INTEGRATION SCHEME FOR SHUNTED JOSEPHSON JUNCTIONS

Materials with etch selectivity with respect to one another and one or more additional etch-stop layers are used in a Josephson junction structure to allow for integration with a Josephson junction with supporting structures such as resistors. Selective etch processes compatible with high volume manufacturing are used to pattern various layers of the Josephson junction structure to provide a Josephson junction, which is electrically coupled to a support structure.

FIELD OF THE DISCLOSURE

The present disclosure is related to Josephson junctions, and in particular to Josephson junctions including a shunt resistor and methods for manufacturing the same.

BACKGROUND

A Josephson junction includes a non-superconducting material sandwiched between two layers of superconducting material. Josephson junctions can be used to build electronic circuitry, and in particular logic circuitry and quantum computing circuitry. Further, Josephson junctions can be arranged to provide superconducting quantum interference devices (SQUIDs), which can be used for extremely sensitive measurement tools. Integrating one or more Josephson junctions along with other circuitry may require supporting components such as a shunt resistor. Generally, it is desirable to integrate these supporting components along with the Josephson junctions. However, processes for fabricating Josephson junctions including one or more support components have generally been defined by low volume, complex fabrication techniques. As demand for circuitry including Josephson junctions continues to increase, there is a need for Josephson junctions integrated with supporting components and processes for providing these Josephson junctions using high volume manufacturing techniques.

SUMMARY

In one embodiment, a method for manufacturing a Josephson junction structure begins with providing a substrate. An insulating layer is provided on the substrate. A support structure is on the insulating layer. A wetting layer is provided on the insulating layer and the support structure. An etch-stop layer is provided on the wetting layer. A base metal layer is provided on the etch-stop layer. The base metal layer comprises a superconducting material and is in electrical contact with the support structure via the etch-stop layer and the wetting layer. A middle layer is provided on the base metal layer. The middle layer comprises an insulating material. A top metal layer is provided on the middle layer. The top metal layer comprises a superconducting material. The top metal layer is patterned with a first etch process that is selective with respect to the middle layer. The middle layer is patterned with a second etch process that is selective with respect to the base metal layer. The base metal layer is patterned with a third etch process that is selective with respect to the etch-stop layer. The etch-stop layer and the wetting layer are patterned with a fourth etch process that is selective with respect to the support structure. The base metal layer, the middle layer, and the top metal layer are patterned such that they provide a Josephson junction that is electrically coupled to the support structure. Providing the etch-stop layer and the wetting layer allows the materials for the support structure and the base metal layer to be chosen independently of their etch selectivity with respect to one another. Providing the etch-stop layer and the wetting layer and patterning the layers using selective etch processes, the Josephson junction structure can be manufactured using high volume manufacturing techniques.

In one embodiment, the support structure is a resistor. The resistor may be a shunt resistor. In various embodiments, the resistor comprises one of titanium tungsten and tungsten. The wetting layer comprises titanium. The etch-stop layer comprises aluminum. The base metal layer and the top metal layer comprise niobium. The middle layer comprises aluminum oxide.

The first etch process and the third etch process may utilize an etching solution comprising fluorine. The second etch process and the fourth etch process may utilize an etching solution comprising chlorine.

In one embodiment, a Josephson junction structure includes a substrate, an insulating layer, a wetting layer, an etch-stop layer, a base metal layer, a middle layer, and a top metal layer. The insulating layer is on the substrate. The support structure is on the insulating layer. The wetting layer is on the insulating layer and the support structure. The etch-stop layer is on the wetting layer. The base metal layer is on the etch-stop layer. The base metal layer comprises a superconducting material and is in electrical contact with the support structure via the etch-stop layer and the wetting layer. The middle layer is on the base layer and comprises a non-superconducting material. The top metal layer is on the middle layer and comprises a superconducting material. Providing the etch-stop layer and the wetting layer allows the materials for the support structure and the base metal layer to be chosen independently of their etch selectivity with respect to one another.

