A method for fabricating a patterned ultrathin YBCO device with zero material removal

Abstract

This invention provides a method for fabricating patterned ultrathin YBCO devices with zero material removal, belonging to the field of single-photon detection technology. This method utilizes hydrogen plasma to selectively modify the exposed YBCO thin film within a specific process window, thereby suppressing its superconductivity, while the photoresist-covered area retains its original high-temperature superconductivity. This achieves high-precision "zero-material-removal" patterning at the sub-micron and even nanometer scales without removing any material.

Description

Technical Field

[0001] This invention belongs to the field of single-photon detection technology, specifically relating to a method for fabricating a patterned ultrathin YBCO device with zero material removal. Background Technology

[0002] Single-photon detectors (SSPDs) can capture and convert light energy at the level of a single photon. Due to their high detection efficiency, extremely low dark count rate, short jitter time, and fast recovery time, they have significant application value in quantum communication and are a strategic technology for the national development of quantum information. Compared with low-temperature superconductors, high-temperature superconductors exhibit faster quasi-particle breakdown and recombination dynamics and operate at higher temperatures, making copper oxide-based high-temperature superconducting nanowire single-photon detectors (SNSPDs) a research hotspot. SNSPDs made of this material can meet the demands of various fields for ultrafast response and faster data transmission, and also help reduce the requirements for cooling environments.

[0003] YBa2Cu3O 7-δ YBCO, a typical copper-based high-temperature superconductor, possesses a superconducting critical temperature close to 90 K and an ultrafast electron-phonon coupling time on the order of picoseconds. Furthermore, it can achieve large-area, high-quality epitaxial film growth on various single-crystal substrates, making it an ideal candidate material for fabricating high-temperature SNSPDs and other quantum-sensitive devices. High-quality micro / nano patterning is crucial for the successful realization of YBCO-based single-photon detectors. Specific patterning techniques are typically used to fabricate continuous epitaxial films into submicron or even nanometer-scale narrow bridges, meandering structures, or other geometries to define current channels and enhance response to localized photothermal perturbations.

[0004] Currently, patterning of YBCO thin films mainly relies on material removal processes, including focused ion beam (FIB) processing, ion beam etching (IBE), and reactive ion etching (RIE). However, these patterning methods still have significant limitations in practical applications. Some studies (Curtz N, Koller E, et al., Superconductor Science and Technology, 2010) have utilized Ga... 3+Ion-based fibrillation (FIB) technology has been used to fabricate 50 nm wide YBCO bridges, but this method has extremely low processing efficiency, making it difficult to achieve large-scale array production. Regarding ion beam etching, researchers (Arpaia R, Andersson E, et al., Physical Review Materials, 2018) have achieved 65 nm wide nanowire patterning on a 10 nm thick YBCO film using Ar ion etching combined with a carbon hard mask. Although this method shows certain advantages in size control, high-energy ion bombardment inevitably causes damage to the material surface and sidewalls, manifesting as oxygen loss, lattice distortion, and defect enrichment. In the field of reactive ion etching, researchers (Ma H, Wang H, et al., SmallStructures, 2025) have successfully fabricated high-quality nanowires with a thickness of 5 nm and a linewidth of 68 nm using low-temperature, low-pressure inductively coupled plasma reactive ion etching (ICP-RIE) technology. However, this type of process still faces challenges: for ultrathin films, it is extremely difficult to accurately monitor the etching rate and detect the endpoint, and the non-volatile byproducts generated by physical collisions can easily cause secondary contamination of the surface.

