Shulze - Surface and Thin Film Characterization of Superconducting Multilayer films for Application in RF Accelerator Cavities

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Operated by the Los Alamos National Security, LLC for the DOE/NNSA SRF Workshop Padua October 2010LA-UR 10-06339

10/1/10

Surface and Thin Film Characterization ofSuperconducting Multilayer films for Application in RFAccelerator Cavities

A.T. Zocco, T. Tajima, M. Hawley, Y.Y. Zhang, N.F. Haberkorn, L. Civale, and R.K. Schulze, LosAlamos National Laboratory, Los Alamos, NM 87545 USA

T. Prolier, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439 USA

B. Moeckly, Superconducting Technologies, Inc., 460 Ward Drive, Santa Barbara, CA 93111 USA

The Fourth International Workshop on: Thin films and New Ideas for Pushing the Limits of RFSuperconductivity, Padua, IT October 4-6, 2010

This work has been supported by the Defense Threat Reduction Agencyand DOE Office of Science Nuclear Physics

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• The RF critical magnetic field HRF in atype-II superconductor is somewherebetween Hc1 and Hc2

• Use thin films with thickness d < λL toenhance the lower critical field

[Gurevich, APL 88 (2006) 012511]

The key idea of using a thin film superconductor is the fact that Bc1increases when the thickness is d< λL (penetration depth)

See Tajima talk for further details

MgB2Coherence length 5 nmPenetration depth 140 nm

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An example: Coating 105 nm MgB2 layer could sustain 355 mT,corresponding to ~100 MV/m with Bpeak /Eacc ~ 3.6 mT/(MV/m)

Simple single layer example• AssumptionsHc1(Nb) = 0.17 Tλ(MgB2) = 140 nmξ(MgB2) = 5 nm• Hc1(MgB2) = 355 mT• d = 105 nm• The film thickness needs to be determined so that the

decayed field at the Nb surface is below the RF criticalfield of Nb (~200 mT).

H0 = 355mTHi = 170mT

d = 105 nm

NbMgB2

Eacc ~ 100 MV/m

Dielectricmaterial

See Tajima talk for furtherdetails

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Materials and Deposition Methods:Polymer assisted deposition (PAD) for NbN - LANLSequential reactive coevaporation for MgB2 - STICoevaporation with 2 e-beam sources for MgB2 - Kagoshima UniversityAtomic layer deposition for dielectrics Al2O3, MgO, Y2O3 - ANLFuture CVD and PECVD for NbN and MgB2 - LANL

Characterization Tools:XRDSEMSPM - STM, AFMXPSAuger spectroscopy and sputter ion depth profilingPPMS - TcMagnetometry - Hc1RF power measurements - SLAC

Materials and thin film characterization carried out in concert with depositionmethods is critical for fine tuning synthesis methods and desired superconducting andRF performance properties:Chemistry and phase at surfaces and interfacesInterface mixingFilm thickness

See Tajima talk for further details

This talk

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B.H. Moeckly and W.S. Ruby, Supercond. Sci.Technol. 19 (2006) L21–L24

Reactive co-evaporation method

Film synthesis methods

Polymer assisted deposition of NbN MgB2

PAD solution:NbCl2, NH4OH, polyethyleneimine, HF,H2O

Spin coat to thin film on substrate -provides basis of thin film structure forstarting material NbCl2

Anneal (~1000°C) in reactive atmosphereto provide oriented growth ofmicrocrystalline domains:NH3 to produce NbNCH4 to produce NbC

Zou, GF, et al., Chem. Comm. 45 (2008) 6022

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XPS high resolution scanNb3d XPS

Before anneal mostly Nb oxideAfter anneal 800°C in UHV, surfaceis mostly Nb metal with a bit ofpartial oxidation (high bindingenergy tailing)

Small amount of oxygen left atsurface after anneal by XPS

Nb substrate conditioning

Required to remove excessivesurface oxide to avoid reactionswith deposited thin films andimproves surface magneticproperties - less dissipation

