Secondary Ion Mass Spectrometry...
Transcript of Secondary Ion Mass Spectrometry...
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Secondary Ion Mass Spectrometry
(SIMS)
Thomas Sky
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Characterization of solar cells
Characterization
•Optimization of processing
•Trouble shooting
1,2
1,0
0,8
0,6
0,4
0,2
0,01E16 1E17 1E18 1E19 1E20
De
pth
(µ
m)
P Concentration (cm-3)
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Characterization of device structure
1014
1015
1016
1017
1018
1019
1020
1021
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
PBAs
Ato
mic
co
nce
ntr
atio
n (
cm
-3)
Depth (um)
Si07
Ge0.3
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Assignment of electrically active defects
– Assignment of an impurity to an electrically active deep level
defect in ZnO
Quemener et al., Appl. Phys. Lett.,102, 232102 (2013)
SIMS (impurities)
DLTS
(electrically active defects)
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Outline
• Basic principle and characteristic features
• Physical processes
– Sputtering
– Ionization
• SIMS instrumentation
– Types of mass spectrometers
– Measurement modes: Mass spectra, Depth profiling, Ion
imaging
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Outline
• Basic principle and characteristic features
• Physical processes
– Sputtering
– Ionization
• SIMS instrumentation
– Types of mass spectrometers
– Measurement modes: Mass spectra, Depth profiling, Ion
imaging
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Secondary Ion Mass Spectrometry
0 20 40 60 80 100
1
10
100
1000
10000
Co
un
ts/s
ec
Mass (AMU)
Li
O
O2
K
Zn
ZnO
ZnO2
NaCr
1014
1015
1016
1017
1018
1019
1020
1021
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
PBAs
Ato
mic
co
nce
ntr
atio
n (
cm
-3)
Depth (um)
Si07
Ge0.3
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SIMS – Basic principle
0 100 200 300 400 500 6001
10
100
1000
10000
Inte
nsity (
coun
ts/s
ec)
Sputter time (sec)
Mass
spectrometer
Se
co
nd
ary
be
am
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Characteristic features
• Quantitative chemical analysis
• High detection sensitivity
– 1016 – 1012 atoms/cm3 (ppm-ppb)
– Can measure H
• Large dynamic range
– > 5 orders of magnitude
• Very high depth resolution
– Resolution < 20 Å can be obtained
• Ion microscopy
– Lateral resolution < 0.5 µm
(NanoSIMS ~ 60nm)
But,
• Limited to concentration <1-5%
• Samples must be vacuum
compatible
• Samples must be partially
conductive
• Destructive technique
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Outline
• Basic principle and characteristic features
• Physical processes
– Sputtering
– Ionization
• SIMS instrumentation
– Types of mass spectrometers
– Measurement modes: Mass spectra, Depth profiling, Ion
imaging
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SIMS – Basic principle
0 100 200 300 400 500 6001
10
100
1000
10000
Inte
nsity (
coun
ts/s
ec)
Sputter time (sec)
Mass
spectrometer
Se
co
nd
ary
be
am
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Ion – solid interaction
Matrix atom
Impurity
atom
Primary ion
Secondary ions are
accelerated by an
applied sample
voltage
Primary
beam
Energy is transferred from the energetic primary ions to
atoms in the sample. Some of these receive enough energy to
escape the sample.
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Sputtering
Sigmund P. Theory of Sputtering, Phys. Rev. 184(2), 383 (1969)
Sputtering Yield:
number of sputtered atoms per incoming ion
it
in
0
iti
E
ES
U
KES
8/3
n 383/1ln5.0 S
Sputtering yield:
50.05for 3
keV5.321
it
6/5
tiit
2/13/2
t
3/2
ititiit
ZZZZK
ZZZZMME
Mi, Zi: Ion mass and atomic number
Mt, Zt: Target mass and atomic number
U0 : Surface escape barrier in eV
Ei : Ion energy
Nuclear stopping cross-section: Sputtering is a multiple
collision process involving a
cascade of moving target
atoms, this cascade may
extend over a considerable
region inside the target.
