WO1989003125A1

WO1989003125A1 - A process for producing an electric circuit including josephson diodes

Info

Publication number: WO1989003125A1
Application number: PCT/DK1988/000156
Authority: WO
Inventors: Ib Johannsen
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-09-22
Filing date: 1988-09-22
Publication date: 1989-04-06
Anticipated expiration: 1990-03-22
Also published as: AU2533088A; DK498087A; DK160382B; DK498087D0; DK160382C

Abstract

A process for producing an electronic circuit, such as an integrated circuit, including a storage circuit comprising Josephson diodes, from a material including metal oxides which during heating can be rendered superconducting by introducing oxygen and rendered insulating by introducing an inert gas. The heating occurs locally and is performed by means of a focused beam also used for trimming.

Description

Title: A process for producing an electric circuit includ¬ ing Josephson diodes

Technical Field

The invention relates to a process for producing an elec- tronic circuit, such as an integrated circuit including Josephson diodes from thin layers of metal oxides which can be rendered superconducting.

Background Art

It is known from the article "The heat is on for supercon- ductors" in New Scientist dated May 7, 1987 to use metal oxides as superconducting materials . According to the article the wiring patterns in the superconducting mate¬ rials are to be provided by conventipnal electron beam vaporization and resistance techniques. -

Furthermore US-PS No. 4,536,781 describes how Nb can be rendered locally superconducting by oxygen ion implanta¬ tion by means of conventional lithographic techniques and a broad irradiation of the material being employed.

Disclosure of Invention

By the process according to the invention the conductive paths of an electronic circuit are provided by a chemical process induced by a local heating by means of a focused irradiation in a controlled atmosphere, whereby oxygen is partly removed from the metal oxides otherwise supercon- ducting at low temperatures to form insulating barriers/areas, whereby the desired wiring patterns are provided.

The controlled atmosphere may for instance be F2 , H2 or CO Brief Description of Drawings

Fig. 1 illustrates a process for producing an integrated circuit from a material which can be rendered superconduct¬ ing,

Fig. 2 is a top view of a superconducting film in form of a Josephson diode,

Fig. 3 is a top view of a SQUID (Superconducting Quantum Interf rence Device) ,

Fig. 4 illustrates an amplified Josephson effect, where a plurality of Josephson diodes are arranged in series,

Fig. 5 an arrangement for providing a controlled transfor¬ mation of a superconductor, and

Fig. 6 illustrates the arrangement of Fig. 5 on a larger scale .

Best Mode for Carrying Out the Invention

In response to the oxygen content a film of for instance YBa2Cu3θ_χ turns superconducting at a temperature below 90 K. It has been demonstrated that the materials can be reversibly transformed between a superconducting phase including a high content of oxygen and an insulating or semiconducting phase including a low content of oxygen by heating the film in an atmosphere of pure oxygen or He, respectively. It has furthermore been demonstrated that the transformation occurs very quickly, i.e. in less than 5 sec. Consequently, these materials include very loosely positioned oxygen atoms entering and leaving the lattice very quickly at a high temperature, i.e. above 400°C. As far as YBa2Cu3θ_χ is concerned x. varies between six and seven during this treatment. The transformation occurs irrespective of the . type of heating involved. Neither are significant differences apparent with respect to the used type of deoxidized gas, provided said gases are inert (He, Ar , N2 etc.) . The use of other reactive gases for modifying the material seems, on the other hand, to be of importance. It has for instance been demonstrated that fluorine can enter the lattice and provide even higher transition temperatures for the superconductivity.

The film used can be manufactured in many different ways.

1) Cathode sputtering of the starting materials or the completed ceramics,

2) electron beam vaporization of the metals either sepa¬ rately or together from an alloy,

3) by means of molecular beam epitaxi,

I 4) by means of plasma spraying,

5) by means of serigraphic techniques, or

6) by means of tape-casting .

The resulting film is of a quality varying from a single- crystal film of a thickness of less than 10 μ to a micro- crystal layer of a thickness of 50-200 μm. Depending on the manufacturing process the film exhibits a superconductivity at a low temperature or is merely semiconducting or insu¬ lating .

