CN111969101A - NbN-based Josephson junction and preparation method thereof - Google Patents
NbN-based Josephson junction and preparation method thereof Download PDFInfo
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Abstract
The invention provides a Josephson junction based on NbN and a preparation method thereof, wherein the preparation method comprises the following steps: providing a substrate, forming a NbN bottom layer film, a metal NbNx barrier layer and a NbN top layer film, etching and defining a bottom electrode and a junction area, and forming an isolation layer and a wiring layer. The invention forms the metal NbNx barrier layer through the ion nitriding process to obtain the SNS structure Josephson junction without parallel resistors, solves the problems of magnetic flux noise and integration level of the SIS structure Josephson junction, improves the process repeatability and stability, can freely regulate and control the resistivity, the thickness and the like of the barrier layer material through parameters such as ion nitriding time, power and the like, effectively avoids the formation of an insulating layer at an S/N interface, has the characteristics of high surface smoothness, good nitriding uniformity and the like, and improves the characteristic voltage I of the SNS junctioncRnAnd the defect of high-frequency application of the device is limited, and the development of a high-quality NbN SNS Josephson junction is facilitated.
Description
Technical Field
The invention belongs to the field of superconducting electronics, and particularly relates to a Josephson junction based on NbN and a preparation method thereof.
Background
Superconducting josephson tunnel junctions have wide applications in the field of superconducting electronics, such as superconducting Single Flux Quantum (SFQ) circuits, superconducting quantum interference devices (SQUIDs), josephson voltage references, and the like. The SFQ circuit represents logic information by utilizing whether a single magnetic flux quantum exists in a Josephson junction or not, is expected to simultaneously cross over the physical bottlenecks of a speed semiconductor circuit and a power consumption semiconductor circuit, and is an important alternative scheme in the post-Mole era internationally. The SQUID device is formed by connecting one or two Josephson junctions in parallel through a superconducting loop, realizes detection of physical quantities of magnetic field, magnetic gradient, voltage, displacement and the like which can be converted into magnetic flux on the basis of the Josephson effect and magnetic flux quantization effect, has magnetic flux sensitivity with the magnitude of mu phi 0/V Hz, and is widely applied to the fields of biomagnetic detection, geophysical detection, nondestructive detection and the like, which relate to low magnetic field detection. In addition, quantum state transport in the Josephson junction enables the Josephson junction to have remarkable advantages in the field of high-precision measurement metrology, the Josephson voltage reference output voltage realized by applying thousands of Josephson tandem junction arrays can reach 10V at most, and the relative uncertainty is lower than 10-10Josephson voltage references have been developed for routine dc calibration and related applications in over 60 laboratories worldwide.
Wherein the josephson junction is a weakly connected superconductor prepared according to the josephson effect. However, in the existing josephson junction, there are many problems in both SIS (superconducting-insulating-superconducting) structure and SNS (superconducting-normal metal-superconductor) structure, for example, in the SIS structure, magnetic flux noise is increased due to parallel resistance, the integration of a superconducting circuit is limited, and a thinner barrier layer and a smaller junction area are required to increase the circuit clock frequency, but when the thickness of the barrier layer is very thin (d-1 nm) or the junction size is in the submicron range, the process repeatability and stability of the josephson junction face serious challenges. In the SNS structure, the characteristic voltage IcRnSmall, limiting the high frequency applications of the device.
Therefore, how to provide a josephson junction based on NbN and a preparation method thereof are necessary to solve the above problems.
Disclosure of Invention
In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide a NbN-based josephson junction and a method for preparing the same, which are used to solve the problems of the prior art, such as large magnetic flux noise, low integration level, and difficult improvement and flexible adjustment of the characteristic voltage of the SNS josephson junction.
To achieve the above and other related objects, the present invention provides a method for preparing a josephson junction based on NbN, comprising the steps of:
providing a substrate;
forming a functional structure material layer on the substrate, wherein the functional structure material layer comprises a NbN bottom layer film, a metal NbNx barrier layer and a NbN top layer film which are formed from bottom to top, the metal NbNx barrier layer is formed on the surface of the NbN bottom layer film, and the metal NbNx barrier layer is formed on the basis of an ion nitriding treatment process;
etching the functional structure material layer based on a first etching process to define a bottom electrode in the NbN bottom layer film;
etching the NbN top layer film and the metal NbNx barrier layer on the bottom electrode based on a second etching process to define a plurality of junction regions, wherein the metal NbNx barrier layer of the junction regions forms a junction barrier layer, and the NbN top layer film of the junction regions forms a top electrode;
forming an isolation layer on the exposed surfaces of the top electrode, the junction barrier layer and the bottom electrode and the substrate around the exposed surfaces, wherein a first connecting hole exposing the top electrode and a second connecting hole exposing the bottom electrode are formed in the isolation layer;
and forming a wiring layer on the isolation layer, wherein the wiring layer comprises a first wiring part electrically connected with the top electrode through the first connecting hole and a second wiring part electrically connected with the bottom electrode through the second connecting hole.
Optionally, the specific step of forming the functional structural material layer includes:
forming an initial NbN underlayer film on the substrate;
performing the ion nitriding treatment process on the initial NbN underlying film to obtain the NbN underlying film and the metal NbNx barrier layer, wherein the ion nitriding treatment process comprises the following steps of: placing the substrate with the initial NbN bottom layer film in a vacuum chamber, forming nitrogen-containing plasma based on the vacuum chamber, and bombarding the surface of the initial NbN bottom layer film by adopting the nitrogen-containing plasma to perform the ion nitriding treatment process; and
forming a NbN top layer film on the metal NbNx barrier layer.
