Dieter Uhl

Multichannel DC SQUID sensor array for biomagnetic applications

A biomagnetic multichannel system for medical diagnosis of the brain and heart has been developed. 37 axial first order gradiometers (manufactured as flexible superconducting printed circuits) are arranged in a circular flat array of 19 cm in diameter. Additionally, three orthogonal magnetometers are provided. The DC SQUIDs are fabricated in all-Nb technology, ten on a chip. The sensor system is operated in a shielded room with two layers of soft magnetic material and one layer of Al. The everyday noise level is 10 fT/Hz/sup 1/2/ at frequencies above 10 Hz. Within two years of operation in a normal urban surrounding, useful clinical applications have been demonstrated (e.g., for epilepsy and heart arrhythmias). For the first time current sources of sporadic events causing epilepsy or ventricular extrasystoles have been localized from coherent recordings of complete biomagnetic field distributions with spatial resolution of millimeters and temporal resolution of 1 ms.

Ultra low noise all niobium DC-SQUIDs

The noise and signal properties of SQUIDs with amorphous silicon barriers and Al/sub 2/O/sub 3/ barriers are studied. The barrier material is found to be of great importance for the value of the 1/f noise component. The best results were obtained for SQUIDs with Al/sub 2/O/sub 3/ barriers and a 1/f noise level at 1 Hz of about 1*10/sup -6/ Phi / square root Hz was found. After integration of coupling coils onto the SQUIDs, a signal limitation and a dramatic increase of the noise were found. Implementation of a damping circuitry over the coupling coil results in optimized signals ( Delta V( Phi /sub 0//2) approximately=I/sub c/R) and a white noise level comparable to the white noise level without a coupling coil. The 1/f noise component for SQUIDs with a damped coupling coil is higher than for 1/f noise component of SQUIDs without a coupling coil. For SQUIDs with Al/sub 2/O/sub 3/ barriers, the 1/f noise level keeps below 3*10/sup -6/ Phi /sub 0// square root Hz at 1 Hz. For SQUIDs with an amorphous silicon barrier the 1/f noise component changes per cooling cycle in an irregular way. The stability for thermal cycling and room-temperature storage is very good for all the devices.

Single-layer and integrated YBCO gradiometer coupled SQUIDs

For many SQUID applications such as biomagnetism or non-destructive evaluation it is convenient or even necessary to work without the restrictions of a magnetically shielded room. This contribution deals with two sensors appropriate for this purpose. In the first concept we present a flip chip arrangement of a single-layer flux transformer and a single-layer SQUID, taking advantage of a simple technology. The SQUID was prepared on a bicrystal and located in the common line of two-parallel-loop arrangements. The flipped antenna was designed as a two-parallel-loop gradiometer with 26 mm baseline on a single-crystal substrate. A field gradient sensitivity of was obtained. We could demonstrate a field gradient resolution of at 1 kHz in an unshielded environment. In the second concept we integrated both the flux antenna and the SQUID on a bicrystal. The tighter coupling scheme results in smaller devices for similar field gradient sensitivities. The integrated SQUID is designed as a device on a bicrystal substrate. The remaining space is used for test structures and SQUIDs without antennae, in order to control the technology as well as the SQUID design. Parallel processed dummy substrates were used to monitor the quality of film growth by x-ray analysis. The quality of our SQUID design will be discussed on the basis of the measured field gradient sensitivity and noise. The reliability of the devices is demonstrated by an NDE type measurement.

Analytical calculation of the I - V characteristics of SQUIDs with parasitic elements

An approximate analytical calculation of the current - voltage characteristics of SQUIDs with parasitic capacitances and inductances is presented. The analytical treatment of the I - V curves is based on the approximation of harmonic signals. For bias currents (critical current of the SQUID) the approximated and the numerically calculated I - V curves are in very good agreement. From the analytical treatment of the characteristics we have derived analytical formulae for three voltages, where there is no flux modulation at the I - V characteristic. The comparison of numerically calculated and experimental characteristics shows that the equation of the upper point without flux modulation of the I - V curves is suitable for calculating parameters such as inductances and capacitances of SQUIDs with parasitic elements.

Noise in All-Niobium DC-SQUIDs

The equivalent flux noise of DC-SQUIDs fabricated with different technologies has been measured. The white noise is fitted very well with a thermal noise model. The coloured noise depends strongly on the preparation technique and the presence of a coupling coil. For Al2O3-barriers, for instance, we find an extremely low coloured noise level of typically 1×10−6 O0/(Hz)1/2 at 1 Hz for autonomous 100 pH-SQUIDs and about 3×10−6 O0/(Hz)1/2 at 1 Hz for SQUIDs with a coupling coil. For amorphous silicon barriers on the other hand these values are about one order of magnitude higher. The different behaviour of SQUIDs with amorphous silicon barriers compared to SQUIDs with Al2O3-barriers will be discussed.

A niobium planar SQUID gradiometer operating in an unshielded environment

We present an integrated thin-film gradiometric SQUID sensor in technology. The dc-SQUID converter is designed as a parallel inductance double-washer SQUID. It is inductively coupled to a planar parallel inductance gradiometer antenna with 15 mm baseline by two series input coils with eight turns each. The field gradient sensitivity is . The intrinsic field gradient resolution down to 1 Hz is , measured in a shielded environment. The sensor can be operated reliably without any magnetic shielding. Under these conditions the transfer function is decreased by about a factor of three, resulting in a field gradient resolution of in the white noise region. Coherent noise sources produced by a power line and railway were suppressed by means of electronic subtraction using a reference channel and selective filtering. The resolution of the system is demonstrated by MCG measurements without any shielding.

Resonances in All-Niobium DC-SQUIDs

The influence of LC-Resonances on the I-V-curves and the noise of DC-SQUIDs has been studied on special purpose SQUID-chips. Every chip (5×10 mm) is provided with 10 Ketchen-SQUIDs of different type. A common modulation coil is integrated in order to operate the SQUIDs in a flux locked loop. The SQUIDs are all-niobium devices with either Al2O3-barriers or amorphous silicon barriers. The resonances are damped with gold resistors.