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2522 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY,VOL. 7, NO. 2, JUNEunction on a Narrow e~ ~ ~ - ~ h e ~ i ~ a ~a n

Shin'ichi M orohashi, Tadashi Utagawa and Youichi En oSuperconduc t iv i ty Research L a b o r a t o r y

In te rna t iona l Superconduc t iv i ty Tcchno logy Ce n10-13 S h i n o n o m e I-chome Koto-ku , Tokyo, 135 J

~ ~ s ~ r a c t - ~ eave Fabricated tw o typ es of Hig h-s on ju n ct i on s u s i n g a f o c u s e d i o n b e a m

H i gh -T c ju n ct i on s fab r i ca ted u s i n g a~ a ~ o w - f Q c u s e ~a- ion beam to damage asubstrate , resu l t ing in a geometrical defect prior

e p o s i t i o n , s h o w f l u x f l o w - l i k ec ~ a r a ~ t e ~ s ~ c so r th e S rT i 0 3 s u b s tra te . T h e

n t - v o ~ t a g e ch aracter i s t i c s o f th e ju n ct i on sh are f a b ~ c a t e d n a n a m w t un gs te n (W )

metal layer d e ~ o s i t e don b o th M gO an d S r T i 0 3substrates us in g a Focused ion beam chem icalep os i l i on t ech n i q u e are qual i tat ivelyc Q n s i s t e ~ t i th a r e s i s ti v e ly s h un t ed j u nc t io nmodel and s h ow S h ap i ro s tep s u n d er mi crowaveirradiat ion .

1. INTRODUCTIONThe development of high-temperature supercondctor

(FITS) digital circuits reqires reproducihle Josephsonjunctions and the integration of these junctions . Several typesof HTS Josephson junctions, such as multilayer junctions[l]-[3], step-edge junctions[4]-[6], junctions on bicrystallinesubstrates[7], 181 and proximity-effect junctions with variousnormal conductors[9], [lo] have been fabricated Among them ,the fabrication of HTS junctions using a narrow-focused Ga-ion beam to damage the MgO substrate, resulting in ageometrical defect prior to HTS layer deposition (FIB-damaged junction), has the advantage of the freedom of choicein the position of each junction with the high accuracy of theion-beam positioning[ 1 I].An integrated circuit ne& the microstrip line structureconsisting of junctions, an insulation layer and asuperconducting ground plane. We plan to use a SrTiO,insulator because it promotes the epitaxial growth of FITSlayers, though it s large dielectric constant is disadvantageousfor high speed operation of HTS digital circuits. However,

j

Manuscript received at Augest I, 1996This work was supported by the New Energy and IndustrialTechnology Developme nt Organization for the RRrD of Industrial Scien ceand Technology Frontier Program .

FTB-chmaged ju ncti

(FIB-CVD junction)

substrate [12]. W e

integration process, wecan be fabricated on SrTi

T I . F DJUFig. 1 shows the I

h a g e d junctions fabricated on SrTiO, sfabrication process for this junction has been publishe&ail elsewhere[ll]. The junction with the I-V characteshown in Fig. l(a) was fabricatedun& the condition thaextraction voltage and the beam current for the Ga-ion bwere 30 keV and 7 PA, and the Ga-ion beam etching tim e200 seconds. Th e junction with the I-V characteristic shin Fig. 1(b)was fabricated under the extractioi vkeV, the Ga-ion beam current of 30 pA and the Ga-ioetching time of 1000 seconds. These conditionsclosely to an etching time of 4000 seconds uncurrent of 7 PA. The c-axis oriented NdBaCuOdeposited using off-axis sputtering. The depositionout using an Ar (8 sccmtO, (2 sccm) gas mixturepressure of IO Pa and substrate temperature of 840 "C . AIFIBdamaged junctions fabricated on SrTi03 substshowed flux flow-like characteristics. In these junctions,deposited NWaCuO layer thickness was 60 nm , COthe thin HTS layer makes it easy to form a weak 1grain boundary. On the other hand, the junction fabricatethe MgO substrate for an etching time of 320 secondsbeam current of 7 pA and with a thicker N a a C u O lay120 nm shows a RSJ-like characteristic as shown in Fig.

1051-8223/97$10.00 0 99 7 IEEE

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2523

Fig. 1. I- V characteristics at 4.2 K of the FIB-damaged junctionsfabricated on SrTi 03 suhstrates. The NdBaCuO layer is 60nm thick. TheGa-ion etching time and the ion-beam current are (a)200 seconds and 7PA, (b)1000 seconds and 30 pA. Vertical scales are 1 mA/div, andhorizontal scales are 100pV/div.

