Preparation method of superconducting nanowire single-photon detector capable of keeping high detection efficiency in liquid helium temperature region

Abstract

The invention discloses a preparation method of a superconducting nanowire single-photon detector capable of keeping high detection efficiency in a liquid helium temperature region. The preparation method comprises the following steps: adopting a double-layer substrate formed by silicon oxide and silicon; the method comprises the following steps: growing a niobium nitride film on a silicon oxide surface of a substrate through radio frequency magnetron sputtering to serve as a buffer layer, growing a niobium nitride film through direct current magnetron sputtering, and accurately measuring and controlling the thickness through XRR; the method comprises the following steps: preparing a double-line parallel nanowire structure in a manner of exposing PMMA A4 electron beam glue by electron beams, and transferring a nanowire pattern to a niobium nitride film in a reactive ion etching manner; the method comprises the following steps of: removing photoresist above a region needing to be etched on a silicon oxide surface of a substrate in a manner of exposing the AZ4620 photoresist through ultraviolet laser direct writing, and then etching silicon oxide and silicon from the silicon oxide surface of a device through reactive ion etching to prepare a suspension bridge structure. The high detection efficiency of the device is ensured while the working temperature of the device is greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of single-photon detection in the communication band, and in particular to a method for preparing a superconducting nanowire single-photon detector capable of maintaining high detection efficiency in a liquid helium temperature region. Background Art

[0002] With the development of science and technology and the deepening of research, traditional multi-photon detection can no longer meet people's needs. People hope to realize photon detection more effectively, so single-photon detection technology came into being. As the most advanced detection technology today, single-photon detection technology has reached the limit of photon detection sensitivity, and has been applied to many research fields, which have enabled these fields to achieve rapid development. For example, with the development of quantum technology, technologies such as optical quantum computing and quantum precision measurement have also developed rapidly. Quantum computing has attracted the attention of researchers all over the world due to its huge "quantum superiority", and the quantized characteristics of photons themselves also make it shine in quantum computing.

[0003] The superconducting nanowire single-photon detector is a relatively popular superconducting detector. It is a single-photon detector composed of one or more lines made of superconducting materials with a width of nanometers. The shapes of nanowires vary, and they can be designed into various shapes such as meandering lines, spirals or fractal curves according to actual application requirements. At present, SNSPD has achieved a detection efficiency of nearly 100% and extremely low dark counts in the communication band. In addition, SNSPD also has extremely low time jitter and a faster detection speed, which is suitable for applications such as laser radar that require high time resolution. The spectral detection range of SNSPD is also very wide, and saturated single-photon detection efficiency has been achieved in the band from X-rays to 29μm.

[0004] However, due to the low superconducting critical temperature of traditional superconducting films, efficient SNSPDs can only work at lower temperatures, usually below 2.5K. Therefore, these SSPD systems are usually used in conjunction with refrigerators. Due to the large size, weight and power consumption of the cooling system, efficient SSPDs are difficult to deploy on airborne or satellite platforms, affecting the further development of applications such as airborne single-photon lidar. If the operating temperature can be raised to 4.2K while still maintaining the high detection efficiency of the detector in the communication band, the device's demand for refrigerators will be greatly reduced, which will not only reduce the application cost, but is also expected to further promote the development of applications such as airborne single-photon lidar. Therefore, it is very necessary to develop superconducting nanowire single-photon detectors that can maintain high detection efficiency in the liquid helium temperature range. Summary of the invention

[0005] The object of the present invention is to provide a method for preparing a superconducting nanowire single photon detector (SNSPD) which can maintain high detection efficiency in the liquid helium temperature region.

[0006] The technical solution to achieve the purpose of the present invention is: a method for preparing a superconducting nanowire single-photon detector that maintains high detection efficiency in the liquid helium temperature region, the preparation comprising the following steps:

[0007] A niobium hexanitride pentanitride film was grown on the silicon oxide surface of the substrate by radio frequency magnetron sputtering as a buffer layer, and a niobium nitride film was grown by direct current magnetron sputtering, and the film thickness on the buffer layer was accurately calibrated and controlled by X-ray reflectivity;

[0008] The double-wire structure of nanowires was prepared by electron beam exposure of PMMA A4 electron beam glue, and the pattern of nanowires was transferred to niobium nitride film by reactive ion etching.