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Sputtering
• Dependence of ion
0 20 40 60 80 100
1
2
3
No
rma
lize
d io
n y
ield
Ge content (%)
• Dependence of target on
sputtering yield: (Si1-xGex)
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• Sputtering of polycrystalline Fe surface
• The erosion rate is different for the different grains: Sputtering yield vary with the crystal orientation
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Sputtering
• Example of sputtering yield:
200 µm
0 100 200 300 400 500
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
De
pth
(µ
m)
Width (µm)
Current: 200 nA
Sputtering time: 700 sec
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Sputtering
• Example of sputtering yield:
200 µm
0 100 200 300 400 500
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
De
pth
(µ
m)
Width (µm)
Material removed: 1200200 µ3 = 410-8 cm3 ≈ 21015 atoms
Current: 200 nA
Sputtering time: 700 sec
Incoming ions: 20010-9A 6.241018 ions/C 700 sec = 9x1014 ions
Sputtering Yield = 2.2 atoms/ion
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Energy distribution of secondary ions
• Accelerated further (5kV) by an external field in SIMS
0 20 40 60 80 10010
100
1000
10000
100000
28Si
4
28Si
3
28Si
2
Se
co
nd
ary
in
ten
sity (
arb
. u
nit)
Energy (eV)
28Si
5kV
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Ionization
• Ion yield/ionization efficiency : The fraction of sputtered ions
that becomes ionized
• Ion yield can generally not be predicted theoretically
• Ion yield can vary by several orders of magnitude depending on
element and chemistry of the sputtered surface
• Oxygen on the surface will increase positive ion yield
• Cesium on the surface will increase negative ion yield
5kV
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Ionization
Negative secondary
Positive secondary vAC
vEC i
/exp YieldIon Negative
/exp YieldIon Positive
Ei A
C±: Constants
v: velocity perpendicular to surface
: work function
(Cs)
(O)
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Ionization
64 66 68 7010
0
101
102
103
104
105
106
107
Se
co
nd
ary
In
ten
sity(c
ps)
M/q (AMU)
Negative mode
Positive mode
Mass spectrum of ZnO, Zn peaks
64Zn (48.6%)
66Zn (27.9%)
67Zn (4.1%)
68Zn (18.8%)
70Zn (0.6%)
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Ionization
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
0 20 40 60 80 100
Norm
aliz
ed P
- - yie
ld
Ge concentration (%)
Phosphorus in Si1-xGex
• Limits the quantification procedure:
– SIMS is mainly a tool for measuring small concentrations
in a given matrix
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General Yield
• Measured intensity It for a specific target atom
TCYII ttPt
IP: Primary ion current
Y : Sputtering yield
(number of sputtered particles
per impinging primary ion)
[Ct]: Concentration of species t
t : Secondary ion formation and
survival probability
(ionization efficiency)
T: Instrument transmission function
t is highly
dependent on
species and
matrix
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General Yield
• Measured intensity It for a specific target atom
TCYII ttPt
IP: Primary ion current
Y : Sputtering yield
(number of sputtered particles
per impinging primary ion)
[Ct]: Concentration of species t
t : Secondary ion formation and
survival probability
(ionization efficiency)
T: Instrument transmission function
tIRSFC t
From
calibration
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Outline
• Basic principle and characteristic features
• Physical processes
– Sputtering
– Ionization
• SIMS instrumentation
– Types of mass spectrometers
– Measurement modes: Mass spectra, Depth profiling, Ion
imaging
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The SIMS instrument
Focused ion beam
Mass Spectrometer
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Types of SIMS instrument
• Instruments are usually classified by the type
of mass spectrometer:
– Quadrupole
• Low impact energy
– Time of Flight
• Simultaneous detection of many elements
• High transmission
• Measures large molecules
– Magnetic Sector
• High mass resolution
• High transmission
• Low detection limit
UiO-MiNaLab
since 2004
MiNaLab/NICE
since 2012
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Quadrupole SIMS
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Time of Flight SIMS
Time Of Flight-SIMS (TOF-SIMS)
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• Analysis is performed by a
short pulse length and
small spot size ion beam
• Sputtering is achieved by a
beam of reactive
species(e.g. O2 or Cs)
• Sputter beam and analysis
beam conditions are
optimized independently!
TOF-SIMS Analysis beam
Sputter beam
Analysis
gun
Extraction
Spectrum
Sputter
gun
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Magnetic sector - mass spectrometer
Lorenz’ force: BvEF qq
Centripetal force: rr
mv rF
2
2
e
0r
mvqE
Electrostatic
sector analyser
2
mr
mvqvB
Magnetic
sector analyser
2
0e
m
Er
Br
q
m
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Secondary ion mass spectrometry
0 100 200 300 400 500 6001
10
100
1000
10000
Inte
nsi
ty (
coun
ts/s
ec)
Sputter time (sec)
Depth
profile
0 20 40 60 80 1001
10
100
1000
10000
Co
un
ts/s
ec
Mass (AMU)
Mass
spectrum
Ion image
20 µm
2
0e
m
Er
Br
q
m
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Magnetic sector vs. TOF-SIMS
• Superior detection limit
(~ppb)
• Better depth limit (>50um)
• Better mass resolution
• Faster depth profiling
• Better lateral resolution
(<130nm)
• Simultaneous mass spectra
• Measures large molecules (<10
000 amu)
• Better for insulating samples
• Can provide molecular
information
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Instrumentation
ion sources
sample chamber
electrostatic sector
analyser magnetic sector
analyser
detectors
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0 20 40 60 80 100
1
10
100
1000
10000
Co
un
ts/s
ec
Mass (AMU)
Li
O
O2
K
Zn
ZnO
ZnO2
NaCr
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Mass spectrum
Mass spectrum of a ZnO-sample with traces of Li, Na, K, and Cr.
