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High-Temperature Superconducting Magnetometers for On-Scalp MEG

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High-Temperature Superconducting Magnetometers for On-Scalp MEG Thesis for the degree of Doctor of Philosophy High-temperature superconducting magnetometers for on-scalp MEG Silvia Ruffieux Department of Microtechnology and Nanoscience – MC2 Quantum Device Physics Laboratory Chalmers University of Technology Göteborg, Sweden 2020 High-temperature superconducting magnetometers for on-scalp MEG Silvia Ruffieux ISBN 978-91-7905-356-7 © Silvia Ruffieux, 2020. Doktorsavhandlingar vid Chalmers tekniska högskola Ny serie nr 4823 ISSN 0346-718X Quantum Device Physics Laboratory Department of Microtechnology and Nanoscience – MC2 Chalmers University of Technology SE–412 96 Göteborg Sweden Telephone: + 46 (0)31 772 1000 Cover: Top left: AFM image of the microbridges forming the Josephson junctions in a bicrystal grain boundary SQUID. Bottom left: Voltage modulation of a SQUID as a function of flux and bias current. Top center: SQUIDs in the 7-channel system. Center: 7-channel system placed to record from the author’s head. Bottom center: Resonance shift in a KIM when applying a dc magnetic field. Top left: Micrograph of a fabricated prototype integrated flux transformer device. Top left: Simulation of the sheet current density in a SQUID washer coupled to a flux transformer input coil. Printed by Chalmers Reproservice Göteborg, Sweden 2020 High-temperature superconducting magnetometers for on-scalp MEG Silvia Ruffieux Department of Microtechnology and Nanoscience – MC2 Chalmers University of Technology Göteborg, Sweden 2020 Abstract In the growing field of on-scalp magnetoencephalography (MEG), brain activity is studied by non-invasively mapping the magnetic fields generated by neuronal currents with sensors that are flexibly placed in close proximity to the subject’s head. This thesis focuses on high-temperature superconducting magnetometers made from YBa2Cu3O7−x (YBCO), which enables a reduction in the sensor-to-room temperature standoff distance from roughly 2 cm (for conventional MEG systems) down to 1 mm. Because of the higher neuromagnetic signal magnitudes available to on-scalp sensors, simulations predict that even a relatively low-sensitivity (higher noise) full-head on-scalp MEG system can extract more information about brain activity than conventional systems. In the first part of this thesis, the development of high critical temperature (high-Tc) super- conducting quantum interference device (SQUID) magnetometers for a 7-channel on-scalp MEG system is described. The sensors are single layer magnetometers with a directly coupled pickup loop made on 10 mm × 10 mm substrates using bicrystal grain boundary Josephson junctions. We found that the kinetic inductance strongly varies with film quality and temperature. Determination of all SQUID parameters by combining measurements and inductance simulations led to excellent agreement between experimental results and theoretical predictions. This allowed us to perform an in-depth magnetometer optimization. The best magnetometers achieve a magnetic field noise level p p of 44 fT/ Hz at 78 K. Fabricated test SQUIDs provide evidence that noise levels below 30 fT/ Hz are possible for high quality junctions with fairly low critical currents and in combination with the optimized pickup loop design. Different feedback methods for operation in a densely-packed on-scalp MEG system were also investigated. Direct injection of current into the SQUID loop was identified as the best on-chip feedback method with feedback flux crosstalk below 0.5%. By reducing the operation temperature, the noise level can be further reduced, however, the effective area also decreases because of the decreasing kinetic inductance contribution. We present a method that allows for one-time sensor calibration independent of temperature. In the second part, the design, operation, and performance of the constructed 7-channel on- scalp MEG system based on the fabricated magnetometers is presented. With a dense (2 mm edge-to-edge) hexagonal head-aligned array, the system achieves a small sensor-to-head standoff p distance of 1-3 mm and dense spatial sampling. The magnetic field noise levels are 50-130 fT/ Hz and the sensor-to-sensor feedback flux crosstalk is below 0.6%. MEG measurements with the system demonstrate the feasibility of the approach and indicate that our on-scalp MEG system allows retrieval of information unavailable to conventional MEG. In the third part, two alternative magnetometer types are studied for the next generation system. The first alternative is magnetometers based on Dayem bridge junctions instead of bicrystal grain boundary junctions. With a magnetometer based on the novel grooved Dayem bridge junctions, a p magnetic field noise level of 63 fT/ Hz could be achieved, which shows that Dayem bridge junctions are starting to become a viable option for single layer magnetometers. The second alternative are high- SQUID magnetometers with an inductively coupled flux transformer. The