The present disclosure relates to superconductors, and in particular to providing one or more barrier layers to protect superconducting structures.
Thin film niobium (Nb) is widely used as a superconducting material in superconducting (SC) electronics, such as SC integrated devices, circuits, and the like. The superconducting properties of niobium may be significantly degraded and often completely destroyed when the niobium is subjected to fabrication processes that involve temperatures at or above approximately 250° C. Other SC materials suffer the same fate at elevated fabrication temperatures. Accordingly, foundries that process niobium-based SC integrated circuits go to significant effort to design fabrication processes that do not exceed potentially damaging temperatures in order to maintain the superconducting properties of the niobium or like SC material. Furthermore, in many designs, the niobium is deposited in a layer, which is patterned to form SC structures. These SC structures are sandwiched between two dielectric layers. A common and desirable material for the dielectric layers is polyimide. Curing polyimide above a certain temperature is believed to lead to the diffusion of oxygen, hydrogen, or other degrading materials into the SC material, thereby greatly diminishing and/or destroying the SC properties of the niobium.
One or more dielectric barrier layers, such as, but not limited to, aluminum oxide (Al2O3), is used to isolate and protect super conductive (SC) structures. The SC structures are formed from SC materials, such as, but not limited to, niobium, from other surrounding materials. Using the barrier layers prevents the loss or degradation of the superconducting properties of the SC structures during subsequent fabrication steps that employ elevated temperatures. As a result, incorporation of the barrier layers relaxes the need to use lower temperature fabrication processes and opens up possibilities for use of different and more desirable processes and/or materials during subsequent fabrication steps.
An exemplary method includes the following steps. Initially, a substrate is provided. Optionally, a lower barrier is provided over the substrate. The lower barrier is formed from a first dielectric material. At least one superconductive structure is formed over the substrate, and if provided, the lower barrier. The at least one superconductive structure is formed from a superconductive material that provides superconductivity within at least a portion of the at least one superconductive structure. An upper barrier is provided over the at least one superconductive structure and portions of the substrate, or if provided, the lower barrier. The lower and upper barriers may be formed from the same or different dielectric materials. A dielectric layer is provided over the upper barrier. The dielectric layer is formed from a potentially destructive dielectric material, which is different from the materials used for the lower and upper barriers. The lower and upper barriers separate the at least one superconductive structure from the dielectric layer. Next, the potentially destructive dielectric layer is heated above a destructive temperature that would normally result in loss or degradation of superconductivity of the at least one superconductive structure without the presence of the lower and/or upper barriers.
In one embodiment, the superconductive material is at least one of a group consisting of niobium (Nb), aluminum (Al), titanium (Ti), tin (Sn), lead (Pb), indium (In), chromium (Cr), palladium (Pd), tantalum (Ta), and a compound containing at least one of niobium (Nb), aluminum (Al), titanium (Ti), tin (Sn), lead (Pb), indium (In), chromium (Cr), palladium (Pd), and tantalum (Ta). In one embodiment, the first material is at least one of a group consisting of aluminum oxide (Al2O3), titanium dioxide (TiO2), and hafnium oxide (HfO).
In another embodiment, a superconducting apparatus comprises a substrate, a lower barrier, at least one superconducting structure, an upper barrier, and a dielectric layer. The lower barrier resides over the substrate and is formed from a first material. The at least one superconductive structure resides over the lower barrier and is formed from a superconductive material that provides superconductivity within at least a portion of the at least one superconductive structure. The upper barrier resides over the at least one superconductive structure and portions of the lower barrier. The upper barrier is formed from the first material, and the lower barrier and the upper barrier encapsulate the at least one superconductive structure. The dielectric layer resides over the upper barrier and is formed from a potentially destructive dielectric material, which is different from the first material. The destructive nature of the dielectric material is triggered when it is heated to relatively high temperatures during fabrication.
Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.
The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.
The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.
It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.
Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
In this disclosure, one or more thin-film dielectric barrier layers, such as, but not limited to, aluminum oxide (Al2O3), are used to isolate and protect superconductive (SC) structures. The SC structures are formed from SC materials, such as but not limited to niobium, from other surrounding materials. Using the barrier layers prevents the loss or degradation of the superconducting properties of the SC structures during subsequent fabrication steps that employ elevated temperatures. As a result, incorporation of the barrier layers relaxes the need to use lower temperature fabrication processes and opens up possibilities for use of different more desirable processes and/or materials during subsequent fabrication steps. Prior to delving into the details of these concepts, an overview of a traditional wafer that includes SC structures is described.
With reference to
A dielectric, such as a polyimide dielectric 16, is provided over the exposed portions of the substrate as well as the SC structures 14. Most polyimide dielectrics 16 should be cured at temperatures at or above 300 to 350° C. for proper and thorough curing; however, to prevent damage to SC structures 14, which are formed from niobium or the like, the curing temperatures are often capped at 250° C. to prevent the SC structures 14 from losing their superconductive properties. Limiting the curing temperatures limits the effectiveness of the curing process for the polyimide dielectric 16, and as such, limits the functional effectiveness of polyimide dielectric 16. Again, while niobium-based SC structures 14 and polyimide dielectrics 16 are discussed in detail, other SC and dielectric materials suffer in the same or similar fashion. The concepts described below allow these same materials to be used in higher temperature fabrication processes without damaging the SC properties of the SC materials that are used for the SC structures 14.
Exemplary, but non-limiting, superconductive materials that may be used for the SC structures 14 include niobium (Nb), aluminum (Al), titanium (Ti), tin (Sn), lead (Pb), indium (In), chromium (Cr), palladium (Pd), and tantalum (Ta), as well as compounds containing one or more of the same, such as, Nb3Ge, Nb3Si, NbN, and the like. Other exemplary, but non-limiting compounds may include MgB2, NbTi, NbTiN, Nb3Al, Nb3Sn, Nb3Ga, and the like. Exemplary, but non-limiting, materials for the upper barrier layer 20 include aluminum oxide (Al2O3) titanium dioxide (TiO2), hafnium oxide (HfO), and the like. Exemplary combinations of materials for the SC structures 14 and the upper barrier layer 20 include niobium and aluminum oxide; niobium and titanium dioxide, and the like. Exemplary materials for the substrate 12 include, but are not limited to silicon (Si), silica (glass), sapphire, fused silica, polyimide (kapton), and the like. While the exemplary dielectric material used for discussion is polyimide, other dielectric materials that may affect the SC structures at higher processing temperatures include, but are not limited to silicon dioxide, silicon nitride, ALX 2010, ALX 2030, benzocyclobutene (BCB), and the like.
The polyimide material used for the polyimide dielectric 22 in the disclosed examples is Dupont's HD-4110 from HD Microsystems. The polyimide dielectric 22 is not limited to HD-4110, as those skilled in the art will appreciate. One goal is to be able to effectivity use those dielectric (and other) materials that require high temperature processing and would normally affect the superconductivity of the SC structures 14 when processed at these high temperatures without the use of the concepts provided herein. While only one SC structure 14 is illustrated in the various embodiments, those skilled in the art will recognize that the wafer 18 and any die, conductors, cabling, or the like formed therefrom will likely include a plurality of such elements that may take on any number of shapes and sizes.
Turning now to
The concepts described herein employ one or more barrier layers, such as the lower and upper barrier layers 24, 20, to protect the superconductive characteristics of the SC structures 14 during high temperature fabrication steps. Superconductivity for a superconductive material is defined herein as having a resistivity of zero below the transition of the superconductive material. An SC structure 14 is defined as an structure that is formed from a SC material and provides superconductivity throughout all or a portion of the structure. Using the concepts described herein, the superconductivity of the SC materials for the SC structures 14 is not lost, but may be marginally effected, even when the SC structures 14 are subjected to subsequent fabrication process temperatures at or above 250° C., 275° C., 300° C., 350° C., and even 375° C.
