Spatially resolved characterization of superconducting films and cryoelectronic devices by means of low temperature scanning laser microscope

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Applied Surface Science 106 (1996) 390-395

Spatially resolved characterization of superconducting films and cryoelectronic devices by means of low temperature scanning laser microscope A.G. Sivakov *, A.P. Zhuravel', O.G. Turutanov, I.M. Dmitrenko B.I. Verkin h~stitutefor Low Temperature Physics and Engineering, National Academy of Sciences of Ukraine, 47 Lenin Ace.. 310164 Kharkoc, Ukraine

Received 17 September 1995; accepted 15 January 1996

Abstract The work describes the low temperature scanning laser microscopy technique used for spatially resolved characterization of superconducting films and film-based cryoelectronic circuits in the temperature range from 2 to 300 K. The determination of superconducting parameters for separate elements of a high Tc Josephson junctions array and imaging of the resistive transition in a high Tc superconducting polycrystalline film are demonstrated. The spatial evolution of the resistive state of a Sn thin film strip associated with the phase slip lines formation is visualized.

1. Introduction Development of techniques capable of determining local values of superconducting parameters in thin film superconductors is important for both fundamental and applied problems. Such techniques would make it possible to study the resistive state structure for elucidation of the nature of superconductivity, for diagnostics of superconducting films while developing the technology of their preparation, as well as for control of the operation of cryoelectronic circuits and devices. The importance of these techniques has grown due to the discovery of high Tc superconductors (HTSCs) whose nature of superconductivity is actively studied and whose properties often prove to be rather inhomogeneous.

Corresponding author. Tel.: +380-572-308507; fax: +380572-322370; e-mail: [email protected].

This work describes the usage, for the above listed goals, the low temperature scanning laser microscopy (LTSLM) technique. The technique is based on a simple principle of scanning a sample with a laser beam (probe) focused on a sample surface, with a simultaneous recording of the response. A change in any characteristic of the sample or the probe, arising as a result of their local interaction, may serve as a response signal. In the case of a currentcarrying superconducting film it is convenient to choose as a response signal the change in the voltage drop across the sample that arises due to the change in the resistance at the illuminated spot. The voltage response dependence on the probe coordinates at the sample surface can be visually represented as a two-dimensional image of resistive domains. The change in voltage due to the illumination of a current loaded superconductor is caused by a considerable number of mechanisms which can be divided

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A.G. Sivakou et aI. / Applied Surface Science 106 (1996) 390-395

into two classes: equilibrium (bolometric) and nonequilibrium (nonbolometric) mechanisms [1]. The effects of the first class are displayed in the resistance change due to the lattice heating mainly caused by low-energy phonons. Non-equilibrium mechanisms are related to the pair-breaking process by photons. This process is accompanied by high energy quasiparticle formation and involves non-equilibrium effects produced by electrons, phonons and Josephson effects at grain boundaries. It is worth noting that similar effects are produced by the action of an electron beam on a superconductor, so the LTSLM and the low temperature scanning electron microscopy (LTSEM) [2] techniques, the latter being now well developed, are rather close as to their research potential. Thus, the variety of response mechanisms creates a problem to be resolved by both techniques.

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2. Experimental Our experiments were carried out with a device whose scheme is shown in Fig. 1. The device consisted of a scanning laser microscope with a low temperature attachment, of the measuring electronics and of a PC with an interface unit. The optical system of the scanning microscope focused the beam of a 2 mW H e - N e laser into a spot of 2 /zm diameter and deflected it by an electromagnetic scanner controlled by a computer, thus shaping a raster of the size up to 250 X 250 /xm on the surface of the sample. Since we used a channel for the reflected light, we could obtain an optical raster image of the sample by a photodetector and thus control the surface visually. The laser intensity was modulated by a sinusoidal signal of 0-100 kHz frequency and was attenuated if necessary by means of a polarizer. For choosing a specific part of the sample, the entire optical system could be shifted with 0.5 /zm step increments parallel to the sample plane by computer controlled stepping motors within an 10 x 10 mm field. A sufficiently large flange focal distance of the focusing system up to 20 mm and no necessity in using a vacuum system for the laser beam, enabled us to attach practically any optical cryostat. For investigating low and high Tc superconductors in high magnetic fields we constructed a low tempera-

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ture attachment comprising of a helium cryostat with a 5 T superconducting solenoid with an inner vacuum chamber of 35 mm diameter. A gas-flow cryostat with a cooling finger tipped with a copper sample holder was set into the inner cavity of the solenoid. The sample was fixed on the holder with a heat conducting vacuum paste to ensure proper thermal contact. The sample temperature was roughly adjusted by the coolant consumption. As a coolant we used liquid helium or nitrogen, depending on the range of operating temperatures. For a precise temperature adjustment we inserted into the sample holder two thermal sensors for different temperature ranges and a bifilar heater to maintain the precise temperature using an electronic PID regulator. Thus, we covered the temperature range from the room temperature (300 K) down to helium (4,5 K) with a precision of 5 mK. The design of the gas-flow cryostat enabled the filling of its inner chamber with

