IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 7, NO. 2, JUNE 1997

Serial Array High Tc SQUID Magnetometer 3347

Soon-Gul Lee Department of Physics, Korea University, 208 Seochang-dong, Jochiwon, Chungnam 339-800, Republic of Korea.

Yunsung Huh, Gwang-Seo Park Department of Physics, Sogang University, C.P.O. Box 1142, Seoul 121-741, Republic of Korea.

In-Seon Kim, Yong Ki Park, Jong-Chul Park Korea Research Institute of Standards and Science, Yusong P.O. Box 102, Taedok Science Town, Taejon 305-600, Republic of Korea.

Abstrucr- SQUID magnetometers have been fabricated from series arrays of YBazCu307 dc SQUIDs by pulsed laser film deposition and ion mill patterning techniques. The array was designed either in linear chain with flux focusers or in meander structure. The arrays consisted of 50-130 step-edge junction SQUIDS and each SQUID had inductance of 20-40 pH with 2-5 pm junction width. A linear array of 50 SQUIDs showed a modulation amplitude of 150 pV which corresponds to a gain of -10. One of 80 SQUID meander arrays had a gain of as high as 50. These results provide feasibility of the series array of SQUIDs in the direct readout SQUID amplifier circuitry with wide bandwidth. Larger number SQUID arrays and processes to increase the gain to the theoretical value need to be studied.

INTRODUCTION

Among various applications of high Tc superconductors, the SQUID is one of the most promising devices and thus has been studied most extensively. In spite of the materials problems associated with the oxide nature and the extremely short coherence length (comparable to the lattice parameters of the high Tc superconductors) SQUID techniques, from design to fabrication, have been progressed enormously in such a short period of time. The thermal noise inherent to the high operation temperature turned out to be not critical for most applications except a few, such as magneto- encephalography. The thermal noise of the high Tc SQUID was trimmed down almost to the theoretical limit by making them from single layers of superconductors. One can always optimize the device for a specific application by a proper design. The design issue is far more important in high Tc devices compared with their low Tc counterparts.

Since the noise level of the SQUID is much lower than that of the room temperature preamplifiers, a good impedance matching of the SQUID output to the preamplifier is necessary to fully utilize the intrinsic high sensitivity of the SQUD. dc SQUID amplifiers are usually operated in a flux-locked loop (FLL) mode, with tank circuits or transformers to match the impedance. The use of the matching circuits not only complicates the system but, more importantly, seriously limits the bandwidth. The frequency response of the FLL mode is at best 100 kHi. In addition, the relatively low reproducibility of the parameters

of high Tc SQUIDs makes the impedance matching more tedious. Junction parameters, such as critical current and normal state resistance are not well controlled yet.

The goal of th is work is to develop high Tc dc SQUID arrays to make the voltage swing large enough so that the SQUID output can be measured directly with a room temperature preamplifier. The elimination of the matching circuit provides a much wider bandwidth as well as simplifies the electronics. The SQUID array can be used either as a second-stage readout amplifying device of the first-stage single sensing SQUID or as the SQUID amplifier itself.

Welty et al. [1,2] studied the series of 100 Nb dc SQUIDs as the readout amplifier and achieved 3 mV voltage swing with 2 ns response time which corresponds to -0.2 GHz. Geren [3] proposed integration of the dc SQUID arrays into rf amplifier circuitry with frequencies up to microwaves. Stawiasz et al. [4] studied the noise properties of the Nb dc SQUID series arrays. All those experimental studies have been performed on conventional low Tc SQUID arrays. In this work we have studied fabrication of the series arrays of YE3a2Cu307 dc SQUIDs and their feasibility in the direct- readout SQUID amplifier circuitry.

FABRICATION OF SQUID ARRAYS

In order to utilize the full capacity of the SQUID the output signel of the SQUID has to be amplified large enough in the intermediate stage so that the noise level exceeds that of the room temperature preamplifier. In a typical scheme of the intermediate stage amplification, tank circuits or transformers are used. In this study, the intermediate amplification is designed to be realized by connecting identical dc SQUIDs in series. Each SQUID of the array participating in the amplification also plays as a sensing SQUID.

