Development of transmon qubits solely from optical lithography on 300 mm wafers

Abstract

Qubit information processors are increasing in footprint but currently rely on e-beam lithography for patterning the required Josephson junctions (JJs). Advanced optical lithography is an alternative patterning method, and we report on the development of transmon qubits patterned solely with optical lithography. The lithography uses 193 nm wavelength exposure and 300-mm large silicon wafers. Qubits and arrays of evaluation JJs were patterned with process control which resulted in narrow feature distributions: a standard deviation of 0.78% for a 220 nm linewidth pattern realized across over half the width of the wafers. Room temperature evaluation found a 2.8 − 3.6% standard deviation in JJ resistance in completed chips. The qubits used aluminum and titanium nitride films on silicon substrates without substantial silicon etching. T1 times of the qubits were extracted at 26 µs -27 µs, indicating a low level of material-based qubit defects. This study shows that large wafer optical lithography on silicon is adequate for high-quality transmon qubits, and shows a promising path for improving many-qubit processors.

Optical Direct Write of Dolan--Niemeyer-Bridge Junctions for Transmon Qubits

“…Photolithography has demonstrated feature-repeatability down to less than 2% relative standard deviation 23,61,62 . For qubit fabrication, careful treatments of every processing step, including lithography, development, ashing, and oxidation has shown relative standard deviations of 3.5% 34 .…”

“…Fabrication based exclusively on photolithography simplifies processing and reduces the need for multiple lithography steps 23 . Photolithography has shown consistency and uniformity across wafers, yet is limited by its relatively large minimum feature size.…”

“…This work joins a growing body of literature studying alternative fabrication techniques for transmon qubits. Other work using photolithography 23,28 demonstrated modest coherence times, and merged-element transmons have used large overlap areas in compact footprints 29,30 . Our work extends elements of these studies, using photolithography to make large overlap area JJs for highly coherent three dimensional transmon qubits.…”

Coherent superconducting qubits from a subtractive junction fabrication process

“…In the commonly used shadow-evaporation processes, free standing bridges 18 or overhangs 19 , and multi-angle evaporation are exploited to generate the desired interface in situ. One drawback of these techniques is a systematic angle dependent parameter spread across larger wavers, where great efforts are necessary to mitigate this spread 20,21 . The need for point-like evaporation sources limits the applicable materials and growth processes.…”

Free-standing silicon shadow masks for transmon qubit fabrication

“…State-of-the-art superconducting qubits are fabricated by patterning an organic resist with e-beam or optical lithography to create a liftoff mask, followed by shadow evaporation of the aluminum layer [12][13][14][15][16][17][18] . Inevitably, this approach introduces contamination to the various interfaces 5 .…”