A mid-infrared wave band wide spectrum detector and a preparation method thereof

Abstract

This invention discloses a broadband detector in the mid-infrared band, comprising, from bottom to top, a silicon wafer, a reflective layer, and a dielectric layer. U-shaped superconducting nanowires are disposed on the surface of the dielectric layer, with the open ends of the U-shaped superconducting nanowires connected to gold electrode one and gold electrode two, respectively. Gold antennas are symmetrically disposed on both sides of the U-shaped superconducting nanowires. Gold electrode one is connected to a constant voltage source, and gold electrode two is grounded. This invention also discloses a method for fabricating the broadband detector in the mid-infrared band. The superconducting nanowire single-photon detector based on gold antenna enhancement of this invention features high field gain, large photosensitive area, high nanowire absorption efficiency, high intrinsic quantum efficiency, and extremely low fill factor, enabling efficient application in mid-infrared band detection. Compared with traditional optical cavity superconducting single-photon detectors, it has advantages such as high fabrication feasibility, high compatibility with planar processes, and field gain, and can detect mid-infrared light signals across a wider band.

Description

Technical Field

[0001] This invention pertains to detectors and their fabrication methods, specifically a broadband detector in the mid-infrared band and its fabrication method. Background Technology

[0002] With the rapid development of various mid-infrared detector technologies, humanity's exploration of the deep universe is accelerating. Many scientific questions, such as the cause of the Big Bang, the secrets of the origin of species, and the search for habitable planets, rely on high-performance photoelectric detectors. The mid-infrared band (3-5 μm), as one of the atmospheric windows, allows for exploration and observation of outer space from Earth. On the one hand, compared to space telescopes, ground-based mid-infrared detectors can meet some of the requirements, and are lower in cost, more controllable from the ground, and more compact in structure. On the other hand, the absorption peaks of key molecular spectral lines related to the origin of species are mainly distributed in the mid-infrared band, and the light signals transmitted from the depths of space to Earth are extremely weak, thus placing higher demands on mid-infrared detectors.

[0003] To evaluate the performance of various mid-infrared detectors, Jones proposed the concept of "normalized detectivity" in 1959. Normalized detectivity is directly proportional to the detection area and response bandwidth, and inversely proportional to the equivalent noise power. Therefore, there is a pressing need for mid-infrared detectors that simultaneously possess a large detection area, wide response range, and low equivalent noise power. Compared to the visible and near-infrared bands, the light signal in the mid-infrared band is weaker and more difficult to detect, thus placing higher demands on the sensitivity of the detector.

[0004] Superconducting nanowire single-photon detectors (SNSPDs) are a new type of high-performance single-photon detector for the 21st century, characterized by high detection efficiency, wide spectral response, low dark count, extremely low equivalent noise power, and extremely low time jitter. The core of an SNSPD lies in a superconducting wire with a linewidth of less than 100 nm, typically etched from a superconducting thin film of less than 10 nm thickness. Placed in a low-temperature environment, when a single photon is absorbed by the nanowire, the interaction of Cooper pairs within the nanowire affects the formation of hundreds or thousands of quasi-particles, creating a hotspot diffusion effect locally within the nanowire. Due to the localized resistive state formed within the nanowire, the current on the nanowire is transferred to an external resistor, and the optical signal is converted into an electrical signal through the detector's readout circuit.

[0005] SNSPDs are essentially micro / nano structures. Achieving large detection areas (centimeter-scale) places extremely high demands on thin film growth uniformity, electron beam lithography masks, and reactive etching conditions. First, magnetron sputtering is typically used to grow superconducting thin films. However, magnetron sputtering is affected by substrate surface defects, making it difficult to control the uniformity of large-area film growth. Second, electron beam lithography is a crucial step in nanowire fabrication. Prolonged operation of electron beam lithography equipment at high voltages can lead to electron beam instability, resulting in inconsistent exposure of the resist to high-energy electrons. Third, reactive etching involves introducing specific reactive gases that react with the exposed superconducting film, generating gaseous products, and then removing excess gas from the chamber. This effectively removes excess film, enabling the final fabrication of individual nanowires. During this process, the gas concentration is spatially limited, and the reaction area should not be too large.

