Quantification of H, C, N, O in Niobium and
Niobium Oxide for SRF Cavities
P. Maheshwari, F. Stevie, M. Rigsbee, D. Griffis
North Carolina State University
G. Myneni, G. Ciovati
Jefferson Laboratory
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Outline
• Interstitial elements in Nb
• Introduction to SIMS analysis
• Surface roughness of Nb
• SIMS detection of significant H
• Quantification of C, N, O in Nb by ion implantation
• Mobility of H and D
• SIMS analysis before and after heat treatment
• SIMS measurements in Nb oxide
• Summary and current work
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Interstitial Elements in Nb
• Nb used as cavity material
• Highest temperature superconducting metal (9.2ºK)
• High critical magnetic field (~200mT)
• Easy formability
• Interstitial elements of interest are H, C, N, O
Superconducting Radio Frequency (SRF) Nb cavity manufactured at JLAB
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Secondary Ion Mass Spectrometry (SIMS)
- Primary ion penetrates surface, energy lost through collision
cascade, primary ion implanted into solid, secondary particles
(including ions) leave surface at low energy
- Escape depth of sputtered species only few Angstroms
2-4
SIMS Analysis Characteristics
•Use O2+ primary beam for metals
(electropositive elements)
•Use Cs+ primary beam for H, C, O, N
•SIMS instrument uses high ion extraction field over short distance
Desire flat surface for analysis
Polycrystalline sample
Optical Image of Nb Surface
Fine grain
Sample W3
(Nomarski)
•Surface is rough
•Poor depth resolution
•SIMS craters not
measurable
mechanical polish + 10min BCP 1:1:1 180C 12 hr in air
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sample
Optical Images Polycrystalline
of Nb Surface
Fine grain
Sample PC3
Surface at higher magnification shows
roughness on order of micrometers
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Large grain sample
Optical Images of Nb
Surface
SIMS craters
Large grain
Sample L18
•Not as rough
as fine grain sample
•Better depth
resolution
•Crater difficult
to measure
Circular features are pits
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Single
crystal
sample
Optical Images
of Nb
Surface
•Large Grain BCP
nanopolished
•Smooth surface
suitable for crater
measurement
Pit
SIMS craters
•Implanted sample used
to quantify C, N, O
Nanopolish provides very smooth surface
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Project Goal
• H, C, N, O are interstitial elements in Nb that can affect
performance (Q-drop)
• Goal is to characterize H, C, N, O in Nb
• Note that only about 60 nm penetration range of fields
into Nb
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SIMS Standards Using Ion Implantation
• All elements and isotopes possible
• Implantation into any substrate or structure
• Vary peak concentration by varying dose
• Vary depth of peak with implant energy
1E+08
28Si+
Energy
58Ni+
1E+04
Dose
1E+02
1E+00
0
200
100
Time (sec)
300
Counts/sec
1E+06
58Ni
in Si
5E14 atoms/cm2
SIMS Conversion of Raw to Processed Data
Raw
Processed
1E+08
28Si+
1E+08
Conc (atoms/cm3)
1E+21
28Si+
1E+20
1E+19
1E+04
58Ni+
1E+06
Counts/sec
in Si
5E14 atoms/cm2
1E+06
Counts/sec
58Ni
Ni
1E+18
1E+04
1E+17
1E+02
1E+02
1E+16
0
200
100
Time (sec)
1E+00
300
DL<1E16at/cm3
1E+15
1E+00
0.0
0.2
0.4
0.6
Depth (µm)
0.8
• Raw data in counts versus cycles or time
• Convert to reduced data in concentration versus depth
• Depth axis with crater depth (use profilometer)
• Concentration axis with RSF (normalized to matrix)
1.0
SIMS Analysis of Interstitial Elements in Si
H, D, C, N, O in Si
28Si Cts
1E+22
Conc. (atoms/cm3)
1E+08
1H
1E+21
1E+07
1E+06
1E+20
1E+05
D
1E+19
16O Cts
1E+18
1E+04
1E+03
1E+17
1E+16
1E+15
0.0
0.2
0.4(um)
Depth
12C
1E+02
28Si14N
1E+01
18O
1E+00
0.6
0.8
Counts (cts/sec)
1E+23
Typical Analysis Conditions
•CAMECA IMS-6F
•Cs+ primary beam 14.5keV impact energy (High sensitivity)
•7- 20nA 120µm x 120µm raster
•30µm diameter detected area
•Cs+ 6keV impact energy (High depth resolution)
•7nA 120µm x 120µm raster
•30µm diameter detected area
•SIMS samples were nanopolished, large grain Nb samples
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SIMS Mass Spectrum Control Sample
SIMS results showed major difference in H content for large
grain samples before and after heat treatment
1E+07
Nb-
1E+06
Counts/sec
1E+05
NbH5-
1E+04
1E+03
1E+02
1E+01
1E+00
90
92
94
96
98
100
Mass (a.m.u.)