[0009] On the silicon oxide surface of the substrate, the photoresist above the area to be etched is removed by exposing AZ4620 photoresist by UV laser direct writing, and then the silicon oxide and silicon are etched away from the silicon oxide surface of the substrate by reactive ion etching to prepare a suspended bridge structure.

[0010] Furthermore, a hexanitrogen pentaniobium film is grown on the silicon oxide surface of the substrate by radio frequency magnetron sputtering as a buffer layer, and the specific conditions are:

[0011] Background vacuum: better than 2×10 -5 Pa;

[0012] Gas: Ar, N2;

[0013] Target material: Nb;

[0014] Sputtering gas pressure: 12mTorr;

[0015] N2:Ar flow ratio: 30sccm:10sccm;

[0016] Sputtering power: 400W constant power RF sputtering;

[0017] Deposition rate: 20nm / min.

[0018] Furthermore, the niobium nitride film is grown by direct current magnetron sputtering, and the specific conditions are:

[0019] Background vacuum: better than 2×10 -5 Pa;

[0020] Gas: Ar, N2;

[0021] Target material: Nb;

[0022] Sputtering gas pressure: 2mTorr;

[0023] N2:Ar flow ratio: 10sccm:90sccm;

[0024] Sputtering current: 1.85A constant current DC sputtering;

[0025] Deposition rate: 41 nm / min.

[0026] Furthermore, the double-wire structure of the nanowires was prepared by electron beam exposure of PMMA A4 electron beam glue, and the specific conditions were:

[0027] Photoresist type: PMMA A4;

[0028] Pre-coating: 600r / min, 6s;

[0029] Main coating: 4000r / min, 60s;

[0030] Pre-baking: 180℃, 4min;

[0031] Exposure dose: 688μA / cm 2 .

[0032] Furthermore, the pattern of the nanowires was transferred onto the niobium nitride film by reactive ion etching, and the specific conditions were as follows:

[0033] Etching material: NbN;

[0034] Reaction gas: CF4;

[0035] Flow rate: 30sccm;

[0036] Pressure: 2Pa;

[0037] Power: 50W;

[0038] Time: 70s.

[0039] Furthermore, the photoresist above the area to be etched is removed by exposing AZ4620 photoresist by UV laser direct writing on the silicon oxide surface of the substrate. The specific conditions are:

[0040] Photoresist: AZ4620;

[0041] Pre-coating: 600r / min, 6s;

[0042] Main coating: 5000r / min, 60s;

[0043] Pre-baking: 90℃, 30min;

[0044] Exposure dose: 450μJ / cm 2 ;

[0045] Development time: 120s.

[0046] Furthermore, silicon oxide and silicon are etched away from the silicon oxide surface of the substrate by reactive ion etching to prepare a suspended bridge structure, and the specific conditions are:

[0047] Etching material: SiO2;

[0048] Reaction gas 1: CF4;

[0049] Reaction gas 1 flow rate: 30 sccm;

[0050] Reaction gas 2: O2;

[0051] Reaction gas 2 flow rate: 10 sccm;

[0052] Pressure: 4Pa;

[0053] Power: 150W;

[0054] Duration: 90s.

[0055] Etching material: Si;

[0056] Reaction gas 1: SF6;

[0057] Reaction gas 1 flow rate: 40 sccm;

[0058] Pressure: 10Pa;

[0059] Power: 70W;

[0060] Time: 18min.

[0061] Furthermore, two meandering lines with a width of 60 nm were connected in parallel to increase the superfluidity and signal amplitude by two times.

[0062] A superconducting nanowire single-photon detector which maintains high detection efficiency in a liquid helium temperature region is obtained by implementing the superconducting nanowire single-photon detector preparation method.

[0063] Compared with the prior art, the present invention has the following significant advantages: 1) XRR technology is used to accurately measure and control the film thickness on the buffer layer, thereby increasing the operable temperature of the SNSPD detector, and on this basis, a suspended bridge structure is prepared to improve the detection sensitivity of the device; 2) the nanowire structure of the device is a double-wire structure, which improves the superconducting critical current of the device while also improving the signal-to-noise ratio of the device; 3) the prepared SNSPD having both a suspended bridge structure and a buffer layer ensures high quantum efficiency in the communication band while increasing the operating temperature to 4.2K, and the quantum efficiency of the device reaches saturation in the 1064nm band and weak saturation in the 1330nm band.