2
m
0 e
Brm
q E r
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Isotopic abundance
11,5 12,0 12,5 13,0 13,5
100
101
102
103
104
105
106
Se
co
nd
ary
in
ten
sity (
cp
s)
Mass (AMU)
Mass spectrum of
graphite
91.6%
8.4%
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Mass interference
• Several ions/ionic molecules have similar mass
to charge ratio:
10B - 30Si3+ = 10 amu
75As - 29Si30Si16O = 75 amu
31P - 30Si1H = 31 amu
The measured SIMS intensity is the sum of
the intensity from each elements
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40
Mass interference
• Several ions/ionic molecules have similar mass
to charge ratio:
10B - 30Si3+
2
0e
m
Er
Br
q
m
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41
Isotopic abundance
11,5 12,0 12,5 13,0 13,5
100
101
102
103
104
105
106
Se
co
nd
ary
in
ten
sity (
cp
s)
Mass (AMU)
Mass spectrum of
graphite
91.6%
8.4%
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42
Mass interference
• Several ions/ionic molecules have similar mass
to charge ratio:
10B - 30Si3+ Monitor 11B
75As - 29Si30Si16O
2
0e
m
Er
Br
q
m
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43
Energy selection
electrostatic sector
analyser
Sec
ondar
y b
eam
re E0
2
e
0r
mvqE
Increasing
kinetic
energy
Energy selection
slit
Ejection energy (eV) lo
g (
ion
in
ten
sit
y)
75As
29Si30Si16O
0 50 100
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44
Mass interference
• Several ions/ionic molecules have similar mass
to charge ratio:
10B - 30Si3+ Monitor 11B
75As - 29Si30Si16O Energy selection
31P - 30Si1H
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45
High mass resolution
30,85 30,90 30,95 31,00 31,05 31,10
100
1000
10000
Inte
nsity (
co
un
ts/s
ec)
Mass (AMU)
magnetic sector analyser
rm
B
2
mr
mvqvB
Exit slit
Discriminating between 31P and 30Si1H:
M(31P) = 30.973761
M(30Si1H) =30.98160
30,85 30,90 30,95 31,00 31,05 31,10
100
1000
10000
Inte
nsity (
co
un
ts/s
ec)
Mass (AMU)
31P
30Si
1H
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46
Mass interference
• Several ions/ionic molecules have similar mass
to charge ratio:
10B - 30Si3+ Monitor 11B
75As - 29Si30Si16O Energy selection
31P - 30Si1H High mass resolution
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47
Isotopes
12,96 13,00 13,04
100
101
102
103
104
105
Se
co
nd
ary
in
ten
sity (
cp
s)
Mass (AMU)
11,5 12,0 12,5 13,0 13,5
100
101
102
103
104
105
106
Se
co
nd
ary
in
ten
sity (
cp
s)
Mass (AMU)
11,5 12,0 12,5 13,0 13,5
100
101
102
103
104
105
106
Se
co
nd
ary
in
ten
sity (
cp
s)
Mass (AMU)
Mass spectrum of
graphite
11,5 12,0 12,5 13,0 13,5
100
101
102
103
104
105
106
Se
co
nd
ary
in
ten
sity (
cp
s)
Mass (AMU)
91.6%
8.4%
98.9%
1.1%
13C 12C1H
High mass
resolution
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Depth resolution
Limited by
• Surface roughness
• Sputter rate
– For large sputter rates or many elements
• Sputter rate/ measurement cycle
• Primary beam energy
– 10-15keV O2+/Cs+ 5-10 nm
– 500 eV O2+ ~2 nm
First 2-10nm gives an artificial
signal (Dynamic SIMS)
O2+/Cs+
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50
0 100 200 300 400 500 600 700
1
10
100
Inte
nsity (
co
un
ts/s
ec)
Sputter time (sec)
Calibration of depth profiles
”Raw” phosphorus profile
0 100 200 300 400 500
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
De
pth
(µ
m)
Width (µm)
Depth calibration
Sputter time: 700 sec
Depth: 9310 Å
Erosion rate: 13,3 Å/sec
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51
0 100 200 300 400 500 600 700
1
10
100
Inte
nsity (
co
un
ts/s
ec)
Sputter time (sec)
Calibration of depth profiles
TCYII ttPt
”Raw” phosphorus profile
t1 CS
S: Sensitivity factor
0 100 200 300 400 500 6000,1
1
10
100
1000
10000
Inte
nsity (
co
un
ts/s
ec)
Sputter time (sec)
Concentration calibration
Ion implanted sample:
P dose 1e15 P/cm2
sensitivity factor:
Relate the intensity to atomic concentration
xxIS
d
Dose
Sensitivity factor:
1 count/sec = 3,41015 P/cm3
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52
Calibration of depth profiles
0,0 0,2 0,4 0,6 0,81E14
1E15
1E16
1E17
1E18
P c
on
ce
ntr
atio
n (
cm
-3)
Depth (µm)
0 100 200 300 400 500 600 700
1
10
100
Inte
nsity (
co
un
ts/s
ec)
Sputter time (sec)
”Raw” phosphorus profile Calibrated
phosphorus profile
Erosion rate:
13,3 Å/sec
Sensitivity factor:
1 count/sec = 3,41015 P/cm3
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53
Ion imaging
Distribution of given atoms at
the surface
Primary beam
Secondary
beam to
detector
Intensity recorded as a
function of primary beam
position
Sample Surface
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54
Ion imaging
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‘Summary’
55