The novel physical properties of high-temperature superconductors (HTS) have placed unusually severe requirements upon substrate materials to be used for thin film applications. To date, the highest quality thin films have only been produced upon a limited class of crystalline substrate materials while film properties have generally been at best mediocre on most conventional substrates. The ongoing development of in situ film deposition techniques has improved this situation to some extent and the further evolution of film processing promises to expand the selection of useful substrates for HTS films and devices. In this paper we review the current status of substrates for HTS thin film electronic applications and discuss a number of issues pertaining to substrates that continue to play a significant role in the development of HTS thin film technology.

High-Tc superconducting thin films of Bismuth layered perovskite were artificially prepared by reactive sequential deposition of Bi, SrCu, and CaCu using DC magnetron multitarget sputtering. This deposition technique achieves a selective growth for Bismuth system composed with four and five Cu-0 planes by controlling the deposition time of CaCu oxide and its input power. These were post annealed and it was made sure that thin films with five Cu-0 planes did not exhibit zero resistivity. In order to obtain the as-deposited superconducting thin films, intermittent deposition technique was investigated. The Details about this fabrication process were studied referring to superconducting properties and crystal structure.

(Rb,Ba)Bi03 is a model material for the application of molecular beam epitaxy (MBE) to the growth of the superconducting perovskites. Synthesis of the superconducting phase can be accomplished in situ at temperatures between 300°C and 400°C, which has allowed the growth of superconducting films on silicon substrates. Measurements of the sticking coefficients for various growth conditions have shown that in a narrow region of flux compositions, adsorption and desorption rates of bismuth and rubidium (and possibly barium) will control the stoichiometry of the film. The epitaxy proceeds in the normal (1 0 0) orientation on [1 0 0) SrTiO3, despite a 10% lattice mismatch. On (10 0) MgO substrates, (1 1 0) epitaxy is obtained at lower substrate temperatures, while lattice matched (10 0) epitaxy occurs at higher substrate temperatures.

Superconducting YBa2Cu3O7-x thin films have been prepared by co-evaporation using an ozone vapor jet to oxidize the films. Films exhibiting zero resistance at 85 K have been fabricated in-situ under high vacuum conditions using substrate temperatures of 700°C without a post-evaporation anneal in oxygen. This process has the advantage that ozone can be made and stored in very pure form, unlike most other commonly used forms of highly reactive oxygen. This makes ozone very useful when a well characterized oxidizing gas is needed for systematic studies of the growth kinetics and oxidation of oxide superconductor films. In this paper, techniques for growing films using ozone are discussed, including the production of pure ozone vapor. Characteristics of the resulting films are also discussed. The superior oxidation efficiency of ozone may facilitate in-situ measurements on fully oxidized superconducting surfaces using techniques such as RHEED and XPS that require nearly UHV conditions. The fabrication in nearly UHV of devices and structures requiring fully oxidized interfaces and surfaces may also be possible using oxidation by ozone.

Work on high temperature superconductor films on silicon is reviewed. High temperature superconductor thin films of YBa2Cu3O7-x, (YBCO) have been made on single crystal SrTiO3 with zero resistance transition temperatures(Tc) of 90 K and critical current densities (Jc) up to 5 x 106 A cm-2 at 77 K. Both thick and thin films of YBCO have been made on silicon or on silicon with a buffer layer with Tc up to 85-90 K. Polycrystalline superconducting films with unoriented grains exhibit weak link behavior and have low values of Jc (a few hun-dred A cm-2 at 77 K), whereas the considerably higher value of 6 x 104 A cm-2 at 77 K has been achieved for YBCO deposited on a double epitaxial buffer layer on silicon.

High-Tc superconducting Ba2YCu3Ox thin films have been epitaxially grown on Si(100) substrates with the epitaxially grown Zr02 or yttria-stabilized zirconia (YSZ) as a buffer layer. The thin films were prepared by rf magnetron sputtering of the stoichiometric Ba2YCu30x target below 700°C. An adequate positive substrate bias was necessary to ensure surface smoothness and to show the superconducting transition above liquid nitrogen temperature without any post-treatment. The highest Tc(onset) and Tc(end) observed were 88K and 84K, respectively. The critical current density was Mx105A/cm2 at 50K. Using the silicon substrates patterned with the trench, the superconducting microbridge junction has been fabricated in as-grown Ba2YCu3Ox thin films by rf-magnetron sputtering. The microbridge junctions with constrictions as small as submicron dimension were obtained. These microbridge junctions behaved as Josephson junction and were observed microwave-induced steps. Based on Likharev's theory, it is suggested that these devices show Josephson effect in the Abrikosov vortex motion region.

