RU2757762C1 - Method for manufacturing thin-film tunnel junctions by the method of separate lithography - Google Patents

Abstract

FIELD: microelectronics.

SUBSTANCE: invention relates to the technology of microelectronics and can be used in the manufacture of thin-film superconducting structures. The method for manufacturing thin-film tunnel junctions by the method for separate lithography includes the formation of the first layer of aluminum by the first lithography, the application of a resist under the second lithography of the second metal layer, exposure in the lithograph, the development of the resist, the formation of a tunnel barrier and the deposition of the second metal layer. Before the formation of the tunnel barrier, the natural oxide is removed and the surface of the first layer of aluminum is cleaned by ion or plasma etching, and the tunnel barrier is formed on the surface subjected to said etching.

EFFECT: simplification of manufacturing technology that does not require chemical etching and reduction of requirements for the choice of materials for layers of normal metal, as well as the formation of a high-quality tunnel barrier of arbitrary controlled transparency.

6 cl, 8 dwg

Description

The invention relates to the technology of microelectronics and can be used in the manufacture of thin-film superconducting structures.

It is known that highly sensitive detectors of microwave and EHF radiation are based on the use of thin-film tunnel junctions, structures of the superconductor-insulator-superconductor (SIS) type, structures of the superconductor-insulator-normal metal (SIN) type, structures of the normal metal-insulator-superconductor (NIS) type , structures of the normal metal-insulator-normal metal (MIM) type.

A method of manufacturing superconducting tunnel junctions and single-electron transistors by the Dolan method with shadow deposition at two angles through a suspended mask of electronic resist is described (G.J. Dolan, Appl. Phys. Lett. 31, 337-339, 1977). A two-layer mask is used to spray two layers of metal at different angles. Oxidation of the lower aluminum layer during the manufacturing process makes it possible to obtain, in particular, high quality Josephson tunnel junctions. The disadvantage of this method is the presence of a suspended resist bridge, which reduces the reproducibility and reliability of manufacture and does not allow ionic cleaning of the substrate before the deposition of films.

A method of manufacturing devices with thin-film tunnel junctions is described (RU 2733330 C1, Kotelnikov Institute of Radioelectronic Power Engineering, Russian Academy of Sciences, 01.10.2020). The essence lies in the deposition of two different layers of metal films in one vacuum cycle and without using the complex structure of a two-layer electronic resist with a suspension bridge made of this resist. The result is achieved by sputtering the film onto the substrate in the desired direction along the corresponding groove and spraying it onto the wall of the second groove, followed by removal along with the resist. The tunnel barrier is formed by oxidation of the first sprayed aluminum film. As a result, only the required superconducting tunnel junction of the required dimensions with the upper and lower electrodes formed along the respective orthogonal grooves in the resist remains on the substrate. In contrast to the previous analogue, this method has a higher productivity and the ability to carry out ionic cleaning of the substrate before the deposition of films. However, this technology is not suitable for industrial production and requires complex technological equipment for spraying films with the possibility of tilting and rotating the substrate inside the working chamber during the spraying process.

A method of manufacturing an SIS of a three-layer Gurvich structure, which is formed in a single vacuum cycle through a window in a photoresist, is described (M. Gurvitch, High quality refractory Josephson tunnel junctions utilizing thin aluminum layers Appl. Phys. Lett., 42, 472-474, 1983). The essence of the technology consists in the deposition of the first layer, oxidation in the same working chamber at certain parameters, pumping out and deposition of the second layer of the structure in one vacuum cycle. This method has become widespread, since the manufactured tunnel passages are of high quality, and there is no need for complex technological equipment for spraying. The disadvantage of this method is the same shape of the upper and lower superconductor layers, which requires additional technological steps when creating complex circuits, the need to anodize the ends of the films in the case of niobium, poor compatibility with aluminum-aluminum transitions dissolving in an alkaline developer for a photoresist.

A known method of manufacturing an ICS tunnel junction in two vacuum cycles (GM Lapir, NI Komarovskikh, Electronic Industry, No. 6, 64, 1973). The essence of the technology is as follows: reverse lithography, deposition of the first metal layer, removal of the resist; re-lithography, cleaning, oxidation, deposition of a second metal film, removal of the resist. The disadvantage of this method is the low quality of the tunnel barrier due to the violation of the vacuum before applying the upper layer of the tunnel junction.

