RU2107358C1 - Process of manufacture of high-temperature superconducting josephson junction - Google Patents

Abstract

FIELD: cryogenic microelectronics. SUBSTANCE: invention refers to manufacture of devices operating in superhigh frequency range with level of inherent noises approaching to quantum threshold which operation is based on phenomenon of high-temperature superconductance and Josephson effect with working temperature near boiling temperature of liquid nitrogen and characteristics compared favorably with characteristics of analogs operating under temperature of 4.2 K. Bicrystal is grown from melt of leucosapphire on to double seed that is manufactured from two monocrystal plates of triangular form cut from monocrystalline ingot of leucosapphire. After this section which plane is ground and chemically and mechanically polished is made on side surfaces of each plate and then plates are interjoined with planes of section in holder. Later double seed is placed into vacuum or inert gas atmosphere at melting temperature of leucosapphire, double seed is brought in touch with melt of leucosapphire and bicrystal is grown on to double seed. Bicrystal substrate is cut from it after cooling. Buffer layer on to which epitaxial high- temperature superconducting film is sprayed is deposited on to surface of substrate after chemical and mechanical polishing. EFFECT: improved efficiency of process. 2 dwg

Description

 The invention relates to cryogenic microelectronics and can be used for the manufacture of electronic devices and devices operating in the microwave frequency range, with a noise floor close to the quantum threshold, the operation of which is based on the phenomenon of high-temperature superconductivity and the Josephson effect, with an operating temperature near the boiling point of liquid nitrogen and characteristics not inferior to those of analogues operating at a temperature of 4.2 K.

Затем на всю поверхность наносят эпитаксиальную ВТСП-пленку YBCO-123 толщиной 1000. . . 2000

, поверхность которой ориентирована по плоскости (001). Буферный слой MgO разворачивает кристаллическую решетку эпитаксиальной пленки YBCO-123 на 45^o и формирует бикристаллическую границу между двумя половинами эпитаксиальной пленки. Это, так называемая, биэпитаксиальная технология изготовления джозефсоновских переходов.A known method of manufacturing a high temperature superconducting (HTSC) Josephson junction [1], which consists in obtaining loosely coupled Josephson junctions at the grain boundaries in HTSC films. These transitions are reliably and reproducibly formed on homogeneous plenary substrates using a sublayer located between the substrate and the superconducting film. The superconductor film is grown on top of the sublayer and is misoriented relative to the rest of the film by an angle between 5 and 90 ^o . The boundary thus formed acts as a high-quality superconducting transition. The characteristics of these transitions are improved by adding buffer layers between the substrate and the superconducting film. A flat crystalline substrate is made from materials such as strontium titanate SrTiO ₃ , leucosapphire Al ₂ O ₃ , yttrium stabilized zirconia, ZrO ₂ (Y ₂ O ₃ ), etc., and a buffer layer is applied to it - a thin MgO thick epitaxial film 200 ... 1000

Then, an YBCO-123 epitaxial HTSC film of 1000 thickness is applied over the entire surface. . 2000

whose surface is oriented along the (001) plane. The MgO buffer layer expands the crystal lattice of the YBCO-123 epitaxial film by 45 ^° and forms a bicrystal border between the two halves of the epitaxial film. This is the so-called biepitaxial technology for making Josephson junctions.

 The disadvantage of this method is the significant imperfection of the structure of the bicrystal boundary of the film in the region of the formed HTSC Josephson junction, and, as a result, this transition has a small characteristic voltage U, a low degree of reproducibility, and a large level of excess noise of the i / f type due to fluctuations critical current in the transition region, which does not allow the use of this method for the manufacture of devices operating in the microwave range.

. Поверхности блоков механически шлифуют и полируют. После этого блоки приводят в контакт и по методу твердофазного сращивания в режиме самодиффузии атомов сваривают, обеспечивая таким образом механическую прочность бикристалла, и изготавливают единый бикристалл. Из полученного таким образом бикристалла, кристаллографические оси [100] и [010] разных половин которого развернуты вокруг оси [001] на угол наклона 36,86^o относительно друг друга, вырезают бикристаллические подложки толщиной 0,5...0,8 мм с ориентацией поверхности (001). Поверхность подложек шлифуется, полируется. Для эпитаксиального роста ВТСП-пленки YBCO-123 осуществляют финишную химико-механическую полировку. Затем выращивают ВТСП-пленку толщиной 100...200 нм, наносят контактные площадки, применяют фотолитографию с последующим химическим травлением пленки для создания геометрии СКВИДа. Лазерной резкой на подложке с пленкой формируют отдельные дискретные приборы.A known method of manufacturing a thin-film HTSC SQUID magnetometer on a SrTiO ₃ bicrystalline substrate [2], which is the closest to the claimed method of manufacturing a Josephson HTSC transition by its technical solution. The known method consists in the fact that at the beginning of the single-crystal blocks of strontium titanate SrTiO ₃ make bicrystal. For this purpose, a single-crystalline ingot of strontium titanate is cut hit along the crystallographic planes (100). Then set the angle of cut equal to 18.43 ^o , going to the atomic plane (130) and

