CN117070892A - Two-step control N 2 Method for reducing internal stress of NbN film and improving superconducting transition temperature by partial pressure - Google Patents
Two-step control N 2 Method for reducing internal stress of NbN film and improving superconducting transition temperature by partial pressure Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
- C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
- C23C14/0641—Nitrides
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
- C23C14/02—Pretreatment of the material to be coated
- C23C14/021—Cleaning or etching treatments
- C23C14/022—Cleaning or etching treatments by means of bombardment with energetic particles or radiation
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
- C23C14/22—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the process of coating
- C23C14/34—Sputtering
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
- C23C14/22—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the process of coating
- C23C14/54—Controlling or regulating the coating process
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B12/00—Superconductive or hyperconductive conductors, cables, or transmission lines
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/60—Superconducting electric elements or equipment; Power systems integrating superconducting elements or equipment
Abstract
The invention discloses a two-step control N 2 The method for reducing the internal stress of the NbN film and improving the superconducting transition temperature by partial pressure comprises the following steps: first use N 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 1-112 s, and the thickness of the NbN buffer film layer is 0.1-5 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 150-450 s, and the thickness of the NbN main film layer is 10-30 nm; finally obtaining NbN superconduction with low internal stress and high superconduction transition temperatureA film. The method controls N through two steps 2 The partial pressure realizes the growth of two thin film layers on the substrate, the preparation process is simple, the improvement effect is good, and the large-scale production can be realized.
Description
Technical Field
The invention belongs to the technical field of superconducting niobium nitride films, and particularly relates to a two-step control method for N 2 Partial pressure is used in reducing internal stress of NbN film and raising superconductive transition temperature.
Background
The superconducting technology is one of the most active and important leading-edge research fields in the contemporary condensed state physics and material science, and has wide application in the aspects of power transmission, electric energy storage, weak magnetic detection, photoelectric high-sensitivity detection, medical magnetic field, high-performance filter, quantum computing application and the like along with the development of refrigeration technology, material technology, electronic technology and the like.
Among many superconducting thin film materials, a Niobium Nitride (NbN) superconducting thin film has relatively high superconducting transition temperature (Tc-17.3K), and the working temperature of a superconducting device based on the NbN superconducting thin film can be realized in a 4.2K liquid helium cryocooler with lower cost, and the superconducting thin film has wide application in low-temperature superconducting devices such as a superconducting hot electron mixer, a superconducting nanowire single photon detector, a superconducting dynamic inductance detector and the like.
The stress problem of NbN superconducting thin films is directly related to yield, stability and reliability of superconducting electronics. In recent years, the action of material stress has become an important field of international physical research on device reliability, and reports on device failure caused by stress are also available in China. Internal stress has direct influence on the quality, crystal structure and superconducting performance of the film, and excessive internal stress can cause the film and the substrate to be broken into pieces, so that the film and the substrate cannot be applied. For phonon-cooled thermionic mixers, the superconductive film must be as thin as possible, so that the film can cool down the thermions generated during the mixing process rapidly and effectively. Although NbN superconducting films generally need to have higher superconducting transition temperatures, the process of preparing thinner films may result in lower superconducting transition temperatures, such that the resistivity of the films increases as the thickness is reduced. Considering that a high superconducting transition temperature is advantageous for improving the performance of the thermionic mixer, it is necessary to try to raise the superconducting transition temperature of the thin film while making the thin film as thin as possible.