[0005] In order to develop high-performance SNSPDs, it is urgent to develop a YBCO device fabrication method that can effectively protect the superconducting properties of YBCO while achieving high-precision patterning. Summary of the Invention

[0006] To address the problems existing in the background technology, the present invention aims to provide a method for fabricating patterned ultrathin YBCO devices with zero material removal. This method utilizes hydrogen plasma to selectively modify the exposed YBCO thin film within a specific process window, thereby suppressing its superconductivity, while the photoresist-covered area retains its original high-temperature superconductivity without being compromised. This achieves high-precision patterning at the submicron and even nanometer scales without removing any material.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] A method for fabricating a patterned ultrathin YBCO device with zero material removal includes the following steps:

[0009] Step 1: Prepare an ultrathin YBCO film on the surface of a single-crystal substrate, and then grow an amorphous YBCO protective layer in situ on the surface of the YBCO film; the single-crystal substrate is a Yttria-Stabilized Zirconia (YSZ) substrate;

[0010] Step 2: A metal electrode is prepared on the surface of the amorphous YBCO protective layer by electron beam evaporation, and then a dense alumina protective layer covering the entire surface is grown by plasma-enhanced atomic layer deposition (PEALD).

[0011] Step 3: Coat the sample surface with electron beam photoresist and develop the nanostructure compensation pattern using electron beam lithography (EBL) technology. The exposed area is the pseudo-insulating region.

[0012] Step 4: Treat the sample obtained in Step 3 with hydrogen plasma;

[0013] Step 5: After the plasma treatment is completed, the remaining photoresist in the sample obtained in step 4 is removed to obtain a patterned ultrathin YBCO device.

[0014] Furthermore, the thickness of the ultrathin YBCO film is 5-10 nm; the thickness of the amorphous YBCO protective layer is 6-10 nm; and the growth method is DC magnetron sputtering.

[0015] Furthermore, in step 2, the background vacuum level during electron beam evaporation is lower than 5 × 10⁻⁶. -6 mbar, with the metal deposition rate controlled between 0.5 and 2 Å / s.

[0016] Furthermore, in step 4, the energy of hydrogen ions in the hydrogen plasma treatment is 470-800 eV, and the treatment time is adjusted according to the thickness of the ultrathin YBCO film, preferably 2-10 min.

[0017] Furthermore, in step 4, the hydrogen plasma treatment uses an inductively coupled plasma reactive ion etching (ICP-RI) device.

[0018] Furthermore, the specific parameters for hydrogen plasma treatment are as follows: hydrogen atmosphere flow rate of 8-12 sccm, ICP power of 800-1200W, RF power of 200-500W, and RF bias voltage of 470-800V.

[0019] The mechanism of this invention is as follows:

[0020] Non-etching patterning is achieved by utilizing the chemical-structural coupling effect between hydrogen plasma and ultrathin YBCO films. In terms of structural dynamics, the injected hydrogen plasma induces a decrease in the long-range order of the lattice through physical displacement damage, causing the atomic arrangement in the exposed regions to shift from ordered to disordered, thus fundamentally disrupting the superconducting current transport path. In terms of chemical thermodynamics, hydrogen ions, due to their extremely high chemical reactivity, strongly interact with the copper-chain oxygen in the YBCO lattice. Because hydrogen has strong reducing properties, the injected hydrogen ions can induce the formation of oxygen vacancies. Hydrogen readily reacts with weakly bound oxygen on the Cu-O chains (forming -OH or trace amounts of H2O), or by occupying oxygen sites and squeezing existing oxygen atoms into interstitial positions. This process significantly reduces the hole carrier concentration within the copper-oxygen plane, promoting the material's transition from a superconducting state to a high-resistivity insulating state, further enhancing the quenching characteristics in the windowed regions. A key feature of this process is that the areas protected by the photoresist completely retain the original film's crystal structure and superconducting properties, while the exposed windowed regions do not contribute to superconducting transport due to the chemical-structural coupling effect. By precisely controlling the plasma energy (accelerating voltage) and dosage (gas flow rate / injection time), localized controlled transformation of the device's electrical properties at the micrometer scale was achieved while maintaining the integrity of the overall film geometry (i.e., zero material removal). Furthermore, this patterning method is somewhat dependent on the substrate material. For LSAT substrates, due to their small lattice mismatch with YBCO (typically less than 1.5%), the epitaxial film interface is relatively intact and the defect density is low. Therefore, the constraint effect on ion migration during hydrogen plasma treatment is relatively limited, which may lead to a certain degree of lateral diffusion of hydrogen ions in the implanted region, thereby reducing the spatial resolution of the modified region. In contrast, YSZ substrates, due to their larger lattice mismatch, higher interface strain, and higher defect density, have a certain limiting effect on ion diffusion, making them more conducive to achieving localized electrical control.