The capability of developing an efficient activated oxygen source in conjunction with molecular beam epitaxy has led to successful thin film synthesis of Y1Ba2Cu3O7-x, high temperature superconductors. The smooth morphology and well ordered surface structures of these in-situ grown films allow to conduct superconducting tunneling experiments directly in a planar junction mode. We review in this paper in-situ film growth, materials and superconducting properties, and quasiparticle tunneling characteristics of Y1Ba2Cu3O7-x/native barrier/Pb junctions.

Pulsed laser deposition is a technique commonly used to deposit high quality thin films of high temperature superconductors. We discuss the results obtained when this technique is applied to the deposition of Tl-CA-Ba-Cu-O thin films using a frequency doubled Nd:YAG laser operating at 532 nm and an excimer laser operating at 248 nm. Films with onset temperatures of 125 K and zero resistance temperatures of 110 K deposited on (100) oriented MgO from a composite T12Ca2Ba2Cu3Ox target were obtained at both wavelengths upon appropriate post deposition annealing. We will discuss the microstructure and composition as a function of laser wavelength and annealing conditions.

I. Introduction: Characteristic Features Of Laser Ablation Method For HTSC Thin Films Among the various methods for the HTSC film formation, the laser ablation method has following advantages. First, the film formation can be performed under a high oxygen or N20 pressure, such as 10-1 torr.1 This condition is not easily satisfied by other techniques such as MBE and evaporation. Second, there is small deviation of the composition of the films from the target composition used for the film formation. This is advantageous for the films with multi-component systems, such as Y-Ba-CL-O2 or Bi-Sr-Ca-Cu-0.3) Third, photochemically excited species produced by ablation can contribute to the low temperature film formation.4) Furthermore, the laser beam can be intentionally used for the substrate excitation to lower the substrate temperature and to increase the crystallinity of the films.

High Tc superconducting YBa2Cu3O7 and Bi2Sr2CaCu2Ox films have been successfully prepared by the Liquid Phase Epitaxial (LPE) technique. Their X-ray diffraction patterns revealed a high degree of (001) preferred orientations with their c-axes perpendicular to the film surfaces. Both these in-situ grown YBa2Cu3O7 and Bi2Sr2CaCu2Ox films showed the zero-resistivity at 80°K, respectively.

A status report is given on the metalorganic chemical vapor deposition (MOCVD) of high-temperature superconducting thin films. The advantages of MOCVD processing manifest themselves in the quality of the films produced, and in the economy of the process. Metalorganic precursor requirements, deposition parameters and film properties are discussed. Also difficulties have been identified in making MOCVD a manufacturing technology. To solve these problems, future research directions are proposed.

Thin films of YBa2Cu3O7-x were grown on MgO(100) by metalorganic chemical vapor deposition (MOCVD). Low pressure growth studies were carried out between 400 and 600°C using metal β-diketonate complexes as source reagents for Y, Ba, and Cu. As-deposited films were amorphous and a two stage annealing protocol was used in which fluorine was first removed in a Ar/H20 stream between 700 and 850°C, followed by calcination in flowing oxygen between 500 and 950°C. Scanning electron microscopy, X-ray diffraction and energy dispersive analysis indicate that good compositional and dimensional uniformity could be achieved. The temperature of the oxygen annealing step was shown to have a dramatic impact on the physical and electrical properties of the YBa2Cu307-x thin films. Annealing temperatures exceeding 910°C gave large crystallites and semiconducting resistivity above Tc; annealing temperatures below 910°C yielded films with metallic conductivity whose density and superconducting transition varied inversely with maximum annealing temperature. Optimized deposition/annealing protocols yielded films with a preferred c-axis orientation, R273/R100 ratios of 2, onsets as high as 94K and zero resistance exceeding 90K.

