Method for building periodic microstructures around htsc films possessing josephson properties

FIELD: building two-dimensional periodic microstructures around high-temperature superconductor films.

SUBSTANCE: proposed method intended for producing loose couplings in the form of two-dimensional periodic microstructures possessing Josephson properties used in high-sensitivity systems of HTSC film SQUID-magnetometers, for instance to produce high-sensitivity magnetic-flux sensors and electric-field detectors for magnetocardiogram recording units in medicine, geophysics, environment control, in oil industry to check oil products for paramagnetic impurities, and the like involves building of periodic microstructures around HTSC films possessing Josephson properties. To this end high-temperature superconductor is applied to substrate and loose-coup[ling regions are formed thereon in the course of HTSC film evaporation at the same time organizing rarefaction and compression regions within substrate to produce impact on film growth process. Periodic rarefaction and compression regions can be organized in substrate by laser-beam ultrasonic pulse, for instance that of nanosecond length, that excites acoustic standing wave of about 45 - 90 μm in length; in the process opposite side of substrate is acted upon by ultrasonic pulse. Provision is made to prevent parasitic loose couplings with critical currents lower than that of bridges in compression region of superconducting components and for dispensing with large units whose substructure has disoriented crystals forming Josephson structures of Dyem bridge type at crystal-to-crystal interface or on embedded microscopic particles of non-superconducting phase YBaCuO.

EFFECT: facilitated manufacture, enhanced operating stability of mentioned components, enhanced reliability and reproducibility of their characteristics.

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The invention relates to a method of creating weak links in the form of two-dimensional periodic microstructures with Josephson properties used in highly sensitive systems film HTS squid-magnetometers, in particular, the creation of highly sensitive sensors of the magnetic flux and electromagnetic field detectors used in devices for registration of magnetocardiogram in medicine, Geophysics, ecology, and control of paramagnetic impurities in the oil, etc.

Currently, we can distinguish two groups of weak ties, forming at the grain boundaries of the single-layer and multilayer structures. For single-layer structures include Josephson junctions, formed on bicrystalline substrate having epitaxially the nature of the grain boundary, stepped-end (step-edge transitions. For single-layer structures are also micromastia, i.e. monocrystalline weak links, the size of which is comparable with the coherence length in high-temperature superconductors. To multilayer structures are transitions end type (edge-junctions), vertical sandwiches, horizontal sandwiches.

When forming bicrystalline weak ties border with Josephson properties is formed by epitaxial growing film on the substrate, the two halves of which are misoriented cu is starecheski lattice (application JP 6080845 B4, IPC 5 H 01 L 39/24) (Proc. ASC 96. Pittsburgh. USA, Preprinnt EKC-5, to appear in IEEE Trans. Appl. Supercond. 7(1997). In one device the weak connection type bridge Diema organized on bicrystalline substrate SrTiO₃or NdGaO₃24 and 36.8° border misorientation of the crystal structures of the substrate, where the sensor is made magnetometer on the basis of the squid on YBaCuO films. The disadvantage is that the Josephson relations type of bridge Diema on bicrystalline substrates, as is the case in the analogue, the effect of concentration of the magnetic flux is weakened due to the presence of boundaries between crystallites, or microstripline non-superconducting phase YBaCuO and thus decreases the sensitivity of the squid magnetometer. Moreover, in the field of narrow superconducting electrodes near the seam of a bicrystal not exclude the possibility of spurious weak links with critical currents, smaller I_withbridges, which may lead to additional contours of quantization, significantly impairing the operation of the interferometer.

Preparation of high-quality bicrystalline substrates is a complex and expensive process, so bicrystalline weak links unpromising. They can be used in practice only for the implementation of simple circuits, for example, a single squids. This is due to the fact that the local arrangement of the light is provodnikov elements is defined by the boundary line of section in the substrate.

During the formation of weak ties on biepitaxial grain boundary, the idea is to use additional seed and buffer layers (CFHT 1989, N 5. T.2, p.97-102), grown on a single crystal substrate at the locations of weak ties. Obtained on the substrate, the boundary between the poly - and monostructural is the basis for the origin of the crystal transition (Japan Journal of Applied Physics Vol 20 No I.January 1990. Pp.74-78). Preparation epitaxially weak ties based on standard photolithography, these links can be placed anywhere on the substrate. Therefore, this approach could be adopted for the base, but the presence of grain boundaries, as shown by our research, leads to a low resistance to thermal Cycling.