[0021] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0022] This invention achieves patterning by controlling the transition between superconducting and insulating states in localized regions of YBCO thin films, rather than relying on physical etching to remove material. This allows for the construction of YBCO structures while maintaining the continuity of the film's geometry, effectively avoiding problems such as sidewall roughness, edge damage, and over-etching. Utilizing the small atomic radius of hydrogen ions, which allows them to enter the crystal lattice and induce oxygen vacancies and local structural transitions, the modification effect is primarily confined to areas not covered by photoresist by controlling plasma power and processing time. This spatially controls the electrical property transition region, forming a clear interface between the superconducting and high-resistivity regions without introducing significant geometric etching. The switching current of the ultrathin YBCO device prepared using this method is approximately 30% higher than that of conventionally etched devices of the same size, indicating that this method helps improve superconducting transport performance while maintaining device geometric accuracy. Attached Figure Description

[0023] Figure 1 The image shows the IV characteristic curves of the ultrathin YBCO microwire with a linewidth of 2 μm prepared in Example 1 of this invention.

[0024] Figure 2 IV characteristic curves of ultrathin YBCO microwires with a linewidth of 2 μm prepared for Comparative Example 1.

[0025] Figure 3 The IV characteristic curve of the ultrathin YBCO microwire with a linewidth of 2 μm prepared for Comparative Example 2.

[0026] Figure 4 IV characteristic curves of the ultrathin YBCO microwire with a linewidth of 2 μm prepared for Comparative Example 3.

[0027] Figure 5 This is an atomic force microscope (AFM) image of the surface morphology of the ultrathin YBCO microwires obtained in Example 1 of the present invention.

[0028] Figure 6 Scanning electron microscope (SEM) image of the ultrathin YBCO microwires prepared for Comparative Example 1. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0030] Example 1

[0031] A method for fabricating a patterned ultrathin YBCO device with zero material removal includes the following steps:

[0032] Step 1: Select a Yttria-Stabilized Zirconia (YSZ) single crystal substrate and ultrasonically clean it sequentially with acetone, ethanol, and deionized water. Then, use high-purity nitrogen to dry the substrate surface to remove residual moisture. Epitaxially grow an 8 nm thick YBCO thin film on the substrate surface using DC magnetron sputtering. The process parameters are set as follows: working atmosphere is a mixture of O2 and Ar (volume ratio 1:3), working pressure is 30 Pa, sputtering power is 125 W, and deposition temperature is 802 °C. After the film growth is completed and the furnace is cooled to room temperature, an amorphous YBCO protective layer with a thickness of 10 nm is grown in situ on the YBCO thin film surface under the same gas environment and power conditions.

[0033] Step 2: Attach a stainless steel electrode mask to the surface of the amorphous protective layer, and deposit metal electrodes (Ag / Au: 20nm / 20nm) using an electron beam evaporation system, requiring a cavity vacuum level below 5×10⁻⁶. -6 The evaporation rate was controlled at 1 Å / s, and the mask was removed after electrode fabrication. A dense alumina passivation layer with a thickness of 10 nm was then grown on the entire surface of the device using plasma-enhanced atomic layer deposition (PEALD). An amorphous YBCO protective layer and an Al2O3 protective layer were used together to protect the single-crystal YBCO thin film and prevent it from degrading due to contact with air.