Superconducting thin films of Bi-Sr-Ca-Cu-O have been grown on single crystal (100) SrTiO3 and MgO substrates using laser ablation at 355 nm. Films have been deposited using compositions of 2:2:1:2 (Bi:Sr:Ca:Cu) and 1.86:0.33:1.93:2.06:3.06 (Bi:Pb:Sr:Ca:Cu) to form primarily the low temperature Bi2Sr2CalCu20, and the high temperature Bi2Sr2Ca2Cu3Ox phases, respectively. Films deposited at 760-765°C substrate temperature in the presence of 1 Torr of oxygen show superconducting onset temperatures of 85 K and 110 K, respectively, for the two compositions, with zero resistance obtained at about 70 K in both cases. The zero resistance temperatures for the films can be improved by subsequent annealing of the films at higher temperatures.

YBa2Cu3Oy thin films have been deposited in-situ on several substrate materials using pulsed excimer laser deposition. On the substrates, SrTiO3, MgO, LaA103, and yttrium-stabilized zirconia (YSZ), excellent films were obtained. These films had high superconducting transition temperatures (91K) with narrow transition widths (≈0.5K), metallic conductivity in the normal state, low room-temperature resistivity ( ~250 µΩ-cm), high critical currents (~3x107 A/cm2 at 4.2K), c-axis orientation perpendicular to the plane of the film, and epitaxial alignment with the substrate. On the more technologically relevant substrates of A12O3 and Si, less optimal results were obtained. The transition temperatures were high (86-88K) and metallic conductivity way obtained in the normal state. However, the room-temperature and microwave surface resistivities were higher and the critical currents were lower than for the above benchmark substrates. These diminished transport properties correlate with the imperfect alignment and epitaxy of the YBCO and substrate. For A12O3 substrates, a narrow substrate-temperature window was found for the best in-situ YBCO films. The poorer transport properties correlate with the lack of registry of the YBCO a-b plane with the sapphire r-plane. For Si substrates, a buffer layer is required due to high reactivity even at substrate temperatures as low as 550 C. YSZ provides a good buffer, and our best results were obtained on clean, hydrogen-terminated surfaces rather than oxidized Si. The amount of Y2O3 in ZrO2 was varied, and the best films were obtained with x near 0.1 where (ZrO2)1-x(Y2O3)x is cubic. Epitaxial alignment of the YBCO with the Si was achieved, but there was a substantial spread in orientations, accounting for the diminished transport properties.

Recent accomplishments such as the demonstration of low microwave surface resistance over a wide range of frequencies and low magnetic flux noise in high quality thin films of the 90 K superconductor YBa2Cu3O7-x (YBCO), have heightened the hope that the new high temperature superconductors (HTSC) can play an important technological role. With improvement in the quality of films, very exciting superconducting state properties which till now were masked by inhomogeneities introduced by the fabrication processes, are beginning to be uncovered. Progress in these areas has been easiest for those in the community who initially chose to focus on understanding the materials issues relating to the growth of high quality thin films and single crystals. Current techniques for studying these materials include measurements of the high frequency response of single layer YBCO films in order to reveal the intrinsic properties of the superconductors and gauge their suitability for technological applications. By alloying the superconductors in the cation sites, we can also extract information on the mechanisms of superconductivity in these systems, generate new families of lattice matched materials, such as the Y1-yPryBa2Cu3O7-x system which for y=0 to 1 possess a variety of electrical properties and can form building blocks for multilayer junctions or superlattices.

The physics of the in-situ laser deposition process is reviewed. Emphasis will be placed on the use of excimer lasers, and the deposition of YBCO thin films. It will be shown that the laser target interaction conditions, and the properties of the laser generated plume are inducive to the formation of high quality films. In addition to the formation of energetic atomic beams, laser deposition is also highly compatible with reactive deposition which make it suitable for oxide and nitride films. An in-situ diagnostic technique is introduced which is capable of detecting the inter-facial boundary layer between the film and the substrate. It is also shown that post-deposition in-situ oxidation is necessary for the formation of superconducting films.

A family of fast time response ion probes has been developed to study the laser-ablation plume dynamics under Y1Ba2Cu3O7-x film-growth conditions. These probes are useful over a wide range of pressures, distances, laser energies, and energy densities. The ion probe measurements are complemented and corroborated by measurements using spatially and temporally resolved optical emission and absorption spectroscopy. The results confirm a long lived ionized component to the plume and a marked slowing of the plasma front at film deposition pressures and distances. Both the ion probe and spectroscopic techniques show promise as sensitive process monitors for film deposition by laser ablation.

