A method for fabricating a niobium-based josephson junction

Abstract

The application discloses a preparation method of a niobium-based Josephson junction and belongs to the technical field of low-temperature superconductivity. x The preparation method mainly comprises the following processes: cleaning a substrate; defining a bottom electrode area through photolithography; preparing an Nb bottom electrode; peeling; preparing an Al-AlO x -Al barrier layer; growing an Nb top layer electrode; defining a junction area and a top electrode area through secondary photolithography; performing reactive ion etching and ion beam etching; and removing glue. The niobium-based Josephson junction is prepared through micro-processing technology, vacuum coating technology and etching technology, so that the process is simplified; the niobium-based Josephson junction is prepared through a process of twice photolithography, so that the possibility of interface degradation of the device in the preparation process is reduced, the process of preparing the niobium-based Josephson junction is simplified, complex processing procedures are avoided, and the mass production and application of the Josephson junction are facilitated.

Description

Technical Field

[0001] This invention belongs to the field of low-temperature superconducting technology, specifically relating to a method for preparing a niobium-based Josephson junction. Background Technology

[0002] With the rapid development of superconductivity-related theories and applications, superconducting Josephson devices have attracted much attention due to their high sensitivity and low power consumption. Niobium-based Josephson junctions as direct detectors are particularly popular research subjects. Superconducting Josephson junctions used in direct detectors require high critical current density, low leakage current, strong noise immunity, and a certain manufacturing yield. Therefore, a mature, stable, and reproducible fabrication scheme for niobium-based Josephson junctions is fundamental to achieving this goal.

[0003] Currently, the fabrication method for niobium superconducting tunnel junctions requires four photolithography steps, making the process relatively complex and increasing the difficulty of applying niobium-based Josephson junctions. How to efficiently and cost-effectively fabricate niobium-based superconducting tunnel junctions remains a significant technical challenge in the research field. Summary of the Invention

[0004] The technical problem solved by this invention is to provide a method for preparing niobium-based Josephson junctions that simplifies the preparation process and avoids complex processing procedures.

[0005] Technical Solution: To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A method for fabricating a niobium-based Josephson junction, characterized by: using alumina as an insulating layer material to isolate the upper and lower superconducting electrodes, and employing two photolithography steps to define the bottom electrode region, as well as the junction region and the top electrode region. Specifically, the method includes the following steps:

[0007] Step 1: Clean the substrate;

[0008] Step 2: Define the bottom electrode region using photolithography;

[0009] Step 3: Fabrication of the Nb bottom electrode;

[0010] Step 4: Peeling;

[0011] Step 5: Preparation of Al-AlO x -Al barrier layer;

[0012] Step 6: Grow the Nb top electrode;

[0013] Step 7: Perform photolithography again to define the junction region and the top electrode region;

[0014] Step 8: Remove the Nb top layer and barrier layer that are not protected by photoresist;

[0015] Step 9: Remove the glue to obtain the finished product.

[0016] Furthermore, in step 1, sapphire is used as the substrate for ultrasonic cleaning, followed by drying and baking.

[0017] Furthermore, in step 2, ultraviolet lithography exposure and development technology is used to prepare the pattern of the bottom electrode on the clean substrate surface: first, photoresist is spin-coated, and the photoresist on the substrate surface is evenly distributed and the thickness reaches the micrometer level, then baking and pattern exposure are performed, and development is performed after exposure.

[0018] Furthermore, in step 3, the method for preparing the Nb bottom electrode is as follows: under vacuum conditions, a niobium film is grown on the substrate surface using a magnetron sputtering device.

[0019] Further, in step 4, the photoresist is removed in an organic solvent and a stripping operation is performed; the substrate with the niobium film is immersed in acetone, and after the photoresist is removed, it is rinsed with deionized water and dried with a nitrogen gas gun to obtain the Nb bottom electrode.

[0020] Furthermore, in step 5, the barrier layer is prepared using aluminum material. Before growing the aluminum film, the sample surface is cleaned to remove the oxide film present on the Nb surface; then it is sent into an electron beam evaporation device to grow the aluminum film.