. The surfaces of the blocks are mechanically sanded and polished. After that, the blocks are brought into contact and, using the method of solid-phase splicing in the self-diffusion mode, atoms are welded, thus providing the mechanical strength of the bicrystal, and a single bicrystal is made. From the bicrystal obtained in this way, the crystallographic axes [100] and [010] of the different halves of which are rotated around the axis [001] by an inclination angle of 36.86 ^° relative to each other, bicrystal substrates 0.5 ... 0.8 mm thick are cut out surface orientation (001). The surface of the substrates is ground, polished. For epitaxial growth of the HTS film, YBCO-123 carries out the final chemical-mechanical polishing. Then, a HTSC film 100 ... 200 nm thick is grown, contact pads are applied, photolithography is applied followed by chemical etching of the film to create SQUID geometry. Laser cutting on a substrate with a film form separate discrete devices.

 In this way, a bicrystalline border is formed with different angles of inclination. The YBCO-123 epitaxial film growing on the surface of the substrate inherits the structure of the bicrystalline boundary of the substrate and forms its bicrystal boundary in the film, which is the weak-coupling region between two superconductors. In such HTSC films, photolithography can produce various electronic circuits with both passive and active elements. The active elements are Josephson junctions, which provide the required current – voltage characteristics, the bicrystal technology for making Josephson junctions is more efficient than the biepitaxial technology, but this requires a high-quality bicrystal substrate with a perfect boundary structure.

The disadvantages of this method of manufacturing a Josephson HTS transition is that the bicrystal boundary obtained by the solid-phase splicing method is strongly violated in its structure. Its imperfection is determined mainly by the relief of the spliced surfaces. When surfaces come into contact over the entire area of the boundary, a large number of microvoids are formed, which in turn are a drain of point defects. Microvoids break the connection between two crystal lattices and therefore the Josephson tunnel effect disappears in these places. Another type of defects of the bicrystal boundary obtained by solid-phase splicing is the violation of the stoichiometry of the composition resulting from the diffusion of atoms in the boundary region. Violations of stoichiometry are attenuated in crystals with a dense packing of atoms in the lattice, for example, in cubic crystals of SrTiO ₃ , MgO, ZrO ₂ (Y ₂ O ₃ ), etc. Therefore, such bicrystals are currently being fabricated and are used in the fabrication of HTSC Josephson junctions. However, their common drawback is the large dielectric loss of the radiation generated by the transition, which sharply limits their use in the microwave frequency range (100 ... 1000 GHz).

 The table shows the frequency characteristics (dielectric loss and tangential loss tg) of some materials.

The table shows that the most promising material for microwave electronics are leucosapphire bicrystals. The manufacture by the known method of the HTSC transition of Josephson from a material promising in microwave electronics - leucosapphire is impossible. That is, it is impossible to make a substrate from bicrystals consisting of light atoms, the spatial lattice of which contains free nodes, ensuring their movement along the crystal lattice, since in solid-phase splicing, the bicrystal boundary is saturated with light atoms, for example, in the case of Al ₂ O ₃ - aluminum atoms.

 During the epitaxial growth of an HTSC film, they diffuse into the film and bypass the transition boundary. At such a bicrystal boundary, it is impossible to fabricate a working Josephson junction.

In addition, the Josephson HTSC transition fabricated on a solid-phase splicing substrate has a low reproducibility of parameters, a high cutoff frequency f _from i / f type noise, and a low characteristic voltage U _c = I _c • R _n , where I _s - critical current, R _n - normal resistance, which leads to that the DC SQUID and the SQUID magnetometers fabricated on this high-Tc Josephson junctions, have a relatively high level of noise equivalent to the magnetic flux and low resolution of agnitnomu field.

