JP2011176000A - Method for preventing deterioration of characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the deterioration of characteristics of a superconductor to achieve ideal characteristics of the superconductor. <P>SOLUTION: An oxide film 13 is formed on the surface of a superconductor 12 using an atomic layer deposition method. A natural oxide film 15 is thereby removed, and a fine and uniform oxide film 13 is formed to prevent the deterioration of characteristics of the superconductor 12 in an electromagnetic field irradiation environment and to achieve ideal characteristics of the superconductor 12. The fine oxide film 13 prevents the aged deterioration of the superconductor 12 caused by oxygen in the atmosphere. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a technique for preventing deterioration of characteristics of a superconductor.

  Superconductors have a characteristic that electric resistance becomes zero and current can flow without loss below the critical temperature at which a superconducting state is reached, and the magnetic field cannot be allowed to enter the superconductor due to the Meissner effect. . With such a feature, it is possible to store a large current or to produce a filter having a very sharp cutoff characteristic.

G. Ciovati, P. Kneisel, and A. Gurevich, "Measurement of the high-field Q drop in a high-purity large-grain niobium cavity for different oxidantion processes", Physical Review Special Topics-Accelerators and Beams, 2007, Vol . 10, 062002 R. Barends, H. L. Hortensius, T. Zijlstra, J. J. A. Baselmans, S. J. C. Yates, J. R. Gao, and T. M. Klapwijk, "Contribution of dielectrics to frequency and noise of NbTin superconducting resonators", Applied Physics Letters, 2008, Vol. 92, 223 Martin M. Frank, Yves J. Chabal, and Glen D. Wilk, "Nucleation and interface formation mechanisms in atomic layer deposition of gate oxides", Applied Physics Letters, 2003, Vol. 82, p.4758-4760 M. Milojevic, R. Contreras-Guerrero, M. Lopez-Lopez, J. Kim, and RM Wallace, "Characterization of the` `clean-up '' of the oxidized Ge (100) surface by atomic layer deposition", Applied Physics Letters, 2009, Vol. 95, 212902 Martin M. Frank, Glen D. Wilk, Dmitri Starodub, Torgny Gustafsson, Eric Garfunkel, Yves J. Chabal, John Grazul, and David A. Muller, "HfO2and Al2O3gate dielectrics on GaAs grown by atomic layer deposition", Applied Physics Letters, 2005, Vol. 86, 152904 ML Huang, YC Chang, CH Chang, YJ Lee, P. Chang, J. Kwo, TB Wu and M. Hong, "Surface passivation of III-V compound semiconductors using atomic-layer-deposition-grown Al2O3", Applied Physics Letters , 2005, Vol. 87, 252104

  However, the above feature is a story in an ideal situation. In reality, as shown in FIG. 5, it is reported that a natural oxide film 15 is formed on the surface of the superconductor 12, and the ideal characteristics are not obtained due to the influence of the natural oxide film 15. For example, in a superconducting cavity resonator, a decrease in Q value due to a weak magnetic field has been reported. Further, it has been reported that a decrease in the Q value can be suppressed by making the surface oxide film dense (Non-patent Document 1). In a superconducting resonator, the situation when an oxide film is artificially deposited is examined, and it has been pointed out that the interface with the surface oxide film causes noise (Non-patent Document 2).

  The present invention has been made in view of the above, and an object of the present invention is to prevent deterioration of characteristics of a superconductor and realize ideal characteristics of the superconductor.

  The method for preventing property deterioration according to the present invention is a method for preventing property deterioration of a superconductor, characterized in that an oxide film is formed on the surface of the superconductor using an atomic layer deposition method.

  In the method for preventing deterioration of characteristics, the method includes a step of alternately supplying a precursor and an oxidant on a carrier gas in a reaction chamber storing the superconductor, after the step of supplying the precursor, and After the step of supplying the oxidizing agent, only the carrier gas is supplied into the reaction chamber.

  In the characteristic deterioration prevention method, the superconductor is any one of niobium, lead, titanium, gallium, and niobium nitride.

  In the characteristic deterioration prevention method, the oxide film is any one of alumina, hafnia, and hafnium aluminate.

  In the method for preventing deterioration of characteristics, the superconductor is niobium, and trimethylaluminum as the precursor and water as the oxidant are supplied into the reaction chamber at a substrate temperature of 150 ° C. to 250 ° C. It is characterized by.

  According to the present invention, it is possible to prevent deterioration of the characteristics of the superconductor and realize ideal characteristics of the superconductor.

