R. B. Laibowitz

Quantum interference devices made from superconducting oxide thin films

We have fabricated superconducting quantum interference devices (dc SQUID’s) from thin films of the superconducting oxide YBa2Cu3Oy. The devices were made by first lithographically patterning an ion implant mask containing a 40 by 40 μm loop and two 17‐μm‐wide weak links over a ∼1‐μm‐thick oxide film. Ion implantation was then used to destroy the superconductivity in the film surrounding the device without actually removing material, resulting in a completely planar structure for the SQUID’s. The SQUID’s were operated in the temperature range from 4.2 to 68 K. The superconducting flux quantum was measured to be h/2e in these materials.

The effect of stress on the dielectric properties of barium strontium titanate thin films

Barium strontium titanate thin films are being developed as capacitors in dynamic random access memories. These films, grown on silicon substrates, are under tensile residual stress. By a converse electrostrictive effect, the in-plane tensile stress reduces the capacitance in the thickness direction of the film. We measured the substrate curvature change upon the removal of the film, and found the magnitude of the residual stress to be 610 MPa. In a separate experiment, we applied a force to vary the stress in a film on a substrate, and simultaneously recorded the capacitance change of the film. The measurements quantify the effect of stress on thin film capacitance. The stress free capacitance was found to be 23% higher than the capacitance under residual stress.

Epitaxial films of YBa2Cu3O7−δ on NdGaO3, LaGaO3, and SrTiO3 substrates deposited by laser ablation

Frequency‐doubled Nd:YAG laser (532 nm) pulses of 1.7 J/cm2 and 10 ns duration were used to deposit thin films of YBa2 Cu3 O7−δ by laser ablation on NdGaO3 , LaGaO3, and SrTiO3 substrates held at 725±5 °C in 0.2 Torr of O2 ambient. Electrical resistivities versus temperature of all films show normal metallic behavior and sharp superconducting transitions with Tc (R=0) at 92–93 K. Critical current densities in 0.3–0.6 μm thick, 200 μm long, and 10–30 μm wide strips were measured to be 106 A/cm2 at 60, 77, and 80 K in the films on LaGaO3, NdGaO3, and SrTiO3, respectively. X‐ray diffraction patterns show that all films grew epitaxially, with domains of only two crystalline orientations rotated 90° with respect to each other in the a‐b plane (consistent with twins), and the c axis perpendicular to the substrates. The closely matched lattice constants of the film and substrates (0.8–2.1%) result in epitaxial growth of the films.Frequency-doubled Nd:YAG laser (532 nm) pulses of 1.7 J/cm/sup 2/ and 10 ns duration were used to deposit thin films of YBa/sub 2/ Cu/sub 3/ O/sub 7-//sub delta/ by laser ablation on NdGaO/sub 3/ , LaGaO/sub 3/, and SrTiO/sub 3/ substrates held at 725 +- 5 /sup 0/C in 0.2 Torr of O/sub 2/ ambient. Electrical resistivities versus temperature of all films show normal metallic behavior and sharp superconducting transitions with T/sub c/ (R = 0) at 92--93 K. Critical current densities in 0.3--0.6 ..mu..m thick, 200 ..mu..m long, and 10--30 ..mu..m wide strips were measured to be 10/sup 6/ A/cm/sup 2/ at 60, 77, and 80 K in the films on LaGaO/sub 3/, NdGaO/sub 3/, and SrTiO/sub 3/, respectively. X-ray diffraction patterns show that all films grew epitaxially, with domains of only two crystalline orientations rotated 90/sup 0/ with respect to each other in the a-b plane (consistent with twins), and the c axis perpendicular to the substrates. The closely matched lattice constants of the film and substrates (0.8--2.1%) result in epitaxial growth of the films.

Laser wavelength dependent properties of YBa2Cu3O7−δ thin films deposited by laser ablation

YBa2Cu3O7−δ thin films were deposited onto (100) SrTiO3 substrates using 1064, 532, 355, 248, and 193 nm laser ablation. Transport measurements show lower normal‐state resistivities and higher critical currents in films deposited by the shorter wavelength lasers. The surface morphology of the films was rough with large particulates when the 1064 nm laser was used whereas much smoother surfaces with fewer and smaller particulates were obtained with the UV lasers. It is suggested that the better film quality obtained when the UV lasers are used is due to a small absorption depth of the UV photons in the ceramic target and to higher absorption by the ablated fragments. This leads to smaller ablated species and further fragmentation in the hotter plume and, therefore, to smoother and denser films.

Effects of radiation damage in ion‐implanted thin films of metal‐oxide superconductors

The effects of ion implantation into thin films of the superconductor YBa2Cu3Ox have been studied. Using oxygen and arsenic ions, the superconducting transition temperature Tc, the change in room‐temperature electrical properties from conducting to insulating, and the crystalline to amorphous structural transition in the films were studied as a function of ion dose. The deposited energy required to change Tc was found to be 0.2 eV/atom, while 1–2 eV/atom was required to affect the room‐temperature conductivity, and 4 eV/atom to render the film amorphous. This hierarchy of effects is discussed in terms of the damage mechanisms involved.