Optionally, the metallic NbNx barrier layer resistivity and thickness are manipulated by at least one of time and power of the ion nitridation process.
Optionally, the interface electron permeability coefficient of the josephson junction is adjusted based on the nitriding treatment, wherein the adjusting the interface electron permeability coefficient comprises: by the formula γ ═ psξs)/(ρnξn) Controlling and controlling the interface electron transmission coefficient, wherein gamma represents a ratio, rho represents resistivity, xi represents a coherence length,nrepresents a metallic NbNx barrier layer,srepresenting the electrode layer.
Optionally, the critical current density and/or the characteristic voltage of the josephson junction is regulated based on the nitridation process, wherein the critical current density and/or the characteristic voltage of the josephson junction is regulated by formula Jc(d,T)＝Jc0exp(-d/ξn(T)) regulating the critical current density, d represents the thickness of the metallic NbNx barrier layer, T represents the temperature, Jc0Representing the critical current density at a barrier thickness of 0,nrepresents a metallic NbNx barrier layer, and xi represents a coherence length; by the formula Vc(d,T)＝Vc0(d/ξn(T))exp(-d/ξn(T)) regulating the characteristic voltage, d represents the thickness of the metallic NbNx barrier layer, T represents the temperature, Vc0Representing the characteristic voltage at a barrier layer thickness of 0,nrepresenting a metallic NbNx barrier layer and ξ representing the coherence length.
Optionally, in the process of forming the metal NbNx barrier layer based on the nitridation process, a lower surface of the metal NbNx barrier layer is directly in contact with the NbN bottom layer film, an upper surface of the metal NbNx barrier layer is directly in contact with the NbN top layer film, and an N element in the metal NbNx barrier layer is formed on an upper surface of the material layer to shield the Nb element.
Optionally, the substrate comprises a single crystal magnesium oxide substrate, the thickness of the substrate being 0.4 mm; the thickness of the metal NbNx barrier layer is between 2nm and 8 nm; the thickness of the NbN top layer film is between 150nm and 250 nm; the shape of the junction region comprises a circle having a diameter between 1.6 μm-3 μm; the diameter of the first connection hole is between 1.2 μm and 2.6 μm; the diameter of the second connecting hole is between 1.2 and 2.6 mu m.
Optionally, simultaneously etching the NbN bottom layer film, the metallic NbNx barrier layer, and the NbN top layer film based on the first etching process; and simultaneously etching the NbN top layer film and the metal NbNx barrier layer based on the second etching process.
Optionally, the first etching process includes step exposure and inductively coupled plasma etching; the second etching process comprises step exposure and inductively coupled plasma etching; the NbN underlayer film is prepared by a direct-current reactive magnetron sputtering method; the NbN top layer film is prepared by a direct-current reactive magnetron sputtering method; the isolation layer is prepared by a plasma enhanced chemical vapor deposition process; the wiring layer is prepared by a direct current reactive magnetron sputtering method.
The invention also provides a NbN-based Josephson junction, which is preferably prepared by the preparation method of the Josephson junction, and of course, can also be prepared by other methods, wherein the NbN-based Josephson junction comprises the following steps:
a substrate;
the functional structure layer is formed on the substrate and comprises a bottom electrode, a junction barrier layer and a top electrode from bottom to top, wherein the junction barrier layer comprises a metal NbNx layer, the bottom electrode comprises a bottom NbN layer, the top electrode comprises a top NbN layer, the metal NbNx layer is formed on the surface of the bottom NbN layer, and the metal NbNx layer is formed on the basis of an ion nitriding treatment process;
the isolation layer is formed on the exposed surfaces of the top electrode, the junction barrier layer and the bottom electrode and on the surrounding substrate, and a first connecting hole exposing the top electrode and a second connecting hole exposing the bottom electrode are formed in the isolation layer;
a wiring layer including a first wiring section electrically connected to the top electrode through the first connection hole and a second wiring section electrically connected to the bottom electrode through the second connection hole.
Optionally, the substrate comprises a single crystal magnesium oxide substrate, the thickness of the substrate being between 0.4 mm; the thickness of the metal NbNx barrier layer is between 2nm and 8 nm; the thickness of the NbN top layer film is between 150nm and 250 nm; the shape of the junction region comprises a circle having a diameter between 1.6 μm-3 μm; the diameter of the first connection hole is between 1.2 μm and 2.6 μm; the diameter of the second connecting hole is between 1.2 and 2.6 mu m.
Optionally, the lower surface of the metal NbNx layer is in direct contact with the bottom NbN layer, the upper surface of the metal NbNx layer is in direct contact with the top NbN layer, and the N element in the metal NbNx layer is formed on the upper surface of the material layer to shield the Nb element.
In addition, the ion nitriding process can realize free regulation and control of the barrier layer, the resistivity, the thickness and the like of the material of the barrier layer can be freely regulated and controlled through parameters such as ion nitriding time, power and the like, the formation of an insulating layer at an S/N interface is effectively avoided, the characteristics of high surface smoothness, good nitriding uniformity and the like are realized, and the characteristic voltage I of the SNS junction is improvedcRnAnd the defect of high-frequency application of the device is limited, and the development of a high-quality NbN SNS Josephson junction is facilitated.
Drawings
Figure 1 shows a process flow diagram illustrating the preparation of NbN-based josephson junctions according to the present invention.
Figure 2 shows a schematic diagram of a substrate provided in the fabrication of an exemplary NbN-based josephson junction of the present invention.
