Pulsed laser deposition (PLD) has been widely used for deposition of high Tc superconducting thin films, and is recognized as one of the best physical vapor techniques for the preparation of these films. The most important advantage of this technique is stoichiometric deposition; films can be made with the same composition as the target. Utilizing PLD, not only thin films but also multilayers and superlattices of high Tc superconductors have been fabricated. In this paper, the performance of the technique will be reviewed, and speculations regarding the future would be made.

Laser deposition is widely used to fabricate thin-films of the YBa2Cu3O7−δ superconductor due to its ease of operation and its reproducibility. The films thus formed do not require post-deposition annealing to be superconducting. The processing route involves many variables which would be expected to exert an influence on the early stages of the film growth and thereby the final microstructure. The parameters which have been varied are the laser energy, laser pulse repetition rate and laser wavelength. The films have been examined using transmission electron microscopy (TEM) in order to determine the film orientation, composition and microstructure.

Excimer laser ablation with an in situ heat treatment was used to prepare high quality superconducting YBa2Cu3O7−x thin films on (100)-SrTiO3 and (100)-LaAlO3 substrates. A pulsed excimer laser (XeCl; 308 nm) was used to ablate a rotating, bulk YBa2Cu3O7−x target at a laser energy density of 2–3 J/cm2. Based on four-probe dc resistance measurements, the films exhibited superconducting transition temperatures (Tc, midpoint) of 88 and 87K with 2K (90–10%) transition widths for SrTiO3 and LaAlO3, respectively. Transport critical current densities (Jc) measured at 77K were 2 × 106 and 1 × 106 A/cm2 in zero field for SrTiO3 and LaAlO3, respectively. X-ray diffraction (XRD) analysis showed the films to be highly oriented, with the c-axis perpendicular to the substrate surface.

Pulsed KrF (248 nm) excimer laser ablation has been used to grow nonsymmetric epitaxial YBCO/PrBCO superlattices containing 1-, 2-, 3-, 4-, and 8-unit-cell-thick YBCO layers, separated by N-cell-thick insulating PrBCO layers (N = 1–16). The 1×N and 2×N structures exhibit a rapid initial decrease of Tco with increasing PrBCO layer thickness, followed by saturation of Tco at a nonzero value, as the YBCO layers' separation is increased further. The limiting Tco values for 1-, 2-, 3-, 4-, and 8-cell-thick YBCO layers isolated in a PrBCO matrix are ∼19 K, ∼54 K, ∼71 K, ∼80 K, and ∼87 K, respectively. Single-cell-thick YBCO layers are found to be superconducting even when separated by ∼19 nm in a PrBCO matrix. These results are discussed in terms of possible Josephson and/or proximity effects coupling the YBCO layers, and the possibility that PrBCO modifies the electronic structure and depresses Tco in YBCO cells that are immediately adjacent to the PrBCO layers. Finally, high-temperature postannealing studies show that these superlattice structures are stable and that little diffusion occurs along the c-axis direction of compositional modulation.

We present data on thin films of YBa2Cu3O7 (YBCO) and YBa2(Cu1−xFex)3O7 prepared by laser ablation using a sequence of targets, including Cu-deficient YBa2Cu2.4O7, CuO, and (CuO)1−y(FeO)y. This technique achieves mixing on an atomic scale. We find that stochiometric films, made using a combination of the Cudeficient target and CuO, have a sharp transition (width < 1K) near 90K although the Cu-deficient material alone has a broad transition at a lower Tc. We have achieved substitution of Fe on the Cu sites, using a sequence of the three targets for a range of Fe concentrations. Tc decreases nearly linearly with concentration and Tc → 0 near 15% Fe. The Tc suppression for the films is slightly less than the corresponding value obtained for bulk samples.

The fabrication of high-quality thin-films often depends on the early stages of the growth process during which epitaxy is established. The substrate structure and orientation generally play a critical role at this stage through epitaxy and interaction or reaction. Many different materials are being used as substrates to support thin-films of the YBa2Cu3O7-δ superconductors. In this study the early stages of the growth of YBa2Cu3O7-δ thin-films have been investigated using transmission electron microscopy (TEM). Using pulsed-laser ablation, ultra-thin films were formed directly onto self-supporting, ion-thinned, single-crystal ceramic substrates. The substrates used were (001)-oriented MgO, (001)-oriented SrTiO3 and (001)-oriented LaAlO3.