Sample before heat treatment: intense NbHx- peaks
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SIMS Mass Spectrum Heated Sample
1E+06
Nb-
1E+05
Counts/sec)
1E+04
1E+03
NbH2-
1E+02
1E+01
1E+00
90
92
94
96
98
100
Mass (a.m.u .)
Sample after heat treatment shows dramatically
reduced NbHx- ion intensity
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Quantification of C, N, O and Mobility of D
D, C, N, O implanted in Si and Nb – no implant peak found
for D in Nb.
Si Implant
(b)
1E+21
1E+22
1E+07
1E+21
1E+06
1E+20
2D
1E+05
1E+19
1E+04
16O
1E+18
1E+03
28Si14N
1E+17
1E+02
12C
1E+16
1H Counts
1E+15
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
H
1E+08
1E+07
1H Counts
16O
1E+20
1E+06
1E+05
1E+19 93Nb Counts
93Nb14N
1E+18
1E+04
12C
1E+03
1E+17
1E+01
1E+16
1E+00
1E+15
1.6
1E+02
2D Counts
1E+01
1E+00
0.0
Depth (um)
(a) Depth profile for D,C,N,O in Si
Nb implant
Counts (cts/sec)
28Si Counts
1E+08
Counts (cts/sec)
Concentration (atom/cm3)
1E+22
Concentration (atom/cm3)
(a)
0.2
0.4
0.6
1.0
0.8
Depth (um)
1.2
1.4
1.6
(b) in Nb using 14.5 keV Cs+ beam
H level is too high to quantify using ion implantation
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Quantification of C, N, O in Nb
Cs+ 6keV impact energy, high depth resolution
Ion implantation
1E+04
93Nb
1E+20
1E+03
16O
1E+19
1E+02
93Nb14N
12C
1E+18
1E+01
1E+17
Counts (cts/sec)
Concentration (atom/cm3)
1E+21
•Dose: atoms/cm2
•C: 1E15
•N: 1E15
•O: 2E15
1E+00
0.0
0.2
0.4
0.6
0.8
Depth (um)
P. Maheshwari, et al., Surface and Interface Analysis (2010, 42)
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Diffusion of H and D in Nb
•D does not show any peak in Nb. Possible causes:
•High diffusion coefficient for H in Nb
•H moves while sputtering
Element
Nb 300K
(m2/s)
520M C-steel Si 300K
300K (m2/s) (m2/s)
H
8.06 x 10-10
2 x 10-11
D
5 x 10-10
1 x 10-32
2 x 10-32
Table showing diffusion coefficients in Nb, steel and Si
Ref: Volkl. J, Wipf H; Hyperfine Interactions 8 ( 1981) 631-638.
Ref:E. Hörnlund et.al. Int. J. Electrochem. Sci., 2 (2007) 82 - 92
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Diffusion Length Comparison
Using the 1-D equation for diffusion we can calculate the
diffusion lengths covered by H in these materials in 1 sec.