[0021] Furthermore, in step 6, the sample obtained in step 5 is transferred to a magnetron sputtering apparatus to grow a top niobium film as the top electrode according to the conditions in step 3.

[0022] Furthermore, in step 7, ultraviolet lithography exposure and development technology defines the junction region and the pattern of the top electrode on the top niobium film.

[0023] Furthermore, reactive ion etching and ion beam etching are used to remove the top Nb film and barrier layer that are not protected by photoresist. The etching process of the top Nb film is carried out in a reactive ion etching apparatus, and the Al-AlO film is etched. x The process of creating the Al barrier layer is carried out in an ion beam etching apparatus.

[0024] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0025] Compared with existing technologies, the fabrication method for niobium-based Josephson junctions employs simple processes including photolithography, high-vacuum deposition, and etching. This method requires only two photolithography steps to fabricate niobium-based Josephson junctions with controllable dimensions, reducing the possibility of interface degradation during fabrication, simplifying the fabrication process, avoiding complex processing steps, and facilitating the large-scale manufacturing of Josephson junctions. Attached Figure Description

[0026] Figure 1 This is a flowchart of the sample preparation process for a niobium-based Josephson junction.

[0027] Figure 2 This is an optical image of the sample under a microscope.

[0028] Figure 3 This is the RT curve of the sample.

[0029] Figure 4 The image shows the IV characteristic curve of the sample at 3.8K.

[0030] Figure 5 The image shows the IV characteristic curves of the sample under microwave irradiation at a frequency of 40 GHz and a power of 14 dBm. Detailed Implementation

[0031] The present invention will be further illustrated below with reference to specific embodiments, which are based on the technical solutions of the present invention.

[0032] In carrying out the invention, it should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0033] The method for preparing the niobium-based Josephson junction of the present invention, as follows: Figure 1 As shown, the steps include the following:

[0034] Step 1. Clean the substrate

[0035] In this invention, ultrasonic equipment is used to clean the substrate: a sapphire substrate is prepared, and it is ultrasonically cleaned in acetone, alcohol and deionized water for 10 minutes in sequence, then dried with a nitrogen air gun, and baked on a 110°C baking table for 5 minutes.

[0036] Step 2. Photolithography defines the bottom electrode region.

[0037] The pattern of the bottom electrode was fabricated on a clean substrate surface using ultraviolet lithography exposure and development technology. A layer of AZ5214 photoresist was spin-coated onto the sample surface using a spin coater. The spin coater conditions were: 600 rpm for 10 s; 3000 rpm for 60 s. The purpose of spin-coating the photoresist was to ensure uniform distribution and a thickness on the micrometer scale on the substrate surface. The sample was then placed on a heated plate.

[0038] The sample was baked at 95°C for 2 minutes, then exposed to UV light for 10 seconds. After exposure, it was developed for 20 seconds. The resulting structure is as follows: Figure 1 As shown in (b).

[0039] Step 3. Prepare the Nb bottom electrode;

[0040] Niobium films were grown on the substrate surface using a high-vacuum deposition apparatus: The substrate was transferred to a magnetron sputtering apparatus with a cavity background vacuum level better than 1x10⁻⁶. -7 The distance between the sample and the Nb target was adjusted using a lifting platform. After multiple experiments, it was determined that an Nb film grown at a distance of 80 mm between the sample and the Nb target exhibited a higher superconducting critical temperature. The argon gas flow rate was set to 5 sccm. After the gas pressure inside the chamber stabilized at 3.5 mTorr, the DC power supply was turned on and set to 200W. After observing a blue glow on the target surface, the argon gas flow rate was adjusted to 2.3 sccm. Once the gas pressure stabilized at 2 mTorr, pre-evaporation was performed for 3 minutes to clean the target surface and ensure stable operation of the equipment, resulting in a purer Nb film. After pre-evaporation, the baffle was opened, and the sample stage was rotated at a speed of 10 rad / s. Under these conditions, the Nb film growth rate was approximately 50 nm / min, and the evaporation time was set to 3 minutes, resulting in an Nb film of approximately 150 nm. Its structure is shown below. Figure 1 As shown in (c).

