Properties of ErBa2Cu3O7−x thin films formed by ionized cluster beam deposition at low pressure

Nuclear Instruments and Meihods in Physics Research 1381 ./61 (1993) 1313-131`.; North-Holland

Ssam Intarx~tion: with laatarrials ti Atams

Properties of ErBa,Cu 3 k)7_x thin films formed by ionized cluster beam deposition at low pressure A . Shuhara, N . Riloh and T . Takvgi

Ion Engmcertng ft-1i Institute Corp., Hirakata . Osaka 573-01, Japan

E . Haraguchi, S . Terai and A . Ryohman

Kansas Electnr Kwer Co ., hic., Amagaraki, Hyogo 661, Japan

The influence of chamber pressure -)n HTS film formation was studied to order to fabricate. mdorm targe-,:rea H IS films for application in coil or tape fabrication . As-deposited ErBa'CU 3 07 -, supereonducting thin films were prepared by ionized cluster beam (IC'B) deposition in a mixture of 6 vol .% ozone in oxygen . X-ray diffraction studies showed that films with preferential c-axis orientation normal to the substrate surface were formed at a total pressure as low a~ I x 10 -° Torr. The properties of films fora-t at I x 10--4 Torr -c allioct identical to those of films formed at 4 x 10 -a Torr . Fabrication at lower pressure reduced the source consumption significantly. As-deposited alms ,repared on a 6 in . sample holder have it composition variation of no more than f5crr and a zero resistance temperature of 78-80 K. 1 . Introduction Sine. discovery of cuprate high-temperature superconductors (HTS) by Bednorz and Müller in 1986, a large number of investigations have been devoted to the development of HTS film deposition . We have investigated HTS films prepared by ionized cluster beam (ICB) deposition it order to apply these films to tape or soil fabrication. ErBa,Ca j0 7 - c films have alrcat:y been produced on a 3 in. diameter simple holder from three ICB sources by Kiwagoe et al . [1,2] . Those films were deposited in a mixture of 6 vol .% ozone in oxygen at a total pressure of 4 x 10 -° Torr [3] . However, film fabrication on a larger diameter substrate is necessary for our purposes . Formation on a larger (e .g . 6 in .) substrate by ICB deposition can be performed basically by increasine the distance between the ICB sources and the substrate . However, because the films are produced at a relatively high pressure, the collision, which take place between the gas molecules and metal atoms present a problem to the fabrication of good HTS thin films, as will be explained . This paper presents the results of studies to lower the pressure during deposition and to evaluate the uniformity of films prepared on a 6 in . sample holder . 2 . Experimental Three ICB sources for Er, Ba and Cu deposition having an acceleration voltage of 300 V used to 0168-583X/93/$06.00© 1993 - Elsevier

deposit 123 thin films on (100) MgO and (100) SiTi0, substrates (10 x 10 mm). The sample holder was heated to 650°C by thermal radiation from a Ta %% ;re heater located above . During film deposition, the h,)Islcr was rotated at 10 rpm to obtain uniform film properties . The oxidation gas, composed of 6 vol .% ozone in oxygen was introduced into the chamber through a narrow tube located in proximity to the substrate . Quartz crystal deposition monitors were used to deterinutc the deposition rate from each source . The film deposition ate was 4 nm/min . Each source had an inner volume of 5 cm'. The weight of metal deposited on the sample was determined by ICP (inductively coupled plasma) measurements of the film . Room temperature resistivity was determined by a standard four point resistivity probe . The transport properties of the film, w~~" re evaluated by a standard four-probe ac: measurement . 3 . Results Fig . 1 shows the deposition rate profiles for Er, Ba and Cu as a function of chamber pressure. Tac data were obtained at a monitor to source distance of 300 mm . Kawagoc performed HTS film fabrication at this distance . As the pressure increased, the normalized sates decreased for all three metals. The rate decrease is caused by collisions between the gas molecules and the metal atoms (incluc ing clusters) and depends on the elemental species. The profile for a metal with

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Va . NOVEL TECIINIQUES (a)

Aid-1- et at.. 1 HTSMir: film fonttarnm or h-premrtms

è a

Y

Cu(64) -4 1® 10-3 10-5 Pressure ( Torn ) Fig . 1 . Normalized deposition ratesfor Er,Ba andCu sources s a function of chamber pressure- ï he numbers in the figu :e indicate the mass numbers of these metals.

smaller mass number drops more rapidly. Collisions were not se serious at this distances, even when the chamber pressure was raised as high as 4X 10 -4 Torr,

under which condition Kawagoe performed ErBCO film formation on a 3 in . s;nnple holder without any

problems. Fig. 2 shows the deposition rate for the Ba source as a function of the distance from the ICB source to the monitor. The data were obtained at a total pressure of 4X 10 -4 Torr. The meats free path (L) for hanum-oxygen collisions was calculat_d to be 250mm from fig. 2 anti was found to obey the ..°_quation 4' = ;N cap(-1/L), where 1 is the distance between the source and the monitor, Nr the number of metal atoms ejected from the source, and N the number of metal atoms reaching the monitor. This value is larger than that for oxygen-

oxygen collisions (130 mm), and the difference can be

explained by the larger mass of barium . These tcvo figures indicate that formation at lower pressure becomes more important with increasing substrate diameter.