A known method of manufacturing devices with thin-film superconducting transitions (RU 2593647 C1, FGBUN IRE named after V.A.Kotelnikov RAS, 08/10/2016). A three-layer superconductor-insulator-normal metal structure (BLUE contact) is applied without breaking the vacuum; apply resist, conduct exposure, manifestation; selective chemical or ionic etching of the three-layer structure, after etching the three-layer structure, the surface is planarized by sputtering through a dielectric mask with a thickness equal to the thickness of the three-layer structure, after which the dielectric is removed outside the region of tunnel junctions and a thin film of a bridge (absorber) is applied from a normal metal or other superconductor, when In this case, this layer of the bridge is applied to the planarized surface and can be significantly thinner than the previous layers, less than 10 nm. The disadvantages of this method include the need to use different metals with selective etching in acids and alkalis and the low quality of such structures.

The closest to the patent is a method of manufacturing tunnel junctions with a titanium oxide barrier (E. Otto, M. Tarasov, L. Kuzmin. Ti-TiO2-Al normal metal-insulator-superconductor tunnel junctions fabricated in direct-write technology. Supercond. Set Technol. 2007, v. 20, 865-869 - prototype). The essence of the technology consists in the following operations: deposition of a titanium film on a substrate, lithography, chemical etching of titanium, removal of resist, purification of a titanium film in oxygen plasma and ion etching, oxidation of a film in air for 10 hours at a temperature of 150 ° C, deposition of an aluminum film, lithography, chemical etching of aluminum, removal of resist.

The disadvantages of this method are that the temperature and oxidation time are too high to create a titanium oxide barrier, the resistance of such a tunnel barrier is too high, and the need for chemical etching.

The present invention is aimed at solving the problem of creating a high-quality tunnel junction (structures of SIS, SIN, NIS or MIM) of arbitrary shape and area by the method of direct separate lithography.

The patented method of manufacturing thin-film tunnel junctions by the method of separate lithography includes the formation of the first layer of aluminum by the first lithography, the application of a resist for the second lithography of the second metal layer, exposure in the lithograph, the development of the resist, the formation of a tunnel barrier and the deposition of the second metal layer.

The difference is that before the formation of the tunnel barrier, the natural oxide is removed and the surface of the first layer of aluminum is cleaned by plasma or ion etching, and the tunnel barrier is formed on the surface subjected to said etching.

The method can be characterized in that the removal of a thick natural oxide is carried out by plasma or ion etching, as well as in that during the formation of the superconductor-insulator-superconductor (SIS) structure, the second metal layer is an aluminum layer.

The method can also be characterized by the fact that during the formation of the SIN structure, a layer of normal metal is applied over an additional sublayer of aluminum, and the thickness of the normal metal layer exceeds the thickness of the aluminum layer.

In addition, the method can be characterized by the fact that during the formation of the MIM structure, an additional sublayer of normal metal is applied under the first layer of aluminum. The method can also be characterized in that the surface of the first layer of aluminum is subjected to nitridation, and the tunnel barrier is formed on the specified surface.

The technical result is a simplification of the manufacturing technology, the use of industrial technologies of mass production that do not require a complex technological installation for shadow spraying, which does not require chemical etching, and a decrease in the requirements for the choice of materials for layers of normal metal, as well as the formation of a high-quality tunnel barrier of arbitrary controlled transparency.

The patented method is based on a technology that implements the deposition of two metal films with a tunnel barrier between them in two vacuum cycles, without the use of complex technological equipment for spraying, and also without the use of chemical etching. There is no need to tilt and / or rotate the substrate inside the working chamber. Optical or electronic lithography, carried out in two technological cycles, as well as the use of additional ionic or plasma etching to remove the oxide that appears due to the violation of the vacuum after the deposition of the first film, makes it possible to create a tunnel junction of arbitrary shape and area. Oxidation of the aluminum surface cleaned after etching allows the creation of a high quality tunnel junction. Separate lithography of the upper and lower layers allows the formation of structures of any shape.

The essence of the invention is illustrated in the figures, where the positions indicate: 1 - silicon substrate; 2 - supply electrodes; 3 - the first layer of the resist; 4 - the second layer of resist; 5 - first layer of metal, 6 - resistive mask; 7 - the second layer of metal; 8 - tunnel crossing.

Fig 1. Silicon substrate (1) and the manufactured lead electrodes (2), with the help of which contact with the tunnel structure will be made.

FIG. 2. Top view: a mask made of a two-layer resist for sputtering the first layer (only the top layer 3 of the PMMA resist is visible in the figure).

FIG. 3. Three-dimensional image in section AA (Fig. 2) layers 3, 4 resist (PMMA (3) and MMA (4)) after exposure and development.

FIG. 4, 5 - the first layer of metal (5): top view and three-dimensional image.

