arX

iv:c

ond-

mat

/061

2164

v2 [

cond

-mat

.sup

r-co

n] 2

Jan

200

7

Low-Tc

Josephson junctions with tailored barrier

M. Weides,∗ C. Schindler, and H. KohlstedtInstitute for Solid State Research (IFF) and Center of Nanoelectronic Systems for

Information Technology (CNI), Research Centre Juelich, D-52425 Juelich, Germany(Dated: February 6, 2008)

Nb/Al2O3/Ni0.6Cu0.4/Nb based superconductor-insulator-ferromagnet-superconductor (SIFS) Josephsontunnel junctions with a thickness step in the metallic ferromagneticNi0.6Cu0.4 interlayer were fabricated.The step was defined by optical lithography and controlled etching. The step height is on the scale of a fewangstroms. Experimentally determined junction parameters by current-voltage characteristics and Fraunhoferpattern indicate a uniform F-layer thickness and the same interface transparencies for etched and non-etchedF-layers. This technique could be used to tailor low-Tc Josephson junctions having controlled critical currentdensities at defined parts of the junction area, as needed fortunable resonators, magnetic-field driven electronicsor phase modulated devices.

PACS numbers: 85.25.Cp, 74.50.+r, 74.78.FkKeywords: π Josephson junctions; superconductor-ferromagnet-superconductor heterostructures, Josephson devices

I. INTRODUCTION

The work horse in superconducting electronics is theJosephson junction (JJ). A Josephson junction consists of twoweakly coupled superconducting metal bars via a constriction,e.g. made up by a normal (N) metal or a tunnel barrier (I).Various types of Josephson junctions are routinely appliedinultra-high sensitive SQUID (Superconducting Quantum Inter-ference Devices) magnetometers, radio astronomy receiversor the voltage standard [1]. EspeciallyNb/Al − Al2O3/Nblow-Tc tunnel junctions attract considerable interest in manyrespects. TheAl over layer technique [2] allows the fabri-cation of high densityNb-based Josephson circuits with com-promising small parameters spreads. Nonetheless, the Joseph-son junction itself is a research subject which enriches ourun-derstanding of superconductivity, transport phenomena acrossinterfaces and tunnel barriers. Here it is worthwhile to note,that with the advent of high-quality magnetic tunnel junc-tions approximately 10 years ago, new so far unexplored de-vices are now under development which make use of bothfabrication techniques and which consists of advanced layersequences compromising superconducting (S) and magneticmaterials (F).At superconductive/magnetic metal (S/F) interfaces the super-conducting order parameterΨ is spatially decaying and os-cillating inside the magnet (coherence lengthξF1, oscillationlengthξF2), whereas for a S/N systemΨ is simplydecayinginside the metal [3]. By combining the low-Tc Nb/Al tech-nology with magnetic tunnel junctions new functionalitiesarepredicted. In this framework so called0–π Josephson junc-tions were recent focus of research activities [4, 5, 6].The supercurrent through an SNS junction is given byI =Ic sin(φ), whereφ = Ψ1 − Ψ2 is the phase difference ofthe superconducting electrode wave functions andIc > 0the maximum supercurrent through the junction [7]. In ab-sence of current (I = 0) through the JJ the Josephson phase

∗Electronic address:m.weides@fz-juelich.de

φ = 0 corresponds to the energy minimum. These junctionsare so-called0 JJs. In an SFS stack with ferromagnetic layerthicknessdF ∝ ξF2/2, the amplitude of order parameterΨvanishes at the center of the F-layer and the order parameterhas the opposite sign at the adjacent superconducting elec-trode. This state is described by a phase shift ofπ and thesejunctions are so-calledπ JJs. SFS-typeπ JJs have a negativecritical current, hence the Josephson relation can be rewritten:I = −Ic sin(φ) = |Ic| sin(φ+π) [8, 9]. Recently, these typesof Josephson junctions have been realized using SFS [10, 11]and SIFS [12, 13] stacks. The kind of coupling can be deter-mined by thejc(dF ) dependence, see Fig.1.