[0034] Step 3: Perform UV lithography on the sample obtained in Step 2. First, clean the sample surface (acetone megasonic lithography for 5 min, isopropanol megasonic lithography for 5 min, isopropanol rinsing and nitrogen drying). Then, pre-bake on a 110℃ hot plate for 1 min, spin-coat AZ5214 photoresist (6000 rpm, 60 s), and bake at 110℃ for 1 min. After UV exposure and ZX238 development (45 s), the micron-scale channel region connected to the electrode is defined, and the positioning region is obtained. A 30 μm wide micron-line device is fabricated using a low-temperature inductively coupled plasma reactive ion etching (ICP-RIE) process: using a mixed etching gas (CHF3 / Ar: 10 / 10 sccm), ICP power 1000 W, RF power 100 W, gas pressure 1 mTorr, and the sample stage temperature maintained at -20℃ to suppress thermal damage. After etching, remove residual photoresist by immersion in acetone and megasonic treatment.

[0035] Step 4: Perform electron beam lithography on the sample obtained in Step 3: First, clean the surface of the sample obtained in Step 3, then pre-bake it on a 180℃ hot plate for 1 min; spin-coat ZEP-520A electron beam photoresist (7000 rpm, 60 s), and bake it on a 180℃ hot plate for 3 min; spin-coat conductive carbon adhesive (4000 rpm, 60 s), and bake it on a 110℃ hot plate for 2 min; the process is carried out under an accelerating voltage of 20 kV and an exposure dose of 30 uC / cm. 2 Under the condition of electron beam exposure, the nanowires are patterned to define a 2μm nanowire pattern; after exposure, the adhesive is washed with deionized water, developed with ZED-N50 (3min) and fixed with isopropanol in sequence.

[0036] Step 5: Place the sample treated in Step 4 into the reaction chamber of the ICP system for hydrogen plasma treatment. The process parameters are: hydrogen flow rate 10 sccm, ICP power 1000W, gas pressure 1 mTorr, RF power 500W, treatment time 150s, and RF bias voltage ~800V. After processing, purge the chamber with gas and remove the sample.

[0037] Step 6: Place the injected sample in an ultraviolet light system for 5 minutes, and finally remove the residual electron beam photoresist on the surface by soaking in acetone to obtain an ultrathin YBCO nanowire structure with complete morphology and high precision.

[0038] Comparative Example 1

[0039] The ultrathin YBCO microwires were prepared according to the steps of Example 1, except that step 5 was adjusted as follows: the exposed sample was subjected to low-temperature (-20℃) low-pressure (1mTorr) inductively coupled plasma reactive ion etching using a mixed etching gas (CHF3 / Ar: 10 / 10sccm), with an ICP power of 1000W, an RF power of 100W, and an etching time of 110s.

[0040] Comparative Example 2

[0041] Ultrathin YBCO microwires were prepared following the steps in Example 1, except that step 1 was modified by using (LaAlO3). 0.3 (Sr2TaAlO6) 0.7The (LSAT) single-crystal substrate was ultrasonically cleaned sequentially with acetone, ethanol, and deionized water, followed by drying with high-purity nitrogen to remove residual moisture from the substrate surface. An 8 nm thick YBCO thin film was epitaxially grown on the substrate surface using DC magnetron sputtering. The process parameters were set as follows: working atmosphere of O2 and Ar (volume ratio 1:3), working pressure of 30 Pa, sputtering power of 125 W, and deposition temperature of 802 °C. After the film growth was complete and the furnace cooled to room temperature, a 10 nm thick amorphous YBCO protective layer was grown in situ on the YBCO film surface under the same gas environment and power conditions.

