Background Art US-PS No. 5,612,290 discloses a method of the above type, where the two horizon- tal planes are connected with one another through a plane presenting an inclination of 5 to 30°. A grain boundary of the oxide superconductor is formed in the contact area between the inclining planes. Such a grain boundary corresponds in the classic sense to a non-conducting joint surface between superconductors. Such a joint surface can be utilized for the manufacture of a Josephson diode because it is possible to utilize the quantum mechanical tunnel effect provided the joint surface is very thin.

When the above inclination exceeds 30°, a substantially stepped junction is formed whereby various parameter values,-such as a critical current, are not reproducible and perhaps even unstable apparently because the grain boundaries become unstable.

The above technique is usually referred to as the step-edge-technology and has for

instance been described in the article"Reproducible Step-Edge Junction SQUIDs" by Y. Q. Shen, Z. J. Sun, R. Kromann, T. Holst, P. Vase and T. Freltoft in IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY. Vol. 5, No. 2, June 1995. The article teaches that a step is etched into the substrate in which the desired diode is to be arranged. Subsequently, a superconductive film is deposited epitaxi- ally. As the step is etched in such a manner that the ab-plane of the film also grows in parallel on the surface of the slope of the step, an edge is formed in the ab-plane of the film at the rim of the step. If the rim is sharp, a well-defined grain boundary is formed at the edge. However, it is often difficult to produce reproducible step-edge-diodes. One reason may be that the rim easily turns blunt during the etch- ing or during the deposition of the film, and it can be difficult to produce a well-de- fined grain boundary on a blunt rim, cf. Fig. 1.

Brief Description of the Invention The object of the invention is to improve the step-edge-technology in such a manner that compared to previously a more well-defined grain boundary is obtained even when the inclination of the inclining plane exceeds 30°.

A method of the above type is according to the invention characterized in that a buffer layer is initially applied so as to provide the inclining plane, viz. the step-edge in the substrate, whereafter a pattern of mask layers are applied onto the buffer layer in such a manner that a rim of the pattern is arranged at the desired position of the inclining plane, viz. the step-edge, and whereby the inclining plane is finally etched through said buffer layer. The step is etched as deep into the substrate as in the situation where no buffer layer applied. The sharp edge on the step in the substrate is then protected by the buffer layer during the etching.

Moreover, the buffer layer may according to the invention be formed by a metal-oxi- de superconductor. Furthermore, a metal-oxide superconductive layer may according to the invention be initially deposited in a thickness of approximately 10 nm at a temperature of 680 to 720° C and then deposited approximately 100 nm at a temperature of approximately 750 to 800°C.

Moreover, the mask layer may include UV-resist, and the pattern may be formed by means of UV-lithography.

Brief Description of the Drawing The invention is explained in greater detail below with reference to the accompanying drawing, in which Fig. 1 illustrates a so-called step-edge in a substrate, Fig. 2 illustrates a method according to the invention for providing a step-edge with a sharp edge, Fig. 3 shows a SQUID provided by means of the method according to the invention, Fig. 4 shows the current-voltage characteristics of the SQUID of Fig. 3, Fig. 5 shows an assembly for providing a step-edge with a sharp edge in a substrate, and Fig. 6 illustrates how the etching is carried out by means of an Ar-ion beam.

Best Mode for Carrying Out the Invention The laser ablation assembly shown in Fig. 5 comprises a deposition chamber 1 and

a frequency-tripled Q-switched Nd: YAG laser 2 arranged outside the deposition- chamber and generating a wavelength of 355 nm and a pulse length of 5 ns. A sub- strate is placed in the deposition chamber 1. The temperature of the substrate 8 is measured by means of a thermoelement. The temperature can furthermore be mea- sured by means of an external pyrometer 18. The substrate 8 is heated by means of a halogen bulb 12 focussed at the end of a quartz rod 14 inside the chamber 1. How- ever, the halogen bulb 12 is placed outside the chamber 1. The quartz rod 14 serves to transmit the light from the halogen bulb 12 to the substrate holder. A more de- tailed description of the deposition chamber appears from the article"Deposition, characterization, and laser ablation patterning of YBCO thin films"by Per Vase, Shen Yuenqiang and Torsten Freltoft in Applied Surface Science 46 (1990) 61-66, North Holland.

