There has been a great deal of interest in building bolometers from hightemperature superconductors due to their high transition temperatures and the associated ease of cooling. High-temperature superconducting (high Tc) bolometers are difficult to fabricate because the standard method of thermal isolation is not compatible with these materials. A method is described that allows a standard thermal isolation technique (using amorphous silicon nitride membranes) to be used with high-temperature superconductors.

Schematic of processing to create thermally isolated YBCO structures on amorphous Si₃N₄.
The solution combines two techniques: ion beam assisted deposition (IBAD), and xenon difluoride (XeF2) gas etching for undercutting Si. IBAD allows for the growth of YBCO (yttrium barium copper oxide) onto amorphous silicon nitride (Si3N4). Then XeF2 gas etching is used to undercut the bulk Si below the Si3N4, creating a thin, thermally isolated membrane of Si3N4 with YBCO on top.

The new idea is the application itself. People have thought of making hightemperature superconducting bolometers, but the method of thermal isolation has always been an issue. For instance, elaborate wafer bonding techniques have been used to accomplish this. The idea of using IBAD buffer layers on Si3N4 and then depositing the high-temperature superconductor significantly eases the thermal isolation process.

The crux of the innovation comes from merging the two technologies. IBAD uses a standard thin film deposition method such as sputtering, and the simultaneous bombardment of the sample with an ion beam at a grazing angle. The ion beam facilitates the formation of polycrystalline surfaces on amorphous ones. This fact allows the deposition of a thin (≈100 nm) buffer layer of polycrystalline MgO onto the amorphous Si3N4. After this buffer layer is deposited, standard techniques can then be used to deposit YBCO (a common high Tc material) on top of the MgO/Si3N4 stack.

After establishing that high-quality YBCO could be deposited onto the MgO/Si3N4 stack, standard photolithography and etching techniques were used to pattern YBCO structures. Then, the process of undercutting the silicon under the MgO/Si3N4 stack was performed. The undercut was accomplished using XeF2 gas etching. Standard lithography and etching techniques were used to open up holes in the MgO/Si3N4 stack so that bare silicon is exposed in the desired regions. A photoresist mask protects the remainder of the structure. The sample is then put into the XeF2 gas etcher and the Si is undercut, resulting in YBCO structures residing on top of a thermally isolated MgO/Si3N4 membrane.

This work was done by Joseph A. Bonetti, Alan W. Kleinsasser, Warren A. Holmes, and Peter K. Day of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it.. NPO-48711

NASA Tech Briefs Magazine

This article first appeared in the December, 2015 issue of NASA Tech Briefs Magazine.

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