For a variety of Josephson junctions a non-uniform criticalcurrent densityjc is desirable, as for example for tunable su-perconducting resonators, toy systems for magnetic flux pin-ning or magnetic-field driven electronic switches similar toSQUIDs. The first considerations [14] of non-uniformjc’swere caused by technological drawbacks leading to variationsof barrier thicknesses by fabrication [15] or of illumination incase of light-sensitive junctions [16]. Later the properties ofJJs with periodic spatially modulations were intensively stud-ied regarding the pinning of fluxons [17, 18, 19], the spectrumof electromagnetic waves [20, 21] or their magnetic field de-pendences [22]. Experimentally the spatial modulation ofjc

was realized lithographically by inserting of artificial defectssuch as insulation stripes across the barrier (ex-situ layer pro-cess,jc = 0) [23, 24], microshorts (jc increased) or microre-sistors (jc decreased). The properties of JJs depend on geo-metrical (width, length, thickness) and the physical (dielectricconstant of insulatorǫ, resistanceρ, magnetic thicknessΛ andjc) parameters. When tailoringjc all other parameters shouldbe unchanged to facilitate calculations and avoid further in-homogeneities in the system. The conventional methods forchangingjc intrinsically modify eitherǫ or ρ, too. Our fab-rication technology permits the controlled change of only theinterlayer thicknessesd1 andd2 = d1 + ∆dF , i.e. the localjc.

The case of non-uniform coupling phase within asingleJosephson junction, i.e. one half is a0 JJ (dF = d1) andthe other half is aπ JJ (dF = d2) (see dashed lines in Fig.

2

1) is of particular interest. In such a0–π junction a sponta-neously formed vortex of supercurrent circulating around the0–π phase boundary with flux|Φ| ≤ ±Φ0/2 inside the JJsmay appear [25]. The sign of flux depends on the direction ofthe circulation and its amplitude equal to|Φ0/2|, i.e. a semi-fluxon, if the junction lengthL is much larger than the Joseph-son penetration depthλJ [26, 27]. The ground state dependson the symmetry ratios of critical currents|jc(d1)|/|jc(d2)|and the effective junction lengthsℓ1/ℓ2 of 0 and π parts(ℓ = L/λJ ). The 0–π junctions have been actively stud-ied both in theory and experiment during the past few years[6, 28, 29, 30, 31, 32, 33, and references herein].0–π junc-tions were studied at so-called tricrystal grain boundaries ind-wave superconductors [28], later inYCu2Cu3O7–Nb rampzigzag junctions [30] andNb based JJ using current injectors[34]. The advantage of SFS/SIFS technology over these sys-tems is that by a proper chosen F-layer thicknessdF the phasecan be set to0 (d1) or π (d2) and the amplitude of the criticalcurrent densitiesjc(0) andjc(π) can be controlled to some de-gree. It can be prepared in a multilayer geometry (thus allow-ing topological freedom of design), can be easily combinedwith the well-developedNb/Al–Al2O3/Nb technology andhas good scalability. For0–π junctions one needs0 andπcoupling inone junction, setting high demands on the fabrica-tion process, as the change of coupling demands exact controlin F-layer thickness.0–π JJs have been realized in SFS-likesystems [4, 5]. However, both systems have the disadvantagesthat the0–π phase boundary was prepared in an uncontrolledmanner and doesnot give information aboutjc in 0 and πcoupled parts. Hence, the ratios of|jc(0)|/|jc(π)| andℓ0/ℓπ

cannot be calculated for these samples and their ground stateis unknown.

In a recent publication [6] the authors presented the firstcontrollable stepped0–π JJ of SIFS type that are fabricatedusing high qualityNb/Al2O3/Ni0.6Cu0.4/Nb heterostruc-tures. These stepped junctions came along with referencejunctions to calculate the ground state of0–π JJs. The re-quirements for SIFS0–π junctions are challenging. Here wepresent our technology background for stepped junctions withthe focus on small parameter spreads. Our approach rep-resents a considerable step forward to fulfil the extreme re-quirements on the implementation of conventional or quantumcomputing devices based on Josephson junctions.

The current and/or coupling phase profile can be modi-fied by a tailoredstepped barrier in SXS/SIXS-type (X=N, F)Josephson junctions. The patterning concept for stepped JJspresented in this article can be either used for JJ with tailoredjc’s and uniform phase0–0 (π–π) JJs (dots in Fig.1) or withsymmetricjc’s and non-uniform phase, i.e.0–π JJs (dashes inFig. 1).

