JP5669064B2 - Superconducting tunnel junction detector - Google Patents

Description

  The present invention relates to a superconducting tunnel junction detector that detects photons using a superconducting tunnel junction (STJ).

As a detector using a conventional superconducting tunnel junction element (hereinafter referred to as “STJ element”), for example, there is an optical sensor described in Patent Document 1. This optical sensor uses an STJ element integrated with a light absorber, which converts light energy into phonons with the light absorber and excites electrons in the superconductor with the converted phonons. Detected.
On the other hand, the present inventors are researching and developing a superconducting tunnel junction detector that can detect photons in a wider band by using a substrate absorption type STJ element having a substrate as an energy absorber. .

  The principle of photon detection by the substrate absorption STJ element will be described as follows. When the substrate absorbs photons (energy), phonons are generated in the substrate. The generated phonon group propagates in the substrate, and a part of the phonon group reaches the lower electrode in close contact with the substrate of the STJ element. When the phonon reaches the lower electrode, the Cooper pair in the lower electrode is dissociated and quasiparticles are generated. The current (tunnel current) that flows when the quasiparticle tunnels the tunnel barrier is used as a photon detection signal.

JP 61-271879 A

By the way, in order to improve the detection efficiency by the substrate absorption type STJ detector, it is conceivable that the lower electrode of the STJ element is first enlarged so that more phonons can reach the lower electrode.
However, only the quasiparticles that reach the tunnel barrier among the quasiparticles generated in the lower electrode contribute to the detection signal, so even if many phonons can reach the lower electrode, that alone does not necessarily It cannot be said that the detection efficiency is improved. That is, it is necessary to make the tunnel barrier small in order to suppress the leakage current, but the distance that the quasiparticles generated in the lower electrode can reach the tunnel barrier (quasiparticle diffusion length) varies depending on the electrode material, but at most. It is several tens to several hundreds μm. For this reason, even if the lower electrode is simply enlarged, there is a high possibility that the increase in quasiparticles associated therewith cannot reach the tunnel barrier, and there is a limit to improving the detection efficiency of the substrate absorption STJ detector. There is.

  The present invention has been made paying attention to the above problems, and by increasing the number of phonons reaching the lower electrode regardless of the size of the lower electrode, the number of quasiparticles generated in the lower electrode is increased. An object of the present invention is to provide a superconducting tunnel junction detector capable of improving photon detection efficiency.

  A superconducting tunnel junction detector according to one aspect of the present invention is provided with a superconducting tunnel junction element in which a lower electrode, a tunnel barrier, and an upper electrode are sequentially stacked on an upper surface of a substrate. A superconducting tunnel junction detector that detects photons based on a metal film embedded on the upper surface of the substrate so as to surround the lower electrode.

  A superconducting tunnel junction detector according to another aspect of the present invention includes a superconducting tunnel junction device in which a lower electrode, a tunnel barrier, and an upper electrode are sequentially stacked on a top surface of a substrate, and phonons generated in the substrate. A superconducting tunnel junction detector for detecting photons based on the above, and a pair of grooves extending so as to be opposed to each other with at least the lower electrode interposed therebetween.

  According to the superconducting tunnel junction detector, phonons propagating in the substrate by a metal film embedded in the upper surface of the substrate or a groove formed in the upper surface of the substrate are reflected at the interface of the metal film, the groove, etc. (Dispersion) is reduced. As a result, more phonons reach the lower electrode. Thereby, the detection efficiency of photons can be improved.

It is a block diagram which shows the structure of the superconducting tunnel junction detector by embodiment of this invention. It is a figure which shows the structure of the board | substrate absorption type STJ element used with the said superconducting tunnel junction detector. It is a figure which shows the manufacturing process of the said board | substrate absorption type STJ element. It is a figure which shows the manufacturing process of the said board | substrate absorption type STJ element. It is a figure which shows the manufacturing process of the said board | substrate absorption type STJ element. It is the figure which showed typically the mode of propagation of the phonon in a board | substrate. It is a figure which shows the modification of the said board | substrate absorption type STJ element.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a block diagram showing a configuration of a superconducting tunnel junction detector according to an embodiment of the present invention. The superconducting tunnel junction detector 1 according to the present embodiment is configured as a superconducting terahertz wave detector that detects terahertz waves, and amplifies the substrate absorption STJ element 10 and the detection signal of the substrate absorption STJ element 10. A preamplifier 20, an A / D converter 30 that performs A / D conversion on the output of the preamplifier 20 and outputs the digital data, and a recording device 40 that records the output of the A / D converter 30 are provided.