[0006] Currently, researchers have demonstrated that SNSPDs exhibit single-photon responses from the near-infrared band to the ten-micrometer band, thus possessing a wide response range. However, the absorption efficiency of the nanowires is limited by the optical cavity at different wavelengths, exhibiting high absorption in narrow bands, while broadband absorption requires further optimization. Low equivalent noise power is related to the dark count and detection efficiency of SNSPDs. Because SNSPDs operate in a low-temperature environment, much noise is shielded, allowing the dark count to remain at a very low level (100 cps). The detection efficiency of SNSPDs consists of three parts: optical coupling efficiency, absorption efficiency, and quantum efficiency. Optical coupling efficiency in astronomical detection is mainly related to the detection area; mid-infrared light sources need to be collimated with the detector to ensure high coupling efficiency. For SNSPD device properties, absorption efficiency and quantum efficiency are key performance characteristics of the chip. Currently, SNSPDs have been proven to possess high quantum efficiency, but low absorption efficiency limits their application in astronomical detection.

[0007] Because photons in the mid-infrared band have low energy, existing detectors relying solely on a single nanowire structure can only absorb a small portion of photons, with most being transmitted or reflected at the interface. Current mid-infrared superconducting nanowires attempt to improve the performance of SNSPDs through optical cavity design; however, the actual fabrication of thick optical cavities is extremely difficult. This is because a thick optical cavity is a three-dimensional structure, and growing it using conventional equipment can lead to cavity cracking and surface inhomogeneity. Furthermore, optical cavities are typically optimized for a narrow wavelength band, with very weak absorption in other bands. Summary of the Invention

[0008] Purpose of the invention: In order to overcome the shortcomings of the existing technology, the purpose of this invention is to provide a broadband detector in the mid-infrared band with high field gain, large photosensitive area and high absorption efficiency. Another purpose of this invention is to provide a simple and convenient method for fabricating a broadband detector in the mid-infrared band that is highly compatible with planar processes.

[0009] Technical solution: The broadband detector in the mid-infrared band of the present invention comprises, from bottom to top, a silicon wafer, a reflective layer and a dielectric layer. U-shaped superconducting nanowires are disposed on the surface of the dielectric layer. The open ends of the U-shaped superconducting nanowires are connected to gold electrode one and gold electrode two, respectively. Gold antennas are symmetrically disposed on both sides of the U-shaped superconducting nanowires. The gold antennas are periodically arranged to increase the response area of ​​the U-shaped superconducting nanowires and gold antennas. Gold electrode one is connected to a constant voltage source, and gold electrode two is grounded.

[0010] Furthermore, the U-shaped superconducting nanowires have a single-segment width of 30–60 nanometers and a thickness of 5–10 nanometers. The U-shaped superconducting nanowires are made of niobium nitride superconducting material, with a spacing between the two wires of 0.1–100 micrometers, a duty cycle of 0.001–0.9, and a nanowire width of 20–100 nanometers. The "dragonfly"-shaped structure formed by the U-shaped superconducting nanowires and antenna structures exhibits high absorption efficiency. While conventional antennas only contain a single through-wire structure, the U-shaped superconducting nanowires designed in this scheme increase the high-absorption region of the nanowires and expand the photosensitive area. Moreover, the larger area of ​​the U-shaped nanowire structure reduces the difficulty of coupling during actual fabrication, effectively improving the yield.

[0011] Furthermore, the reflective layer is a gold reflective layer. The thickness of the reflective layer is 20~300 nanometers.

[0012] Furthermore, both gold electrode one and gold electrode two are trapezoidal. The shape affects the transmission of electrical signals and needs to be matched with impedance.