When forming the stepped-end transition (step-edge junctions) to the Josephson processes on poor communication leads misorientation not only along the a-b plane and along any other plane. To do this, first ion or chemical etching on a single crystal substrate is formed with a step, with a fixed angle of inclination and height. Then on top of the grown epitaxial film HTS. The presence of steps on the substrate leads to the formation of two grain boundaries at the top and the foot of the stairs. These boundaries define the properties of micromastia perpendicular to the stupa is like.

In step-edge junctions can be achieved by increasing the value of V_ccompared to epitaxially, due to the fact that j_cyou can change by three orders of magnitude, varying three process parameters (application JP 6080845 B4, IPC 5 H 01 L 39/24):

- change the angle of the stairs;

- height of the step;

the ratio of film thickness to the height of the step.

On step-edge junctions was achieved value of the critical current density j_c=10⁵A/cm and the highest value of V_c=(5-8)μkV at T=77,6 K. the Best features on the Squids, which includes step-edge transitions, obtained the following optimal parameters: the height of the step h=300-400 nm, the angle of the stairs ≥60°the HTS film thickness equal to h/2. The disadvantage of this type of weak ties is the strong dependence of j_cfrom the above technological factors, which leads to poor reproducibility and controllability of the method.

There is a method of creating weak ties by forming bridges of hyperfine HTS film (patent RF N 2133525, IPC H 01 L 39/22). The results of studies of the dependence of the superconducting properties of YBaCuO films on the thickness d show that after overcoming the percolation threshold there is some interval of values of d at which the critical current density J_cjumps by almost two orders of magnitude. When the value is in d˜ 10÷20 nm the magnitude of the critical current density is of the order of 10³A/cm². Right from the jump when d>25 nm the value of J_c>10⁵A/cm²and, with further growth reaches saturation (J_c=10⁶A/cm²).

Thus, by varying the film thickness d can choose the desired current value. The disadvantage of this method is that the Josephson relations type of bridge Diema on ultrathin films the effect of concentration of the magnetic flux is weakened due to the presence of boundaries between crystallites, or microstripline non-superconducting phase YBaCuO, thus raises the noise level and decreases the sensitivity of the squid magnetometer. Moreover, in the field of narrow superconducting electrodes does not exclude the possibility of spurious weak links with critical currents, smaller I_cbridges. There may appear additional contours of quantization, significantly impairing the operation of the interferometer.

Closest to the claimed method is a method of creating weak links in the system on film HTS-squids named authors way "controlled quenching associated with relaxation of mechanical stresses during rapid cooling of the film during sputtering (RF patent N 2199796, IPC H 01 L 39/22). The main idea of this method was with edusim: technological options for growing films are chosen in which for normal cooling process after deposition is high quality monocrystalline film, i.e. temperature deposition 840°C, the intensity of the laser radiation 10⁹W/cm²the pulse duration of 20-40 NS, the pressure of air or oxygen to 0.1 Torr. Further, increasing the cooling rate of the film, choose a time of cooling, which produces the desired critical current J_c. In the method of "controlled quenching" used detected a significant effect on the physical properties of YBaCuO films such technological parameters as the cooling rate of the film after the coating process and the dwell time in the furnace at a predetermined temperature associated with the time of relaxation of mechanical stresses. High values of the superconducting parameters, in particular the critical current density J_cand single-crystal structure suggest that the mechanical stresses in superconducting YBaCuO thin film in the process of growing, almost time to recover completely (film cools down for quite a long time - about 30 minutes). However, at high speeds the cooling of the superconducting properties of the film, in particular the critical current density is significantly reduced, which is associated with freezing fur is practical stress the formation of domains of stress that are associated with oxygen deficiency in the grown thin film structure.

