Nbn thin film with double buffer layer, superconducting tunnel junction and preparation method therefor and use thereof

Abstract

The present invention relates to an NbN thin film with a double buffer layer, a superconducting tunnel junction and a preparation method therefor and the use thereof. In the present invention, a double buffer layer of MgO and NbTiN is prepared on a Si substrate, and then an NbN thin film is obtained by means of preparation on the buffer layer; subsequently, a barrier layer and an NbN superconducting layer are sequentially prepared on the NbN thin film, a junction region of a superconducting tunnel junction is defined by means of a combination of ultraviolet lithography and reactive-ion etching, and an insulating isolation layer and a lead layer are prepared, thereby finally obtaining a high-quality NbN superconducting tunnel junction. Compared with the prior art, by means of arranging on a Si substrate a double buffer layer having gradually varied lattice parameters, the present invention alleviates the defect caused by a lattice mismatch between Si and NbN; thus, an NbN thin film with a high superconducting transition temperature and a superconducting tunnel junction with a high critical current density are prepared, and the astronomical detection signal collection frequency of an NbN frequency mixer is improved.

Description

A NbN film with double buffer layer, superconducting tunnel junction, and preparation method and application thereof Technical Field

[0001] The present invention relates to the field of astronomical detection technology, and in particular to a NbN film with a double buffer layer, a superconducting tunnel junction, and a preparation method and application thereof. Background Art

[0002] Niobium nitride (NbN) thin films have a high superconducting critical transition temperature (approximately 16K) and a large band gap (approximately 5.6 meV). They can replace traditional Nb-based superconducting tunnel junctions (SISs) in astronomical detection devices operating in the millimeter and submillimeter wavebands (0.1-3 THz). They also overcome the approximately 700 GHz band gap limit of Nb-based SIS mixers and enable applications in the terahertz (THz) frequency range. MgO is the most widely used substrate material for NbN thin film fabrication due to its low lattice constant mismatch with NbN. This is because the lattice mismatch between MgO and NbN films is between 4% and 6%, and the MgO substrate is thermally stable, making NbN films grown at higher temperatures immune to diffusion effects from substrate elements. However, the high dielectric constant of the MgO substrate leads to high losses in the THz band, making it unsuitable for the design of waveguide-type superconducting mixers. MgO is also highly hygroscopic and difficult to fabricate in micro- and nanostructured environments.

[0003] Compared with NbN devices prepared on MgO substrates, the advantages of NbN devices prepared on Si substrates are mainly reflected in the excellent dielectric properties of silicon and extremely low losses in the THz band. Si has a small thermal expansion coefficient and high thermal conductivity, which is suitable for superconducting low-temperature environments for astronomical detection. At the same time, the micro-nano processing technology applied to Si substrates is mature and low-cost, making it easy to integrate superconducting detection devices with industrial semiconductor devices. However, due to the large lattice mismatch between NbN and Si substrates (0.446nm vs. 0.543nm), it is difficult to grow high-quality NbN films on Si substrates, which will cause the superconducting critical transition temperature of NbN to drop, and the performance of the NbN superconducting mixer prepared with it will be seriously reduced. CN 103165811 A discloses a silicon-based niobium nitride thin film superconducting material and a method for making the same. While a single-layer TiN transition layer improves the superconducting performance of the NbN film to some extent, there is still a certain lattice mismatch compared to a two-layer NbN superconducting layer, and the performance also lags behind that of MgO-based niobium nitride films. Therefore, it is necessary to explore better transition layer materials for preparing Si-based NbN films to improve their superconducting performance.

[0004] Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a NbN film with a double buffer layer, a superconducting tunnel junction and its preparation method and application, so as to improve the performance of the NbN film and its superconducting tunnel junction and superconducting mixer prepared on a Si substrate.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A NbN film with a double buffer layer is prepared on a Si substrate and comprises a MgO buffer layer, a NbTiN buffer layer and a NbN superconducting layer which are sequentially deposited on the Si substrate.