The deposition characteristics of YBa2Cu3O7 thin films formed by pulsed laser evaporation (PLE) method have been found to be significantly different from films obtained by other vaporization methods primarily because of very high evaporation flux which absorbs the incoming laser irradiation. Based on the experimentally obtained deposition characteristics, the physics of the PLE process is analyzed, and a hydrodynamic gas expansion model is proposed for the PLE process. In this model, the laser generated partially ionized plasma, initially under high temperature and pressure, expands anisotropically into vacuum. The plasma expansion characteristics determine the nature of the deposition process. Solutions governing the plasma expansion are obtained, and the calculated deposition characteristics are compared with results obtained on PLE deposited YBa2Cu3O7 films on silicon substrates. This model is able to explain most of the salient features of the pulsed laser deposition technique. The non-equilibrium nature of the PLE technique has been utilized for in-situ fabrication of superconducting YBa2Cu3O7 thin films on (100) SrTiO3, (100) YS-ZrO2, and (100) LaA1O3 substrates in the temperature range of 500-650°C. A positively biased ring between the substrate and the target has been found to reduce the processing temperatures to 500°C, although the epitaxial quality of the films deteriorated considerably below 550°C. The films formed on lattice matched SrTiO3 and LaA1O3 substrates are virtually defect-free with minimum channeling yields values equal to the single crystal value. Critical current densities values over 6.0 x 106 amps/cm2 (at 77 K and zero magnetic field) were obtained for epitaxial silver doped films YBa2Cu3O7 films on (100) LaA1O3 substrates. The effect of the processing parameters on the superconducting properties and microstructure of thin films is discussed in detail.

High Tc superconductor films were prepared by laser deposition. Epitaxial YBaCuO films with high critical current density are obtained for a uniform laser beam energy density of about 4 J/cm2, a substrate temperature above 730°C and an oxygen pressure of 0.3 to 0.5 mbar. Using identical parameters, LaA103 interlayers grow epitaxially on the YBaCuO films and can therefore be used as insulating layers in multi-layer structures. For the Bi(Pb)SrCaCuO system, both the 80 K and the 110 K phase can be prepared in-situ with inductively measured 111's of 77 K and 80 K, respectively. A subsequent annealing step improves these values. For the 110 K phase a Tc(R = 0) of 98 K was obtained. Whereas the critical current densities in the Bi(Pb)SrCaCuO films must still be improved, the epitaxial YBaCuO films show high critical current densities as 5 x 106 A/cm2 at 77 K and zero field or 5 x 107 A/cm2 at 4.2 K and 7 T (for the magnetic field in the film plane). The critical current densities are affected by defects introduced by ion irradiation.

Scanning electron microscopy (SEM), Auger electron spectroscopy (AES), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS) are providing a detailed understanding of the oxide superconductors. The SEM has revealed the causes of surface morphologies such as excessive roughness, graininess, and cracking, as well as the mechanisms of particle deposition and the charges on these particles; at low temperatures, it has revealed the reasons for the non-linear superconductive-resistive transitions in current carrying films. AES has been used to study interfacial layers and multi-layer superstructures, such as the atomically abrupt junctions between differently substituted Y-Ba-Cu-O hetero-epitaxial layers. TEM has revealed the defect structure of "epitaxial" films, as well as the nature of the atomic accommodation at film-substrate interfaces, and the surprising complexities of the epitaxial growth processes. TEM measurements of the correct microstructure and local stoichiometry of Bi-Sr-Ca-Cu-O phases have focused attention on the doping mechanisms in these apparently "valence balanced" superconductors, that may be different from the majority of oxide superconductors that are doped by the conventional valence imbalance mechanism. XPS measurements of non-superconducting surface layers on superconducting films can detect layers only 0.1 nm thick and laser deposited Y-Ba-Cu-O films can have surface layers only 1 nm thick after exposure to air. XPS has been used to show that extremely thin superconducting films (< 10 nm thick), having a negligible area of pin-holes, can be made, demonstrating that laser deposition can produce extremely thin films of high integrity. The valencies of all the elements in all major families of oxide superconductors have been measured, which leads to a better understanding of the doping mechanism, the electronic structure, and the nature of the carriers responsible for superconductivity.

