A method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector

Abstract

This invention discloses a method for fabricating a highly sensitive long-wavelength infrared superconducting single-photon detector. It fully utilizes and amplifies the acoustic mismatch effect between the superconducting thin film and the substrate to achieve phonon localization enhancement on the superconducting nanowires. The effect of this innovative design is to increase the energy utilization rate of long-wavelength infrared photons on the superconducting nanowires, thereby improving the response sensitivity of the superconducting nanowires to long-wavelength infrared photon signals. Compared to existing designs that reduce the cross-sectional size of the superconducting nanowires, create holes beneath the nanowires, or increase the buffer layer, this invention is easier to implement and has higher scalability.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a high-sensitivity long-wave infrared superconducting single-photon detector and belongs to the technical field of infrared single-photon detection. BACKGROUND

[0002] The long-wave infrared band (8-14 mu m) carries key information such as celestial radiation, molecular vibration and environmental thermal radiation, and has important scientific significance and practical value for the development of multiple frontier disciplines and field applications. At present, the development of a high-sensitivity long-wave infrared detector is still a very challenging hot topic. A superconducting nanowire single-photon detector (SNSPD) has an extremely low superconducting energy gap (<5 meV) and an intrinsic dark count rate, can realize single-photon detection, is praised as the single-photon detector with the best comprehensive performance in the near-infrared band, and theoretically, the detection band of the SNSPD can completely cover the long-wave infrared. Therefore, the SNSPD provides a very potential solution to the development difficulty of the high-sensitivity long-wave infrared detector.

[0003] As one of the core indicators for measuring the performance of the SNSPD, the quantum efficiency directly determines the ability of the SNSPD to capture a single photon and convert it into a detectable electrical signal, and the performance directly affects the working efficiency and reliability of the entire quantum system. Compared with near-infrared photons, long-wave infrared photons have lower energy and are more difficult to detect. The quantum efficiency of a conventional near-infrared SNSPD decreases exponentially in the long-wave infrared band, and it is difficult to obtain high sensitivity. How to develop a high-quantum-efficiency long-wave infrared SNSPD has become a widely concerned topic at home and abroad. In the prior art, methods such as reducing the superconducting energy gap and reducing the cross-sectional size of the superconducting nanowire are usually used to improve the quantum detection efficiency. These methods have been successful in experiments. However, there are certain limitations. First, the regulation of the superconducting energy gap is limited by factors such as the superconducting transition temperature and the difficulty of film preparation, and the optimization space is limited. Second, reducing the cross-sectional size of the superconducting nanowire greatly increases the difficulty of preparation and reduces the superconducting transition current, thereby resulting in a low signal-to-noise ratio of the response signal and being difficult to identify. In addition, simply adjusting the film thickness or process cannot fundamentally solve the core problem of the dissipation of photon energy to the substrate in the form of phonons. In addition, some research attempts to regulate the interface characteristics by introducing an intermediate layer or a hole, but this also increases the difficulty of device preparation, limits the process expandability, and is difficult to achieve the purpose of improving the quantum efficiency of the long-wave infrared SNSPD. SUMMARY

[0004] Purpose of the invention: In response to the needs of the cutting-edge field of long-wave infrared detection, the core objective of this invention is to provide a method for fabricating a highly sensitive long-wave infrared superconducting single-photon detector (SNSPD). By means of material and substrate selection and atomic-level doping control, the acoustic mismatch between the superconducting thin film and the substrate is increased in a targeted manner. This suppresses the escape of phonons from the superconducting-substrate interface to the substrate during the down-conversion of photons on the superconducting nanowires, extends the quasiparticle lifetime, promotes hot spot growth, and ultimately significantly improves the quantum efficiency of the SNSPD.

[0005] Technical solution: The present invention provides a method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector, comprising the following steps:

[0006] (1) Superconducting thin films are grown on substrates at low Debye temperature and low bandgap using atomic-level doping control technology;

[0007] (2) Highly sensitive long-wave infrared SNSPDs were developed using electron beam lithography or reactive ion etching processes.

[0008] Furthermore, the relative atomic mass of the substrate material is less than half that of the superconducting thin film; the Debye temperature of the substrate material is twice that of the superconducting thin film.

