Method for preparing nanowire Josephson junction based on nanometer laser direct writing exposure

Abstract

The invention discloses a method for preparing a nanowire Josephson junction based on nanometer laser direct writing exposure, and belongs to the technical field of micro-nano processing, a superconducting material film is deposited on a substrate, photoresist is uniformly spin-coated on the superconducting material film, a photoresist pattern containing a microbridge and an electrode is obtained through photoetching exposure and development, and the nanowire Josephson junction is prepared. Transferring the pattern on the photoresist to the superconducting material film through first etching to form a micro-bridge and electrode structure, performing ultrasonic cleaning to remove the residual photoresist, and spin-coating the photoresist on the pattern of the superconducting material film again to form a micro-bridge and electrode structure; the method comprises the following steps of: exposing and developing photoresist covering a superconducting material film again through nano laser direct writing photoetching, performing micromachining on a microbridge by combining laser shrinkage control, and finally transferring a nanowire pattern from a photoresist structure to the superconducting material film through re-etching to form the nanowire Josephson junction. According to the invention, high-precision preparation of the nanowire Josephson junction is realized through combination of a nanometer laser direct writing technology and laser shrinkage control. A laser shrinkage control technology is utilized, accurate nanoscale laser direct writing is carried out on the two sides of a micro-bridge area, and the nanowire structure in the middle of the stroke is gradually shrunk. The process ensures that the geometric dimension of the nanowire can be accurately controlled, so that the design requirement of a specific device is met. The application of laser shrinkage greatly improves the uniformity and consistency of the nanowire form, and avoids the possible problems of dimensional deviation and instability in the traditional process.

Description

Technical Field

[0001] The invention belongs to the technical field of micro-nano processing, and in particular relates to a method for preparing a nanowire Josephson junction based on nano laser direct writing exposure. Background Art

[0002] Existing superconducting nanowire and Josephson junction manufacturing processes, such as focused ion beam (FIB), electron beam lithography (EBL) and nanoimprint lithography (NIL), face many challenges in application, including complex processes, slow processing speeds and the possible introduction of ion contamination, which seriously affect the performance and large-scale manufacturing capabilities of superconducting devices. Summary of the invention

[0003] The purpose of the present invention is to provide a method for preparing a nanowire Josephson junction based on nano laser direct writing exposure to solve the problems existing in the above-mentioned prior art. The present invention aims to solve the problems of complex process, slow speed and ion pollution in the existing process through a series of micro-nano processing technology steps. Yttrium barium copper oxide (YBCO) film is deposited on a strontium titanate (STO) substrate, and microbridge and electrode patterns are made using ultraviolet lithography technology. Combined with ion beam etching and cleaning treatment, the formation of patterned YBCO film is achieved. Afterwards, a nanowire structure is accurately prepared on the microbridge by nano laser direct writing technology, and a nanowire Josephson junction is finally formed. This method can avoid the influence of thermal effects while retaining the performance of the superconducting film, and supports fast and large-area device manufacturing, providing a new solution for the efficient preparation and performance testing of superconducting Josephson junctions.

[0004] A method for preparing a nanowire Josephson junction based on nano laser direct writing exposure comprises the following steps: depositing a superconducting material film on a substrate, uniformly spin coating a photoresist on the superconducting material film, performing photolithographic exposure and development to obtain a photoresist pattern containing a microbridge and an electrode, transferring the pattern on the photoresist to the superconducting material film by first etching to form a microbridge and an electrode structure, performing ultrasonic cleaning to remove the residual photoresist, spin coating the photoresist again on the pattern of the superconducting material film, performing re-exposure and re-development on the photoresist covering the superconducting material film by nano laser direct writing lithography, micro-machining the microbridge in combination with laser shrinkage control, and finally transferring the nanowire pattern from the photoresist structure to the superconducting material film by re-etching to form a nanowire Josephson junction.