Figure 3 shows a schematic diagram of the initial NbN formation in the preparation of an exemplary NbN-based josephson junction of the present invention.
Figure 4 shows a schematic diagram of the formation of metallic NbNx in the fabrication of an exemplary NbN-based josephson junction of the present invention.
Figure 5 is a schematic diagram illustrating the formation of a NbN overlayer film in the preparation of a NbN-based josephson junction of the present invention.
Figure 6 shows a schematic diagram of the bottom electrode defined in the preparation of an exemplary NbN-based josephson junction of the present invention.
Figure 7 is a schematic diagram illustrating the definition of junction regions in the preparation of NbN-based josephson junctions in accordance with an embodiment of the present invention.
Figure 8 shows a schematic diagram of the formation of an isolation layer in the fabrication of an NbN-based josephson junction according to an embodiment of the present invention.
Fig. 9 is a schematic diagram illustrating the formation of a wiring layer in the fabrication of a NbN-based josephson junction according to an embodiment of the present invention.
FIG. 10 shows a typical I-V curve for an SIS junction.
FIG. 11 shows a typical I-V curve for an SNS junction.
Description of the element reference numerals
101 substrate
102 original NbN underlayer film
103 metal NbNx barrier layer
104 NbN underlayer film
105 NbN top layer film
106 bottom electrode
107 junction barrier layer
108 top electrode
109 barrier layer
109a first connection hole
109b second connection hole
110 wiring layer
110a first wiring part
110b second wiring part
S1-S6
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention.
As in the detailed description of the embodiments of the present invention, the cross-sectional views illustrating the device structures are not partially enlarged in general scale for convenience of illustration, and the schematic views are only examples, which should not limit the scope of the present invention. In addition, the three-dimensional dimensions of length, width and depth should be included in the actual fabrication.
For convenience in description, spatial relational terms such as "below," "beneath," "below," "under," "over," "upper," and the like may be used herein to describe one element or feature's relationship to another element or feature as illustrated in the figures. It will be understood that these terms of spatial relationship are intended to encompass other orientations of the device in use or operation in addition to the orientation depicted in the figures. Further, when a layer is referred to as being "between" two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. "between … …" means that two endpoint values are included.
In the context of this application, a structure described as having a first feature "on" a second feature may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features are formed in between the first and second features, such that the first and second features may not be in direct contact.
It should be noted that the drawings provided in the present embodiment are only for illustrating the basic idea of the present invention, and the drawings only show the components related to the present invention rather than being drawn according to the number, shape and size of the components in actual implementation, and the type, quantity and proportion of each component in actual implementation may be changed freely, and the layout of the components may be more complicated.
As shown in fig. 1, the present invention provides a method for preparing a josephson junction based on NbN, comprising the steps of:
s1, providing a substrate;
s2, forming a functional structure material layer on the substrate, wherein the functional structure material layer comprises a NbN bottom layer film, a metal NbNx barrier layer and a NbN top layer film which are formed from bottom to top, the metal NbNx barrier layer is formed on the surface of the NbN bottom layer film, and the metal NbNx barrier layer is formed on the basis of an ion nitriding treatment process;
s3, etching the functional structure material layer based on a first etching process to define a bottom electrode in the NbN bottom layer film;
s4, etching the NbN top layer film and the metal NbNx barrier layer on the bottom electrode based on a second etching process to define a plurality of junction regions, wherein the metal NbNx barrier layer of the junction regions forms a junction barrier layer, and the NbN top layer film of the junction regions forms a top electrode;
s5, forming isolation layers on the exposed surfaces of the top electrode, the junction barrier layer and the bottom electrode and the substrate around the top electrode, wherein the isolation layers are formed with a first connection hole exposing the top electrode and a second connection hole exposing the bottom electrode;
s6, forming a wiring layer on the isolation layer, the wiring layer including a first wiring portion electrically connected to the top electrode through the first connection hole and a second wiring portion electrically connected to the bottom electrode through the second connection hole.
The method for preparing the NbN-based josephson junction of the present invention will be described in detail with reference to the accompanying drawings, wherein it should be noted that the above sequence does not strictly represent the preparation sequence of the NbN-based josephson junction protected by the present invention, and those skilled in the art can change the sequence according to the actual process steps, and fig. 1 shows only one example of the preparation steps of the NbN-based josephson junction, and those skilled in the art can change the design according to the conventional choice in the art.
First, as shown in S1 in fig. 1 and fig. 2, step S1 is performed to provide the substrate 101. The substrate 101 may be a single material layer or a stacked structure composed of multiple material layers. In an example, the substrate 101 is selected to be magnesium oxide (MgO), and may be a single crystal magnesium oxide substrate, and is further preferably a single crystal magnesium oxide substrate in a (100) direction, and of course, may be selected to be other common substrates that can achieve the functions of the present invention. Optionally, the substrate 101 has a thickness of 0.4mm to accommodate the requirements of the stepper apparatus.
Next, as shown in S2 in fig. 1 and fig. 3-5, step S2 is performed to form a functional structure material layer on the substrate 101, wherein the functional structure material layer includes a NbN bottom layer film 104, a metal NbNx barrier layer 103 and a NbN top layer film 105, which are formed from bottom to top, as shown in the structure of fig. 5, wherein the metal NbNx barrier layer 103 is formed on the surface of the NbN bottom layer film 105, and the metal NbNx barrier layer 103 is formed based on an ion nitriding process.