[0009] Furthermore, in step (1), the substrate material needs to possess the following physical properties:

[0010] 1) The substrate material can have a certain conductivity at room temperature, but the surface resistivity is > 10 MΩ at low temperature (<10 K) to prevent electrical crosstalk with the superconducting thin film grown on its surface;

[0011] 2) The substrate material has no absorption of visible or infrared light or has an absorption rate of less than 5%;

[0012] 3) The substrate material is a single crystal or polycrystalline structure with a relatively small atomic mass and a high Debye temperature.

[0013] Furthermore, in addition to the above requirements, the substrate must also possess the following physical properties:

[0014] 1) The surface roughness of the substrate is < 0.5 nm;

[0015] 2) The substrate is not easily hydrolyzed;

[0016] 3) The substrate will not change during any of the processes, nor will it affect the physical and chemical properties of the thin film.

[0017] Furthermore, the substrate includes, but is not limited to, one of Si substrate, SiO2 substrate, MgO substrate, Al2O3 substrate, MgF2 substrate and Al2O3 substrate.

[0018] Furthermore, in step (1), the superconducting thin film needs to possess the following physical properties:

[0019] 1) Superconductivity can be controlled by atomic-level doping of thin films. The main body of superconducting thin film materials is composed of metal atoms with relatively large atomic masses;

[0020] 2) Superconducting thin films have an amorphous structure and a low Debye temperature;

[0021] 3) The thickness of the superconducting thin film does not exceed 20 nm.

[0022] Furthermore, the superconducting thin film includes, but is not limited to, one of nitrogen-doped amorphous metal thin films, NbN thin films, WSi thin films, MoSi thin films, and NbTiN thin films.

[0023] Furthermore, the growth methods of superconducting thin films include, but are not limited to, magnetron sputtering, ion beam sputtering, electron beam evaporation, and physicochemical vapor deposition.

[0024] Furthermore, in step (1), the conditions for growing superconducting thin films on the substrate at low Debye temperature and low bandgap include: a background vacuum level lower than 1×10⁻⁶. -4 Pa; the neutralizing gas is argon, and the doped atoms include N atoms, Si atoms and / or Ge atoms. The film growth rate is controlled to be no more than 1 nm / s to ensure the film quality.

[0025] Furthermore, in step (2), the control conditions for developing a highly sensitive long-wave infrared SNSPD using electron beam lithography or reactive ion etching include: the electron beam resist used will not chemically react with the superconducting thin film; electron beam lithography with multiple scans (more than 1 scan) is used to improve the structural uniformity of the nanowires; and during reactive ion etching, the etching rate ratio between the electron beam resist and the superconducting thin film is less than 3:1.

[0026] This invention proposes a method for fabricating a highly sensitive long-wavelength infrared superconducting single-photon detector based on the acoustic mismatch effect between a superconducting thin film and a substrate. This method effectively increases the degree of acoustic mismatch between the superconducting thin film and the substrate, reducing the energy dissipation of photon-excited quasi-particles through phonons, thereby improving the quantum efficiency of the device. Phonons, as collective excitations of atomic thermal vibrations in crystals, have propagation characteristics in different media determined by the acoustic properties of the medium (such as sound velocity, density, and elastic modulus). The higher the degree of acoustic matching between the superconducting thin film and the substrate, the easier it is for phonons generated during quasi-particle relaxation to transfer from the superconducting thin film to the substrate, resulting in energy loss. Conversely, if the acoustic mismatch between the two increases, the reflection probability of phonons at the interface will significantly increase, reducing phonon escape to the substrate, thereby extending the lifetime of quasi-particles, increasing the probability of quasi-particles participating in hotspot formation, and ultimately improving the quantum efficiency of the detector.

[0027] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0028] (1) Starting from the microscopic mechanism of photon detection, this invention innovatively proposes a new method for fabricating high-sensitivity long-wave infrared SNSPDs based on phonon localization enhancement. This avoids the complex processes of additional steps such as hole drilling and adding thermal isolation layers. It effectively improves the energy conversion efficiency of photons on superconducting nanowires by utilizing only the acoustic mismatch effect between the superconducting thin film and the substrate, thereby improving the quantum efficiency.

[0029] (2) This invention does not change the conventional structure of SNSPD, is simple to implement, has strong scalability, and can be widely applied in the field of large-scale integrated nano-optoelectronic device technology.