It is sectional drawing which shows the example of the superconductor apparatus to which the characteristic degradation prevention method in this Embodiment is applied. It is a schematic diagram which shows the structure of the atomic layer deposition apparatus used for the characteristic degradation prevention method in this Embodiment. It is a graph for demonstrating the process of the characteristic degradation prevention method in this Embodiment. It is a top view which shows the structure of a superconducting resonator. It is sectional drawing which shows the example of the conventional superconductor device.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention eliminates the natural oxide film formed on the surface of the superconductor, and instead forms a protective film with a dense oxide film, thereby preventing the deterioration of the superconductor characteristics and the ideal characteristics of the superconductor. Is a method for preventing characteristic deterioration. As a method for removing the natural oxide film on the surface of the superconductor and forming an oxide film on the surface of the superconductor, atomic layer deposition (ALD) was used.

  FIG. 1 is a cross-sectional view showing an example of a superconductor device to which the characteristic deterioration prevention method according to the present embodiment is applied. A superconductor device 10 shown in FIG. 1 is obtained by forming a superconductor 12 on a substrate 11 and removing a natural oxide film 15 as shown in FIG. 5 by using an atomic layer deposition method. The oxide film 13 is grown on the surface of the superconductor 12. The thickness of the oxide film 13 formed by the atomic layer deposition method needs to be 3 nm to 10 nm, and about 5 nm is optimal.

As the superconductor 12, niobium (Nb), lead (Pb), titanium (Ti), gallium (Ga), and niobium nitride (NbN) can be considered. As the oxide film 13 formed by the atomic layer deposition method, alumina (Al ₂ O ₃ ), hafnia (HfO ₂ ), and hafnium aluminate (HfAlO _x ) can be considered. The alumina oxide film 13 is formed from trimethylaluminum (TMA) (atomic formula: (CH ₃ ) ₃ Al) and water (H ₂ O). The hafnia oxide film 13 is formed from tetrakisethylmethylaminohafnium (TEMAH) (atomic formula: Hf [N (CH ₃ ) (C ₂ H ₅ )] ₄ ) and water (H ₂ O). The hafnium aluminate oxide film can be grown by alternately supplying TMA and TEMAH.

  Next, the atomic layer deposition apparatus 100 used for the characteristic deterioration prevention method in the present embodiment will be described.

  This characteristic deterioration prevention method is executed using, for example, the atomic layer deposition apparatus 100 shown in FIG. In the atomic layer deposition apparatus 100 shown in the figure, pulsing valves 102 and 103, a reaction chamber 104, and a dry pump 105 are connected by a pipe 106. A sample (superconductor device 10) is placed in the reaction chamber 104. The carrier gas 101 is always introduced into the reaction chamber 104 via the pipe 106. The precursor 20 is introduced into the pipe 106 by opening the pulsing valve 102, and reaches the reaction chamber 104 by the carrier gas 101. The oxidant 30 is introduced into the pipe 106 by opening the pulsing valve 103, and reaches the reaction chamber 104 by the carrier gas 101. The pulsing valves 102 and 103 have a structure that can be opened and closed for a desired time, and the pulsing valves 102 and 103 do not open at the same time. The dry pump 105 exhausts the carrier gas 101, excess precursor 20, reaction products, and the like from the reaction chamber 104 to the outside.

  Next, the process of the characteristic deterioration prevention method in this embodiment will be described.

  In this characteristic deterioration prevention method, first, the precursor 20 made of a metal compound is placed on the carrier gas 101 and introduced into the reaction chamber 104 in a pulsed manner, and then only the carrier gas 101 is supplied to the reaction chamber 104. Thereafter, the oxidant 30 is placed on the carrier gas 101 and introduced into the reaction chamber 104 in a pulsed manner, and the precursor 20 introduced first and the oxidant 30 are reacted to form an oxide. The characteristic deterioration preventing method is characterized in that the natural oxide film 15 on the surface of the sample (superconductor 12) is removed, and the oxide film 13 is further increased at the atomic level by the self-limiting mechanism of the precursor 20 and the oxidant 30 itself. In addition, the oxide film 13 grows uniformly on all surfaces of the superconductor 12. Hereinafter, the process of this characteristic deterioration prevention method will be specifically described with reference to the drawings.

  FIG. 3 is a graph showing the relationship between the pressure in the reaction chamber 104 and time in the method for preventing characteristic deterioration. The vertical axis represents the pressure in the reaction chamber 104, and the horizontal axis represents time.

First, the sample is stored in the reaction chamber 104. The sample is, for example, a superconductor device 50 in which a natural oxide film 15 is formed as shown in FIG. The temperature in the reaction chamber 104 is kept at about 50 ° C. to 350 ° C., and the carrier gas 101 is introduced into the reaction chamber 104. Nitrogen (N ₂ ) or helium (He) is used for the carrier gas 101. The pressure _Pc of the carrier gas 101 is about 3 to 6 hPa.