198200202204206208210212214216218220

0

2

4

6

8

10

12

14

16

18

x 104

Binding Energy (eV)

c/s

Before anneal (red)

After anneal (blue)

Nb2O5

Nbmetallic

small amount ofNb sub-oxide

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Angle Resolved XPS used to determine Nb2O5 oxide layer thickness resulting fromBCP treatment on Nb metal crystal plate

ARXPS reveals an oxide layer that is 27-30Å thick resulting from the BCP treatment

3 different photoelectron take off angles (TOA) relative to the surface plane: 90°, 45°, and 20°. The Nb3d manifold is curve fit toextract intensity data for the Nb in the form of Nb2O5 (oxide overlayer) and Nb in the form of metal (base substrate). The spin orbitcouple peaks were constrained to a ratio of 3/2, expected theoretically. The metal peaks were fit using asymmetric broadeningfollowing theory from Doniac and Suncic, and the oxide peaks were simple Gaussian-Lawrencian.

!

iI = ioI " i# " exp($l / i%

a

b

& )dl

XPS intensities for photoemission peaks associated with the oxide overlayer, and the underlyingintrinsic metal were used. The intensity, I, of photoelectron emission from each layer, i, can bedescribed by the equation, where Io is the bulk intensity, which is dependent on the atomvolume density and is taken as unity for the base metal and some lower fraction for the oxidebased on material densities. l is the distance that the electron travels through the materialbefore exiting the surface into the vacuum and is described as l=d/sinθ, where d is the thicknessof the oxide overlayer, and θ the angle of electron emission relative to the surface plane. λ isinelastic mean free path of the electron in the solid. For the oxide overlayer we integrate froml=0 to l=d/sinθ, and for the base metal we integrate from l=d/sinθ to ∞ for the bulk substrate.

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NbN Surface and Thin Film Analysis

• NbN intrinsic Tc = 16K• thin superconducting films produced by PAD method• with current deposition and annealing parameters films are N poor• low oxygen content critical for yielding superconductivity• incomplete coverage (pinhole) issues need to be resolved - AFM and XPS• annealing conditions critical in determining micro-nanostructure of films

grain size and surface roughness - AFM

• relative atomic sensitivity factors in Auger spectroscopy not yet correct - needstandard

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NbN Surface and Thin Film Analysis - surface morphology by AFM

Topographic Image Phase Image

SRF-NbN6-1 1 x 1 µm

RMS = 10.6 nmon Al2O3

SRF-NbN6-2 1 x 1 µm

RMS = 5.1 nmon SrTiO3

SRF-NbN3-34 x 4 µm

RMS = 21.6 nmon Al2O3

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Comment: sample NbN3_2, NbN on sapphire produced by PAD process Atomic Concentration Table C1s N1s O1s Al2p Nb3d [0.296] [0.499] [0.711] [0.193] [3.127] 1.30 25.38 17.45 11.58 44.29

on surface

after 10 nm sputter clean

XPS spectroscopy measurement on surfaceand after sputter ion clean of 10 nm (intomain bulk of film) shows relatively highoxygen (17.45% atomic) and a smallamount of carbon (1.3% atomic). Some ofthe O signal may be from the incompletecoverage of sapphire.

Na, Si, and most of the C at the surface arejust surface impurities from processing or airexposure.

Nb:N ratio here is measured to be 1.7. TheNbN films tend to be nitrogen deficient.

The balance in the nitrogen deficiency maybe made up by the O and C impurity levels.

NbN films - surfaces vs.bulk film

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Auger survey spectrum taken at 12 nm pointin profile shows O, C, and Al in addition tothe Nb and N. C is in a metal carbidechemical form.

Relatively high O (>5%) and C (~5%) levelin bulk of film

No superconductivity

NbN film - profile

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XPS survey spectrum taken at 8 nm point inprofile shows a very clean film.