[0030] (3) Compared with existing technical solutions that require reducing the width of nanowires to improve quantum efficiency, this invention reduces the difficulty of process preparation and makes it easier to develop large photosensitive surface long-wave infrared SNSPDs. Attached Figure Description

[0031] Figure 1 This is a phenomenological diagram of the photoresponse of a nitrogen-doped amorphous tungsten thin film (NAW) superconducting single-photon detector in Example 1;

[0032] Figure 2 This is a phonon reflection and projection diagram at the interface between the NAW and the Si substrate in Example 1;

[0033] Figure 3 A comparison of interfacial thermal conductivity at the interface between NAW and several superconducting thin films and Si substrates in Comparative Example 1.

[0034] Figure 4 The graph shows a comparison of the superfluid hysteresis ratios of superconducting single-photon detectors made from NAW and several different materials in Comparative Example 1 at different operating temperatures.

[0035] Figure 5 The graph shows the detection results of NAW at different wavelengths, where the horizontal axis represents the normalized bias current and the vertical axis represents the normalized photon count rate. Detailed Implementation

[0036] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0037] Example 1

[0038] First, we analyze the phenomenological process of photon detection by a superconducting single-photon detector. Figure 1In the initial stage, photons are absorbed by the superconducting nanowires and Cooper pairs are broken up. Subsequently, hot electrons generate a large number of electronic excitations through electron-electron interactions and radiate high-energy (Debye) phonons through electron-phonon interactions. Some of the energy absorbed by the quasiparticles contributes to hot spot formation, while phonon energy is dissipated through two main pathways. Phonons transfer energy back to the electronic system through phonon-electron interactions or escape into the substrate, and cause energy loss through phonon-phonon interactions. When energy loss is minimized, the overall photon energy conversion efficiency is improved. Incident phonons can propagate in longitudinal or transverse modes and are reflected or refracted at the superconductor-substrate interface, a process that follows the acoustic form of Snell's law. Strong acoustic impedance mismatch at the interface leads to a critical angle θ. max This critical angle is relatively small, defining the condition for total internal reflection. In other words, phonons with an incident angle greater than this critical angle will be completely confined inside the nanowire. θ max It can be expressed as equation (1):

[0039] θ max = sin -1 (v1 / v2) (1)

[0040] In equation (1), v1 is the group velocity of phonons in the superconductor, and v2 is the group velocity of phonons in the substrate.

[0041] If the critical angle θ max With a smaller ion density, more phonons will preferentially reflect back to the superconductor at the interface, continuing to break up Cooper pairs before their energy falls below the condensation energy threshold. When there is a significant acoustic impedance mismatch between the superconductor and the substrate, phonon transport at the interface is strongly suppressed, and excited phonons can be effectively confined within the superconductor, reducing phonon escape losses to the substrate.

[0042] Secondly, research and analysis revealed that Si has a relatively small atomic mass (~28), and the Debye temperature of Si substrates is 640 K, which is higher than most substrate materials. Furthermore, Si substrates are inexpensive and readily available; therefore, Si substrates are used in this embodiment. In addition, research found that tungsten atoms have a relatively large atomic mass (~184), and tungsten is a superconductor. Nitrogen-doped amorphous tungsten (NAW) superconducting films can be prepared by doping with a small amount of nitrogen atoms. The calculated Debye temperature of this superconducting film is ~144 K, which is significantly different from the Debye temperature of Si substrates (640 K), thus meeting the core design requirements of this invention. Figure 2 In addition, NAW films have a high superconducting transition temperature (~4.4 K) and strong oxidation resistance, making them ideal materials for developing highly sensitive long-wave infrared SNSPDs.

