atom-probe tomographic analyses of …...• analysis volumes of, for example, 50 nm x 50 nm x 400...

David N Seidman13 July 2005

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ATOM-PROBE TOMOGRAPHIC ANALYSES OF NIOBIUM SUPERCONDUCTING RF CAVITY MATERIALS

Jason T. Sebastian*

David N. SeidmanNorthwestern University Department of Materials Science and Engineering

Cook Hall, 2220 North Campus Drive, Evanston, IL 60208Northwestern University Center for Atom-Probe Tomography (NUCAPT)

http://arc.nucapt.northwestern.eduJim Norem

High Energy Physics Division, Argonne National Laboratory (ANL), Argonne, IL 60439Pierre Bauer

Technical Division, Fermi National Accelerator Laboratory (FNAL), Batavia, IL 60510

*Now at Imago Scientific Instruments, Madison, Wisconsin


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Agenda

• Background• Superconducting Nb rf cavities• Atom-Probe Tomography (APT)

• Specimen preparation

• Preliminary APT results• Three analyses (separate tips)

• Conclusions and next steps

Agenda


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The Nb-O phase diagram - a variety of different oxide phases can form

• NbO, NbO2, and Nb2O5 are the thermodynamic equilibrium phases• Other phases (e.g., Nb2O3 and Nb2O) also reported• Kinetic effects must also be considered

Superconducting Nb rf cavities

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

From Binary Alloy Phase Diagrams (2nd Ed.), ASM (1990)

Phase Composition (at. % O)

Pearson symbol Space group Prototype

(Nb) 0 to ~9 cI2(bar) Im3(bar)m W

NbO 50 cP6 Pm3(bar)m NbO

NbO2 66.7 tI96 I41/a

Nb2O5 ~71.4 mP99 P2/m


4

Atom-probe tomography (APT) - schematic

The time-of-flight (TOF) of

each field-evaporated ion is measured

simultaneously with the

position of the impact on the 2D detector

Atom-probe tomography (APT)

E = V / (k r)[field

enhancement]

• Time of flight Chemical nature

• Impact Position Atom position on tip surface(projection microscope)

• T = 20 - 100 K• p <= 10-10 torr

(predominantly hydrogen)


5

Field evaporation - schematic

Field evaporation

• A thermally-activated process involving the excitation of a surface atom over a Schottky hump in the presence of an electric field

– In an atom probe, evaporation is controlled via the use of a high-voltage pulser, or high frequency (femtosecond) pulsing laser

In the presence of an, electric field, the ionic state is energetically

favorable to the atomic state at distances away from the tip surface

Without electric field With electric field


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NUCAPT’s new local electrode atom-probe (LEAP) tomograph takes atom-probe microscopy to a new level

• Increased data collection rate (up to 72 million ions hr-1)• Expanded specimen geometry capabilities• Analysis volumes of, for example, 50 nm x 50 nm x 400 nm (= 106 nm3 =

10-3 µm3)

Figure courtesy of Imago Scientific

Instruments NUCAPT’s LEAP Tomograph

Atom-probe tomography (APT)

xy

Vpulse z

Vex

Vaccel

V = 0

Pos.

Neg.

xy

Vpulse z

Vex

Vaccel

V = 0

Pos.

Neg.


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Specimen preparation

1. Electropolishing• 10% HF (48%) in 90% HNO3 (68%)• DC voltage• “Drop off” technique• Keep things as clean as possible

• Rinsing (UHP water)• Minimal time in air (stored in desiccator under dry N2)

2. Buffered Chemical Polishing (BCP)• 1 : 1 : 2

HF (49%) : HNO3 (68%) : H3PO4 (85%)• No voltage• Replicating the setup at FNAL

3. Focused ion beam (FIB) milling at ANL to be done soon• “Site-specific” specimen preparation



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Specimen preparation - electropolishing and buffered chemical polishing


Step 1 - Machining Blanks(EDM at FNAL from Nb

cavity material)

~8 mm

Step 2 - Polishing at FNAL

Rectangular cuvette with

polishing solution (BCP)

Stereo-microscope


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Specimen preparation - electropolishing and buffered chemical polishing


~500 µmNb specimen

Localelectrode

Step 4 - Alignment in the LEAP tomograph

Step 3 - Heat treatment at FNAL


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Dire

ctio

n of

ana

lysi

s

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10

Distance into bulk (nm)

F

Atomic sensitivity of the technique - individual fluorine atoms on and near the surface (propensity for the oxide)

Nb LEAP analysis

27 x 26 x 9 nm3

~95 k atoms

Oxygen

Niobium

Fluorine


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Mass Spectrum

0

200

400

600

800

1000

1200

10 20 30 40 50 60 70 80 90 100 110 120 130 140

Mass-to-charge state (a.m.u.; 0.2 a.m.u. bins))