In potential applications, the recently discovered high transition temperature (high-Tc) ceramic superconductors (Bednorz and Muller, 1986, Wu et al., 1987, Cava et al., 1987) may experience large mechanical stresses and strains. These can be imposed by magnet fabrication, high magnetic fields, and, in the case of superconducting films, also by thermal contraction mismatch with the substrate material (see, for instance, Baynham, 1988, Severin and de With, 1988). Although mechanical strength of a superconductor may appear to be not as important a property, as, say, high superconducting transition temperature, high upper critical magnetic field or high critical current density, it may play a decisive role, when a superconducting material is used for practical purposes. Since ceramics are brittle materials, and break quite easily when stretched, bent or hit, use of ceramics as practical superconductors requires that they possess high ultimate stress and strain, sufficient fracture toughness and good shock resistance. It is also important that the actual stresses and strains arising in superconducting ceramics at low temperatures can be predicted and, if possible, minimized.

In order to fabricate superconducting films which have large critical currents while maintaining high Tc values it is necessary to understand the role and origin of flux-line pinning centers. The dissipation associated with imperfect pinning is being studied with both conventional transport and mechanical oscillator measurements on high-Tc films.

High-resolution patterning of thin-film high temperature superconductors is necessary in the development of device applications of these new materials. Such fine-line structures are necessary in the fabrication of devices such as superconducting quantum interference devices (SQUIDS), transmission lines and others. Furthermore, high spatial resolution patterning can be an important tool in studying the fundamental properties of the films themselves. Focused ion beam patterning has recently been applied to YBaCuO thin films. The finely focused ion beam can produce patterns by local physical sputtering or through implant damage. These techniques have proven to be valuable for producing submicron superconducting structures in a reliable way.

The discovery of high critical temperature (Tc) superconductors has spawned an extensive research effort to understand and apply these new materials. One potentially promising area is in microelectronics. High quality superconducting thin films on a variety of substrates have been demonstrated by several authors. However, useful application of these thin films requires subsequent processing to fabricate microelectronic structures. These structures can range from relatively simple superconducting lines for interconnects to as yet undiscovered electronic devices. In this paper, we present our work on the use of a focused laser beam to modify the transport properties of a superconducting thin film. By placing the film in a vacuum chamber and scanning a focused Ar+ laser beam normal to the surface, we can selectively remove oxygen from the film on a micron scale. The resulting transport properties reveal a reduced Tc and critical current density (Jc). By monitoring the room temperature electrical resistance, one can control the degree of oxygen removal which allows one to modify the film properties from metallic-like to semiconducting-like. This technique shows promise for forming device structures based upon a superconducting-semiconducting-superconducting geometry. Current progress towards the realization of a fast photodetector fabricated by this technique will be discussed.

We studied plasma treatment for application to high Tc superconducting devices. The electrical and physical properties of high Tc thin films can be modified by Ar plasma treatment. The Ar plasma irradiated films showed super-normal transition. We confirmed by X-ray photoelectron spectroscopy that Ar plasma treatment changed Cu valence of high Tc films significantly. We tried to fabricate super-normal-super (S-N-S) structures from high Tc thin films using selective Ar plasma treatment.

Investigation of the effects of ion bombardment and subsequent annealing of YBa2Cu3O7-x thin films indicates that a major part of the damage for low ion doses is associated with low activation energy processes. This conclusion is based on the partial recovery of the superconducting properties (TR.n and 3c) and the c-axis lattice parameter by annealing at temperatures as low as 300°C. The recovery can be substantial; in some cases superconduc-tivity reappeared in films where it had been completely lost and the c-parameter recovered more than half the increase that had occurred due to ion bombardment. This suggests that the likely damage mechanism at low doses is in the oxygen sublattice, such as the destruction of the long range order in the Cu-0 chains and generation of vacancies in the Cu-0 square planes. Almost full recovery of the 0+ implanted films occurs with 900°C annealing. However, in Si+ implanted films recovery occurs only up to 700-750°C followed by severe degradation of Tc and L for anneals at 900°C.