[0005] The term "Josephson junction" is also called a superconducting tunnel junction. It is generally a structure composed of two superconductors sandwiched by a very thin barrier layer (thickness ≤ the coherence length of Cooper electron pairs), such as the S (superconductor) - I (semiconductor or insulator) - S (superconductor) structure, referred to as SIS. In it, superconducting electrons can pass through the semiconductor or insulator film from one side to the other side through the tunnel effect. However, in fact, as long as two superconductors with weak coupling (coupling area size ≤ the coherence length of Cooper electron pairs) can form a Josephson junction, it does not necessarily need to be in the form of a tunnel junction.

[0006] The term "microbridge" refers to the micrometer-wide bridge structure left on YBCO after UV lithography and ion beam etching. The subsequent use of a nano-laser direct writing system to manufacture a contraction-type Josephson junction is based on the fine processing of this microbridge.

[0007] The term "laser shrinkage control" refers to a micro-nano processing technology that interacts with a material through a nano laser direct writing system and controls the distance between exposure tracks, so that the middle area of ​​the material reaches the required scale. In the present invention, it specifically refers to the process of spin coating photoresist on a YBCO micro bridge, shrinking the laser of the nano laser direct writing system along a direction parallel to the width of the micro bridge, exposing and writing the photoresist on the micro bridge, and leaving a photoresist of a scale of hundreds of nanometers after development. Then, the photoresist is removed by ion beam etching to form a shrinkage-type nanowire Josephson junction.

[0008] Nano laser direct writing exposure technology has significant advantages. It can avoid the influence of thermal effects on superconducting films through precise laser control, thereby maintaining the superconducting properties of the material, especially at the nanoscale. In addition, this technology has the ability to process large areas quickly, which can meet the needs of superconducting devices in large-scale applications and significantly improve manufacturing efficiency. At the same time, through laser contraction control, this technology can achieve high-precision processing of complex nanostructures, making the preparation of nanowire Josephson junctions more efficient and pollution-free, suitable for the integrated development and large-scale production of superconducting devices.

[0009] Method for making Josephson junction by laser direct writing exposure to photoresist: The core technical innovation of the present invention is to use nano laser direct writing technology to directly expose on photoresist, accurately control the shape and size of nanostructures, and thus prepare Josephson junctions. This method is different from traditional electron beam lithography and other technologies, and can efficiently realize the patterning of micro-nano structures, which is particularly suitable for the manufacture of superconducting devices.

[0010] Nanowire formation technology based on laser contraction control: This invention uses the laser contraction in the nano laser direct writing technology to accurately control the spacing between two adjacent lasers in the photoresist to achieve high-precision construction of the intermediate nanowire. The control of the laser contraction ensures the processing accuracy and structural consistency at the nanoscale, and is an important technical means for preparing nanowire Josephson junctions.

[0011] The nano-laser direct writing, developing and etching parameters are determined by the preset nanowire geometric parameters.

[0012] The present invention provides a method for preparing a nanowire Josephson junction based on nano laser direct writing exposure, which realizes the efficient preparation of a nanowire Josephson junction through a series of micro-nano processing steps, combining nano laser direct writing technology with traditional photolithography technology. The process specifically includes the following steps:

[0013] YBCO thin film deposition: First, a layer of superconducting material yttrium barium copper oxide (YBCO) thin film is deposited on a strontium titanate (STO) substrate using DC magnetron sputtering as the basic material for the Josephson junction. The strontium titanate (STO) substrate has good lattice matching, ensuring the high-quality growth of the YBCO thin film and guaranteeing superconducting properties.

[0014] Photoresist spin coating: Adjust the speed of the coating machine to the set value, and use the dynamic glue dripping method to evenly spin coat a thin layer of photoresist on the YBCO film. The thickness and uniformity of the photoresist spin coating are crucial to the subsequent exposure and formation of nanostructures.

