Single-junction Josephson probe and preparation method thereof
By fabricating a single-junction Josephson probe at the tip of a nanoneedle, the problems of low yield and difficulty in reducing the tip size in the prior art are solved, enabling near-field microwave imaging with high spatial resolution, simplifying the fabrication process and improving detection sensitivity.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- NANJING UNIV
- Filing Date
- 2026-04-20
- Publication Date
- 2026-05-19
AI Technical Summary
In the existing technology, the fabrication process of SQUID-on-tip probe devices suffers from junction parameter dispersion and ring structure defects, resulting in low yield and difficulty in further reducing the tip size, thus failing to achieve submicron-level high spatial resolution electromagnetic field measurement.
By employing a single-junction Josephson probe structure and utilizing a custom-grooved quartz rod and nanotip pulling technology, combined with two magnetron sputtering processes, a single SsS-type microbridge Josephson weak junction is formed at the tip of the probe. This simplifies the probe structure, avoids complex photolithography and etching steps, and improves the reliability of the fabrication.
This achievement brings the probe tip size close to the 10-nanometer level, improving yield and spatial resolution, providing possibilities for near-field microwave imaging, simplifying the fabrication process, and enhancing detection sensitivity and reliability.
Smart Images
Figure CN122069945A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of superconducting electronics and scanning probe microscopy, specifically relating to a single-junction Josephson probe and its preparation method. Background Technology
[0002] With the rapid development of microwave photonics, quantum computing, and high-frequency integrated circuits, electronic devices are constantly evolving towards miniaturization and high integration, posing unprecedented challenges to the characterization of their electromagnetic properties. Traditional far-field detection techniques are limited by the optical diffraction limit, making it difficult to accurately measure the local electromagnetic field distribution of devices at the sub-micron scale. Although near-field scanning probe microscopy has overcome this limitation, conventional probes lack sufficient sensitivity and spectral resolution in the microwave to millimeter-wave band.
[0003] Josephson junctions, as superconducting devices exceptionally sensitive to electromagnetic fields, operate based on the quantum tunneling effect of Cooper pairs. When external electromagnetic radiation acts on a Josephson junction, a microwave-induced constant-voltage step, known as the Shapiro step, appears in its current-voltage characteristic curve. The step voltage and radiation frequency satisfy the Josephson frequency-voltage relationship. This characteristic makes the Josephson junction one of the most sensitive electromagnetic wave detectors to date, and it is widely used in deep space exploration, terahertz imaging, and quantum voltage references.
[0004] Josephson junctions typically consist of two superconducting electrodes sandwiched by a barrier layer. Based on the barrier layer material, they can be classified into SIS junctions (insulating layer barrier), SNS junctions (normal metal barrier), and SsS junctions (superconducting weak junctions). The advantage of SsS junctions is that they require only a single superconducting material for construction, utilizing the superconducting properties at geometrical changes to weaken and form a natural barrier layer. Niobium (Nb) has become the most widely used material system in superconducting electronic device research due to its high superconducting transition temperature (Tc ~ 9.2 K) and mature thin-film fabrication technology.
[0005] In recent years, superconducting quantum interference devices (SQUID-on-tip) based on nanotips have made significant progress in magnetic field detection, with the smallest tip size reaching the 50-nanometer scale. However, SQUID-on-tip fabrication typically requires the fabrication of a superconducting ring structure containing two Josephson junctions at the tip. This configuration places extremely high demands on the electrical symmetry of the two junctions and the geometric regularity of the superconducting ring. In practical fabrication, the discreteness of junction parameters and the random defects in the ring structure often lead to low device yields, and the complex geometric configuration also limits further reduction in tip size. Summary of the Invention
[0006] The technical problem solved by this invention is to provide a single-junction Josephson probe that simplifies the probe structure, reduces the probe tip size, and provides a method for fabricating a single-junction Josephson probe that reduces the difficulty of probe fabrication and improves the device yield.
[0007] Technical Solution: To solve the above-mentioned technical problems, the present invention adopts the following technical solution:
[0008] A single-junction Josephson probe includes a needle body and a niobium thin film layer on the needle body. The needle body has two symmetrically arranged grooves. The niobium thin film layer includes a first electrode and a second electrode arranged symmetrically. The first electrode and the second electrode are connected at the top of the needle body through an intermediate connection region. The first electrode, the intermediate connection region and the second electrode form a single Josephson junction.
