Hybrid Subtractive-Additive Method Of Producing A Superconducting Multi-Layer Transition-Edge Sensor

patent description

The proposed invention is a method of producing a superconducting multi-layer transition sensor in which the first layer of the multilayer is deposited and patterned completely with a subtractive process before the subsequent layers are deposited and patterned using separate additive processes.

A commonly used superconducting multi-layer for transition edge sensors is a bilayer consisting of one superconducting layer and one normal metal layer. Currently, both layers of the superconducting bilayer are deposited in the same deposition step without breaking vacuum.  This ensures that interface between the two layers is of high quality. However, it also places limits on the available deposition and patterning techniques. The deposition method either has to be the same for both layers or a specialized chamber that can accommodate multiple deposition approaches must be used.

Currently, superconducting bilayers are patterned using an additive approach during deposition or with subtractive approaches after deposition. Subtractive methods are difficult with noble metals frequently used in superconducting bilayers.  In both of the current approaches, additional steps may be required to eliminate superconducting shorts on the edge of the device.

With the proposed invention, the superconducting metal can be deposited and patterned first, and the normal metal layer can be designed to slightly overhang over the edge of the superconductor. This completely eliminates the requirement for additional normal metal features to suppress superconductivity on the edge of the device. The additional normal metal features are known to impact the current distribution in present transition edge sensor structures.

Additionally, the proposed invention allows the formation of new bilayer transition edge structure types not possible with current approaches. For example, the superconducting film can be patterned into a meandering wire, parallel filaments, or a mesh before the normal metal film is deposited. These new structures may provide a means to tune the properties of the superconducting transition without significantly modifying the resistance of the structure. The proposed method is easily extended to multilayer structures.

The main limitation of the proposed invention is that any metal oxide on the first layer must be removed with a process that is highly uniform across the wafer (better than a few percent). We have already demonstrated two methods that meet this requirement. The first is the use of deposition system designed to have a high uniformity in vacuum energetic ion based clean. The second is the use of an atmospheric plasma clean that can remove the metal oxide and provide a thin passivation layer that lasts for a long enough duration to deliver the wafer to the vacuum deposition system. If the process used to remove the metal oxide can thin down the metal layer with high uniformity, there is the potential to adjust the superconducting transition temperature of the multilayer.