Specifically, in this step, the metal NbNx barrier layer 103 is formed on the basis of an ion nitriding process, and is used as a barrier layer of a josephson junction by a subsequent process. As an example, the specific steps of forming the functional structure material layer include:
first, as shown in fig. 3, an initial NbN underlayer film 102 is formed on the substrate 101. The initial NbN underlayer film 102 is formed over the entire substrate 101 surface, in one example, the initial NbN underlayer film 102, which is an underlying superconducting material, is grown by a dc reactive magnetron sputtering method, optionally, the initial NbN underlayer film 102 has a thickness between 150nm and 250nmAnd may be 180nm, 200nm or 220 nm. NbN means having a high superconducting transition temperature (T)c16.5K), large superconducting energy gap (delta-3 meV) and high characteristic frequency (1.4 THz). The initial NbN bottom layer film 102 serves as a structural basis for preparing the metal NbNx barrier layer 103 on one hand, and is used for subsequently manufacturing a josephson junction bottom electrode on the other hand.
Next, as shown in fig. 4, the ion nitridation process is performed on the initial NbN underlayer film 102 to obtain the NbN underlayer film 104 and the metallic NbNx barrier layer 103, wherein the ion nitridation process includes: placing the substrate with the initial NbN underlayer film 102 formed thereon in a vacuum chamber and forming a nitrogen-containing plasma based on the vacuum chamber, i.e., a nitrogen-containing plasma such as N formed by high-pressure discharge in the vacuum chamber2 +、N+、N2 2+And ions, bombarding the surface of the initial NbN bottom layer film 102 by adopting the nitrogen-containing plasma so as to perform the ion nitriding treatment process. In one example, the initial NbN underlayer film 102 is treated by in situ ion nitridation. For example, the initial NbN underlayer film 102 is grown by a direct current reactive magnetron sputtering method, and then the ion nitridation treatment process is performed in the same chamber to realize in-situ ion nitridation, and based on the same process chamber, the N source of the initial NbN underlayer film 102 and the N source forming the metal NbNx barrier layer 103 can be continuously introduced and integrated in the same process program (recipe), thereby simplifying the process and improving the film quality.
After the initial NbN underlayer film 102 is subjected to the above-described ion nitridation, a part of the initial NbN underlayer film 102 is converted into the metallic NbNx barrier layer 103 by the action of nitrogen-containing plasma, and the remaining part is used as the NbN underlayer film 104. During this treatment, N is present in the plasma2 +、N+And N2 2+The ion bombards the surface of the sample at a certain speed to effectively remove the surface oxide layer, and the sample is surrounded by nitrogen and ionized nitrogen plasma when the ion nitriding process is carried out, so that the oxidation of the bottom layer is effectively prevented, namely, an insulating layer is prevented from being formed between a metal barrier layer formed subsequently and a bottom electrode, namely, the ion nitriding technologyEffectively preventing the formation of an insulating layer at the interface of S (the NbN underlying film 104)/N (the metallic NbNx barrier layer 103), preventing the formation of a superconductor-insulator-normal metal-insulator-superconductor (SINIS) junction, and avoiding JcToo low and poor process stability. The metal NbNx barrier layer 103 formed by the nitridation processing can form a high-flatness NbNx barrier layer and has the advantage of good uniformity. The high flatness and the ultrahigh uniformity of the ion nitriding technology overcome the difficult problems of poor interface diffusion and uniformity of the conventional three-layer film, and are beneficial to the research and development of high-quality NbN SNS Josephson junctions.
By way of example, the metallic NbNx barrier layer 103 may have a thickness between 2nm and 8nm, and may be, for example, 4nm, 5nm, or 6 nm. The resulting metallic NbNx barrier layer eventually saturates with increasing nitridation time to a thickness between 2-8nm, with a reduction in thickness of the initial layer of between 2-6 nm. Currently, the mainstream of SNS josephson junctions is the Nb-based SNS junction, and NbSi having the same element must be used as a barrier layer in order to obtain a good interface. Compared with Nb junctions, NbN junctions have great potential in high operating temperatures and high frequency applications, while NbN SNS junctions require nitrides as barrier layers, so the ion nitridation technique can only be applied in the preparation of NbN SNS junctions. The development of NbN josephson junctions is in the primary exploration stage both internationally and domestically, and the ion nitriding method is not yet applied in the field. The ion nitriding technology effectively prevents the formation of an insulating layer at an S/N interface, and has high nitriding uniformity and good process stability.
