Physica C 201 (1992) 337-339 North-Holland PHYSICA

TiBaCaCuO-films for passive microwave devices

M. Manzel a, H. Bruchlos a, E. SteinbeiB a, T. Eick a, M. Klinger b, j . Fuchs c and B. Kley c a InstitutfurPhysikalischeHochtechnologie, Helmholtzweg4, D-6900Jena, Germany b Forschungsgesellschaft far lnformationstechnik, Bodenburger Str., D- 3202 Bad Salzdetfurth, Germany c Friedrich-Schiller-Universitiit Jena, IAP, Max-Wien-Platz 1, 1)-6900 Jena, Germany

Received 25 July 1992 Revised manuscript received 26 August 1992

Superconducting T1BaCaCuO-fflms were prepared by a two-step method. The TI-HTSC films on LaA103-substrates are single- phase, highly c-axis oriented, and have critical temperatures up to 116 IC The films have surface resistances of 35 mfl at 77 K and 92 GHz and were tested for microwave devices. Furthermore we propose a new structuring method for the fabrication of microwave devices made of T1-HTSC-Fflms.

1. Introduction

The fabrication of passive microwave components using high-temperature superconducting films (HTSC) requires films with high critical tempera- tures (To) and low surface resistances (Rs) at op- erating frequencies. The ability to grow the super- conducting films on both sides of large area substrates can enhance the performance of RF microstrip de- vices. Therefore a film preparation technique, which produces highly c-axis oriented T1BaCaCuO-films on both sides of a LaA103 substrate, is very desirable. Such films were used in the fabrication of resonators and filters in the UHF region.

2. Thin film preparation

The films were p repared by a two-step me thod [ 1,2 ]. As a first step B a - C a - C u - O precursor f i lms were sput ter depos i ted on both sides o f a LaAIO3 substrate and then Tl was incorpora ted by post an- nealing in a Tl-oxide loaded a tmosphere . The pre- cursor fi lms were react ively depos i ted f rom a me- tallic B a - C a - C u alloy target by means o f a DC high rate sputtering. Selective resput t ing effects can be suppressed at high depos i t ion rates ( R > 5 0 n m / min ) , because the desorp t ion rate o f the alkal ine

ear th-metal components was found to be nearly in- dependent o f the deposi t ion rate [ 2 ]. The DC sput- tering o f the precursor fi lms was ca rded out under the condi t ions l isted in table 1.

The as-deposi ted fi lms on (100) LaA103 substrate were amorphous , isolating, and appear mirror- smooth. T1 was incorpora ted in the precursor fi lms by post anneal ing in a Tl-oxide loaded a tmosphere for l0 min at 1140 K and subsequent cooling in 02 with a special t empe ra tu r e - t ime programm. During this step the sample, depos i ted on both sides of the substrate, was enclosed in a gold foil bag together with two pres intered pellets of T1-HTSC bulk ceramic. T1- diffusion exper iments with pellets of different com- posi t ions (2223, 2222, and 2201 ) have shown that T12Ba2Ca2CuEOx is an useful composi t ion with re- spect to the durabi l i ty o f the Tl-diffusion source. Dep th profiles, measured by means of secondary neutral mass spectrometry, indicate that Tl is ho-

Table 1 Sputtering conditions

Target ~ = 80 mm Ba2Ca2Cu3-aUoy DC power 275 W Sputtering gas 90% Ar+ 10% 02 Sputtering pressure 1 Pa Substrate temperature 500 K Substrate-target spacing 50 mm

0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

338

Table 2 Structure parameters

M. Manzel et al. / TIBaCaCuO-films for passive microwave devices

Crystal structure Dominant phase Lattice constant Halfwidth of rocking curve

polycrystalline, grain size about 10 lain 2223 c= 35, 648 A <1.5 °

Table 3 Superconducting properties

Transition temperature AC-susceptibility transition width (90-10% ) Critical current density at 77 K (H = 100 kA/m perpendicular to the film normal ) Surface resistance at 77 K and 92 GHz

