Fabrication and noise properties of high-Tc SQUIDs with multilayer superconducting flux transformers

Doktorsavhandling, 2013

The thesis describes the development of highTc superconducting quantum interference devices (SQUIDs) with multilayer thin film flux transformers. High-Tc SQUID magnetometers are promising in various biomedical applications, including magnetoencephalography (MEG) and ultra-low field magnetic resonance imaging (ulf-MRI). Both MEG and ulf-MRI demand magnetic field sensitivity of less than 10 fT/Hz1/2 at frequencies as low as 10 Hz. The magnetic field sensitivity of single-layer high-Tc SQUID magnetometers is typically about 50 fT/Hz1/2. To improve the magnetic field sensitivity, superconducting flux transformer with a multiturn input coil should be used. The flux transformer requires multilayer superconducting structures, which is a significant challenge for high-Tc superconducting materials due to the anisotropy of the material and high temperatures required for a deposition of thin films. Chemical-mechanical polishing (CMP) of high-Tc superconducting films has been developed in this work to fabricate multilayer structures. CMP improves surface smoothness of films, thereby reducing galvanic shorts between top and bottom superconducting electrodes. It has been shown that edge slope angles of about 2 degree can be fabricated using CMP, meaning that crossovers with very high critical current densities 2*10$6 A/cm2 can be obtained. These results were important for successful fabrication of high-quality multilayer structures with high yield. The CMP technique was used to fabricate multiturn magnetic flux transformers on 10*10 mm2 STO substrates. A flip-chip magnetometer based on the developed multilayer flux transformer and a bicrystal SQUID was designed, fabricated and characterized. Magnetic field sensitivity of 8 fT/Hz1/2 at 2 kHz and 80 fT/Hz1/2 at 10 Hz has been demonstrated. Low-frequency magnetic flux noise was investigated and related with the microstructure of the flux transformer. The developed multilayer flip-chip flux transformer was used in ulf-NMR experiments and demonstrated the improvement in the signal-to-noise ratio (SNR) when compared to a planar SQUID magnetometer. The demonstrated gain in SNR indicates the new multilayer structures are a promising technology for high-Tc SQUID-based ulf-MRI systems. Lastly, high-Tc superconducting quantum interference filters (SQIFs) were designed and fabricated. The SQIF consisted of array of 50 SQUID loops connected in series along the bicrystal grain boundary. Electrical characterizations revealed a large spread of individual SQUID parameters that lead to the situation when only few SQUIDs from the whole array are operational at a certain bias current and voltage-to-field response demonstrates an absence of voltage dip at the zero external field. At the same time, it has been demonstrated that an array of identical SQUIDs benefit from higher voltage-to-flux transfer function as compared with a single SQUID.