Micro refrigerator

Abstract: The micro refrigerator comprises two NIS, i.e. normal metal/insulator/superconductor tunnel junction refrigerators (2, 2') formed on a substrate (1) connected in series in a SINIS structure, i.e. with a common normal metal element (4, 4') connected between and the superconducting elements (3, 3') outside. The refrigerator includes an insulating membrane (6) thermally isolated from the substrate (1) and the common normal metal element (4, 4') includes a protrusion (5) extending to this insulating membrane (6), which refrigerates the insulating membrane (6) and thus the aforesaid object (X).

Description: The present invention relates to a micro refrigerator, which comprises two normal metal/insulator/superconductor (NIS) tunnel junctions formed on a substrate connected in series in a SINIS form, i.e. a common normal metal element connected between and superconductor elements outside, when the bias voltage induced between the opposing superconductor elements creates an electronic current in both tunnel junctions, which cools the normal metal element, which is located close to the object to be cooled. Certain radiation sensors, bolometers, used in space research demand a temperature of less than 1 Kelvin, if good energy resolution is wanted. The temperature range 100 - 300 mK cannot be achieved using simple devices. A temperature of about 0.3 - 1 K can be achieved using a Helium-3 (He 3 ) evaporator, but even this is insufficient in certain cases. A Peltier refrigerator utilizing a superconductor element is described in the paper Nahum, M. et al; Applied Physics Letters 65 , 1994, page 3123. Normal metal/insulator/superconductor, i.e. NIS junctions operate at extremely low temperatures of less than 10 Kelvin. An NIS refrigerator has the drawback that it is difficult to thermally insulate the cooled normal metal element. An improved refrigerator construction is described in the paper Leivo, M.M.; Pekola, J.P.; Averin, D.V. 'Efficient Peltier refrigeration by a pair of normal metal/insulator/superconductor junctions', Applied Physics Letters 68 (14), 1996, page 1996. When two NIS junctions are connected in a SINIS structure, the connected normal metal element is thermally insulated on both sides. In addition to the cooling element, the object itself that is to be cooled must be thermally insulated from the substrate. On the other hand, the heat transmitted to the superconductor in the NIS junctions must be effectively conducted away, so that it does not limit the cooling power. In known solutions, the cooling has only been a few milliKelvins at 300 mK, though in theory a considerably greater temperature difference could have been achieved. The object of the invention is to create an improved micro refrigerator, by means of which a temperature in the order of 100 mK can be achieved in a desired object more simply than previously. The characteristic features of the invention are described in the accompanying Patent Claims. When the cooling NIS junctions are on the substrate, heat is transmitted there with little resistance. The superconductor elements heated up are at the bottom. Advantageously, the thermal conductivity of the normal metal element is much greater than that of the surfacing of the substrate and the insulating membrane formed. from it. It then efficiently collects the heat from the insulating membrane, which is thermally separated from the substrate. This takes place advantageously by cutting the direct connection of the insulating membrane at the boundary area of the opening and by supporting it from the substrate by means of narrow strips. Other advantages and embodiments of the invention appear in connection with the later examples of applications. In the following, the invention is illustrated by reference to the accompanying figures, which show one micro refrigerator according to the invention. Figure 1 shows the layout of the SINIS refrigerator on the substrate Figure 2 shows a cross-section of Figure 1 at point II - II. The micro refrigerator is fabricated on substrate 1, the material of which is silicon, with a thickness of 0.5 mm. On its surface, there is a thin (50 - 500 nm) layer 8 of silicon nitride, basic structure Si 3 N 4 . In the middle of the chip, there is a 400 x 400 µm 2 window of silicon nitride, which is fabricated by etching silicon away, creating an opening 7 that penetrates the silicon layer. The scale of Figure 2 is distorted to improve the illustration. A highly efficient thermally insulating membrane 6 has been created in substrate 1 by cutting openings 9 in the original membrane. These leave the insulating membrane 6 resting on thin supports 10 only. The size of the insulating membrane is advantageously 200 x 200 µm 2 ± 50%, Figure 1. Series connected NIS tunnel junction refrigerators 2 and 2' are made as close as possible to the edge of opening 7. They are fabricated by electron beam lithography as follows. Firstly superconducting comb-like elements 3 and 3' are 30 nm thick aluminium wires. Niobium (Nb) or some high temperature superconductor (YBCO) can also be used. The aluminium elements (Al) are oxidized, providing the insulating layers of the tunnel junctions. Next, common normal metal elements 4 and 4' are made, along with a copper protrusion 5 extending to the insulating membrane, with a width at its narrowest of 0.5 - 1 µm and a thickness of about 0.05µm (20 - 250 nm). The end 12 of protrusion 5 is widened to improve thermal conductivity to insulating membrane 6. The surface area of the widened section 12 is advantageously at least twice the surface area of the rest of the normal metal element 4, 4'. Silver (Ag) or gold (Au) can also be used in place of copper (Cu). The material of the surface layer 8 has been selected so that it will form, as an insulating membrane 6, a base with a reasonably even temperature for the object to be cooled, which in Figure 1 is shown diagrammatically by the letter X. It is essential that as small a part of the normal metal element 4, 4' as possible remains outside the membrane, so that heat is not conducted into it from the substrate. In the normal metal element, heat is transmitted along with the electrons. These have a poor contact with the lattice of the substrate. Superconducting elements 3 and 3' are connected thermally through their lattice to the substrate, through the thin insulating layer. According to Figure 1, the insulating membrane 6 is supported by four (generally 2 - 4 pcs.) narrow strips 5. Their width is 1 - 10% of the average diameter of the insulating membrane 6. Both protrusion 5 and the leads required by object X are led through these strips. By way of example, the leads required by the temperature sensor are drawn in the Figure. These are led away along another strip. These other leads are advantageously of superconducting aluminium, which does not transport heat to the insulating membrane 6. Electron beam lithography permits a large number of parallel tunnel junctions of similar quality (3 - 100 pcs.). Photolithography, on the other hand, can be used to make tunnel junctions with a large surface area, when they need be no more than 1 or 2 pcs. There may be more than one SINIS refrigerator. Their normal metal protrusions are located in different strips. Alternatively, the insulating membrane is supported by only one or more normal metal protrusions. The object to be cooled is attached to the membrane and may form a unit that has a structure that is essentially thicker than the membrane. The thin edges and narrow supports of the membrane form thermal insulation. In the publication Manninen-Leivo-Pekola, Applied Physics Letters 70(14) , 1997, p. 1885, there is a description of the background of a corresponding SINIS refrigerator and the dimensioning of tunnel junctions.

