A method for preparing narrow-linewidth superconducting nanowires with negative electron beam adhesive adhesion enhancement

Abstract

This invention discloses a method for fabricating narrow-linewidth superconducting nanowires using negative electron beam adhesive adhesion enhancement. The negative adhesive adhesion enhancement structure includes several adhesion enhancement units, which are evenly spaced on both sides of the superconducting nanowire to be fabricated, and each unit is integrally connected to the nanowire. This negative adhesive adhesion enhancement structure significantly enhances the adhesion between the nanowire and the substrate, solving the problem of developer-induced deformation encountered in the fabrication of extremely narrow-linewidth nanowires using negative adhesive processes.

Description

Technical Field

[0001] This invention discloses a method for preparing narrow-linewidth superconducting nanowires with negative adhesive adhesion enhancement structure and negative electron beam adhesive adhesion enhancement, and the superconducting nanowires themselves, belonging to the field of superconducting nanowire single-photon detection (SNSPD) technology. Background Technology

[0002] Superconducting nanowire single-photon detectors can be applied in various fields involving single-photon or high-speed weak-light detection, such as quantum communication, single-photon source calibration, long-distance ranging, non-destructive integrated circuit testing, fiber optic time-domain reflectometers and their sensing, and time-of-flight depth imaging. The detector is biased close to the superconducting critical current during operation. It absorbs a single photon to generate an electrical pulse, which is then detected for photon detection. The detector is composed of meandering superconducting nanowires, typically 60–120 nm wide and 6–8 nm thick. The length depends on the detector's photosensitive area, typically several hundred micrometers. The detector's typical operating wavelength is in the near-infrared band. Theoretical and experimental studies have shown that reducing the nanowire width can effectively improve the detector's quantum efficiency and operating wavelength, extending its applications to the mid- and far-infrared bands and enhancing its overall performance.

[0003] The linewidth of detector nanowires depends on the nanowire fabrication process. The general fabrication process for nanowires is as follows: first, a superconducting thin film is prepared on a silicon substrate; then, gold electrodes are fabricated using photolithography and lift-off processes; next, an electron beam resist is spin-coated, followed by electron beam exposure, development, and reactive ion etching to produce the superconducting nanowires. In this process, spin-coating the electron beam resist and electron beam exposure are the key steps that directly determine the width and uniformity of the nanowires. Two types of electron beam resists are commonly used experimentally: positive and negative. Negative electron beam resists offer high resolution and contrast, providing a more intuitive and convenient design for nanowire patterns, and are therefore often the preferred electron beam resist for fabricating SNSPDs. A typical negative electron beam resist (referred to as a negative resist), such as silsesquioxane, has a resolution below 10 nm, making it the highest-resolution electron beam resist currently available. During electron beam exposure, the high-energy electron beam breaks the molecular bonds of the negative resist, forming a stable compound. The unexposed portions are dissolved during development, leaving only the exposed parts.

[0004] However, when using negative photoresist processes to fabricate extremely narrow linewidth nanowires, the narrow nanowire width results in a small effective contact area between the nanowire pattern and the substrate, leading to weak adhesion. During development, the lateral impact force from the developer easily disrupts the nanowire pattern, causing bending and distortion, severely compromising the uniformity and straightness of the nanowires, and impairing the performance of SNSPDs. This developer impact effect is less noticeable when the nanowire width is greater than 80 nm; however, when the nanowire width is less than 50 nm, the developer impact effect is particularly significant, severely limiting the application of negative photoresist processes in fabricating extremely narrow nanowires. Figure 1 Taking a scanning electron microscope (SEM) image as an example, the nanowire width is designed to be 30 nm. The lateral impact effect of the developing solution causes the nanowire to bend and deform severely after development. Summary of the Invention

[0005] Purpose of the invention: To address the problem of developer-induced deformation encountered during the fabrication of extremely narrow linewidth nanowires using negative electron beam adhesive bonding, this invention proposes a method for fabricating narrow linewidth superconducting nanowires with enhanced negative electron beam adhesive bonding. By designing a nanowire pattern structure with enhanced negative electron beam adhesive bonding, the adhesion between the nanowire and the substrate is strengthened, enabling the fabrication of superconducting nanowire single-photon detectors with linewidths less than 20 nm using negative electron beam adhesive bonding, thereby improving the detector's quantum efficiency and operating wavelength.

[0006] Technical solution: In the first aspect, the present invention proposes a negative adhesive adhesion enhancement structure, comprising a plurality of adhesion enhancement units, wherein the adhesion enhancement units are equally spaced on both sides of the superconducting nanowire to be prepared, and each adhesion enhancement unit is integrally connected with the superconducting nanowire to be prepared.