Complex mass spectrum - lots of complex ionsNb LEAP analysis - mass

spectrum

Nb3+

NbO3+

NbO22+

Nb2+

NbO2+

NbO1+ NbO21+

O+

+1 +2 +3

O 16.00 8.00 5.33

Nb 93.00 46.50 31.00

NbO 109.00 54.50 36.33

NbO2 125.00 62.50 41.67

Mass-to-charge (amu)

~5,200

• Flight path = 128 mm• Resolution ≈ 1/250 (FWHM)• T = 50K• f = 15%• p ≈ 8 x 10-11 torr

Element Isotope Abundance

Nb 93 100%

16 99.74%

O 17 0.04%

18 0.22%

Nb4+


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Evaporation of complex ions (very schematic) - many different complex ions are field evaporated from the oxide layer of the tip

Nb LEAP analysis - field evaporation

Oxygen

Niobium

Nb

Nb

Nb

Nb

Nb

Nb

Nb

Nb

Nb

O

O

O

O

Nb

Nb

Nb

Nb

Nb

Nb

Nb

Nb

Nb

O

O

O

O

Nb

Nb

Nb

Nb

Nb

Nb

Nb

Nb

Nb

O

O

Nb

Nb

Nb

Nb

Nb

Nb

Nb

Nb

Nb

O

O

Nb

Nb

Nb

Nb

Nb

Nb

Nb

Nb

Nb

O

O

O

O

Nb

Nb

Nb

Nb

Nb

Nb

Nb

O

O

O

O

Nb

Nb

Nb

Nb

Nb

Nb

Nb

O

O

O

O

O

O

Nb

Nb

Nb

Nb

NbO

O

O

O

Bulk Nb

Nb-oxide layer

Nb++

Nb+++

NbO++

NbO+++

NbO2++


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Atomic reconstruction of the transition through the oxide layer

Nb LEAP analysis

23 x 21 x 11 nm3

~112 k atoms

Oxygen

NiobiumDire

ctio

n of

ana

lysi

s

We see a clear transition from oxygen (cyan) to niobium (magenta) as we move along the analysis direction


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A 30 atomic percent Nb isosurface gives a better representation of the spatial gradients in atomic

concentration

Nb atoms with30 at. % Nb isosurface

Nb and O atoms with30 at. % Nb isosurface

O atoms with30 at. % Nb isosurface

30 at. % Nb isosurface

Oxygen

Niobium

Extraneoussurfaces (not important)

Nb LEAP analysis

23 x 21 x 11 nm3

~112 k atoms


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The proximity histogram (proxigram) allows for the determination of quantitative concentration profiles relative to the position of an irregularly-shaped interface

23 x 21 x 11 nm3

Proxigram



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0

0.1

0.2

0.3

0.4

0.5

0.6

0 1 2 3 4 5 6 7 8 9 10 11 12


H

C

N

(15 amu)

O

(21 amu) ca. H2F

(29 amu) ca. N2H

Nb

(35 amu) ca. O2H3

30 atomic percent Nb proxigram - we see the transition from oxygen to niobium

Nb LEAP analysis


Nb

O

H

ca. H2F

ca. N2H

ca. O2H3


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0

0.05

0.1

0.15

0.2

0.25

0.3

0 1 2 3 4 5 6 7 8 9 10 11 12


H

C

N

(15 amu)

O

(21 amu) ca. H2F

(29 amu) ca. N2H

Nb

(35 amu) ca. O2H3

Hydrogen, residual gas atoms, and other “UFO” peaks make up nearly 50% of the ions collected

Gas atom signals


H

N (red)

Nb

ca. H2F

ca. N2H

ca. O2H3


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Hydrogen and residual gas atoms can come from “outside the specimen”

Gas atom signals

From Miller et al., (1996)

Electric field distribution on a atom-probe specimen (schematic)

Ball model of an atom-probe tip surface (schematic)

Field-ionization of gas atoms on an atom-probe tip surface


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Diffusivities

1E-27

1E-26

1E-25

1E-24

1E-23

1E-22

1E-21

1E-20

1E-19

1E-18

1E-17

1E-16

1E-15

1E-14

1E-13

1E-12

1E-11

1E-10

1E-09

1E-08

1E-07

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004

1/T (K)

0

500

1000

1500

2000

T (

K)

O LB-1 NbO LB-2 NbO LB-3 NbO LB-4 NbC LB-1 NbC LB-2 NbC LB-3 NbH LB-1 NbH LB-2 NbH LB-3 NbH LB-1 FeH LB-2 FeH LB-3 FeH LB-4 FeH LB-1 TiH LB-2 TiH LB-3 TiT (K) [right axis]

H in Fe

H in Nb

H in Ti

O in Nb

C in Nb

T (rightaxis)

~Room T

Hydrogen is extremely mobile in niobium (even at room temperature)

Gas atom signals

At room temperature, after ten seconds, (6D

t)1/2 for H in Nb is ~0.1 millimeter (cf. ~ 1 picometer for O in

Nb)

Diffusivity = D = Do exp{-Q/(R T)}


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0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8 9 10 11 12 13