[0041] Step 4. Peel off;

[0042] The photoresist was removed in an organic solvent, followed by a stripping operation: the sample was removed from the chamber, immersed in acetone, and after the photoresist was removed, it was rinsed with deionized water and dried with a nitrogen gas gun to obtain the niobium bottom electrode, the structure of which is as follows. Figure 1 As shown in (d).

[0043] Step 5. Preparation of Al-AlO x -Al barrier layer:

[0044] Aluminum was used to prepare the barrier layer. Before growing the aluminum film, the sample surface needed to be cleaned using an ion beam etching apparatus to remove the oxide film present on the Nb surface. After etching, the sample was transferred to an electron beam evaporation apparatus. The entire process was performed in an in-situ vacuum system, controlled by a LabVIEW program. The growth rate was set to 0.1 nm / s, and the program was started to grow a 6 nm Al film on the sample surface. After growth, the sample was moved to an oxidation chamber and oxidized with 7 Torr of oxygen for 120 minutes to obtain an intermediate barrier layer. To prevent damage to the barrier layer during subsequent preparation processes, the sample was transferred back to the electron beam evaporation apparatus to grow a 2 nm aluminum film as a protective layer. Its structure is as follows. Figure 1 As shown in (e).

[0045] Step 6. Grow the Nb top electrode;

[0046] A niobium film was grown using magnetron sputtering as the top electrode: The sample was transferred to the magnetron sputtering apparatus, and the top niobium film was grown according to the conditions in step three, with a growth rate of approximately 50 nm / min and a growth time of 3 min. Its structure is shown below. Figure 1 As shown in (f).

[0047] Step 7. Re-lithographically define the junction region and top electrode region:

[0048] The conditions for UV lithography exposure and development are the same as in step two. A layer of AZ5214 photoresist is spin-coated onto the sample surface using a spin coater. The spin coater conditions are: 600 rpm for 10 s; 3000 rpm for 60 s. The purpose of spin-coating the photoresist is to ensure uniform distribution and a thickness on the micrometer scale on the surface of the Nb top electrode. The sample is then baked at 95°C for 2 minutes on a heating platform, followed by pattern exposure on a UV exposure machine for 10 s. After exposure, development is performed for 20 s. The second photolithography simultaneously defines the junction region and the top electrode region. The junction region connects to the top electrode at the edge of the three-layer structure. Due to the Al-AlO₂... x The presence of the Al layer prevents short circuits between the bottom and top electrodes, eliminating the need for additional insulating materials to isolate them. Its structure is as follows: Figure 1 As shown in (g).

[0049] Step 8. Reactive ion etching and ion beam etching;

[0050] Reactive ion etching (RIE) and ion beam etching were used to remove the top Nb film and barrier layer not protected by photoresist. The Nb etching process was performed in a reactive ion etching apparatus using SF6 gas at a pressure of 30 mTorr and an etching power of 100 W. Under these conditions, the etching rate for Nb was approximately 50 nm / min, while for Al and AlO₂... x Reactive ion etching showed no significant etching.

[0051] To ensure complete etching of Nb, the etching time was set to 4 minutes. (Al-AlO) etching. x The Al process is performed in an ion beam etching apparatus for 20 seconds, and its structure is as follows: Figure 1 As shown in (h).

[0052] Step 9. Remove the adhesive to obtain the sample.

[0053] The photoresist was removed in an organic solvent to obtain the sample: The sample was immersed in acetone solution, and after the photoresist was removed, it was rinsed with deionized water and dried with a nitrogen gas gun to obtain the sample, the structure of which is as follows. Figure 1 As shown in (i). Figure 2 This is an optical image of the sample under a microscope.

[0054] The test results of the niobium-based Josephson junction prepared according to the method of the present invention are as follows:

[0055] Figure 3 The RT curve of the sample was measured during the cooling process, with a temperature range of 5K to 12K. A LabVIEW program controlled a current source to input a 10μA current, read the voltage signal of the sample, and calculated the sample's resistance. The superconducting critical temperature T0 of the sample is also shown. c It is 8.74K.