[0015] UV lithography: UV lithography is used to expose the photoresist spin-coated on the YBCO film. This step is followed by a development process to form the photoresist pattern of the required structure such as microbridges and electrodes. UV lithography ensures accurate patterning of a large area.

[0016] Ion beam etching: Ion beam etching technology is used to accurately transfer the pattern on the photoresist to the YBCO film to form the structure of the electrode and microbridge. Acetone ultrasonic cleaning is then used to remove the residual photoresist to ensure the cleanliness of the surface after etching.

[0017] Secondary photoresist spin coating: Spin coat a layer of photoresist again on the patterned YBCO film to prepare for subsequent nanowire preparation.

[0018] Nano laser direct writing: The photoresist covering the YBCO film is finely exposed through nano laser direct writing exposure technology. In this step, the high resolution of the laser beam and the laser shrinkage control are used to form a nanowire structure on the microbridge area. This process not only avoids the influence of thermal effects on the superconducting film, but also ensures the accuracy and consistency of the nanostructure.

[0019] Ion beam etching and cleaning: Ion beam etching is performed again to transfer the nanowire pattern from the photoresist structure to the YBCO film to form the final nanowire Josephson junction, followed by ultrasonic cleaning with acetone to remove the photoresist residue.

[0020] Compared with the prior art, the present invention has the following advantages and technical effects:

[0021] The present invention realizes high-precision preparation of nanowire Josephson junctions by combining nano laser direct writing technology with laser shrinkage control. Using laser shrinkage control technology, precise nanoscale laser direct writing is performed on both sides of the microbridge area, and the nanowire structure in the middle is gradually shrunk. This process ensures that the geometric dimensions of the nanowires can be precisely controlled to meet specific device design requirements. The application of laser shrinkage greatly improves the uniformity and consistency of the nanowire morphology, avoiding the dimensional deviation and instability problems that may occur in traditional processes.

[0022] The present invention effectively reduces the adverse factors such as thermal effects and ion implantation that may occur during micro-nano processing by optimizing laser parameters (such as reducing laser power), thereby protecting the superconducting properties of the YBCO superconducting film. This not only improves the quality and stability of the device, but also enhances the electrical properties of the nanowire Josephson junction, such as IV characteristics, V-Φ characteristics, etc. At the same time, thanks to the high processing speed and large-area processing capability of nano laser direct writing technology, the method for preparing nanowire Josephson junctions provided by the present invention can meet the needs of large-scale superconducting device manufacturing.

[0023] The present invention further verifies the high quality and consistency of the nanowire Josephson junction by characterizing the morphology and materials of the nanowires using a scanning electron microscope (SEM). The present invention provides a reliable technical path for the efficient preparation of nanowire Josephson junctions and related superconducting devices, and effectively solves the problems of complex processes, slow manufacturing speed, and inaccurate size control in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0025] Figure 1 This is a process flow chart of preparing a nanowire Josephson junction based on nano laser direct writing exposure in the present invention;

[0026] Figure 2 The microbridge portion of the primary structure of the Josephson junction after the first etching in Example 1;

[0027] Figure 3 is a scanning electron microscope image (SEM) of the nanowires in Example 1;

[0028] Figure 4 This is the current-voltage characteristic curve of the nanowire Josephson junction prepared in Example 1. DETAILED DESCRIPTION

[0029] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0030] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0031] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0032] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to the skilled artisan. The present invention description and examples are exemplary only.

[0033] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0034] The embodiment of the present invention provides a method for preparing a nanowire Josephson junction based on nano laser direct writing exposure:

[0035] Step 1: Determine the nanowire geometry parameters

[0036] According to the required nanowire Josephson junction parameter index, the Josephson junction geometric parameters are determined. According to the Josephson junction geometric parameters, the parameters of nano laser direct writing are determined.

