Secondary Ion Mass Spectrometry
(SIMS)
Thomas Sky
1
Characterization of solar cells
1E16
1E17
1E18
1E19
1E20
0,0
0,2
Depth (µm)
0,4
0,6
0,8
1,0
1,2
-3
P Concentration (cm )
Characterization
•Optimization of processing
•Trouble shooting
2
Characterization of device structure
21
10
Si Ge
07
0.3
10
-3
Atomic concentration (cm )
20
19
10
P
B
As
18
10
1017
1016
1015
14
10
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Depth (um)
3
Assignment of electrically active defects
– Assignment of an impurity to an electrically active deep level
defect in ZnO
SIMS
DLTS
(impurities)
(electrically active defects)
Quemener et al., Appl. Phys. Lett.,102, 232102 (2013)
Outline
• Basic principle and characteristic features
• Physical processes
– Sputtering
– Ionization
• SIMS instrumentation
– Types of mass spectrometers
– Measurement modes: Mass spectra, Depth profiling, Ion
imaging
5
Outline
• Basic principle and characteristic features
• Physical processes
– Sputtering
– Ionization
• SIMS instrumentation
– Types of mass spectrometers
– Measurement modes: Mass spectra, Depth profiling, Ion
imaging
6
7
Secondary Ion Mass Spectrometry
O
Zn
O2
10000
Counts/sec
1000
Cr
Na
K
ZnO
Li
100
ZnO2
10
1
0
20
40
60
80
100
Mass (AMU)
21
10
Si Ge
07
0.3
-3
Atomic concentration (cm )
20
10
19
10
P
B
As
18
10
1017
1016
1015
14
10
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Depth (um)
8
Mass
spectrometer
Secondary beam
Intensity (counts/sec)
SIMS – Basic principle
10000
1000
100
10
1
0
100
200
300
400
500
600
Sputter time (sec)
9
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
But,
• Limited to concentration <1-5%
• Samples must be vacuum
compatible
• Samples must be partially
conductive
• Destructive technique
• Ion microscopy
– Lateral resolution < 0.5 µm
(NanoSIMS ~ 60nm)
1
Outline
• Basic principle and characteristic features
• Physical processes
– Sputtering
– Ionization
• SIMS instrumentation
– Types of mass spectrometers
– Measurement modes: Mass spectra, Depth profiling, Ion
imaging
11
Mass
spectrometer
Secondary beam
Intensity (counts/sec)
SIMS – Basic principle
10000
1000
100
10
1
0
100
200
300
400
500
600
Sputter time (sec)
12
Ion – solid interaction
Matrix atom
Impurity
atom
Primary
beam
Secondary ions are
accelerated by an
applied sample
voltage
Primary ion
Energy is transferred from the energetic primary ions to
atoms in the sample. Some of these receive enough energy to
escape the sample.
13
Sputtering
Sputtering Yield:
number of sputtered atoms per incoming ion
K it  Ei 
S Ei  
S n  
U 0  Eit 
Sputtering yield:
Sputtering is a multiple
collision process involving a
cascade of moving target
atoms, this cascade may
extend over a considerable
region inside the target.
Nuclear stopping cross-section:

Sn    0.5 ln 1       / 383

Eit  1  M i M t Z i Z t Z i2 / 3  Z t2 / 3
K it  Z i Z t 
5/ 6
3/ 8


1/ 2
32.5
keV
3 for 0.05  Z t Z i  5
Mi, Zi: Ion mass and atomic number
Mt, Zt: Target mass and atomic number
U0 : Surface escape barrier in eV
Ei : Ion energy
Sigmund P. Theory of Sputtering, Phys. Rev. 184(2), 383 (1969)
14
Sputtering
• Dependence of target on
sputtering yield: (Si1-xGex)
3
Normalized ion yield
• Dependence of ion
2
1
0
20
40
60
80
100
Ge content (%)
• Sputtering of polycrystalline Fe
surface
• The erosion rate is different for the
different grains: Sputtering yield vary
with the crystal orientation
15
Sputtering
• Example of sputtering yield:
0,0
Depth (µm)
Current: 200 nA
Sputtering time: 700 sec
-0,2
-0,4
-0,6
-0,8
-1,0
0
200 µm
100
200
300
400
500
Width (µm)
16
Sputtering
• Example of sputtering yield:
0,0
Depth (µm)
Current: 200 nA
Sputtering time: 700 sec
-0,2
-0,4
-0,6
-0,8
-1,0
0
200 µm
100
200
300
400
500
Width (µm)
Material removed: 1200200 µ3 = 410-8 cm3 ≈ 21015 atoms
Incoming ions: 20010-9A  6.241018 ions/C  700 sec = 9x1014 ions
Sputtering Yield = 2.2 atoms/ion
17
Energy distribution of secondary ions
Secondary intensity (arb. unit)
100000
5kV
10000
28
Si
28
1000
Si2
28
Si3
28
Si4
100
10
0
20
40
60
80
100
Energy (eV)
• Accelerated further (5kV) by an external field in SIMS
18
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
19
Ionization


Negative Ion Yield  exp  C   A / v 
Positive Ion Yield  exp  C  Ei    / v
Positive secondary
(O)
Negative secondary
(Cs)

