Air-bridge Fabrication Research Articles

Lower-temperature fabrication of airbridges by grayscale lithography to increase yield of nanowire transmons in circuit QED quantum processors

Quantum hardware based on circuit quantum electrodynamics makes extensive use of airbridges to suppress unwanted modes of wave propagation in coplanar-waveguide transmission lines. Airbridges also provide an interconnect enabling transmission lines to cross. Traditional airbridge fabrication produces a curved profile by reflowing resist at elevated temperature prior to metallization. The elevated temperature can affect the coupling energy and even yield of pre-fabricated Josephson elements of superconducting qubits, tunable couplers, and resonators. We employ grayscale lithography to enable reflow and thereby reduce the peak temperature of our airbridge fabrication process from 200 to 150 °C and link this change to a substantial increase in the physical yield of transmon qubits with Josephson elements realized using Al-contacted InAs nanowires.

Fabrication of airbridges with gradient exposure

In superconducting quantum circuits, airbridges are critical for eliminating parasitic slotline modes of coplanar waveguide circuits and reducing crosstalks between direct current magnetic flux biases. Here, we present a technique for fabricating superconducting airbridges. With this technique, a single layer of photoresist is employed, and the gradient exposure process is used to define the profile of airbridges. In order to properly obtain the bridge profile, we design exposure dosage based on residual photoresist thickness and laser power calibrations. Compared with other airbridge fabrication techniques, the gradient exposure fabrication technique provides the ability to produce lossless superconducting airbridges with flexible size and, thus, is more suitable for large-scale superconducting quantum circuits. Furthermore, this method reduces the complexity of the fabrication process and provides a high fabrication yield.

Fabrication and characterization of aluminum airbridges for superconducting microwave circuits

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.

Broadband CPW Feed for Millimeter-Wave SIW-Based Antipodal Linearly Tapered Slot Antennas

To achieve broadband performances in the millimeter-wave range, antipodal linearly tapered slot antenna (ALTSA) designs with new combined substrate integrated waveguide (SIW) and regular coplanar waveguide (CPW) feeds are presented and studied. This feed structure eliminates the fabrication of air bridges in direct CPW-fed tapered slot antennas (TSAs). Two millimeter-wave design techniques are introduced for the selected 41-61 GHz and 90-120 GHz frequency ranges, demonstrating very good impedance match and nearly constant gain, beamwidth, and cross-polarization levels over bandwidths of 39% and 28%, respectively. The design procedure is validated by comparing simulated results with measurements performed on a 21-31 GHz (38% bandwidth) prototype. Very good agreement between measured and calculated performance characteristics is obtained with only cross-polarization levels slightly higher than predicted. The structural design parameters and dimensions of all three designs are given.

Simple e-beam air-bridge technology for mm-wave applications

► A new and simple air-bridge fabrication technology. ► Simple resist selection gives fewer and simpler fabrication processes. ► No special e-beam dose or process calibration required. ► The process latitude of UVIII resist and polyamide is exploited. ► Excellent experimental demonstration for mm-wave application. We describe a new air-bridge fabrication technology using electron beam lithography. This novel technique greatly simplifies the lithographic process, eliminating the requirement for complex writing control procedures used in established electron-beam lithography based processes. The new technique is demonstrated using single-dose per layer electron beam lithography and is applied to the fabrication of microwave and millimeter-wave waveguide structures.

Studies on key nano-fabrication processes for GaAs-based air-bridge-type two-dimensional photonic-crystal slab waveguides

We report successful studies on improved key nano-fabrication processes for AlGaAs-based air-bridge-type two-dimensional photonic-crystal (2DPC) slab waveguides. The first topic is proximity-effect correction in electron beam lithography for the definition of a uniform and precise periodic/non-periodic 2DPC waveguide pattern. The second topic is optimization of the selective wet-etching condition in an air-bridge fabrication process. The resultant nano-fabrication processes have been proved to be quite effective for the realization of 2DPC slab waveguides such as modified and topology-optimized bend waveguides having broad-band and high-transmittance spectra in excellent agreement with the calculated ones.

Metallic air-bridges on non-planar transport structures

Peculiarities of metallic air bridge fabrication on non-planar surfaces along a step or across a trench were investigated. These particular reliefs feature prominently in many geometries used to study electron transport in nanostructures. The bridges were fabricated by a one-step electron beam exposure followed by metal evaporation and subsequent lift-off using a three-layer positive electron resist system. The method is based on varying the exposure dose for the pillars and the span of the air-bridge. The peculiarities for a substrate with a relief are directly associated to a topography induced thickness variation of the resist layers after spin-coating. In case of a step, the dose redistribution and the correct location are sufficient to form a stable bridge. To fabricate a stable air-bridge across a trench, it is very desirable that the bottom layer of the resist system provides good planarization of the substrate.

Fabrication of air bridges using electron beam lithography

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Design, fabrication, and characterization of a hot-electron vacuum transistor

The design and fabrication of a vacuum transistor is described which uses hot-electron emission through a thin Schottky metallization as a source of electrons for current flow. A thin titanium metallization on p-type ion-implanted gallium arsenide is used to form a Schottky contact. Application of a reverse bias greater than the avalanche breakdown voltage results in the generation of hot-electrons which are extracted by a collector metallization suspended above the Schottky metal. The collector is fabricated using a standard air-bridge fabrication process which is compatible with standard GaAs monolithic microwave integrated circuit technology. Measurements on fabricated prototype devices confirm a collector current which can be modulated by varying the potential applied to the Schottky metallization and consequently the resulting hot-electron generation. Small-signal modeling suggests that the device can be used for voltage and power gain, although the required load resistances are high for the first-generation prototype devices. This transistor may find application as a high-speed amplifier or in switching circuits for display applications.

T-gate, Γ-gate, and air-bridge fabrication for monolithic microwave integrated circuits by mixed ion-beam, high-voltage electron-beam, and optical lithography

Mixed lithography combining a 140 keV Be++ focused ion beam and a 60 keV electron beam has been applied to fabricate T-section and Γ-section structures for gate electrodes in GaAs metal–semiconductor field effect transistors for monolithic microwave integrated circuits. The gates are formed by lift-off metallization, following successive electron and ion beam exposures of a single resist layer and a single stage of development. Gates with lengths down to 170 nm have been fabricated. Gate resistance parameters given by reverse modeling with data from microwave measurements are consistent with direct measurements on test structures. Forward modeling shows the benefits of the Γ-gate structure for well-balanced device design. High-voltage lithography has been applied also to a novel procedure for generating air-bridges for monolithic microwave integrated circuits.

T-gate and airbridge fabrication for MMICs by combining multi-voltage electron-beam lithography and ion-beam lithography

Mixed lithography combining a 140 keV Be ++ focussed ion beam and a 60kV electron beam has been applied to fabricate T-section structures for use as gate electrodes in GaAs MESFETs intended to operate in monolithic microwave integrated circuits. Gate metallization is formed by lift-off using a single resist layer and a single development stage, after successive electron and ion beam exposures. Aluminium T-gates with gate lengths of 0.22μm have been fabricated with resistances of 90Ω/mm. Air bridges have been fabricated using high-voltage and low-voltage electron-beam lithography on thick resist layers and lift-off in two stages; firstly to form pillars 6μm high, and secondly to join them by a span 5μm above the substrate.