DETAILED DESCRIPTION

FIG.1shows a cross-sectional view of a Josephson junction structure10including an integrated shunt resistor according to one embodiment of the present disclosure. The Josephson junction structure10includes a substrate12, an insulating layer14on the substrate12, a resistor structure16on the insulating layer14, a wetting layer18on the insulating layer14and a portion of the resistor structure16, an etch-stop layer20on the wetting layer18, a base metal layer22on the etch-stop layer20, a middle layer24on the base metal layer22, and a top metal layer26on the middle layer24. As shown, the wetting layer18, the etch-stop layer20, and the base metal layer22are patterned into separate sections that are separated by the resistor structure16. The middle layer24and the top metal layer26are patterned in a desired shape to form a Josephson junction28including the base metal layer22, the middle layer24, and the top metal layer26.

As discussed in detail below, the materials for the resistor structure16, the wetting layer18, the etch-stop layer20, the base metal layer22, the middle layer24, and the top metal layer26are chosen not only for one or more electrical characteristics thereof (e.g., superconducting, non-superconducting, resistivity, etc.), but also for their selectivity with respect to one another in certain etch processes used to form the Josephson junction structure10. In particular, etch processes for the base metal layer22and the top metal layer26are selective with respect to the middle layer24and the etch-stop layer20, and vice-versa. Similarly, etch processes for the etch-stop layer20and the wetting layer18are selective with respect to the resistor structure16. As defined herein, an etch process is selective between materials when the etching rate of the etching process between the material for which etching is not desired (e.g., a lower layer) and a material for which etching is desired (e.g., an upper layer) is less than 1. In some embodiments, however, the etching rate between the material for which etching is not desired and the material for which etching is desired may be less than 0.75, less than 0.5, and less than 0.1.

To satisfy the above requirements, the resistor structure16may comprise titanium tungsten (TiW) or tungsten. However, the resistor structure16may also comprise any other suitable material, and in various embodiments can comprise any non-superconducting metal that is not magnetic and is also selective in an etch process with respect to the etch-stop layer20and the wetting layer18. Previously, the base metal layer22was provided directly on the resistor structure16. Such a structure required etch selectivity between the base metal layer22and the resistor structure16. Due to the superconducting requirements of the base metal layer22to provide the Josephson junction28, the materials available for the resistor structure16were very limited (e.g., to noble metals including gold that have a low sheet resistivity). Due to the low sheet resistivity of the materials available for the resistor structure16, a large area or footprint was required to achieve a desired resistance. The use of the wetting layer18and the etch-stop layer20described herein effectively decouples the etch selectivity requirements of the base metal layer22and the resistor structure16, which enables the use of additional categories of materials for the resistor structure16. In particular, this may enable the use of materials with a high sheet resistivity for the resistor structure16. As a result, the resistor structure16can have a much smaller area or footprint compared to previous approaches while providing a desired resistance. In one embodiment, an area of the resistor structure16may be less than 90 μm2, and may be as low as 10 nm2in some embodiments.

As discussed above, to provide the Josephson junction28the base metal layer22and the top metal layer26must comprise superconducting materials, while the middle layer24must comprise a non-superconducting material. Further, the top metal layer26must be selective with respect to the middle layer24in an etch process, and the middle layer24must be selective with respect to the base metal layer22in an etch process. In one embodiment, the base metal layer22and the top metal layer26comprise niobium, while the middle layer24comprises aluminum oxide (Al2O3). The base metal layer22and the top metal layer26may also comprise other niobium class superconductors such as niobium-titanium nitride (Nb—Ti)N, niobium nitride (NbN), or any other superconducting material that meets the etch selectivity requirements discussed herein.

The wetting layer18and the etch-stop layer20may comprise titanium and aluminum, respectively. However, the wetting layer18and the etch-stop layer20may comprise any suitable material that is selective in an etch process with the base metal layer22and the resistor structure16while also being electrically conductive such that the base metal layer22electrically contacts the resistor structure16via the etch-stop layer20and the wetting layer18. The wetting layer18must also be capable of smooth deposition onto the insulating layer14and the resistor structure16.

The substrate12may comprise any suitable material such as silicon. The insulating layer14may similarly comprise any suitable material such as silicon oxide (SiO2).