Raman spectroscopy can provide information about stoichiometry, oxygen content, and crystal structure of high-Tc thin films in a rapid, contactless, non-destructive manner at room temperature. Data are presented for both polycrystalline and single crystal-like films. An example is given of the potential utility for direct local analysis of patterned films with features as small as 1µm.

The optical response of granular films of YBa2Cu3O7-x to pulsed far infrared radiation has been measured. The infrared light energies ranged from 2.5 to 19 meV. Direct coupling to intergrain Joseph-son currents dominates the 3.2 meV photoresponse. Such direct coupling effects are considerably reduced for 10.9 and 19.1 meV light. These results will be interpreted in terms of possible magnitudes for the superconducting energy gap in YBCO.

Milling with Xenon ions directed at grazing angles onto the surfaces of c-axis oriented YBa2Cu3O7 films is shown to give smooth films which superconduct at thicknesses approaching a lattice constant. The observed linear dependence of the room temperature sheet conductance with milling time allows a determination of the effective electrical thickness after each milling stage. The resistance transitions of all films having a superconducting transition show a metallic behavior with a positive temperature coefficient of resistance. Degradation of superconducting properties becomes significant for films less than 100A thick although this degradation is appreciably less for films which initially (before milling) have sharp resistance transitions and a strong c-axis orientation. Reproducible and controlled fabrication of ultrathin films using this milling technique shows promise as an important step in the post-deposition processing of high- Tc thin films for electronic device applications.

Micro-bridges are fabricated with YBa2Cu3O7-8 thin films prepared by CVD method. The thickness of the bridge is ranging from 0.2 to 1 µm and the width and length are both about 1 µm. The critical current density of the bridge is ranging from 104 to 105 A/cm2 at 77K in zero field. The bridge shows clear ac and dc Josephson effects. However, temperature dependences of critical current show Ic oc (1-T/Tc)3/2, which suggests the transport current is superconducting bulk current and not Josephson current. The single bridge looks like a typical Josephson junction and all transport current in the bridge behaves as Josephson current.

Electron tunneling for YBa2Cu3O7-x(YBCO) thin film prepared by activated reactive evapo-ration technique has been studied using normal conductor-insulator-superconductor (NIS) tunnel junctions and using superconductor-insulator-superconductor(SIS) tunnel junctions. Based on data of NIS using single crystal thin film of YBC0(001), reproducible gap parameter ▵⊥(4.6K) of 9.0 ± 1.8meV and a coupling constant of 2.4 ± 0.5 < 2▵⊥(0)/kTc < 3.5 ±0.7 in perpendicular direction to the Cu-0 planes were obtained. The quasi-particle density of states exhibits a lifetime broadening picture. The recombination lifetime t r of quasiparticle decreases as the temperature approaching to the transition point from low temperatures. 77, at 60K is estimated to be 10-13 s. Multipeaks which are often observed in differential tunnel conductance spectra of (110) or (103) oriented thin film of YBCO are understood as not to be an intrinsic property of YBCO.

Many of the applications envisioned for the high temperature superconductors (HTSC) lie in the area of Josephson electronics, but realization of this technological promise requires a controllable multi-layer process which would admit the possibility of insulating layers, useful for applications from ground-plane isolation to tunnel-barrier formation, in combination with high quality superconducting or normal-conducting layers. We have begun investigating the growth and processing techniques necessary to realize these structures by studying a prototype multi-layer system based on Y Pr1-xBa2Cu307-x alloys. This class of materials is of great interest due to two facts: By varying the Pr concentration, x, the material's resistivity can be tuned. For x=0, we have pure YBa2Cu307-x (YBCO) which is a 90K superconductor; with increasing x, Tc decreases until at x =1 with pure PrBa2Cu307_ (PBCO) we find a divergent resistivity with decreasing T. Secondly, over the full alloy range, 0<x <1, the material remains orthorhombic in structure and is lattice matched to YBCO to a fraction of a percent. 1 Our major effort to date has been to study Josephson weak-link formation by using YBCO for electrodes and PBCO as a lattice-matched semiconducting barrier layer. 2 The first step is the growth of a heteroepitaxial four-layer structure of YBCO - PBCO YBCO - Au, using our standard laser deposition process. "All layers are grown during a single cycle of the vacuum system. We use Rutherford backscattering, both in the random and channeling modes, and X-ray scattering to verify layer stoichometry and heteroepitaxial growth of the orthorhombic perovskite structure, c-axis normal to the substrate. Both YBCO layers have sharp superconducting transitions above 80K upon removal from the vacuum system. Single devices with areas, A, from 2.5x10-5cm2 to 2x10-7crn2 are isolated with a four-step fabrication procedure involving standard photolithography and Ar ion milling. These devices show Current-Voltage characteristics similar to those observed for low temperature Superconductor /Normal metal /Superconductor (SNS) devices. We find good scaling of the critical currents. I. with A and scaling of the resistances, R7, with 1/A; the typical values of the /c.R7 product of 3.5rnV are consistent with traditional SNS behavior. Further, we observe Shapiro steps in response to 100GHz mm-wave radiation and oscillation of the d.c. supercurrent in a transverse magnetic field thus demonstrating that both the a.c. and d.c. Josephson effect occurs in these devices. These preliminary results suggest that our multi-layer process can be optimized for a variety of Josephson devices essential in producing electronic circuitry from the HTSC systems.

