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4.1 Microbridge Fabrication

To more closely monitor the electrical properties of a thin film superconductor, often it is useful to confine the conduction route to a narrow region.  This forces all of the current to travel through the user defined pathway.  The formation of a microbridge is one method that can be used to achieve this objective.  A superconductor microbridge is made when two conducting pads (usually of the superconductor) are connected only by a narrow pathway of superconductor, between 1-100 mm wide.  In this geometry, seen in Illustration 4.1, current is passed through the microbridge and the potential is measured on each side of the bridge.  This type of conductivity measurement minimizes the resistance due to the contacts.  Thin films of YBa₂Cu₃O_7-x deposited on insulating substrates, as described in Chapter 2, can be patterned into microbridge structures by a variety of methods.  The simplest method is to use the diamond tip of a micromanipulator to scribe the microbridge into the thin film by isolating the bridge from the rest of the superconductor.  This leaves behind the microbridge and two isolated superconductor sections.  The second method is to create the microbridge through laser patterning.¹⁶  Here, a thin film superconductor is placed under a mask so that the shadowed region is left behind.  A narrow Pt wire (either 25, 50, 100 mm) masks the bridge portion of the assembly.  The superconductor thin film and mask are positioned in front of the excimer laser (UV light, 248 nm).  The YBa₂Cu₃O_7-x material that is not covered by the mask is then ablated off by 3-5 laser pulses with an energy density of 0.5 J/cm².  The width of the microbridge is approximately the thickness of the Pt wire used.  Other researchers^{17, 18} have fabricated even smaller features (1 mm) by using a tightly focused beam of laser light onto the superconductor’s surface and moving the light’s position.  In this manner, thermal lithographic patterns may be formed.  The light thermally heats the superconductor so that those regions are deoxygenated.  This lowers their electrical conductivity as well as destroys superconductivity within that region.  Other routes employed in patterning superconductors are to use standard lithographic methods such as ion-milling^{19, 20} and wet-etching.²¹ 

Due to the chemical reactivity of the YBa₂Cu₃O_7-x, as discussed in Chapter 2, ion-milling and wet-etching techniques have not been used.  The thermal lithographic method avoids these chemical complications, but leaves behind material that is still somewhat resistive due to an unavoidable loss of oxygen from selected regions of the film.  The patterning methods were, therefore, not used for the electropolymerization depositions as well as mapping isolating electronic interactions.  Rather, diamond scribing with a micromanipulator and laser ablation patterning have been exploited.