Researchers at the Institute for Quantum Computing (IQC) have found a new one-step process to construct tiny bridge structures on microchips with superconducting circuits, which are essential for future quantum computers based on superconductors.
Currently, superconducting circuits are believed to be one of the most promising candidates for information storage on future quantum computers. These superconducting circuits may have the processing ability to make efficient breakthroughs in complex problems – such as pharmaceutical drug design – that are too time consuming to be computed on today’s computer systems.
These tiny bridge structures, known as air bridges, are used to create complex designs, as they enable circuit crossings and mitigate loss of information in superconducting circuits on microchips.
A scanning electron microscope image of a 36um long, aluminum air bridge fabricated using a single-step grayscale electron beam lithography process. The air bridge, viewed from a tilted angle, is shown spanning a co-planar waveguide on a superconducting chip.
“Our new method creates robust air bridges with a single lithography patterning step,” said Noah Janzen, a PhD student in the Department of Physics and Astronomy and at IQC, and lead author of this study. “By using a single step process, the time, cost, and amount of materials needed to create these structures is significantly reduced and optimized compared to previously known air bridge fabrication processes.”
The air bridges are miniscule, with a width of 6 µm, over 10 times smaller than a strand of hair, and a length ranging from 20 – 100 µm (at their largest, the size of a single hair). These bridges improve the quality of microchip devices by reducing quantum level deterioration effects, known as quantum decoherence, of the superconducting currents.
To create the air bridges, the research team layered a polymer on top of the microchip, then created a 3-dimensional template for the bridge by etching the template into the polymer layer using a focused beam of electrons through a process called electron beam lithography. A thin layer of aluminum was then deposited on top of the bridge template, and the polymer template was removed, leaving only the aluminum bridge suspended in the air. Support by staff at the Quantum-Nano Fabrication and Characterization Facility and the quality of the infrastructure at this facility have been essential for this development.
Air bridges are one important ingredient for improving the quality of the superconducting chips, and they also open up the design space on these chips from 2-dimensions to 3-dimensions. Combined with the newly reported simple method for creating air bridges, this provides researchers with a process to create more complex microchips and devices.
“This new method has already enabled us to develop a new device for the exploration of fundamental topics in quantum information. We expect it will be useful in a broad range of efforts in the field,” said Adrian Lupascu, professor in the Department of Physics and Astronomy and at IQC.
The paper Aluminum air bridges for superconducting quantum devices realized using a single-step electron beam lithography process was published in Applied Physics Letters August 29, 2022, and selected as an Editor’s pick.