J. Flokstra

A capacitive RF power sensor based on MEMS technology

An environment decontaminating system has an air cleaning and deodorizing function. It includes a housing having an inlet and an outlet for passage of air. A DC high voltage dust collector and a dust catching filter are provided adjacent the inlet. An ozone generator is positioned adjacent the filter. An odor/ozone turbulent mixing plate is positioned adjacent the ozone generator. A deodorizing catalyst is positioned adjacent the mixing plate. An acid gas absorbent, a suction scavaging fan, and an ozone concentration sensor are positioned adjacent an outlet of the housing.

Elimination of flux-transformer crosstalk in multichannel SQUID magnetometers

Multichannel SQUID magnetometers are being developed for signal-field mapping in biomagnetic experiments. A problem that becomes more serious as the number of channels is increased is the crosstalk caused by the mutual inductances between the individual sensing coils. A simple and effective method for eliminating this crosstalk is presented in this Paper. The method is based on a rearrangement of the feedback loops which causes the flux-transformer circuits to become currentless. The feasibility of the method is verified experimentally.

Preparation of YBaCuO thin films on various substrates by laser ablation: Influence of the substrate temperature

High-quality YBa2Cu3O7?? films were prepared in situ by laser ablation on SrTiO3 (100), on MgO (100) and on Si (111) + ZrO2 buffer layer. We especially investigated the influence of the substrate temperature. In similar preparation circumstances zero resistivity has been achieved for films on SrTiO3 up to 89.5 K, on MgO up to 84.6 K and on Si(+ZrO2) up to 83.5 K. X-ray diffraction analysis was used to study the growth conditions of the films, and to determine the oxygen stoichiometry. The films deposited at substrate temperatures above 720°C show pure c-axis orientation with an oxygen deficiency ?<0.1. Interdiffusion between substrate and layer, analyzed with Auger sputter profiling, could not be observed. The grains of the films on Si+ZrO2, studied by scanning electron microscopy, are caused by the polycrystalline buffer layer. These grains largely determine the physical properties of the deposited layer. n nCritical current measurements were carried out using wet chemically etched microbridges on the different substrates. At T=77 K the values are in the range from 3x106 A/cm2 for films on SrTiO3 to 2x103 A/cm2 in the case of Si+ZrO2. The extrapolated resistivity at T=0 is found to be proportional to the temperature width of the superconducting transition.

Field dependence of the Josephson current and Fiske resonances in specially shaped Josephson junctions

We investigated the magnetic-field dependence of the Josephson current and Fiske resonances in specially shaped Josephson junctions. In order to be able to use junctions for high-resolution X-ray spectroscopy, a very good suppression of the sidelobes of both the Josephson current and the Fiske resonances must be achieved. n nIn a theoretical argument we show that a properly chosen junction shape leads to the sidelobe suppression of both the critical current and Fiske resonance amplitudes. n nThe Josephson current and Fiske resonance amplitudes were measured as a function of the magnetic field, for junctions fabricated in Nb/Al technology. As expected, a very good sidelobe suppression was obtained for quartic-shaped junctions. For junctions with anodized structures within the tunneling area, the shape of the internal structures is reflected in the field dependence of both the Josephson current and the Fiske resonances. Finally, Fiske modes in these junctions have been imaged with low-temperature scanning electron microscopy, and we conclude that a quartic junction can be approximated by a rectangle, to describe the lower-order Fiske modes, whereas the high-order modes are specific to the exact shape of the junction.

On the suppression of the sidelobes of the supercurrent in small Josephson tunnel junctions

The critical currents of Nb/Al, Al-oxide, Al/Nb tunnel junctions of various shapes have been measured as a function of the applied magnetic field. For the square junction and for some special shapes like the diamond, ?1 + cosine? and quartic junctions the Ic(B) pattern falls off theoretically as 1/Bn, with n respectively equal to 1, 2, 3 and 4. In general the measurements are in good agreement with the theoretical predictions. For the 1 + cosine and quartic shapes we found a sidelobe suppression that is even larger than that obtained in theory. For the quartic junction the first sidelobe is only 0.3% of the zero-field current. An Ic(B) modulation with a small, only slowly decreasing amplitude is observed for the diamond, 1 + cosine, and quartic junctions, that can be explained by rounding of the sharp edges of the junction shapes, due to the fabrication process.