As an example, the metallic NbNx barrier layer resistivity and thickness are manipulated by at least one of time and power of the ion nitridation process. Thereby, a free control of the metallic NbNx barrier layer 103 is achieved, for example, the resistivity and thickness of the metallic NbNx barrier layer 103 as described above can be freely controlled according to the parameters of the nitridation process. Thereby preparing a high-quality over-damped SNS Josephson junction with a non-hysteresis I-V curve. Wherein, the thickness of the metal NbNx barrier layer is gradually and rapidly increased along with the increase of nitridation power and time, and finally the metal NbNx barrier layer is in saturation, and the electricity of the metal NbNx barrier layerThe resistivity increases slowly and then rapidly as the nitridation power and time increase. The invention regulates and controls the nitrogen content of the bottom layer superconducting NbN by the ion nitriding technology, and forms a layer of normal metal NbN on the surfacexThe resistivity, thickness and the like of the barrier layer and the barrier layer material can be freely regulated and controlled by parameters such as ion nitriding time, power and the like. The barrier layer is an NbNx film with an N/Nb stoichiometric ratio of less than 0.5, such as 0.2 and 0.3, a body-centered cubic or hexagonal structure and normal metal behavior at low temperature. The Jc and Vc are characterized in the formula, the Jc and Vc are controlled by the thickness of the barrier layer and the coherence length, the larger the resistivity of the barrier layer is, the smaller the coherence length is, the nitrogen content regulation corresponds to the change of the coherence length of the barrier layer, and the thickness corresponds to d in the formula
As an example, the interface electron permeability coefficient of the josephson junction is controlled based on the nitridation process, wherein the manner of controlling the interface electron permeability coefficient includes: by the formula γ ═ psξs)/(ρnξn) Controlling and controlling the interface electron transmission coefficient, wherein gamma represents a ratio, rho represents resistivity, xi represents a coherence length,nrepresents a metallic NbNx barrier layer,sthe electrode layer may represent a top electrode or a bottom electrode. In one example, the metal NbNxThe coherence length of the barrier layer depends on the resistivity of the barrier layer, with the greater the resistivity, the smaller the coherence length of the barrier layer. In another example, the electrode layers are provided at 200nm both above and below, and the coherence length of the electrode layers is considered to be as large as necessary. The SNS Josephson junction realizes electron transfer by Andrew reflection, junction performance is related to the resistivity of the barrier layer material and S/N interface characteristics, and the interface electron transmission coefficient depends on the ratio gamma (rho) of the resistivity of the superconducting material and the barrier layer material multiplied by the coherence lengthsξs)/(ρnξn). The method is used for researching barrier layer material parameters, such as the film resistivity and the change trend of the S/N interface electron transmission coefficient when the film thickness changes, and quantitatively disclosing the rule between performance parameters such as the characteristic voltage and the critical current density of the SNS junction and the macroscopic parameters of the barrier layer material.
AsExample, the critical current density of a josephson junction is regulated based on the nitridation process, wherein the critical current density is regulated by formula Jc(d,T)＝Jc0exp(-d/ξn(T)) regulating the critical current density, d represents the thickness of the metallic NbNx barrier layer, T represents the temperature, Jc0Representing the critical current density at a barrier thickness of 0,nrepresenting a metallic NbNx barrier layer and ξ representing the coherence length. Thereby realizing the free regulation and control of the critical current density of the Josephson junction. In another example, a characteristic voltage of a josephson junction is modulated based on the nitridation process, by formula Vc(d,T)＝Vc0(d/ξn(T))exp(-d/ξn(T)) regulating the characteristic voltage, d represents the thickness of the metallic NbNx barrier layer, T represents the temperature, Vc0Representing the characteristic voltage at a barrier layer thickness of 0,nrepresenting a metallic NbNx barrier layer and ξ representing the coherence length. Thereby realizing the free regulation and control of the characteristic voltage of the Josephson junction. In one example, the metal NbNx barrier layer can be adjusted based on various process parameters, the smaller the resistivity of the metal NbNx barrier layer is, the larger the coherence length is, the smaller the thickness is, the larger the interface electron transmission coefficient is, the larger Jc is, and the larger Vc is, which is more beneficial to the development of a high-quality SNS junction.
Finally, as shown in fig. 5, the NbN overlayer film 105 is formed on the metallic NbNx barrier layer 103. In one example, the NbN top layer film 105, the underlying superconducting material, is grown by a dc reactive magnetron sputtering method, and optionally, the NbN top layer film 105 has a thickness of 150nm to 250nm, which may be 180nm, 200nm, or 220 nm.
As an example, in the process of forming the metal NbNx barrier layer based on the nitridation process, the metal NbNx barrier layer 103 is in direct contact with the NbN underlying film 104, which effectively prevents the formation of an insulating layer at the S (the NbN underlying film 104)/N (the metal NbNx barrier layer 103) interface. The N element in the metal NbNx barrier layer 103 is formed on the upper surface of the material layer to shield the Nb element, and Nb is nitrided immediately after being bombarded out in the process, so that the Nb element is not exposed, and the oxidation of the upper layer of the barrier layer is effectively prevented, namely, the formation of an insulating layer at the interface of S (the NbN top layer film 105)/N (the metal NbNx barrier layer 103) is effectively prevented, and a superconductor-insulator-normal metal-insulator-superconductor (SINIS) junction is prevented from being formed.
Next, as shown in S3 of fig. 1 and fig. 6, step S3 is performed to etch the functional structure material layer based on a first etching process to define a bottom electrode 106 in the NbN bottom layer film 104. As an example, the NbN bottom layer 104, the metal NbNx barrier layer 103, and the NbN top layer 105 are simultaneously etched based on the first etching process to obtain the bottom electrode 106, where the NbN bottom layer 104, the metal NbNx barrier layer 103, and the NbN top layer 105 are composed of the same elements, and materials of the same elements may be etched to the bottom in one step to ensure the etching steepness of the junction region. As an example, the first etch process includes step exposure and inductively coupled plasma etching. The etching gas may be CF4 and Ar.
Next, as shown in S4 of fig. 1 and fig. 7, step S4 is performed to etch the NbN top layer 105 and the metal NbNx barrier layer 103 on the bottom electrode 106 based on a second etching process to define a plurality of junction regions, wherein the metal NbNx barrier layer 103 of the junction regions forms a junction barrier layer 107, and the NbN top layer 105 of the junction regions forms a top electrode 108. As an example, the NbN underlayer 104 and the metal NbNx barrier layer 103 are simultaneously etched based on the second etching process to form respective josephson junctions, the NbN underlayer 104 and the metal NbNx barrier layer 103 are composed of the same elements, and materials of the same elements can be etched to the bottom in one step to ensure the etching steepness of the junction region. As an example, the second etching process includes step exposure and inductively coupled plasma etching.