[0013] Furthermore, the gold antenna has a thickness of 10–300 nanometers, a length of 10–1000 nanometers, and a width of 1–1000 nanometers. The number of these antennas is equal to the length of a single U-shaped superconducting nanowire divided by the radiation period of the gold antenna. The gold antenna can collect incident light signals onto localized areas of the nanowires. The absorption effect is strongest at the point of direct contact between the antenna and the nanowire, decreasing with distance. Therefore, periodically arranged gold antennas are needed to fully cover the nanowire region.

[0014] Furthermore, the silicon wafer is preferably a single crystal, which results in better uniformity of the grown film. The dielectric layer is any one of silicon nitride, silicon oxide, germanium, and silicon, preferably silicon nitride.

[0015] The above-mentioned method for fabricating a broadband detector in the mid-infrared band includes the following steps:

[0016] Step 1: Deposit a reflective layer onto the cleaned silicon wafer using an electron beam evaporator;

[0017] Step 2: Prepare a dielectric layer on the reflective layer using electron beam evaporation or chemical vapor deposition.

[0018] Step 3: A superconducting material thin film is prepared on the dielectric layer using magnetron sputtering.

[0019] Step 4: Spin-coat positive photoresist onto the surface of the superconducting material thin film, expose the positive photoresist in a patterned manner, and obtain a positive photoresist mask with electrode shapes through development. Deposit gold electrode 1 and gold electrode 2 on the positive photoresist mask using a lift-off method.

[0020] Step 5: Fabricate a gold antenna on a dielectric layer using electron beam lithography.

[0021] Step six: Using electron beam lithography, U-shaped superconducting nanowires are fabricated at corresponding positions on the dielectric layer.

[0022] Furthermore, in step one, the silicon wafer is cleaned with low-power ultrasonic cleaning using acetone and ethanol respectively, and finally dried with nitrogen gas.

[0023] Further, in step five, a positive electron beam resist is spin-coated onto the surface of the superconducting material thin film. The positive electron beam resist is patterned and exposed using an electron beam exposure machine. After development and fixing, a positive photoresist mask with a gold antenna shape is obtained. A uniform gold layer is deposited on the positive photoresist mask using electron beam evaporation. The gold layer is then peeled off using a chemical solution water bath heating method to obtain the gold antenna.

[0024] Further, step two specifically includes: cleaning the silicon wafer containing the gold reflective layer with acetone and ethanol using low-power ultrasound in sequence, drying the cleaned silicon wafer containing the gold reflective layer with nitrogen gas to obtain a high-cleanliness silicon wafer containing the gold reflective layer, and depositing a uniform dielectric layer on the gold reflective layer using electron beam evaporation or chemical vapor deposition.

[0025] Further, step three specifically includes: cleaning the sample containing the dielectric layer with acetone and ethanol using low-power ultrasound in sequence, drying the cleaned sample containing the dielectric layer with nitrogen gas to obtain a sample containing the dielectric layer with high cleanliness, and depositing a thin film of superconducting material with good uniformity on the dielectric layer by magnetron sputtering.

[0026] Further, step six specifically includes: spin-coating a positive electron beam resist onto the surface of the dielectric layer, patterning the positive electron beam resist using an electron beam exposure machine, obtaining a positive photoresist mask with nanowire shape through development and fixing, and using reactive ion etching to etch excess superconducting material film to obtain U-shaped superconducting nanowires.

[0027] Working Principle: The device operates below 100 mK, where the nanowires are in a superconducting state. Mid-infrared light signals reach the photosensitive region of the device via free-space coupling; this region refers to the area between the gold antenna and the nanowires. At the antenna, surface plasmons appear at the interface between the antenna and the nanowires. These surface plasmons confine the light signal within the nanowire region. Through overall structural optimization and control, high absorption in the nanowire region is achieved. The nanowires are typically fabricated to be very thin, enabling even higher sensitivity to mid-infrared light signals. When the superconducting nanowire absorbs photon energy, an electrical pulse is generated at both ends. A specific readout circuit reads this electrical pulse signal, completing a mid-infrared photon detection process. Combining high absorption and high sensitivity response, a high-efficiency mid-infrared superconducting single-photon detector is achieved. The key to the detector's structure lies in adjusting the dimensions of the gold antenna structure, the superconducting nanowire unit structure, and the dielectric layer thickness to regulate the incident mid-infrared light onto the dielectric layer surface. Due to the enhanced response of the antenna field, the absorption efficiency of the nanowires is maximized.