II. EXPERIMENT

A Leybold Univex 450b magnetron sputter system with 8targets in the main chamber, a load-lock including an etchingstage, as well as a separate oxidation chamber was used forthe junction preparation. Together with a transfer chamber, a

0.1

1

10

100

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

stepped JJ:0-π phase andsymmetric j

c's

stepped JJ:uniform phase (0-0)

and tailored jc's

jc(d

2)

jc(d

1)

dF

dF

d2

d2 d1d

1

F1=0.78 nm

F2=1.35 nm SIFS JJ @ 4.2K

0 coupling π coupling

crit

ical

cur

rent

den

sity

j c (A

/cm

2 )

thickness of the F-layer dF (nm)

FIG. 1: (color online)jc(dF ) dependences and fitting curve forSIFS-JJs. F-layer thicknesses chosen asd1 and d2 in stepped JJsyields0–0 coupled junction with asymmetry injc (dotted lines) orsymmetric0–π junction (dashed lines). Data from Ref. [13].

TABLE I: Deposition (DC-sputtering) and etching parameters forSIFS stacks. The rates were determined by profiler measurements.

metal Ar pressure power density rate parameters

[10−3 mbar] [W/cm2] [nm/s]

Nb 7.0 5 2.0 static

Al 7.0 1.9 0.05 rotation

NiCu 4.2 0.6 ≤ 0.34 target shifted

Cu 4.2 1.9 0.1 rotation

SF6 onNb 15 0.6 ∼ 1 RF-source

SF6 onNiCu 15 0.6 <0.001 RF-source

Ar onNiCu 5 0.6 ∼ 0.01 RF-source

robot handler and a Siemens Simatic control unit, the clustertool is able to deposit automatically (pre-programmed) tunneljunction layer sequences for superconducting and spintronicsapplications.The deposition and patterning of the stepped SIFS junctionswas performed by a four level photolithographic mask proce-dure. Here we present an improved version of an earlier fabri-cation sequence for planar SIS junctions [2]. The magnetronsputtering system was capable of handling4–inch wafers andhad a background pressure of5 · 10−7 mbar was used.NbandNiCu were statically deposited, whileAl andCu weredeposited during sample rotation and at much lower deposi-tion rates to obtain very homogeneous and uniform films, seetableI. Although the actual stack sequence for the junctions isSINFS, the N-layer (Cu) was introduced to provide the growthof a uniform and homogenous F-layer thickness [35]. It isnot relevant for the electric and magnetic properties discussedhere and will be neglected.The F-layer was deposited with a gradient of thickness alongy-axis on the S/I stack [35]. The increase of thickness overthe junctions width (≤ 100 µm) is estimated as less than0.02 nm, i.e. the F-layer can be treated as planar for an indi-

3

d1

d2

L2

L1

(e)

Nb

Al-Al O2 3

Ni Cu0.6 0.4

Nb

d2

resistSF

6

Ni Cu0.6 0.4

Ar

d1

(a) (b)

Nb

Nb

(c)

DdF

(d)

FIG. 2: (color online) The complete SIFS stack was protectedinpart by photoresist. TheCu-layer was necessary for uniform currenttransport: (a) reactive etching ofNb with SF6 down toNiCu layer,(b) ion-etching ofNiCu to set0 coupling, (c) in situ deposition ofcapNb layer. Schematic layouts of stepped JJ based on SFS/SIFStechnology (d) and of stepped JJ along with planar referencejunc-tions (e). The F-layer (blue) thickness increases from leftto right.

vidual junction. After the deposition of40 nm Nb as cap layerand subsequent lift-off the complete SIFS stack with wedge-shaped F-layer thickness, but without steps in F-layer, wasobtained.

The patterning of the desired step-like variation indF wasdone after the complete deposition of the SIFS stack. Theparts of the JJ that were supposed to have a larger thicknessd2 were protected by photoresist, see Fig.2. Ion-etching aloneof bothNb andNiCu to define the step in F-layer did not pro-vide a good control over the final F-layer thickness, as thisunselective and long-timed etching has the disadvantages ofnon-stable etching rates and an non-uniform etching front.Therefore it was not possible to achieve in such way a definedstep.