Laser-Ablation (LA) had received little attention prior to the first published report of depositing Y1Ba2Cu3O7-x) (YBCO) thin films with this novel method.[1] However, LA has been used to produce films of infrared materials for some time[2], and the recent discovery of 1-igh Temperature Superconductors (HITS) has sparked considerable interest in this relatively obscure Physical Vapor Deposition (PVD) technique. Over the past three years, a variety of in-situ LA processes for producing films of HTS compounds, as well as other materials, have been reported in the literature.[3,4,5,6,7] Like any other PVD technique, LA has its own unique advantages anid disadvantages. The main advantages of LA include: 1) its ability to accurately replicate the stoichiometry of the ablation target within the laser-deposited film; 2) the high energy of the ablated species which may enhance the quality of film growth; 3) it does not require hot filaments which allow a number of reactive gases to be present in the chamber during deposition; and 4) a wide array of complex chemical compounds can be deposited. Two of the main problems facing this emerging PVD technique have been: 1) it has been applied mostly to small area deposition (<6 cm2) with poor uniformity, and 2) the ablated films typically display a large number- of particles ranging in size from 0.5 μm to over 10 μm, whose presence may significantly hamper a number of microelectronic device applications.

We report here on the pulsed laser deposition of high quality, superconducting, 200 nm thick films of YBa2Cu3O7-x at rates approaching 15 nm/s. Film crystallinity and electrical properties were studied as a function of deposition rate from 0.1 to 14.5 nm/s. Though some degradation in the film crystallinity is observed by RBS channeling and X-ray rocking curve measurements, critical current densities (Jc = 4 × 106 A/cm2, 77 K, B = 0) are effectively unchanged in going to the higher deposition rates.

Superconducting films of YBa2Cu3O7-δ have been fabricated by pulsed laser deposition technique applying bias voltage to a substrate up to +/-500 V. It has been found that substrate bias voltages are effective for preparing the fidms with high Tc and high crystalline quality at lower substrate temperatures. Positive bias is more effective than negative one. Time resolved observation has been carried out in order to investigate the influence of biasing on the plume and revealed three kinds of light emission. The first is an ordinary plume with a velocity of 6×105 cm/s, which is independent of biasing. It suggests that most of the luminous particles in the plume are electrically neutral. The second is oxygen plasma with a short life time, which is triggered by laser pulse under the positive bias and triggered by arrival of the plume in 8 ps after the laser pulse under the negative bias. The third is an additional plume with a velocity of 3×105 cm/s. The oxygen plasma and additional plume have improved superconducting properties of films deposited with biasing to the substrate.

Thin epitaxial films of cubic - fluorite structured PrO2 and YSZ (yttria- stabilized zirconia) were grown on single crystal silicon substrates using the laser ablation - deposition technique. X-ray diffraction theta two - theta, omega rocking and phi scans indicate a high degree of epitaxial orientation of the films to the Si lattice. The highest quality of epitaxy was obtained with the PrO2 [111] oriented normal to Si(111) surfaces and the cubic YSZ [100] normal to Si(100) surfaces. For both PrO2 and YSZ, high epitaxial quality required the removal of the Si native oxide prior to deposition and careful control of the deposition environment. It was further found that the YSZ films on Si(100) were an excellent surface for subsequent laser ablation of YBCO films by the usual in situ process. The resistivity of this YBCO was ≈ 250 micro-ohm-cm at 300 K, extrapolated to the resistivity -temperature origin, showed a sharp transition to zero resistance at ≈ 85 K and was nearly identical to high quality YBCO films deposited on (bulk) YSZ substrates.

We report here on two aspects of non-chemical, post-deposition processing of laser-deposited superconducting Yba2Cu3O7-δthin films. One patterning process consists of placing a contact mask over the film and removes the surrounding material by excimer laser ablation. Good edge definition for 30–100 μm etch features is obtained. However, some redeposition of material is observed. In separate patterning experiments, oxygen-deficient films were re-oxygenated by a direct-write laser heating method. This process produces very smooth edges between the superconducting patterns and the surrounding material.

