The response of a Hall-effect sensor to a spatially dependent magnetic field is of importance for many applications such as magnetic microscopy and nondestructive testing. Using the analytical expression of the response of a Greek cross Hall sensor response to an ideal field dot published a few years ago, we have calculated its sensitivity and its full width at half maximum for the field produced by a magnetic dipole and by two coplanar lines. The experimental results are in good agreement with theory. They show that the spatial resolution is roughly equal to the dimension of the central part of the Greek cross and that a flux-meter approximation is not appropriate for modeling such Hall-effect sensors for very close field sources.

In this paper the signal to noise ratio of the magneto optical (MO) imaging technique is calculated. As an example of the sensitivity that can be achieved, MO images of flux penetration in YBa2Cu307-δ films at low field (ranging from 0.2G up to 5G) and near Tc (81K) are presented.

The correlation of the outputs of two dc-SQUIDs connected to the same solid washer was studied. A dedicated electronic system was used in order to operate both SQUIDs at the same time. It was found that the temporal correlation of the SQUID outputs is strongly dependent on the geometry of the SQUID loop. For a solid washer, the flux noise is mainly due to vortices located close to the SQUIDs. However, by surrounding the SQUID loop with a hole patterned in the superconducting solid washer, it is possible to drastically reduce the flux penetration around the loop. The resulting flux noise becomes dominated by vortices located inside the washer far from the SQUIDs. Magneto-optical images were also used to visualize the flux penetration inside the washer around the slit of such SQUIDs. These confirm previous descriptions of vortex penetration deduced from the electrical measurements.

We have developed a configuration for precision magnetic measurements associating a /spl mu/-Hall sensor, a modulated ferromagnetic antenna, and an analog electronic circuitry. This association is not only able to remove the 1/f noise of the Hall sensor, but also to achieve an ultimate noise level even lower than thermal white noise of the Hall sensor. The system main characteristics are the following: bandwidth 645 Hz, noise level white and lower than 10 nT//spl radic/Hz above 0.1 Hz, slew rate 10/sup -1/ T/s and system dynamic 84 dB in a 1-Hz bandwidth. The performances of the modulated sensor are compared to those of a dc operated sensor.

The objective of our work is to achieve the fabrication of fully integrated monolithic semiconducting preamplifier/superconducting YBa2Cu3O7−δ (YBCO) microbolometers in order to enhance overall sensor performance. To our knowledge this paper reports the first cofabrication of a c-axis-oriented YBCO film with a critical temperature of 86 K and an active semiconducting device on the same silicon substrate. We proposed a process fabrication where the p-MOS field effect transistors (p-MOSFET) with a Pt-based metallization are fabricated first and the YBCO and buffer layers are deposited as the final step. After YBCO deposition the Pt-based multilayers present interconnect sheet resistivity of 0.45 Ω per square and specific contact resistivity of 2 × 10−4 Ω cm2. The drain current versus gate voltage (Vgs) and versus drain–source voltage (Vds) characteristics of a 30 µm wide 10 µm long transistor are finally given. It presents a transconductance gm = 8.7 × 10−5 A V−1 (at Vds = 10 Vand Vgs = −20 V) and a threshold voltage Vt = −6.53 V at 300 K.

We report the process technology for the modular integration of YBa2Cu3O7−δ (YBCO) and p-type metal-oxide-semiconductor (p-MOS) devices on the same silicon substrate. Basic test structures consisting of single p-MOS field-effect transistors with a Pt-based metallization and YBCO bridges were fabricated. After completion of the p-MOS fabrication, highly c-axis-oriented YBCO films showing a critical temperature of 86 K were grown on a free silicon surface. The electrical characteristics at 77 K of the p-MOS transistors were comparable to those of this technology with Al metallization. This means that no fatal degradation was introduced into the p-MOS process by the YBCO one. The cofabrication on the same silicon substrate of devices using such disparate technologies as YBCO and MOS is a very promising starting point for a new generation of monolithic integrated circuits combining the advantages of oxide and semiconductor properties.

Mo, Pt, Pt/Mo and Pt/Ti thin films have been deposited onto Si and SiO 2 substrates by RF sputtering and annealed in the YBa 2 Cu 3 O 7-δ (YBCO) growth conditions. The effect of annealing on the sheet resistance of unpatterned layers was measured. A Pt-based multilayered metallization for the PMOS devices was proposed and tested for a compatible monolithic integration of semiconducting devices and YBCO sensors on the same silicon substrate. The best results were obtained with a Pt/Ti/Mo-silicide structure showing 0.472 Ω interconnect sheet resistivity and 2 × 10 -4 Ω cm 2 specific contact resistivity after annealing for 60 min at 700 °C in 0.5 mbar O 2 pressure.