Reported flaws:
the noise level i / t and the characteristic voltage value are largely determined by the quality of the technology of cross-linking the bicrystal border.

 In the known solution, a bicrystalline boundary is formed by solid-phase splicing, separating two single-crystal blocks with different orientations of the crystallographic axes, which, when epitaxially growing an HTSC film, forms the same bicrystalline boundary in it. Moreover, the quality of the bicrystal boundary of the film (structural perfection, plane, etc.) is completely determined by the quality of the boundary of the bicrystal substrate.

 The common essential features of the claimed technical solution and the well-known are the following: a bicrystal is formed from which a bicrystal substrate is cut, grinding, chemical-mechanical polishing of the substrate surface, deposition of an epitaxial high-temperature superconducting film, contact pads on the substrate, and then photolithography of its high-temperature superconducting film and its are carried out.

-плоскостью, а другая - с

-плоскостью, после чего на боковых поверхностях каждой из пластин выполняют срез, плоскость которого перпендикулярна

-плоскости, а угол α между плоскостью среза и боковой поверхностью пластин составляет 18,4 или 14, или 11,3^o, после чего плоскость среза каждой из пластин шлифуют, химико-механически полируют и соединяют пластины между собой плоскостями среза в держателе так, что

-плоскости пластин параллельны, а угол между

-плоскостями составляет 2α , затем двойную затравку помещают в вакуум или атмосферу инертного газа при температуре плавления лейкосапфира, вводят двойную затравку в соприкосновение с расплавом лейкосапфира так, что направления фронтов кристаллизации совпадают с

-направлениями кристаллических решеток пластин, производят последовательное наращивание бикристалла на двойную затравку, из которого после его охлаждения вырезают бикристаллическую подложку, ориентированную по

-плоскости, причем после химико-механического полирования на поверхность подложки наносят буферный слой, на который затем напыляют эпитаксиальную высокотемпературную сверхпроводящую пленку.Distinctive essential features of the claimed technical solution are the following: a bicrystal is grown from a leucosapphire melt for double seed, which is made from two triangular single crystal plates cut from a single crystal leucosapphire ingot so that one of the surfaces of each of the plates coincides with

-plane, and the other with

- a plane, after which a cut is made on the lateral surfaces of each of the plates, the plane of which is perpendicular

-planes, and the angle α between the cut plane and the lateral surface of the plates is 18.4 or 14, or 11.3 ^o , after which the cut plane of each of the plates is ground, chemically mechanically polished and the plates are interconnected with the cut planes in the holder so that what

- the plane of the plates are parallel, and the angle between

α-planes is 2α, then the double seed is placed in a vacuum or inert gas atmosphere at the melting point of the leucosapphire, the double seed is introduced into contact with the leucosapphire melt so that the directions of the crystallization fronts coincide with

-directions of the crystal lattices of the plates, the bicrystal is sequentially double-grown, from which, after cooling, a bicrystal substrate oriented along

-planes, and after chemical-mechanical polishing, a buffer layer is applied to the surface of the substrate, onto which an epitaxial high-temperature superconducting film is then sprayed.

The technical result that is achieved by implementing this method is that the Josephson HTSC transition, the bicrystal substrate of which is made of leucosapphire, has low dielectric losses in the microwave range, which makes it possible to effectively use it for the manufacture of devices operating in the microwave frequency range. Moreover, the formed HTSC junction has high parameters, a good degree of reproducibility, a low value of the cutoff frequency of noise type i / t, a sufficiently high value of the characteristic voltage U _c = I _c • R _n , where I _c is the critical current, R _n is normal resistance: at T = 77 K and a bicrystal misorientation angle of 24 ^o, it reaches 0.2 mV, and the value of t _{c is} close to 10 Hz. The electronic properties of the Josephson transition are completely determined by the process of electron scattering at the interface. Therefore, the presence of structural defects of the boundary, such as voids, accumulation of impurity atoms, dislocations, etc., cause strong scattering of electrons, reducing the number of electrons crossing the energy barrier at the boundary. The non-flatness (roughness) of the boundary also causes additional diffusion scattering of electrons. The quality of the boundary of an HTSC film and its structural perfection are uniquely determined by the boundary of the substrate, the structural defects of which are inherited by the film during its epitaxial growth.