[0037] For example, the desired Josephson inductance is L. To observe Josephson-like behavior, the geometric dimensions of the nanowire need to satisfy

[0038] t<<λ L ;

[0039] w<<λ P ;

[0040] Where t is the film (nanowire) thickness; w is the nanowire width; λ L is the London penetration depth; P is the penetration depth of the pearl. The value of t can be roughly determined according to this formula. Considering that the inductance L of the nanowire is mainly composed of the dynamic inductance L k The ratio of the length l and width w of the nanowire can be determined according to the calculation formula of the dynamic inductance:

[0041]

[0042] When the ratio remains unchanged, the length l and the width w can be selected to be as small as possible, but should not be lower than the minimum line width of nanolaser direct writing (generally 100 nm).

[0043] Step 2: Thin film deposition

[0044] The relevant parameters of thin film deposition are determined, and then a layer of yttrium barium copper oxide (YBCO) thin film is deposited on a strontium titanate (STO) substrate by using a DC magnetron sputtering method.

[0045] For example, if you want to get a film with a thickness of t, you can conduct a short-term preliminary experiment under the same conditions, measure the thickness of the deposited film, calculate the deposition rate, and then make a YBCO film of ideal thickness. The sputtering atmosphere is argon (Ar). In order to improve the oxidation state and quality of the film, a certain proportion of oxygen (O2) can be added; the deposition temperature can be selected from 700-800℃; the sputtering power can be selected from 50-200W; the working pressure can be selected from 1-10mTorr. After the deposition is completed, oxygen annealing is performed. Annealing can be performed at 500-600℃ in a pure oxygen atmosphere for 30-120min.

[0046] Step 3: Make the microbridge and electrodes on both sides

[0047] The microbridge and two-side electrodes were fabricated on the YBCO film using ultraviolet lithography and ion beam etching (IBE). The specific steps are as follows: a thin layer of photoresist was spin-coated on the YBCO film using the dynamic glue drop method; the photoresist was exposed and developed using ultraviolet lithography to obtain a photoresist structure containing patterns such as microbridges and electrodes; these patterns were then transferred to the YBCO film using ion beam etching, and the residual photoresist was finally removed using acetone ultrasonic cleaning.

[0048] Step 4: Photoresist spin coating and pre-baking

[0049] Spin-coat a thin layer of photoresist on the YBCO film and bake it on a hot plate for a period of time. The specific parameter selection and precautions are as follows: In the coating stage, in order to improve the resolution of the photolithography process, the photoresist should be uniform and as thin as possible. Therefore, a dynamic glue dripping method should be adopted to prevent excessive photoresist accumulation at the edge of the substrate and the formation of edge beads; the speed of the glue machine should be as high as possible and last for more than 1 minute. Taking the positive photoresist AR-P 3700 as an example, the glue can be dripped at a speed of 2000rpm and then maintained at a speed of 10000rpm for 1min. In the drying stage, in order to obtain the thinnest photoresist, the drying time should be long enough to allow the solvent to fully evaporate. But the time should not be too long to prevent the photoresist from being too hard, the photosensitivity will decrease, and ultimately affect the pattern quality. Taking the positive photoresist AR-P 3700 as an example, it can be baked on a hot plate at 100℃ for 90s.

[0050] Step 5: Nanolaser direct writing lithography

[0051] The parameters of nano laser direct writing, developing and etching are determined according to the desired nanowire geometric parameters, and then the micro bridge is micro-processed using a laser shrinkage controlled process.

[0052] For example, the desired nanowire length is l and width is w

[0053] The laser spot radius R is determined by the laser wavelength λ and the numerical aperture NA of the optical system:

[0054]

[0055] Relationship between effective exposure radius r and spot energy distribution Laser energy distribution usually obeys Gaussian distribution. The effective exposure radius r of the spot refers to the energy density E exceeding the photoresist exposure threshold E. th The radius of the area.