A thickness of the base metal layer22and the top metal layer26may be between 1 nm and 500 nm. The thickness of the resistor structure16should be at least 1.5 times less than the thickness of the base metal layer22and the top metal layer26, and can be up to 5.0 times less, depending on a desired resistance of the resistor structure16. A thickness of the wetting layer18may be between 1 nm and 100 nm. A thickness of the etch-stop layer20may be between 1 nm and 100 nm. A thickness of the middle layer24may be between 1 nm and 20 nm.

The Josephson junction structure10discussed herein is capable of being made in a high-volume manufacturing facility with existing semiconductor fabrication tooling (e.g., for complementary metal-oxide semiconductor (CMOS) devices). In addition to the constraints discussed above, the materials for the substrate12, the insulating layer14, the resistor structure16, the wetting layer18, the etch-stop layer20, the base metal layer22, the middle layer24, and the top metal layer26may thus be chosen for their compatibility with a particular fabrication tooling scheme, which may restrict the available pool of materials for each of the above. For example, gold is generally not compatible with CMOS fabrication processes due to the affinity thereof for diffusion into semiconductor materials.

FIG.2shows a cross-sectional view of the Josephson junction structure10according to an additional embodiment of the present disclosure. The Josephson junction structure10shown inFIG.2is substantially similar to the one shown inFIG.1, but further includes a metal contact layer30and an additional insulating layer32. The metal contact layer30is patterned to contact the top metal layer26of the Josephson junction28in order to provide a signal path to/from the Josephson junction28. While not shown, the metal contact layer30may be further patterned to provide a signal path to/from the base metal layer22directly and/or via the resistor structure16(e.g., by contacting the base metal layer22at the right side of the resistor structure16as shown).

FIG.3is a flow diagram illustrating a method for manufacturing a Josephson junction structure according to one embodiment of the present disclosure. The steps described inFIG.3are illustrated sequentially inFIGS.4A through4N, and are therefore discussed along withFIG.3. First, the substrate12is provided (step100andFIG.4A). The substrate12may be provided by any suitable means. For example, the substrate12may be grown by a suitable crystal growth process and subsequently processed (e.g., cleaned, textured, etched) to provide a suitable shape, size, and surface quality. The insulating layer14is provided on the substrate12(step102andFIG.4B). The insulating layer14may be provided by any suitable process, and in some embodiments may include multiple steps such as a deposition and subsequent oxidation.

A resistor layer34is provided on the insulating layer14(step104andFIG.4C). The resistor layer34may be provided by any suitable means, including a deposition process such as physical vapor deposition (PVD). The resistor layer34is patterned to provide the resistor structure16(step106andFIG.4D). The resistor layer34may be patterned by any suitable process. For example, the resistor layer34may be patterned using a mask and etch process, the details of which will be readily understood by those skilled in the art and thus are not included herein. As discussed above, the resistor layer34may comprise a material that is selective with respect to the insulating layer14in an etch process. Accordingly, the resistor layer34can be etched into a desired pattern to provide the resistor structure16without impacting the insulating layer14.

Subsequent to the patterning of the resistor layer34to provide the resistor structure16, the Josephson junction structure10is cleaned (step108andFIG.4E). In particular, the exposed portions of the insulating layer14and the resistor structure16may be cleaned. As discussed above, the resistor structure16may comprise titanium tungsten (TiW) or tungsten, which may oxidize relatively easily. Cleaning the Josephson junction structure10may remove said oxidation and thus allow for higher quality surfaces to be achieved in the subsequently deposited layers. Notably, steps108through120below may be performed as an integrated process in a physical vapor deposition (PVD) chamber such that the Josephson junction structure10does not need to be removed from the chamber between steps. The cleaning step may prepare the Josephson junction structure10for the subsequent steps of the manufacturing process to provide high quality surfaces and thus electrical connections between the various parts thereof. In one embodiment, the cleaning process comprises a sputter clean. However, any suitable cleaning process may be performed.