As an example, the shape of the junction region comprises a circle, which is directly between 1.6 μm-3 μm, e.g. may be 1.8 μm, 2 μm, 2.5 μm. Therefore, the appropriate critical current density can be obtained, and the high-frequency application of the device is facilitated. Of course, in other examples, the shape of the junction region may also be made into a square junction.
Next, as shown in S5 of fig. 1 and fig. 8, step S5 is performed to form an isolation layer 109 on the exposed surfaces of the top electrode 108, the junction barrier layer 107, and the bottom electrode 106 and on the surrounding substrate 101, wherein a first connection hole 109a exposing the top electrode 108 and a second connection hole 109b exposing the bottom electrode 106 are formed in the isolation layer 109. As an example, the diameter of the first connection hole is between 1.2 μm-2.6 μm, e.g. may be 1.5 μm, 2 μm, 2.2 μm; the diameter of the second connection hole is between 1.2 μm and 2.6 μm, for example, 1.5 μm, 2 μm, 2.2 μm. The positions and the number of the first connection holes 109a and the second connection holes 109b can be set according to actual requirements, so as to electrically lead out the top electrode 108 and the bottom electrode 106 in the following step.
In an example, after forming the junction region, an isolation material layer covering the entire surface may be deposited on the substrate 101, and the isolation material layer may be etched by a photolithography process to obtain the first connection hole and the second connection hole, for example, an opening process is performed by using step exposure and reactive ion etching, and SiO uncovered by the photoresist is etched by using the reactive ion etching2And a thin film forming a connection hole connecting the upper film and the wiring layer. The isolation layer 109 is made by a chemical vapor deposition method, and the material thereof includes, but is not limited to, a silicon dioxide thin film, and the deposition thickness thereof may be between 180nm and 300nm, such as 200nm, 220nm, and 250nm, which are selected according to the actual device layout.
Finally, as shown in S6 of fig. 1 and fig. 9, step S6 is performed to form a wiring layer 110 on the isolation layer 109, where the wiring layer 110 includes a first wiring portion 110a electrically connected to the top electrode 108 through the first connection hole 109a and a second wiring portion 110b electrically connected to the bottom electrode 106 through the second connection hole 109 b.
In one example, the wiring layer 110 is grown by a dc reactive magnetron sputtering method, and the material thereof includes but is not limited to NbN, and the deposition thickness thereof may be between 300nm and 400nm, such as 350nm and 380nm, which is selected according to the actual device layout. In an example, after the isolation layer is formed, a wiring material layer covering the entire device surface is formed on the isolation layer, and then the wiring material layer is etched through a photolithography etching process to obtain the wiring layer. Optionally, the wiring layer is prepared by step exposure and inductively coupled plasma etching.
The invention prepares the SNS structure Josephson junction of the freely-regulated barrier layer based on a nitridation process. Josephson junctions are weakly connected superconductors prepared according to the josephson effect. When a superconducting-insulating-superconducting (SIS) structure is formed by a thin film insulating layer between two superconductors, a superconducting tunnel current I with zero voltage related to the phase difference theta of electron wave-pair functions in the superconductors appearss(Is＝Icsin θ) in which IcIs the critical current of the josephson junction. If a voltage difference V exists between the superconductors, the phase difference theta changes with time
At this time IsWill be of amplitude IcAnd f is 2 eV/h. The actual josephson junction can be considered as an ideal josephson junction with a parallel resistor R and a capacitor C, the electrical behavior of which can be explained with the RCSJ model. The parallel network has two characteristic times RC and LJ/R, introduction of a damping parameter betac＝RC/(LJ/R)＝2πIcR2C/Φ0: when beta iscWhen the damping coefficient is more than 1, the junction is under-damped, and the I-V curve has hysteresis; when beta iscWhen the value is less than 1, the knot is over-damped, and the I-V curve is a single-value curve. SIS Josephson junctions are usually underdamped junctions and require the addition of a shunt resistor to make the junction betacLess than 1, however, the parallel resistance not only increases the flux noise but also limits the integration of the superconducting circuit. On the other hand, increasing the circuit clock frequency requires thinner barrier layers and smaller junction areas, but when the barrier layer thickness is very thin (d-1 nm) or the junction size is in the submicron range, the process repeatability and stability of the josephson junction are both severely challenging. FIG. 10 is a typical I-V curve for an SIS junction.
The SNS junction is prepared by the method,the nitrogen content of the bottom layer superconducting NbN is regulated and controlled by the ion nitriding technology, and a layer of normal metal NbN is formed on the surface of the bottom layer superconducting NbNxThe resistivity, thickness and the like of the barrier layer and the barrier layer material can be freely regulated and controlled by parameters such as ion nitriding time, power and the like. The ion nitriding mode not only effectively avoids the formation of an insulating layer at an S/N interface, but also has the characteristics of high surface flatness, good nitriding uniformity and the like, and is beneficial to the research and development of high-quality NbN SNS Josephson junctions. The Cooper pair in a superconductor-normal metal-superconductor (SNS) junction realizes the coupling of superconductivity on two sides in the form of Andrew reflection at an S/N interface and has a non-hysteresis I-V curve. The SNS junction is the junction of the intrinsic shunt resistor, so that the area required by an external shunt resistor is effectively saved. In addition, SNS junctions have a high transmission of electrons at the S/N interface and have a high J comparable to that of SIS junctions when the N layer is relatively thick (d 10nm)cAnd the repeatability and stability of the process are ensured. However, the characteristic voltage I of SNS junctions compared to SIS junctionscRnSmall, limiting the high frequency applications of the device. Research shows that the characteristic voltage of the SNS junction is closely related to the material characteristics of the barrier layer and the S/N interface characteristics. The scheme of the invention can ensure the clarity of the interface of the SNS Josephson junction while freely regulating and controlling the barrier layer. FIG. 11 is a typical I-V curve for an SNS junction. In addition, compared with the current mainstream Josephson junction, the invention has obvious effect, the current mainstream of the NbN Josephson junction is NbN/AlN/NbN junction, the leakage current is small, and the energy gap voltage is large (the>5mV)，JcIn the range of several tens to several thousands A/cm2The range varies. AlN can realize the transformation from an insulating state to a normal state through a stoichiometric control ratio, however, AlN shows a piezoelectric effect due to the enhanced quantum-phonon coupling effect, and the crystal structure of the AlN thin film needs to be accurately controlled to inhibit the influence of the piezoelectric effect on the performance of the Josephson junction, so AlN is not the best selection scheme of the SNS Josephson barrier layer in NbN system.