Diffusion Length = 2√Dt
Matrix
Nb(300k)( nm)
520M C steel Si (300K)
(300K) (nm) (nm)
Diffusion
Length
5.7 x 104
4.5 x 102
1.4 x 10-5
Table showing diffusion lengths in 1 sec. of H in Nb, steel and Si
Diffusion rate for H in Nb = 5.7E4 nm/s
Hypothesis for H Removal
• Nb2O5 surface layer (<10nm) is removed at high
temperatures in vacuum (above 600oC)
• H is free to leave the Nb
• Cool down, bake at 120oC and exposure to air causes the
surface oxide layer to form rapidly
• Oxide prevents reintroduction of H from the atmosphere at
room temperature
• To test this theory:
•Heat treated sample etched with HF to remove oxide layer
and rinsed in H2O
•Hydrogen allowed to reenter
•H analysis by SIMS
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Etching Experiment (Raw Data)
Large grain 800C/3hr, 400C/20min
BeforeL10Etch
L10 (etched with HF and rinsed with water)
H
After
Etch
1E+07
Nb
1E+06
Counts (cts/sec)
Counts (cts/sec)
1E+07
1E+05
1E+04
1E+03
1E+02
H
1E+01
1E+06
1E+05
1E+04
1E+03
Nb
1E+02
1E+01
1E+00
1E+00
0
200
400
600
800
1000
1200
0
200
400
600
800
1000
1200
Time (s)
Time (s)
H appears to return to former level after etch
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>25µm analysis shows no decrease in H
Non Heat treated Nb Sample
1E+08
1H Cts
Counts (cts/sec)
1E+07
93Nb Cts
1E+06
1E+05
93Nb1H5 Cts
1E+04
D Cts
1E+03
1E+02
1E+01
1E+00
0.0
5.0
10.0
15.0
Depth
(um)
20.0
25.0
Cs+ 8.5 nm /sec compared to 5.7E4 nm/sec for H in Nb
30.0
TEM analysis of Oxide layer on fine grain Nb sample
W3 poly – mechanical polish + 10min BCP + 180C 12hr in air
Surface protected with sputtered 60nm Au-Pd and 2µm FIB W
Focused Ion
Beam (FIB)
sample
preparation
sample W3
HD2300 STEM (bright field)
TEM Analysis of Oxide layer on fine grain Nb sample
Sample W3
Au-Pd
Nb2O5
Nb
Oxide is uniform and no apparent O region below oxide
HD2300 STEM (bright field)
TEM and SIMS Analyses in Nb2O5
• Surface oxide appears to play an important role in
controlling H levels in Nb
• TEM measurements in single crystal Nb show typical oxide
<10nm*
• Anodization used to produce a 120 nm thick oxide on Nb
•H, D, 18O then implanted into oxide
• H 1E16atoms/cm2
• D 2E15 atoms/cm2
• 18O 5E15 atoms/cm2
Nb
*A. D. Batchelor, D. N. Leonard, P. E. Russell, F. A. Stevie, D. P. Griffis, G. R. Myneni, Proceedings of Single
Crystal Niobium Technology Workshop, Brazil, AIP Conference Proceedings, Melville, NY (2007) 72-83
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SIMS Analyses in Nb2O5
Before implantation
Oxide
After implantation
Substrate
Counts (cts/sec)
1E+10
1E+08
H Cts
1E+06
Nb Cts
1E+04
16O Cts
D Cts
18O Cts
1E+02
1E+00
0
100
200
300
Time (s)
400
500
600
D peak at the interface, H peak in the oxide
H and D not mobile in oxide
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SIMS Analysis in Nb2O5
Analysis at low Cs impact energy (6keV)
1E+08
16O Cts
Concentration (atom/cm3)
1E+22
18O
1E+21
1H
Peak shape if D
1E+07
not mobile in Nb
1E+06
1E+20
1E+05
1E+19
93Nb
D
1E+04
1E+18
1E+03
1E+17
1E+02
Counts (cts/sec)
1E+23
• Peaks were
observed for H, O
• D has peak at
interface
12C Cts
1E+16
1E+01
1E+15
1E+00
0.0
0.1 Depth (um) 0.2
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0.3
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Estimate of H in Nb Based on H in Nb2O5
•Calculate RSF for H in Nb2O5
•Note that Nb matrix signal very similar in Nb2O5 and Nb
•Assume same RSF for H in Nb
•Result is 2E22 atoms/cm3 or 40% atomic
Estimate of H in Nb using RSF from Nb2O5
Sample without heat treatment
1H
1E+22
1E+05
2E22 atoms/cm3
Concentration (atom/cm3)
93Nb Cts
1E+04
1E+21
1E+20
16O
1E+19
1E+03
12C
1E+02
93Nb14N
1E+18
D Cts
1E+17
Counts (cts/sec)
1E+23
1E+01
18O
1E+16
1E+15
1E+00
0.0
0.1 Depth (um) 0.2
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0.3
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Results Summary
• H detected at high concentration in control samples using
SIMS
• Heat treated samples show much reduced H
• Heat treated sample after HF etch shows high H
• H not mobile in Nb oxide (diffusion barrier for H)
• C, N, O do not show significant differences after heat
treatment and do not appear to be of major importance with
respect to cavity performance
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Experiments in Progress
•Analyze H in Nb using TOF-SIMS and temperature
controlled stage (Sample temperature can be varied from
cryo to 300C)
•XRD measurements to check for lattice expansion when Nb
is charged with H
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