Oxygen <2% atomic

Tc = 9.5K

NbN film - profile

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0200400600800100012000

0.5

1

1.5

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2.5

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3.5

4

4.5x 105 NbN4_6.spe

Binding Energy (eV)

c/s

-O K

LL

-O1s

-N K

LL

-N1s

-Nb4

p

-Nb3

s

-Nb3

p3 -N

b3p1

-Nb3

d

-Al2

s -A

l2p

File Name: NbN4_6.spe

Comment: PAD NbN on sapphire from YYZ sampleNbN4-2

--------------------------

Atomic Concentration Table - RSF in [brackets]

--------------------------

N1s O1s Al2s Nb3d[0.499] [0.711] [0.312] [3.127]

32.55 9.53 4.43 53.49

at 10 nm sputter depth

NbN film - excessive oxygen in film

Al and some of the O signal is from the sapphire substrate due toincomplete coverage (holes) of the NbN film

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MgB2 Surface and Thin Film Analysis

• MgB2 intrinsic Tc = 39K• thin superconducting films produced by codeposition methods• high quality films are being produced - Tc, stoichiometry, interfaces good, RF

performance, Hc1

• some issues with stability and interface mixing (inter reactions)• oxygen from substrate or dielectric may cause chemical interference at

interfaces

• for Auger spectroscopy and Auger thin film profiling there exists an overlap inthe low energy Nb and B Auger peaks. Principal component analysis used toeffectively separate signals for these two elements. The Mg chemical states ofMgB2 and MgO may also be separated.

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MgB2 Surface and Thin Film Analysisprincipal component analysis (PCA) in Auger profiling spectroscopy

Separating B and NbAuger peaks

Separating Mg inMgB2 and Mg in MgOAuger signals

B

NbB

sum to fitexperiment

Nb

Mg in MgB2

Mg in MgO

Mg in MgB2

Mg in MgO

sum to fitexperiment

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MgB2 Surface Analysis - surface alteration due to air exposure for a thick film (100 nm)

Note:Ultrathin films show full depletionof B from altered surface layer -see below

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0 10 20 30 40 50 60 70 800

0.5

1

1.5

2

2.5

3x 105

Sputter Time (min)

Inte

nsity

C1O1

Mg2B1Nb1

surfa

ce o

xide

MgB2

Mg-

B o

xide

Mg-

B o

xide

Mg

oxid

e

Nb

Thin film structure complicated:

1) Nb substrate

2) Thin Mg oxide

3) First layer of thin Mg-B oxide

4) Second layer of thin Mg-Boxide

5) Thicker MgB2 layer

6) Thin surface oxide layer

MgB2 film structure

Intended:

100nm MgB2 on 10nm B on Nbsubstrate

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MgB2

surfa

ce o

xide

Mg

oxid

e

Nb

Thin film structure:

1) Nb substrate

2) Mg oxide (MgO)

3) Thicker MgB2 layer

4) Thin surface oxide layer

MgO layer relatively thick

Substantial mixing at interface ofMgO and MgB2

MgB2 film structure

Intended:

1000nm MgB2 on Nb substrate

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Auger spectroscopy sputter depth profile: peak intensity profilewith Mg chemical states resolved using principal componentanalysis (PCA) / target factor analysis (TFA)

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0 100 200 300 400 500 600 700 8000

10

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80

90

100SRF45_7.pro

Sputter Depth (nm)

Atom

ic C

once

ntra

tion

(%)

O1Mg2Al2B1

Nb1

MgB2 film of ~230 nm thickness showsvery low oxygen and close to Mg:B =0.5 stoichiometry

Layer of MgO at interface which seemsfairly sharp

Al2O3 layer of ~370 nm thicknessshows poor stoichiometry of ~Al1O1instead of Al2O3

Interface of Al2O3 layer with Nb seemsto be very broad, indicatinginterdiffusion of Al2O3 with Nb