The current method for reducing the internal stress of the NbN superconducting film mainly comprises the following steps: epitaxial growth of the film is realized by adopting a monocrystalline substrate with smaller lattice mismatch with the film; a transition buffer layer is inserted between the substrate and the film to reduce lattice distortion; the multilayer film is adopted to offset tensile stress and compressive stress alternately. The current method for improving the superconducting transition temperature of the NbN superconducting film mainly comprises the following steps: the method of heating the substrate in the preparation of the film is usually above 500 ℃; a buffer layer is prepared on the substrate and then a film is prepared. The substrate heating method can improve the superconducting transition temperature of the film, but on one hand, the high deposition temperature limits the preparation process of the superconducting detector, and cannot be compatible with subsequent devices such as lift-off process; on the other hand, the deposition temperature is high, the crystal nucleus grows fast, the compactness of the film is affected, and in the process of cooling from high temperature to room temperature after film coating, the thermal stress is additionally introduced due to the difference of the thermal expansion coefficients between the film and the substrate. The method of the buffer layer generally requires that the lattice constant of the buffer layer is relatively close to that of an NbN superconducting film, and common buffer layers comprise magnesium oxide, silicon carbide, aluminum nitride, titanium nitride and the like, but the method of adopting the buffer layer generally relates to various deposition methods, and the preparation process of the buffer layer also requires system optimization, and has complex process and low efficiency. Therefore, the regulation and optimization of internal stress and superconducting transition temperature of the ultrathin NbN superconducting film grown at room temperature are realized on the substrate by improving the film growth condition, and the problem to be solved in the current NbN superconducting film preparation technology is urgent.
Disclosure of Invention
In view of the above, an object of the present invention is to provide a two-step control N 2 Method for reducing internal stress of NbN film and improving superconducting transition temperature by adjusting N 2 The mass flow ratio of Ar gas is used for growing film layers with different thicknesses, and the simple preparation of NbN superconducting films with low stress and high superconducting transition temperature is realized under the condition of ensuring the thinner films.
In order to achieve the above purpose, the technical scheme provided by the invention is as follows:
the embodiment of the invention provides a two-step control N 2 The method for reducing the internal stress of the NbN film and improving the superconducting transition temperature by partial pressure comprises the following steps:
first use N 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 1-112 s, and the thickness of the NbN buffer film layer is 0.1-5 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 150-450 s, and the thickness of the NbN main film layer is 10-30 nm; finally, the NbN superconducting film with the internal stress range of-650 to-350 MPa and the superconducting transition temperature of 6 to 10K is obtained.
Preferably, N is used first 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 60%, wherein the sputtering time is 1-112 s, and the thickness of the NbN buffer film layer is 0.1-5 nm; reuse of N 2 Depositing an NbN main film layer with the mass flow ratio of Ar gas of 20%, wherein the sputtering time is 150-450 s, and the thickness of the NbN main film layer is 10-30 nm; finally, the NbN superconducting film with the internal stress range of-650 to-350 MPa and the superconducting transition temperature of 6 to 10K is obtained.
Another embodiment of the present invention provides a pair ofStep control N 2 The method for reducing the internal stress of the NbN film and improving the superconducting transition temperature by partial pressure comprises the following steps:
first use N 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 1-112 s, and the thickness of the NbN buffer film layer is 0.1-5 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 375-450 s, and the thickness of the NbN main film layer is 25-30 nm; finally, the NbN superconducting film with the internal stress range of-500 to-350 MPa and the superconducting transition temperature of 7 to 10K is obtained.
Preferably, N is used first 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 60%, wherein the sputtering time is 1-112 s, and the thickness of the NbN buffer film layer is 0.1-5 nm; reuse of N 2 Depositing an NbN main film layer with the mass flow ratio of Ar gas of 20%, wherein the sputtering time is 375-450 s, and the thickness of the NbN main film layer is 25-30 nm; finally, the NbN superconducting film with the internal stress range of-500 to-350 MPa and the superconducting transition temperature of 7 to 10K is obtained.
Another embodiment of the present invention provides a two-step control N 2 The method for reducing the internal stress of the NbN film and improving the superconducting transition temperature by partial pressure comprises the following steps:
first use N 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 1-45 s, and the thickness of the NbN buffer film layer is 0.1-2 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 300-450 s, and the thickness of the NbN main film layer is 20-30 nm; finally, the NbN superconducting film with the internal stress range of-550 to-400 MPa and the superconducting transition temperature of 6-9K is obtained.