The principle is based on a local change of the composition of a film including metal oxides/fluorides with respect to the transformation between

superconducting - insulating superconducting - semiconducting superconducting - conventional, metallic conductivity one type of superconductivity - another type of supercon¬ ductivity.

The transformation can be provided by a local heating by means of for instance a focused laser beam, electron beam or the like in the presence of either

an oxydizing gas (O2, O3 , F2 , CI2 a reducing gas (H2 , CO, SiH^.) an inert gas (He, Ar, N , N2, CO2) or vacuum a different reactive gas (CF₃C1, C2F4CI2, SiCl4, Fe(C0)g, Ni(C0)₆, BF3, H₂S) or

through direct bombardment with neutral or ionized irra¬ diation, such as ion -beam implantation.

Under these circumstances the great mobility of the oxygen atoms in the material and the free positions in the lattice are also employed. The irradiation can be performed by means of neutral atoms Ar , He etc. during a "mechanical" exhaustion of for instance oxygen from the lattice or ionized irradiation in form of low-weight ions (0^"", FT_. H⁺, C0⁺) or heavy-weight ions (Cu⁺⁺, La⁺⁺⁺ etc.), the latter being part of the lattice, if necessary.

A combination of these techniques enables the production of all types of electrical components from superconducting components to insulating components from the same starting ilm.

Example

The raw powder is produced by co-precipitating Cu⁺⁺, Y⁺⁺⁺,

Ba⁺⁺, as carbonates with K2CO3. The washed precipitate is dried and calcinated at 900°C for 6 h. The precipitate is crushed and screened into a fine powder subsequently plasma sprayed (He/Ar plasma) onto an aluminium oxide sub¬ strate. The resulting film of a thickness of approximately 50 μτa. is insulating/semiconducting. It can be rendered superconducting at 70 K by heating in pure oxygen (to 900°C) and rendered semiconducting again by heating to more than 400^βC in for instance He. The process is com¬ paratively quick due to the great mobility of the oxygen atoms at these temperatures. According to the invention the above heating is preferably carried out by means of a focused laser beam.

A short-wave laser (VIS, UV) renders it possible to focus in very small areas (D approx. 500 nm) and thus performing a locally defined heating. In an atmosphere of for instance He the oxygen diffuses out of a superconducting ceramics when the .temperature is sufficiently high, i.e. above 400"C. A simple arrangement is illustrated in Fig. 1. The laser beam or the sample is moved to form the desired wiring patterns. Typical patterns include Josephson diodes of superconducting film separated by an insulating barrier. An amplified Josephson effect is achieved by means of several Josephson junctions in series, cf. Fig. 4.

Fig. 5 illustrates an arrangement for a controlled trans¬ formation of a superconductor. The arrangement can also be used at a low temperature in such a manner that the super¬ conductivity and optionally Josephson effects are directly measured during the formation of the barrier. A cooling is performed on the back of the sample, for instance to 77 K while either vacuum or He is maintained above the sample. Such a technique may be used for trimming Josephson diodes, which constitutes an essential feature. One of the main reasons for the delay in developing supercon¬ ductors based on Josephson diodes is that it has not been possible to match the diodes sufficiently, which is very important because the Josephson diodes are not amplifying per se. Fig. 5 shows in addition that these metal oxide- based films can be used for several different components, and for instance SQUIDs or integrated circuits can be produced by this technique.

Preparation

In a specific example the plasma spraying has been used as applying method. By such a. technique a sintered powder of Ba2Cu3θ_χ is led into the flame/the plasma, whereby the particles are heated and accelerated onto a substrate. It has been demonstrated that a coherent film can be provided on several different substrates I2O (β-alumina sapphire), Si (single- and poly-crystal) glass, quarz , copper, aluminium, steel etc. The present tests use in particular substrates of AI2O3 in form of plates of a thickness of 0.5-1.0 mm of the type used for the prepara¬ tion of thick-film circuits. The resulting films are not particularly even. The thickness of the layers varies from 50 to 100 μm . The material is not superconducting after the plasma spraying and has probably lost some oxygen. By this technique a good contact between the crystals is not necessarily ensured. Nevertheless a superconducting film can be achieved at a critical temperature of about 70 K by a careful heating of the produced films. The process comprises the steps of the film being quickly heated in pure oxygen to 900°C, kept at this temperature for about 1 h, and subsequently slowly cooled (approximate¬ ly 100-200° C/hour) . The- plasma spraying has not been op¬ timized yet. Beyond the plasma spraying, which is a coarse technique resulting in relatively thick and uneven films, other film producing techniques are involved too:

Cathode sputtering (DC, AC/RF, Magnetron) based on targets from the individual starting metals, from an alloy of starting metals, from the oxides, and from the completed ceramics . Superconducting films of a good quality have been achieved.