MgB2 + dielectric film multilayers

Intended:

200nm MgB2 on 300nm Al2O3 on Nb substrate

MgB2 Al2O3

Nb

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Auger sputter depth profile

Surface layer >10 nm is fully Mgoxide and completely depleted of B

MgB2 layer (~40 nm) is slightly Bpoor except at 50 nm depth wherestoichiometry is close to correct

Mg oxide layer (~20 nm)

Aluminum oxide (~15 nm)

The small amount of oxygen (~2%)in the MgB2 film is real

Al is actually at ~0 atomic% in MbB2layer - nonzero signal arises fromspectral noise

0 20 40 60 80 100 1200

10

20

30

40

50

60

70

80

90

100

Sputter Depth (nm)

Atom

ic C

once

ntra

tion

(%)

O1Mg2Al2B1Nb1

MgB2

surf

ace

Mg

oxid

e

buri

ed M

g ox

ide

alum

inum

oxi

de

Nb substrate

MgB2 + dielectric filmmultilayers

MgB2 50 nm / ALD Al2O3 10 nm / Nb

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0 20 40 60 80 100 1200

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100MgOald4_5.pro

Sputter Depth (nm)

Atom

ic C

once

ntra

tion

(%)

O1Mg2B1Nb1MgB2

surf

ace

Mg

oxid

e

ALD

Mg

oxid

e

Nb

subs

trat

e


MgB2 50 nm / ALD MgO 10 nm / Nb

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Sputter Depth (nm)

0 20 40 60 80 100 1200

10

20

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80

90

100MgB2Y_6.pro

Atom

ic C

once

ntra

tion

(%)

O1Mg2Y2B1Nb1

MgB2

surf

ace

Mg

oxid

e

ALD

Y o

xide

Nb

subs

trat

e

buri

ed M

g ox

ide


MgB2 50 nm / ALD Y2O3 10 nm / Nb

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Comparison of Auger sputter depth profiles for MgB2 films onALD dielectrics on baked Nb substrates

MgB2 50 nm / ALD Al2O3 10 nm / Nb MgB2 50 nm / ALD MgO 10 nm / Nb MgB2 50 nm / ALD Y2O3 10 nm / Nb

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0 50 100 150 200 250 300 350 400 450 5000

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100MgB2Nb5_11.pro

Sputter Depth (nm)

Atom

ic C

once

ntra

tion

(%)

O1Mg2B1

Nb1

B

MgB2

B B B B Nb

MgB2 MgB2 MgB2

65nm 37nm

Auger sputter ion profile

Top layer of nominally pure B 10nmplus4x double layers of MgB2 50nm / B 10nmonNb substrate

Top layer of nominally pure B approximately10nm in thickness, but shows Mg signal also

Individual layers and total film thickness arethicker than predicted

I believe that the “less than sharp” interfacesand incomplete stoichiometry gain (Mgfound in the pure B layers) are due tointermixing of the layers during thedeposition process. Not an artifact from thesputtering during analysis - note therelatively sharp interface at the Nb substrate.

First MgB2 layer slightly Mg rich, other layersslightly B rich.

MgB2 + dielectric film many multilayers

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Summary:•Lots of materials and thin film information available in surface analysis, sputterdepth profiles, and full spectroscopy

•Stoichiometry (with proper calibration), film thickness, material interfaceinteractions

•In the NbN system, oxygen content in the films is one critical factor indetermining proper phase and superconductivity (<5% atomic need)

•Stoichiometry to be improved in PAD produced NbN by adjustment of annealingconditions

•MgB2 thick films on Nb crystal plate show promising results

•Ongoing progress in producing ultra-thin MgB2 dielectric multilayers

•Additional methods to produce thin films being investigated - CVD and PECVDtowards the primary goal of conformal coatings on RF cavity interiors

Technology

Shulze - Surface and Thin Film Characterization of Superconducting Multilayer films for Application in RF Accelerator Cavities