SQUIDs and multi-turn input coils of YBaCuO for integated squids

Applied Superconductivity Vol.1, Nos 10-12, pp. 1675 - 1680, 1993 Printed in Great Britain. All rights reserved

SQUIDS AND MULTI-TURN YBaCuO FOR INTEGATED R Wunderlich

J. banger

J. Koriath

Technical University Hamburg-Hasburg; Ei%endorferstr. 42

2100 Hamburg 90

B. Meyer

0964-1807193 $6.00 + 0.00 Copyright @ 1993 Pergamon Press Ltd

INPUT COILS OF SQUIDS J. Miller

Dept. of Semiconductor Technology Federal Republic of Germany

- For the application of a SQUID as a sensor for biomagnetic nx~ure

ABSTRACT

nrznts we have developed de SQUHIs consisting of reproducible SNS step edge junctions. The superconductor is YBaCuO and for the normal m&l we used Ag. Modulation is observed beyond 77 K. As the upper limit for the magnetic flux noise a CPM of < 80 $@~/a

could determined. Using a multitarget cosputtering process from

n&a&c targets structured multilayers for input coils has been developed with MgO as interlayer. The films show good electrical properties as well as extrernly snxxkh surfaces without any defects acrcss a large deposition area of 3 inch in dianxter. Alectrical measurerrxnts, XHD, EDX, SEM and STM studies show the high quality of the multilayer.

INTRODUCTION

DC superconducting magnetic

flux detectors

cal measurements

quantum interference devices ‘SQUIDS’ offers a number of applications

to biomagnetism,

as ultrasensitive

ranging from geophysi-

i.e. measurements of magnetic fields inside

the human body generated by electric currents. Here especially heart and brain activities

are of interest.

are low, for biomagnetic

While for geophysical

applications

detection

demands

measurements SQUID rings combined with detector and

input coils are required to obtain enough sensitivity and dynamic response in the frequency range of interest. The detector coils that collects the magnetic flux and the input coils which couples to the SQUID inductance L via mutual inductance M form the superconducting form an integrated

flux transformer.

SQUID.

Together

with the SQUID

ring they

The original SQUIDS and most commercial available

SQUIDS consist of three dimensional structures, but modern High-TV-SQUID sors are to be fabricated

sen-

in thin film planar technology with multilayer processes.

To realize the Josephson junction of the SQUID ring in planar technology the following approaches are practical:

(1) grain boundary junctions nucleated at defined

substrate locations e.g. bi-epitaxial 112)or step edge junctions3); (2) junctions with artifical barriers e.g. normal metals4n5)or semiconductor barrier&. In this paper we describe the fabrication, electrical properties and noise measurements of SNS-type step edge SQUID s with Ag as normal metal. We chose this process for SQUID

fabrication

because we expected

good reproducibi~ty

as the

process allows to adjust the properties of the Josephson junction easily by &mging the geometrical

dimensions or the process parameters. 1675

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Furthermore, in regard to the application of these SQUIDS for biomagnetic measurements we developed a multitarget co-sputtering process for the preparation of multilayer structures for input coils. This process allows both the reproducible in situ deposition of extremly smooth large area YBaCuO films of excellent’electrical properties and the fabrication of epitaxially grown MgO bufferlayers as interlayers. Using these processes input coils from structured YBaCuO/MgO/YBaCuO multilayers have been produced. EXPERIMENTAL The SNS-DC-SQUID s consist of thin a YBaCuO layer sputtered across a step etched into a substrate. The films are deposited by on-axis sputtering using a RF magnetron process and show a critical temperature offset beyond 89 K with a transition width of less than 1 K and critical current densities up to 6 *lo6 A/cm2 at 77 K on MgO substrates. The step in the MgO substrate is produced by ion milling in a plate reactor using a titanium mask. The titanium mask is structured by standard photolithography and wet etching. The damage to the MgO substrate induced by the patterning of the step was removed by low power oxygen ion milling as proved by channeling measurements (RBS). Because of the small residual damage of the MgO substrate remaining after generating the step, YBaCuO films deposited on these substrates exhibit Tc values up to 85 K with the critical current beyond 1.5 *lo6 A/ cm2 at 77 K. After cooling in oxygen a silver layer were deposited in situ at room temperature. Finally the SQUID structure was patterned by Ar milling with a photoresist mask. Fig. 1 shows the obtained structure with stipewidth of 0.8 pm in an SEM micrograph.

Figure 1: SEM micrographs of a SQUID and a single junction across a step in the substrate

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On a single MgO substrate 6 SQUIDS, 4 single Josephson junctions microbridges to determine the electrical properties were fabricated.