[0009] Preferably, the needle body diameter is 0.5mm-1.2mm, the needle tip diameter is 100nm-200nm, and the thickness of the first electrode and the second electrode is 40nm-60nm.
[0010] This invention also provides a method for preparing a single-junction Josephson probe, comprising the following steps:
[0011] Step 1, Substrate pretreatment: Select a solid quartz rod with two symmetrical longitudinal grooves. The quartz rod is then subjected to ultrasonic cleaning, high-purity nitrogen drying and baking to remove surface contaminants.
[0012] Step 2, Nanoneedle tip drawing: The pretreated quartz rod is drawn by laser melting using a laser needle drawing machine to obtain nanoneedles with a tip diameter of 100nm-200nm, which are called needle bodies;
[0013] Step 3, First magnetron sputtering: Deposit a niobium thin film with a thickness of 40nm-60nm on the surface of the needle to form the first electrode;
[0014] Step 4, In-situ Flipping: Flip the needle body after the first electrode has been deposited 180° so that the other side of the needle body faces the sputtering direction;
[0015] Step 5, Second magnetron sputtering: Deposit a niobium film of the same thickness as the first electrode on the other side surface of the needle body to form the second electrode; after the two depositions, an intermediate connection region is formed at the connection part between the first electrode and the second electrode at the tip of the needle body, thereby forming a single SsS-type microbridge Josephson weak junction at the tip of the needle body.
[0016] Preferably, the quartz rod has a length of A, 20mm≤A≤60mm, a diameter of D, 0.5mm≤D≤1.2mm, a groove depth of B, 0.2mm≤B≤0.5mm, and a groove width of C, 0.05mm≤C≤0.2mm.
[0017] Preferably, the cross-section of the groove is rectangular, semi-circular, trapezoidal, or triangular.
[0018] Preferably, the process parameters for magnetron sputtering in steps 3 and 5 are: background vacuum ≤ 1 × 10⁻⁶. -7 Torr, sputtering pressure 1.9 × 10⁻⁶ -3 Torr, argon flow rate 5 sccm, sputtering DC power 200W, deposition rate 0.7 nm / s.
[0019] Preferably, in step 4, the in-situ flipping is performed under vacuum conditions to avoid oxidation and contamination of the niobium film surface.
[0020] Beneficial effects: Compared with the prior art, the present invention has the following advantages:
[0021] 1. The single-junction Josephson probe of the present invention has a simple structure and the tip size of the single-junction probe has the potential to be further reduced; the smallest size of the existing SQUID-on-tip is about 50 nanometers, while the single-junction probe of the present invention is expected to approach the theoretical limit of such tip devices (about 10 nanometers), providing the possibility for near-field microwave imaging with ultra-high spatial resolution.
[0022] 2. The single-junction structure eliminates the need for superconducting rings and double-junction symmetry matching, fundamentally avoiding the common problems of junction parameter discrepancies and ring structure defects in SQUID-on-tip device fabrication, significantly improving device yield and fabrication reliability;
[0023] 3. For the first time, a single Josephson junction was fabricated at the tip of a nanoneedle. The simplest nanobridge-type weak connection structure was used, and the junction region was located at the very tip of the needle, which is beneficial to improving the spatial resolution of near-field detection.
[0024] 4. By using a custom-grooved quartz rod combined with needle pulling technology, the geometry of the tip can be precisely controlled, providing a structural basis for the formation of weak junctions. Through two in-situ flipping magnetron sputtering processes, a continuous superconducting thin film is formed on the tip surface, while a barrier layer is naturally formed at the narrow junction at the tip. The process is simple and controllable, without the need for complex photolithography and etching steps.
[0025] 5. The prepared probe exhibited a clear Shapiro step at 3.6K, confirming the existence of the Josephson junction and its high sensitivity response to microwaves.
[0026] 6. This probe can be applied to near-field microwave microscope systems to achieve local electromagnetic field distribution imaging and spectrum analysis of devices such as high-frequency integrated circuits and superconducting quantum chips, providing a key tool for electromagnetic compatibility research and fault diagnosis. Attached Figure Description
[0027] Figure 1 This is a schematic diagram of the external structure of a single-junction Josephson probe according to an embodiment of the present invention.
