New Fabrication Methods for Josephson Junctions with Large IcRN Products Luis Gmez2006/12/08Dept. of Basic SciencesThe University of Tokyo

CollaboratorsShinichi Kitamura (M1)Takahiro Kubo (M2)Haruhisa Kitano (Research Associate)Atsutaka Maeda (Associate Professor)

Work supported by:

JST CREST

Creation of Ultrafast, Ultralow Power, Super-performance Nanodevices and Systems. (Prof. Hiroyuki Sakaki)

Single-Flux-Quantum Terahertz ElectronicsProf. Akira Fujimaki, Prof. Masayoshi Tonouchi, and Prof. Atsutaka Maeda

OutlineMotivationOverview: Josephson junctions (JJ) and SFQ Survey of HTS JJsImportant design parameters for SFQ logicFerromagnet/Superconducting microbridgesFabricationResultsPossible explanations for the JJ formation (Theory)Summary

Motivation45nmSub-Terahertz ElectronicsLSI Bottleneck (Power Density, Long Interconnects)Pin Bottleneck (in Parallel Architecture)

Overview: Josephson junctions (JJ) and SFQJosephson JunctionsSFQAfter K.K. Likharev, Rev. Mod. Phys. 51, 101 (1979)Tunnel junction& Weak linksAfter Likharev et al, IEEE Trans. on Appl. Supercond. 1, 3 (1991)RCSJ model

Survey of HTS JJsGrain boundary junctionsStep-edgeS-I-S junctionsSandwichS-N-S junctionsCo-planar bridge, Sandwich, Edge or Ramp, Step-edge microbridgeE-beam damage junctionsCo-planar (microbridge) After K.A. Delin and A. W. Kleinsasser, Supercond. Sci. Technol. 9, 227 (1996)After S. Tolpygo and M. Gurvitch, Appl. Phys. Lett. 69, 3914 (1996)

Important design parameters for SFQ logicSmall spread of individual JJ Ic, RN valuesReproducibleLarge IcRN productsMinimal parasitic inductance and capacitance For Phase Mode SFQ logicUsed overdamped JJs (C

Ferromagnet/Superconducting microbridgesW=2mts=65nml=400nmtf=60nmJST patent (2006)A. Maeda, L. Gomez

Ferromagnet/Superconducting microbridgesSample fabrication (Ferromagnet evaporation)

Ferromagnet/Superconducting microbridgesTop view

Ferromagnet/Superconducting microbridgesSample fabrication (Etch)

Ferromagnet/Superconducting microbridgesTop view

Ferromagnet/Superconducting microbridge JJsFinished sample

Ferromagnet/Superconducting microbridge JJsSample dimensions

Junctions electrical characterization: Experimental setupNew Cryo-ProbeDC-electrical characterizations as a function of :Temperature (4K-100K)Magnetic field (350G)Microwaves (20GHz)

R vs. T (Josephson junction signature).E-beam voltage = 50KVW=2mtS=65nml=400nmtF=60nm

Comparison with E-beam damaged JJsS. Tolpygo et al, Appl. Phys. Lett. 63, 1696 (1993)E-beam voltage = 120KV

Photolithography made samples

I-Vs (IcRN product, a THz Oscillator?)

RTs and IVsIcRN ~ 4 mV

Ic vs. T (Wide vs. narrow junctions)For T=15KS. Tolpygo et al, Appl. Phys. Lett. 69, 3914 (1996)

Ic vs. B (Unconventional junctions)

Ic vs. B (as a function of temperature)

I-Vs + microwaves (Shapiro steps)

I-Vs + microwaves (Shapiro & Photon induced steps)

Possible explanations for the junction formation (theory)Ferromagnetic proximity effect in F/S multilayer

V. Pea et al, PRB. 69, 224502 (2004)Buzdin, RMP. 77, 77935 (2005)LTSHTS

Possible explanations for the junction formation (theory)Inverse proximity effect in S near F material