Preferably, N is used first 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 60%, wherein the sputtering time is 1-45 s, and the thickness of the NbN buffer film layer is 0.1-2 nm; reuse of N 2 Depositing an NbN main film layer with the mass flow ratio of Ar gas of 20%, wherein the sputtering time is 300-450 s, and the thickness of the NbN main film layer is 20-30 nm; finally, the NbN superconducting film with the internal stress range of-550 to-400 MPa and the superconducting transition temperature of 6-9K is obtained.
Another embodiment of the present invention provides a two-step control N 2 The method for reducing the internal stress of the NbN film and improving the superconducting transition temperature by partial pressure comprises the following steps:
first use N 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 45-112 s, and the thickness of the NbN buffer film layer is 2-5 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 150-450 s, and the thickness of the NbN main film layer is 10-30 nm; finally, the NbN superconducting film with the internal stress range of-650 to-350 MPa and the superconducting transition temperature of 6 to 10K is obtained.
Preferably, N is used first 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 60%, wherein the sputtering time is 45-112 s, and the thickness of the NbN buffer film layer is 2-5 nm; reuse of N 2 Depositing an NbN main film layer with the mass flow ratio of Ar gas of 20%, wherein the sputtering time is 150-450 s, and the thickness of the NbN main film layer is 10-30 nm; finally, the NbN superconducting film with the internal stress range of-650 to-350 MPa and the superconducting transition temperature of 6 to 10K is obtained.
Another embodiment of the present invention provides a two-step control N 2 The method for reducing the internal stress of the NbN film and improving the superconducting transition temperature by partial pressure comprises the following steps:
first use N 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 45-112 s, and the thickness of the NbN buffer film layer is 2-5 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 375-450 s, and the thickness of the NbN main film layer is 25-30 nm; finally, the NbN superconducting film with the internal stress range of-500 to-350 MPa and the superconducting transition temperature of 7 to 10K is obtained.
Preferably, N is used first 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 60%, wherein the sputtering time is 45-112 s, and the thickness of the NbN buffer film layer is 2-5 nm; reuse of N 2 Depositing an NbN main film layer with the mass flow ratio of Ar gas of 20%, wherein the sputtering time is 375-450 s, and the thickness of the NbN main film layer is 25-30 nm; finally obtaining the internal stress with the range of-500 to-350 MPa,NbN superconductive film with superconductive transition temperature of 7-10K.
Preferably, when depositing the NbN buffer film layer and the NbN main film layer, a metal Nb target material with the purity of 99.99 percent and a Si-based substrate are adopted, the temperature of the fixed substrate is room temperature, the sputtering power is 100-300W, and the deposition air pressure is 1.0-5.0 mTorr.
Preferably, before depositing the NbN buffer film layer, the metal Nb target and the Si-based substrate are put into a coating cavity of a high-vacuum magnetron sputtering system and vacuumized to ultra-high vacuum until the background vacuum degree of the coating cavity is reached<5.0×10 -8 Torr。
Preferably, the Si-based substrate is ion cleaned before being used in the film plating cavity, the ion beam for ion cleaning is argon ion beam, and the ion cleaning vacuum environment<5.0×10 -8 The Torr, argon flow is 20-100 sccm, ion source power is 30-100W, working air pressure is 1.0-10.0 mTorr, and ion cleaning time is 60-300 s.
Preferably, the NbN buffer film layer is pre-sputtered before being deposited, N is pre-sputtered 2 The mass flow ratio of Ar gas is 5-50%, the pre-sputtering power is 50-800W, the deposition air pressure is 1.0-10.0 mTorr, and the pre-sputtering time is 60-300 s.
The embodiment of the invention also provides an NbN superconducting film, which controls N through the two steps 2 The partial pressure is used for reducing the internal stress of the NbN film and improving the superconducting transition temperature.
Compared with the prior art, the invention has the beneficial effects that at least the following steps are included:
(1) The method of the invention firstly uses high N on the substrate 2 Growing a thinner NbN buffer film layer under the condition of Ar gas mass flow ratio, and having high N 2 The film deposition rate is lower under the condition of Ar flow ratio, the film roughness of the obtained buffer film layer is very low, the lattice distortion internal stress is released in advance, and atoms between the buffer film layer and the substrate are mutually fused to form a high-reliability interface, so that the interface stress is reduced, and the adhesion of the NbN main film layer is enhanced.