We present an original method for studying the low frequency flux noise due to vortices in superconducting quantum interference device (SQUID) systems. We use two SQUIDs connected to the same washer in order to study the correlation of their outputs. A dedicated electronic system has been built so as to operate both SQUIDs at the same time. It was thus possible to distinguish fluctuations due to vortices located in the film far from the SQUID from fluctuations originating from vortices located close to the SQUID itself. In this article, the experimental setup is described and some preliminary noise measurements are presented.

In nuclear physics experiments, a decision maker named as “trigger” gives a bit pattern which allows the fired detectors identification. As the data acquisition dead time is greater than the time between physical events, timing information is essential. We add to the trigger function a Time to Digital Converter (TDC) in order to make a separation between events. The paper describes the architecture chosen for the TDC and illustrates the contribution of each element to the TDC performance. An eight-bit counter is used for the dynamic range of the TDC (in microsecond) associated to a delay line improving the resolution (in nanosecond). The study shows that exploiting the two system clock states (high and low) allows to reduce the number of delay line cells. The Differential Nonlinearity Measurements are given for different resolutions (1, 2 and 5 ns) and illustrate the clock period, the clock duty cycle and the delay line contributions to the TDC performances.

Any magnetic sensor placed in a spatially inhomogeneous magnetic field delivers a signal proportional the mean field value taken over an effective area or volume which depends on the type of sensor considered. In the case of the field produced by a magnetic dipole and detected by a square or circular planar sensor, the overall measured spatial resolution ideally depends on the ratio of the mean dipole-sensor distance z0 to the square root of the effective sensor area AE. For Z0/AE≪1, the spatial resolution is limited by the size of the sensor, whereas for z0/AE≫1 the dipole-sensor distance is the predominant factor. To compare various low noise magnetic sensors operating either at low temperature or at room temperature, we have measured their sensitivities and spatial responses to the field produced by a magnetic moment having the form of a tiny circular current loop. The sensors could be moved in all directions with respect to the current loop. The transfer of each sensor to the magnetic dipole field was compared to their response in a homogeneous field so as to deduce their effective area and compare this area to that deduced from independent spatial resolution measurements. We report the experimental results given by four types of sensors namely a dc-SQUID, a Hall effect sensor, a giant magneto-resistive sensor and a flux-gate sensor and discuss them by mean of a “figure of merit” criterion combining their spatial resolution and their sensitivity.

We have designed and built a liquid helium immersion insert which is magnetically shielded from the ambient magnetic field fluctuations by the use of a superconducting lead can. The insert has a variable temperature stage made of sapphire which is optically heated by the light guided from an external lamp via a glass rod. The temperature regulation is achieved by the use of a commercial PID controller: the temperature of the sapphire ranges from 50 K to 100 K with peak to peak fluctuations less than 20 mK over a 3 minutes observation period. The magnetic noise inside the insert has been measured with a low T-c dc-SQUID and the white noise level above 200 Hz is less than 4 fT/root Hz. Furthermore, the magnetic attenuation factor due to the superconducting lead can is greater than 26 500 for frequencies above 15 Hz. This cryogenic insert is used for studying low frequency vortex noise in high T-c superconducting thin films at different temperatures and the field penetration in the films as a function of the applied magnetic field during its transition state.

This note describes the design and realization of a liquid helium cryostat insert comprising a variable temperature regulated stage. The latter is magnetically shielded by a superconducting lead pot bathing in liquid helium. This apparatus is intended for the study of high Tc superconducting films and superconducting quantum interference devices submitted to different applied magnetic fields for temperatures ranging from 50 to 100 K.

Magneto-optical (MO) studies have been carried out on superconducting thin films. In this paper, we give the features that determine the signal to noise ratio of the imaging system and optimize the image contrast. The flux distribution of vortices in thin films are given, which determines the requirements to satisfy for vortex visualization using a MO layer. Images of partial flux penetration through the slit of a DC SQUID are presented. Flux focusing efficiency can be deduced from these images.

In compliance with international agreements on nuclear weapons limitation, the French ground-based nuclear arsenal has been decommissioned in its totality. One of its former underground missile control centers, located in Rustrel, 60 km east of Avignon (Provence) has been converted into the "Laboratoire Souterrain a Bas Bruit de Rustrel-Pays d'Apt" (LSBB). The deepest experimental hall (500 m of calcite rock overburden) includes a 100 m(2) area of sturdy flooring suspended by and resting on shock absorbers, entirely enclosed in a 28 in long, 8 m diameter, 1 cm thick steel capsule. This results in an unparalleled combination of shielding against cosmic rays, acoustic, seismic and electromagnetic noise, which can be exploited for rare event searches using ultra low-temperature and superconducting detectors. The first characterization measurements in this unique civilian site are reported. (C) 2000 Elsevier Science B.V. All rights reserved.