While the voltage signal of an N SQUID array is amplified by N times, the noise of the array increases as N’12 because of its uncorrelated nature. The resultant signal to noise ratio increases by N*/2. Typical voltage noise of high Tc SQUIDS is roughly in the range of 100 pV per unit band- width and the input noise of best preamplifiers is -1 nV/Hz’/2. Therefore, the magnitude of the voltage modula-

1051-8223/97$10.00 0 1997 IEEE

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(b)

Fig. 1 of array on 1 an’ SrTiO3 substrate. SQUIDs.

Series array of 50 step-edge junction YBCO dc SQUIDS. (a) Design (b) Optical microscope picture of the

tion, which is -10 pV, needs to be amplified to millivolt range to ensure that the array noise exceeds that of the preamplifier for N=lOO.

The number of SQUIDs in the array was chosen to be 50- 130. The series is either in a linear chain or in a square meandering pattern with a minimum device area. The meandering array can be used to detect uniform fields. If one intends to use a flux focuser to enhance the field responsivity of the array, a uniform flux coupling to the meander array is extremely difficult. For this purpose we designed a linear chain array with two flux focusers and a control line as shown in Fig. 1. Central two parallel lines of the focusers ensure uniformity of the field threading each SQUID along the chain. According to our estimation the magnetic field generated by the induced screening currents running oppositely in the two lines is uniform within 1 % between the center SQUID and the end SQUID of the array which are 0.5 mm apart.

The overall dimension of the device is 8.4x8.4 mm’. The line width of the pickup loop is 1 mm except the central part which has 100 pm width. The gap between the two focusers is 50 pm wide. Each SQUID has 6x20 pm2 hole and a corresponding inductance of -20 pH. The screening parameter will be -1 for 21,=100 PA. To avoid interloop flux coupling, neighboring SQUIDs are connected via a 4 pm line as shown in Fig. 1 (b). Step-edge junctions

comprising the SQUID 4 pm width. Ladder type arrays with common edges were also fabricated to compare the properties.

About 2,000 A step was formed on a SrTiO3(1OO)l

yBa,Cu307 film with about the same thickness as the step, height was deposited by pulsed laser ablation. 200-500 A thick SrTiOs layer was deposited in situ to protect the film from degrading during following patterning processes. Aui contact pads were prepared by wet etching of the windows inl 10 % aqueous €IF solution and subsequent Au deposition by sputtering and lift-off. into a SQUID array by devices they were usu atmospheric pressure at 450 “c for more than an make up for possible o in vacuum.

substrate using Ar’ milling with a photoresist mask. I

RESULTS AND DISCUSSION

Fig. 2 and 3 show I-V c linear chain array of 50 modulation curve was low noise preamplifier. At 77 K the critical array was about 20 pA and the modulation voltage was 30 pV. If we assume that 21, of the SQUID is 20 pA, PLz0.2.

I

Fig. 2 I-V curve ofthe 50 SQUID array at 70 K.

Fig. 3 Voltage modulation ofthe 50 SQUID array. T=70 K and Ib=133 pA

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At a lower temperature, 70 K, 21,~lOO pA which corresponds to PLzl and the swing voltage increases to 150 pV (Figs. 2 and 3). The modulation amplitude of a single SQUID was measured to be 10-20 pV, so the amplitude gain of the array is about 10, which is much smaller than the designed value, 50. The gain discrepancy between theory and measurement is discussed below. One of our meander arrays of SO SQUIDs has shown a gain of about 50 with 45 pV amplitude at 77 K.