[0028] Figure 2 This is a schematic diagram of the cross-sectional structure of a single-junction Josephson probe in an embodiment.
[0029] Figure 3 This is a schematic diagram of the longitudinal section structure of a single-junction Josephson probe in an embodiment.
[0030] Figure 4 This is a schematic diagram of the cross-sectional structure of an existing double-junction Josephson probe.
[0031] Figure 5 This is a diagram showing the tip structure of an existing double-junction probe and a single-junction probe in this embodiment.
[0032] Figure 6 This is a diagram of the tip structure of the single-junction probe in this embodiment.
[0033] Figure 7 This is a schematic diagram of the cross-sectional structure of the quartz rod in an embodiment of the present invention.
[0034] Figure 8 (a) is a schematic diagram of steps 3 and 5 of the preparation process of the single-junction Josephson probe of the present invention; (b) is a SEM image of the single-junction Josephson probe.
[0035] Figure 9 This is the resistance-temperature characteristic curve of the probe prepared in the embodiment of the present invention.
[0036] Figure 10 The current-voltage characteristic curve of the probe prepared for the embodiments of the present invention is shown at 3.6K temperature and 15GHz@-5dBm microwave irradiation.
[0037] Figure 11 This is a schematic diagram of the four-terminal wiring method in this embodiment. Detailed Implementation
[0038] The present invention will be further illustrated below with reference to specific embodiments. These embodiments are implemented based on the technical solutions of the present invention, and it should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.
[0039] Example
[0040] like Figure 1 , Figure 2 , Figure 3 and Figure 5As shown, a single-junction Josephson probe includes a needle body 1 and a niobium thin film layer on the needle body 1. The needle body 1 is a solid needle made of quartz, with a diameter of 0.5mm-1.2mm, which is 0.6mm in this embodiment. The needle body 1 has two symmetrically arranged grooves 101, each 0.1mm wide and 0.25mm deep, extending along the length of the needle body 1 to the tip (needle tip). The tip diameter is between 100nm and 200nm. The niobium thin film layer includes a symmetrically arranged first electrode 2 and a second electrode 3, with a thickness of 40nm-60nm, which is 50nm in this embodiment. The first electrode 2 and the second electrode 3 are connected at the tip of the needle body 1 via an intermediate connection region 4 (i.e., a narrow connection region). The intermediate connection region 4 is located in the middle of the tip of the needle body 1. The width of the narrow connection region (… Figure 6 In the single-junction probe, the vertical distance of the narrow connection region (width) is less than 100 nm, and the first electrode 2, the middle connection region 4, and the second electrode 3 form a single Josephson junction.
[0041] This embodiment also provides a method for preparing a single-junction Josephson probe, including the following steps:
[0042] Step 1, Substrate Pretreatment: A solid quartz rod 9 with two symmetrical longitudinal grooves 901 is selected. The quartz rod 9 is sequentially subjected to ultrasonic cleaning, high-purity nitrogen drying, and baking to remove surface contaminants. The length of the solid quartz rod 9 is A, 20mm≤A≤60mm. In this embodiment, as shown... Figure 7 As shown, A=50mm, the diameter is D, 0.5mm≤D≤1.2mm, in this embodiment, D=0.6mm, the cross-section of groove 901 is rectangular (it can also be semi-circular, triangular or trapezoidal), the depth of groove 901 is B, 0.2mm≤B≤0.5mm, in this embodiment, B=0.25mm, the width of groove 901 is C, 0.05mm≤C≤0.2mm, in this embodiment, C=0.1mm. The quartz rod is ultrasonically cleaned in acetone and anhydrous ethanol for 5 minutes each, then dried with high-purity nitrogen and baked in a 95℃ oven for 5 minutes.
[0043] Step 2, Nanoneedle tip drawing: The pretreated quartz rod 9 is loaded into the sample holder of a laser needle drawing instrument (such as SutterInstrument P-2000). The quartz rod 9 is locally heated and melted by laser while axial tension is applied, so that the quartz rod 9 is quickly drawn off in the heated area to obtain a pair of nanoneedles with a tip diameter of 100nm-200nm, i.e., two needle bodies 1. The original groove 901 structure can be retained on both sides of the drawn needle body 1. The groove 901 becomes narrower closer to the tip of the needle body 1. The tip of the needle body 1 forms a connecting area, and the groove 901 on the side of the needle body 1 forms a groove 101. After drawing multiple quartz rods 9, the needle bodies 1 with good morphology are preliminarily selected using an optical microscope for later use.
