In-situ growth of epitaxial YBa2Cu3O7 thin films by on-axis unbalanced d.c. magnetron sputtering

182

Thin Solid Films, 228 (1993) 182-185

In-situ growth of epitaxial YBa2Cu307 thin films by on-axis unbalanced

d.c. magnetron sputtering N. Savvides and A. Katsaros Division of Applied Physics, Commonwealth Scientific and Industrial Research Organization, Sydney, N.S. I4/. 2070 (Australia)

Abstract

YBa2CH307 thin films are grown epitaxially onto MgO(100) substrates by on-axis d.c. magnetron sputtering. The magnetron uses an unbalanced magnetic field configuration to circumvent resputtering effects, and high quality thin films are realized with excellent reproducibility. A stoichiometric target is sputtered in an Ar-O2 mixture (Ar-to-O2 ratio of 15 to 1) and films are deposited onto the heated substrates placed 40 mm directly in front of the target. We report on the growth and properties of films prepared as a function of sputtering pressure p~ = !-100 Pa, and as a function of the heater temperature Ts = 600-860 °C. c axis epitaxy is obtained over a wide range of deposition conditions (T~ >/700 °C; Pt >i 30 Pa). Typical films have excellent crystalline quality, a transition temperature T c = 8 5 - 9 0 K , a critical current density 1¢77K~106Acm-2 and a resistance ratio R3oo K/Rmoo K = 2.0--3.1. In-situ superconducting

1. Introduction

2. Experimental details

For many of the small-scale device applications at present being considered, high quality in-situ superconducting YBa2Cu3OT_x thin films must be reproducibly prepared and used to develop multilayer devices and heterostructures [ 1, 2]. Currently the most successful methods for in-situ growth of YBa2Cu30 7_x thin films are magnetron sputtering [3-7], laser ablation [1, 8] and reactive evaporation [9]. Magnetron sputtering is a very reproducible and easily controlled high deposition rate technique [10] that can be adapted to deposit multilayers or heterostructures combining superconducting, metallic, semiconducting and insulating layers. However, when standard or balanced magnetrons are used to prepare YBa2Cu307_ x thin films, severe resputtering of the film during deposition alters its composition and degrades its superconducting properties. We have developed an unbalanced magnetron sputtering source which circumvents resputtering [7, 11] and have operated it under both d.c. and r.f. power to deposit superconducting YBa2Cu307_ x and insulating PrBazCu3OT_ x thin films respectively. In this paper we report on the in-situ growth and the properties of YBa2Cu307_ x thin films. We use a single stoichiometric target and on-axis geometry to deposit devicequality films at high deposition rates, and at oxygen partial pressures well below those used by other techniques.

In conventional magnetron sputtering, energetic O ions and neutrals produced at the target, and Ar ÷ and O2 ÷ ions from the plasma, bombard the growing film and preferentially resputter Ba and Cu from its surface [6, 11]. Methods to minimize resputtering, such as placing the substrate in an off-axis position and sputtering at very high pressures [3-6], suffer from very low deposition rates, typically 30 nm h -I or less, or the geometry is incompatible with standard (planar) thin film deposition processes. To offset these problems while maintaining the advantages of magnetrons, we use an unbalanced magnetic field configuration to direct the plasma away from the substrate and eliminate resputtering effects. Figure 1 serves to illustrate the difference between a conventional or balanced magnetron and the unbalanced magnetron. Further details of the source and its operation have been given elsewhere [7, 11]. The stoichiometric YBa2Cu307_x target was sputtered in an A r - O 2 gas mixture at a fixed partial pressure ratio PAr/P02 = 15. The discharge was operated at 120-150 W and 110-130 V. The deposition rate varied with the total sputtering pressure Pt from 3 lain h - t at Pt-----1 Pa to 0.3 lam h -~ at pt = 50 Pa for a 150W discharge. The single-crystal MgO(100) substrates were mechanically clamped at the centre of a Haynes alloy heater which is fixed directly opposite the target. The deposition temperature T s was monitored with a

0040-6090/93/$6.00

© 1993 -- Elsevier Sequoia. All rights reserved

N. Sauuides, A. Katsaros / ln-situ growth of YBa2Cu307 by d.c. magnetron sputtering

Balanced Magnetron

Unbalanced Magnelron

183

Sputtering Pressure Pt (Pa) 0

90_

20

40

60

80

I

I

I

I

I00

YBa2Cu3OT_x/MgO (100)

":'.:: :"- - - - - - - -

I

I

FILM

i ...............