2A and 2B show the configuration of the substrate absorption type STJ element 10. FIG. 2A is a plan view, and FIG. 2B is a cross-sectional view taken along line AA in FIG.
As shown in FIG. 2, the substrate absorption STJ element 10 includes a substrate 11 and an STJ element 13 provided on the substrate 11.
The substrate 11 is a single crystal substrate made of LiNbO ₃ (lithium niobate), LiTaO ₃ (lithium tantalate), or the like that easily absorbs the terahertz wave to be detected.
The STJ element 13 includes a lower electrode 31 made of a single layer of a superconducting electrode material or two layers of different superconducting energy gaps, a tunnel barrier (tunnel barrier) 33 made of an insulating film, It is an element having a structure in which an upper electrode 35 made of a single layer of a superconducting electrode material or two layers of films having different superconducting energy gaps is sequentially laminated. As the superconductive electrode material, for example, Al (aluminum) / niobium (Nb) two-layer film, and as the insulating film serving as a tunnel barrier, for example, AlOx (aluminum oxide) is used. Here, the superconducting electrode material is a two-layer film having a different superconducting energy gap because a layer of a material having a smaller superconducting energy gap value collects quasiparticles generated in a layer of a larger material ( This is because an increase in the number of quasiparticles due to the collapse of the Cooper pair near the tunnel barrier can be expected.

  A metal film 12 is embedded on the upper surface of the substrate 11 so as to surround the lower electrode 31 of the STJ element 13. Specifically, a groove 11a surrounding the lower electrode 31 is formed on the upper surface of the substrate 11, and a metal film 12 is formed so as to fill the groove 11a. The groove 11a can be formed by digging the upper surface of the substrate 11 using, for example, a photolithography technique and an etching apparatus. The metal film 12 can be formed by depositing metal on the surface of the groove 11a using, for example, a vapor deposition apparatus or a sputtering apparatus.

Further, a ground layer 15 is formed on the substrate 11, and the ground layer 15 is connected to the lower electrode 31. The STJ element 13 and the ground layer 15 are covered with an interlayer insulating film 17 made of SiO ₂ (silicon dioxide) or the like, so that electrical insulation between the lower electrode 31 and the upper electrode 35 of the STJ element 13 is reduced. It has been. Furthermore, a wiring 19 connected to the upper electrode 35 is formed on the substrate 11 through a contact hole 17a formed in the interlayer insulating film 17, and this wiring 19 is connected to a signal detection PAD 19a.

3 to 5 show an outline of a manufacturing process of the substrate absorption type STJ element 10.
In the first step shown in FIG. 3A, a rectangular groove 11a is formed on the upper surface of the substrate 11 by photolithography and etching. Specifically, for example, the high aspect ratio groove 11a is formed by reactive ion etching (RIE) using a Bosch process. The depth of the groove 11a is adjusted by the number of STJ elements provided on the substrate 11, the interval between them, and the like, but is preferably approximately half the thickness of the substrate 11 or slightly larger than that.

  In the second step shown in FIG. 3B, a metal is deposited in the groove 11a formed on the upper surface of the substrate 11 by photolithography and vapor deposition or sputtering. Thereby, the metal is buried in the groove 11a to form the metal film 12. Here, the metal to be deposited in the groove 11a is not particularly limited. For example, Nb (niobium), Cr (chromium), nickel (Ni), Al (aluminum), or an alloy thereof can be considered.

  In the third step shown in FIG. 3C, a thin film 51 having an SIS (Superconducting-Insulator-Superconducting) structure in which a thin insulator is sandwiched between superconductors by sputtering, that is, an Nb / Al-AlOx-Al / Nb structure. Is deposited on the substrate 11. Here, the upper layer Nb / Al is the upper electrode layer of the STJ element 13, the intermediate layer AlOx is the tunnel barrier layer of the STJ element 13, and the lower layer Al / Nb is the lower electrode layer of the STJ element. The tunnel barrier layer (AlOx) can be obtained by oxidizing the Al film by leaving it in an oxygen atmosphere for a long time.