[0044] Step 3, First Magnetron Sputtering: Place the drawn needle body 1 horizontally on the sample holder of the DC magnetron sputtering equipment, so that the axis of the needle body 1 is at 90 degrees to the sputtering direction, and the line connecting the two grooves 101 is also perpendicular to the sputtering direction; evacuate to a background vacuum level better than 1×10⁻⁶. -7 Torr was used, with high-purity argon gas introduced as the working gas. The sputtering pressure, sputtering power, and deposition time were adjusted, with a sputtering pressure of 1.9 × 10⁻⁶. -3 Torr, argon flow rate 5 sccm, sputtering DC power 200W, pre-sputter for 5 minutes to clean the target surface, then open the sample baffle for deposition, deposition rate 0.7 nm / s, deposition time controlled at 3 minutes, deposit a niobium (Nb) thin film with a thickness of 40 nm-60 nm on the surface of needle 1 to form the first electrode 2.
[0045] Step 4, In-situ Flipping: After depositing the first electrode 2, the needle 1 is placed in a vacuum environment with a vacuum degree ≤1×10⁻⁶. -7 The Torr is transferred to the in-situ flipping device inside the vacuum tube, where the needle tip is mechanically flipped 180° so that the undeposited side faces the sputtering direction. In this embodiment, the in-situ flipping device (the in-situ flipping device adopts the in-situ flipping system in the invention patent application publication document with publication number CN121398453 A, which includes a vacuum chamber, a magnetron injection baffle, a sample transfer platform, a transfer rod, a flipping rotation frame, a probe fixing bracket, an external knob, a vacuum pipe, and a linear slide rail; the specific structure will not be described in detail here) completes the flipping of the needle body 1. The in-situ flipping device is located in the injection chamber or vacuum tube of the magnetron sputtering equipment, and the flipping operation of the needle body 1 is completed under vacuum conditions to avoid oxidation and contamination of the niobium film surface.
[0046] Step 5, Second Magnetron Sputtering: The flipped needle 1 is reintroduced into the magnetron sputtering chamber. Using the same process parameters as in Step 3, a niobium film of the same thickness is deposited on the other side surface of the needle 1 to form the second electrode 3. After the two depositions, a narrow connection region, i.e., the intermediate connection region 4, is formed at the top of the needle 1 where the first electrode 2 and the second electrode 3 meet. The intermediate connection region 4 is a microbridge-type weak connection region with a width of less than 100 nm, thus naturally forming a single SsS-type microbridge Josephson weak connection junction, such as... Figure 8 As shown.
[0047] like Figure 9 and Figure 10 As shown, sample testing: The prepared single-junction Josephson probe was mounted on a low-temperature test probe stage, and electrical measurements were performed using the standard four-terminal method. Figure 11 As shown, two gold electrode wires are connected near the tip of the probe at the coated locations on both sides, with voltages V+ and V- applied, and currents I+ and I- applied on the sides further away from the tip. Resistance-temperature characteristic tests show that the sample undergoes two superconducting transitions during cooling: the first transition temperature is approximately 7.1 K, corresponding to the Nb film in the wider region of the tip sidewall; the second transition temperature is approximately 3.9 K, corresponding to the weakly connected region at the tip tip. Current-voltage characteristic curves measured at 3.6 K show typical Josephson junction characteristics. Further application of a 15 GHz microwave signal reveals clear Shapiro steps in the IV curve, with the step voltage interval satisfying the Josephson frequency-voltage relationship, confirming the successful fabrication of a single Josephson junction.
[0048] Comparative Example 1 (Comparison of groove-free structures)
[0049] A standard ungrooved quartz rod was used instead of a custom-grooved quartz rod, and the remaining preparation steps were exactly the same as in Example 1. The results showed that the drawn needle tip was a smooth cone shape, and no effective geometrically narrow junction region was formed at the tip after two magnetron sputtering operations. No two superconducting transitions were observed in low-temperature electrical tests, and no Shapiro steps were observed under microwave irradiation. This indicates that the custom-grooved structure plays a crucial role in creating a geometrical abrupt change at the needle tip, thereby inducing the formation of a superconducting weak junction.