In addition, as shown in fig. 9, and with reference to fig. 1-8, the present invention also provides a NbN-based josephson junction, preferably prepared by the method of preparing a josephson junction of the present invention, although other methods may be used, including:
a substrate 101;
a functional structure layer formed on the substrate 101, the functional structure layer including, from bottom to top, a bottom electrode 106, a junction barrier layer 107 and a top electrode 108, wherein the junction barrier layer 107 includes a metal NbNx layer (corresponding to the metal NbNx barrier layer in the description of the manufacturing method), the bottom electrode 106 includes a bottom NbN layer (corresponding to the NbN bottom layer film in the description of the manufacturing method), the top electrode 108 includes a top NbN layer (corresponding to the NbN top layer film in the description of the manufacturing method), the metal NbNx layer is formed on the surface of the bottom NbN layer, and the metal NbNx layer is formed based on an ion nitriding process; here, part of the names in the structures are different from those described in the manufacturing method due to the manufacturing process, and it can be understood by those skilled in the art based on the manufacturing process and general knowledge herein that the scope of the present invention should not be excessively limited.
An isolation layer 109 formed on the exposed surfaces of the top electrode 108, the junction barrier layer 107 and the bottom electrode 106 and the surrounding substrate 101, wherein a first connection hole 109a exposing the top electrode 108 and a second connection hole 109b exposing the bottom electrode 106 are formed in the isolation layer 109;
a wiring layer 110 including a first wiring section 110a electrically connected to the top electrode 109b through the first connection hole 109a and a second wiring section 110b electrically connected to the bottom electrode 106 through the second connection hole 109 b.
As an example, the substrate 101 comprises a single crystal magnesium oxide substrate, the thickness of the substrate 101 being between 0.2mm-0.6 mm; the thickness of the metal NbNx barrier layer 103 is between 2nm and 8 nm; the thickness of the NbN top layer film is between 150nm and 250 nm; the shape of the junction region comprises a circle having a diameter between 1.6 μm-3 μm; the diameter of the first connection hole is between 1.2 μm and 2.6 μm; the diameter of the second connecting hole is between 1.2 and 2.6 mu m.
As an example, the lower surface of the metallic NbNx layer is in direct contact with the bottom NbN layer, the upper surface of the metallic NbNx layer is in direct contact with the top NbN layer, and the N element in the metallic NbNx layer is formed on the upper surface of the material layer to shield the Nb element.
In summary, the NbN-based josephson junction and the method for manufacturing the same according to the present invention form the metal NbNx barrier layer as the barrier layer of the josephson junction by the ion nitridation process to prepare the SNS structure josephson junction without the need of parallel resistors, thereby solving the problems of magnetic flux noise and integration of the SIS structure josephson junction, and improving the problems of poor process repeatability and stabilitycRnAnd the defect of high-frequency application of the device is limited, and the development of a high-quality NbN SNS Josephson junction is facilitated. Therefore, the invention effectively overcomes various defects in the prior art and has high industrial utilization value.
The foregoing embodiments are merely illustrative of the principles and utilities of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or change the above-mentioned embodiments without departing from the spirit and scope of the present invention. Accordingly, it is intended that all equivalent modifications or changes which can be made by those skilled in the art without departing from the spirit and technical spirit of the present invention be covered by the claims of the present invention.
Claims (12)
1. A method of preparing a josephson junction based on NbN, comprising the steps of:
providing a substrate;
forming a functional structure material layer on the substrate, wherein the functional structure material layer comprises a NbN bottom layer film, a metal NbNx barrier layer and a NbN top layer film which are formed from bottom to top, the metal NbNx barrier layer is formed on the surface of the NbN bottom layer film, and the metal NbNx barrier layer is formed on the basis of an ion nitriding treatment process;
etching the functional structure material layer based on a first etching process to define a bottom electrode in the NbN bottom layer film;
etching the NbN top layer film and the metal NbNx barrier layer on the bottom electrode based on a second etching process to define a plurality of junction regions, wherein the metal NbNx barrier layer of the junction regions forms a junction barrier layer, and the NbN top layer film of the junction regions forms a top electrode;
forming an isolation layer on the exposed surfaces of the top electrode, the junction barrier layer and the bottom electrode and the substrate around the exposed surfaces, wherein a first connecting hole exposing the top electrode and a second connecting hole exposing the bottom electrode are formed in the isolation layer;
and forming a wiring layer on the isolation layer, wherein the wiring layer comprises a first wiring part electrically connected with the top electrode through the first connecting hole and a second wiring part electrically connected with the bottom electrode through the second connecting hole.