A.G. Sit,akot, et al. / Applied Surface Science 106 (1996) 390-395

392

liquid helium and maintaining a temperature below 4.5 K by pumping out the vapour during experimentation with low T~ superconductors. Ac voltage of the sample response to the modulated laser beam was detected phase sensitively by a lock-in amplifier that received the reference voltage from the generator which modulated the laser beam intensity. The detected signal was acquired by the computer through an analog-to-digital converter together with other data from the auxiliary digital meters of the experiment and control equipment. If needed, we could use two converters in parallel (e.g., for recording signals from the sample and from the photodetector). While scanning the sample, the response was displayed on the monitor screen as a grey-scale, two-dimensional or quasi-three-dimensional ('landscape') picture.

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3. Results The critical temperature and the critical current are the important characteristics of any superconducting device. Using the LTSLM technique, the local values of these characteristics can be obtained by taking SLM images at several temperatures or bias currents. The wide-range and low temperature attachment enables the investigations of high Tc samples with highly scattered critical temperatures and also low Tc superconductors. In the present work we investigated high Tc Josephson junction arrays and superconducting Sn thin films. 25

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Fig. 3. Schematic top view of ramp Josephson junction array (a) and LTSLM images of the array at temperatures corresponding to the midpoints of superconducting transitions of bottom electrodes (89 K) (b), top electrodes (85.5 K) (c), and junctions (55 K) (d). The resistive areas contributing to response are darker.

The arrays consisted each of 24 Josephson ramptype junctions connected in series prepared at Twente University (The Netherlands) by the technology reported in Ref. [3]. The junctions were made by overlapping base and top HTSC electrodes through a thin barrier layer in the vicinity of the ramp in the base electrodes. Fig. 2 exhibits a resistance versus temperature plot for the normal-to-superconducting transition of one of the tested junction arrays. It is seen from the curve that the array transition is step-like which is typical for all samples. By using LTSLM we have spatially resolved the elements responsible for those partial transitions. Fig. 3 shows the sample schematics (a) and LTSLM images of a fragment of the junction array at three different temperatures ( b - d ) which correspond

A.G. Sit,akou et al. / Applied Surface Science 106 (1996) 390-395

approximately to the midpoints of the R(T) curve steps. The darker areas correspond to the higher response voltages. The response voltage is caused by the variation of the sample resistance due to the sample overheating in the illuminated spot. It is seen from the picture that at the temperature of 89 K (b), the highest response is observed at the bottom electrodes while the top ones are still in the normal state contributing little to the response because of the small value of d R / d T . The picture is reversed at the temperature of 85.5 K (c). This temperature corresponds to the transition temperature of the top electrodes and hence they display higher response while the measuring current of the bottom electrodes is below the critical value and thus they add nothing to the response voltage. The third step is associated with the superconducting transition of the junctions themselves. The corresponding LTSLM image is taken at the temperature of 55 K. Besides obtaining the spatial distribution of the critical parameters of a superconductor the LTSLM technique enables also spatially resolved studies of a superconductor in a resistive state when the superconductivity coexists with the electric field in a wide range of transport currents. We used the described technique to image the process of destroying the superconductivity by a current in thin film superconductors. As was shown in Ref. [4], along with a dynamic mixed state, in wide (as compared with the penetration depth of a magnetic field) films a resistive state associated with the formation of spatially localized non-equilibrium regions is achieved. These regions, called phase slip lines (PSL), are similar to phase slip centres observed in quasi-one-dimensional superconducting channels. Fig. 4a shows the I - V curve of the Sn thin film strip at a temperature below T~. It is known that the initial part of the I - V curve up to the current denoted by I * in the figure is associated with the dynamic mixed state. The linear part of the curve prior to the transition into the normal state reflects a resistive state due to the PSLs. The intermediate region with the large differential resistance was not studied theoretically or experimentally. We visualized the resistive state of the film strip just in this region.

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Fig. 4. An I-V curve of Sn thin film strip recorded at a temperature of 3.8 K (a) and response images obtained at several transport currents of 800 /~A (b), 850 /~A (c), 920 /~A (d), 960 /~A (e) (indicated in the 1-V curve by full points), the film critical current being 750 p,A. The phase slip lines are the darker areas in b-e. A set of visual images taken at the currents indicated by the points on the I - V curve is presented in Fig. 4b-e. The comparison of the visual pictures obtained for various currents with the I - V curve appearance leads us to the conclusion that the current I * corresponds to the shaping of the first PSL in the film, while the interval of a large differential resistance is caused by the increase in the number of PSLs. The formation of a subsequent PSL is accompanied by their rearrangement after which the lines

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A.G. Sicakou et al. /Applied Surface Science 106 (1996) 390-395

are located symmetrically relative to the centre of sample to occupy the largest volume. The process of spatial rearrangement of the PSL system is not obvious and is not related with the I - V curve shape. This rearrangement should be accounted for by a future theory of the resistive state with PCLs.