The use of selective etching inNb/Al–Al2O3/Nb stackfabrication processes, such asCF4 andSF6 reactive ion etch-ing (RIE) or other techniques are reported in Ref. [36]. Inparticular, it was shown thatSF6 provides an excellent RIEchemistry for low-voltage anisotropic etching ofNb with highselectivity towards other materials. The inertSF6 dissociatedin a RF-plasma and the fluor diffused to the surface of thesubstrate, where it reacted with niobium5F+ Nb −→ NbF5.The volatileNbF5 was pumped out of the etching-chamber.WhenSF6 was used as process gas all non-metallic etchingproducts such as fluorides and sulfides from the top-layer oftheNiCu-layer had to be removed by subsequent argon etch-ing.

The patterning process of the step is depicted in Fig.2 (a)–(c). The key points were i)selective reactive etching of Nb,

ii) argon etching of NiCu to defined1 = d2 − ∆dF and iii)subsequentin situ deposition ofNb.

TheNb cap layer was removed by reactive dry etching us-ing SF6 with a high selectivity to the photoresist (AZ5214E).A few tenth of nanometer∆dF of NiCu wereAr ion etched ata very low power and rate to avoid any damaging of theNiCufilm under the surface and to keep a good control over thestep height. When the F-layer thickness was reduced downto the thicknessd1 the etching was stopped and40 nm ofNb were deposited. The complete etching and subsequentNbdeposition was done in situ at a background pressure below2 · 10−6 mbar. The chip contained stacks with the new F-layer thicknessesd1 (uniformly etched),d2 (non-etched) andwith step in the F-layer thickness fromd1 to d2.

After the preparation of steps in F-layer the actual junc-tion areas were defined by aligning the photo mask on thevisible step-terraces (ramp of≤ 20 nm height and∼ 1 µmwidth), followed byAr ion-beam etching of the upperNb,NiCu andAl layers. The lengthL1 andL2 of a stepped junc-tion are within lithographic alignment accuracy of∼ 1 µm.The etching was controlled by a secondary ion mass spec-trometry (SIMS) and stopped after the complete etching ofthe Al2O3 tunnel barrier. Afterwards the mesas were insu-lated by SNEAP (Selective Niobium Etching and Anodiza-tion Process) [2]. In the last photolithographic step the wiringlayer was defined. After a short argon etching to reduce thecontact resistance a300 nm thick Nb wiring was deposited.Fig. 2 (d) sketches a stepped SIFS junction and (e) the com-pletely structured chip with sets of stepped and planar refer-ence junctions and wedge shaped F-layer alongy-axis. Fig.1depicts theIc(dF ) dependence of planar SIFS JJs withNiCuas ferromagnetic interlayer. Note that a simple decay ofIc

can generally be achieved by increasing the interlayer thick-ness, independent of its magnetic properties, i.e. in SNS orSINS junctions. The wedge shaped interlayer (Fig.2 (e)) fa-cilitates the quick fabrication of samples with various F-layerthickness and, at the same time, a low junction to junctiondeviation.

III. RESULTS AND DISCUSSION

On test samples multiple steps with2 µm gaps were struc-tured for analysis with scanning electron (SEM) and atomicforce (AFM) microscopy after etching andNb deposition, seeFig. 3. For SEM the photoresist was left on the sample andfor AFM it was removed. While etching withSF6 theNiCu-layer served as an etching barrier. Thus, it facilitated theover-etching of theNb to ensure its complete removal despite theshadow effects from resist walls (∼ 1 µm height). The short-timed argon etchedNiCu-layer was slightly non-uniform nearthe resist walls due to the anisotropic etching front. Sinceinreal stepped JJs the half withdF = d1 has dimensions about10 µm or larger, the non-uniformity of theNiCu layer nearthe step, created by shadow-effects of the resist, was aver-aged out in transport measurements. The presence of resistcaused a decrease of theNb deposition rates by∼ 10%, espe-cially near the asymmetric resist walls (seen in SEM). Thus,

4

FIG. 3: (color online) Topography of test sample with multiple stepsand2 µm wide gaps after etching andNb deposition. (a) SEM imagebefore removal of photoresist (protectingπ coupled parts); (b) AFMimage (7×7 µm2) after removal of photoresist; (c) Profile measuredby AFM (dotted line).

the non-uniform deposition ofNb after the reactive and argonetching led to a non-uniform cap layer, seen in the differenceof stack heights (much larger than∆dF ) in the AFM image ofthe SIFS stack, Fig.3(b).