[0064] Description of the accompanying tables and graphs

[0065] Figure 1 Optical microscope and SEM images of the device.

[0066] Figure 2 It is the quantum efficiency test result of the device working at 4.2K for light of different wavelengths in the communication band. Quantum efficiency (IDE) is the most important parameter to measure the detection sensitivity of SNSPD. It refers to the probability of generating a voltage pulse after a photon is absorbed by the nanowire.

[0067] Figure 3 It is a flow chart of the device process preparation proposed by the present invention. DETAILED DESCRIPTION

[0068] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0069] The present invention uses X-ray reflectivity to accurately calibrate and control the thickness of the film on the buffer layer. Due to the high sensitivity requirements of the device, the thickness of the grown film is relatively thin, and the uniformity and stability of the magnetron growth are required to be high. The common method of measuring film thickness is to characterize the thickness of the film on the buffer layer by TEM, but it will damage the sample. The device can be non-destructively detected by the method of X-ray reflectivity. For the same batch of grown films, after the thickness is accurately calibrated by XRR, the device with a more suitable thickness is screened out, which can take into account the device operating temperature and detection sensitivity at the same time, and is used for subsequent device preparation. In addition, since the film is thin, the line width of the device with high detection sensitivity is narrow, so the output signal amplitude is low, resulting in a low signal-to-noise ratio. Therefore, the specially designed and prepared nanowire structure is a double-wire structure to increase the signal amplitude and improve the signal-to-noise ratio. Connecting two 60nm wide winding lines in parallel can increase the superfluidity to twice the original value, and the signal amplitude is also twice the original value. At the same time, the method of preparing the suspended bridge structure was improved. A double-layer substrate composed of silicon oxide and silicon was used, and reactive ion etching was used to etch from the silicon oxide surface of the substrate, which not only improved the device sensitivity but also reduced the probability of device damage.

[0070] A method for preparing a superconducting nanowire single photon detector (SNSPD) that works at liquid helium temperature and maintains high detection efficiency in a communication band, wherein the substrate used is a double-layer substrate composed of 270 nm thick silicon oxide and 360 μm thick silicon, and the preparation method comprises the following steps:

[0071] Step 1: Place the substrate with silicon oxide facing upward into the sub-chamber of the magnetron sputtering system for Ar + The purpose of milling is to remove molecular-level impurities on the surface of the substrate and make it easier for the film to combine with the substrate. The conditions are shown in Table 1.

[0072] Table 1 Ion milling conditions

[0073] Gas Type Gas flow Working air pressure Ion beam current Cleaning time Ar 10sccm <![CDATA[5.5×10 -2 Well]]> 20mA 10s

[0074] Step 2, the ion-milled substrate is sent into the main chamber, and the Nb5N6 film is grown by radio frequency magnetron sputtering. The sputtering parameters are shown in Table 2.

[0075] Table 2 RF sputtering growth conditions of Nb5N6 thin films

[0076]

[0077]

[0078] Step 3: After the film growth is completed, the niobium nitride film is in-situ magnetron sputtered in a vacuum chamber, and the sputtering parameters are shown in Table 3.

[0079] Table 3 DC sputtering growth conditions of NbN thin films

[0080] Background vacuum <![CDATA[Better than 2×10 -5 Pa]]> gas <![CDATA[Ar(99.999%),N2(99.999%)]]> Target Nb (purity 99.999%) Sputtering pressure 2mTorr N2:Ar flow ratio 10sccm:90sccm Sputtering current 1.85A constant current DC sputtering Deposition rate 41nm / min

[0081] Step 4, take out the sample from the chamber of the magnetron sputtering system, then spin-coat PMMA A4 electron beam resist on the surface of the sample, and then use a RAITH EBPG5200 exposure machine to perform electron beam writing on the electron beam resist to form a nanowire pattern on the electron beam resist. The lithography conditions are shown in Table 4.

[0082] Table 4 Electron beam lithography conditions

[0083]

[0084] Step 5: Next, the sample after the electron beam exposure is etched by reactive ion etching to form nanowires. The etching machine used is Samco RIE-10, the etching gas is CF4, and the specific etching parameters are shown in Table 5.