[0008] Furthermore, the thickness of the MgO buffer layer is 2-20 nm, preferably 2-10 nm.

[0009] Furthermore, the thickness of the NbTiN buffer layer is 2-20 nm, preferably 2-10 nm.

[0010] In order to epitaxially grow the NbN thin film and improve the quality of the NbN thin film, the surface roughness of the MgO buffer layer and the NbTiN buffer layer is less than 1 nm, preferably less than 0.7 nm.

[0011] Furthermore, the thickness of the NbN superconducting layer is 10-300 nm, preferably 80-150 nm.

[0012] The present invention also provides a NbN superconducting tunnel junction, comprising a NbN film, a NbN superconducting layer, a barrier layer and an insulating isolation layer, wherein the barrier layer is sandwiched between the NbN film and the NbN superconducting layer.

[0013] Furthermore, the barrier layer includes one of AlN, MgO, Nb2O5, Al2O3 or amorphous silicon, preferably AlN, and has a thickness of 1-10 nm, preferably 2-5 nm.

[0014] Furthermore, the insulating isolation layer has a thickness of 100-500 nm and includes one of SiO2, AlOx, AlN or TiN.

[0015] The present invention also provides a method for preparing a NbN superconducting tunnel junction, comprising the following steps:

[0016] S1. Preparation of NbN thin film: depositing a MgO buffer layer on a Si substrate, and depositing a NbTiN buffer layer on the surface of the MgO buffer layer to form a double buffer layer; depositing a NbN superconducting layer on the double buffer layer to obtain a NbN thin film;

[0017] S2. Preparation of NbN superconducting tunnel junction: a barrier layer and a NbN superconducting layer are sequentially deposited on the NbN thin film obtained in step S1, followed by a first etching to obtain a superconducting tunnel junction structure; after a second etching, an insulating isolation layer is deposited on the side of the superconducting tunnel junction, and a lead layer is deposited on the top of the superconducting tunnel junction.

[0018] Furthermore, the crystal plane orientation of the Si substrate in step S1 is one of (100), (111), or (110), and the Si substrate is flat or curved. Preferably, before depositing the buffer layer, the surface of the Si substrate is cleaned using a standard RCA cleaning method to remove organic matter and stains on the surface.

[0019] Furthermore, in step S2, before etching, photoresist is coated by spin coating, scraping, spraying, etc., and the photoresist includes one of positive or negative photoresists such as AZ4620 and AZ1500, and is exposed using an ultraviolet photolithography machine in conjunction with a mask, and the developer includes one of ZX-238 or AZ300MFI.

[0020] Furthermore, reactive ion etching (RIE) is used to etch away the surface NbN and AlN layers not protected by the photoresist using CF4 and Ar, respectively, and then an insulating isolation layer is deposited.

[0021] Furthermore, the insulating isolation layer in step S2 includes one of SiO2, AlOx, AlN or TiN, and has a thickness of 100-500 nm, preferably 200-300 nm.

[0022] Furthermore, the lead layer includes one of conductive materials such as Nb, NbTiN, Al, Au, Cu or Ag, and has a thickness of 50-500 nm, preferably 200-400 nm.

[0023] Furthermore, the MgO buffer layer, NbTiN buffer layer, NbN superconducting layer, barrier layer and Nb lead layer described in steps S1 and S2 can be prepared by one of magnetron sputtering, ion beam sputtering, atomic layer deposition, organic vapor deposition, sol-gel or nanoparticle spin coating.

[0024] Furthermore, the present invention preferably uses a magnetron sputtering method with a power between 100-900 W, preferably 250-500 W, and a sputtering intensity of 1-5 mTorr.