[0028] Beneficial effects: Compared with the prior art, the present invention has the following significant features:

[0029] 1. The superconducting nanowire single-photon detector based on gold antenna enhancement of the present invention has the characteristics of high field gain, large photosensitive area, high nanowire absorption efficiency, high intrinsic quantum efficiency, extremely low fill rate, and wide spectral response, and can be efficiently applied to the mid-infrared band detection application field.

[0030] 2. Compared with traditional mid-infrared optical cavity superconducting single-photon detectors, it has advantages such as high fabrication feasibility, high compatibility with planar processes, and field gain, and can detect mid-infrared light signals in a wider band.

[0031] 3. Compared with conventional superconducting single-photon detectors, this invention solves the problem of small photosensitive area in superconducting nanowire single-photon detectors, fundamentally breaking through the limitation of micron-scale photosensitive area;

[0032] 4. By using a "metal-dielectric-metal" sandwich structure, it has the advantages of uniform thin film deposition and large area, which effectively broadens the application field of superconducting nanowire detection in the mid-infrared band.

[0033] 5. The "dragonfly"-shaped structure composed of U-shaped superconducting nanowires and antenna structures has high absorption efficiency, which reduces the difficulty of actual process preparation and effectively improves the yield. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the structure of the present invention;

[0035] Figure 2 This is a schematic diagram of the cross-section at point P of the present invention;

[0036] Figure 3 This is a graph showing the relationship between the absorption rate and wavelength of the present invention;

[0037] Figure 4 This is a graph showing the absorption rate ratio of the present invention at a wavelength of 4 micrometers;

[0038] Figure 5 This is a graph showing the relationship between the electric field intensity at point P and the wavelength in this invention.

[0039] Figure 6 This is a graph showing the relationship between the electric field strength gain at point P and the wavelength in this invention.

[0040] Figure 7 This is a scanning electron microscope image of the U-shaped superconducting nanowire 4 of this invention. Detailed Implementation

[0041] like Figures 1-2 The mid-infrared broadband high-efficiency detector consists of a silicon wafer 1 topped with a reflective layer 2 and a dielectric layer 3. The silicon wafer 1 is a single crystal, resulting in good film uniformity. The reflective layer 2 is a gold reflective layer with a thickness of 20-300 nanometers. The dielectric layer 3 is any one of silicon nitride, silicon oxide, germanium, and silicon, preferably silicon nitride. The dielectric layer 3 has a thickness of 20-500 nanometers. The upper surface of the dielectric layer 3 has U-shaped superconducting nanowires 4, a first gold electrode 5, a second gold electrode 6, and a gold antenna 7. The U-shaped superconducting nanowires 4 are in direct contact with the gold antenna 7 on both sides. The gold antenna 7 is periodically arranged to increase the response area of ​​the U-shaped superconducting nanowires 4 and the gold antenna 7. This allows for the fabrication of a broadband, high-efficiency mid-infrared superconducting nanowire single-photon detector, significantly improving sensitivity and process compatibility, thereby expanding its application areas.

[0042] The U-shaped superconducting nanowire 4 is connected to the constant voltage source 8 via gold electrode 5, and gold electrode 6 is grounded. The width of a single segment of the U-shaped superconducting nanowire 4 is 30–60 nm, and its thickness is 5–10 nm. The U-shaped superconducting nanowire 4 is in the form of a meandering grating, made of niobium nitride superconducting material (NbN), with a radiation period of 0.1–100 μm, a duty cycle of 0.001–0.9, and a nanowire width of 20–100 nm. Both gold electrodes 5 and 6 are trapezoidal. The gold antenna 7 has a thickness of 10–300 nm, a length of 10–1000 nm, and a width of 1–1000 nm; its number is equal to the length of a single U-shaped superconducting nanowire 4 divided by the radiation period of the gold antenna 7. The gold antenna 7 can collect incident light signals onto localized areas of the nanowire. The absorption effect is strongest at the point of direct contact between the antenna and the nanowire, decreasing with distance. Therefore, periodically arranged gold antennas 7 are needed to fully cover the nanowire region.

