness reading and the actual film thickness deposited on the sample. A further important aspect of the work involved the progressive design and optimization of the full device mask, including contact pads, alignment markers, and the active junction region. The evolution of the layout, from initial test structures to the final configuration, was guided by experimental observations of pattern distortions and dose non-uniformities. It was found that the writing order of the electron-beam lithography pattern generator could significantly influence the final pattern quality, introducing systematic variations in feature definition. To mitigate this effect, a constrained writing sequence was implemented, ensuring improved reproducibility of the exposure process. Once all process parameters were stabilized, a comprehensive Standard Operating Procedure (SOP) was established, covering both the fabrication of Cr/Au contact structures and the subsequent lithography, angled evaporation, oxidation, and lift-off steps required for the realization of the Josephson junction. Finally, prior to fabricating fully oxidized junctions, a preliminary electrical validation of the process was carried out by producing test structures without the intermediate oxidation step. This allowed for the verification of electrical continuity in the aluminum structures. Measurements performed using a probe station confirmed ohmic behavior and the absence of macroscopic discontinuities in the conduction path. 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For enhancing operating speed of a superconducting integrated circuit (IC), the device size must be reduced into the submicron level. For this purpose, we have introduced electron beam (EB) direct writing technique into the fabrication process of a Nb/AlOx/Nb Josephson IC. A two-layer (PMMA/(alpha) M-CMS) resist method called the portable conformable mask (PCM) method was utilized for having a high aspect ratio. The electron cyclotron resonance (ECR) plasma etching technique was utilized. We have fabricated micron or submicron-size Nb/AlOx/Nb Josephson junctions, where the size of the junction was varied from 2 micrometer to 0.5 micrometer at 0.1 micrometer intervals. These junctions were designed for evaluating the spread of the junction critical current. We achieved minimum-to-maximum Ic spread of plus or minus 13% for 0.81-micrometer-square (plus or minus 16% for 0.67-micrometer-square) 100 junctions spreading in 130- micrometer-square area. The size deviation of 0.05 micrometer was estimated from the spread values. We have successfully demonstrated a small-scale logic IC with 0.9-micrometer-square junctions having a 50 4JL OR-gate chain, where 4JL means four junctions logic family. The circuit was designed for measuring the gate delay. We obtained a preliminary result of the OR- gate logic delay, where the minimum delay was 8.6 ps/gate.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jul 7, 1997•2 citations
We fabricate different-sized Al/AlO x /Al Josephson junctions by using a simple bridge-free technique, in which only single-layer E-beam resist polymethyl methacrylate (PMMA) is exposed at low accelerate voltage (below 30 kV) and the size of junction can be varied in a large range. Compared with the bridge technique, this fabrication process is very robust because it can avoid collapsing the bridge during fabrication. This makes the bridge-free technique more popular to meet different requirements for Josephson junction devices especially for superconducting quantum bits.
We fabricate high-quality Al/AlO x /Al junctions using improved bridge and bridge-free techniques at 30-keV e-beam voltage, in which the length of undercut and the size of junction can be well controlled by the pre-exposure technique. The dose window is 5 times as large as that used in the usual Dolan bridge technique, making this technique much more robust. Similar results, comparable with those achieved using a 100-keV e-beam writer, are obtained, which indicate that the 30-keV e-beam writer could be an economic choice for the superconducting qubit fabrication.
A minimal method to fabricate Al/AlOx_xx​/Al Josephson junctions (JJs) using photolithography and argon etching, before metallization and oxidation, is demonstrated. JJs with areas ranging from 1 to 6 μμμm2^22 can be fabricated and, with the appropriate oxidation conditions, the junction resistance can be varied by $\sim$2 orders of magnitude. Transmission electron microscopy reveals the successful fabrication of JJs with few grain boundaries suggesting reduced energy loss from two-level-systems. Superconducting QUantum Interference Devices (SQUIDs) fabricated from this methodology exhibit reduced resistance variation over multiple chips, compared with electron beam lithography, and the devices can sustain repeated thermal cycles to 10 mK with the excellent flux response remaining unchanged. The quantum applications of this technology are demonstrated by embedding a SQUID resonator into a 3D cavity and parametrically amplifying low photon numbers with gains of $\sim$40 dB. This work establishes the simplest approach to fabricating JJs to date, and it could prove pivotal to the widespread utilization of superconducting circuit-based quantum technologies.