[0025] The present invention also provides a NbN superconducting mixer, comprising the above-mentioned NbN superconducting tunnel junction.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) The present invention provides two buffer layers of different materials, MgO and NbTiN, on a Si substrate. Compared with a single-layer buffer layer, the lattice constant of the double-layer buffer layer changes step by step until it approaches the lattice constant of NbN. This can more effectively solve the problem of large lattice mismatch between Si and NbN, reduce the leakage current of the NbN superconducting tunnel junction, and improve the collection efficiency of astronomical detection signals of the NbN mixer.

[0028] (2) The NbTiN buffer layer material near NbN has a low resistivity above the superconducting transition temperature, and the dynamic inductance of the device prepared is small. Combined with the ingenious arrangement of the MgO buffer layer, the quality of the NbN film and the superconducting transition temperature are ultimately improved, thereby improving the quality of the NbN superconducting tunnel junction and the detection capability of the superconducting mixer in the millimeter and submillimeter wavebands (0.1 to 3 THz). It has good application prospects in the field of astronomical detection.

[0029] (3) The present invention limits the thickness and surface roughness of MgO and NbTiN, confirms the appropriate buffer layer thickness range, and helps to better achieve the preparation of high-quality NbN films and improve the quality and superconducting transition temperature of NbN films. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG1 is a schematic structural diagram of a NbN film with a double buffer layer and a superconducting tunnel junction according to the present invention.

[0031] FIG2 is an atomic force microscope image of the NbN thin film of Example 1.

[0032] Explanation of the marks in the figure: 1. Si substrate, 2. MgO buffer layer, 3. NbTiN buffer layer, 4. NbN superconducting layer, 5. Barrier layer, 6. Insulation isolation layer, 7. Lead layer. DETAILED DESCRIPTION

[0033] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0034] Example 1:

[0035] Example 1: A NbN film with a double buffer layer and a NbN superconducting tunnel junction are prepared on a Si substrate 1. The structural schematic diagram is shown in FIG1 . The operation steps are as follows:

[0036] S1. Preparation of NbN thin film

[0037] A Si(100) substrate was selected as the substrate. The surface of Si substrate 1 was first cleaned. The standard RCA cleaning method was used to clean the surface of Si substrate 1 to remove any organic matter and stains. The cleaning process was as follows: acetone, alcohol, and deionized water were used for 10 minutes each. The washed Si wafer was then placed in an 80°C RCA solution No. 1 water bath for 15 minutes, followed by a final 80°C RCA solution No. 2 water bath for 15 minutes.

[0038] This embodiment uses magnetron sputtering to deposit a 3nm thick layer of MgO onto the surface of the Si substrate 1 in an Ar gas environment at a sputtering power of 100W and a sputtering pressure of 2mTorr. Subsequently, a 3nm thick layer of NbTiN is deposited onto the surface of the MgO buffer layer 2 in a mixture of N2 and Ar at a sputtering power of 350W, a sputtering pressure of 2mTorr, and an Ar to N2 flow ratio of 4:1, forming a double buffer material layer. Subsequently, a 120nm thick NbN superconducting layer 4 is formed by magnetron sputtering of a Nb target in an N2 and Ar gas mixture (Ar:N2 = 10:1) at a sputtering power of 250W and a sputtering pressure of 2mTorr.

[0039] S2. Preparation of NbN superconducting tunnel junction

[0040] A 2nm thick AlN barrier layer 5 was deposited on the surface of the NbN film prepared in S1 by magnetron sputtering at a sputtering power of 300W and a volume ratio of 10:1 for an Ar / N2 mixture. A 120nm thick NbN superconducting layer 4 was then formed on the surface of the AlN barrier layer 5 by sputtering again at a sputtering power of 250W, a sputtering pressure of 2mTorr, and a sputtering atmosphere of a N2 / Ar mixture (Ar:N2=10:1), forming a NbN / AlN / NbN surface.