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4. Discussion

In the previous section we showed that the visualization of the process of normal-to-superconducting state transition enables one to find how the total resistive transition curve depends on the transitions of individual elements of the thin film structure. The bolometric response is exploited in this case due to the strong dependence of the resistance on temperature. The technique's resolution in the bolometric mode depends on the size of the perturbed part of the film rather than on the diameter of the illuminated spot. The size of the perturbed region is determined by the frequency-dependent thermal healing length. Typical values of this parameter for most HTSC films lie within the range of several to tens of micrometers. This is also true for the scanning electron microscopy, hence the spatial resolution of both techniques in this mode is practically the same in spite of the more sharp focusing of the electron beam. Thus the abilities of the electron and the laser scanning microscopy in most cases give similar results. However, the results obtained by the two methods above may differ when non-equilibrium mechanisms of the response are involved. To measure the local critical temperature of a sample part there is no need to obtain a set of LTSLM images of the whole sample at several temperatures. The local critical temperature at the point of interest can be determined by recording the temperature dependence of the response voltage for a fixed laser beam position. In this way the transition temperatures of each electrode in the array were determined individually. To determine the critical temperature of a whole electrode it must be overheated by the beam uniformly. For this sake the beam modulation frequency was chosen such that the size of the thermally perturbed region defined by the frequency-dependent thermal healing length would

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be comparable with the electrode dimensions. The local film overheating did not exceed O. ! K. The measurements showed that the quality of base and top electrodes was different, presumably due to the deterioration of the substrate surface after etching

A.G. Sit,ako~, et al. / Applied Surface Science 106 (1996) 390-395

of the base electrode. Along the junction chain the Tc gradients of the base and the top films of 1 K / m m and 10 K / m m , correspondingly, were observed. The transition widths were 1.2 K for the base film and 5 K for the top, the corresponding T~s being 89.7 K and 86.2 K at one of the chain ends and reducing to 89.2 K and 82.4 K at the others. The Tc gradient of electrodes resulted in a gradient of the critical current of the junctions. We point out that for the LTSLM studies of the ready-made Josephson junction array there was no need to prepare any additional potential taps, while the information on T~ of each junction was obtained by measuring the response voltage across the whole sample. The higher beam modulation frequency, the smaller thermally perturbed region governed by the temperature healing length, and hence the better spatial resolution of the technique was limited by the light spot diameter only. Since the probe diameter is less than the sample dimensions the response image is essentially non-one-dimensional and its interpretation becomes then ambiguous. Nevertheless, even in this case some useful qualitative information could be derived from a set of the SLM images obtained at fixed values of a systematically changed parameter like temperature, current and magnetic field. Fig. 5 displays a number of SLM images representing the transition of a polycrystalline Bi based HTSC film into the superconducting state. In these images one can observe the transitions of the separate grains of the film and the network of weak links formed at intergrain boundaries. This information could be useful in studies of the resistivity mechanisms, e.g., percolation, in highly inhomogeneous superconductors, and from the technological point of view for determining the film homogeneity.

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5. Conclusion The electron and laser scanning microscopy ability for studying the spatial distribution and the local values of superconducting parameters in thin film structures was found to be quite similar. The obvious advantage of the low temperature scanning laser microscope is its setup simplicity. With the newly developed wide-range low temperature attachment the spatially resolved studies of the highly scattered critical temperatures of high Tc Josephson junctions arrays became possible. The results obtained were used for the understanding of the array behavior in other experiments. Exploiting the local interaction between the laser beam and non-equilibrium resistive state in the superconducting Sn film we visualized the evolution of the spatial structure of this resistive state associated with the formation of the phase slip lines. The extremely low perturbance of the film by the laser beam allowed us to observe the rearrangement of the unpinned phase slip lines in the film strip.

References [1] A. Frenkel, Phys. Rev. B 48 (1993) 9717. [2] R. Gross and D. Koelle, Preprint University of Tubingen (1993). [3] E.M.C.M. Reuvekamp, G.J. Gerritsma, D. Terpstra, M.A.J. Verhoeven, H. Rogalla, J.G. Wen and H.W. Zandbergen, in: Applied Superconductivity, Ed. H.C. Freyhardt, Proc. EUCAS, GtJttingen, Germany,Vol. 2 (1993) p. 1231. [4] V.G. Volotskaya,I.M. Dmitrenko and A.G. Sivakov, Sov. J. Low Temp. Phys. 10 (1984) 179.