Magnetic field maps of YBCO thin films obtained by ...

Supercond. Sci. Technol. 10 (1997) 366–370. Printed in the UK

PII: S0953-2048(97)82434-7

Magnetic field maps of YBCO thin films obtained by scanning SQUID microscopy for HTSC microelectronics K E Andreev†, A V Bobyl‡, S A Gudoshnikov§, S F Karmanenkok, S L Krasnosvobodtsev¶, L V Matveets§, O V Snigirev†, R A Suris‡ and I I Vengrus§ † Physics Department, Moscow State University, Moscow 119899, Russia ‡ Ioffe Physico-Technical Institute, St Petersburg 194021, Russia § Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Troitsk 142092, Moscow Region, Russia k Electronics Department, Electrotechnical University, St Petersburg 197376, Russia ¶ Lebedev Institute of Physics, Moscow 117924, Russia Received 10 March 1997 Abstract. Scanning SQUID microscopy has been extended to record current-induced magnetic field maps of superconductor topologies based on YBCO film structures. The technique described can be used as a powerful tool for the diagnostics of superconductor integrated circuits and highly sensitive bolometers. This kind of diagnostics predicts the degree of integration which can be achieved at the modern level of HTSC microelectronics technology. One would expect an overall yield of 100-element 1 mm2 chips at a level of ∼0.1–0.5% in the production of superconducting YBCO/MgO bolometers.

1. Introduction Scanning SQUID microscopy (SSM) occupies a prominent place among methods of spatial inhomogeneity visualization, owing to its unique sensitivity of several pT [1–3]. Nevertheless, this extreme sensitivity makes interpretation of magnetic field maps (MFMs) of device structures difficult, and additional investigations of the test structures should be performed. The problem of growth and patterning of homogeneous epitaxial films, as well as quality control in present-day technological processes, may be thought of as a key problem of HTSC microelectronics [4, 5]. In the present work SQUID diagnostics was extended to recording MFMs of highly sensitive bolometer test structures examined with a bias current. Moreover, the results obtained allowed us to predict the degree of integration which can be achieved at the modern level of HTSC technology and to estimate the prospects of using very large-scale integration (VLSI) technology [6] in superconductor electronics. 2. Scanning SQUID microscope Numerous techniques of superconductor film characterization use a magnetic field as a means to display sources of c 1997 IOP Publishing Ltd 0953-2048/97/050366+05$19.50

residual losses. A static magnetic field applied to the surface of an HTSC film placed in a resonator permits one to determine physical mechanisms of microwave residual losses [7]. Other methods such as Hall sensors, magnetooptic films and magnetic force microscopy [8–10] have been developed to obtain MFMs. Measuring the shielding of an applied a.c. magnetic field by a wire coil is frequently used as a qualitative method for revealing holes and nonsuperconducting inclusions [8]. The coil diameter should be no less than 1 mm, so the resolution is relatively poor. Furthermore, the a.c. magnetic field strength is to be no less than a few gauss. In this case the field strength at the film edges could be higher than the first critical field, owing to the Meissner effect. It is therefore rather difficult to investigate high-quality films having some small regions in a mixed state and containing some micrometre-sized holes or inclusions. SSM can operate on the microscopic level [1–3] in a magnetization field of about a few tenths of microtesla or less. To realize this kind of microscopic superconductor film diagnostics, a sensitive d.c. SQUID detector was used to scan the sample surface over the X–Y coordinates. The detector had a magnetic field sensitivity of up to a few pT in the bandwidth, a spatial resolution of the order of a few tens of micrometres and a scanning area of about 1 cm2 . The d.c. SQUID was fabricated from a thin YBCO