Compared to Stirling or Gifford-Mac Mahon crycoolers, the pulse tube refrigerator (PTR) has the advantage of having no moving parts in the cold stage. This results in lower levels of vibration, which is more suitable for the design of highly sensitive superconducting magnetometers. We describe a PTR system in which the compressor is connected to a cold part with a semi-flexible one meter long tube, under an average helium gas pressure of 3 MPa. Using a frequency of 30Hz and a high to low pressure ratio of 1.5, the lowest temperature achieved is 56K. The cooling power is 200 mW at 77K with a cooling down time of about of 45min. A bare YBaCuO dc SQUID has been successfully operated at a temperature close to 77K. The SQUID was mounted onto a copper block, attached to the cold head of the PTR.

The sensitivity of a bolometer increases if the thermal conductance between the part of the sensor heated by the incident radiation and the heat sink decreases. Then, our work deals with the modelling and the fabrication of suspended YBaCuO microstructures by silicon micromachining. High quality YBaCuO films showing completely c-oriented texture and single inplane epitaxial orientation were obtained on CeO2 / YSZ-buffered Si substrates. Critical temperatures up to 88 K were measured for the as grown films. The micromachining technique using the Reactive Ion Etching (RIE) of silicon substrates was very reproducible. A thermal model was established on single suspended bridges and verified experimentally. The results enabled to optimise the detector geometry, a meander constituted of 17 suspended bridges in series and covering a detection surface of 100 x 100 mu m(2). The performances of the optimised detector, measured at 85 K under irradiation from a blackbody are among the best reported for YBaCuO micrometers (NEP = 4,0 10(-12) W root Hz and detectivity = 2,5 10(9) cm root Hz / W, with a thermal time constant of 564 mu s). The etching of Separated by Implanted Oxygen (SIMOX) substrates enabled to fabricate suspended membranes constituting of the YBaCuO / CeO2 (/)YSZ / Si multi-layers and then to obtain an additional liberty degree in the membrane composition.

We propose a transportable Single Photon Emission Computed Tomograph (SPECT) device dedicated to the reconstruction of a radioactive tracer distribution in a process. The experimental device Is realized with 36 collimated sodium iodide detectors mounted around a bi-dimensional cylindrical box which simulates a mixing reactor vessel. The reconstruction is based on a statistical algorithm applied to a small number of views and detectors. Dynamic acquisitions coupled to video monitoring were performed after the injection of radioactive and dye tracers in the vessel. The analysis of first results show that the experimental SPECT method, based on a limited number of detectors, is hopeful for industrial needs. (C) 1997 Published by Elsevier Science Ltd. All rights.

Suspended epitaxial YBaCuO microbolometers were successfully fabricated by two silicon micromachining techniques. The first one used the reactive ion etching (RIE) of Si substrates and the second one the etching of the SiO2 layer in separated by implanted oxygen (SIMOX) substrates. This work aims at the modeling and the measurement of the bolometric performances of IR pixels (100×100 μm2 detection area) constituted by suspended bridges in series. The influence of both the dimensions and the thermophysical properties of the materials constituting the membrane is discussed. Thermal conductances and time constants were measured as functions of the length and the width of different suspended bridges fabricated by RIE. Comparison of a “RIE type” bridge with a bridge of same dimensions fabricated from a SIMOX substrate shows that the sensitivity-bandwidth product of the SIMOX bridge is improved by one order of magnitude. All measurements on suspended bridges are consistent with calculations from thermal model. The specific detectivity D* of a 4-μm-wide suspended meander line, measured at 85 K under irradiation from a blackbody, is 2.5×109 cmHz/W, with a thermal time constant of 564 μs. These performances are among the best reported for YBaCuO microbolometers.

Suspended epitaxial YBaCuO bolometers were successfully fabricated by two silicon micromachining techniques. The first one used the RIE (Reactive Ion Etching) of Si substrates and the second one the etching of the SiO/sub 2/ layer in SIMOX (Separated by IMplanted OXygen) substrates. Thermal conductances and time constants of different suspended bridges fabricated by RIE were measured as functions of length and width. The influence of the materials constituting the membrane was discussed by comparing a "RIE bridge" to a similar "SIMOX bridge". All measurements were consistent with calculations from thermal model. Experimental results and model lead to the optimization of a 100/spl times/100 /spl mu/m/sup 2/ IR detector. Its detectivity D*, measured at 85 K under irradiation from a blackbody, was 2.5 10/sup 9/ cm/spl radic/(Hz)/W, with a time constant of 564 /spl mu/s. These performances are among the best reported for YBaCuO bolometers.