We have conducted two separate experiments in the UV laser ablation of a sintered Yba2Cu3Ox+6wafer. We have measured the photoejected population distributions using selected UV laser wavelengths (4.01, 4.17 and 4.35 eV) near the 4.1 eV optical transition in Yba2Cu3Ox+6 In addition, we have measured the change in the ejected species kinetic energy as a function of the target bulk temperature. All the experiments were conducted at laser fluences well below the plasma formation threshold, and near that for product formation. Our results show that the UV laser ablation, at threshold fluences, proceeds via a nonthermal electronic excitation mechanism.

Target modification by excimer laser exposure has been investigated. Under conditions typical in the fabrication of superconducting thin films, deposition rate decreases with exposure and significant physical and chemical modifications occur on the target surface. These modifications do not inhibit congruent evaporation.

Spatial and temporal measurements of the optical absorption, optical emission and ion probe response in the ablation plume formed following pulsed 248 nm irradiation of Y1Ba2Cu3Ox are reported over laser energy densities from near threshold into the film growth regime. Time of flight absorbance-velocity profiles in vacuum indicate the formation and acceleration of a plasma front, with ions leading neutrals on the edge of the expanding plume. Ion probe screening measurements show that the laser plume is a well-shielded plasma with Debye lengths <10 μm at film deposition distances. Velocity distributions and estimates of ground state Ba+, Ba, Y+, and Y densities indicate that the populations of the ions outnumber those of the neutrals at high energy densities in vacuum. Measurements of the slowing of the plasma front and attenuation of the total charge reaching the substrate are reported for laser ablation in background pressures of oxygen. Absorption by ground state YO and BaO in the region close to the pellet indicates oxide densities ˜5 × 1013 cm−3 close to the pellet.

Using a sintered Yba2Cu3Ox+6 wafer in high vacuum, we have measured the photo-ejected products and kinetic energies at selected UV laser wavelengths (351 nm, 248 nm, and 193 nm) where the laser fluence is maintained near product formation threshold. At these fluences, we are well below the above surface plasma formation threshold and do not detect; any evidence for surface melting. Our results show that, for a specific laser wavelength, both the ion and the neutral mass spectrum agree. The measured spectrum shows that the photablated products consist only of atomic and diatomic species. In addition, the oxygen is bound only to the yttrium and barium. Our measurements further show that, for a specific ejected species, the yield is dependent on both the laser fluence and wavelength. However, the measured KE distribution is independent of the laser fluence for fluences near threshold. The species are ejected with mean KE between 6–9 eV and no species has kinetic energy in excess of 13 eV Our results imply that for UV laser threshold fluence ablation, the excitation/desorption process is nonthermal.

We have deposited thin films of the electron-doped, high temperature superconductor Nd1.85Ce0.15CuO4−y by pulsed laser deposition. Films were deposited from a stoichiometric target using a KrF excimer laser (248 nm, 250 mJ/pulse, −2 J/cm2) as a function of substrate temperature, oxygen pressure and vacuum anneal conditions. The film composition and structure, as determined by RBS and XRD, were very sensitive to the deposition and subsequent anneal conditions. Stoichiometric films were deposited at low substrate temperatures (740 °C) but contained other orientations and phases. Predominantly c-axis oriented films were formed at high substrate temperatures (900 °C) and high background pressures of oxygen (200 mtorr). These films were semiconducting when quenched on oxygen or nitrogen following deposition. Slow cooling in a vacuum, yielded superconducting films with a maximum Tc-(onset) of 15 K and Tc(R=0) of 11 K. A variation of the deposition and anneal conditions indicated that loss of copper competed with the optimization of the carrier concentration.

A study of the pulsed laser evaporation process of thin superconducting films using both Nd:YAG and ArF eximer lasers is presented. Nuclear reaction analysis, Rutherford backscattering spectrometry (RBS) and scanning electron microscopy were used to characterize composition, thickness and surface morphology of the deposited films. The influence on the properties of the films, of the various parameters of the method such as photon wavelength, laser power density and geometrical conditions are presented and discussed.