Scanning SQUID microscopy for HTSC microelectronics

film grown on a bicrystal strontium titanate substrate with 36◦ boundary matching [11]. The superconductor film was patterned in the form of a quasi-square washer with a 50 × 50 m2 inner hole and an outer size of about 0.5 mm. The effective pick-up area was about 200 mm2 . The transfer function (magnetic field divided by output voltage) equalled 140 nT V−1 . The dynamic range of the magnetometer electronics was 110 dB. Both the SQUID and a high-precision X–Y –Z spatial feed were placed in a liquid-nitrogen cryostat. The X–Y scanning area was 8 × 8 mm2 ; the Z-distance between the detector and sample could be adjusted in the range 0.1– 2 mm [12]. The room-temperature part consisted of three stepper motors, the stepper-drive electronics and SQUID electronics. Measurements were carried out in automatic computer-driven mode. Two cylindrical metal shields around the cryostat, an optical coupler and a distance of about 1 m between the stepper motors and the cryogenic part were used to suppress the interference from the digital circuits of the computer, stepper motors and ambient magnetic signals. Under these conditions the measurements were done in the usual laboratory environment without any radio-frequency shielding of the room. The residual magnetic field inside the cryostat was less than 200 nT and the magnetic noise from the moving parts of the microscope less than 1 nT. 3. Samples Despite the significant success of HTSC film technology, the preparation of homogeneous epitaxial YBCO films on large-area substrates (2–3 in in diameter) is still very difficult [5, 13]. Most interest in the field of superconductor electronics, including microwave electronics, is attracted by substrates with minimal values of permittivity and dielectric loss angle, widely used in conventional microelectronics. These demands are satisfied to the largest extent by sapphire and magnesium oxide substrates. Nowadays, the efforts of many laboratories are directed to preparing large-area YBCO/CeO2 /α-Al2 O3 (r-cut) structures [13, 14]. However, MgO substrates show much promise for the preparation of epitaxial YBCO films and the fabrication of various electronic devices having the highest currentcarrying capacity and minimum microwave losses [14]. The crucial stage in epitaxial film growth is preparation of the substrate surface. Truly epitaxial growth was realized on MgO substrates after a two-stage preparation procedure. Preliminarily polished substrates were subjected to dynamic chemical polishing with H3 PO4 solution and then annealed at 1100 ◦ C for 3–5 h. Superconducting YBCO films were grown by d.c. magnetron sputtering of stoichiometric targets. The deposition was done in an Ar:O2 (∼1:1) gas mixture at pressures of about 100–120 Pa. The substrate temperature was in the range 670–690 ◦ C, with a power density applied to the target of approximately 5–6 W cm−2 . In these regimes the deposition rate was 0.8–1.0 nm min−1 . After deposition the samples were slowly cooled in oxygen during a period of no less than 1 h. The crystallographic orientation and structural perfection of the resulting YBCO films were controlled by Raman spectroscopy [15].

Figure 1. Yield of YBCO/MgO superconductor microstrips versus root-mean-square microstrain fluctuations hεi; the curve corresponds to polynomial fitting.

4. Results and discussion In the first stage, local measurements of intrinsic microstrain fluctuations hεi were performed on 10×10 mm2 samples, using x-ray diffraction analysis with a resolution of 3 mm [16, 17]. The quantity hεi can be expressed in terms of the c-axis lattice parameter as follows: hεi = [h(1c/c)2 i]1/2 . The photolithography on HTSC films included a stage of wet etching in an ethylenediaminetetraacetic acid solution. The samples patterned as microstrips were used in flicker noise intensity and film resistivity measurements. Commonly, the microstrips were 1 mm long and about 15 mm wide. An analysis of the relationship between the number of various defects in the strips expressed through the parameter hεi on the one hand and flicker noise intensity on the other gave the dependence presented in figure 1. This correlation made it possible to estimate the sample quality and to select more homogeneous films with a satisfactory level of flicker noise prior to carrying out the photolithographic procedure. Two kinds of film chip topologies were used in YBCO film patterning. Figure 2(a) shows the film-patterning topology of a 4 × 4 mm2 chip, together with the common microstrip topology. An electron-microscopic image of this element is presented in figure 2(b). A set of silver contacts is depicted in the sketch and these contacts are clearly seen in the photograph. MFM measurements were carried out in a residual magnetizing field of about 170 nT, directed perpendicularly to the film surface. Figure 3 demonstrates the MFM of the film topology registered at about 200 mm above the film surface. It should be noted that very small deviations of the sample plane from being perpendicular to the magnetic field axis distort the magnetic image of the sample. The fact that the MFM is non-uniform at the top and bottom of the image may indicate that the sample is bent somewhat, so that the scanning and sample planes are non-parallel. The structure 367

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Figure 3. Magnetic field map of the test chip element in a residual field of about 170 nT. The special marks and image size are the same as in figure 2(a ).

(b) Figure 2. (a ) Layout of superconductor chip used as a test element. The distance between the special marks at the corners of the chip element along the X - and Y -directions is 4 mm. The four-contact microstrip marked by the arrow was subjected to more detailed investigations. (b ) Electron-microscopic image of the electronic chip.

of the MFM of the above test structure corresponds to the case of Meissner shielding of currents flowing mainly along peripheral parts. The difference between neighbouring isofield lines in the map is equal to 40 nT. The maximum field strength recorded in the map centre is about 680 nT, which is considerably greater than the strength of the magnetizing field. The reason for this effect is that the superconducting film acts back on the mutual inductance between the SQUID and its feedback coil [1]. Note that the image contains no local peaks corresponding to holes or non-superconducting inclusions of about 50 mm in size. The microstrip marked by the arrow in figure 2(a) was chosen for detailed investigations because this configuration allowed four-probe measurements. This microstrip was about 50 mm wide and 500 mm long. Figure 4 shows the transition of this microstrip to the superconducting state, the transition region width of about 1.3 K indicating the high quality of the film investigated. The diamagnetic behaviour of this and other strips in the chip was not displayed on the 368