[0007] Furthermore, the adhesion enhancement unit has a T-shaped structure.

[0008] Furthermore, the material of the adhesion enhancement unit is the same as the material of the superconducting nanowire to be prepared.

[0009] Secondly, this invention proposes a method for preparing narrow-linewidth superconducting nanowires with negative electron beam adhesive adhesion enhancement, comprising the following steps:

[0010] Step 1: Deposit a niobium nitride thin film on a double-sided thermally oxidized silicon substrate using DC magnetron sputtering; then fabricate gold electrodes on both sides of the niobium nitride thin film using photolithography.

[0011] Step 2: Spin-coat negative electron beam photoresist HSQ onto the surface of the niobium nitride thin film to form a negative electron beam photoresist HSQ layer;

[0012] Step 3: The nanowire pattern is transferred to the negative electron beam photoresist HSQ layer using electron beam lithography; the nanowire pattern is a negative photoresist adhesion enhancement structure.

[0013] Step 4: Develop using TMAH developer to obtain the negative electron beam photoresist HSQ layer pattern;

[0014] Step 5: The negative electron beam photoresist HSQ layer pattern is transferred onto the niobium nitride thin film using a reactive ion etching process to prepare narrow-linewidth superconducting nanowires with enhanced negative electron beam photoresist adhesion.

[0015] Thirdly, this invention proposes a narrow-linewidth superconducting nanowire, comprising a structure based on negative adhesive adhesion enhancement. Fourthly, this invention proposes a superconducting nanowire single-photon detector, composed of meandering superconducting nanowires, wherein the superconducting nanowires are the narrow-linewidth superconducting nanowires described in claim 5.

[0016] Furthermore, the linewidth of the superconducting nanowire is less than 20 nm.

[0017] Beneficial effects: The negative adhesive adhesion enhancement structure prepared by the method of the present invention can effectively enhance the adhesion of ultra-narrow linewidth nanowires to the substrate, reduce the width of nanowires prepared by negative electron beam adhesive, thereby improving the quantum efficiency of nanowires for photon detection and improving the overall performance of SNSPD. Attached Figure Description

[0018] Figure 1 This is a schematic diagram illustrating the severe bending and deformation of nanowires caused by the impact effect of the developer.

[0019] Figure 2 Schematic diagram of negative adhesive adhesion enhancement structure;

[0020] Figure 3 Simulation results of superconducting current density distribution in negative adhesive adhesion reinforced structures;

[0021] Figure 4 SEM image of narrow-linewidth nanowires with a negative adhesive adhesion enhancement structure;

[0022] Figure 5 for Figure 4 The IV characteristic curves of the negative adhesive adhesion-enhanced nanowire structure are shown. Detailed Implementation

[0023] The technical solution of the present invention will now be further described in conjunction with the accompanying drawings and embodiments.

[0024] Example 1:

[0025] This invention proposes a negative adhesive adhesion enhancement structure, such as... Figure 2As shown, the fabrication process includes: T-shaped attachment reinforcement structures are placed at regular intervals *l* on both sides of a conventional superconducting nanowire. These structures are rectangular in shape and tightly connected to the nanowire. The length of the attachment reinforcement structure is *d*, and the width is *w*. In actual fabrication, the width of the superconducting nanowire can be set to 20-100 nm, the interval *l* to 200-1000 nm, the length *d* to 50-1000 nm, and the width *w* to 30-100 nm. The specific dimensions can be adjusted according to the geometry of the superconducting nanowire itself. During fabrication, the attachment reinforcement structure can be made of the same superconducting material as the nanowire, such as niobium nitride film, to ensure that the attachment structure and the nanowire are integrated and to enhance adhesion.

[0026] The adhesion enhancement structure proposed in this invention can effectively increase the adhesion of nanowires to the substrate, reduce the impact effect from the lateral developer during the development process, and ensure the integrity of the nanowire geometry.

[0027] Figure 3 Simulation results of a superconducting current density distribution are presented, with a nanowire width of 100 nm and an attached reinforcement structure length d of 700 nm and a width w of 100 nm. The simulation results demonstrate that this "T"-shaped structure avoids the current crowding effect of the nanowire, ensuring that the superconductivity of the nanowire itself remains unaffected. The superconducting current density at local locations where the attached reinforcement structure connects to the nanowire is slightly reduced, but this does not affect the superconducting current density at the main nanowire location, thus ensuring the sensitivity of the nanowire's photon response.