On planar SIFS-JJs (reference junctions) the actual stepheight ∆dF and the coupling is estimated by comparingjc(d1) with the knownjc(dF ) dependence, see Fig.1. d2

is determined from a reference sample with wedge-shaped F-layer. Our experimental results suggested that a continuouslyvariable-thickness model was more suitable for the junctionsthan a monolayers-thickness model (radius of neutralNi, Cuis∼ 0.15 nm) [37].By the IVC andIc(H) of the reference JJs one can estimateparameters for the stepped junction, such as the ratio of asym-metry|jc(d1)|/|jc(d2)| and the quality of the etched and non-etched parts. The uniformity of the supercurrent transportin aJosephson junction can be judged qualitatively from the mag-netic field dependence of the critical currentIc(H). The mag-netic fieldH was applied in-plane and along one junction axis.The magnetic diffraction pattern depends in a complex wayon the effective junction lengthℓ and on the current distribu-tion over the junction area [38]. The ideal pattern of a short(ℓ ≤ 1) JJ is symmetric with respect to both polarities of thecritical current and the magnetic field with completely vanish-ing Ic at the minima. If the pattern is not symmetric, irregu-lar or has a current offset, the current transport over the non-superconducting interlayers is non-uniform. If the trapping ofmagnetic flux can be excluded this effect is attributed to non-uniformity of the tunnel barrier, the ferromagnetic layer or theinterface transparencies over the junction area.

A. Reference junctions

Fig. 4 shows IVC andIc(H) dependences for a non-etchedjunction (dot) with the F-layer thicknessd2 = 5.05 nmand a uniformly etched junction (star) with thicknessd1 =

-0.4 -0.2 0.0 0.2 0.4-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

-1000 -500 0 500 1000-100

-50

0

50

100

-40 -20 0 20 40

-1.0

-0.5

0.0

0.5

1.0

T=4.2 K

JJ with d1

(shifted along I-axis)

criti

cal c

urre

nt I c

(mA

)

magnetic field (mT)

0 coupled JJs without etching

dF=d

2=5.05 nm

after etching d

F=d

1=4.75 nm

JJ with d2

JJ with d1

JJ with d2

voltage (µV)

bias

cur

rent

(mA)

bias

cur

rent

(mA)

voltage (µV)

FIG. 4: (color online)Ic(H) of etched (star) and non-etched (dots)JJs. The insets show IVCs for small and large bias current ranges inzero magnetic field. Both JJs are in the short JJ limit. Measurementswere done at4.2 K.

d2 − ∆dF = 4.75 nm. Both junctions were0 coupled. Thestep height∆dF = 0.3 nm is calculated by comparingIc(d1)with jc(dF ) from Fig. 1. The polycrystalline structure of theroom-temperature sputtered layers and the very low etchingrate ofNiCu led to a good control over∆dF . However, onehas to keep in mind that the local variation of F-layer thicknessmight exceed this value, and the values ford1, d2 and∆dF arejust the mean thicknesses seen by the transport current. Theinsets of Fig.4 show the IVCs for small and large ranges ofbias current. Besides the difference inIc, theIc(H) depen-dence and the IVCs (right inset) showed no evidence for aninhomogeneous current transport for both samples. The largerIc, but same resistanceR and capacitanceC led to a slightlyhysteretic IVC of the etched sample, as the width of hystere-sis is determined by McCumber parameterβc ∝ I2

c RC. TheresistanceR is nearly independent fromdF as the voltagedrop over the tunnel barrier is much larger than the serial re-sistance of a few nanometer thick metal [13]. However, anetching-induced change of transparency at the F/S interfacemight modify R. No change ofR is visible in the IVCs ofboth JJs in Fig.4, apart from the change inIc. A change ofcapacitanceC would require a change ofR, as both are deter-mined by the dielectric tunnel barrier.