(2) The method of the invention uses low N on NbN buffer film layer 2 Growth under the condition of Ar flow ratioThe NbN main film layer fills surface defects and pores after stress release under the action of the NbN buffer film layer to form a new phase nucleation center of the film so as to increase the nucleation rate of the film, and a compact film is rapidly formed under the optimal sputtering parameters, so that the prepared NbN superconducting film reduces the internal stress and simultaneously improves the superconducting transition temperature, and the method is simple and easy to implement and has high usability and popularization.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic view of a NbN superconducting thin film structure according to an embodiment of the present invention;
FIG. 2 is a diagram of N provided by an embodiment of the present invention 2 NbN buffer film layer thickness and N grown on Si substrate with Ar flow ratio of 60% 2 An influence change curve of the thickness of the NbN main film layer grown when the Ar flow ratio is 20% on the internal stress of the NbN film;
FIG. 3 is a graph showing the variation of the normalized resistance of NbN thin films with different thicknesses and temperatures, which is provided by the embodiment of the invention, for directly growing NbN main thin film layers on a Si substrate;
FIG. 4 is a graph showing the variation of the normalized resistance of NbN thin films with different thicknesses of NbN main thin film layers grown on a 2nm buffer thin film layer according to an embodiment of the present invention;
FIG. 5 is a graph showing the variation of the normalized resistance of NbN thin films with different thicknesses of NbN main thin film layers grown on a 5nm buffer thin film layer according to an embodiment of the present invention;
FIG. 6 is a diagram of N provided by an embodiment of the present invention 2 NbN buffer film layer thickness and N grown on Si substrate with Ar flow ratio of 60% 2 And the effect change curve of the thickness of the NbN main film layer grown at the Ar flow ratio of 20% on the superconducting transition temperature of the NbN film.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the detailed description is presented by way of example only and is not intended to limit the scope of the invention.
The invention is characterized in that: in order to obtain a thinner film and simultaneously solve the problems of large internal stress and reduced superconducting performance caused by large lattice mismatch degree of a substrate and an NbN superconducting film, the embodiment of the invention provides a two-step control method for N 2 Method for reducing internal stress of NbN film and raising superconducting transition temperature by partial pressure 2 Ar flow ratio depositing ultrathin NbN buffer film layer, releasing internal stress caused by lattice distortion in advance and providing nucleation center for new phase of film, and subsequently utilizing low N 2 The Ar flow ratio deposits the adhesive force of NbN main film layer, forms a compact film structure rapidly, reduces the internal stress of NbN superconducting film and improves the superconducting transition temperature.
Embodiments provide a two-step control N 2 The method for reducing the internal stress of the NbN film and improving the superconducting transition temperature by partial pressure comprises the following steps:
step 1, preparing and processing a target material:
preparing a metal Nb target with purity of 99.99%, loading the metal Nb target into a coating cavity of a high-vacuum magnetron sputtering system, vacuumizing to ultrahigh vacuum, and when the vacuum degree is insufficient, possibly influencing the movement of plasma to reduce the controllability and repeatability of the film deposition process, thereby waiting for the background vacuum degree of the coating cavity<5.0×10 -8 Torr。
Step 2, selecting and processing a substrate:
the substrate is a high-resistance Si substrate, the Si substrate is compatible with a mature semiconductor process, and a film with low stress and high superconducting transition temperature is grown on the Si substrate, so that the preparation and application of materials in the superconducting detector are facilitated.
Regarding the treatment of Si substrate, placing the Si substrate into a sample transferring cavity of a magnetron sputtering device, vacuumizing, and waiting for the vacuum degree<5.0×10 -6 When Torr, the substrate is placed in a film plating cavity of a magnetron sputtering device, and before pre-sputtering, the substrate is further required to be subjected to ion cleaning to remove impurity ions on the surface of the substrate, wherein the ion beam for ion cleaning can be an argon ion beam, and the ion cleaning vacuum environment is adopted<5.0×10 -8 The Torr, argon flow is 20-100 sccm, ion source power is 30-100W, working air pressure is 1.0-10.0 mTorr, and ion cleaning time is controlled at 60-300 s.
