University of Glasgow
case study: Optimising
Process Fabrication ofSuperconducting Qubits

November 2021

Optimising Process Fabrication ofSuperconducting Qubits

With over 11 years of collaboration with
University of Glasgow, we continue to support
the innovative fabrication processes for power
semiconductor, optoelectronic, quantum andmany more devices that take place at thestate‑of‑the‑art James Watt Nanofabrication
Centre (JWNC) cleanroom within the university.
We had a highly informative conversation with
two researchers from the University of Glasgow
who have shared their innovative projects
related to the fabrication of superconductingqubits. Dr Jharna Paul is a Research Associate
and Valentino Seferai is a PhD student within
the Quantum Circuits Group which is led byProfessor Martin Weides. Both researchers use
the Oxford Instruments PlasmaPro® 100 Cobra
ICP RIE system for the fabrication ofsuperconducting qubits.

Developing Superconducting Qubits
In one of their previous projects in association
with other researchers, Dr Jharna and Valentino
have focused on the creation of integrated
and highly coherent cavity magnon–polariton
systems. These have strongly coupled spin‑wave
and microwave photonic components, making
them a good candidate for quantum information
processing with hybrid quantum devices. The
result of this research was “the achievement ofstrong coupling on chip devices comprising asuperconducting resonator and a YIG sample atsub‑Kelvin temperatures”1. The superconducting
resonators were fabricated from niobium nitride(NbN) films, using electron‑beam lithography andOxford Instruments’ reactive ion etching systems.

© Oxford Instruments plc, 2021. University of Glasgow case study: Developing Superconducting Qubits 2

Dr Jharna’s current research involves wafer‑scale
nanofabrication of quantum qubit devices
along with integrated on‑chip components
for quantum information processing andcomputing applications. The first stage of thefabrication process includes the deposition ofsuperconducting metal on intrinsically doped
silicon wafers with a magnetron sputter tool.
The metal is then patterned using e‑beamlithography and the exposed metal etched using
Oxford Instruments’ PlasmaPro® 100 Cobra ICP
system which achieves a clean etch surface withhigh selectivity to the underlying substrate.
Valentino is focussed on the fabrication ofsuperconducting quantum circuits, using
Josephson junctions which are well‑established forthe construction of qubits in quantum information
processing applications. Previously he was involvedin a research project led by Professor Martin
Weides in which the research team achieved “goodcoherence properties of four transmon qubits
with subtractive Josephson junctions”2, where thequbit life and coherence time of their best device
was more than 20 µs, a significant step for future
large‑scale fabrication of superconducting circuits.
Currently, Valentino Seferai is optimising anotherfabrication recipe for an emerging material, using
tantalum to make superconducting resonators
which provides the high‑quality factors to better
control the qubits.

“Once you optimise the process,the ICP system is reliable withrepeatable results”.

Process Optimisation is the key forsuccessful research

the process, the ICP system is reliable withrepeatable results”.

For both researchers, meticulous processoptimisation is fundamental. They usuallydevelop their own recipes which requires deep
understanding of the process.

The research team investigates dry etch recipesthat produce high coherence qubit devices inorder to build the superconducting circuit andthis device fabrication needs high selectivity andaccurate control of the plasma. The goal is toetch the metals and other materials as cleanly
as possible without leaving any residues andwithout damaging the metals in order to get agood etch profile. The PlasmaPro® 100 Cobra ICP“effectively controls the etch rate and makes theprocess more stable”, as stated by Valentino.

“We use Oxford Instruments
PlasmaPro® 100 Cobra ICP etching system,
as you can effectively control the plasma, thepower, the gas delivery, the chamber pressure
and the substrate temperature”, Dr Jharna Paulstates, and she continues “Once you optimise© Oxford Instruments plc, 2021. University of Glasgow case study: Developing Superconducting Qubits 3

100 nm

200 nm

300 nm

400 nm

500 nm

600 nm

700 nm

800 nm

900 nm1000 nm

The PlasmaPro® 100 Cobra ICP“effectively controls the etch rate
and makes the process morestable”.

Figure 1:Profile Post Etch of Al with
Cl2/ HBr/ BCl3. Very high selectivity toZEP mask. Al etch profile circa > 88◦.
Courtesy of Dr James Grant, Senior
Plasma Process Research Engineer inJames Watt Nanofabrication Centre.

100 nm

600 nm

200 nm

700 nm

300 nm

800 nm

400 nm

900 nm

500 nmOxford Instruments PlasmaPro® 100 Cobra ICP
provides application related flexibility with
controlled high etch rates for deep features andlow etch rates for shallow features with highprecision and low damage. According to Dr
Jharna “it produces high quality vertical sidewalls
which are very important to minimise losses”.
In the images to the left, the Quantum Circuits
Group has successfully achieved a vertical etchin the circuits with Oxford Instruments’ ICP system,etching Al and NbN thin films.

1000 nm

Figure 2: Profile Post Etch of NbN
with CF4 Ar RIE (ZEP mask still on).
NbN etch profile circa > 88◦. Courtesyof Dr James Grant, Senior Plasma
Process Research Engineer in James
Watt Nanofabrication Centre.

Finally, by using endpoint detectors where theresearchers can monitor the process in real‑time,
Dr Jharna and the research team can detect
the point where all the etched film has been
removed, achieving excellent reproducibility.

© Oxford Instruments plc, 2021. University of Glasgow case study: Developing Superconducting Qubits 4

Driving the Future of Quantum
Industry

Emailplasma@oxinst.com

Recently, the University of Glasgow launched
a dedicated Centre for fundamental andtranslational research in quantum technology3 fora wide range of applications including computing,information processing, photonics and sensors.
For device fabrication, the University of Glasgow
uses various plasma systems, including
PlasmaPro® 100 RIE and FlexAL® ALD system.
Oxford Instruments is an intrinsic part of thequantum community and to enable the quantum
devices to be characterised at millikelvintemperatures, in January 2021, the university
announced the purchase of Oxford Instruments’next generation Cryofree® refrigerator, Proteox.
Oxford Instruments is a long‑term strategic
partner with the university, and we will continue
to support in their cutting‑edge research anddevelopment projects into quantum and manyother devices.

PhoneReferences

Dr Jharna Paul
Research Associate,University of Glasgow

1. Appl. Phys. Lett. 119, 033502 (2021); https://doi.org/10.1063/5.0054837
Submitted: 22 April 2021. Accepted: 24 June
2021. Published Online: 19 July 2021
2. Appl. Phys. Lett. 117, 124005 (2020); https://doi.org/10.1063/5.0023533
Submitted: 30 July 2020. Accepted: 08
September 2020. Published Online: 24
September 2020
3. University of Glasgow Launches Centre forQuantum Technology, University of Glasgow,
29 September 2021, https://futurescot.
com/scottish-university-launches-quantumtechnology-centre/

Valentino Seferai
PhD student, University of Glasgow

UK, Yatton+44 (0) 1934 837000
Germany, Wiesbaden+49 (0) 6122 937 161
India, Mumbai+91 22 4253 5100
Japan, Tokyo+81 3 5245 3261
PR China:• Beijing• Guangzhou• Shanghai+86 400 678 0609
Singapore+65 6337 6848
Taiwan+886 3 5788696
US, Canada & Latin America
Concord, MA+1 800 447 4717

plasma.oxinst.com© Oxford Instruments plc, 2021. University of Glasgow case study: Developing Superconducting Qubits 5