[0042] Comparative Example 3

[0043] Ultrathin YBCO microwires were prepared following the steps in Example 1, with only step 1 modified as follows: LSAT single-crystal substrates were used, and the substrates were ultrasonically cleaned sequentially with acetone, ethanol, and deionized water, followed by drying with high-purity nitrogen to remove residual moisture from the substrate surface. An 8 nm thick YBCO film was epitaxially grown on the substrate surface using DC magnetron sputtering. The process parameters were set as follows: background atmosphere of a mixture of O2 and Ar (volume ratio 1:3), working pressure of 30 Pa, sputtering power of 125 W, and deposition temperature of 802 °C. After the film growth was complete and the furnace cooled to room temperature, an amorphous YBCO protective layer with a thickness of 10 nm was grown in situ on the YBCO film surface under the same gas environment and power conditions.

[0044] Step 5 is adjusted as follows: The exposed sample is subjected to low-temperature (-20℃) low-pressure (1mTorr) inductively coupled plasma reactive ion etching, using a mixed etching gas (CHF3 / Ar: 10 / 10sccm), with an ICP power of 1000W, an RF power of 100W, and an etching time of 110s.

[0045] Figure 1 The IV characteristic curves of an ultrathin YBCO micrometer-line with a linewidth of 2 μm prepared using Example 1 of this invention are shown. Analysis reveals that for the YSZ substrate device, when the bias current is below a critical value, the voltage across the device remains zero, exhibiting a perfect zero-resistance state. As the current further increases, the device undergoes a superconducting-resistive transition, and the voltage rapidly jumps to a finite value (tested at a voltage limit of 1V). The experimentally measured critical switching current of this linewidth device is approximately 647 μA.

[0046] Figure 2The IV characteristics of an ultrathin YBCO microline with a linewidth of 2 μm, fabricated using Comparative Example 1 (conventional CHF3 / Ar reactive ion etching process), are presented. The results show that the device maintains zero voltage output at low bias current, exhibiting a typical superconducting state; when the current increases to the critical switching current, the device undergoes a superconducting-resistive transition, and the voltage rapidly jumps to a finite value, exhibiting a significant voltage jump characteristic. The experimentally measured critical switching current of this device is approximately 503 μA. Figure 1 Compared with devices fabricated using hydrogen plasma modification, nanowires fabricated by conventional etching processes have relatively lower switching currents under the same linewidth conditions. This indicates that while conventional etching processes achieve pattern transfer, they also cause edge damage to the nanowires, thus affecting the overall current-carrying capacity. The hydrogen plasma treatment used in this invention can effectively suppress superconductivity in a predetermined region, while ensuring that the high-temperature superconducting properties of the YBCO thin film are fully preserved in the nanowire region protected by the photoresist.

[0047] Figure 3 The IV characteristic curves for Comparative Example 2 are presented. The results show that the device treated with hydrogen plasma on the LSAT substrate does not exhibit obvious IV hysteresis, but rather shows typical non-superconducting characteristics, that is, the voltage increases continuously and linearly with increasing current.

[0048] Figure 4 The IV characteristic curves for Comparative Example 3 are presented. Experimental results show that the nanowires fabricated on the LSAT substrate using conventional etching processes exhibit a large switching current and a wide hysteresis window, with a critical switching current of approximately 1763 μA, demonstrating strong superconducting current-carrying capacity. This figure shows that the thin film on the LSAT substrate still achieves well-defined superconducting transport characteristics after conventional etching, thus proving that the LSAT substrate thin film itself possesses a patternable basis. Combined with... Figure 1 , Figure 3 The data results indicate that the patterning effect of hydrogen ion treatment exhibits significant selectivity for the substrate material. This is because the lattice mismatch between the LSAT substrate and YBCO is extremely small (typically <1.5%). While this is beneficial for growing high-quality epitaxial films, during hydrogen plasma treatment, this high-quality interface lacks effective dislocation traps to hinder impurity migration. Consequently, when hydrogen ions are implanted into the exposed area, they readily diffuse laterally along the interface and enter the edge of the protected area shielded by the photoresist. In contrast, the YSZ substrate, due to its larger lattice mismatch and interfacial strain, provides a certain degree of constraint on ion diffusion, making it more conducive to achieving localized electrical control.