 The bicrystalline boundary obtained by the growth of a crystal from a melt is natural and therefore the most perfect. Therefore, the boundary of the film grown on such a bicrystal substrate is also the most perfect and contains a minimum number of structural defects. However, dislocations, even if they are formed, do not play a noticeable role in such an artificial boundary in the formation of the energy barrier for electron tunneling. The growth of bicrystals from leucosapphire is currently the only possible way of manufacturing bicrystal substrates for Josephson's HTSC junctions operating in the microwave frequency range. Josephson's HTSC junctions formed on bicrystalline leucosapphire substrates also allow the creation of various electronic devices, such as direct current squids, squid magnetometers with a relatively low level of equivalent noise in magnetic flux and a record high resolution in magnetic field at T = 77 K.

-плоскости, 2 - плоскости среза; на фиг. 2 изображен чертеж, поясняющий сущность способа выращивания бикристалла лейкосапфира из расплава, где 1 - двойная затравка, 2 - бикристаллическая граница, 3 - расплав, 4 - молибденовый контейнер (а, б).In FIG. 1 schematically depicts single-crystal triangular double-seed plates, where 1 are atomic

-planes, 2 - cut plane; in FIG. 2 is a drawing explaining the essence of a method for growing a leucosapphire bicrystal from a melt, where 1 is a double seed, 2 is a bicrystal border, 3 is a melt, 4 is a molybdenum container (a, b).

 The inventive method of manufacturing a HTSC transition is as follows.

. Двойную затравку (1, фиг. 2) изготавливают из монокристаллического слитка лейкосапфира, из которого вырезают две монокристаллические пластины треугольной формы (фиг. 1), вырезанные из слитка лейкосапфира таким образом, что одна из поверхностей каждой из пластин совпадает с

-плоскостью, а другая поверхность совпадает с

-плоскостью (1, фиг. 1), после чего на боковых поверхностях каждой из пластин выполняют срезы (2, фиг. 1), плоскости которых перпендикулярны

-плоскости, а угол α между плоскостью среза и боковой поверхностью пластины составляет 18,4^o, после чего плоскость среза каждой из пластин шлифуют, полируют и соединяют пластины между собой плоскостями среза в держателе, в котором с помощью поступательного перемещения и осуществляют взаимную ориентацию пластин так, что

-плоскости пластин параллельны, а угол 2α между

-плоскостями составляет величину 36,8^o. Точность установки углов поворота и вращения не превышает 0,01^o. После этого двойную затравку вставляют в молибденовый контейнер (4, фиг. 2) с исходным сырьем лейкосапфира, контейнер с двойной затравкой (1, фиг. 2) помещают в камеру с атмосферой инертного газа или в вакуумную камеру ростовой установки, затем температуру в нагревателе поднимают до температуры плавления лейкосапфира t = 2054^oC, вводят двойную затравку в соприкосновение с расплавом лейкосапфира (3, фиг. 2) так, что направления фронтов кристаллизаций совпадают с

-направлениями кристаллических решеток пластин двойной затравки (1, фиг. 2), и производят последовательное наращивание кристалла на двойную затравку со скоростью кристаллизации 10. . . 12 мм/ч, из которого после его охлаждения вырезают бикристаллические подложки толщиной 0,8 мм, ориентированные по

-плоскостям, причем после химико-механического полирования на подложки наносят буферный слой CeO₂, после чего напыляют эпитаксиальную высокотемпературную сверхпроводящую пленку YBCO-123 толщиной 100...200 нм.A double seed 1 is made (Fig. 2) from a leucosapphire crystal (Al ₂ O ₃ ) for the Sapphire-2 growth unit (designed and manufactured at the Institute of Crystallography of the Russian Academy of Sciences). The Sapphire-2 installation works according to the horizontal Bridgman method with a tungsten heater. The container 4 (Fig. 2) for growing a leucosapphire bicrystal is made of sheet molybdenum in the form of a “boat”. Raw materials are poured into container 4 (the battle of crystals of leucosapphire grown from powder on a Verneuil growth unit), the container is inserted into the unit on a special platform equipped with a horizontal movement mechanism. Double seed 1 (Fig. 2) is made of a certain geometry with a given crystallography. For epitaxial growth of HTSC films with a perovskite structure, surface-oriented leucosapphire substrates are used

. The double seed (1, Fig. 2) is made of a single crystal leucosapphire ingot, from which two triangular single crystal plates (Fig. 1) are cut from a leucosapphire ingot so that one of the surfaces of each of the plates coincides with