The scattering of the critical currentIc and resistanceR onthe etched junctions was, just like for the non etched SIFSjunctions, of the order of2%. The trapping of magnetic fluxmight cause a larger variation in critical current density thanthe interface asymmetry stemming from the etching process.For all thicknesses of ferromagnetic layer the same homo-geneity of the etched junctions were observed.

A set of junctions denoted by the dashes in Fig.1 was mea-sured by the authors, too. TheIc(H) dependences of this0,π and0–π JJs are depicted in Fig. 3 of Ref. [6]. These junc-tions show the same quality of parameters for the etched andnon-etched samples as the samples depicted in Fig.4.

5

0 1 2 3 4 5 6 7 8 9 100.0

0.2

0.4

0.6

0.8

1.0

symmetric stepped JJj2/j

1

1.00.8

0.60.4

0.20.0

shifted by 10% along I-axiscr

itica

l cur

rent

I c/I m

ax

magnetic field h

FIG. 5: (color online) CalculatedIc(h) dependence for various ratiosof j2/j1 and centered step injc profile.

B. Stepped junctions

1. Calculated Ic(h) of stepped JJ

The magnetic diffraction patternIc(H) of a JJ depends onits jc profile, see Ref. [38, 39]. The analytic solution for ashort (ℓ < 1) stepped junction with different critical currentdensityj1 andj2 in both halves is given by

Ic(h)

A=

j1 cos (φ0 − h) − j2 cos (φ0 + h) + (j2 − j1) cosφ0

2h,

whereφ0 is an arbitrary initial phase,h = 2πΛµ0LH/Φ0 thenormalized magnetic flux through the junction cross section,Λ the magnetic thickness of junction andA the junction area.The phase-field relation for maximumIc is reached for

φ0 = arctan [j2h sinh − j1h sinh

2hj2 sin2(

h

2

)

+ 2hj1 sin2(

h

2

) ].

The calculatedIc(h) for various ratios ofj2/j1 is depicted inFig.5. Characteristic features are the centered maximum peakand the appearance of periodic minima of the supercurrent forh = n for integern. The height of the odd-order minima (n =1, 3, 5...) depends on the asymmetry ratiojc(d2)/jc(d1) andincreases for decreasingjc(d2). Ic(h) is completely vanishingfor magnetic flux equal to multiples of2Φ0. The maximumcritical current atIc(0) decreases linearly down toIc/Imax =0.5 for jc(d2) = 0. The correspondingIc(h) pattern becomesthat of a junction with half the width and uniformjc.

2. Measured Ic(H) of stepped JJ

In Fig. 6 the measured magnetic diffraction patternIc(H)of a stepped JJ along with calculatedIc(h) curves are de-picted. Both junctions halves are0 coupled. The magneticfield axis h was scaled to fit the first measured minima ofIc(H). dF is determined by comparingjc of reference junc-tion with the knownjc(dF ) dependence in Fig.1, yielding

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

-100

-50

0

50

100-1000 -500 0 500 1000-40 -20 0 20 40

-1.0

-0.5

0.0

0.5

1.0

calculated d

1=4.68 nm, d

2=4.98 nm

d1=4.68 nm, d

2=4.92 nm

d1=4.72 nm, d

2=4.92 nm

T=4.2 K

criti

cal c

urre

nt I c

(mA

)

magnetic field (mT)

stepped JJ with different jc(d

F)

100x100 µm2

centered step in F-layer measured

voltage (µV)

bias

cur

rent

(mA)

bias

cur

rent

(mA)

voltage (µV)

FIG. 6: (color online)Ic(H) of a stepped0–0 JJs (square shapedwith 100 µm junction length) withd1 = 4.68 nm and d2 =4.98 nm (determined from reference JJs) plus calculatedIc(h). Thejunction is in short JJ limit.

d1 = 4.68 nm and d2 = 4.98 nm. Due to the rathersteep slope of thejc(dF ) curve near the0 to π crossoverat dF = 5.21 nm, jc is very sensitive tod1 and d2. Avariation ofd1 and d2 by 0.05 nm changesjc up to 30%.The sputter rate may vary slightly over the∼ 500 µm dis-tance to the reference junctions, preventing the exact esti-mation of d1 and d2. We calculatedIc(h) (dashes) usingd1 = d2 − ∆dF = 4.68 nm andd2 = 4.98 nm determinedfrom reference junctions. Thend2 was decreased by0.04 nm(dots) and finally∆dF increased by the same thickness, too(dashes-dots). The final calculation has the best agreementwith data, although the total interface roughness (rms) of themultilayers should exceed0.04 nm by far. The measurementand simulation in Fig.6 show the good estimation and controlof F-layer thicknesses.

