Method for manufacturing superconducting detector with thin-film tunnel junctions and suspended absorber

Abstract

FIELD: microelectronics technology.SUBSTANCE: invention can be used in the creation of thin-film superconducting structures. A method for producing a detector with thin-film tunnel junctions and a suspended absorber includes applying a thin-film superconductor - insulator - normal metal - insulator - superconductor (SINIS) structure to a dielectric substrate without breaking the vacuum, wherein the SINIS structure is formed in a single vacuum cycle on the surface of the dielectric substrate by sputtering an aluminium layer, which is oxidized to form aluminium oxide, on top of which a copper film and a sacrificial layer of silicon dioxide are sputtered, then a suspended absorber is formed from the copper film by photolithography by etching silicon dioxide to form a mask for etching the copper film, an insulating layer of aluminium oxide and a layer of aluminium; after which the copper film is etched, and subsequent layers of aluminium oxide and aluminium are etched, after which, by means of a second photolithography, pads of the superconductor -insulator - normal metal type superconducting junction are formed by etching the sacrificial layer of silicon dioxide to form a hard mask for etching the copper film; after which a suspended absorber is formed from a copper film by etching off the sacrificial layer of aluminium and the sacrificial layer of silicon dioxide until the copper film can be etched.EFFECT: invention provides a reliable technology for creating devices with high-quality tunnel junctions and a suspended absorber, which does not have high requirements for the resist and the composition of etching liquids, does not require a complex technological installation for shadow sputtering, allows for the production of suspended absorbers with reinforcement in the form of SiO2, and is also capable of withstanding fluorine-containing plasma and cleaning in various solvents.1 cl, 12 dwg

Description

[0001] The invention relates to microelectronics technology and can be used in the creation of thin-film superconducting structures.

[0002] The article describes technologies for creating thin-film superconducting structures for highly sensitive bolometers, electron coolers, single-electron transistors containing a free-hanging microbridge of normal metal and superconducting junctions of the superconductor-insulator-normal metal (SIN), superconductor-insulator-other superconductor (SIS) type, as well as Andreev contacts (superconductor-Andreev contact-normal metal) and structures with a Schottky barrier (superconductor-Schottky barrier-semiconductor).

[0003] A similar method is known: the fabrication of an integral sensitive bolometer, in which a freely hanging microbridge of amorphous silicon is formed by etching an oxide layer deposited underneath it [1]. According to this method, an oxide strip is first formed on an oxidized silicon substrate using photolithography and etching, a first layer of amorphous silicon is formed using photolithography and etching, a second layer of amorphous silicon is formed using photolithography and etching, it is doped, and a top layer with holes for etching the first oxide layer is formed by etching. The result is a thin hanging bridge of doped amorphous silicon with the required temperature coefficient of resistance. The disadvantage of this analog is the need for three photolithography operations, three different depositions, and four etching operations.Three vacuum cycles followed by exposure to atmosphere and photolithography chemicals in between results in parasitic contact resistances, reduced structure quality, and poor device reproducibility.

[0004] Similar methods are known in which freely hanging semiconductor bridges are formed by various modifications according to the similar method [1]. In patent [2], the technology is modified for thicker insulator films and a greater suspension height. Patent [3] presents a modification for using a supporting layer of silicon nitride, which allows the use of thinner resistive bridge films. Patent [4] describes a spectrometer using such bridges in the form of a pixel matrix on a polyimide (Mylar) substrate. Patent [5] describes the design of a differential detector using pairs of hanging bridges on a polyimide base to eliminate the influence of drifts and create frequency-selective receivers. Patent [6] describes a design with the support of such hanging microbridges using micropillars. The patent [7] proposes to select a suspension thickness equal to a quarter of the wavelength and to form a reading circuit on the same substrate.

[0005] A similar method is known: the production of a freely suspended bridge, in which the bridge is placed above the substrate on top of a pre-exposed resist [8]. A three-layer resist is used, in which the middle layer is completely exposed. In contact lithography, the shape of the bridge is determined by the upper and lower resist layers. Electrical contacts 100 nm thick are deposited at a right angle, and the bismuth bridge is deposited at angles of 50° on each side of the bridge. Thus, 100 nm of bismuth is deposited under the resist bridge and supported above the substrate by the lower resist layer. After deposition, the substrate is placed in acetone to dissolve the photoresist. Unwanted metal on top of the resist is removed, leaving a bolometer suspended at its ends, and the resist underneath is also removed. This technology is characterized by the complexity of the exposure and development processes of the resist, poor reproducibility, and low quality of the films deposited on top of the three-layer photoresist.