[0085] Table 5 Reactive ion etching conditions

[0086]

[0087] Step 6: AZ4620 photoresist is then spin-coated on the silicon oxide surface of the sample. Then, an ultraviolet direct writing lithography machine is used for overlay, and the exposure metering is 450 μA / cm 2 Then, the positive photoresist is placed in a developer for 120 seconds, and then the designed pattern of the area to be etched is formed on the photoresist. The photolithography conditions are shown in Table 6.

[0088] Table 6 UV lithography conditions

[0089]

[0090] Step 7: Next, the part not protected by the photoresist is etched by reactive ion etching to prepare a suspended bridge structure. The etching is carried out in two steps. The first step is to etch SiO2 using CF4 and O2, and the second step is to etch Si using SF6. The specific etching parameters are shown in Table 5.

[0091] In summary, the present invention can increase the working temperature of the device by adding a buffer layer, and the detection sensitivity of the device can be improved by adding a suspension structure. Combining the two control methods can achieve the high detection efficiency of the detector in the communication band while increasing the working temperature of the SNSPD to 4.2K. The quantum efficiency of the device reaches saturation in the 850nm and 1064nm bands, and reaches weak saturation in the 1330nm band.

[0092] Example

[0093] In order to verify the effectiveness of the scheme of the present invention, the following experiment was carried out.

[0094] (A) Sputtering

[0095] In the laboratory DE500 magnetron sputtering system, Nb5N6 thin films were grown on the silicon oxide surface of the substrate by radio frequency magnetron reactive sputtering. After ultrasonic cleaning with acetone, alcohol, and deionized water for 10 minutes each, the substrate was sent to the sub-chamber through the sample delivery chamber, decontaminated by ion milling in the sub-chamber, and then sent to the main chamber by the push rod. -5 When the ratio of N2 to Ar is 3:1, the gas flow meter is used to adjust N2:Ar=3:1. The gate valve of the main chamber is controlled to make the sputtering gas pressure 12mTorr. The RF power of the RF source is set to 400W. A 30nm Nb5N6 film is sputtered on the substrate. During the entire sputtering process, the chassis is cooled by circulating water (T~300K). Next, the NbN film is magnetron sputtered in situ. Ar and N2 with a ratio of 9:1 are filled into the sputtering chamber. A 2.8nm NbN film is sputtered on the Nb5N6 film at a constant current of 1.85A. After the film is sputtered, a certain amount of nitrogen (30sccm) is filled into the main chamber again to fully nitride it. Figure 3 As shown in the figure.

[0096] (B) Preparation of Nanowires

[0097] The nanowire pattern was written using the EBPG5200 high-performance electron beam nanolithography system from Raith, Germany. The electron beam acceleration voltage was 100 kV, the beam current was 0.2 nA, and the exposure dose was 688 μA / cm 2, the exposure dose is obtained by comparative test. After photolithography, it is developed in MIBK developer for 90s, fixed in isopropyl alcohol (IPA) for 60s, cleaned with deionized water, and finally blown dry with N2 air gun. Then the uncovered NbN film is etched away by reactive ion etching. The reactive ion etcher model Samco RIE-10 is used. CF4 gas is introduced. At a flow rate of 30sccm, a power of 50W, and a gas pressure of 2Pa, etching is performed for 70s. Figure 3 As shown in the middle figure, the sample was then immersed in a beaker containing N-methylpyrrolidone solvent, and then the beaker was placed in a constant temperature water bath at 80°C for about 1 hour to remove the glue.

[0098] (C) Preparation of suspended bridge structure

[0099] First, AZ4620 photoresist was spin-coated on the silicon oxide surface of the substrate at a speed of 5000RPM, and then exposed using a laser direct writing UV lithography machine with an exposure dose of 450μA / cm 2 After developing with positive photoresist developer for 120s, remove the photoresist above the area to be etched.

[0100] (D) Etching silicon oxide and silicon substrate

[0101] The sample's silicon oxide surface was etched using a reactive ion etcher, and the etching process was divided into two steps. The first step was to etch SiO2, with CF4 and O2 gases introduced at gas flow rates of 30 and 10 sccm, gas pressure of 4 Pa, and etching for 90 seconds at a power of 150 W. The second step was to etch Si, with SF6 gas introduced at a gas flow rate of 40 sccm, gas pressure of 10 Pa, and etching for 1080 seconds at a power of 70 W, to prepare a suspended bridge structure, such as Figure 3 As shown in the picture on the right.