The gain discrepancy between design and data is believed to originate from both nonuniform junction critical currents among SQUIDs and incoherent modulation due to unequal fluxes threading the SQUIDS. The normal state resistance of the junctionwas observed to be relatively uniform in the same batch. In case the junction critical current is different among SQUIDs, the modulation amplitude of each SQUID at the same bias is dispersed below its maximum and thus the array amplitude is suppressed. The significant rounding of the I-V curve near the transition in Fig. 2 is due to critical current inhomogeneity. Fig. 4 shows a severe case of multiple transition due to nonuniform junction critical currents. The inhomogeneous I, might come from variation of the junction parameters due to lithographic error or different step conditions.

Fig. 4 Severe case ofmultiple transition observed in another array.

The phase incoherence of the modulation can also reduce he amplitude severely by canceling signals of different SQUIDS. This effect becomes larger as the magnetic field ncreases. Fig. 5 shows the amplitude modulation of the trray output voltage due to distributed SQUID flux. Li et 11. 151 studied amplitude modulation by variation of wiodicity. According to the same analysis as in Ref. 5 the Fariation of the modulation period in Fig. 5 is estimated to

Fig. 5 Amplitude modulation ofthe array output signal. T=74 K and &=30 PA.

as that from Fig. 5. Figs. 6 and 7 show a linear ladder array with its voltage

modulation signals at several bias currents. The area of the loop is S X 20 pm2 and the junction width is 2 pm. Modulation amplitude is modulated with the bias current. At Ib=390, 580 pA, the modulation is almost nulled. In ladder arrays, the screening current can circle a loop containing more than one SQUIDs, thus the resultant signal is a linear combination of signals with different periods. The amplitude reduction is h r more serious in ladder arrays. In Fig. 7, noise of the signal becomes very large at large biases. The large noise is believed to be due to flux jumping in the SQUID loops.

Most of those errors are extrinsic and can be reduced by careful preparation of the sample and by proper design. Fabrication of practical large number (W-100) SQUID arrays and the process to increase the gain to the theoretical limit are under study.

Fig. 6 Ladder type SQUID array. Each SQUID has 8 X 20 }un2 area and )e -5 %. The cause of the period variation is unequal flux n each SQUID which is due to either variation of the Leometrical SQUID area or nonuniform field distribution in he array. Since the field is uniform within 1 % for the inear array with flux focusers, the variation causing the mplitude modulation in Fig. 5 is ascribed to the variation Of ‘E SQUID loop area. The SQUD area from the optical fliCrOSCOpe Picture had a lithographic error of the Same Size

SUMMARY

we have studied the series SQUID array as a way to fully take advantage of the sensitivity of high Tc SQUIDs without losing the large bandwidth. The gain ofthe array was

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I

Fig.7 Voltage modulation of a ladder array at several biases

measured to be 0.2-0.6 of the designed value. The gain discrepancy is due to variation of the critical current and the modulation period, which are extrinsic and thus reducible. These results indicate the feasibility of the SQUID array in the SQUID amplifier circuitry. Larger number (N>100) arrays and optimum processes are under study.

REFERENCES

[I] R. P. Welty and J. M. Martinis, “A Series Array of DC SQUIDS: IEEE Trans. M a p . vol. 27, pp. 2924-2926,1991.

[2] R. P. Welty and J. M. Martinis, “Two-Stage Integrated SQUID Amplifier with Series Array Output,” IEEE Trans. Appl. Supercon. vol. 3, pp. 2605- 2608,1993.

[3] W. P. Geren, “Application of Small-Signal Model of DC SQUID Cicuit Design,” IEEE Trans. Appl. Supercon. vol. 5 , pp.2770-2773,1995.

[4] K. G. Stawiasz and M. B. Ketchen, “Noise Measurements of Series SQUID Arrays,” IEEE Trans. Appl. Supercon. vol. 3, pp. 1808-1811, 1993.

[SI K. Li and P. Hubbell, “Measurement and Siniulation of the Voltage-Flux Transfer Function of SQUID Arrays,” IEEE Trans. Appl. Supercon. vol. 5, pp. 3255-3258,1995.