-plane, and the other surface coincides with

- a plane (1, Fig. 1), after which sections on the lateral surfaces of each plate are performed (2, Fig. 1), the planes of which are perpendicular

-planes, and the angle α between the cutting plane and the side surface of the plate is 18.4 ^o , after which the cutting plane of each of the plates is ground, polished and the plates are connected to each other by the cutting planes in the holder, in which the plates are moved relative to each other by translational movement so that

-planes of the plates are parallel, and the angle 2α between

-planes is 36.8 ^o . The accuracy of setting the rotation and rotation angles does not exceed 0.01 ^o . After that, a double seed is inserted into a molybdenum container (4, Fig. 2) with leucosapphire feedstock, a double seed container (1, Fig. 2) is placed in an inert gas atmosphere or in a vacuum chamber of a growth unit, then the temperature in the heater is raised up to the melting point of leucosapphire t = 2054 ^o C, a double seed is introduced into contact with the leucosapphire melt (3, Fig. 2) so that the directions of the crystallization fronts coincide with

-directions of the crystal lattices of the double seed plates (1, Fig. 2), and sequentially build up the crystal on a double seed with a crystallization rate of 10.. . 12 mm / h, from which, after cooling, 0.8 mm thick bicrystalline substrates oriented along

-planes, and after chemical-mechanical polishing, a CeO ₂ buffer layer is applied to the substrates, after which an epitaxial high-temperature superconducting YBCO-123 film with a thickness of 100 ... 200 nm is sprayed.

 Thin YBCO HTSC film is produced by double-beam laser spraying. The two-beam laser sputtering method employs synchronized emissions of two AIG: Nd lasers, whose rays passing through the lenses are focused (energy density at the focus of the order of 10 J / cm) on the surface of rotating superconducting targets of the same composition (YBCO-123), prepared according to standard technologies from a mixture of powders of oxides of elements forming HTSC ceramics. Under the action of radiation, the material of each target is sprayed, and due to the specific geometry of the setup, the radiation power, and the mutual orientation of the laser beams, a thin YBCO HTSC film is deposited on the surface of the bicrystal substrate with subsequent epitaxial growth.

 Then, gold contact pads are sprayed onto the YBCO film through a special patch mask that is firmly pressed to the surface, which are then annealed in the same chamber where the HTSC film is sprayed. Preliminarily, the surface of an HTSC film is cleaned in an RF discharge by argon ions (Leybold L-560 installation, power 150 W, pressure 3 • 10 mbar, cleaning time 1-2 min). The resistance of the obtained contacts averaged 0.2 Ohms at T = 77 K.

 The technical result that is achieved by implementing this method is that the Josephson HTSC transition, the bicrystal substrate of which is made of leucosapphire, has low dielectric losses in the microwave range, which makes it possible to effectively use it for the manufacture of devices operating in the microwave frequency range.

 The relevance of the problem and the high technical characteristics of the HTSC Josephson junction ensure the industrial applicability of this invention.

Claims (1)

A method of manufacturing a Josephson's high-temperature superconducting transition, which consists in forming a bicrystal from which a bicrystal substrate is cut out, followed by grinding, chemical-mechanical polishing of the surface of the substrate, deposition of an epitaxial high-temperature superconducting film, contact pads on the substrate, and then high-temperature photolithography is performed by photolithography and its etching, characterized in that the bicrystal is grown from a leucosapphire melt on double seed, which is made of two triangular single crystal plates cut from a single crystal leucosapphire ingot so that one of the surfaces of each of the plates coincides with

- the plane, and the other with

- a plane, after which a cut is made on the side surfaces of each of the plates, the plane of which is perpendicular

- the plane, and the angle α between the cut plane and the side surface of the plate is 18.4 or 14, or 11.3 ^o , after which the cut planes are chemically-mechanically polished, connected in the holder so that

- planes are parallel, and the angle between

- the planes is 2α, then the double seed is placed in a vacuum or inert gas atmosphere at the melting point of leucosapphire, the double seed is brought into contact with the leucosapphire melt so that the directions of the crystallization fronts coincide with

- by the directions of the crystal lattices of the plates, a bicrystal is sequentially double-grown, from which, after cooling, a bicrystal substrate oriented to

- planes, moreover, after chemical-mechanical polishing, a buffer layer is applied to the surface of the substrate, on which an epitaxial high-temperature superconducting film is then sprayed.

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