  In the fourth step shown in FIG. 3D, a photosensitive photoresist is applied onto the thin film 51 by a spin coater, a spray coater, or the like, and the upper electrode is formed at a substantially central position in the region inside the groove 11a using a photomask. After patterning in the shape of 35 and exposing to ultraviolet light, the resist 52 is formed by developing with a positive developer.

  In the fifth step shown in FIG. 4A, the upper electrode layer, the tunnel barrier layer, and a part of the lower electrode layer are shaved by reactive ion etching (RIE) and ultrasonically cleaned with an organic solvent such as acetone. The remaining resist 52 is removed. Thereby, the upper electrode 35 and the tunnel barrier 33 of the STJ element 13 are formed.

In the sixth step shown in FIG. 4B, a resist 53 patterned in the shape of the lower electrode 33 and the ground layer 15 is formed by the same method as in the fourth step (FIG. 3D).
In the seventh step shown in FIG. 4C, the lower electrode layer is shaved by the same method as in the fifth step (FIG. 4A), and then the remaining resist 53 is removed. Thereby, the lower electrode 31 and the ground layer 15 are formed.

In the eighth step shown in FIG. 4D, an interlayer insulating layer (SiO ₂ or the like) 54 is deposited by sputtering.
In the ninth step shown in FIG. 4E, the resist 55 is formed by patterning into a shape excluding the portions to be the contact holes 17a and the PAD 19a by the same method as in the fourth step (FIG. 3D).

  In the tenth step shown in FIG. 5A, the interlayer insulating layer 54 is removed by the same method as in the fifth step (FIG. 4A), and then the remaining resist 55 is removed. Thereby, the interlayer insulating film 17 and the contact hole 17a are formed.

In an eleventh step shown in FIG. 5B, a wiring layer (for example, Nb layer) 56 is deposited by sputtering.
In the twelfth step shown in FIG. 5C, a resist 57 patterned in the shape of the wiring 19 and the wiring PAD is formed by the same method as in the fourth step (FIG. 3D).
In the thirteenth step shown in FIG. 5D, the wiring layer 56 is cut by the same method as in the fifth step (FIG. 4A), and then the remaining resist 57 is removed. Thereby, the wiring 19 and the PAD 19a are formed.
The substrate absorption type STJ element 10 is manufactured through the above first to thirteenth steps.
In the above, each layer is deposited by sputtering, but the present invention is not limited to this, and each layer may be deposited by other methods (for example, vapor deposition).

Here, a series of actions of the superconducting tunnel junction detector 1 according to the present embodiment will be described.
When a terahertz wave is irradiated on the lower surface of the substrate 11, that is, the surface of the substrate 11 where the STJ element 13 is not provided, the substrate 11 absorbs the terahertz wave (that is, photon energy). Phonons are generated in the substrate 11. The generated phonon group propagates in the substrate 11 and a part of the phonon group reaches the lower electrode 31 of the STJ element 13. When the phonon reaches the lower electrode 31, the Cooper pair in the lower electrode 31 is destroyed and quasiparticles are generated. The substrate absorption type STJ element 10 outputs, as a detection signal, a tunnel current that flows when the quasiparticles generated in the lower electrode 31 tunnel through the tunnel barrier 33. The detection signal (tunnel current value) output from the substrate absorption type STJ element 10 is amplified by the preamplifier 20 and then converted into digital data by the A / D converter 30 and recorded in the recording device 40.

  In the present embodiment, the metal film 12 is embedded on the upper surface of the substrate 11 so as to surround the lower electrode 31 of the STJ element 13. For this reason, as shown in FIG. 6, diffusion (dispersion) of phonons propagating in the substrate 11 is reduced, and more phonons can reach the lower electrode 31. That is, the phonon (part of the phonon) that has reached the portion of the upper surface of the substrate 11 where the lower electrode 31 is not formed is made to reach the lower electrode 31, and the phonon lower electrode 31 generated in the substrate 11. You can increase your reach to. The size of the lower electrode 31 is set in consideration of the size of the tunnel barrier 33 and the quasiparticle diffusion (scattering) length of the lower electrode 31, and is generated when the amount of phonons reaching the lower electrode 31 increases. Quasiparticles and tunneling currents also increase. As a result, the terahertz wave detection efficiency is improved, and the terahertz wave detection time can be shortened.