The wetting layer18is then provided on the exposed portions of the insulating layer14and the resistor structure16(step110andFIG.4F). The wetting layer may prepare the surface of the insulating layer14and the resistor structure16for deposition of the etch-stop layer20, providing a barrier between the etch-stop layer20and the insulating layer14. This may be important because the materials of the insulating layer14and the etch-stop layer20may interact in such a way that the etch-stop layer20would not otherwise form a smooth surface on the insulating layer14. For example, aluminum tends to agglomerate on silicon oxide (SiO2). Providing the wetting layer18may prevent said agglomeration and therefore allow for the formation of a smooth surface on which to build the Josephson junction28above. Since Josephson junction performance is directly correlated to the morphology or smoothness of the barrier layer between the superconducting layers, and the morphology of the barrier layer is dependent on the morphology of all of the layers below it, providing the wetting layer18may significantly improve the performance of the Josephson junction structure10. The wetting layer18may be provided by any suitable means including a deposition process such as PVD.

The etch-stop layer20is provided on the wetting layer18(step112andFIG.4G). As discussed above, the etch-stop layer20allows for patterning of the base metal layer22without requiring etch selectivity between the base metal layer22and the resistor structure16, thereby opening up more combinations of materials and allowing the Josephson junction structure10to be manufactured in high volume facilities. The etch-stop layer20may be provided by any suitable means including a deposition process such as PVD.

The base metal layer22is provided on the etch-stop layer20(step114andFIG.4H). The base metal layer22may be provided by any suitable means including a deposition process such as PVD. The middle layer24is provided on the base metal layer22(step116andFIG.4I). The middle layer24may be provided by any suitable means including a deposition process such as PVD. Providing the middle layer24may comprise multiple steps including a deposition step and an oxidation step (e.g., to provide aluminum oxide as discussed above), the details of which will be readily understood by those skilled in the art and thus are not included herein. The top metal layer26is provided on the middle layer24(step118andFIG.4J). The top metal layer26may be provided by any suitable means including a deposition process such as PVD.

While not shown in the flow diagram ofFIG.3, additional protective layers may be provided on the top metal layer26. The additional protective layers may include, for example, silicon nitride (SiN). The additional protective layers may prevent oxidation of the top metal layer26during subsequent patterning and processing thereof, which may decrease manufacturing duration and complexity and allow for a more simplified fabrication process.

The top metal layer26is patterned (step120andFIG.4K). The top metal layer26may be patterned by any suitable process. For example, the top metal layer26may be patterned using a mask and etch process, the details of which will be readily understood by those skilled in the art and thus are not included herein. Notably, the etch process used to pattern the top metal layer26is selective with respect to the middle layer24. In one embodiment, the etch process used to pattern the top metal layer26utilizes an etching solution comprising fluorine. While etching solutions including fluorine (e.g., hydrofluoric acid) will readily etch niobium, they will not etch aluminum oxide (or do so much more slowly compared to niobium) and are therefore selective with respect to niobium and aluminum oxide. Accordingly, the top metal layer26can be etched into a desired pattern with minimal effect on the middle layer24.

The middle layer24is patterned (step122andFIG.4L). The middle layer24may be patterned by any suitable process. For example, the middle layer24may be patterned using a mask and etch process, the details of which will be readily understood by those skilled in the art and thus are not included herein. Notably, the etch process used to pattern the middle layer24is selective with respect to the base metal layer22. In one embodiment, the etch process used to pattern the middle layer24utilizes an etching solution comprising chlorine. While etching solutions including chlorine (e.g., hydrochloric acid) will readily etch aluminum oxide, they will not etch niobium (or do so much more slowly compared to aluminum oxide) and are therefore selective with respect to aluminum oxide and niobium. Accordingly, the middle layer24can be etched into a desired pattern with minimal effect on the base metal layer22.