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

Rat

io N

b/O

O (left axis)

Nb (left axis)

Nb/O (right axis)

Removal of the hydrogen and residual gas atoms from the concentration gives a better picture of the transition through the oxide layer

Nb2O5 (0.4)

NbO0.5 (2.0)

NbO (1.0)

NbO2 (0.5)

Nb LEAP analysis

We see a clear and smooth transition from Nb2O5 to NbO0.5 (= Nb2O)

Nb

O

Nb/O


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The profile corresponds, to first order, to the current models of oxide and suboxide formation on Nb

Nb LEAP analysis

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8 9 10 11 12 13


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

Rat

io N

b/O

O (left axis)

Nb (left axis)

Nb/O (right axis)

Nb

O

Nb/O


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Second Nb analysis - more Nb-rich

25 x 25 x 6 nm3

Nb LEAP analysis

Oxygen

Niobium

Dire

ctio

n of

ana

lysi

s


23

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9


0

2

4

6

8

10

12

14

16

18

20

Rat

io N

b/OO (left axis)

Nb (left axis)

Nb/O (right axis)

Second Nb analysis proxigram - a glimpse at the transition from oxide to Nb metal (?)

Nb LEAP analysis

The transition from oxide to metal may be marked by a

rapid increase in the ratio of Nb and O concentrations

Dissolved atomic oxygen in Nb metal (~5-10 at. %)

Nb

O

Nb/O


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Second analysis … a “continuation” of the first analysis

Nb LEAP analysis

First analysis (Nb2O5 to Nb2O) Second analysis (Nb2O to Nb metal [?])

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9


0

2

4

6

8

10

12

14

16

18

20

Rat

io N

b/OO (left axis)

Nb (left axis)

Nb/O (right axis)

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8 9 10 11 12 13


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

Rat

io N

b/O

O (left axis)

Nb (left axis)

Nb/O (right axis)

Nb

ONb/O

Nb

O

Nb/O


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Nb LEAP analysis

Third Nb analysis - an oxide metal interface


Dire

ctio

n of

ana

lysi

s

Oxygen

Niobium

27 x 26 x 9 nm3

~95 k atoms


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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9 10 11 12 13


0

2

4

6

8

10

12

14

16

18

20

Rat

io N

b/O

O (left axis)

Nb (left axis)

Nb/O (right axis)

55% Nb isosurface and proxigram

Nb LEAP analysis

27 x 26 x 9 nm3

~95 k atoms

Dissolved atomic oxygen in Nb metal

(~10 at. %)

Nb

O Nb/O


27

Dire

ctio

n of

ana

lysi

s

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10


F

Atomic sensitivity of the technique - individual fluorine atoms on and near the surface (propensity for the oxide)

Nb LEAP analysis

27 x 26 x 9 nm3

~95 k atoms

Oxygen

Niobium

Fluorine


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Conclusions and next steps

• Nanochemical, atomic scale analyses of the oxide surface and of the near-surface bulk niobium are being performed• “Smooth” transition from surface Nb2O5 to Nb2O (and into the

bulk Nb)• Ability to detect small number of contaminant atoms in the

oxide surface and in the near-surface bulk niobium• Levels of oxygen in the near-surface bulk niobium (metal) of

5-10 atomic percent, which is consistent with bulk Nb-O phase diagram

• More analysis to come• Interpretation of mass spectra

• Improved analysis conditions• Improved specimen preparation techniques (reliability and

repeatability)• Focused ion beam (FIB) milling and/or femtosecond laser

ablation • Many classes of samples

Conclusions and next steps


29

Bulk

Thin-film

“Large-grain” (artificial)

“Small grain” (artificial)

BCP

EP

Combinations EP-BCP

With mild bake

Without mild bake

Purified w. Titanium

Non-purified

Cold-worked

Not-cold worked

Manufacturer A

Manufacturer B

High RRR

Low RRR

Non-weld

Weld

Classes of samples - effects of many, many variables on the atomic chemistry

Sample classes


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A “more complete” run would capture much more of the atomic chemistry of the oxide surface and of the near-

surface bulk niobium

Nb LEAP analysis

Present analyses

“More complete”analyses


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Specimen preparation - focused ion beam (FIB) milling of “site-specific” LEAP specimens from niobium cavity materials

FIB specimen preparation

REFERENCEStrategies for Fabricating Atom Probe Specimens with a Dual Beam FIB,M.K. Miller, K.F. Russell and G.B. ThompsonUltramicroscopy, 102 (2005) 287-298.

Possibility of centering a specimen on a region of

interest (e.g., a grain boundary)

We will use the ANL Zeiss 1540XB dual-beam

FIB to attempt “site-specific” specimen

preparation


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Studies of Nb cavity materials

• SEM micrographs from P. Bauer et al.

Superconducting Nb rf cavities

atom-probe tomographic analyses of …...• analysis volumes of, for example, 50 nm x 50 nm x 400...

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