IV. CONCLUSIONS

Josephson junctions with a step in the ferromagnetic layerwere fabricated. Using a wedge-shaped F-layer in a SIFSstack on a4–inch wafer along with stepped and referencejunctions it was possible to trace out regimes of differentcouplings (0, π), depending on the initial F-layer thicknessd2 and step∆dF . The etched and non etched SIFS junctionsdiffer only by F-layer thickness. No inhomogeneities canbe seen in the current transport characteristics of the etchedjunctions.The patterning of stepped JJs allows free lateral placementof well-definedjc’s and/or local coupling regimes within asingle junction. If decreasing temperature the slope∂jc/∂Tdepends on the interlayer thickness (observed in SIFS JJs[13]), and the ratioj1/j2 could be varied thermally. Stepped

6

junctions can be realized inNb based JJs with any interlayermaterial (N,F,I) which is chemically stable towards thereactive etching gas. The patterning process could be adjustedto all thin film multilayer structure providing that the reactiveetching rates of the layer materials differ. Replacing the opti-cal lithography with electron beam lithography may enhancethe lateral accuracy of the step down to the dimension ofe-beam and decrease the non-uniformity near the resist walldue to the thinner resist height.JJs with varyingjc and planar phase could be used for deviceswith special shapedIc(H) pattern [38], toy systems forflux pinning or tunable superconducting resonators.0–π JJsbased on low-Tc superconductors with stepped F-layer offer agreat flexibility for the integration of this devices, as it offersadvantages over the existing0–π junctions based on d-wavesuperconductors [30, 40] or current injectors [34] such asthe low dissipation of plasma oscillations, no restrictions intopology, no additional bias electrodes and easy integrationinto the matureNb/Al − Al2O3/Nb technology.

The 0–π SIFS JJs with stepped F-layer allow to study thephysics of fractional vortices with a good control of the ratioof symmetry between0 andπ parts. The change in magneticdiffraction pattern between short to the long0–π JJ limit [29]or the formation of spontaneous flux in the ground state ofmultiple 0–π phase boundaries in a long JJ could be studied[32]. 0–π JJs may be used as the active part in the qubit, aswas recently proposed [33].

Acknowledgement

The authors thank B. Hermanns for help with fabricationand E. Goldobin, R. Kleiner, D. Koelle and A. Ustinov forfruitful discussions. This work was supported by ESF pro-gram PiShift and Heraeus Foundation.

[1] W. Buckel and R. Kleiner,Superconductivity. Fundamentalsand Applications., Wiley-VCH (2004).

[2] M. Gurvitch, M. A. Washington, H. A. Huggins and J. M. Row-ell, IEEE Trans. Magn.19, 791 (1983).

[3] A. I. Buzdin, Rev. Mod. Phys.77, 935 (2005).[4] M. L. Della Rocca, M. Aprili, T. Kontos, A. Gomez and

P. Spatkis, Phys. Rev. Lett.94, 197003 (2005).[5] S. M. Frolov, D. J. Van Harlingen, V. V. Bolginov, V. A.

Oboznov and V. V. Ryazanov, Phys. Rev. B74, 020503 (2006).[6] M. Weides, M. Kemmler, H. Kohlstedt, R. Waser, D. Koelle,

R. Kleiner and E. Goldobin, Phys. Rev. Lett.97, 247001 (2006).[7] B. D. Josephson, Phys. Lett.1, 251 (1962).[8] L. Bulaevskii, V. Kuzii and A. Sobyanin, JETP Lett.25, 7

(1977).[9] A. I. Buzdin and M. Yu. Kupriyanov, JETP Lett.53, 321 (1991).

[10] A. V. Veretennikov, V. V. Ryazanov, V. A. Oboznov, A Yu. Ru-sanov, V. A. Larkin and J. Aarts, Physica B284, 495 (2000).