The main disadvantage of this method is that the cause of the mechanical stress is misalignment parameters of the lattices of the substrate material and the film and the difference in their coefficients of thermal expansion. As substrates we used the single crystals LaAlO₃(100), SrTiO₃and other micrographs of the surfaces of the epitaxial film and the hard film and show that the structure of the epitaxial film (1)is homogeneous and defect-free, and in the case of the hard film is observed fragmentation of the material (figure 2), i.e. is formed block structure. While the blocks have two characteristic dimension: ˜10 μm and ˜1-3 μm. Large blocks have a substructure consisting of misoriented crystallites. It was observed that large blocks are located quite close to each other, so they can form Josephson structure type bridge Diema at the boundaries between crystallites or microstripline non-superconducting phase YBaCuO. Thus, it is not excluded the possibility of a spurious weak links with critical currents, smaller I_withbridges, this increases noise and decreases chustvitelnos the ü PT-squid magnetometer. You may also receive additional contours of quantization, which appears uncontrolled noise signal that will interfere with the measurements on PT-squid-magnetometer.

For the implementation of the method proposed in the prototype, the deposition of films was carried out in situ at a pressure of the air ˜0.1 Torr in the vacuum chamber of the post. For spraying were used solid-state pulsed Nd:YAG laser with wavelength Λ=1.06 µm, pulse duration t=20 NS, pulse repetition rate f=12 Hz. The laser beam was incident on the target, which is focused through the optical system and the quartz window of the vacuum chamber. The diameter of the focal spot on the target was ˜1 mm as the substrate was used monocrystalline LaAlO₃(100), SrTiO₃(100). During deposition the substrate was heated in the vacuum chamber in a cylindrical quartz furnace to a temperature of t=820-860°C. the cooling Rate of the film after deposition ranged from 10 to 30°C/min, the power Density of the laser radiation pulse W on the surface of the target was changed from 2.5·10⁸W/cm to 1.5·10⁹W/cm and, accordingly, the pulse energy E is from 40 to 200 MJ/pulse. Distance from target to substrate remained unchanged and amounted to 3 cm, the plasma torch was directed normal to the substrate.

Object of the invention is the development of the processing method of forming a periodic microstructures on HTSC films with Josephson properties providing at simplifying technology elements Josephson increase the stability of these items, the reliability and reproducibility of characteristics by preventing in narrow superconducting elements the possibility of spurious weak links with critical currents, smaller I_withbridges and lack of large blocks having a substructure consisting of misoriented crystallites which form the Josephson structure type bridge Diema at the boundaries between crystallites or microstripline non-superconducting phase YBaCuO.

This technical result is achieved in that in the method of formation of periodic microstructures on HTSC films with Josephson properties, comprising applying to the substrate a high-temperature superconductor and the formation of areas with weak link region with a weak bond is formed simultaneously with the deposition of HTSC films by creating in the substrate regions of rarefaction and compression affecting the growth process of the film. Of the periodic suction and compression in the substrate to create laser pulses of nanosecond duration, exciting acoustic standing wave with a length of about 45-90 μm, and an ultrasonic pulse impinges on the opposite side on the spoon. An ultrasonic pulse is created by a laser used for the deposition of films, directing part of the light to the opposite side of the substrate using a transparent quartz plate and mirror with silver outer coating. In the substrate is formed of mechanical stress, which define the epitaxial growth film with a periodic structure and Josephson properties that do not have misoriented crystallites and not subject to relaxation during operation of these systems during thermal Cycling.

The method is illustrated by the photos, layout and graphics, where figure 1 and 2 shows the micrograph of epitaxial and hard films according to the prototype, figure 3 and 4 - micrograph of HTS films at ultrasonic pulse exposure on the substrate at different magnification, figure 5 shows the diagram of a device for implementing the method, figure 6 diagram for removal of the volt-ampere characteristics of HTS tapes under microwave irradiation received Josephson structures, 7 - WAH with steps Shapiro, confirming the existence of Josephson junctions in the resulting films.