8o

i

/ / / S o f t Iron ~i/ PermanentMagnet

[.~

g

3. Results and discussion

Figure 2 shows midpoint transition temperatures Tc and the resistance ratios RaOoK/R]ooK for films deposited at T~ = 740 °C as a function of the sputtering pressure Pt, and for films deposited at Pt = 42 Pa as a function of the deposition temperature T~. The films were 200-250nm thick, with the thinner films deposited at Pt t> 80 Pa or T~ >I 800 °C. Figure 3 compares the X-ray diffraction spectra of some of these films;

p l

Fig. l. Schematic diagrams of the magnetic field configurations in a conventional (balanced) magnetron and the unbalanced magnetron.

chromel-alumel thermocouple embedded in a hole in the heater block; the actual substrate temperature was estimated to be 50-70 °C below T,. After each deposition, the oxygen pressure was raised to 50 Pa and the specimen held at T~ for 30 min. The oxygen pressure was then raised to 1 arm while the temperature was lowered to 550 °C. The specimen was held at this temperature for about 1 h and then cooled to room temperature. Epitaxial growth and film quality are generally sensitive to the deposition temperature, the gas composition and pressure, and the degree of energetic particle bombardment. The process can be optimized to yield thin films suitable for a specific application; for example, multilayer devices operating at 77 K would require films with very smooth surface morphology while high critical current density applications would require films with a dense network of defects to enhance flux pinning. The object of the present work was to establish optimum deposition conditions for in-situ growth of epitaxial c axis films for superconducting quantum interference devices (SQUIDS) and other devices. Films for investigation were prepared (a) as a function of sputtering pressure (Pt = 1-100 Pa) for a fixed deposition temperature Ts = 740 °C and (b) as a function of temperature (Ts= 600-860 °C) for a fixed sputtering pressure Pt = 42 Pa.

z,_-,401c ~

2Pa

PAr/P02 "-" 15:1

'°i//

,,.,a,\

.

3

6o

600

'/

,

,

,

700 800 Substrate Heater Temperature T s (°C)

1

i

.

Fig. 2. Variation in the transition temperature Tc (midpoint) and the resistance ratio R3oo g/Rloo K of YBa2Cu307 -x thin films with del~Osition sputtering pressure Pt and substrate heater temperature Ts.-

Films deposited at a fixed temperaturb (T, = 740 °C) and Pt ~ 20 Pa consist of a mixture of c axis and a axis crystallites, with the a axis being domiriant at 3-5 Pa. These films have a low Tc and 10w resistance ratio but can be improved by post-deposition annealing in oxygen. At higher sputtering pressures the c axis becomes dominant so that for Pt >/35 Pa the films show very strong (00/) reflections, indicating a very tiigh degree of c axis crystalline alignment. These films have a midpoint Tc = 87-90 K with sharp transitions, and an excellent metallic resistivity in the normal state with R3ooK/RtooK = 2.5-3.1 comparable with the best bulk material. For films deposited as a function of temperature the onset of crystallization was observed, at Ts ~ 600 °C. At Ts = 650 °C the films consisted mainly of c axis crystallites with some non-aligned crystallites. At higher temperaturesthe c axis orientation became dominant, with films deposited at Ts/> 700 °C being ;highly c axis aligned. X-ray diffraction measurement of the full width a t half-maximum (FWHM) of the (005) line of c axis films gave 0.10-0.15 °. Rocking curves (omega scans) On the

N. Sauuides, A. Katsaros /In-situ growth of YBa2Cu307 by d.c. magnetron sputtering

184

A

CuO

/M 0

4 - - / I YBa2 3 7-x

.oo (200) T

g

I I

!

,oo., lit ,oo.,

r

,oo, 10021

2 --

•~>'

i II,]Li

10011 (004)

Pt (Pa)

(007)

100

Ig 0-

35

(a)

10

I

8

~~,'

] (oo,~ (]

02) (ooz)

2

o

I

Ill

IlLII ,00,,I

(oo4)

-

10 (h)

7'

20

30

40

Ts (*C) 850

~ =50= ~ ~ ~60

20 (degree)

Fig. 3. Typical X-ray diffraction spectra of YBa2 Cu307_ x thin films deposited as a function of (a) sputtering pressure Pt and (b) substrate heater temperature 7",.

same line gave a FWHM of 0.2-0.6 °, compared with 0.2 ° for single crystals. More qualitative studies of the X-ray data reveal an interesting relationship between the c axis lattice parameter and the preparation conditions (Fig. 4). Films deposited at Ts t> 680 °C and/or Pt >140 Pa have c cell length parameters close to that of bulk material ( 11.688 A). Films deposited at lower temperatures and pressures show c axis lattice parameter expansion which is believed to be due to crystalline defects (e.g. stacking faults) resulting from the non-optimum growth conditions. It is significant to note that all the films were grown at oxygen partial pressures well below the Hammond-