[11] Y. Blum, A. Tsukernik, M. Karpovski and A. Palevski, Phys.Rev. Lett.89, 187004 (2002).

[12] T. Kontos, M. Aprili, J. Lesueur and X. Grison, Phys. Rev. Lett.89, 137007 (2002).

[13] M. Weides, M. Kemmler, E. Goldobin, D. Koelle, R. Kleiner,H. Kohlstedt and A. Buzdin, Appl. Phys. Lett.89, 122511(2006).

[14] M. Russo and R. Vaglio, Phys. Rev. B17, 2171 (1978).[15] K. Schwidal and R. D. Finnegan, J. Appl. Phys.40, 213 (1969).[16] A. Barone, G. Paterno, M. Russo, and R. Vaglio, Phys. Stat.

Solidi (a)41, 393 (1977).[17] D. W. McLaughlin and A. C. Scott, Phys. Rev. B18, 1652

(1978).[18] A. N. Vystavkin, Yu. F. Drachevskii, V. P. Koshelets andI. L.

Serpuchenko, Sov. J. Low Temp. Phys.14, 357 (1988).[19] B. A. Malomed and A. V. Ustinov, J. Appl. Phys.67, 3791

(1990).[20] M. V. Fistul, P. Caputo and A. V. Ustinov, Phys. Rev. B60,

13152 (1999).[21] N. Lazarides, Supercond. Sci. Technol.18, 73 (2005).[22] N. Lazarides, Phys. Rev. B68, 92506 (2003).

[23] A. Golubov, A. Ustinov and I. L. Serpuchenko, Phys. Lett. A130, 107 (1988).

[24] M. A. Itzler and M. Tinkham, Phys. Rev. B51, 435 (1995).[25] L. N. Bulaevskii, V. V. Kuzii and A. A. Sobyanin, Solid State

Commun.25, 1053 (1978).[26] J. H. Xu, J. H. Miller and C. S. Ting, Phys. Rev. B51, 11958

(1995).[27] E. Goldobin, D. Koelle and R. Kleiner, Phys. Rev. B66, 100508

(2002).[28] C. C. Tsuei and J. R. Kirtley, Rev. Mod. Phys.72, 969 (2000).[29] J. R. Kirtley, K. A. Moler and D. J. Scalapino, Phys. Rev.B 56,

886 (1997).[30] H. Hilgenkamp, Ariando, H. J. H. Smilde, D. H. A. Blank,

G. Rijnders, H. Rogalla, J. R. Kirtley and C. C. Tsuei, Nature(London)422, 50 (2003).

[31] E. Goldobin, D. Koelle and R. Kleiner, Phys. Rev. B67, 224515(2003).

[32] A. Zenchuk and E. Goldobin, Phys. Rev. B69, 024515 (2004).[33] E. Goldobin, K. Vogel, O. Crasser, R. Walser, W. P. Schleich,

D. Koelle and R. Kleiner, Phys. Rev. B72, 054527 (2005).[34] E. Goldobin, A. Sterck, T. Gaber, D. Koelle and R. Kleiner,

Phys. Rev. Lett.92, 057005 (2004).[35] M. Weides, K. Tillmann and H. Kohlstedt, Physica C437-438,

349 (2006).[36] A. W. Lichtenberger, D. M. Lea and F. L. Lloyd, IEEE Trans.

Appl. Supercond.3, 2191 (1993).[37] M. Weides, Ph. D. thesis, Universitat zu Koln, Germany (2006).[38] A. Barone and G. Paterno,Physics and Applications of the

Josephson Effect, John Wiley & Sons (1982).[39] TheIc(h) characteristic for a0-π JJ can be obtained in compact

form from a general expression valid for N0–π phase bound-aries, forN = 1 and equal lengths of the0- andπ-parts, foundin N. Lazarides, Supercond. Sci. Technol.17, 585 (2004).

[40] J. R. Kirtley, C. C. Tsuei, M. Rupp, J. Z. Sun, L. S. Yu-Jahnes,A. Gupta, M. B. Ketchen, K. A. Moler and M. Bhushan, Phys.Rev. Lett.76, 1336 (1996).