[0006] A similar method is known: the fabrication of bolometers with suspended resistive elements thermally coupled to suspended tunnel SIS junctions [9]. The structures are formed on top of a silicon oxide layer, and the silicon substrate itself is etched in CF6 plasma. A disadvantage of this method is the presence of a silicon oxide sublayer, which increases thermal contact with the substrate and reduces the ultimate performance compared to fully suspended metal bridges.

[0007] A similar method is known: the fabrication of a suspended aluminum single-electron transistor

[10] . In this case, a 50-nm-high pedestal is initially formed from the organic polymer calixarene, similar to Bakelite, which can then be etched in oxygen plasma after the single-electron transistor is formed. A disadvantage of this method is the deposition of bridge and junction films on top of this organic polymer, which reduces the film quality and the fabrication accuracy of the structures.

[0008] The closest to the patented method is the production method of devices with freely hanging microbridges

[11] (RU 2632630 C1, FGBUN IRE RAS, 06.10.2017 - prototype). The technology includes the deposition of a three-layer thin-film structure of the superconductor-insulator-normal metal (SIN) type without breaking the vacuum; resist deposition, exposure, development; selective chemical etching of the lower electrode of the three-layer structure, while photolithography is carried out before the deposition of the three-layer structure, the topology of the SIN structure is formed by the explosion method, and a single etching is carried out in an alkaline developer, combined with the development of a resist with a window pattern. The essence of the technology: deposition of a three-layer structure of aluminum (superconductor) - oxidation of aluminum (insulator) - copper (normal metal), lithography for etching aluminum in the absorber area, lithography for etching copper.The disadvantages of this method include the criticality of selecting a specific resist due to the requirement for high adhesion of the resist to copper; the need to etch copper and aluminum in a weak alkali and acid, respectively, since etching in strong alkalis and acids causes the copper layer to peel off from the aluminum oxide surface. Furthermore, there is no option to clean the sample in solvents during the intermediate stages.

[0009] The present invention is aimed at solving the problem of creating a detector of the superconductor-insulator-normal metal-insulator-superconductor (SINIS) structure with high reproducibility, high-quality tunnel junctions with a small spread of parameters, without high requirements for the resist and etching liquids. The method for manufacturing a superconductor detector with thin-film tunnel junctions and a suspended absorber consists in applying an additional sacrificial layer of SiO2, lithography according to a given pattern, etching off the sacrificial layer of SiO2 in the required places, separate etching of copper and aluminum in acid and alkali, repeated lithography to form a tunnel junction, etching SiO2 and etching copper, etching off the sacrificial layer of aluminum under suspended copper bridges (creation of a suspended absorber), characterized in that:

[0010] - the sacrificial layer of SiO2 prevents the etching liquid from leaking between the copper and the resist due to poor adhesion;

[0011] - the sacrificial layer of SiO2 allows you to “fix” the copper layer, which facilitates any further wet etching;

[0012] - thanks to the hard mask, which is obtained from the sacrificial layer of SiO2, it becomes possible to clean samples in solvents, such as acetone, as well as etch in fluorine-containing plasma, since the etching rate of SiO2 is significantly lower than the etching rate of the resistive mask.

[0013] Patented method

[0014] The technical result is a reliable technology for creating a device with high-quality tunnel junctions and a suspended absorber. This technology does not impose high requirements on the resist or etchant composition, does not require a complex shadow-sputtering setup, and enables the fabrication of suspended absorbers reinforced with SiO2. This technology is also capable of withstanding fluorine-containing plasma and cleaning in various solvents. The patented method is based on the application of an additional sacrificial SiO2 layer, which functions as a hard mask. Due to its good adsorption, the SiO2 layer helps prevent under-etching of metal films, reducing the number of resist and solution requirements required for this technology. The silicon dioxide hard mask also allows the use of various solvents, as well as fluorine-containing plasma etching, if required by the manufacturing process.