For implementing the method experimental setup for the deposition of films whose schema is presented in figure 5, containing the vacuum chamber 1 is placed inside a cylindrical quartz is the second furnace 2, installed in the furnace HTS target 3, which was sprayed with a laser 4 with the air pressure P˜0.1 to 0.2 Torr., and a stable temperature of 860°C. installing used solid-state pulsed Nd:YAG laser with wavelength Λ=1.06 µm, pulse duration from 20 to 40 NS, pulse repetition rate f=12 Hz. The power density of the laser radiation pulse W on the surface of the target was 2.5·10⁸W/cm to 1.5·10⁹W/cm and, accordingly, the pulse energy E from 40 to 200 MJ/pulse. The laser beam was incident on the target 3, after passing through the optical system 6 and the quartz window 5 of the vacuum chamber 1. We use this setup was possible to obtain films in situ. As a target 3 was used polycrystalline YBaCuO samples made by developed by the authors of the technology described in patent N 21044939 published 20.02.1998, Superconducting parameters of the target 3 were as follows: the critical temperature T=90 K, the width of the transition T=2 K, the critical current density j=10²A/cm². Targets were made in the form of tablets with a diameter of 1-3 cm, thickness 0.5 cm as the substrate 7 was used monocrystalline LaAlO₃(100), SrTiO₃and other Distance from target 3 to the substrate 7 was changed from 1 to 5 cm remained Unchanged orientation of the target relative to the laser beam (the angle between the normal to the Ergneti target and the direction of the incident laser beam was 30° ), as well as the orientation of the substrate 7 on target 3 (plane surface of the substrate and the target are parallel, so that the axis of the plasma torch was normal to the substrate). During deposition, the substrate 7 is heated to 840-860°inside a cylindrical furnace 2, which was controlled by thermocouple 8. For excitation in the surface acoustic standing wave part of the laser radiationthat is about 4% of the energy of incident radiation reflected a transparent quartz plate 9 on the mirror 10 with a silver outer coating, which reflects the laser light on the substrate 7 with the reverse nenapysani side. Under the influence of the laser pulse with different duration t=(20-40) na in the substrate is excited by an acoustic standing wave length L=(45-90 microns), calculated by the formula

L=v·t/2,

where t=(20-40) NS duration of the laser radiation; v=4,5·10³m/s the speed of sound in the substrate (Physical directory edited Isegoria, Ezekiela, Moscow, Energoizdat, 1991).

The growth of a periodic structure on the HTS film shown in Fig. 3 and 4 with different zoom scale is through the impact on the epitaxial growth film regions of the periodic suction and compression in the substrate, generated acoustic standing wave.

To confirm that the floor is Chennai structure on the HTS film has a Josephson properties use a standard scheme for removal of the I-V characteristics in the microwave field (see Fig.6). As a constant current source was used a power supply with a linear scan. The current control was carried out by the milliammeter M, which allowed to detect currents from 1μAnd up to 60 mA. The potential was recorded by the voltage comparator R and displayed on a chart recorder. The microwave part of the experimental complex represented by the generator GC-115 with an operating range 25,84-37,5 GHz. The maximum power of the generator in the continuous generation mode is 50 mW. Supply and concentration fields of the microwave directly in the area of the bridge was carried out using a rectangular waveguide. With the known power transmitted by the waveguide, the use of this device allows to estimate the amplitude of the electric field component of the microwave directly on the film.

On the current-voltage characteristics shown in Fig.7, it can be concluded that the parameters of the Shapiro steps are in good agreement with theoretical calculations, when at low levels of exposure steps Shapiro on WAH satisfy the relation Vn=nv/2e, n is an integer (number of steps), v is the frequency of microwave radiation, e is the electron charge.

For example WACH periodic microstructure when offline (without irradiation) and the irradiation of microwave.

In resulttable installed, on the resulting microstructure with period L=45 μm and critical current density j_c=10³A/cm²steps Shapiro have the most pronounced form, and therefore, a film with such settings are best exhibit Josephson properties. This conclusion is of great practical importance, because it allows to evaluate the quality HTS films in the manufacture of devices containing weak links, such as PT - and RF-squids.

1. The method of formation of periodic microstructures on HTSC films with Josephson properties, comprising applying to the substrate a high-temperature superconductor and the formation of areas with weak coupling, characterized in that the formation of areas with weak communication carried out during deposition of HTSC films by simultaneous creation in the substrate regions of rarefaction and compression affecting the growth process of the film.

2. The method according to claim 1, characterized in that region of the periodic suction and compression create ultrasonic pulse laser radiation causing acoustic standing wave.

3. The method according to claim 2, characterized in that the ultrasonic pulse impinges on the opposite coating film side of the substrate.

4. The method according to claim 3, characterized in that an ultrasonic pulse is created by a laser used for applied what I film.

5. The method according to claim 4, characterized in that to generate an ultrasonic pulse directed part of the laser radiation on the opposite side of the substrate using a transparent quartz plate and mirror with silver outer coating.

6. The method according to any of claim 2 to 5, characterized in that the ultrasonic pulse generate an acoustic standing wave in length 45-90 microns.