>IlOK < I K 2 × 105 A/cm 2

35 mfZ

~100

c 2

10

4 ""---- r7

./

J

/ /

¢

10 20 30 40 50 60 70 80 90 100 110 Temperature / K

Fig. 1. Temperature dependence of the surface resistance Rs for a T1BaCaCuO-film, measured at 92 GHz.

mogeneously distributed in the annealed thin T1- HTSC-fdm. However, the annealing process is a very critical technology step, in which the precursor film is leaded with TI, simultaneously crystallized, and fully oxidized. The two-step (deposi- tion + annealing) can also be used successfully for the preparation of large-area TIBaCaCuO-film on both sides of monocrystalline substrates.

3. Film properties

The superconducting properties of the T1Ba- CaCuO-films depend sensitively on the morphology, phase composition, and texture of the films. We could demonstrate [3 ] that T1-HTSC-films with low sur- face resistance Rs are characterized by a well-devel- oped coarse-grain crystal structure, high degree of c- axis orientation, and high phase purity, expressed not only in the X-ray diagram but also in the sharpness o f the AC-susceptibility transition near To. Typical parameters of the structure are listed in table 2.

The films are approximately 1 ~tm thick and are grown on both sides of the LaA103 substrates. The T1-HTSC-films having the mentioned structure pa- rameters show the following superconducting prop- erties (table 3).

The temperature dependence of the surface resis- tance Rs of a T1BaCaCuO-film, measured with a Ho~ l- resonator, is shown in fig. 1. The reached Revalues are sufficiently low for microwave device fabrication [5,6].

4. Film patterning method

For the preparation o f planar passive microwave devices made of TI-HTSC films the structuring pro- cess of the accomplished films has the disadvantage that the structuring equipments are contaminated with toxic Tl-compounds. Moreover, the edges o f the striplines, carrying the highest current densities [ 4 ],

M. Manzel et aL / TIBaCaCuO-films for passive microwave devices 339

are disturbed by the structuring process. Therefore we have tested a new method for the fabrication of planar microwave devices. We pattern the Tl-free precursor films and adjust the superconducting properties of the structured film by annealing in the well known manner. The usefulness of this technol- ogy was proved on functioning 4 GHz half-wave- length microstrip resonators. We obtained unloaded quality factors at 77 K up to 1000 at - 50 dBm input power, which are lower than expected from the Rs values. Therefore the improvement of the annealing process of the patterned precursor films is now un- der investigation.

Acknowledgements

The authors thank J,H. Hinken, F.I.T., Bad Salz- detfurth, for valuable discussions and B. Conrad for

technical assistance. This work was supported by BMFT under Grant Nr. FKZ 13 N 5927.

References

[ 1 ] C.X. Qiu and I. Shih, Appl. Phys. Lett. 53 (1988) 1122. [2] M. Manzel, H. Bruchlos, G. Bruchlos, T. Eick, E. SteinbeiB

and L. Illgen, Phys. Status Solidi A 128 ( 1991 ) 175. [3] M. Manzel, H, Bruchlos, G. Brnchlos, T. Eick, E. SteinbeiB,

M. Klinder and G. Eimbeck, Phys. Status. Solidi. A 131 ( 1992 ) K43.

[4] J. Wolff, Einf'tihrung in die Mikrostrip-Leitungstechnik, Teil I.: Die Leitungen (Wolff, Aachen, 1985 ) p. 78.

[5] R.B. Hammond, G.V. Negrete, L.C. Bourne, D.D. Strother, A.H. Cardona and M.M. Eddy, Appl. Phys. Lett. 57 (1990) 825.

[6] W.L. Holstein, L.A. Parisi, D.J. Kountz, C. Wilker, A.L. Matthews, P.N. Arendt and R.C. Taber, IEEE rans. Magn. 27 (1991) 1568.