Fig. 2.EV characteristic at 4.2 K of the FIB-damaged junctions fabricatedon the MgO ubstrate.The NdBaCuO layer is 120nm hick. The Ga-ionetching time and the ion-beam current are 300 seconds and 7 PA. Thevertical sc ale is 500 pA/div, and the horizontal scale is 200 pV/div.nI, FIR-CVD JUNCTION

The fabrication process of the junction on a narrow W lineformed by FIB-CVD &position is shown in Fig. 3. First, theAu protection layer (30-nm thickness) on a SrTiO, (or aMgO) substr ate was etched in an a m f 30 pm x 0.2 pm byscanning a narrow-focused Ga-ion beam with an energy of 30keV (Fig. 3(a)). Subseqentially, keeping the narrow-focusedGa-ion beam scanning, W(CO), gas was sprayed through a

gas gun. As shown in Fig. 3(b), a nmow W metal line isformed only on the area of the SrTiO3 substrate on which thenarrow-focused Ga-ion beam is scanned After etching theexcess Au layer by the Ar ion-milling technique at anextraction voltag e of 250 V (Fig, 3(c)), a NdBaCuO layer isdeposited on the SrTiO, substla te. Finally , Au electrodes aredeposited through a metal m ask, followin g the patterning andetching of the N dBaCuO layer (Fig. 3(d)).Fig. 4 shows a section analysis of the narrow W metallayer deposited on an MgO substrate using the Atomic ForceMicroscope. The cross-section of the W metal layer wasfound to be almost triangular, resulting from the gaussiancontribution of the Ga-ion beam. The shape of the cross-section is favorable for junction formation, because we thinkthat the weak link of the junction is formed by a grainboundary of the HTS ayer deposited on the narrow W metalline. An optical micrograph of an FIB-CVD junction isshown in Fig. 5. The thin, 30 pm long l ine is the narrow Wline formed by FIB-CVD deposition, and the 5 pm wid:superconducting line over the narrow W line can also be seen.Ga-ion beam scanning+ +a)

Au layerSubstrate

a-ionbeam scanningekl layer

Fig. 3. Fabricationprocess of the W-C Wju nc t io n .

Eg. 4. Section analysis of the narrow W metal layer deposited on the M gOsubstrate using the Atomic Force microscope.

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2524

ine thicknesse

/---30 pm

crograph of a FIB-CVDunction.Fig. 6 shows the I-V characteristics at 4.2 K of the F BCVD junctions, fabricated on MgO substrates. The narrow W

lines are 30 nm (Fig. 6(a)) and 50 nm (Fig, 6@)) hick; thethicknesses are controlled by the &position time for aconstant extraction voltage of 30 keV and ion beam current of7 PA. The NdBaCuO layer on the narrow W line was 120 nmthick. The junction with rhe W line thickness of 30 nmshows a flux flow-like charactaistic. The junction with theW l ine thickness of 50 nm shows more like a RS Jcharacteristic, though an excess current is still large.

onVOP Akick and (b)SO nm hick. Vertical scales are (a ll &div and (b)500pAfdiv. Horizonfa1 scales are 100pVldiv.

Fig. 7. I-V characteristics a t 4.2 K of lhc FIB-CVD junctions faSrTiO, subsbafes. The NdBaCuO layevoltage and the ion-heam current for the Ga-ion bea mpA in the FIB-CVD deposition. The narrow W metal lachick, (b)30nm thick, and (c)60 nm thick. Vertical scal(h)200 pA/div, and (c) 50 pA/div. Horizontal scales are 100 pV/div.

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2525Fig. 8 shows the I-V characteristics at 4.2 K of the FIB-CVD unctions, which were fabricated on SrTiO, substrates

with a 60 nm thick NcU3aCuO layer. The FIB-CVDdeposition conditions are the same as for the results indicatedin Fig. 7. Both junctions with the nmow W line thicknessesof 30 and 50 nm show RSJ-like characteristics, resultingfrom the thin TITS layer fac ilitat ing the form ation of we&link junctions. F ig. 713)shows the I-V characteristic with andwithout microwave inLadiation of 7.784 GITz. The Shapirosteps appear at th e theoretically p d c t e d voltages, but arerounded due to thermal noise.

IV.CONCLUSIONOur newly devised FIB-CVD unctions can be fabricated

with RSJ characteristics even on SrTiO, substrates, on whichthe FIB-damaged junctions only show flux flowcharacteristics. The weak link of th e FTR-chmaged junc tion isthought to be formed by a , d n bounchry similar to thejunctions using a step in th e substmte[l I]. The depth of t h egeometrical ckfect in the substrate formed using the Ga-ionbeam is thought to be controllable in principle by varying theetching time, the extraction voltage and the beam cunent forthe Ga-ion beam. So, we think i t is possible to adjust someof the properties of each junction by changing the defect depth.However, only th e junction s fabricated on the MgO substrateshowed RS J-lik e characteristics. On SrTiO, substrates, i t maybe difficult to make a deep geometrical defect, or to make aweak link based on that defect.On th e other hand, th e narrow W line using the FTB-CVDdeposition can be formed without depending on thecharacteristics of the substrates, and the weak link of thejunctions can be formed by a grain boundary of the HTS layerdepositedon the narrow W metal line. It was confirmed thatthe FTB-CVD junction is more amenable for the integrationof HTS junctions compared with the FTBQmaged junction,though both types of HTS junction fabrication methockmaking use of the focused ion beam technique have th eadvantage of having the freedom of choice in the position ofeach junction with the high accumcy of the ion-beampositioning. As the next step, it is necessary to study thereproducibility of th e FIB-CVDunction.