As illustrated in
The second scenario (Nb/HD-4110 @ 225° C.-circle line) is one in which the polyimide dielectric 22 was provided; however, the polyimide dielectric 22 was cured at a reduced temperature of 225° C. in a nitrogen (N2) atmosphere. With the reduced curing temperature, only a small amount of degradation occurs, which corresponds to a small reduction in the critical transition temperature Tc. In other words, the transition from the normal state (highly resistive) to the superconductive state is shifted only slightly to the left (i.e. lower in temperature). Further, only a minimal degradation in the critical current Ic and residual resistance ratio (RRR) measurements (not shown) occurs. The critical current Ic drops from 186 mA to 182 mA, and the RRR drops from 3.58 to 3.34.
The third scenario (Nb/HD-4110 @ 375° C.-dashed line) is one in which the polyimide dielectric 22 was provided and cured at an elevated temperature of 375° C. in a nitrogen (N2) atmosphere. In this scenario, superconductivity of the SC structures 14 was destroyed, as the SC structures 14 remained highly resistive at temperatures below 9K.
The second scenario (Al2O3/Nb/Al2O3/HD-4110 @ 225° C.-circle line) is one in which the polyimide dielectric 22 was provided; however, the polyimide dielectric 22 was cured at a reduced temperature of 225° C. in a nitrogen (N2) atmosphere. With the reduced curing temperature, only a small amount of degradation occurs, which corresponds to a small reduction in the critical transition temperature Tc. In other words, the transition from the normal state (highly resistive) to the superconductive state is shifted only slightly to the left (i.e. lower in temperature). Further, only a minimal degradation in the critical current Ic and residual resistance ratio (RRR) measurements (not shown) occurs. The critical current Ic drops from 186 mA to 182 mA, and the RRR drops from 3.58 to 3.34. These findings show that the use of the lower and upper barriers 24, 20 have an insignificant impact at reduced processing temperatures.
The third scenario (Al2O3/Nb/Al2O3/HD-4110 @ 375° C.-triangle line) is one in which the polyimide dielectric 22 was provided and cured at an elevated temperature of 375° C. in a nitrogen (N2) atmosphere. Notably, superconductivity of the SC structures 14 was maintained, which is in stark contrast with the scenarios above that did not employ the lower and upper barriers 24, 20 to protect the SC structures 14. In fact, only a small amount of degradation occurs, which corresponds to a small reduction in the critical transition temperature Tc. In other words, the transition from the normal state (highly resistive) to the superconductive state is shifted only slightly to the left (i.e. lower in temperature) and remained above 8 K.
The critical current Ic and the RRR for the first scenario of
With reference to
As illustrated in
Next, a dielectric, such as a polyimide dielectric 22, is provided over the upper barrier 20, using an appropriate application or deposition process. For the polyimide dielectric 22, a polyimide paste or gel is applied over the upper barrier 20 and then heated, as illustrated in
However, with the lower and/or upper barriers 24, 20, such diffusion is prevented and the SC structures 14 may be subjected to elevated temperatures and maintain their superconductive properties. Notably, the superconductivity of niobium and like materials are not affected, even when the polyimide dielectric 22 is cured at temperatures at or above 250° C., 275° C., 300° C., and 350° C. The concepts herein allow for heating the potentially destructive dielectric polyimide 22 or like layer above a destructive temperature that would otherwise result in the loss or degradation of superconductivity, without the presence of the lower and/or upper barriers 24, 20.
The process above may be repeated to form a second tier of like layers, as shown in
Notably, there may be intervening layers that are not shown as well as additional layers provided over the layers shown in the above embodiments. Various SC structures 14 as well as non-SC structures may be provided in, on, or between any of the illustrated or non-illustrated layers of the wafer 18, as those skilled in the art will appreciate. Any one or more of the layers of the wafer 18 may be removed during fabrication. For example, the substrate 12 may be removed after forming the desired layers over the substrate 12.
Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.