[0028] Example 2:

[0029] Based on Example 1, this invention proposes a method for preparing narrow-linewidth superconducting nanowires with negative electron beam adhesive adhesion enhancement, mainly including the following steps:

[0030] Step 1:

[0031] A niobium nitride thin film with a thickness of 6-8 nm was deposited on a double-sided thermally oxidized silicon substrate using DC magnetron sputtering. Subsequently, gold electrodes were fabricated on both sides of the niobium nitride thin film using photolithography for pulse signal readout.

[0032] Step 2: Spin-coat a negative electron beam photoresist (HSQ) layer onto the niobium nitride thin film surface. The spin-coating process parameters are: forward spin 600~800 rpm for 6~8 s; backward spin 4000~5000 rpm for 60~80 s; and baking at 90℃ for 240~260 s.

[0033] Step 3: Transfer the nanowire pattern to the negative electron beam photoresist HSQ layer using electron beam lithography. The electron beam exposure dose is 1500-2500 μC / cm. 2 The nanowire pattern used in this step is the adhesion-enhanced structure proposed in Example 1.

[0034] Step 4: Use TMAH developer for 150-180 seconds, then soak in deionized water for 60-80 seconds.

[0035] Step 5: The negative electron beam photoresist HSQ layer pattern is transferred onto the niobium nitride thin film using reactive ion etching (RIE) to finally fabricate nanowires. The etching process is as follows: etching gas: CHF3 / SF6; gas flow rate: 20~30 / 40~50 sccm; gas pressure: 4~6 Pa; reaction power: 80~100 W; etching time: 32~36 s.

[0036] The above process steps produce a scanning electron microscope image of an extremely narrow linewidth nanowire, as shown in the image. Figure 4 As shown, the nanowires are designed with a width of 20 nm, a spacing l of 800 nm for the attached reinforcement structures, a width w of 30 nm for the attached reinforcement structures, and a length d of 60 nm. Nanowires with attached reinforcement structures exhibit uniform morphology and a straight distribution. Nanowires lacking attached reinforcement structures show significant bending and deformation, affecting their performance.

[0037] Figure 5 The current-voltage (IV) characteristics of the aforementioned nanowires are shown. The measurement temperature was 2.3 K, the superconducting transition current of the nanowires was 4.7 μA, and the calculated superconducting critical current density of the nanowires was approximately 4.2 × 10⁻⁶. 10 A / m 2 This value matches the typical superconducting critical current density value of niobium nitride nanowires, indicating that the structure has no significant impact on the IV properties of the nanowires, thus ensuring the superconductivity of the nanowires.

Claims

1. A negative adhesive adhesion enhancement structure, characterized in that: It includes several attachment enhancement units, which are equally spaced on both sides of the superconducting nanowire to be prepared, and each attachment enhancement unit is integrated with the superconducting nanowire to be prepared.

2. The negative adhesive adhesion enhancement structure according to claim 1, characterized in that: The adhesion enhancement unit has a T-shaped structure.

3. The negative adhesive adhesion enhancement structure according to claim 1, characterized in that: The material of the adhesion enhancement unit is the same as the material of the superconducting nanowire to be prepared.

4. A method for preparing narrow-linewidth superconducting nanowires with negative electron beam adhesive adhesion enhancement, characterized in that: Includes the following steps: Step 1: Deposit a niobium nitride thin film on a double-sided thermally oxidized silicon substrate using a DC magnetron sputtering method; Gold electrodes were then fabricated on both sides of the niobium nitride thin film using photolithography. Step 2: Spin-coat negative electron beam photoresist HSQ onto the surface of the niobium nitride thin film to form a negative electron beam photoresist HSQ layer; Step 3: The nanowire pattern is transferred to the negative electron beam photoresist HSQ layer using electron beam lithography; the nanowire pattern is a negative photoresist adhesion enhancement structure as described in any one of claims 1 to 3; Step 4: Develop using TMAH developer to obtain the negative electron beam photoresist HSQ layer pattern; Step 5: The negative electron beam photoresist HSQ layer pattern is transferred onto the niobium nitride thin film using a reactive ion etching process to prepare narrow-linewidth superconducting nanowires with enhanced negative electron beam photoresist adhesion.

5. A narrow-linewidth superconducting nanowire, characterized in that, This includes a negative adhesive adhesion enhancement structure based on any one of claims 1 to 3.

6. A superconducting nanowire single-photon detector, characterized in that, It is composed of meandering superconducting nanowires, wherein the superconducting nanowires are the narrow linewidth superconducting nanowires described in claim 5.

7. A superconducting nanowire single-photon detector according to claim 6, characterized in that: The linewidth of the superconducting nanowire is less than 20 nm.

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