best device Tc p with bicrystal grain boundary junctions reaches a magnetic field noise level below 11 fT/ Hz and outperforms the best single layer device for frequencies above 20 Hz. In the last part, the potential of kinetic inductance magnetometers (KIMs) is investigated. We demonstrate the first high- KIMs, which can be operated in fields of 9-28 µ and achieve a noise p Tc T level of 4 pT/ Hz at 10 kHz. Keywords: high-Tc SQUID, magnetoencephalography, on-scalp MEG, magnetometer, SQUID optimization, multi-channel system, crosstalk, SQUID magnetometer calibration, kinetic inductance, kinetic inductance magnetometer. i To my love Jonathan my little sunshine Felicia and the heart beating below mine "The mind is not a book, to be opened at will and examined at leisure. Thoughts are not etched on the inside of skulls, to be perused by any invader. The mind is a complex and many-layered thing, Potter. Or at least most minds are..." J.K.Rowling ii Acknowledgement The past five years have been among the best of my life (so far) and I consider myself very lucky to have found this PhD position. The time here would not have been half as great if it were not for the people by my side and I would like to thank them for being there. First of all, I would like to thank my supervisors Dag Winkler, Alexei Kalaboukhov, and Justin Schneiderman for giving me the opportunity to work on this interesting and diverse project. Dag, you were a great source of ideas and you gave me a lot of freedom in planning my PhD and deciding what to do. Thank you also for bringing us to the weekly innebandy training – your priorities are an inspiration. Alex, your broad scientific expertise has been invaluable. Although you were constantly busy with experiments or fabrication (or running from one to another), you always found some time to help me and give me advice. I appreciate your logical way of thinking and reasoning. Thank you also for ’healing’ my SQUIDs with a CeO2 buffer layer. Thank you, Justin, for bringing the medical aspect into my PhD project and giving the superconducting sensors a greater purpose. I very much enjoyed that my project has a useful application. Thank you also for the advice and polishing my writing – your comments generally significantly improved my written work. Next I would like to thank my friends and colleagues in the SQUID lab – Maxim Chukharkin, Minshu Xie, Christoph Pfeiffer and Sobhan Sepehri – as well as the YBCO team – Edoardo Trabaldo, Eric Wahlberg (Andersson), Riccardo Arpaia, Marco Arzeo, Floriana Lombardi, Thilo Bauch – for making my PhD very enjoyable and fun. Thank you also for the advice, the common measurements, the help in the lab and the cleanroom, and the common conference visits. Maxim, I am very grateful that you introduced me to the art of fabricating high-temperature superconducting devices, taught me cleanroom techniques, and generously shared your knowledge about YBCO SQUID magnetometers. I very much enjoyed working together, including your life wisdom, encouragements when films were bad, and comments such as "There is no magic – you just don’t understand it yet. Try again." Minshu, I have learned a lot from you and enjoyed our discussions, measurements and cleanroom visits. Thank you also for introducing me to the inductance simulation – it has been of great use to me. Chris, thank you for the great teamwork getting the 7-channel system to work and being my friend also outside of work. Figuring out why things were not working was much more fun together. Sobhan, you were my iii friend here from day one. I will always remember our road trip together through the US. Thank you also for taking care of 3-month-old Felicia while I was teaching. I would like to thank Visa Vesterinen from VTT Finland for his two month visit at Chalmers where we measured kinetic inductance magnetometers (KIMs) together. Thank you for teaching me how to read out KIMs, answering all my questions (also after you left), introducing me to lab instrument control, and for being the postdoc I wished we had. Kiitos! This work would not have been possible without the support of the cleanroom staff at MC2 and their dedication to keeping the equipment running. I would especially like to thank Niclas Lindvall for the help with electron beam lithography and the nice conversations (and leaving me a tidy office), Henrik Frederiksen for the big help with the DCA system and especially the YBCO PLD system, Mats Hagberg for maintenance of the argon ion milling system (including getting rid of algae in the water cooling system with me), and finally Johan Karl Andersson for help with the laser writer. I normally learned most about the systems when they failed, and you were all there to solve my problems and teach me more about the tools. Thank you, Lars Jönsson, for your excellent work on the cryostats, and the quick and elegant fixes to make my measurements work. I appreciate the administrative support from Debora (Debbie) Perlheden, Susannah Carlsson, Maria Tremblay, Linda Brånell, Annika Holtringer, and the IT support from Henric Fjellstedt.
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