[0056] Figure 4 This is the IV characteristic curve of the sample at temperature T = 3.8 K. The input current range for the sample is -5 mA to +5 mA. The Josephson junction area is 40 × 30 μm. 2 The critical current I of the sample c Given a current density of 0.91 mA, the critical current density j of this sample was calculated. c 75.8 A / cm 2 A significant jump occurs at V = 2.56 mV. Furthermore, the curves obtained from different current scan directions do not coincide. This is because the Nb electrode generates resistance after quenching, and the current flowing through it generates heat, increasing the actual temperature on the sample and causing a hysteresis effect. Subgap resistance R sg and normal state resistance R N The quality of the junction can be evaluated. This invention defines the subgap resistance R. sg This represents the ratio of voltage to current at 2mV for the sample. The current I of this sample at 2mV is... sg The subgap resistance R is calculated to be 456 μA. sg The value is 4.4Ω. Normal state resistance R N It is 38.3Ω.

[0057] Figure 5 The image shows the IV characteristic curves of the sample under microwave irradiation at a frequency of 40 GHz and a power of 14 dBm. A distinct Shapiro step appears at V = 0.84 mV, confirming that this sample is a superconducting Josephson junction.

[0058] The method for fabricating a niobium-based Josephson junction of the present invention uses alumina as the insulating layer material to isolate the upper and lower superconducting electrodes, which simplifies the fabrication process of the niobium-based Josephson junction. Only two photolithography steps are required to fabricate a niobium-based Josephson junction with controllable dimensions. This reduces the possibility of interface degradation during device fabrication, avoids complex processing procedures, and is conducive to the large-scale production and manufacturing of Josephson junctions.

[0059] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a niobium-based Josephson junction, characterized in that: Alumina was used as the insulating layer material to isolate the upper and lower superconducting electrodes. The bottom electrode region and the junction and top electrode regions were defined using two separate photolithography steps. Specifically, the steps included: Step 1: Clean the substrate; Step 2: Photolithography defines the bottom electrode region; UV photolithography exposure and development technology is used to prepare the bottom electrode pattern on the clean substrate surface; first, spin-coating photoresist is performed, ensuring that the photoresist on the substrate surface is evenly distributed and reaches a thickness on the micrometer scale, then baking and pattern exposure are performed, and development is performed after exposure is completed; Step 3: Fabrication of Nb bottom electrode; Niobium film is grown on the substrate surface under vacuum conditions using magnetron sputtering equipment; Step 4: Stripping; Remove the photoresist in an organic solvent and perform the stripping operation; Immerse the substrate with the niobium film in acetone, and after the photoresist is removed, rinse with deionized water and dry with a nitrogen gas gun to obtain the Nb bottom electrode; Step 5: Preparation of Al-AlO x -Al barrier layer; the material used to prepare the barrier layer is aluminum. Before growing the aluminum film, the sample surface is cleaned using an ion beam etching device to remove the oxide film present on the Nb surface. After etching, the sample is transferred to an electron beam evaporation device to grow an aluminum film on the sample surface. After oxidation, an intermediate barrier layer is obtained, and then a 2nm aluminum film is grown in the electron beam evaporation device as a protective layer. Step 6: Grow the Nb top electrode; Step 7: Perform photolithography again to define the junction region and top electrode region; use ultraviolet photolithography to define the junction region and top electrode pattern on the top niobium film; Step 8: Reactive ion etching and ion beam etching are used to remove the Nb top layer and barrier layer that are not protected by photoresist; the etching process of the top Nb film is carried out in a reactive ion etching apparatus, etching Al-AlO x The process of forming the Al barrier layer is carried out in an ion beam etching apparatus; Step 9: Remove the glue to obtain the finished product.

2. The method for preparing a niobium-based Josephson junction according to claim 1, characterized in that: In step 1, sapphire is used as the substrate for ultrasonic cleaning, followed by drying and baking.

3. The method for preparing a niobium-based Josephson junction according to claim 1, characterized in that: In step 6, the sample obtained in step 5 is transferred to a magnetron sputtering instrument, and a top niobium film is grown according to the conditions in step 3 to serve as the top electrode.

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