We present a novel shadow evaporation technique for the realization of junctions and capacitors. The design by e-beam lithography of strongly asymmetric undercuts on a bilayer resist enables in situ fabrication of junctions and capacitors without the use of the well-known suspended bridge (Dolan 1977 Appl. Phys. Lett. 31 337-9). The absence of bridges increases the mechanical robustness of the resist mask as well as the accessible range of the junction size, from 10(-2) µm(2) to more than 10(4) µm(2). We have fabricated Al/AlO(x)/Al Josephson junctions, phase qubit and capacitors using a 100 kV e-beam writer. Although this high voltage enables a precise control of the undercut, implementation using a conventional 20 kV e-beam is also discussed. The phase qubit coherence times, extracted from spectroscopy resonance width, Rabi and Ramsey oscillation decays and energy relaxation measurements, are longer than the ones obtained in our previous samples realized by standard techniques. These results demonstrate the high quality of the junction obtained by this bridge-free technique.
The present thesis focuses on the laboratory fabrication of Al/AlOₓ/Al Josephson junctions using a bridge-free shadow evaporation technique, developed within the broader context of superconducting technologies for quantum computing. The interest in such devices arises from the central role played by the Josephson junction in superconducting qubits: unlike a purely harmonic LC oscillator, characterized by equally spaced energy levels, the presence of the junction introduces a nonlinearity that leads to an anharmonic energy spectrum. This allows for the selective addressing of the fundamental transition, which is a necessary condition for implementing a circuit-based qubit. In this framework, the quality of the AlOₓ tunnel barrier and of the metal interfaces is of crucial importance, as it directly affects not only the static electrical parameters of the junction but also dielectric losses and, more generally, the performance of the device in quantum applications. Following a theoretical overview of superconducting qubits and the physics of Al/AlOₓ/Al Josephson junctions, the work focuses on the experimental implementation and optimization of a bridge-free fabrication approach, adopted as an alternative to conventional suspended-bridge techniques. This method preserves the advantages of in-situ shadow evaporation for barrier formation while replacing the mechanical fragility of the suspended bridge with a geometrical control of the resist undercut. The operating principle relies on engineering an asymmetric profile in a bilayer resist system such that, during the two angled evaporations, only one deposition reaches the substrate along the connection lines, while the other is deposited on the resist sidewalls and subsequently removed during lift-off. In the central region, the overlap of the two evaporated aluminum layers, separated by an intermediate oxidation step, defines the Al/AlOₓ/Al superconductor–insulator–superconductor structure. This approach results in a more robust masking scheme compared to suspended-bridge processes, albeit at the cost of increased sensitivity to the precise definition and reproducibility of the undercut. The experimental activity was structured in several stages. First, the positive resists PMMA and MMA, used to form the bilayer lithographic stack, were characterized in order to determine their response as a function of electron-beam exposure dose and development conditions. A calibration mask consisting of an array of square patterns exposed at varying doses was employed to extract contrast curves and identify suitable exposure parameters. The development depth was quantitatively measured using atomic force microscopy (AFM), enabling a direct correlation between exposure dose and resulting resist profile. This step proved essential for selecting the most appropriate resist configuration capable of producing a controlled undercut compatible with the requirements of the bridge-free process. Subsequently, the lithographic process and evaporation geometry were calibrated through iterative refinement of the mask design, exposure parameters, and development conditions. Cross-sectional analysis, obtained via controlled sample cleaving followed by scanning electron microscopy (SEM), played a key role in assessing the three-dimensional profile of the resist, verifying the presence and quality of the undercut, and identifying potential structural instabilities such as resist collapse. At the same time, modifications were introduced to the evaporation system to enable deposition at the desired angles. This required the design of a new sample holder and a complete recalibration of the quartz crystal microbalance, as the altered geometry significantly affected the relationship between the nominal thickness reading and the actual film thickness deposited on the sample. A further important aspect of the work involved the progressive design and optimization