[0041] AZ4620 photoresist was spin-coated on the NbN / AlN / NbN surface and exposed using a UV photolithography machine with a mask. ZX-238 was used as the developer. Reactive ion etching (RIE) was then performed to obtain the bottom electrode pattern of the NbN / AlN / NbN three-layer structure. Photoresist was spin-coated again on the bottom electrode pattern of the NbN / AlN / NbN three-layer structure to define a 4μm 2 The tunnel junction area is exposed using a UV photolithography system with a mask. Reactive ion etching (RIE) is used to remove the surface NbN superconducting layer 4 and AlN barrier layer 5, which are not protected by photoresist, using carbon tetrafluoride (CF4) and Ar, respectively. SiO2 is then sputtered to form the side insulating isolation layer 6 at a sputtering power of 250W and a sputtering pressure of 2mTorr. Finally, the remaining photoresist is removed using the non-polar solvent acetone, and Nb is sputtered on top of the NbN / AlN / NbN to form the Nb lead layer 7 at a sputtering power of 300W and a sputtering pressure of 2mTorr.

[0042] Example 2:

[0043] In Example 2, a NbN thin film with a double buffer layer and an NbN superconducting tunnel junction were fabricated on a Si substrate 1. The difference from Example 1 is that in step S1, the thickness of both the MgO buffer layer 2 and the NbTiN buffer layer 2 were 10 nm. The other process steps were the same as in Example 1.

[0044] The present invention also provides the following comparative examples to verify the beneficial technical effects of the present invention.

[0045] Comparative Example 1:

[0046] Comparative Example 1 provides a NbN film and a NbN superconducting tunnel junction without the MgO buffer layer 2 and the NbTiN buffer layer 3 , that is, the NbN superconducting layer 4 is directly sputter-deposited on the Si substrate 1 , and the other processes are the same as those in Example 1.

[0047] Comparative Example 2:

[0048] Comparative Example 2 provides a single buffer layer NbN film without a NbTiN buffer layer 3 and a NbN superconducting tunnel junction, that is, after depositing a MgO buffer layer 2 on a Si substrate 1 , a NbN superconducting layer 4 is directly sputter-deposited. Other processes are the same as in Example 1.

[0049] Comparative Example 3:

[0050] Comparative Example 3: A NbN thin film with a double buffer layer and a NbN superconducting tunnel junction were fabricated on a Si substrate 1. The difference from Example 1 is that the thickness of the MgO buffer layer 2 and the NbTiN buffer layer 3 in step S1 is 30 nm. The other steps are the same as in Example 1.

[0051] The thickness of each film layer in the NbN film prepared by the present invention can be measured by using a probe-type surface step profiler to measure the average thickness, and the surface roughness can be measured using an atomic force microscope (AFM). The film thickness of the above-mentioned Examples 1-2 and Comparative Examples 1-3 was measured by a scanning step profiler (Bruker Dektak XT, Germany). The atomic force microscope (AFM) was used to measure the area of ​​4 μm 2 The average surface roughness of the NbN film is shown in Figure 2. The superconducting transition temperature of the NbN film and the critical current density of the NbN superconducting tunnel junction are measured by a comprehensive physical property measurement instrument (PPMS).

[0052] Table 1 summarizes the superconducting transition temperatures and critical current densities of NbN films and NbN / Al / NbN superconducting tunnel junctions with and without double buffer layers, and with varying buffer layer thicknesses, for Examples 1-2 and Comparative Examples 1-3. It can be seen that Examples 1-2 significantly increase the superconducting transition temperature of the NbN films and the critical current density of the tunnel junctions compared to Comparative Examples 1-3.