It becomes a growth start time _{t 0,} opening the pulsing valve 102 of the precursor 20 by time _{t 1P.} The precursor 20 is transported into the reaction chamber 104 by the carrier gas 101 and is adsorbed on the sample surface. The pressure P ₁ of the precursor 20 is about 7 to 10 hPa.

Thereafter, the pulsing valve 102 is closed, and only the carrier gas 101 is supplied to the reaction chamber 104. The excess precursor 20 in the reaction chamber 104 and the reaction product generated during the adsorption of the precursor 20 are exhausted by the dry pump 105 during the precursor purge time t _1M .

Subsequently, the pulsing valve 103 of the oxidant 30 is opened for a time t _0P . The oxidant 30 is transported into the reaction chamber 104 by the carrier gas 101 and reacts with the precursor 20 adsorbed on the sample surface to form the oxide film 13 for one atomic layer. The pressure _{P 2} of the oxidant 30 may be set to be approximately 12hPa 7.

Then, the pulsing valve 103 is closed and only the carrier gas 101 is supplied to the reaction chamber 104. The excess oxidant 30 in the reaction chamber 104 and the reaction product produced by the reaction with the precursor 20 are exhausted by the dry pump 105 during the oxidant purge time t _0M .

Thereafter, the pulsing valve 102 of the precursor 20 is opened again, and the above process is repeated. As shown in FIG. 3, the atomic layer growth time t _cycle is t _cycle = t _1P + t _1M + t _0P + t _0M .

  The characteristic deterioration prevention method according to the present embodiment is described below for the removal of a natural oxide film, the protective film effect for preventing oxygen from entering from the formed oxide film, and the good interface state between the oxide film and the superconductor. There are three main effects.

  The removal of a natural oxide film by atomic layer deposition has already been reported for silicon (Si) substrates, germanium (Ge) substrates, and compound semiconductor (GaAs, InGaAs) substrates (Non-Patent Documents 3, 4, and 5). , And 6). In the atomic layer deposition method, the precursor (mainly organometallic compound) initially supplied to the substrate surface is very easily oxidized, so oxygen atoms on the substrate surface are absorbed on the precursor side, and the oxide film on the substrate surface is removed. It is. In the experiments by the inventors, it was found that the natural oxide film was similarly removed in the case of the superconductor.

  Further, it has been found that an oxide film formed by an atomic layer deposition method is a dense oxide film with very few defects. For this reason, oxygen is not adsorbed in the oxide film from the outside. That is, the superconductor 12 is not oxidized again. In addition, the oxide film 13 formed by this characteristic deterioration prevention method is very stable because it is formed under a thermal equilibrium condition. Supplying oxygen from the oxide film 13 itself to the superconductor 12 has a temperature of 400. Does not happen unless raised above ℃.

  Furthermore, since the oxide film 13 is formed by this characteristic deterioration prevention method after the natural oxide film 15 of the superconductor 12 is removed, the interface is very clear. This eliminates problems due to electromagnetic field distribution due to non-uniform interface.

  As an oxide film growth method, there are a chemical vapor deposition (CVD) method and a molecular beam epitaxy (MBE) method in addition to the atomic layer deposition method. In the chemical vapor deposition method, film formation is basically performed at a higher temperature than the atomic layer deposition method. For this reason, since the film itself is not in thermal equilibrium, the possibility of oxidizing the superconductor surface again is not necessarily zero. In addition, since chemical vapor deposition is not a growth method sensitive to the surface state, surface reaction cannot be expected so much. Since the molecular beam epitaxy method is very sensitive to the surface state, it is necessary to perform film formation after removing the original natural oxide film. There is also a problem that the method itself is not suitable for industrialization. Therefore, it is considered that the oxide film formation by the atomic layer deposition method is most suitable. In addition, since the atomic layer deposition method can form a uniform oxide film in a three-dimensional structure, it can be applied to any three-dimensional structure. For example, in the superconductor device 10 shown in FIG. 1, an oxide film 13 </ b> A having the same thickness as that of the upper surface is formed on the side wall of the superconductor 12. Such oxide film growth is impossible by chemical vapor deposition or molecular beam epitaxy.

[Example]
A superconducting resonator as shown in FIG. 4 is fabricated on a sapphire substrate using niobium, which is a superconducting material, and is the same as the superconducting resonator that has been subjected to this property deterioration prevention method, that is, niobium on the niobium surface. The temperature dependence of the characteristics of a superconducting resonator with an oxide film was measured.