FIG. 6 - top view: resistive mask (6) for the deposition of the second layer of metal (7). In the figure, only the top layer of the PMMA mask (6) is visible. Before the deposition of the next layer, ionic etching of the oxide formed due to the rupture of the vacuum after the deposition of the first metal layer 5 is carried out.

FIG. 7 is a top view of the sprayed structure after removing the resistive mask, the first layer of metal (5); the second layer of metal (7). In the region of intersection of metal layers 5 and 7, a tunnel junction is formed (8).

FIG. 8. - Three-dimensional image of the tunnel junction (8) after removal of the resist.

The manufacturing process of devices with thin-film tunnel junctions, consisting of two layers of metal, separated by a tunnel barrier, is carried out as follows.

1. On the surface of the dielectric substrate 1, the first layer 4 of the resist (for example, copolymer MMA) is applied and baked (at a temperature of 130-170 ° C for 5 min) to create a negative resist profile for the first layer of the structure.

2. Apply the second layer 3 of the resist (for example, PMMA brand) and bake it (at a temperature of 130-170 ° C for 5 minutes) to form the topology of the first layer.

3. Exposure of the resist is carried out with overexposure of the areas in which the resist will be removed to form the topology of the first layer.

4. The two-layer resist is developed.

5. Spray the first layer 5 of the metal:

- superconductor (for example, aluminum) for SIS and SIN transitions;

- normal metal (for example, copper) for NIS and MIM transitions.

6. The two-layer resist is removed together with the metal layers deposited on it.

7. The substrate is cleaned, a two-layer resist is applied, an exposure is carried out, and a two-layer resist is developed (similarly to items 1 - 4) to form the topology of the second layer.

8. Plasma or ion etching is carried out (200 W, 4.5 min, post, displacement 900 V, argon pressure 6 × 10 ^-3 ) to remove the oxide layer arising from the violation of the vacuum after the deposition of the first layer 5, as well as removal adjacent thin layer 5 of the first metal.

9. Tunnel passage 8 is formed by carrying out the following operations:

- oxidation in a spray chamber (pressure 10 Torr, 10 minutes) - for SIS and SIN tunnel junctions;

- dusting of a thin layer of aluminum (~ 5 nm) and its oxidation (pressure 10 Torr, 10 minutes) - for NIS and MIM tunnel junctions,

10. Spend the spraying of the second layer 7 of the metal:

- superconductor (for example, aluminum) - for SIS and NIS tunnel junctions;

- normal metal (for example, copper) - for SIN and MIM tunnel junctions; a buffer metal layer is preliminarily deposited (for example, palladium ~ 5 nm thick),

11. At the final stage, the two-layer resist is removed together with the metal layers deposited on it to obtain the desired tunnel junction.

The technical result of the proposed solution is to achieve the set goals - the creation of high-quality tunnel passages (SIS, SIN, NIS, MIM) of arbitrary shape and area without the need to use complex technological equipment for shadow spraying. Direct lithography is the most reliable method and can be used to fabricate structures on an industrial scale. Ion etching is carried out before sputtering the second tunnel junction film. For structures like SIN and MIM, a palladium buffer layer can be used.

The achievement of the technical result is confirmed by the implementation of superconducting quantum interferometers (SQUIDs) with Josephson tunnel junctions Al / AlOx / Al with a resistivity of 1 kOhm per 1 sq. Km and a chain of up to 350 such SQUIDs, characterized by high accuracy of reproduction of the characteristic parameters of asymptotic resistance and critical current.

Claims (7)

1. A method of manufacturing thin-film tunnel junctions by the method of separate lithography, including the formation of the first layer of aluminum by the first lithography, the application of a resist under the second lithography of the second metal layer, exposure in the lithograph, the development of the resist, the formation of a tunnel barrier and the deposition of the second metal layer, characterized in that before the formation of the tunnel barrier, the natural oxide is removed and the surface of the first layer of aluminum is cleaned by etching, and the tunnel barrier is formed on the surface subjected to said etching. 2. A method according to claim 1, characterized in that the natural oxide is removed by plasma or ion etching. 3. The method according to claim 1, characterized in that when the superconductor-insulator-superconductor (SIS) structure is formed, the second metal layer is an aluminum layer. 4. The method according to claim 3, characterized in that during the formation of the superconductor-insulator-normal metal (SIN) structure, a layer of normal metal is applied over an additional sublayer of aluminum, and the thickness of the normal metal layer exceeds the thickness of the aluminum layer. 5. The method according to claim 3, characterized in that during the formation of the normal metal-insulator-normal metal (MIM) structure, a normal metal layer is applied under the first aluminum layer. 6. A method according to claim 1, characterized in that the surface of the first layer of aluminum is subjected to nitridation, and a tunnel barrier is formed on said surface.

Images