Bormann thermodynamic stability line [3]. For example, stoichiometric films with excellent c axis alignment are produced at Ts = 700-800 °C using Po2 = 2.6 Pa compared with values of Po2 = 5-80 Pa required by the stability line. Reactive sputter deposition using oxygen supplies both plasma-activated molecular oxygen and 02 + ions [9, 10]. In our technique the film develops a self-bias of about - 2 V, while the plasma is at about + 2 V with respect to ground. Some of the 02 + ions, of energy about 4 eV, are accelerated out of the plasma and undergo coUisional dissociation at the film surface [12], producing highly reactive atomic oxygen. Consequently lower overall oxygen pressures are

N. Savvides, A. Katsaros / 1n-situ growth of YBa2Cu307 by d.c. magnetron sputtering !1.8

I YBa2Cu3OT_x/MgO (100)

11.7

Bulk

°5 T s = 740"C O

11.6 t~ o U

0

(a)

I 50 Sputtering Pressure Pt (Pa)

11.8

I

100

!

185

unbalanced magnetic field configuration to circumvent resputtering, and high quality thin films were grown on MgO(100) substrates. Significantly our technique operates at oxygen partial pressures (P0e~0.1-6Pa) well below those given by the Hammond-Bormann thermodynamic stability line. Activated molecular and atomic oxygen species reach the growing film and impart increased chemical activity to the synthesis so that film growth takes place at low oxygen partial pressures. Other technological advantages are high deposition rates and in-situ epitaxy of c axis films over a wide range of deposition temperatures (Ts = 700-860 °C).

o~

Acknowledgment 11.7

I------ Bulk

We thank Matthew P. James for his assistance with SEM. Pt = 42 Pa

I I 1 i.6 }00 700 800 600 (b) Substrate Heater Temperature T s (°C)

Fig. 4. Variation in the c axis lattice parameter with (a) sputtering pressure Pt and (b) substrate heater temperature Ts.

required to achieve fully superconducting and stable films. The critical current density Jc of highly c-axis-oriented films, determined from four-point transport measurements using a 1 ~tV criterion, was typically 106 A cm -2 or higher at 77 K. The surface morphology of films, generally sensitive to initial nucleation and substrate quality, was studied by scanning electron microscopy (SEM); results will be presented elsewhere. At high deposition temperatures (Ts>750°C), rapid lateral growth, in the a - b plane, produces plate-like crystallites which interconnect to form essentially single-crystal films with growth spirals clearly visible on the surface. 4. Conclusion In-situ superconducting Y B a 2 C u 3 0 7 _ x thin films

were prepared by on-axis magnetron sputtering from a single stoichiometric target. The magnetron uses an

References 1 D. B. Chrisey and A. Inam, Mater. Res. Soc. Bull. •7(2) (1992) 37. 2 K. Char, M. S. Colclough, L. P. Lee and G. Zaharchuk, Appl. Phys. Lett., 59 (1991) 2177. 3 C. B. Eom, J. Z. Sun, B. M. Lairson, S. K. Streiffer, A. F. Marshall, K. Yamamoto, S. M. Angage, J. C. Bravman, T. H. Geballe, S. S. Laderman, R. C. Tabor and R. D. Jacowitz, Physica C, 171 (1990) 354. 4 L. H. Greene, B. G. Bagley, W. L. Feidmann, J. B. Barner, F. Shokoohi, P. Miceli and B. J. Wilkens, Appl. Phys. Lett., 59 (1991) 1629. 5 C. Blue and P. Boolchand, Appl. Phys• Lett., 58 (1991) 2036. 6 X. X. Xi, G. Linker, O. Meyer, E. Nold, B. Obst, F. Ratzel, R. Smithey, B. Strehlau, F. Weschenf¢lder and J. Creerk, Z. Phys• B, 74 (1989) 13. 7 N. Savvides and A. Katsaros, Appl. Phys• Lett., 62 (1993) 528. 8 B. Oh, R. H. Koch, W. J. Gallagher, R. P. Robertazzi and W. Eidelloth, Appl. Phys. Lett., 59 (1991) 123. 9 V. Matijasevic, P. Rosenthal, K. Shinohara, A. F. Marshall, R. H. H a m m o n d and M. R. Beasley, J. Mater. Res., 6 (1991) 682. l0 N. Savvides, Thin Solid Films, 163 (1988) 13. I1 N. Savvides, C. Andrikidis, D. W. Hensley, R. Driver, J. C. Macfarlane, N. X. Tan and A. J. Bourdillon, in D. Christan, J. Narayan and L. Schneemeyer (eds.), High-Temperature Supercon-

ductors: Fundamental Properties and Novel Materials Processing, Materials Research Society Syrup. Proc., Vol. 169, Materials Research Society, Pittsburgh, PA, 1990, p. 655. 12 N. Savvides and B. Window, J. AppL Phys., 64 (1988) 225.