N C Y 6

5

40

20

60

20 ( deg .) Fig. 3. 20 ){-ray diffraction data using Cu K  radiation for films formed ai (a) 4X10 -s , (b) 1x10 -4 and (c) ErBCO 4X 10 -J Torr.

Fig. 3 shows the X-ray diffraction patterns of 100 rim thick films formed at 650°C. The spectrum for the tilm formed at 4 x 10 -` Torr indicates poor film crystallinity, while the spectra for films formed at 1 X 10 -4 and 4 X 10 -4

Torr are sirtually identical and show preferential c-axis orientation . Although this c-axis orientation seemed to appear at a pressure below 1 X l0 -4 Ton, the total pressure was then fixed at 1 X 10 -4 Torr hereafter, in order to confirm c-axis orientation. The electrical properties of these films depended on the cool-down conditions. The room-temperature resistivity of film cooled for 20 min at l X 10-4 and 4 X 10 -4 Torr was 1.6 mit cm, while that for film cooled for 40 min at 4X 10 -4 Torr was 500 p.11 cm . The latter value is a reasonable room-temperature . resistivity for HT S film. The lattice constants provided by X-ray diffraction data forfilm with 20 mincool-down 10-4 Torr at 4 X and with 40 min cool-down at 4 X 10-4

Torr were 11 .85 and 11 .7C A. respectively. The decrease in the c-axis lattice constant indicates that oxygen is incorporated into the filin and that conversion from orthorhombic to tetragocal structure has occurred . Furthermore the 1 Vin thl'k ErBCO film incor-

â ü C c0

9 E O z

Y N .N ü) N GC

0

600

1300

Distance( mm ) Fig. ' Normalized deposition rate fo: a Ba source as a furcJoa of thedistance from monitor to source .

0

100 Temperature (

200

.: ) Fig. 4. Resistance-temperature curve of 100 rim thick ErBCO film formed on (100) MgO.

A. Shuhara et

al. / HTS thin film fannatim at low pressures

Er

c0 N 0. 0 U

Ba

v

d N m t: o zZ

Cu

1 .0 0.9 0.8 20

0

20

Position

.

40 ' 60 ( mm

I 80

)

Fig. 5. Elemental composition profiles versus position fur as-deposited ErBCO films grown at 65(rC on a rotating sample holder with a diameter of 150 mm. porated sufficient oxygen during the 40 min coo'.-dawn at 4 x 10 - 4 Torr. A typical resistivity versus tetrperaiu ü cu vc for a 100 atn Er-13a-Cu oxide film on (t90) MgO with a Tc of 83 K is shown in fig . 4. The J. value of filir on (100) SrTi0 3 was measured using three kinds of micro-bridge patterns khich were 17 pin wide, and 0 .2, 2 .0 and 20 mm long [2] . The Jc value did not change very much with increasing pattern length and was as high as _ i06 A/cm2 at 77 K. The fabrication of I-rI films on a 6 in . substrate had to be performed aL the distance of 430 mm, but each source rate decreased significantly at this distance, if one fabricated them at 4 x 10 -° Torr. The rate for the Ba source, far example, fell as low as 20% . The source temperature therefore had to be raised to recover the rate reduction and, as a result, green plasma was generated by the Ba source. In addition, the Ba source used up the metal charged in the crucible by depositing 100 nm thick films o.:h1 two or three 100 H N ro H N a

F .iN 0

Û

0

50 Film Thidwss :100 m Substrate: (100) MgO ,-20

60 0 20 40 80 Position ( mm )

Fig . 6. T, versus position for as-deposited Erg:O films grown at 650°C on a rotating sample holder with a diameter of 150 mm.

times. In contrast to these, the rate reduction far the Ba source was held to about 50% at I x 10 - ` Tcrr. At this pressure, the Ba source is capable of debiting 100 rim, thick films more than 10 times. Furthermore, the physical and electrical properties cf ErBCO thin films produced at i x 10-4 Torr were not infers to those of films formed at 4 x 10 - Torr. With these results in mind, we examined the uniformity of HTS films formed at I x 10-4 Torr on a 6 in . sample holder. Fig . 5 shows the III- composition profiles obtained over a rotating 6 in. holder. Data were normalized by a metal weight detected from a sample positioned on the center. The results indicate that the variation in elemental composition