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arXiv:1101.4576 · Nanotechnology

Junction fabrication by shadow evaporation without a suspended bridge

Florent Lecocq, Ioan M. Pop, Zhihui Peng, I. Matei, Thierry Crozes, Thierry Fournier, Cécile Naud, Wiebke Guichard, O. Buisson

Published Jul 8, 2011• 103 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1088/0957-4484/22/31/315302

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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.
Fabrication of sub-micron Josephson junctions is demonstrated using standard processing techniques for high-coherence, superconducting qubits. These junctions are made in two separate lithography steps with normal-angle evaporation. Most significantly, this work demonstrates that it is possible to achieve high coherence with junctions formed on aluminum surfaces cleaned in situ with Ar milling before the junction oxidation. This method eliminates the angle-dependent shadow masks typically used for small junctions. Therefore, this is conducive to the implementation of typical methods for improving margins and yield using conventional CMOS processing. The current method uses electron-beam lithography and an additive process to define the top and bottom electrodes. Extension of this work to optical lithography and subtractive processes is discussed.
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.
Josephson tunnel junctions are the centerpiece of almost any superconducting electronic circuit, including qubits. Typically, the junctions for qubits are fabricated using shadow evaporation techniques to reduce dielectric loss contributions from the superconducting film interfaces. In recent years, however, sub-micrometer scale overlap junctions have started to attract attention. Compared to shadow mask techniques, neither an angle dependent deposition nor free-standing bridges or overlaps are needed, which are significant limitations for wafer-scale processing. This comes at the cost of breaking the vacuum during fabrication, but simplifies integration in multi-layered circuits and implementation of vastly different junction sizes and enables fabrication on a larger scale in an industrially standardized process. In this work, we demonstrate the feasibility of a subtractive process for the fabrication of overlap junctions. In an array of test contacts, we find low aging of the average normal state resistance of only 1.6% over 6 months. We evaluate the coherence properties of the junctions by employing them in superconducting transmon qubits. In time domain experiments, we find that both, the qubit life- and coherence time of our best device, are, on average, greater than 20 μs. Finally, we discuss potential improvements to our technique. This work paves the way toward a more standardized process flow with advanced materials and growth processes, and constitutes an important step for the large scale fabrication of superconducting quantum circuits.
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.
Besides serving as promising candidates for realizing quantum computing, superconducting quantum circuits are one of a few macroscopic physical systems in which fundamental quantum phenomena can be directly demonstrated and tested, giving rise to a vast field of intensive research work both theoretically and experimentally. In this paper we report our work on the fabrication of superconducting quantum circuits, starting from its building blocks: Al/AlO x /Al Josephson junctions. By using electron beam lithography patterning and shadow evaporation, we have fabricated aluminum Josephson junctions with a controllable critical current density ( j c ) and wide range of junction sizes from 0.01 μm 2 up to 1 μm 2 . We have carried out systematical studies on the oxidation process in fabricating Al/AlO x /Al Josephson junctions suitable for superconducting flux qubits. Furthermore, we have also fabricated superconducting quantum circuits such as superconducting flux qubits and charge-flux qubits.