Electron beam vaporization especially based on targets from the individual metals (Y, Ba, La, Cu etc.) layered and followed by heating in oxygen. This technique result- ed in the best ilms.

Molecular beam epitaxi has also been used.

The quality of the produced films varies from single- crystal and very thin layers to the above plasma- sprayed films .

Finally spinning and tape-casting should also be mentioned, said techniques having been used for the preparation of sheets/tapes of the material as a layer atop a carrier material. The tapes can subsequently be shaped as parti¬ cular articles, whereafter they are burnt to form a super- conducting film.

Serigraphic techniques (thick-film techniques) . A Japanese group has reported on successful experiments with pre¬ paration of superconducting paths by serigraphy.

The process according . to the invention is applicable in connection with all these types of film and layers of oxide superconductors .

Examples of oxide superconductors are

^Lal,8-±0,1 ^Ba0,2±0,l ^Cu°4

^{• La}l,85-±0,1 ^Sr0,15±0,l ^CU°4

where M is a trivalent ion, such as Sc, La, Sm, Gd, Ho, Er, Tb , Pr, Yb , ... or combinations thereof or BaB i„Pb , _n . 0 -3 . ^x .( 1 -x) ^'

All these superconductors are characterised by having a slightly too high oxidation step on the metals. For inst¬ ance Cu has usually the oxidation step 1 or 2, and in the present compounds approximately 2.3 applies. In other words copper has a tendency to involve its usual oxidation step 2.0 and thereby to release oxygen from the unit cell (and to remain insulating) .

The latter so-called 1, 2, 3 ion compounds are furthermore characterised by comprising a high number of vacancies in the crystal lattice. The vacancies can probably be filled with small ions, such as fluorine (ion implantation).

Fig. 6 illustrates how each Josephson diode is trimmed. The current-voltage-characteristics is simply measured, where it is particularly desired to measure the so-called "tunnel voltage". The trimming per se is then carried out by one or more scannings by means of the irradiation. It is simultaneously possible to vary the intensity of the irradiation. As a result it is possible to vary the geo- metric dimensions, including the width of the barrier and consequently the tunnel voltage.

Claims

Claims :

1. A process for producing an electronic circuit, such as an integrated circuit including Josephson diodes from thin layers of metal oxides which can be rendered supercon- ducting, c h a r a c t e r i s e d in that the conductive paths of the electronic circuit- are provided by a chemical process induced by a local heating by means of a focused irradiation in a controlled atmosphere, whereby oxygen is partly removed from the metal oxides otherwise supercon- ducting at low temperatures to form insulating barriers/areas.

2. A process as claimed in claim 1, c h a r a c t e r¬ i s e d by using an inert gas, such as He, Ar , N2 or vacuum as controlled atmosphere.

3. A process as claimed in claim 1 or 2, c h a r a c¬ t e r i s e d by using a reactive gas, such as F2 , H2 , CO as controlled atmosphere.

4. A process as claimed in any one of the preceding claims 1 to 3 , c h a r a c t e r i s e d by using a laser beam as focused irradiation.

5. A process as claimed in any one of the preceding claims 1 to 3 , c h a r a c t e r i s e d by using an electron beam as focused irradiation.

6. A process as claimed in any one of the preceding claims 1 to 5 , c h a r a c t e r i s e d by locally heating to more than 400°C.

7. A process as claimed in any one of the preceding claims l to δ, c h a r a c t e r i s e d by further using the focused irradiation for trimming.

8. A process as claimed in any one of the preceding claims 1 to 7 , c h a r a c t e r i s e d in that the resulting insulating area between the two areas supercon¬ ducting at low temperatures is very narrow, whereby a Josephson diode is formed and the focused irradiation in addition can be used for trimming the Josephson diode.