REFERENCES[I 1 C.T. Rogers, A. h a m , M S. Ilegde, B. b t t a . X D. Wu and T. Venkatesdn. Rhricationof heteroepitaxial YBa?Cu307-x-RBa2Cu307-x~YBa~Cu107-xoseph.wn devices grown bylaser deposition, Appl. Phys. Left. vol. 55. pp, 2032-2034. 1989121 T. Flnshimoto. M Sagoi. Y. Mimtani, 1. Ymhida and K. Mindtima. Jncrphmncharacteristics in a-axis a i e n t e d Y B ~ ~ C U ~ ~ ~ - S / P ~ B ~ Z C U ~ ~ ~ - ~ I Y B ~ ~ C U ~ ~ - Sunct ionsAppl. Phys. Leu. vol. 60, p. 1756-1737. I992131 R. I.. f ink, M Thompmn. C. IIilhrvl and 11 Krnger. rlysieretic Jmephcon junc tion s fromYBa?Cu307-xISrTi03/BaI-xKxBi03nlayer films. Appl. Phys. O f f . ol. 61. pp. 595-597.

Fig. 8. J- V characteristics a i 4.2 K of the FIB-CVDunctions fabricatedon SrTiO, substrates. The NdBaCuO la yer is 60 nm thick. The extractionvoltage and the ion-beam current for the Ga-ion beam are 30 kcV and 7pA in the FIB-CVD eposition. The narrow W metal layers are (a130 nmthick and (b)SO nm thick. Fig. 8(b) shows the characteristics with andwithout microwave irradiation of 7.784 G Hz. Vcrfical scalcs are (a)SOOpA/div and (b)50 pA/div. Horizontal scalcs are (a)20 pV/div and (b)50pV/div.1992

I41 J. S. Martens. V. M IIietala, T. E. Zippcrian, G. A Vawter, D. S.Ginley, C. P. T i gge rsand G. K 0. Hchenwarter, Fahication o fTlCa BaC u0 stepedge Josephson junctions withhysteretic behavior, Appl. Phys. Lett. vol. 60, p. 1013-1015. 1992[SIJ. A Edwards J. S. Satehell, N. G. Chew, R. G. Humphrey, M X Keene and 0.. Ibsser.YBa2 Cu3M thin-film step junctions on MgO substrates, Appl. Phys. Lett. vol. 60, p.2433-2435. 1992[a] 1. Luine. I. Bulman. I. Burch. K Daly. A Lee, C. Pettiette-Hall, S. Schwarzbeck and D.Miller. Characterisiics of high performance YBa2Cu307 stepe dge junctions, Appl. Ph.vs./aft . ol. 61. pp. 1128-1130, 1992

171 D. E+ P. Chaudhari. J. Mannhari and E K LeGoues, (Xientation dependence of grain-boundary critical currents in YBa2Cu307-d bicrystals, Phys. Rev. Lef t . vol. 61, pp. 219-[XI R. Cnw. P. Olaudhari, U Kawasaki, U B. Ketchen and A. Gupta. Characterisics ofYDa2Cu307-S Grain Boundary Junction Dc-SQUIDS, IEEE Tmru.Mng~: . ol. 27. pp, 2565-

2568. 1991[91R. H. Ono. J. A. Beall. M W. Cromar, T. E. Harvey, U E. Johansron, C..D. Reintsema

an d D. A. Rudman. TEgh-Tc superconductor-normal metal-suprconducta Josephsonmicroh ridges with high-resistance no rmal metal links,Appl. Phys. Lrtt. vol. 59, pp. 1126-1128. 1991[IO] U. Kahasdwa. Y. Tarutani. A T.wkamoto, M Hiratani and K Takagi, Electricalcharacterisiics of HoBa2Cu3lYl-x-PcBa2Cu307-y-HoBa2Cu307-xunctions with planar-

type structures. Physicn C. vol. 194, pp. 261-267, 1992Fahicat ion of high IC X Rn YBCO-Josephson-junctions on MgO substrates using afocused-ion-beam system,. Physiea C. ol. 210, pp. 138-146, 1993

1121 S. Morohashi. T. lhagawa and Y. Enomoto. A High-Tc Joseph son Junction on a Narrow WLine Formed hy NB-CVD Deposition, in press. 1996INTERNATIONALWORKSHOPONS1 rPFRCONDPCTIVITY(June 24-27, Hachimantai Royal Hotel Iwate. Japan )

221. 19x8

1111 Ch.Neumann. K. Yamaguchi. E. ayashi, K. uzuki, Y.Enomoto and S. Tanaka,