An Alternate Mechanism for Creating Functional Sub-micrometer Superconducting Quantum Interference Devices



Journal Title

Journal ISSN

Volume Title



Localized detection of very small regularly placed magnetic systems, such as an array of tiny magnetic islands, has been of great interest to scientists for years because of their applications to data storage media. One such detection device, the micro-SQUID (microscopic superconducting quantum interference device), can be used to detect very small changes in magnetic flux. Most low Tc micro-SQUIDs (LTS) are made from aluminum or niobium. While an aluminum SQUID is relatively easy to fabricate, one often needs a low temperature system than can be cooled to 1K to see the critical current phenomenon. As a contrast, niobium which has a higher critical temperature, and a more complicated fabrication procedure due to its need to be fabricated in a extremely clean environment to achieve a reproducible value for its critical temperature, which is about 9.25 K. Such a SQUID will only need to be immersed in a helium bath for the superconducting transition to occur. Alternatives such as tin, indium and lead, which are soft superconductors, do not wet a silicon/silicon dioxide surface as easily as niobium and aluminum. However, the benefits of a successful implementation of these soft superconductors as SQUIDs could greatly outweigh their drawbacks in terms of reducing the amount of time necessary for fabrication and measurement as well as the low temperature system requirements. In this dissertation, the successful development of functional square and rectangular tin sub-microscopic SQUIDs for use as magnetometers is reported. The application of a germanium pre-nucleation layer, as a means of creating a electrically continuous path, offers an alternative to micro-SQUIDs fabricated under more in- volved methods as used in niobium SQUIDs. An image of the device surface showed that the roughness consisted of defects such as holes which gives rise to critical current fluctuations and vortex pinning due to magnetic hysteresis. However, the oscillations observed from several of these devices, were found to be smooth with sharp edges but with a diminished period of oscillation. Several devices were tested and their fabrication, measurement and characterization methods are described. Another important study incorporated in our analysis of these tin germanium SQUID included its reduction from the micrometer regime to its lowest functional ge- ometry. Moreover, to avoid the operational breakdown of a SQUID due to magnetic hysteresis and a diminution in sensitivity, the condition 2IcL / 0 had to be satisfied, where Ic is the critical current and L is the inductance of the device and 0 is one flux quantum 0 = h 2e = 20:86 gauss mm2. Experimental measurements showed that all of these devices had magnetic hysteresis and operated outside of this constraint. In addition, several devices exhibited extremely high critical currents when the temper- ature was lowered a few milli-Kelvin past the transition temperature. Furthermore, unstable regions were present in the minima of the modulations indicating that ad- ditional quantum effects were incorporated into the device as a result of screening currents and magnetic hysteresis behavior.