[0056] The energy density distribution is:

[0057]

[0058] Where: E0: energy density at the center of the light spot;

[0059] The effective exposure condition is E(r)≥E th , solve for the effective exposure radius r:

[0060]

[0061] In addition, the relationship between the center energy density E0, laser power P, exposure time t, and spot area A = πR 2 Related:

[0062]

[0063] Combining the above formulas, r can be expressed as a function of P, t, λ, and NA

[0064]

[0065] It can be seen that the effective exposure area can be controlled by adjusting the laser power P and the action time t.

[0066] Then the length of the nanowire l = 2r, and the width w = w0-2r, where w0 is the distance between adjacent laser action center points.

[0067] After laser direct writing, the sample is developed. Taking positive photoresist AR-P 3700 as an example, the sample needs to be immersed in AR 300-47 at room temperature for 60 seconds.

[0068] Step 6: Ion Beam Etching

[0069] According to the thickness of the YBCO film, set the ion beam etching parameters. After setting the parameters, perform ion beam etching on the developed sample. For example, when the argon flow rate is 10sccm, the ion energy is 300eV, and the beam current is 60mA, the YBCO etching rate is about 1.43nm / min. Assuming the thickness of YBCO is t nm, the etching time needs to be set to

[0070] After etching, the sample needs to be ultrasonically cleaned in time to remove residual photoresist.

[0071] Figure 1 The present invention is a process flow chart for preparing nanowire Josephson junctions based on nano laser direct writing exposure.

[0072] Example 1 A method for preparing a high temperature superconducting Josephson junction

[0073] S1. Deposition of YBCO film and gold layer: Select STO as the substrate, and use magnetron sputtering to deposit a 50nm thick YBCO film on the STO surface. Select argon as the sputtering gas, the sputtering power is 100mW, the chamber working pressure is 10mTorr, the temperature is controlled at 700℃ during the deposition process, and after the YBCO film deposition is completed, anneal for 120min at 600℃ and pure oxygen atmosphere. After annealing, a 30nm thick gold layer is deposited on the YBCO surface in an argon atmosphere as a protective layer.

[0074] S2. Spin-coating photoresist: After the YBCO covered with gold layer is prepared by S1, AR-P 3700 photoresist is spin-coated on the surface, the photoresist is dripped at a speed of 2000 rpm, and the rotation speed is accelerated to 10000 rpm to evenly coat the photoresist for 60 seconds. After the spin coating is completed, it is dried at 100°C on a heating platform for 90 seconds to evaporate the solvent in the photoresist;

[0075] S3. UV lithography: Place the chrome-plated mask designed for preparing the primary structure pattern of the high-temperature superconducting Josephson junction at the exposure hole of the UV lithography machine, and place the sample spin-coated with photoresist prepared above at the corresponding exposure position for UV exposure. Use ultraviolet light with a wavelength of 365nm and a power of 100mW for 10s. After exposure, develop in AR 300-47 developer for 30s, and finally place on a heating platform to dry at 100℃ for 60s to enhance the etching resistance of the photoresist. At this point, the primary structure of the Josephson junction on the mask, namely the microbridge and the electrode, has been transferred to the photoresist;

[0076] S4. Ion beam etching: Place the sample in an ion beam etcher, set the argon flow rate to 10 sccm, etch for 120 seconds with an ion energy of 500 eV and a beam current of 100 mA, and remove the gold and YBCO that are not protected by the photoresist. After etching, use acetone solution, anhydrous ethanol, and deionized water to clean the sample in turn to remove the photoresist and other stains on the surface. Figure 2 That is the microbridge part of the primary structure of the Josephson junction;

[0077] S5. Secondary spin coating of photoresist: After the YBCO sample with microbridges and electrodes is prepared by the above process, AR-P 3170 photoresist is spin coated on its surface, and the spin coating parameters are a rotation speed of 4000rpm and a spin coating time of 60s. After the spin coating is completed, it is dried at 100℃ for 60s on a heating platform to evaporate the solvent in the photoresist, and a sample coated with ultra-thin photoresist with a thickness of about 120nm is obtained;

[0078] S6. Nano laser direct writing lithography: Use a nano laser direct writing system to expose the photoresist on the micro bridge part of the above sample. Use a laser with a wavelength of 405nm, a power of 80nW, a point delay of 10ms, and a laser pulse width of 2ns for laser writing. The laser writes and shrinks from both sides to the middle on the nano micro bridge parallel to the bridge width, leaving only the middle part of the photoresist unexposed. After exposure, develop in a solution of AR 300-47 developer and deionized water diluted 4:1 for 60s, and finally place it on a heating platform and dry it at 100℃ for 60s to enhance the etching resistance of the photoresist.