2. The method of preparing a NbN-based josephson junction according to claim 1, wherein the specific step of forming the layer of functional structural material comprises:
forming an initial NbN underlayer film on the substrate;
performing the ion nitriding treatment process on the initial NbN underlying film to obtain the NbN underlying film and the metal NbNx barrier layer, wherein the ion nitriding treatment process comprises the following steps of: placing the substrate with the initial NbN bottom layer film in a vacuum chamber, forming nitrogen-containing plasma based on the vacuum chamber, and bombarding the surface of the initial NbN bottom layer film by adopting the nitrogen-containing plasma to perform the ion nitriding treatment process; and
forming a NbN top layer film on the metal NbNx barrier layer.
3. The method of claim 1, wherein the metallic NbNx barrier layer resistivity and thickness are modulated by at least one of time and power of the ion nitridation process.
4. The method of claim 3, wherein the interface electron transmission coefficient of the Josephson junction is controlled based on the nitridation process, wherein the interface electron transmission coefficient is controlled by: by the formula γ ═ psξs)/(ρnξn) Controlling and controlling the interface electron transmission coefficient, wherein gamma represents a ratio, rho represents resistivity, xi represents a coherence length,nrepresents a metallic NbNx barrier layer,srepresenting the electrode layer.
5. The method of claim 3, wherein the critical current density and/or characteristic voltage of the Josephson junction is controlled based on the nitridation process, wherein the critical current density and/or characteristic voltage is controlled by formula Jc(d,T)＝Jc0exp(-d/ξn(T)) regulating the critical current density, d represents the thickness of the metallic NbNx barrier layer, T represents the temperature, Jc0Representing the critical current density at a barrier thickness of 0,nrepresents a metallic NbNx barrier layer, and xi represents a coherence length; by the formula Vc(d,T)＝Vc0(d/ξn(T))exp(-d/ξn(T)) regulating the characteristic voltage, d represents the thickness of the metallic NbNx barrier layer, T represents the temperature, Vc0Representing the characteristic voltage at a barrier layer thickness of 0,nrepresenting a metallic NbNx barrier layer and ξ representing the coherence length.
6. The method of claim 1, wherein during the formation of the NbNx barrier layer based on the nitridation process, a lower surface of the NbNx barrier layer is in direct contact with a NbN underlayer film, an upper surface of the NbNx barrier layer is in direct contact with a NbN overlayer film, and the N element in the NbNx barrier layer is formed on an upper surface of the material layer to shield the Nb element.
7. The method of preparing a NbN-based josephson junction according to claim 1, wherein the substrate comprises a single crystal magnesium oxide substrate, the substrate having a thickness of 0.4 mm; the thickness of the metal NbNx barrier layer is between 2nm and 8 nm; the thickness of the NbN top layer film is between 150nm and 250 nm; the shape of the junction region comprises a circle having a diameter between 1.6 μm-3 μm; the diameter of the first connection hole is between 1.2 μm and 2.6 μm; the diameter of the second connecting hole is between 1.2 and 2.6 mu m.
8. The method of fabricating a NbN-based josephson junction according to any of claims 1-7, wherein the NbN bottom layer film, the metallic NbNx barrier layer, and the NbN top layer film are etched simultaneously based on the first etching process; and simultaneously etching the NbN top layer film and the metal NbNx barrier layer based on the second etching process.
9. The method of preparing a NbN-based josephson junction according to claim 8, wherein the first etching process comprises step exposure and inductively coupled plasma etching; the second etching process comprises step exposure and inductively coupled plasma etching; the NbN underlayer film is prepared by a direct-current reactive magnetron sputtering method; the NbN top layer film is prepared by a direct-current reactive magnetron sputtering method; the isolation layer is prepared by a plasma enhanced chemical vapor deposition process; the wiring layer is prepared by a direct current reactive magnetron sputtering method.
10. An NbN-based Josephson junction, comprising:
a substrate;
the functional structure layer is formed on the substrate and comprises a bottom electrode, a junction barrier layer and a top electrode from bottom to top, wherein the junction barrier layer comprises a metal NbNx layer, the bottom electrode comprises a bottom NbN layer, the top electrode comprises a top NbN layer, the metal NbNx layer is formed on the surface of the bottom NbN layer, and the metal NbNx layer is formed on the basis of an ion nitriding treatment process;
the isolation layer is formed on the exposed surfaces of the top electrode, the junction barrier layer and the bottom electrode and on the surrounding substrate, and a first connecting hole exposing the top electrode and a second connecting hole exposing the bottom electrode are formed in the isolation layer;
a wiring layer including a first wiring section electrically connected to the top electrode through the first connection hole and a second wiring section electrically connected to the bottom electrode through the second connection hole.
11. The NbN-based josephson junction of claim 10, wherein the substrate comprises a single crystal magnesium oxide substrate, the substrate having a thickness of 0.4 mm; the thickness of the metal NbNx barrier layer is between 2nm and 8 nm; the thickness of the NbN top layer film is between 150nm and 250 nm; the shape of the junction region comprises a circle having a diameter between 1.6 μm-3 μm; the diameter of the first connection hole is between 1.2 μm and 2.6 μm; the diameter of the second connecting hole is between 1.2 and 2.6 mu m.
12. The NbN-based josephson junction of any of claims 10-11, wherein a lower surface of the metallic NbNx layer is in direct contact with the bottom NbN layer, an upper surface of the metallic NbNx layer is in direct contact with the top NbN layer, and the N element in the metallic NbNx layer is formed at an upper surface of a material layer that shields the Nb element.
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