Step 3, pre-sputtering an NbN film:
the main reasons of the pre-sputtering are that the target is easy to attach impurities when not used for a long time, and the surfaces of a plurality of targets are easy to oxidize after contacting air, if the sputtering is directly carried out, the components of the film are easy to be impure, the quality is poor, and the purity of the sputtering of the target can be ensured only by a certain pre-sputtering time.
The reactive gas N is firstly arranged during the pre-sputtering 2 And the gas mass flow ratio of the working gas Ar is 5-50%, then the power supply of the power source is turned on to set the sputtering power to be 50-800W, the working pressure of the chamber is regulated to be 1.0-10.0 mTorr, the power source is turned on to start, a layer of glow is seen on the surface of the target material from the observation window after the successful starting, at the moment, the working pressure is reduced, the pre-sputtering is carried out, and the pre-sputtering time is 60-300 s.
Step 4, high N 2 Ar flow ratio depositing NbN buffer film layer:
after the pre-sputtering is finished, all oxide layer impurities on the surface of the target are sputtered, so that the purity of the surface of the target is kept. Fixing the Si substrate to room temperature, and at this time, re-inspecting and adjusting the reaction gas N 2 And the gas mass flow ratio of the working gas Ar is 40-65%, the sputtering power is 100-300W, the deposition air pressure is 1.0-5.0 mTorr, the sputtering time is set to be 1-112 s according to the expected sputtering rate, a baffle below a target is opened, and the NbN buffer film layer sputtering is carried out, wherein the thickness of the obtained NbN buffer film layer is 0.1-5 nm.
Step 5, low N 2 Ar flow ratio deposition of NbN main film layer:
after the NbN buffer film layer is deposited, the temperature of the Si substrate is fixed at room temperature, the sputtering power is 100-300W, and the deposition air pressure is 1.0-5.0 mTorr, at this time, recheck and adjust the reaction gas N 2 And the gas mass flow ratio of the working gas Ar is 15-35%, the sputtering time is set to be 150-1500 s according to the expected sputtering rate, a baffle above the target is opened, and the NbN main film layer is sputtered, wherein the thickness of the obtained NbN main film layer is 10-100 nm.
Step 6, sampling:
after the set sputtering time is reached, the sputtering is ended. When the instrument timing is 0, the power source can be automatically closed, then the baffle plate is closed, the gate valve is closed, the substrate is conveyed into the sample conveying cavity, the air inlet valve of the sample conveying cavity is opened for ventilation until the atmospheric pressure in the sample conveying cavity is recovered, and the cavity door is opened and the sample is taken out. The structure of the NbN superconducting thin film is shown in figure 1.
The NbN superconducting films of the specific following examples and comparative examples were prepared based on the above steps 1 to 6, as shown in tables 1, 2 and 3:
TABLE 1
TABLE 2
TABLE 3 Table 3
Fig. 2 shows the internal stress curves of the NbN films prepared in examples 1 to 8 and comparative examples 1 to 11, and the internal stress of the NbN film prepared in the presence of the NbN buffer film layer is significantly smaller than that of the film deposited directly on the substrate, and the internal stress reduction is more remarkable particularly in the thinner film. For example, in the case where the thickness of the NbN main thin film layer is 10nm, the internal stress of the NbN thin film directly deposited on the Si substrate is-1263.5 MPa without depositing the NbN buffer thin film layer, whereas the internal stress of the NbN thin film obtained when the thickness of the NbN buffer thin film layer is 5nm is-603.3 MPa, which is reduced by about 50%. This is mainly because, when N 2 Ar flow is relatively large>40%) is slower (2.7 nm/min), and the surface roughness of the formed NbN buffer film layer is very low (Ra is 0.11 nm), meanwhile, the ultrathin NbN buffer film layer releases the internal stress caused by lattice mismatch in advance, reduces interface stress, and in addition, the existence of the NbN buffer film layer can provide a seed layer with lattice perfect matching for the growth of the subsequent NbN main film layer and provide an excellent template for the nucleation of new phases of the film, thereby greatly reducing growth stress.