[0043] Following the research and analysis in step 1, the target substrates (Si) and superconducting thin films (NAW) were established. The experimental fabrication process will now begin:

[0044] (1) Before depositing the NAW thin film, the 2-inch silicon substrate was sequentially cleaned with acetone, ethanol, and deionized water for 15 minutes to remove impurities from the substrate surface. The substrate was then placed in the vacuum chamber of the magnetron sputtering equipment and evacuated to approximately 10 °C. -5 Pa. A NAW thin film was deposited under an argon (100 sccm) / nitrogen (3 sccm) atmosphere and a chamber pressure of 0.67 Pa. The tungsten target was powered by a DC power supply of 60 W. The thickness of the NAW film was controlled by the deposition time; in this embodiment, the NAW thickness was 8 nm, and the surface roughness RMS < 0.5 nm. This embodiment does not require an additional protective layer to prevent oxidation on the NAW film. This is because a surface oxynitride passivation layer spontaneously forms in situ on the NAW film surface. This passivation layer ensures a smooth and uniform surface morphology at the wafer scale and provides excellent chemical stability for subsequent device fabrication.

[0045] (2) Using the well-grown superconducting thin film, a long-wave infrared SNSPD chip was fabricated: The sample was cleaned with acetone, ethanol, and deionized water for 2 minutes each. The sample was spin-coated with PMMA using electron beam photoresist at a spin speed of 4000 rpm and a spin time of 60 s. The pre-baking temperature was set to 180 ℃ and the time was 240 s to remove residual solvent. The chip was then cooled to room temperature with nitrogen and then placed into an electron beam exposure system (EBL, Raith). The exposure resolution, beam step, beam current, and dose were set to 2 nm, 2 nm, 100 pA, and 800 μC / cm², respectively. After electron beam exposure, the sample was first developed in MIBK / IPA (1:3) developer for 90 s and then developed in IPA for 60 s. Reactive ion etching (RIE) was performed using a mixed etching gas of SF6 (40 sccm) and CHF3 (20 sccm), with a chamber pressure of 4 Pa, an etching power of 80 W, and an etching time of 32 s. The sample was then immersed in an N-methylpyrrolidone (NMP) solution at 80°C for 30 minutes to remove residual PMMA photoresist. Finally, the chip was diced into SNSPD chips using a dicing process.

[0046] (3) Constructing optical and circuit systems to test the performance of the SNSPD chip: First, the current-voltage characteristic curves of the chip at different operating temperatures are measured, and the data points of hysteresis current change with temperature are extracted from the measurement results. The relevant data points are fitted using a parametric equation that includes interface thermal conductivity, thereby obtaining the interface thermal conductivity β (unit W / m). 2 ·K 4 ).

[0047] Comparative Example 1: Preparation of several mainstream superconducting thin films, including NbN, TiN, MoSi, and WSi.

[0048] The preparation processes for several mainstream superconducting thin films, including NbN, TiN, MoSi, and WSi, are the same as in Example 1.

[0049] The interfacial thermal conductivity of the NAW-Si prepared in Example 1 was compared with that of several mainstream superconducting thin films prepared in Comparative Example 1, namely NbN (Debye temperature (622 K), TiN (Debye temperature (936 K), MoSi (Debye temperature (364 K), and WSi (Debye temperature 384 K)). (The substrates of NbN, TiN, MoSi, and WSi were all Si substrates, and the difference between the Debye temperature of NbN, TiN, MoSi, and WSi and the Debye temperature of Si substrates was much smaller than that of the NAW-Si combination.) Figure 3 As shown in the figure. Comparison reveals that the NAW-Si interface has the lowest thermal conductivity, significantly lower than other interface architectures, confirming the technical advantages of the structure of this invention. Superfluid hysteresis ratio (I SW / I R The sensitivity of the SNSPD is related to its photodetector sensitivity; generally, the higher the superfluid hysteresis ratio, the higher the sensitivity of the SNSPD. For example... Figure 4 As shown, compared with other mainstream superconducting films in Comparative Example 1, the NAW-Si device prepared in Example 1 has the highest superfluidity-hysteresis ratio at different temperatures, which is also the result of strong interface acoustic mismatch control.