[0102] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0103] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A method for preparing a superconducting nanowire single-photon detector that maintains high detection efficiency in the liquid helium temperature region, characterized in that: The preparation comprises the following steps: A niobium hexanitride pentanitride film was grown on the silicon oxide surface of the substrate by radio frequency magnetron sputtering as a buffer layer, and a niobium nitride film was grown by direct current magnetron sputtering, and the film thickness on the buffer layer was accurately calibrated and controlled by X-ray reflectivity; The double-wire structure of nanowires was prepared by electron beam exposure of PMMA A4 electron beam glue, and the pattern of nanowires was transferred to niobium nitride film by reactive ion etching. On the silicon oxide surface of the substrate, the photoresist above the area to be etched is removed by exposing AZ4620 photoresist by UV laser direct writing, and then the silicon oxide and silicon are etched away from the silicon oxide surface of the substrate by reactive ion etching to prepare a suspended bridge structure.

2. The method for preparing a superconducting nanowire single-photon detector according to claim 1, characterized in that: A niobium hexanitride pentaoxide film was grown as a buffer layer on the silicon oxide surface of the substrate by radio frequency magnetron sputtering. The specific conditions were: Background vacuum: better than 2×10 -5 Pa; Gas: Ar, N2; Target material: Nb; Sputtering gas pressure: 12mTorr; N2:Ar flow ratio: 30sccm:10sccm; Sputtering power: 400W constant power RF sputtering; Deposition rate: 20nm / min.

3. The method for preparing a superconducting nanowire single-photon detector according to claim 1, characterized in that: The niobium nitride film was grown by DC magnetron sputtering. The specific conditions were: Background vacuum: better than 2×10 -5 Pa; Gas: Ar, N2; Target material: Nb; Sputtering gas pressure: 2mTorr; N2:Ar flow ratio: 10sccm:90sccm; Sputtering current: 1.85A constant current DC sputtering; Deposition rate: 41 nm / min.

4. The method for preparing a superconducting nanowire single-photon detector according to claim 1, characterized in that: The double-wire structure of nanowires was prepared by electron beam exposure of PMMA A4 electron beam glue. The specific conditions were: Photoresist type: PMMA A4; Pre-coating: 600r / min, 6s; Main coating: 4000r / min, 60s; Pre-baking: 180℃, 4min; Exposure dose: 688μA / cm 2 .

5. The method for preparing a superconducting nanowire single-photon detector according to claim 1, characterized in that: The pattern of the nanowires was transferred onto the niobium nitride film by reactive ion etching, and the specific conditions were: Etching material: NbN; Reaction gas: CF4; Flow rate: 30sccm; Pressure: 2Pa; Power: 50W; Time: 70s.

6. The method for preparing a superconducting nanowire single-photon detector according to claim 1, characterized in that: On the silicon oxide surface of the substrate, AZ4620 photoresist is exposed by UV laser direct writing to remove the photoresist above the area to be etched. The specific conditions are: Photoresist: AZ4620; Pre-coating: 600r / min, 6s; Main coating: 5000r / min, 60s; Pre-baking: 90℃, 30min; Exposure dose: 450μJ / cm 2 ; Development time: 120s.

7. The method for preparing a superconducting nanowire single-photon detector according to claim 1, characterized in that: The silicon oxide and silicon are etched away from the silicon oxide surface of the substrate by reactive ion etching to prepare a suspended bridge structure. The specific conditions are: Etching material: SiO2; Reaction gas 1: CF4; Reaction gas 1 flow rate: 30 sccm; Reaction gas 2: O2; Reaction gas 2 flow rate: 10 sccm; Pressure: 4Pa; Power: 150W; Duration: 90s. Etching material: Si; Reaction gas 1: SF6; Reaction gas 1 flow rate: 40 sccm; Pressure: 10Pa; Power: 70W; Time: 18min.

8. The method for preparing a superconducting nanowire single-photon detector according to claim 1, characterized in that: Two 60nm wide meandering lines were connected in parallel to increase the superfluidity and signal amplitude by a factor of two.

9. A superconducting nanowire single-photon detector that maintains high detection efficiency in the liquid helium temperature region, characterized in that: The superconducting nanowire single-photon detector is obtained by implementing the method for preparing the superconducting nanowire single-photon detector according to any one of claims 1 to 8.

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