Figure 4. Temperature dependence of the film resistance in the vicinity of Tc for the microstrip marked by the arrow in figure 2(a ).

magnetic map. To reveal some features of the diamagnetic behaviour of the microstrips, more detailed measurements were carried out inside the square marked in figure 3. In this case the magnetizing field strength was raised to 2 µT. A magnetic image of the square (1.2×1.2 mm2 ) is presented in figure 5. Four contact pads (numbered 1–4 in figure 2(a)) can be clearly seen in the vicinity of the strip line, but no local magnetic field perturbations are produced by the microstrip. Presumably, the signal from the strip is very weak (below the noise level of the SQUID). To locate precisely the nonsuperconducting region it may be necessary to minimize the separation between the sensor and the sample. The MFM changes drastically if a transport current of 100 µA is passed along the microstrip at a frequency of 110 Hz and this can be seen in figure 6. There is a clear

Scanning SQUID microscopy for HTSC microelectronics

as magnetic shielding and critical current density. For example, an average non-parallelism of 10% at a distance of about 1 mm corresponds to a film homogeneity of about 90% in the same area. If the square occupied by a chip element were 100 × 100 µm2 in size then it would be possible to fabricate 100 chip elements in the chosen film area. As mentioned above, 10 × 10 mm2 YBCO/MgO structures were used in our experiments. It was found that the overall yield of high-quality superconductor films having hεi close to (2–3) × 10−3 was about 10–20%. With this kind of YBCO film a yield of sensitive bolometers as high as 40–50% was possible (see figure 1). However, after structure patterning the SSM diagnostics of microinhomogeneities also shows a yield as high as 10%. Thus, taking into account the percentage of selected input films and the conditions established in cryoelectronics [4] and VLSI circuit production [6], we estimated the yield of similar superconducting chips at 0.1–0.5%. This agrees well with the experimental yield of highly sensitive bolometers. Figure 5. Magnetic field map of the microstrip marked in figure 2 in magnetizing fields of up to 2 µT.

5. Conclusions

Figure 6. Magnetic field map of the microstrip marked in figure 2 in a magnetizing field at a bias current of 100 µA.

image of the current flow path between current pads 1 and 3 of figure 2(a). The indicated current is not high, but it gives an opportunity to increase the sensitivity of magnetic mapping by more than two orders of magnitude. In this case the dimensions of the mapping area were 3 × 3 mm2 and the magnetic line separation is 40 mT. The correlation between the structure of the induced MFM images and the bias current strength is of major practical and physical interest, and it will be studied using appropriate model representations [2, 11, 18]. Within the limits of the presentday understanding of magnetic images it may be suggested that the non-parallel arrangement of magnetic lines in the MFM images shown in figures 2 and 4 corresponds to the non-uniformity of such physical properties of the film

In the present work, epitaxial YBCO/MgO structures were grown and patterned as sensitive microbolometers. The test chip elements were investigated at 77 K with a scanning d.c. SQUID microscope based on a bicrystal YBCO film. The scanning measurements made on the chip elements confirmed the high quality of the films investigated, since no holes or non-superconducting inclusions ≥50 µm in size were indicated. The magnetic image of a 50 × 500 µm2 strip was detected with high accuracy in a weak external magnetic field at low bias current. Analysis of the MFM images shows that SSM can be successfully used to investigate film HTSC structures in magnetic fields Hc1 at low bias currents in connection with sensitive bolometer technology. Photolithographically patterned YBCO/MgO structures show much promise for the production of highly sensitive superconducting microbolometers. The yield of high-quality chips was estimated at 0.1–0.5% and taking the dimensions of a single chip element to be 100 × 100 µm2 it is possible to predict a yield of about 10 suitable chips for a 1 mm2 structure. The developed SSM testing technique could serve as very effective diagnostics for selecting highquality superconducting chip elements. Acknowledgments This work was supported by the Russian Foundation for Basic Research (grant 93-0217435) and Russian State Programme ‘Superconductivity’, projects 94048 and 94051. The authors are grateful to Professor O G Vendik for helpful discussions. References [1] Mathai A, Song D, Gim Y and Wellstood F C 1993 IEEE Trans. Appl. Supercond. 3 2609 [2] Vu L N, Wistrom M S and Van Harlingen D J 1993 Appl. Phys. Lett. 63 1693 369

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