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Chinese Physics B

Fabrication of Al air-bridge on coplanar waveguide

Zhen-Chuan Jin, Haiteng Wu, Haifeng Yu, Yang Yu

Published Oct 1, 2018• 12 citations
Publisher

Abstract

Superconducting coplanar waveguide (CPW) can be widely used as two-dimensional (2D) resonator, transmission line or feedline, providing an important component for superconducting quantum circuit which is a promising candidate for quantum information processing. Due to the discontinuities and asymmetries in the ground planes, CPW usually exhibits the spurious resonance, which is a common source of decoherence in circuit quantum electrodynamics experiments. To mitigate the spurious resonance, we fabricated superconducting aluminum air-bridges on Nb CPW. The fabricated air-bridges are approximately 3 μm high and up to 120 μm long. Compared with other methods, the fabrication procedures of our air-bridges are simpler, and the air-bridge can withstand strong ultrasound.

DOI
10.1088/1674-1056/27/10/100310

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Superconducting coplanar waveguide (CPW) can be widely used as two-dimensional (2D) resonator, transmission line or feedline, providing an important component for superconducting quantum circuit which is a promising candidate for quantum information processing. Due to the discontinuities and asymmetries in the ground planes, CPW usually exhibits the spurious resonance, which is a common source of decoherence in circuit quantum electrodynamics experiments. To mitigate the spurious resonance, we fabricated superconducting aluminum air-bridges on Nb CPW. The fabricated air-bridges are approximately 3 μm high and up to 120 μm long. Compared with other methods, the fabrication procedures of our air-bridges are simpler, and the air-bridge can withstand strong ultrasound.
Proper grounding between different ground planes in coplanar superconducting qubit circuits is important to avoid spurious resonances which increase decoherence. Here, the authors present a possible solution to suppress such undesired modes using superconducting aluminum air-bridges which have been fabricated on top of aluminum coplanar waveguide transmission lines. 3D electromagnetic simulations were done to guide the design of the air-bridges such that the input reflection (S11) of the bridges was kept at a minimum level. A fabrication method based on optical lithography techniques was developed and it resulted in air-bridges with a height of approximately 10 μm and lengths of up to 500 μm. The method can be generalized to arbitrary length air-bridge with heights even exceeding 15 μm.
Superconducting quantum electrical circuit based on Co-planar waveguide technology is considered one of the most promising candidates for practical realization of a\nquantum computer in future. For purpose of designing a multi-qubit quantum processor which is the heart of a quantum computer, it is required to couple qubits to a large number of superconducting transmission lines and resonators. Placing all these transmission lines and\nresonators in a practical design unavoidably creates bending and discontinuities in the ground\nplanes which have higher order parasitic modes that can be excited. These undesired parasitic\nmodes are detrimental for quantum measurement, for example during readout of a qubit.\nUnwanted modes can easily interfere with a resonator frequency when coupling to the qubit.\nThese parasitic modes can propagate throughout CPW structure and make unequal ground\npotential at the corresponding ground planes. The easiest way to suppress these parasitic\nmodes is to connect two unequal ground potential with superconducting air-bridges in order\nto bring them under equal potential in superconducting state. However, connecting airbridges on top of CPW adds shunt capacitor which may cause undesired reflection in the\nCPW.\nIn this thesis work, we designed superconducting air-bridges on top of\nsuperconducting CPW transmission line which is a subsystem of large quantum electrical\ncircuit. Our focus was to reduce added shunt capacitance by choosing the right height of these\nair-bridges. To keep the reflection coming from the air-bridge at minimum level, an airbridge\nheight of 8.5μm~15μm was chosen. The performance was verified by EM simulation\nand these heights were selected for device fabrication. We developed a new fabrication\nmethod for processing 8.5μm~15μm high aluminum air-bridges for quantum electrical\ncircuits. The method is compatible with the quantum circuit and easily reproducible. Finally,\ninput reflection measurement (S11) at cryogenic temperature was performed for 8.5μm high,\n300μm long aluminum air-bridges with aluminum CPW transmission line. Negligible amount\nof input reflection (S11) was observed from these measurements which is consistent with the\nresult of EM simulation.
Extraneous high frequency chip modes parasitic to superconducting quantum circuits can result in decoherence when these modes are excited. To suppress these modes, superconducting air bridges (AB) are commonly used to electrically connect ground planes together when interrupted by transmission lines. Here, we demonstrate the use of two-photon photolithography to build a supporting 3D resist structure in conjunction with a liftoff process to create AB. The resulting aluminum AB have a superconducting transition temperature of Tc=1.08 K and exhibit good mechanical strength up to lengths of 100 μm. A measurable amount of microwave loss is observed when 35 AB were placed over a high-Q Ta quarter-wave coplanar waveguide resonator.
We successfully designed and fabricated an absorption-type of superconducting coplanar waveguide (CPW) resonators. The resonators are made from a niobium film (about 160 nm thick) on a high-resistance Si substrate, and each resonator is fabricated as a meandered quarter-wavelength transmission line (one end is short to the ground and another end is capacitively coupled to a through feedline). With a vector network analyzer we measured the transmissions of the applied microwave through the resonators at ultra-low temperature. The obtained loaded quality factors are significantly high, i.e. up to ∼10 6 . When the temperature increases slowly from the base temperature (20 mK), the resonance frequencies of the resonators are blue shifted and the quality factors are lowered slightly. In principle, this type of device can integrate a series of CPW resonators with a common feedline, making it a promising candidate as the data bus for coupling distant solid-state qubits and the sensitive detector of single photons.
Superconducting microwave circuits based on coplanar waveguides (CPW) are susceptible to parasitic slotline modes which can lead to loss and decoherence. We motivate the use of superconducting airbridges as a reliable method for preventing the propagation of these modes. We describe the fabrication of these airbridges on superconducting resonators, which we use to measure the loss due to placing airbridges over CPW lines. We find that the additional loss at single photon levels is small, and decreases at higher drive powers.
Coplanar Waveguides (CPW) are ideally suited for coherently interfacing resonators with superconducting qubits. However, integrating CPWs with circuit elements on quantum chips involves curvature and discontinuities of the central conductors and corresponding ground planes, which may generate undesired parasitic modes. Experiments have demonstrated that air-bridges can effectively suppress this unwanted effect. Nevertheless, as quantum processors increase in size, manual air-bridge placement on the chip layout becomes increasingly time-consuming and error-prone. Automation of this process is therefore highly desirable, especially when the center line (channel) of the CPWs is not pre-established. In this paper we propose a parametric EDA method for coplanar waveguide channel identification and air-bridge construction in quantum chips. Our approach applies to both separated and full-package air-bridges and scales efficiently, with running time linear in the number of points and quadratic in the number of arcs in the corresponding chip layout. We evaluate our approach on a set of open-source quantum chip layouts, generating air-bridges in times ranging from 0.05 to 0.5 seconds—a practical speedup of over 10,000 times compared to manual generation. Furthermore, We propose two original quantitative metrics, accuracy and overlap, and verify that our method yields reliable results, producing air-bridges in the required shapes and locations. We fabricate a 13-qubit chip using our method for air-bridge placement, and observe excellent performance with minimal microwave and flux crosstalk. This research enables rapid generation of air-bridges when CPW channels are not pre-specified, paving the way for more flexible, automated, and modular design of superconducting quantum EDA.