We describe the measurement of microwave surface resistance, Rs, of thin films of high-transition-temperature superconductors by the stripline resonator method. This method allows measurement of Rs as a function of frequency from 0.5 GHz to 20 GHz and has a sensitivity of greater than 1 x10-7 Ω. Rs for films deposited by various methods has been measured, and a limited temperature dependence for one YBCO film is also reported. The results are related to the deposition methods and film properties.

Thick (10-15 µm) Tl-Ba-Ca-Cu-O films have been deposited onto yttria-stabilized zirconia and Ag substrates by d.c. magnetron sputtering techniques. Direct deposition onto 1" diameter yttria-stabilized zirconia yields films with typical 22-GHz surface resistance (Rs) values of 5.2 ± 2 mS2 and 52 ± 2 mK2 at 10 K and 77 K, respectively. For comparison, Rs of Cu at this same frequency is 10 mK2 at 4 K and 22 mg2 at 77 K. Tl-Ba-Ca-Cu-O films have also been deposited onto 1" diameter Ag substrates using Au/Cu, Cu, and BaF2 buffer layers. The lowest Rs values were obtained on films with a BaF2 buffer layer, typical values being 7.8 ± 2 mC2 and 30.6 ± 2 mC/ (measured at 22 GHz) at 10 K and 77 K, respectively. Larger films (1.5" diameter) with similar Rs values were prepared using this same technique, demonstrating that the fabrication process can be scaled to larger surface areas. These films are promising for radiofrequency cavity applications because they are thick (50 -75 times the London penetration depth), have relatively large surface areas, are fabricated on metallic substrates, and have Rs values that are competitive with Cu at 77 K and are lower than Cu at 4 K. Because they are polycrystalline and unoriented, it is anticipated that their Rs values can be lowered by improving the processing technique. High-quality films of YBa2Cu3O7 have been electron-beam deposited onto 1" LaGa03 and 1.5" LaA103 substrates. The 1" sample is characterized by Rs values of 0.2 ± 0.1 mS2 at 4 K and 18.6 ± 2 mΩ at 77 K. The 4-K value is only 2 - 4 times higher than Nb. The 1.5" sample has Rs values (measured at 18 GHz) of 0.93 ± 2 Ω and 71 ± 3 mΩ at 10 K and 77 K, respectively.

The microwave surface resistance and dispersion of laser deposited YBaCuO films patterned in the form of a meander line have been measured from L through Ku-band at temperatures from 5 to 90K. Surface resistance values below 1 mΩ were demonstrated at 10 GHz and 79K. The results show great promise for the realization of effective high-Tc superconductive passive microwave components in the near future.

An essential need for all superconductor devices is to make ohmic, low resistivity contacts with normal conductors. Contacts made with indium solder, silver paint or epoxy, direct wire bonds, and pressure contacts have contact surface resistivities typically in the range 10-2-10 0-cm2, several orders of magnitude too high for many practical applications and critical current measurements. Contact resistivity levels on the order of 10-4-10-5 12-cm2 or lower are needed for critical current measurements to keep contact heating from becoming a problem (unless pulsed-current methods and sample necking techniques are used). For thin-film package inter-connects, contact resistivities in the 10-8 12-cm2 range are needed and finally, for on-chip interconnects, resistivities in the 10-10 Ω-cm2 range are required.