[0050] Comparative Example 2 (hollow quartz rod with two or four grooves)
[0051] The invention uses a quartz tube as described in the publication document of patent application CN 116981341 A, and sets two or four symmetrical grooves (when four grooves are set, the cross-sectional view is as shown). Figure 4 As shown), the remaining preparation steps are exactly the same as in Example 1. Since Comparative Example 2 is a hollow quartz tube, two Josephson knots will inevitably form at the tip of the needle. Figure 5The double-junction probe (which differs from the technical solution in this application) cannot form a single-junction probe.
[0052] The single-junction Josephson probe prepared by this invention can serve as the core detection unit for near-field microwave microscopy, used for local electromagnetic field distribution imaging, spectrum analysis, and electromagnetic compatibility testing in fields such as high-frequency integrated circuits, superconducting quantum chips, and spintronic devices. The probe fabrication process is highly compatible with existing micro / nano fabrication technologies, and its simplified structure and high yield are expected to find wide application in superconducting electronic devices and quantum measurement.
[0053] 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 single-junction Josephson probe, characterized in that, The device includes a needle body (1) and a niobium thin film layer on the needle body (1). The needle body (1) has two symmetrically arranged grooves (101). The niobium thin film layer includes a first electrode (2) and a second electrode (3) arranged symmetrically. The first electrode (2) and the second electrode (3) are connected at the top of the needle body (1) through an intermediate connection area (4). The first electrode (2), the intermediate connection area (4) and the second electrode (3) form a single Josephson junction.
2. The single-junction Josephson probe according to claim 1, characterized in that, The needle body (1) has a diameter of 0.5mm-1.2mm, the tip diameter of the needle body (1) is 100nm-200nm, and the thickness of the first electrode (2) and the second electrode (3) is 40nm-60nm.
3. A method for preparing a single-junction Josephson probe, characterized in that, Includes the following steps: Step 1, Substrate pretreatment: Select a solid quartz rod (9) with two symmetrical longitudinal grooves (901). The quartz rod (9) is subjected to ultrasonic cleaning, high-purity nitrogen blowing and baking treatment in sequence to remove surface contaminants. Step 2, Nanoneedle tip drawing: The pretreated quartz rod (9) is drawn by laser melting using a laser needle drawing instrument to obtain nanoneedles with a tip diameter of 100nm-200nm, which are called needle bodies (1). Step 3, First magnetron sputtering: Deposit a niobium thin film with a thickness of 40nm-60nm on the surface of the needle (1) to form the first electrode (2); Step 4, In-situ flipping: Flip the needle (1) after the first electrode (2) has been deposited by 180° so that the other side of the needle (1) faces the sputtering direction; Step 5, Second magnetron sputtering: Deposit a niobium film of the same thickness as the first electrode (2) on the other side surface of the needle body (1) to form the second electrode (3); After the two depositions, an intermediate connection region (4) is formed at the top of the needle body (1) where the first electrode (2) and the second electrode (3) are connected, thereby forming a single SsS-type microbridge Josephson weak connection junction at the top of the needle body (1).
4. The method for preparing a single-junction Josephson probe according to claim 3, characterized in that, The quartz rod (9) has a length of A, 20 mm ≤ A ≤ 60 mm, a diameter of D, 0.5 mm ≤ D ≤ 1.2 mm, a groove (901) depth of B, 0.2 mm ≤ B ≤ 0.5 mm, and a groove (901) width of C, 0.05 mm ≤ C ≤ 0.2 mm.
5. The method for preparing a single-junction Josephson probe according to claim 3, characterized in that, The groove (901) has a rectangular, semi-circular, trapezoidal or triangular cross section.
6. The method for preparing a single-junction Josephson probe according to claim 3, characterized in that, The process parameters for magnetron sputtering in steps 3 and 5 are: base vacuum ≤ 1 × 10⁻⁶. -7 Torr, sputtering pressure 1.9 × 10⁻⁶ -3 Torr, argon flow rate 5 sccm, sputtering DC power 200W, deposition rate 0.7 nm / s.
7. The method for preparing a single-junction Josephson probe according to claim 3, characterized in that, In step 4, the in-situ flipping is performed under vacuum conditions to flip the needle (1) and avoid oxidation and contamination of the niobium film surface.