M. A. Sillanpaa et al, Europhys. Lett. 56, 590 (2001)LTS

SummaryWe are fabricating Fe / LSCO microbridge junctionspromising Josephson characteristics (S-S-S) and large IcRN products.The fabrication method is: It is simple and fast. Compatible with modern lithographic technology. Suitable for large integration of Josephson junctions.Junctions may address all the SFQ requirements Reproducibility of junctions is still an open questionMechanism for junctions is still an open question It should work for both HTS and LTS

PLD growth of LSCO filmsSchematic Figure of PLDKrF Excimer Laser=248nmLSCOSubstrate Holder250mJ,1HzDeposition condition:79010-1torrSubstrate: LSAO

PLD growth parameters

Sheet1

my casedoc

Substrate Temp()680-790650-750

Constituent ElementO2O2 with 10%O3

Chamber Pressure(torr)3.2310^-1310^-5

Growth Rate (/s)1.480.5-3

Thickness of Film ()4000150-4500

Post Anneal613, 60minnone

Cool Down (/min)-19.2-20

After Evapo Press (torr)310-4

Sheet2

Sheet3

Film thickness

LSCO StructureH.Sato et al. : Phys. Rev. B61 12447.Y.Maeno et al. : Physica C173 322.J. B. Torrance et al. : Phys. Rev. Lett.68 3777.

LSCO characterization- TX-rayc-axis length:13.16 (x=0.15)12.83 (x=0.20)

LSCO thin film properties M. Suzuki and M. Hikita, PRB 44, 249 (1991)

THz AntennasBroadband antenna (detector)Resonant narrowband twin dipole antenna (transmitters)Develop both antennae in chip

THz AntennasTwo twin dipole antenna (transmitter and receiver)Develop both antennas in chipAntenna MaskTwo Antennas

SQUIDE-Beam MaskFinished SQUID

R-Ts forLSCO15%060811_7.5minC_45sec

At he current growths of Internet traffic, by year 2010 there will be a technology gap which would not be fulfill by any semiconductor electronics, due to power density and interconnect delay problems. A new technology is needed. This technology could be based on superconductors, which are faster and consume less power.LSI= Large Scale IntegrationTDM=time-division multiplexedWDM=wavelength-division multiplexedSR=Shift registerDFF=Delayed Flip FlopA JJ is when there is a small overlap of quantum-mechanical wavefunctions between two weakly couple superconductorsRCSJ model, Current-phase relationship, voltage-phase relationship, Quantum flux, fj = Josephson frequencythe ac Josephson Effect SIS underdamped exhibits hysteretic dc IVsSNS overdamped no hysteretic IVsLatching or Volta state Logic uses a Tunnel junctiondynamic SFQ circuits, first discussed by Nakajima et al, (1976)information between logic devices is passed ballistically, along either passive microstrip lines or active Josephson transmission lines, in the form of very short (picosecond) "quantized" voltage pulses V(t) with the fixed area Int V(t)dt = 0 = 2.07 mV-ps. "SFQ" pulses can be quite naturally generated, reproduced, amplified, memorized, and processed by elementary circuits comprised of overdamped Josephson junctions. An alternate family of dynamic SFQ devices was suggested, under the name "Phase Mode Josephson System", by Professor K. Nakajima and his collaborators at Tohoku University in Sendai, Japan This system used a single basic cell, the "ICF gate", and seemed much less flexible than the RSFQ family.According to the fundamental Josephson phase-to-voltage relation d/dt= (2e/h)V(t), such a "2-jump" of corresponds to generation of the SFQ voltage pulse across the junction. For typical present-day fabrication technologies, duration of the pulse is a few picoseconds, while the pulse amplitude is a few hundred microvolts. Josephson-junction RSFQ circuits can perform logic and arithmetic functions at extremely high (sub-terahertz) clock frequencies, just a few times lower than the maximum internal speed to = h/D(T) of the superconductors employed. These circuits seem to represent the fastest digital technology currently available. Grain boundary, Step-edge, SNS, SNS, Edge junctions, E-beam madecommentscommentscommentscommentscommentscommentscomments