ELECTRICAL

AND

NOISE

and 3

~EASU~ME~TS

The normal resistance of the SQUID

could be varied &om 6-1000 rnR either

by changing the step height, the film thickness or the interface conditions between YBaCu.0

and silver layers. For SQUIDS with doubled stripewidth

Ic and RN were also doubled and halved, respectively.

the values for

It is noteworthy,

that the

variations for the values of Ic and RN on a substrate were less than 20 %. The response of an applied magnetic field is summarized in Tab.

1 for three

SQUIDS.

Table 1: Parameters

of three SQUIDS

First noise measurements were obtained from SQUID # 1 in direct mode. The output voltage was directly amplified by a low noise transformer followed by a low noise preamplifier.

The voltage noise was measured with a spectrum analyser. This setup was optimized for measurements in the white noise region, so l/f noise at low frequencies (< IkHx) was not measured. The system noise was determined to be 150 pV/&

for f > 1 kHz. When measuring the SQUID noise no rise of the

noise spectrum was detectable, pV/m

could be specified.

noise of @N < 80 ,&o/6 determined.

i.e. only an upper limit of the SQUID

noise of 75

Using this value an upper limit for the magnetic flux and for energy sensitivity

of CtN< 7*10-28 J/Hz were

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MULTILAYER Multilayers

were deposited by an RF sputtering process in a multisource ma-

gnetron sputter chamber containing two planar metallic targets with a composition of Y:Cu 50:50 and Ba:Cu 50:50 for the YBaCuO

films and a MgO target for the

isolation layer. Each target was 100 mm in diameter. each target by individual sted separately.

RF power was supplied to

RF generators with the phases for each target are adju-

During deposition the substrate temperature

was kept at 7500 C

by a resistive heater and the substrate was continuously moved at lo-30 rpm. In our experiments we deposited YBaCuO

at 40 Pa at an argon to oxygen ratio

of 7:3 which resulted in maximum deposition rates of 30 m-n/h. The YBaCuO have a zero resistance temperature

films

in excess of 86 K and critical current densities

of 2*106 A/cm2 (77 K) on MgO and SrTiO substrates. These values were obtained across a diameter

of 3 inches, which was limited by the heater dimensions only.

Processes at these parameters exhibited excellent reproducibility. One of the great advantages of the co-sputtering

process is that the stoichio-

metry can be varied easily by changing the RF power of the YCu- and the BaCutarget, respectively. Moreover,

Thus the optimum film stoichiometry

films sputtered

with slightly higher power values at the BaCu target

than for the optimum stoichiometry outgrowths

was readily obtained.

required were extremly

as can be seen in the SEM picture in Fig.

smooth without

any

2. Increasing the BaCu

power results in a reduction of Tc,o.

Figure 2: SEM picture of YBaCuO

surface sputtered with slightly higher power

at the BaCu target than for optimum stoichiometry

For a multilayer technology the epitaxial deposition of isolation layers is necessary. In our experiment we used MgO interlayers because they are also suitable for

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high frequency applications. The layers were deposited in the co-sputtering system by sputtering from a MgO target. MgO layers deposited at low temperatures exhibit a non epitaxial (llO)-orientation. The maximum intensity of the (200)-peak of an epitaxially grown MgO layer was obtained at a substrate temperature near 600°C. Fig. 3 shows a series of w-scans of such a multilayer structure at different values of 4 for a slightly slanted substrate. 1 Y19CO (006)

Figure 3: w-scan for a slightly slanted substrate of YBaCuO/MgO/YBaCuO multilayer proves epitaxial growth

The orientation of the YBaCuO layer as well as of the MgO layer on top follows the orientation of the SrTiOs substrate which proves epitaxial growth. Interlayers of this kind were used for the fabrication of epitaxial YBaCuO/MgO/YBaCuO multilayers. Both YBaCuO layers showed good electrical properties (T,,, > 82 K). STM studies of the upper YBaCuO layer revealed an average roughness of about 3 nm. Multiturn coils were fabricated using these multilayer processes. The layers were structured by Ar etching in a parallel-plate reactor using a photoresist mask. Fig.4 shows a micrograph of such a coil. SUMMARY In summary a successful process for SNS dc step edge SQUIDS have been developed. The SQUID properties could be varied across a wide range (RN = 3rnfi - 1 R andIc= 0.5 mA - 3 mA (10 K)) an d modulations were observed beyond 77 K. An upper limit for flux noise was ascertained to be @PN < 80 @o/m at 77 K and 1 kHz. For the fabrication of input coils a new in-situ co-sputtering process was presented which allows the reproducible preparation of large area (3” in diameter) YBaCuO

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Figure 4: Optical micrograph of an input coil films with extremely smooth surfaces. Epitaxial YBaCuO/MgO/YBaCuO multilayers have been fabricated with good electrical properties. Electrical properties of structured input coils are under study. ACKNOWLEDGEMENT This work was supported by the federal ministry of Research and Technology under contract No. 13N5708.

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