[0015] The sacrificial layer of SiO2 prevents the etching liquid from leaking between the copper and resist due to poor adhesion, and allows the copper layer to be “fixed,” which facilitates any further liquid etching. Thanks to the solid mask obtained from the sacrificial layer of SiO2, it becomes possible to clean samples in solvents such as acetone. Etching in fluorine-containing plasma is possible, since the etching rate of SiO2 is significantly lower than the etching rate of the resistive mask.

[0016] The essence of the invention is explained in the figures.

[0017] Fig. 1. A silicon substrate (1) and a three-layer structure deposited on top of it, consisting of an aluminum film (2), an insulating aluminum oxide layer (3), and a copper film (4).

[0018] Fig. 2. The sacrificial etched SiO2 layer (5) applied on top and the resistive mask (6) to form the suspended absorber.

[0019] Fig. 3. same as in Fig. 2 - top view.

[0020] Fig. 4. Etched SiO2(5) layer forming a hard mask.

[0021] Fig. 5. Three-layer structure of aluminum film (2), insulating aluminum oxide layer (3) and copper film (4), etched on a hard mask of SiO2 layer (5).

[0022] Fig. 6. Resistive mask (7) for forming a superconductor-insulator-normal metal transition. 3D image.

[0023] Fig. 7. same as in Fig. 6 - top view.

[0024] Fig. 8. Three-layer structure of aluminum film (2), insulating aluminum oxide layer (3) and copper film (4), etched on a resistive mask (7).

[0025] Fig. 9. Three-layer structure of aluminum film (2), aluminum oxide insulating layer (3), and copper film (4) after removing the resistive mask. The sacrificial aluminum layer (2) is placed under the normal metal to form a suspended absorber (8).

[0026] Fig. 10. The formed structure (9) of the superconductor-insulator-normal metal type after etching the sacrificial layer of aluminum (2) to form a suspended absorber.

[0027] Fig. 11. The resulting superconductor detector with thin-film tunnel junction structure (9) and suspended absorber (10) after removing the sacrificial SiO2 layer. 3D image.

[0028] Fig. 12 same as in Fig. 11 - side view.

[0029] The following positions are indicated: 1 - silicon substrate; 2 - aluminum layer; 3 - insulating aluminum oxide layer; 4 - copper film; 5 - sacrificial layer of silicon dioxide (SiO2); 6 - resistive mask for forming a suspended absorber; 7 - resistive mask for forming a superconductor-insulator-normal metal junction; 8 - sacrificial aluminum layer under normal metal to form a suspended absorber; 9 - superconductor-insulator-normal metal structure; 10 - suspended absorber.

[0030] The manufacturing process of a superconducting detector with thin-film tunnel junctions and a suspended absorber is carried out as follows:

[0031] 1. A layer of aluminum 2 is deposited on the surface of the dielectric substrate 1.

[0032] 2. The sprayed aluminum layer 2 is oxidized in oxygen to form an insulating aluminum oxide layer 3.

[0033] 3. Without disturbing the vacuum cycle, a copper film 4 is deposited on the aluminum oxide layer 3.

[0034] 4. A sacrificial layer 5 of silicon dioxide is applied to the copper film 4.

[0035] 5. A single-layer photoresist is applied over sacrificial layer 5. The single-layer photoresist is necessary for forming the suspended absorber from copper film 4.

[0036] 6. The photoresist is exposed by illuminating the areas where the photoresist will be removed, and then developed.

[0037] 7. The sacrificial layer 5 of silicon dioxide is etched in a fluorine-containing plasma to form a hard mask for etching copper films 4, insulating layer 3 of aluminum oxide, and aluminum layer 2.

[0038] 8. The sample is cleaned from photoresist in dimethylformamide, acetone and oxygen plasma.

[0039] 9. Film 4 is etched in an acid solution using a hard mask on layer 5 of silicon dioxide.

[0040] 10. Etching of films 2 and 3 is carried out in an alkaline solution using a hard mask of layer 5 of silicon dioxide.

[0041] 11. Apply a single-layer photoresist over the entire structure. The single-layer photoresist is necessary to form the superconducting transition pads of the superconductor-insulator-normal metal type.

[0042] 12. The photoresist is exposed by illuminating the areas where the photoresist will be removed, and then developed.

[0043] 13. The silicon dioxide layer 5 is etched in fluorine-containing plasma to enable the etching of copper film 4.

[0044] 14. The copper film is etched in an acid solution, which leads to the formation of superconducting transition areas of the superconductor-insulator-normal metal type.