FIG. 3 is a graph showing the normalized resistance versus temperature of the NbN films prepared in comparative examples 1-7, and shows that the NbN films are not superconducting when the NbN main film layer thickness is 10nm and 15nm, the superconducting transition temperature is very low and is only 3.42K when the NbN main film layer thickness is 20nm, and the superconducting transition temperature is increased from 7.26K to 10.05K when the NbN main film layer thickness is increased from 25nm to 100nm.
FIG. 4 is a graph showing the normalized resistance versus temperature of the NbN films prepared in examples 1-3 and comparative examples 8-9, and shows that the NbN film exhibits no superconductivity when the NbN main film layer thickness is 10nm, but the superconducting transition temperature of the NbN film is 4.07K when the NbN main film layer thickness is 15nm, and the superconducting thickness window is increased compared to that of the NbN film directly on the Si substrate. As the thickness of the NbN main film layer increases from 20nm to 30nm, the superconducting transition temperature of the NbN film increases from 6.38K to 8.71K.
FIG. 5 is a graph showing the normalized resistance of the NbN films prepared in examples 4-8 and comparative examples 10-11 above as a function of temperature, wherein the superconducting transition temperature of the NbN film was greatly increased to 6.71K when the thickness of the NbN main film layer was 10 nm. As the NbN main film layer continues to increase, the superconducting transition temperature of the NbN film increases from 7.43K to 10.54K as it increases from 15nm to 100nm.
As can be seen from the comprehensive analysis of fig. 3, fig. 4 and fig. 5, the superconducting transition temperature of the NbN film prepared in the presence of the NbN buffer film layer is significantly higher than that of the film deposited directly on the substrate, and the superconducting performance of the thinner film is significantly improved.
FIG. 6 shows the variation of the superconducting transition temperature of the NbN films prepared in examples 1-8 and comparative examples 1-11, and shows that the superconducting transition temperature of the NbN film prepared in the presence of the NbN buffer film layer is significantly higher than that of the film directly deposited on the substrate, especially the superconducting performance of the thinner film is improved more significantly. For example, when the thickness of the NbN main film layer is 20nm, the superconducting transition temperature of the NbN film directly deposited on the Si substrate is 3.42K, and when the thickness of the NbN buffer film layer is 5nm, the superconducting transition temperature of the NbN film obtained is 8.48K, which is improved by about 2.5 times. This is mainly because, on the one hand, by a high N 2 The NbN buffer film layer under the control of the Ar flow ratio greatly reduces the internal stress of the NbN film; on the other hand, the ultra-thin NbN buffer film layer is completely matched with the growth lattice of the subsequent NbN main film layer, which is helpful for nucleation and crystallization of new NbN phases, thus greatly improving the superconducting transition temperature.
In conclusion, the method of the invention controls N through two steps 2 The partial pressure designs an NbN buffer film layer between the substrate and an NbN main film layer, reduces lattice mismatch, simultaneously, provides a lattice matched seed layer, reduces growth stress by releasing internal stress caused by lattice distortion in advance, reducing interface stress and forming a nucleation center of a new phase of the film, and ensures thatThe two films are combined optimally, the internal stress of the NbN film is reduced, the superconducting transition temperature is improved, and the performance of the ultrathin film is improved obviously. The method can be popularized to other substrates to improve the superconducting performance of the niobium nitride film, has simple preparation process and good improvement effect, and can realize mass production.
The foregoing detailed description of the preferred embodiments and advantages of the invention will be appreciated that the foregoing description is merely illustrative of the presently preferred embodiments of the invention, and that no changes, additions, substitutions and equivalents of those embodiments are intended to be included within the scope of the invention.