  The superconducting terahertz wave detector that detects terahertz waves has been described above. However, the present invention is not limited to this, and it is configured as a superconducting tunnel junction detector that detects visible light, radiation, and the like. You can also. In this case, a substrate made of a material that easily absorbs the detection target may be employed, and for example, a sapphire substrate may be used. That is, the present invention uses a substrate as an energy absorber, and the substrate absorbs a detection target, thereby generating phonons in the substrate, and the generated phonons reach the lower electrode of the STJ element to generate quasiparticles. The present invention can be widely applied to a superconducting tunnel junction detector having a configuration as described above.

  In the above embodiment, the metal film 12 is formed by depositing metal in the groove 11a formed so as to surround the lower electrode 31 on the upper surface of the substrate 11. However, the diffusion of phonons in the substrate 11 is caused by the groove 11a. Therefore, only the groove 11 a may be formed on the upper surface of the substrate 11. In this case, a groove surrounding the lower electrode 31 as described above may be formed, but a pair of grooves extending so as to face each other with the lower electrode 31 interposed therebetween, in other words, at least two opposite electrodes of the lower electrode 31 are opposed to each other. Two grooves extending along each of the two sides may be formed. Here, considering both the reduction of phonon diffusion and the ease of manufacturing the substrate absorption type STJ element 10, grooves are formed on the upper side and the lower side of the lower electrode 31 in FIG. That is, two grooves are formed in a region that does not affect the formation of the ground layer 15, the wiring 19, the PAD 19a, and the like of the STJ element 13 on the upper surface of the substrate 11, and the lower electrode 31 is disposed between the two grooves. Is preferred. Of course, metal may be further deposited in these grooves to form a metal film embedded in the upper surface of the substrate 11. Even in this case, the arrival rate of the phonons generated in the substrate 11 to the lower electrode 31 can be improved.

  Further, in the above embodiment, the metal film 12 is formed by depositing metal in the groove 11a formed (digged) in a direction perpendicular to the upper surface of the substrate 11, but the groove 11a and the metal film are formed. The shape of 12 is not limited to this. For example, the grooves 11a and / or the metal film 12 are formed to be inclined so that the gaps between the grooves 11a and / or the metal films 12 increase toward the lower side of the substrate 11 (FIG. 7A). Alternatively, the metal film 12 may be formed to be curved (FIG. 7B). When a plurality of STJ elements are arranged on the substrate 11, as shown in FIG. 7 (c), a groove 11a having a triangular cross section is formed between the STJ elements, or the triangular cross section is formed. A metal film 12 having a V-shaped cross section may be formed in the groove 11. According to these modified examples, the manufacturing process of the substrate absorption type STJ element is slightly complicated as compared with the above embodiment, but the arrival rate of the phonons generated in the substrate 11 to the lower electrode 31 is further increased. Improvements can be made.

  DESCRIPTION OF SYMBOLS 1 ... Superconducting tunnel junction detector, 10 ... Substrate absorption type STJ element, 11 ... Substrate, 11a ... Groove, 12 ... Metal film, 13 ... STJ element, 31 ... Lower electrode, 33 ... Tunnel barrier, 35 ... Upper electrode

Claims (3)

A superconducting tunnel junction detector in which a lower electrode, a tunnel barrier, and an upper electrode are sequentially laminated on a top surface of a substrate is provided, and is a superconducting tunnel junction detector that detects photons based on phonons generated in the substrate. And
A superconducting tunnel junction detector in which a metal film is embedded on the upper surface of the substrate so as to surround the lower electrode. The superconducting tunnel junction detector is provided with a superconducting tunnel junction element in which a lower electrode, a tunnel barrier, and an upper electrode are sequentially laminated on the upper surface of the substrate, and detects photons based on phonons generated in the substrate. And
A superconducting tunnel junction detector, wherein a pair of grooves extending to face each other with at least the lower electrode interposed therebetween is formed on the upper surface of the substrate.   The superconducting tunnel junction detector according to claim 2, wherein a metal film is embedded in the groove.