[0049] Figure 5Atomic force microscopy (AFM) morphology images of the microwires prepared in Example 1 are shown. The double-C regions represent the quenching areas resulting from plasma treatment, with the obtained ultrathin YBCO microwires located between the two double-C regions. It is clearly observed that the plasma-treated areas are raised overall compared to the untreated areas, forming a height difference of approximately 17.3 nm. This distinct step structure indicates that the hydrogen plasma treatment process did not lead to material removal, but rather caused local morphological changes through structural rearrangement and volume expansion. This height increase may result from the synergistic effect of multiple factors: firstly, hydrogen plasma treatment induces lattice distortion and defect accumulation, causing local crystal structure expansion; secondly, the introduction of hydrogen may be accompanied by the formation of oxygen vacancies and –OH groups, altering local chemical bond lengths and coordination environments, thus leading to volume effects; furthermore, the implantation-induced orthorhombic-to-tetragonal phase transformation itself may also bring about changes in lattice parameters, manifested as a macroscopically measurable height difference.

[0050] Figure 6 Scanning electron microscope (SEM) images of the micrometer-sized wires prepared in Comparative Example 1 are shown. As can be seen from the images, the micrometer-sized wires have relatively clear edge contours, while the windowed areas form distinct groove-like depressions after conventional etching, with their surface height lower than the surrounding unetched areas, exhibiting typical material removal characteristics. Figure 5 A comparison of the hydrogen plasma treatment results shows that traditional etching forms patterns by directly removing thin film material, while the method of this invention mainly achieves patterning through local electrical property modulation and structural rearrangement. There are obvious differences in the morphological appearance between the two.

[0051] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features and / or steps.

Claims

1. A method for fabricating a patterned ultrathin YBCO device with zero material removal, characterized in that, Includes the following steps: Step 1: Prepare an ultrathin YBCO thin film on the surface of a single crystal substrate, and then grow an amorphous YBCO protective layer in situ on the surface of the YBCO thin film; the single crystal substrate is a YSZ substrate; Step 2: First, a metal electrode is prepared on the surface of the amorphous YBCO protective layer using electron beam evaporation, and then a dense alumina protective layer covering the entire surface is grown using plasma-enhanced atomic layer deposition. Step 3: Coat the sample surface with electron beam photoresist and develop the nanostructure compensation pattern using electron beam exposure technology. The exposed area is the pseudo-insulating area. Step 4: Treat the sample obtained in Step 3 with hydrogen plasma; Step 5: After the plasma treatment is completed, the remaining photoresist in the sample obtained in step 4 is removed to obtain a patterned ultrathin YBCO device.

2. The preparation method according to claim 1, characterized in that, The thickness of the ultrathin YBCO film is 5-10 nm; the thickness of the amorphous YBCO protective layer is 6-10 nm; the growth method is DC magnetron sputtering.

3. The preparation method according to claim 1, characterized in that, In step 2, the background vacuum level during electron beam evaporation is lower than 5 × 10⁻⁶. -6 mbar, with the metal deposition rate controlled between 0.5 and 2 Å / s.

4. The preparation method according to claim 1, characterized in that, In step 4, the energy of hydrogen ions in the hydrogen plasma treatment is 470-800 eV, and the treatment time is adjusted according to the thickness of the ultrathin YBCO film.

5. The preparation method according to claim 4, characterized in that, The processing time is 2 to 10 minutes.

6. The preparation method according to claim 1, characterized in that, In step 4, the hydrogen plasma treatment uses an inductively coupled plasma reactive ion etching (ICP-RI) device.

7. The preparation method according to claim 6, characterized in that, The specific parameters for hydrogen plasma treatment are: hydrogen atmosphere flow rate of 8-12 sccm, ICP power of 800-1200W, RF power of 200-500W, and RF bias of 470-800V.

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