Claims: A micro refrigerator, comprising two NIS, i.e. normal metal/insulator/superconductor tunnel junction refrigerators (2, 2') formed on a substrate (1) connected in series in a SINIS structure, i.e. with a common normal metal element (4, 4') connected between and the superconducting elements (3, 3') outside, when the bias voltage induced between the superconducting elements (3, 3') creates an electronic current in both tunnel junctions (2, 2'), cooling the normal metal element (4, 4'), which is located close to the object (X) to be cooled, characterized in that the refrigerator includes an insulating membrane (6) thermally isolated from the substrate (1), on which the aforesaid object is placed, and the common normal metal element (4, 4') includes a protrusion (5) extending to this insulating membrane (6), which cools the insulating membrane (6) and thus the aforesaid object (X). A micro refrigerator according to Claim 1, characterized in that the aforesaid insulating membrane (6) is part of the surface layer (8) on the substrate (1), which continues over an opening (7) formed in the substrate (1) and in which in the area of the opening (7) the insulating membrane (6) is supported from the substrate (1) by means of one or more narrow strips (10). A micro refrigerator according to Claim 2, characterized in that there are 2 - 4 strips (10) and that they have a width of 1 - 10% of the average diameter of the insulating membrane (6). A micro refrigerator according to one of Claims 1 - 3, characterized in that the width of the protrusion (5) is, at its narrowest, 0.5 - 1 µm and its thickness is 0.05 µm (20 - 250 nm) and that it includes a section (12) that widens on the insulating membrane (6), the surface area of which is at least double the surface area of the rest of the normal metal element (4, 4'). A micro refrigerator according to one of Claims 1 - 3, characterized in that the insulating membrane (6) is supported from the substrate (1) by means of one or more protrusions (5) of the normal metal element pair (4, 4'). A micro refrigerator according to one of Claims 1 - 5, characterized in that the size of the insulating membrane is 200 x 200 µm 2 ± 50%. A micro refrigerator according to one of Claims 1 - 6, characterized in that the thickness of the insulating membrane (6) is 50 - 500 nm. A micro refrigerator according to one of Claims 1 - 7, characterized in that the insulating membrane (6) is essentially of silicon nitride, with a basic structure Si 3 N 4 . A micro refrigerator according to one of Claims 1 - 8, characterized in that the superconducting elements (3, 3') are fabricated from a substance from the following group: aluminium (Al), niobium (Nb), high-temperature superconductors (YBCO). A micro refrigerator according to one of Claims 1 - 9, characterized in that the normal metal elements (4, 4') and the part protruding from them to the insulating membrane (6) are fabricated from a substance from the following group: copper (Cu), silver (Ag), gold (Au). A micro refrigerator according to one of Claims 1 - 10, characterized in that in both of the NIS refrigerators of the SINIS unit (2, 2') there are 3 - 100 parallel tunnel junctions fabricated by electron beam lithography. A micro refrigerator according to one of Claims 1 - 10, characterized in that in both of the NIS refrigerators of the SINIS unit there are 1 - 2 tunnel junctions fabricated by photolithography.

Inventors: LEIVO MIKKO, MANNINEN ANTTI, PEKOLA JUKKA, LEIVO, MIKKO, MANNINEN, ANTTI, PEKOLA, JUKKA
Applicants: NANOWAY OY
Application year: 1997
Main classification: H01L35
IPC codes: H01L35