[0079] S7. Secondary ion beam etching: Place the sample in an ion beam etcher, set the argon flow rate to 10 sccm, etch for 300 seconds with an ion energy of 400 eV and a beam current of 80 mA, and remove the gold and YBCO that are not protected by the photoresist. After etching, use acetone solution, anhydrous ethanol, and deionized water to clean the sample in turn to remove the photoresist and other stains on the surface, and obtain a nanowire with a width of 346.5 nm, successfully preparing a shrinkage nanowire Josephson junction.

[0080] Figure 3 This is a scanning electron microscope image (SEM) of the nanowire in Example 1, which characterizes the morphology and material of the nanowire. The results show that the width of the nanowire is 346.5nm, which meets the requirements for preparing a contracted nanowire Josephson junction and has a relatively clear material boundary and a smooth surface morphology.

[0081] The current-voltage characteristic curve of the nanowire Josephson junction prepared in Example 1 was tested by a low-temperature circulation cooling system. The test results are shown in Figure 4 ,from Figure 4 It can be seen that under the condition of 70K, the current-voltage characteristic curve of the prepared sample has the characteristics of a typical Josephson junction, and its superconducting critical current is 110μA. When the current flowing through the Josephson junction is lower than the critical current, it is in a superconducting state, and the voltage is always zero as the current changes; when the current flowing through the Josephson junction is higher than the critical current, it first passes through an intermediate state and then changes to a resistive state, and the voltage and current change linearly.

[0082] The above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for preparing a nanowire Josephson junction based on nano laser direct writing exposure, characterized in that: The following steps are involved: A superconducting material film is deposited on a substrate, a photoresist is evenly spin-coated on the superconducting material film, a photoresist pattern containing a microbridge and an electrode is obtained by photolithographic exposure and development, the pattern on the photoresist is transferred to the superconducting material film by a first etching to form a microbridge and an electrode structure, ultrasonic cleaning is performed to remove the residual photoresist, photoresist is spin-coated again on the pattern of the superconducting material film, the photoresist covering the superconducting material film is exposed and developed again by nano laser direct writing lithography, the microbridge is micro-processed in combination with laser shrinkage control, and finally the nanowire pattern is transferred from the photoresist structure to the superconducting material film by etching again to form a nanowire Josephson junction.

2. The method for preparing a nanowire Josephson junction based on nano laser direct writing exposure according to claim 1, characterized in that: The nano-laser direct writing, developing and etching parameters are determined by the preset nanowire geometric parameters.

3. The method for preparing a nanowire Josephson junction based on nano laser direct writing exposure according to claim 1, characterized in that: The substrate is a strontium titanate substrate.

4. The method for preparing a nanowire Josephson junction based on nano laser direct writing exposure according to claim 1, characterized in that: The superconducting material film is a superconducting material yttrium barium copper oxide film.

5. The method for preparing a nanowire Josephson junction based on nano laser direct writing exposure according to claim 1, characterized in that: The photolithography exposure is ultraviolet photolithography exposure.

6. The method for preparing a nanowire Josephson junction based on nano laser direct writing exposure according to claim 1, characterized in that: Both the first etching and the second etching are ion beam etching.

7. The method for preparing a nanowire Josephson junction based on nano laser direct writing exposure according to claim 1, characterized in that: It also includes ultrasonic cleaning of the sample after etching to remove residual photoresist.

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