[0053] Table 1

[0054] As shown in Table 1, the present invention can prepare a buffer layer structure with precise thickness and small surface roughness. Compared with the comparative example 1 without double buffer layer, the superconducting transition temperature of the NbN film prepared in Examples 1-2 is increased by 2.5-2.9K, and the critical current density of the NbN superconducting tunnel junction is increased from 0.8kA / m 2 Increased to 2.3-2.5kA / m 2 , the superconducting performance has been greatly improved. Compared with the comparative example 2 having a single buffer layer of MgO, the superconducting transition temperature of the NbN film prepared in Example 1-2 and the critical current density of the NbN superconducting tunnel junction also have a significant improvement, which proves that by setting a double-layer buffer layer of MgO and NbTiN with a step-by-step change in lattice constant, the lattice mismatch between Si and NbN can be more effectively reduced, and the superconducting performance can be significantly improved. Comparative Example 3 prepared a double buffer layer structure with a thicker thickness, and as the thickness of the buffer layer increases, large grains and defects will appear on the surface of the buffer layer, which will reduce the current carrying capacity of the NbN film and the critical current density of the NbN superconducting tunnel junction. Therefore, the thickness of the MgO and NbTiN buffer layers in the present invention are respectively controlled at 2-20nm, preferably 2-10nm, so that a NbN superconducting tunnel junction with a better superconducting effect can be obtained.

[0055] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention, without departing from the scope of the present invention, should be within the scope of protection of the present invention.

Claims

1. A NbN film with a double buffer layer, prepared on a Si substrate, characterized in that: The invention comprises a MgO buffer layer, a NbTiN buffer layer and a NbN superconducting layer which are sequentially deposited on a Si substrate.

2. The NbN thin film with double buffer layers according to claim 1, characterized in that: The thickness of the MgO buffer layer is 2-20nm, and the surface roughness is below 1nm; the thickness of the NbTiN buffer layer is 1-20nm, and the surface roughness is below 1nm; the thickness of the NbN superconducting layer is 10-300nm.

3. A NbN superconducting tunnel junction, characterized in that: It comprises the NbN film according to any one of claims 1 to 2, a NbN superconducting layer, a barrier layer and an insulating isolation layer; the barrier layer is sandwiched between the NbN film and the NbN superconducting layer.

4. The NbN superconducting tunnel junction according to claim 4, characterized in that: The barrier layer includes one of AlN, MgO, Nb2O5, Al2O3 or amorphous silicon, and has a thickness of 1-10nm; the insulating isolation layer has a thickness of 100-500nm and includes one of SiO2, AlOx, AlN or TiN.

5. A method for preparing a NbN superconducting tunnel junction as claimed in claim 3, characterized in that: The following steps are involved: S1. Preparation of NbN thin film: depositing a MgO buffer layer on a Si substrate, and depositing a NbTiN buffer layer on the surface of the MgO buffer layer to form a double buffer layer; depositing a NbN superconducting layer on the double buffer layer to obtain a NbN thin film; S2. Preparation of NbN superconducting tunnel junction: a barrier layer and a NbN superconducting layer are sequentially deposited on the NbN film obtained in step S1, and then a superconducting tunnel junction structure is obtained by etching once, and an insulating isolation layer is deposited on the side of the superconducting tunnel junction after etching twice, and a lead layer is deposited on the top of the superconducting tunnel junction after removing the glue.

6. The method for preparing a NbN superconducting tunnel junction according to claim 5, characterized in that: The crystal plane orientation of the Si substrate in step S1 is one of (100), (111) or (110), and the Si substrate is a flat surface or a curved surface.

7. The method for preparing a NbN superconducting tunnel junction according to claim 5, characterized in that: In step S2, a photoresist is coated before etching, including a positive photoresist or a reverse photoresist, and etching is performed using a reactive ion etching method.

8. The method for preparing a NbN superconducting tunnel junction according to claim 5, characterized in that: The lead layer described in step S2 has a thickness of 50-500 nm and includes one of Nb, NbTiN, Al, Au, Cu or Ag conductive materials.

9. The method for preparing a NbN superconducting tunnel junction according to claim 5, characterized in that: The MgO buffer layer, NbTiN buffer layer, NbN superconducting layer, barrier layer and Nb lead layer described in steps S1 and S2 are prepared by one of magnetron sputtering, ion beam sputtering, atomic layer deposition, organic vapor deposition, sol-gel or nanoparticle spin coating.

10. Use of the NbN superconducting tunnel junction according to claim 3 in a NbN superconducting mixer.

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