of the full device mask, including contact pads, alignment markers, and the active junction region. The evolution of the layout, from initial test structures to the final configuration, was guided by experimental observations of pattern distortions and dose non-uniformities. It was found that the writing order of the electron-beam lithography pattern generator could significantly influence the final pattern quality, introducing systematic variations in feature definition. To mitigate this effect, a constrained writing sequence was implemented, ensuring improved reproducibility of the exposure process. Once all process parameters were stabilized, a comprehensive Standard Operating Procedure (SOP) was established, covering both the fabrication of Cr/Au contact structures and the subsequent lithography, angled evaporation, oxidation, and lift-off steps required for the realization of the Josephson junction. Finally, prior to fabricating fully oxidized junctions, a preliminary electrical validation of the process was carried out by producing test structures without the intermediate oxidation step. This allowed for the verification of electrical continuity in the aluminum structures. Measurements performed using a probe station confirmed ohmic behavior and the absence of macroscopic discontinuities in the conduction path. The measured resistance values were higher than those predicted using bulk aluminum resistivity; however, this discrepancy is consistent with the well-known behavior of thin evaporated films, in which surface scattering, grain boundaries, and interface effects lead to an increased effective resistivity. Although these measurements do not yet constitute a full characterization of the Josephson junction in the quantum regime, they provide an essential validation of the fabrication process and the structural integrity of the devices. In conclusion, this work enabled the development and calibration of a bridge-free fabrication process for Al/AlOₓ/Al Josephson junctions, highlighting the strong interplay between lithographic control, evaporation geometry, barrier formation, and electrical response. The main contribution of the thesis lies in transforming a conceptual fabrication approach into a structured and reproducible experimental procedure, based on resist characterization, cross-sectional analysis, instrumental recalibration, and preliminary electrical validation. The results obtained provide a solid foundation for further optimization of the tunnel barrier, cryogenic characterization, and integration of these junctions into superconducting circuits for quantum computing applications.
Abstract Josephson tunnel junctions form the basis for various superconductor electronic devices. For this reason, enormous efforts are routinely taken to establish and later on maintain a scalable and reproducible wafer-scale manufacturing process for high-quality Josephson junctions. Here, we present an anodization-free fabrication process for Nb/Al-AlO x /Nb cross-type Josephson junctions that requires only a small number of process steps and that is in general intrinsically compatible with wafer-scale fabrication. We show that the fabricated junctions are of very high quality and, compared to other junction types, exhibit not only a significantly reduced capacitance but also an almost rectangular critical current density profile. Our process hence enables the usage of low capacitance Josephson junctions for superconductor electronic devices such as ultra-low noise dc-superconducting quantum interference devices (SQUIDs), microwave SQUID multiplexers based on non-hysteretic rf-SQUIDs and RFSQ circuits.
For enhancing operating speed of a superconducting integrated circuit (IC), the device size must be reduced into the submicron level. For this purpose, we have introduced electron beam (EB) direct writing technique into the fabrication process of a Nb/AlOx/Nb Josephson IC. A two-layer (PMMA/(alpha) M-CMS) resist method called the portable conformable mask (PCM) method was utilized for having a high aspect ratio. The electron cyclotron resonance (ECR) plasma etching technique was utilized. We have fabricated micron or submicron-size Nb/AlOx/Nb Josephson junctions, where the size of the junction was varied from 2 micrometer to 0.5 micrometer at 0.1 micrometer intervals. These junctions were designed for evaluating the spread of the junction critical current. We achieved minimum-to-maximum Ic spread of plus or minus 13% for 0.81-micrometer-square (plus or minus 16% for 0.67-micrometer-square) 100 junctions spreading in 130- micrometer-square area. The size deviation of 0.05 micrometer was estimated from the spread values. We have successfully demonstrated a small-scale logic IC with 0.9-micrometer-square junctions having a 50 4JL OR-gate chain, where 4JL means four junctions logic family. The circuit was designed for measuring the gate delay. We obtained a preliminary result of the OR- gate logic delay, where the minimum delay was 8.6 ps/gate.