Single crystalline MgO, 10 mm x 10 mm x 0,5 mm, < 001 > oriented and burnished on the front side is for instance used as substrate 8. This substrate is available for instance from Crys Tec GmbH, Köpenicker Str. 525, D-12555 Berlin. The substrate 8 is placed in the holder inside the deposition chamber 1 and heated by means of the halogen bulb 12 to 680 to 720°C, preferably 700°C. The pressure inside the chamber 1 must be between 0.1 and 1 mbar, preferably 0.33 mbar. Subsequently a layer of YBCO of a thickness of approximately 10 nm is deposited on the substrate 1. The substrate 8 and the deposited layer are then heated by means of the halogen bulb 12 to 750 to 800°C, preferably 780°C. Then a layer of YBCO of a thickness of approxi- mately 100 nm is deposited. This layer 20 serves as buffer, cf. Fig. 2. Immediately following the latter deposition, the chamber 1 is filled with 1 atm oxygen, whereafter the substrate 8 is cooled to 600°C at a rate of 200°C per min. Subsequently, the substrate 8 is further cooled to room temperature at a speed of approximately 10°C per min. or at a lower speed. The-YBCO layer is grown epitaxially on the MgO substrate 8 and with its CuO plane parallel to the surface of said substrate 8.

It is possible to use SrTiO3, LaAl2O3 or other substances as buffer instead of YBCO.

Positioning of step-edge The step-edge is positioned by placing a pattern of mask layers, for instance a square, on the buffer layer 20 in such a manner that one of the edges of the pattern is placed at the desired position of the step-edge 21. The mask layer includes UV-resist, and the pattern is formed by means of UV-lithography. The UV-resist is for instance of the type AZ 5214 available from Hoechst. The photomask is for instance available from Align-Rite Ltd., 1 Technology Drive, Bridgend Science Park, Bridgend, Mid Glamorgan CF 31 3LU, UK. The exposure is carried out by means of a mask-aligner of the type NJB3 available from Karl Suss.

As an alternative, the mask layer can include other materials, such as e-beam resist, coal or metal. The only condition is that these materials are removable without damaging the substrate 8 and the buffer layer 20. Other types of lithography, such as e-beam lithography, can also be used for producing the pattern.

Etching of step-edge The substrate 8 with the buffer layer 20 and mask are placed in an Ar-ion etching chamber. Inside the Ar-ion etching chamber, an Ar-ion gun of the type IS 300 avail- able from Anateck Ltd., 6621-F Electronic Drive Springfield, VA 22151 USA for generating an Ar-ion beam.

The direction of incidence of the Ar-ion beam towards the substrate 8 with the buffer layer 20 must be perpendicular to the edge 22 defining the step-edge 21, and it must present an angle of approximately 45'or less relative to the mask pattern, cf. Fig. 6. The Ar-ion beam is accelerated at 450 V and presents a current intensity of ap- proximately 10 mA.

Following the Ar-ion etching for approximately 180 min in the ion etching chamber,

the portion of the substrate 8 with the buffer 20 not protected by the mask pattern has been reduced by approximately 400 nm, and a step is formed at the rim of the pat- tern. The approximately 100 nm thick YBCO buffer layer 20 has now been removed, and the step on the substrate 8 presents a height of approximately 300 nm. The edge 22 of the step 21 presents a flat slope with an angle of less than 45 °. However, this flat slope applies only to an MgO substrate or the like in case it is a question of NaCl crystal. When other substrates of for instance SrTiO3, LaAl2 O3 are used, the slope must be more steep and present an angle exceeding 70°.

Other etching methods, such as RIE or chemical etching, can also be used provided the resulting slope has the desired angle.

Remaining resist material is removed by means of reactive oxygen plasma in a Plasma-Preen assembly available from Plasmatic Systems Inc., 1327 Aaron Road, North Brunswich, NJ 08902 USA.

Above it has been described how a Josephson diode can be provided on the basis of a substrate with two horizontal planes an an inclining plane therebetween. Nothing, however, prevents the use of only one horizontal plane and an inclining plane pro- vided a sharp edge has been formed between the two planes in such a manner that a well-defined grain boundary is formed after deposition of the metal-oxide superconductive film.

The embodiment involving two horizontal planes does not necessarily include a sharp edge at the bottom. The deposited superconductor is always parallel to the surface of the substrate, and the ab-plane of said superconductor form a number of small edges at the adjacent round rim. In practice these small edges do not matter for the function of the completed Josephson diode.

The step-edge technique according to the invention can furthermore be used for the manufacture of SQUIDs 23, cf. Fig. 3 and the associated current-voltage characteris- tics shown in Fig. 4. A SQUID 23 is used for measuring magnetic fields.

During operation, these SQUID's 23 present a temperature corresponding to the temperature of liquid nitrogen, viz. 77°K.