[0043] The fabrication method of a broadband high-efficiency detector in the mid-infrared band includes the following steps:

[0044] Step 1: The silicon wafer 1 is cleaned sequentially using acetone and ethanol with low-power ultrasonic cleaning; the cleaned silicon wafer 1 is dried with nitrogen gas to obtain a high-cleanliness silicon wafer 1; a uniform reflective layer 2 is deposited on the silicon wafer 1 by electron beam evaporation.

[0045] Step two: The silicon wafer 1 sample containing the reflective layer 2 is cleaned sequentially using acetone and ethanol with low-power ultrasonic cleaning; the cleaned silicon wafer 1 sample containing the reflective layer 2 is then dried with nitrogen gas to obtain a high-cleanliness silicon wafer 1 sample containing the reflective layer 2. A uniform dielectric layer 3 is then deposited on the reflective layer 2 using electron beam evaporation or chemical vapor deposition.

[0046] Step 3: The sample containing dielectric layer 3 was cleaned sequentially using acetone and ethanol with low-power ultrasonication. The cleaned sample containing dielectric layer 3 was then dried with nitrogen gas to obtain a highly clean sample containing dielectric layer 3. A uniform superconducting material thin film was deposited on dielectric layer 3 using magnetron sputtering. During magnetron sputtering, under nitrogen and argon atmospheres, the grown superconducting material thin film was a 5-nanometer thick polycrystalline niobium nitride material with a superconducting transition temperature of 7.2 K and a sheet resistance of 325 Ω / □ at room temperature. This niobium nitride material exhibits stable properties and good performance at both room temperature and low temperatures.

[0047] Step 4: Spin-coat positive photoresist AZ1500 onto the surface of the superconducting material thin film; pattern the positive photoresist using an ultraviolet lithography machine; obtain a positive photoresist mask with electrode shapes through development; deposit an electrode layer on the positive photoresist mask; and prepare gold electrode 5 and gold electrode 6 using a lift-off process.

[0048] Step 5: Spin-coat a positive electron beam resist (PMMA4) onto the surface of the superconducting material thin film; pattern the positive electron beam resist using an electron beam lithography machine; through development and fixing (MIBK) processes, obtain a positive photoresist mask with the shape of a gold antenna 7; deposit a uniform gold layer on the positive photoresist mask using electron beam evaporation at a growth rate of 0.2 Å / s. Use a chemical solution, n-methylpyrrolidone, heated in a water bath to peel off the gold layer, obtaining the gold antenna 7.

[0049] Step 6: Spin-coat positive electron beam resist PMMA4 onto the surface of dielectric layer 3; pattern the positive electron beam resist using an electron beam exposure machine; obtain a positive photoresist mask with the shape of U-shaped superconducting nanowires 4 through development and fixing (MIBK) treatment; and obtain the U-shaped superconducting nanowires 4 by using reactive ion etching to remove excess superconducting material film.

[0050] The detector's U-shaped superconducting nanowire 4, made of niobium nitride, is set to have a period of 60 nm, a duty cycle of 0.5, and a nanowire width of 30 nm. The silicon nitride dielectric layer 3 has a thickness of 200 nm, the gold reflective layer 2 has a thickness of 50 nm, and the gold antenna 7 has a thickness of 40 nm, a length of 576 nm, and a width of 50 nm. The detector's absorptivity is simulated, and the simulation results are as follows: Figure 3 As shown, the nanowires exhibit an absorption rate of 98.1% at a wavelength of 4.06 micrometers. Furthermore, the light absorption efficiency remains above 50% within the wavelength range of 3.5 to 4.5 micrometers. Figure 4 As shown, at a wavelength of 4 micrometers, the absorption rate of the superconducting nanowire unit is 88.8%, ensuring that the superconducting nanowire unit is the main absorber. Figure 5 As shown, the trend of the relationship between the electric field intensity at point P and the wavelength is... Figure 3 The trend is consistent, indicating that increasing the electric field strength is beneficial to improving the absorption rate of the superconducting nanowire unit. For example... Figure 6 As shown, the field enhancement gain at point P is 4.4 × 10⁻⁶. 3 Gain greater than 2.5 × 10⁻⁶ in the 3.57 μm and 3.45 μm ~ 4.89 μm spectral bands. 3 .