[0050] Example 2

[0051] This embodiment further characterizes the detection sensitivity of the NAW device developed in Embodiment 1 in the infrared band. A free-space coupled infrared test optical path is constructed. Infrared photons are emitted from the light source, collimated, and then pass through the optical window of the dilution cooler. The optical power is attenuated to the single-photon level by a set of neutral density filters before finally reaching the photosensitive surface of the detector. When the photon trigger device generates an electrical pulse, the bias circuit transmits the pulse to the amplifier port, and finally, a high-speed counter counts the amplified pulses. Figure 5 This paper presents the relationship between the normalized photon count rate (measured photon count rate / saturated photon count rate, equivalent to the device quantum efficiency) and bias current for the NAW detector fabricated in Example 1 across a wide wavelength range of 1.75–10 μm. The results show that the detector achieves saturated quantum efficiency across the 1.75–6 μm wavelength range, and exceeds 70% at a wavelength of 10 μm. The detection cutoff wavelength λ of the device is calculated using a diffusion hotspot model. c ∝[N(0)(k B T c ) 2 ] -1 N(0), k B and T cThe values ​​represent the electronic density of states at the Fermi surface, the Boltzmann constant, and the superconducting critical transition temperature, respectively. Calculations show that, without considering the interface acoustic mismatch modulation technique, the theoretical detection cutoff wavelength of the detector is 3.3 μm, but it actually measured 6 μm. This can be attributed to the incremental wavelength gain brought about by the interface acoustic mismatch modulation technique, which to some extent surpasses other mainstream superconducting thin-film devices in Comparative Example 1.

Claims

1. A method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector, characterized in that, Includes the following steps: (1) Superconducting thin films are grown on substrates at low Debye temperature and low bandgap using atomic-level doping control technology; (2) Highly sensitive long-wave infrared SNSPDs were developed using electron beam lithography or reactive ion etching processes.

2. The method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector according to claim 1, characterized in that, The relative atomic mass of the substrate material is less than half that of the superconducting thin film; the Debye temperature of the substrate material is twice that of the superconducting thin film.

3. The method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector according to claim 2, characterized in that, In step (1), the substrate material needs to possess the following physical properties: 1) The substrate material can have a certain conductivity at room temperature, but the surface resistivity is > 10 MΩ at low temperature to prevent electrical crosstalk with the superconducting thin film grown on its surface. 2) The substrate material has no absorption of visible or infrared light or has an absorption rate of less than 5%; 3) The substrate material is a single crystal or polycrystalline structure with a relatively small atomic mass and a high Debye temperature.

4. The method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector according to claim 3, characterized in that, The substrate also needs to have the following physical properties: 1) The surface roughness of the substrate is < 0.5 nm; 2) The substrate is not easily hydrolyzed; 3) The substrate will not change during any of the processes, nor will it affect the physical and chemical properties of the thin film.

5. The method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector according to claim 4, characterized in that, The substrate is one of Si substrate, SiO2 substrate, MgO substrate, Al2O3 substrate, MgF2 substrate and Al2O3 substrate.

6. The method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector according to claim 1, characterized in that, In step (1), the superconducting thin film needs to possess the following physical properties: 1) Superconductivity can be controlled by atomic-level doping of thin films. The main body of superconducting thin film materials is composed of metal atoms with relatively large atomic masses; 2) Superconducting thin films have an amorphous structure and a low Debye temperature; 3) The thickness of the superconducting thin film does not exceed 20 nm.

7. The method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector according to claim 6, characterized in that, Superconducting thin films include, but are not limited to, one of nitrogen-doped amorphous metal thin films, NbN thin films, WSi thin films, MoSi thin films, and NbTiN thin films.

8. The method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector according to claim 1, characterized in that, In step (1), the growth methods of superconducting thin films include, but are not limited to, magnetron sputtering, ion beam sputtering, electron beam evaporation, and physicochemical vapor deposition.

9. The method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector according to claim 1, characterized in that, In step (1), the conditions for growing superconducting thin films on the substrate at low Debye temperature and low bandgap include: a background vacuum level of less than 1×10⁻⁶. -4 Pa; the neutralizing gas is argon, and the doped atoms include N atoms, Si atoms and / or Ge atoms. The film growth rate is controlled to be no more than 1 nm / s to ensure the film quality.

10. The method for fabricating a high-sensitivity long-wavelength infrared superconducting single-photon detector according to claim 1, characterized in that, In step (2), the control conditions for developing a highly sensitive long-wave infrared SNSPD using electron beam lithography or reactive ion etching include: the electron beam resist used will not react chemically with the superconducting thin film; electron beam lithography with multiple scans (more than 1 scan) is used to improve the structural uniformity of the nanowires; and during reactive ion etching, the etching rate ratio between the electron beam resist and the superconducting thin film is less than 3:1.

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