The base metal layer22is patterned (step124andFIG.4M). The base metal layer22may be patterned by any suitable process. For example, the base metal layer22may be patterned by a mask and etch process, the details of which will be readily understood by those skilled in the art and thus are not included herein. Notably, the etch process used to pattern the base layer22is selective with respect to the etch-stop layer20. In one embodiment, the process used to pattern the base metal layer22utilizes an etching solution comprising fluorine. While etching solutions including fluorine (e.g., hydrofluoric acid) will readily etch niobium, they will not etch aluminum (or do so much more slowly compared to niobium) and are therefore selective with respect to niobium and aluminum. Accordingly, the base metal layer22can be etched into a desired pattern with minimal effect on the etch-stop layer20and thus the resistor structure16below.

The etch-stop layer20and wetting layer18are patterned (step126andFIG.4N). The etch-stop layer20and wetting layer18may be patterned by any suitable process. For example, the etch-stop layer20and wetting layer18may be patterned by a mask and etch process, the details of which will be readily understood by those skilled in the art and thus are not included herein. Notably, the etch process used to pattern the etch-stop layer20and the wetting layer18is selective with respect to the resistor structure16. In one embodiment, the process used to pattern the etch-stop layer20and the wetting layer18utilizes an etching solution comprising chlorine. While etching solutions including chlorine (e.g., hydrochloric acid) readily etch aluminum and titanium, they will not etch titanium tungsten or tungsten (or do so much more slowly compared to aluminum and titanium). Accordingly, the etch-stop layer20and the wetting layer18can be etched into a desired pattern with minimal impact on the resistor structure16. In some embodiments, a single mask layer may be used to pattern multiple layers in the Josephson junction structure10. For example, a single mask layer and multi-step etch process may be used to pattern the middle layer24, the base metal layer22, the etch-stop layer20, and the wetting layer18in some embodiments.

As discussed above, contacts may be provided to various parts of the Josephson junction structure10such as those discussed above with respect toFIG.2. The processes for doing so will be readily understood by those skilled in the art and thus are not included herein.

The process described above is compatible with high volume manufacturing such as processes designed for semiconductor devices, and in particular CMOS devices. The choice of materials allows for the creation of a desired pattern at each layer and thus provides a Josephson junction that is integrated with a support structure such as the resistor structure16. Notably, the principles of the present disclosure are not limited to the integration of Josephson junctions with resistor structures, but rather are more broadly applicable to the integration of Josephson junctions with any support structures. The wetting layer18and the etch-stop layer20enable the integration of a Josephson junction with any desired support component (e.g., series resistors, other semiconductor devices such as diodes, transistors, and the like) using a similar process to the one discussed above.

While the above shows the base metal layer22directly on the etch-stop layer20, which is directly on the wetting layer18, which in turn is directly on the resistor structure16, there may be additional intervening layers such as an additional inter-layer dielectric layer between the base metal layer22and the resistor structure16. Vias or other metal connecting structures may be used to electrically couple the base metal layer22to the resistor structure16. Further, while the resistor structure16is shown on the insulating layer14and described as being provided before manufacture of the Josephson junction28, the resistor structure16may instead be provided on the additional insulating layer30such that the Josephson junction28is first provided and the resistor structure16is subsequently provided. In such an embodiment, the resistor structure16may contact the base metal layer22by any number of vias and/or additional metallization layers. The process for forming the Josephson junction28and the resistor structure16is the same as described above, with the order of manufacturing steps being changed. The selectivity of etch chemistries and selection of materials discussed throughout the present disclosure enables the manufacture of the Josephson junction structure10in any number of desired configurations.

To illustrate,FIG.5shows the Josephson junction structure10according to an additional embodiment of the present disclosure. The Josephson junction structure10is substantially similar to that shown inFIG.2above, except that the resistor structure16is moved on top of the additional insulating layer32rather than on the insulating layer14. The metal contact layer30electrically connects to the Josephson junction16using one or more vias34, and further extends onto a second additional insulating layer36. The metal contact layer30further electrically connects to the resistor structure16by one or more additional vias34.FIG.5is meant to illustrate that the placement of the resistor structure16is flexible. That is, the principles of the present disclosure enable a number of Josephson junction structures10with support structures that can be provided and located in several different ways. The Josephson junction structure10shown inFIG.5can be manufactured according to the same principles discussed above with respect toFIGS.3and4, with the order of steps changed.