Claims

1. A broadband detector in the mid-infrared band, characterized in that: The structure consists of a silicon wafer (1), a reflective layer (2), and a dielectric layer (3) from bottom to top. A U-shaped superconducting nanowire (4) is disposed on the surface of the dielectric layer (3). The open ends of the U-shaped superconducting nanowire (4) are connected to gold electrode one (5) and gold electrode two (6) respectively. Gold antennas (7) are symmetrically disposed on both sides of the U-shaped superconducting nanowire (4). The gold antennas (7) are arranged periodically. Gold electrode one (5) is connected to a constant voltage source (8), and gold electrode two (6) is grounded. The width of a single segment of the U-shaped superconducting nanowire (4) is 30~60 nanometers, and the thickness is 5~10 nanometers. The U-shaped superconducting nanowires (4) are made of niobium nitride superconducting material, with a spacing of 0.1~100 micrometers and a duty cycle of 0.001~0.

9. The dielectric layer (3) is silicon nitride with a thickness of 20~500 nanometers.

2. A broadband detector in the mid-infrared band according to claim 1, characterized in that: The reflective layer (2) is a gold reflective layer.

3. A broadband detector in the mid-infrared band according to claim 1, characterized in that: The thickness of the reflective layer (2) is 20~300 nanometers.

4. A broadband detector in the mid-infrared band according to claim 1, characterized in that: Both the gold electrode one (5) and the gold electrode two (6) are trapezoidal.

5. A broadband detector in the mid-infrared band according to claim 1, characterized in that: The thickness of the gold antenna (7) is 10~300 nanometers, the length is 10~1000 nanometers, and the width is 1~1000 nanometers.

6. A method for fabricating a broadband detector in the mid-infrared band according to any one of claims 1 to 5, characterized in that, Includes the following steps: Step 1: A reflective layer (2) is deposited on the cleaned silicon wafer (1) using an electron beam vapor deposition. Step 2: Prepare a dielectric layer (3) on the reflective layer (2) using electron beam evaporation or chemical vapor deposition. Step 3: A superconducting material thin film is prepared on the dielectric layer (3) by magnetron sputtering. Step 4: Spin-coat positive photoresist onto the surface of the superconducting material thin film, expose the positive photoresist in a patterned manner, and obtain a positive photoresist mask with electrode shapes through development treatment. Deposit on the positive photoresist mask and obtain gold electrode 1 (5) and gold electrode 2 (6) by stripping method. Step 5: Using electron beam lithography, U-shaped superconducting nanowires are fabricated at corresponding positions on the dielectric layer (4). Step 6: Gold antenna (7) is fabricated on dielectric layer (3) using electron beam lithography.

7. The method for fabricating a broadband detector in the mid-infrared band according to claim 6, characterized in that: In step one, the silicon wafer (1) is cleaned with low-power ultrasonic cleaning using acetone and ethanol respectively, and finally dried with nitrogen.

8. The method for fabricating a broadband detector in the mid-infrared band according to claim 6, characterized in that: In step six, a positive electron beam resist is spin-coated onto the surface of the superconducting material thin film. The positive electron beam resist is patterned and exposed using an electron beam exposure machine. After development and fixing, a positive photoresist mask with the shape of a gold antenna (7) is obtained. A uniform gold layer is deposited on the positive photoresist mask using an electron beam evaporation method. The gold layer is then peeled off using a chemical solution water bath heating method to obtain the gold antenna (7).

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