[0045] 15. The sample is cleaned from photoresist in dimethylformamide, acetone and oxygen plasma.

[0046] 16. The sacrificial layer of aluminum 8 is etched away to form a suspended absorber.

[0047] 17. The remains of the sacrificial layer 5 of silicon dioxide are removed in fluorine-containing plasma.

[0048] The technical result of the patented solution is the achievement of the stated objectives: the creation of a superconducting detector with thin-film tunnel junctions and a suspended absorber. The method of fabricating the absorber with an additional sacrificial layer of silicon dioxide in the form of a solid mask allows for sample cleaning in various solvents, prevents etching fluid from leaking between the copper film layer and the resist due to poor adhesion, and also makes it possible to use fluorine-containing plasma in the fabrication process. This method is undemanding of the alkaline and acidic solutions used in the process, and is also undemanding in the selection of the photoresist. An additional advantage of the presented technology is the ability to use direct deposition.

[0049] Literature

[0050] 1. United States Patent 5,260,225, Integrated infrared sensitive bolometers, M. Liu, J. Haviland, C. Yue, date Nov. 9.1993.

[0051] 2. United States Patent 5,627,112, Method of making suspended microstructures, W.Tennant, I.Gergis, C.Seabury, date May 6,1997.

[0052] 3. United States Patent 5,777,328, Ramped foot support, R.Gooch, date Jul. 7, 1998.

[0053] 4. United States Patent 5,777,329, Bolometer array spectrometer, G.Wectphal, M.Wadsworth, date Jul. 7, 1998.

[0054] 5. United States Patent 5,841,137, Duplicative detector sensor, J.Whitney, date Nov. 24,1998.

[0055] 6. United States Patent 5,962,909, Microstructure suspended by a microsupport, H.Jerominek, M.Renaud, N.Swart, date Oct. 5,1999.

[0056] 7. United States Patent 7,262,413 B2, Photoconductive bolometer infrared detector, C.Kauffman, S.S. Yoo, T.Beystrum, date Aug. 28, 2007.

[0057] 8. D.Neikirk, D.Rutledge, Air-bridge microbolometer for far-infrared detection, Appl. Phys. Lett., 44 (2), pp. 153-155 (1984).

[0058] 9. A. Timofeev, P. Helisto, L. Gronberg, A. Luukanen, H. Seppa, J. Hassel, Suspended tunnel junction bolometers for Terahertz range, The Open Applied Physics Journal, 2012, 5, 34-40.

[0059] 10. T.Li, Yu.Pashkin, O.Astafiev, Y.Nakamura, J.Tsai, H.Im, Low-frequency charge noise in suspended aluminum single-electron transistors, Appl. Phys. Lett., 91, 033107 (2007).

[0060] 11. M. Tarasov, A. Chekushkin, R. Yusupov, Method for manufacturing devices with freely hanging microbridges, patent for invention No. 2632630. Positive decision dated 08 / 25 / 2017 2 632 630.

Claims

A method for manufacturing a detector with thin-film tunnel junctions and a suspended absorber, which includes applying a thin-film superconductor-insulator-normal metal-insulator-superconductor (SINIS) structure with a suspended absorber made of copper film onto a dielectric substrate without breaking the vacuum, characterized in that the SINIS structure with a suspended absorber is formed in a single vacuum cycle on the surface of a dielectric substrate by sputtering a layer of aluminum, which is oxidized in oxygen to form an insulating layer of aluminum oxide, on top of which a copper film and a sacrificial layer of silicon dioxide are sputtered; by means of a first photolithography with a single-layer photoresist, a suspended absorber is formed from a copper film by etching a sacrificial layer of silicon dioxide in a fluorine-containing plasma until a hard mask is formed for etching the copper film, an insulating layer of aluminum oxide and an aluminum layer; then, the copper film is etched in an acid solution, and the insulating aluminum oxide layer and the aluminum layer are etched in an alkaline solution, after which, by means of a second photolithography, the superconducting transition areas of the superconductor-insulator-normal metal type are formed by etching the sacrificial silicon dioxide layer in a fluorine-containing plasma until a hard mask is formed for etching the copper film; then, the copper film is etched in an acid solution; after which a suspended absorber is formed from a copper film by etching the sacrificial layer of aluminum in a fluorine-containing plasma and etching the sacrificial layer of silicon dioxide until it becomes possible to etch the copper film.

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