P. M. Mankiewich

Reproducible technique for fabrication of thin films of high transition temperature superconductors

We report on a new process to make films of Y1Ba2Cu3O7 using coevaporation of Y, Cu, and BaF2 on SrTiO3 substrates. The films have high transition temperatures (up to 91 K for a full resistive transition), high critical current densities (106 A/cm2 at 81 K), and a reduced sensitivity to fabrication and environmental conditions. Because of the lower reactivity of the films, we have been able to pattern them in both the pre‐annealed and post‐annealed states using conventional positive photoresist technology.

Penetration depths of high Tc films measured by two‐coil mutual inductances

A fundamental parameter of sample quality in epitaxial films of high Tc oxides is the effective penetration depth λ∥ of the superconducting sheet. A contactless audio‐frequency method is described, in which an epitaxial YBa2Cu3O7 film is sandwiched between sets of stacked coils, and the procedure is given for computing the complex sheet impedance and hence λ∥ from the change in mutual inductance produced by screening currents in the film. Temperature dependence of the complex impedance is presented.

A method to measure the complex permeability of thin films at ultra-high frequencies

A swept frequency method for measuring the magnetic permeability of thin films for frequencies up to 2 GHz with a resolution at 1 GHz of /spl sim/0.1 /spl mu/m in permeance is described. It is based on the measurement of impedance of a strip loop loaded with the sample under test. The method employs a commercial impedance or network analyzer, is simple, and suitable for permeability measurements of thin magnetic films which are used in recording heads and microwave components.

Propagation of terahertz bandwidth electrical pulses on YBa2Cu3O7−δ transmission lines on lanthanum aluminate

We study the propagation of terahertz bandwidth electrical pulses on high critical current density c‐axis oriented YBa2Cu3O7−δ (YBCO) coplanar transmission lines deposited epitaxially on the low dielectric loss substrate lanthanum aluminate (LaAlO3). The losses on the YBCO transmission line are lower than on an equivalent gold line on the same substrate at temperatures below 50 K. At higher temperatures, absorption and dispersion of the ultrashort electrical pulses are observed in reasonable agreement with Mattis–Bardeen theory.

Fabrication of 20‐nm structures in GaAs

Structures as small as 20 nm have been fabricated in GaAs by high‐resolution electron beam lithography and reactive ion etching. NiCr patterns were formed on the semiconductor surface by liftoff of a single‐layer electron beam resist. This metal mask pattern was transferred into the III‐V material by etching in a SiCl4 plasma

High resolution electron beam lithography on CaF2

We have fabricated 30‐nm lines on 200‐nm centers in CaF2 using a scanning transmission electron microscope. The lines were written by electron beam radiolysis of a fine grain polycrystalline CaF2 film and reaction to CaO followed by development in H2O.

Thin‐film CaF2 inorganic electron resist and optical‐read storage medium

Polished silicon substrates were e‐gun deposited with 100‐nm films of CaF2 in a molecular beam epitaxy vacuum station. Measurements of this material system reveal a factor of 35 reduction in reflectivity for films exposed to electron irradiation. Exposure is accomplished by simply viewing the desired area using a 3‐kV Auger electron microprobe. It is further reported that this exposed inorganic material can be developed (selectively washed away) in H2O. In addition, both over and under exposures have been observed. Optimum electron exposure dose has been determined to be 0.1–0.2 C/cm2.

Laser crystallization of Si films on glass

By using glass substrates which approximate the thermal expansion of a deposited Si film, thermal stress cracking during laser crystallization is eliminated. Results on three types of transparent silicate glass are reported. Chemical vapor deposition is used to coat these substrates first with a buffer layer of SiO2 or Si3N4 and then with a Si film. When the Si films are melted with an argon ion laser beam, large crystal grains are produced. Secondary ion mass spectroscopy is used to determine the effectiveness of the buffer layer in isolating the Si from impurities in the glass.

The observation of the AC Josephson effect in a YBa/sub 2/Cu/sub 3/O/sub 7//Au/YBa/sub 2/Cu/sub 3/O/sub 7/ junction

The authors have demonstrated proximity effect coupling between a high-transition-temperature superconductor and a normal metal. In a device with a 1- mu m long gap in a YBa/sub 2/Cu/sub 3/O/sub 7/ film spanned by an Au shunt, the authors observed a DC supercurrent and the AC Josephson effect under microwave irradiation from 2 GHz to 15 GHz. Preliminary work has also begun with Ag shunts. It is concluded that these high quality S-N (superconductor-normal) interfaces should be applicable both to probing the superconducting state in oxide superconductors and to building high-T/sub c/ electronic devices. >

Electron beam lithography from 20 to 120 keV with a high quality beam

We have performed electron beam lithography studies on thick substrates using beam energies of 20–120 keV and a nominal beam diameter of 2 nm in a Philips 400 electron microscope with scanning capability. Metal lines as narrow as 10 nm were fabricated on Si and GaAs substrates using liftoff of a single thin layer of resist. High resolution (approximately 10 nm) patterns could be written at all beam energies with an exposure latitude that remained approximately constant up to energies for which the range of the backscattered electrons became significantly larger than the pattern area. For large area patterns written with the small beam, the proximity effect is greatly reduced, even at 20 keV, because of the sharp edge of the exposure profile. At high beam energies, the range of backscattered electrons is large enough that they contribute only a slowly varying background dose, leading to a relatively simple proximity correction even for complex patterns.