The UT 19-channel DC SQUID based neuromagnetometer

A 19-channel DC SQUID based neuromagnetometer is under construction at the University of Twente (UT). Except for the cryostat all elements of the system are developed at the UT. It comprises 19 wire-wound first-order gradiometers in a hexagonal configuration. The gradiometers are connected to planar DC SQUIDs fabricated with a Nb/Al, AlO kappa/Nb technology. For this connection we developed a method to bond a Nb wire to a Nb thin-film. The SQUIDs are placed in compartmentalised Nb modules. Further, external feedback is incorporated in order to eliminate cross talk between the gradiometers. The electronics basically consist of a phase-locked loop operating with a modulation frequency of 100 kHz. Between SQUID and preamplifier a small transformer is used to limit the noise contribution of the preamplifier. In the paper the overall system is described, and special attention is paid to the SQUID module (bonding, compartments, external-feedback setup, output transformer).

Fabrication of Nb/Al,AlOx/Al/Nb Josephson tunnel junctions using reactive ion etching in SF6

High quality Nb/Al,AlOx/Al/Nb Josephson tunnel junctions have been made with the help of a fabrication process based on reactive ion etching of Nb in SF6. The Vm value of these junctions is typically 60–70 mV at 4.2 K. At 1.6 K, a Vm of 4.1 V has been measured, which is the highest value that has ever been reported for this type of junction. The area of the junctions ranges from 1 to 25 μm2. By burying the Nb/Al,AlOx/Al/Nb trilayer in the substrate, a planarized junction configuration has been obtained. Reactive ion etching of Nb in SF6 plasmas has been studied in detail. Anisotropic etch profiles can be obtained because of the formation of a resistant layer during etching, which prevents etching of Nb under the photoresist. The etching process has been monitored with a spectrometer. The fluorine emission at 703.7 nm is shown to be suitable for end point detection.

1:30000 cryogenic current comparator with optimum SQUID readout

We developed a 1:30000 Cryogenic Current Comparator for SET current amplification. A dedicated low-noise, directly-coupled SQUID was used for the readout, which allowed reaching a sensitivity close to ideal. The ratio error was <9/spl times/10/sup -9/. The CCC-SQUID equivalent current input noise was 3.0 fA/Hz/sup 1/2/, measured down to 0.1 Hz.

Electronic balancing of multichannel SQUID magnetometers

An electronically balanced three-channel SQUID magnetometer is presented. The electronic balancing reduces the imbalance of the gradiometers by at least a factor of 100. The authors realised in this way an imbalance of 10-5, without the necessity of prebalancing by means of superconducting tabs. Details of the design considerations and the construction of the three-channel system are given, focusing especially on the electronic balancing unit. Some illustrating experiments are also described and the possibilities of electronic balancing discussed

Multichannel heart scanner based on high-T/sub c/ SQUIDs

A 7-channel magnetometer for magnetocardiography based on high-T/sub c/ SQUIDs has been realized. This magnetometer is used for test experiments in the development of a multichannel high-T/sub c/ SQUID based heart-scanner for clinical applications. The intrinsic noise level of the channels in the 7-channel system is typically 120 fT//spl radic/(Hz) down to 1 Hz. Magnetocardiograms were recorded inside a magnetically shielded room. Introductory experiments were performed on the suppression of noise by combining magnetometers to form planar gradiometers. The noise suppression that can be established appeared to be limited by the imbalance of the gradiometric configuration, which is roughly 2%. This relatively poor balance of the system is caused by inaccuracies in the transfer functions of the individual SQUID magnetometers, and by deviations from the planar geometry.