Eriko Takeda

Enhancement of lateral solid phase epitaxy over SiO2 using a densified and thinned amorphous Si layer

Formation of a thin‐film silicon‐on‐insulator structure by lateral solid phase epitaxy of amorphous Si is described. Thinning of the amorphous Si layer after deposition and densification in an ultrahigh vacuum, prior to solid phase epitaxy, successfully enhances the lateral growth length. In addition, the crystallinity of thin silicon‐on‐insulator layers formed by this technique is found to be better than that achieved by the conventional method.

Study on the operation of a dual channel digital superconducting quantum interference device with direct‐current‐powered Josephson multiplexer readout

Operation of a dual channel digital superconducting quantum interference device (SQUID) with a dc‐powered Josephson multiplexer (MUX) readout has been observed successfully using a test circuit. In the circuit, output signals of two SQUIDs are digitized by Josephson comparators and read out by the MUX. The noise of the SQUIDs under the operation of the whole circuit is 0.54Φ0/√Hz at 1 kHz. Advantage of using a dc‐powered circuit to realize a low noise SQUID measurement system is confirmed.

Washer size and shape dependencies of current steps due to standing waves in superconducting quantum interference devices (SQUIDS)

To find out the quantitative relationship between the voltages of current steps caused by standing waves and the size of the SQUID washer, experimental and numerical studies have been conducted. The wavelength of the standing wave depends on both the width and the hole size of the washer. A model treating a multiturn input coil as a plate explains the measured systematic change in the step voltages. For shortening the wavelength, a new figure of SQUID with an incision in the washer is proposed. The figure is effective for increasing the output voltage of the SQUID.

Study of the magnetic coupling between a SQUID and wires integrated on a chip

Magnetic coupling between a SQUID and wires in an integrated circuit has been studied. Using test integrated circuits, the coupling is measured by varying the distance from the SQUID to each wire. The coupling between the SQUID and the wires with and without a groundplane decreases with an increase in the distance. The coupling for the wires with the groundplane is smaller than that for the wires without the groundplane at distances less than 595 /spl mu/m. However, the rate of the decrease in the coupling for the wires with the groundplane falls off for distances more than 595 /spl mu/m and the coupling converges to around 2.9 pH. From two dimensional simulations for the magnetic flux coupling, the origin of the residual inductance is found to be the coupling between the SQUID, and shielding current in the groundplane. The value of the distance, 595 /spl mu/m, does not depend on the size of the SQUID. To decrease the coupling, the use of wires with stripline structure or coplanar structure is desirable.

Design of a SQUID sensor having a SQUID amplifier

The frequency dependent characteristics of a SQUID amplifier have been studied by numerical simulation using a model circuit consisting of a SQUID sensor and SQUIDs connected in series. Optimization of the circuit parameters from the simulated results makes it possible to design the amplifier with wide bandwidth and low distortion.

Operation of Josephson comparator under Josephson noise from SQUID

Operation of a Josephson comparator for analog-to-digital conversion of signal from a SQUID is studied. In the study, a test circuit consisting of the SQUID and the comparator is used. The measured result shows that the comparator responds to AC noise due to the AC Josephson effect. Numerical simulation shows amplitude, which is determined from the response of the comparator, is smaller than the original AC noise amplitude. The difference comes from low-pass characteristics of the Josephson junction in the comparator due to the turn-on delay. The delay is effective in reducing the AC noise, and provides a means for the accurate conversion.<<ETX>>

Device-structure dependence of shift in SQUID characteristics by flux trapping

Abstract Shifts in voltage-flux characteristics of a SQUID by flux trapping have been measured to study effectiveness of guard ring structure on shielding of magnetic field. The measurements are made under controlled magnetic field. Magnitude of the shift depends on the device structure. It is found that there exists a threshold field for the flux trapping, and the field is reduced by introducing the guard-ring in the SQUID. Comparing to the SQUID without the structure, the SQUID with it needs higher-grade shielding to prevent the flux trapping during cooling down.

Reduction in inductance of an integrated SQUID

Abstract In an integrated-SQUID, a change in its inductance caused by a superconductor placed in proximity has been measured to design the inductance accurately. The measured inductance of the SQUID is reduced by the existence of the superconductor. The measured inductance is explained well by numerical results based on a model taking account of flux coupling between the superconductor and the SQUID. The origin of the reduction is found to be mirror current that flows in the superconductor by the coupling.