The superconducting resonator 40 shown in FIG. 4 includes a zigzag inductor 41 called a meander line and a capacitance 42 formed by alternately combining two comb shapes called interdigital capacitors. The manufactured superconducting resonator 40 is designed so that the niobium film thickness is 100 nm and the resonance frequency is 5 to 6 gigahertz. In the superconducting resonator 40 subjected to this characteristic deterioration prevention method, an Al ₂ O ₃ film having a thickness of 20 nm was deposited on the capacitance 42. As a condition during deposition, the substrate temperature is preferably 150 ° C. to 250 ° C. The surface oxide film of niobium is in a condition where oxygen is easily released at 130 ° C. or higher, and the oxide film having a rough surface becomes a dense film at a higher temperature of 350 ° C. or higher. Therefore, in order to remove oxygen from the oxide film on the niobium surface, the substrate temperature is suitably 150 ° C. or higher and 250 ° C. or lower. Further, trimethylaluminum (TMA) is used as the precursor 20, and water (H ₂ O) is used as the oxidant 30. TMA is a material that has a stronger ability to deprive oxygen than other materials. Typical material supply timings are, for example, t _1P = 0.1 seconds, t _1M = 4.0 seconds, t _0P = 0.1 seconds, and t _0M = 4.0 seconds.

The characteristic evaluation of the two superconducting resonators 40 was performed at a temperature T = 3K to 300 mK. The temperature dependence of the resonance frequency of the superconducting resonator 40 can be described by the change in the conductivity of the superconductor and the dielectric constant of the capacitance 42 (Non-Patent Document 2). In the high temperature region, the conductivity changes due to the presence of many thermally excited quasiparticles in the superconductor, but about 15% of the critical temperature at which the material exhibits superconductivity. At the following temperatures, the number of quasiparticles decreases exponentially and becomes almost constant. In such a region, the change in the dielectric constant based on the two-level fluctuation is dominant. The two-level fluctuation is a phenomenon caused by an atom or molecule that fluctuates between two levels in a quantum mechanical manner, and becomes remarkable at an extremely low temperature where quantum coherence is maintained. The two superconducting resonators 40 showed the same behavior in the high temperature region, but showed different behaviors in the low temperature region, that is, the region where the two-level fluctuation was dominant. It has been found that the magnitude of the two-level fluctuation is proportional to the number of defect levels. From the analysis of the experimental results, it was found that the superconducting resonator 40 on which Al ₂ O ₃ was deposited by this characteristic deterioration prevention method had two orders of magnitude fewer defect levels than the intact superconducting resonator 40.

  A superconducting cavity is an indispensable device in an accelerator experiment, and a superconducting cavity having a higher Q value is desirable for increasing the acceleration voltage. The superconducting resonator is expected to be applied in combination with the superconducting qubit, and in order to see the response of each photon one by one, a further increase in the Q value is desired. The present invention can raise the Q value of a superconducting cavity resonator or a superconducting resonator by a relatively inexpensive method, and further prevent aged deterioration due to oxygen in the atmosphere.

  As described above, according to the present embodiment, the oxide film 13 is formed on the surface of the superconductor 12 using the atomic layer deposition method, so that the natural oxide film 15 is removed and a dense and uniform oxidation is performed. Since the film 13 is formed, it is possible to prevent deterioration of the characteristics of the superconductor 12 in the electromagnetic field irradiation environment and realize ideal characteristics of the superconductor 12. In addition, the dense oxide film 13 can prevent the superconductor 12 from aging due to oxygen in the atmosphere.

DESCRIPTION OF SYMBOLS 10,50 ... Superconductor apparatus 11 ... Substrate 12 ... Superconductor 13, 13A ... Oxide film 15 ... Natural oxide film 100 ... Atomic layer deposition apparatus 101 ... Carrier gas 102, 103 ... Pulsing valve 104 ... Reaction chamber 105 ... Dry pump 106 ... Piping 20 ... Precursor 30 ... Oxidant 40 ... Superconducting resonator 41 ... Inductor 42 ... Capacitance

Claims (5)

A method for preventing deterioration of characteristics of a superconductor,
A method for preventing deterioration of characteristics, wherein an oxide film is formed on the surface of the superconductor using an atomic layer deposition method. Supplying a precursor and an oxidant alternately in a carrier gas in a reaction chamber containing the superconductor;
The method for preventing deterioration of characteristics according to claim 1, wherein only the carrier gas is supplied into the reaction chamber after the step of supplying the precursor and the step of supplying the oxidant.   3. The method for preventing deterioration of characteristics according to claim 1, wherein the superconductor is any one of niobium, lead, titanium, gallium, and niobium nitride.   4. The method for preventing deterioration of characteristics according to claim 1, wherein the oxide film is any one of alumina, hafnia, and hafnium aluminate. The superconductor is niobium,
The method for preventing deterioration of characteristics according to claim 2, wherein trimethylaluminum as the precursor and water as the oxidant are supplied into the reaction chamber at a substrate temperature of 150 ° C or higher and 250 ° C or lower.

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