(Ba,Sr)TiO 3 dielectrics for future stacked- capacitor DRAM

Thin films of barium-strontium titanate (Ba,Sr)TiO3 (BSTO) have been investigated for use as a capacitor dielectric for future generations of dynamic random-access memory (DRAM). This paper describes progress made in the preparation of BSTO films by liquid-source metal-organic chemical vapor deposition (LS-MOCVD) and the issues related to integrating films of BSTO into a DRAM capacitor. Films of BSTO deposited on planar Pt electrodes meet the electrical requirements needed for future DRAM. The specific capacitance and charge loss are found to be strongly dependent on the details of the BSTO deposition, the choice of the lower electrode structure, the microstructure of the BSTO, the post-electrode thermal treatments, BSTO dopants, and thin-film stress. Films of BSTO deposited on patterned Pt electrodes with a feature size of 0.2 µm are found to have degraded properties compared to films on large planar structures, but functional bits have been achieved on a DRAM test site at 0.20-µm ground rules. Mechanisms influencing specific capacitance and charge loss of BSTO films are described, as are the requirements for the electrode and barrier materials used in stacked-capacitor structures, with emphasis given to the properties of the Pt/TaSi(N) electrode/barrier system. Major problems requiring additional investigation are outlined.

Epitaxial films of YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ on NdGaO/sub 3/, LaGaO/sub 3/, and SrTiO/sub 3/ substrates deposited by laser ablation

Frequency‐doubled Nd:YAG laser (532 nm) pulses of 1.7 J/cm2 and 10 ns duration were used to deposit thin films of YBa2 Cu3 O7−δ by laser ablation on NdGaO3 , LaGaO3, and SrTiO3 substrates held at 725±5 °C in 0.2 Torr of O2 ambient. Electrical resistivities versus temperature of all films show normal metallic behavior and sharp superconducting transitions with Tc (R=0) at 92–93 K. Critical current densities in 0.3–0.6 μm thick, 200 μm long, and 10–30 μm wide strips were measured to be 106 A/cm2 at 60, 77, and 80 K in the films on LaGaO3, NdGaO3, and SrTiO3, respectively. X‐ray diffraction patterns show that all films grew epitaxially, with domains of only two crystalline orientations rotated 90° with respect to each other in the a‐b plane (consistent with twins), and the c axis perpendicular to the substrates. The closely matched lattice constants of the film and substrates (0.8–2.1%) result in epitaxial growth of the films.Frequency-doubled Nd:YAG laser (532 nm) pulses of 1.7 J/cm/sup 2/ and 10 ns duration were used to deposit thin films of YBa/sub 2/ Cu/sub 3/ O/sub 7-//sub delta/ by laser ablation on NdGaO/sub 3/ , LaGaO/sub 3/, and SrTiO/sub 3/ substrates held at 725 +- 5 /sup 0/C in 0.2 Torr of O/sub 2/ ambient. Electrical resistivities versus temperature of all films show normal metallic behavior and sharp superconducting transitions with T/sub c/ (R = 0) at 92--93 K. Critical current densities in 0.3--0.6 ..mu..m thick, 200 ..mu..m long, and 10--30 ..mu..m wide strips were measured to be 10/sup 6/ A/cm/sup 2/ at 60, 77, and 80 K in the films on LaGaO/sub 3/, NdGaO/sub 3/, and SrTiO/sub 3/, respectively. X-ray diffraction patterns show that all films grew epitaxially, with domains of only two crystalline orientations rotated 90/sup 0/ with respect to each other in the a-b plane (consistent with twins), and the c axis perpendicular to the substrates. The closely matched lattice constants of the film and substrates (0.8--2.1%) result in epitaxial growth of the films.

Lanthanum gallate substrates for epitaxial high‐temperature superconducting thin films

We demonstrate that lanthanum gallate (LaGaO3) has considerable potential as an electronic substrate material for high‐temperature superconducting films. It provides a good lattice and thermal expansion match to YBa2Cu3O7−x, can be grown in large crystal sizes, is compatible with high‐temperature film processing, and has a reasonably low dielectric constant (e≂25) and low dielectric losses. Epitaxial YBa2Cu3O7−x films grown on LaGaO3 single‐crystal substrates by three techniques have zero resistance between 87 and 91 K.

Dielectric relaxation of Ba0.7Sr0.3TiO3 thin films from 1 mHz to 20 GHz

The dielectric relaxation of Ba0.7Sr0.3TiO3 thin films was investigated up to K band (20 GHz) using time domain and frequency domain measurements. Our results show that from 1 mHz to 20 GHz, the dielectric relaxation of the complex capacitance of Ba0.7Sr0.3TiO3 thin films can be understood in terms of a power law dependence known as the Curie–von Schweidler law. The small dispersion (less than 7% decrease in capacitance from 1 mHz to 20 GHz) and low loss (loss angle less than 0.006 at 20 GHz) measured in Ba0.7Sr0.3TiO3 thin films indicate that these films are applicable to device application up to at least K band.

Reliable single‐target sputtering process for high‐temperature superconducting films and devices

We report a simple, single‐target magnetron sputtering process for films of high‐temperature superconductors involving an off‐axis sputtering geometry. The process lends itself both to film growth with high‐temperature post‐anneals and to low‐temperature in situ film growth. The post‐anneal process routinely yields YBa2Cu3O7−x films on SrTiO3 substrates that are fully superconducting at 86–89 K. Current densities at 77 K range from 104 to 8×105 A/cm2. A single‐level superconducting quantum interference device (dc SQUID), made by photolithographically patterning a low current density film, has a flux noise level at 77 K of 3×10−4 Φ0/(Hz)1/2 at 20 Hz, dominated by low‐frequency noise associated with flux motion in the film.