Claims (10)
1. Two-step control N 2 The method for reducing the internal stress of the NbN film and improving the superconducting transition temperature by partial pressure is characterized by comprising the following steps of:
first use N 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 1-112 s, and the thickness of the NbN buffer film layer is 0.1-5 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 150-450 s, and the thickness of the NbN main film layer is 10-30 nm; finally, the NbN superconducting film with the internal stress range of-650 to-350 MPa and the superconducting transition temperature of 6 to 10K is obtained.
2. Two-step control N 2 A method for reducing internal stress of NbN film and raising superconducting transition temp. is characterized by that firstly, N is used 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 1-112 s, and the thickness of the NbN buffer film layer is 0.1-5 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 375-450 s, and the thickness of the NbN main film layer is 25-30 nm; finally, the NbN superconducting film with the internal stress range of-500 to-350 MPa and the superconducting transition temperature of 7 to 10K is obtained.
3. Two-step control N 2 A method for reducing internal stress of NbN film and raising superconducting transition temp. is characterized by that firstly, N is used 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 1-45 s, and the thickness of the NbN buffer film layer is 0.1-2 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 300-450 s, and the thickness of the NbN main film layer is 20-30 nm; finally, the NbN superconducting film with the internal stress range of-550 to-400 MPa and the superconducting transition temperature of 6-9K is obtained.
4. Two-step control N 2 A method for reducing internal stress of NbN film and raising superconducting transition temp. is characterized by that firstly, N is used 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 45-112 s, and the thickness of the NbN buffer film layer is 2-5 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 150-450 s, and the thickness of the NbN main film layer is 10-30 nm; finally, the NbN superconducting film with the internal stress range of-650 to-350 MPa and the superconducting transition temperature of 6 to 10K is obtained.
5. Two-step control N 2 A method for reducing internal stress of NbN film and raising superconducting transition temp. is characterized by that firstly, N is used 2 Depositing an NbN buffer film layer with the Ar gas mass flow ratio of 40-65%, wherein the sputtering time is 45-112 s, and the thickness of the NbN buffer film layer is 2-5 nm; reuse of N 2 Depositing an NbN main film layer with the Ar gas mass flow ratio of 15-35%, wherein the sputtering time is 375-450 s, and the thickness of the NbN main film layer is 25-30 nm; finally, the NbN superconducting film with the internal stress range of-500 to-350 MPa and the superconducting transition temperature of 7 to 10K is obtained.
6. Two-step control N according to any one of claims 1-5 2 A method for reducing internal stress of NbN film and raising superconductive transition temperature by partial pressure is characterized by that in the deposition of NbN buffer film layer and NbN main film layer, the metal Nb target material with 99.99% purity and Si base are usedThe substrate is fixed at room temperature, the sputtering power is 100-300W, and the deposition air pressure is 1.0-5.0 mTorr.
7. The two-step control N of claim 6 2 The method for reducing internal stress of NbN film and raising superconductive transition temperature by partial pressure is characterized by that before depositing NbN buffer film layer, the described metal Nb target material and Si base substrate are placed into the film-plating cavity of high-vacuum magnetron sputtering system, and vacuum-pumped to ultrahigh vacuum until the background vacuum degree of film-plating cavity is reached<5.0×10 -8 Torr。
8. The two-step control N of claim 7 2 The method for reducing internal stress of NbN film and raising superconductive transition temperature is characterized by that before the Si-base substrate is used in film-plating cavity, it needs to make ion cleaning, and the ion beam for ion cleaning is argon ion beam, and the ion cleaning vacuum environment<5.0×10 -8 The Torr, argon flow is 20-100 sccm, ion source power is 30-100W, working air pressure is 1.0-10.0 mTorr, and ion cleaning time is 60-300 s.
9. Two-step control N according to any one of claims 1-5 2 A method for reducing internal stress of NbN film and raising superconductive transition temp. is characterized by that before depositing NbN buffer film layer, the pre-sputtering is carried out 2 The mass flow ratio of Ar gas is 5-50%, the pre-sputtering power is 50-800W, the deposition air pressure is 1.0-10.0 mTorr, and the pre-sputtering time is 60-300 s.
10. An NbN superconducting thin film, characterized in that it is produced by the method according to any one of claims 1 to 9.
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