C±: Constants
v: velocity perpendicular to surface
: work function
Ei
A
20
Ionization
Mass spectrum of ZnO, Zn peaks
7
Secondary Intensity(cps)
10
64Zn
(48.6%)
66Zn
6
10
68Zn
(27.9%)
67Zn
5
10
(18.8%)
Positive mode
Negative mode
(4.1%)
70Zn
(0.6%)
4
10
3
10
2
10
1
10
0
10
64
66
68
M/q (AMU)
70
21
Ionization
Phosphorus in Si1-xGex
1.1
-
Normalized P - yield
1
0.9
0.8
0.7
0.6
0.5
0.4
0
20
40
60
80
100
Ge concentration (%)
• Limits the quantification procedure:
– SIMS is mainly a tool for measuring small concentrations
in a given matrix
22
General Yield
• Measured intensity It for a specific target atom
I t  I PY Ct  tT
I P:
Y:
[Ct]:
t :
T:
Primary ion current
Sputtering yield
(number of sputtered particles
per impinging primary ion)
Concentration of species t
Secondary ion formation and
survival probability
(ionization efficiency)
Instrument transmission function
t is highly
dependent on
species and
matrix
23
General Yield
• Measured intensity It for a specific target atom
I t  I PY Ct  tT
Ct   RSF  I t
From
calibration
IP:
Y:
[Ct]:
t :
T:
Primary ion current
Sputtering yield
(number of sputtered particles
per impinging primary ion)
Concentration of species t
Secondary ion formation and
survival probability
(ionization efficiency)
Instrument transmission function
24
Outline
• Basic principle and characteristic features
• Physical processes
– Sputtering
– Ionization
• SIMS instrumentation
– Types of mass spectrometers
– Measurement modes: Mass spectra, Depth profiling, Ion
imaging
25
The SIMS instrument
Mass Spectrometer
Focused ion beam
26
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
MiNaLab/NICE
• High transmission
since 2012
• Measures large molecules
– Magnetic Sector
• High mass resolution
• High transmission
• Low detection limit
UiO-MiNaLab
since 2004
Quadrupole SIMS
28
Time Of Flight-SIMS
Time of Flight SIMS
(TOF-SIMS)
29
TOF-SIMS
• 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!
Analysis beam
Analysis
gun
Extraction
Spectrum
Sputter
gun
Sputter beam
Magnetic sector - mass spectrometer
Magnetic
sector analyser
Electrostatic
sector analyser
mv
qE0 
re
Centripetal force:
Lorenz’ force:
mv2 r
F
r r
2
mv
qvB 
rm
m rm B 

q
re E0
2
2
F  qE  q  v  B 
33
Secondary ion mass spectrometry
Counts/sec
10000
Mass
spectrum
1000
100
10
1
0
20
40
60
80
100
Intensity (counts/sec)
Mass (AMU)
10000
Depth
profile
1000
100
10
1
0
100
200
300
400
500
600
Sputter time (sec)
m rm B 

q
re E0
20 µm
2
Ion image
34
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
ion sources
electrostatic sector
Instrumentation
analyser
magnetic sector
analyser
detectors
sample chamber
36
Mass spectrum
O
10000
Counts/sec
Zn
O2
1000
Cr
Na
K
m  Brm 

q
E0 re
ZnO
Li
100
2
ZnO2
10
1
0
20
40
60
80
100
Mass (AMU)
Mass spectrum of a ZnO-sample with traces of Li, Na, K, and Cr.
37
Isotopic abundance
6
91.6%
Secondary intensity (cps)
10
8.4%
5
10
4
10
3
10
2
10
1
10
0
10
11,5
12,0
12,5
Mass (AMU)
13,0
13,5
Mass spectrum of
graphite
38
Mass interference
• Several ions/ionic molecules have similar mass
to charge ratio:
10B
75As
31P
-
30Si3+
= 10 amu
29Si30Si16O
= 75 amu
-
30Si1H
= 31 amu
The measured SIMS intensity is the sum of
the intensity from each elements
39
Mass interference
• Several ions/ionic molecules have similar mass
to charge ratio:
10B
-
30Si3+
m rm B 

q
re E0
2
40
Isotopic abundance
6
91.6%
Secondary intensity (cps)
10
8.4%
5
10
4
10
3
10
2
10
1
10
0
10
11,5
12,0
12,5
Mass (AMU)
13,0
13,5
Mass spectrum of
graphite
41
Mass interference
• Several ions/ionic molecules have similar mass
to charge ratio:
10B
75As
-
-
30Si3+
Monitor 11B
29Si30Si16O
m rm B 

q
re E0
2
42
Energy selection
electrostatic sector
analyser
Increasing
kinetic
energy
E0
Secondary beam
re
75As
2
log (ion intensity)
mv
qE0 
re
Energy selection
slit
29Si30Si16O
0
50
100
Ejection energy (eV)
43
Mass interference
• Several ions/ionic molecules have similar mass
to charge ratio:
10B
75As
31P
-
-
30Si3+
29Si30Si16O
Monitor 11B
Energy selection
30Si1H
44
High mass resolution
magnetic sector analyser
Discriminating between
31P and 30Si1H:
B
M(31P) = 30.973761
M(30Si1H) =30.98160
rm
Exit slit
mv
qvB 
rm
2
Intensity (counts/sec)
10000
31
P
1000

30
1
Si H
100
30,85
30,90
30,95
31,00
31,05
31,10
Mass (AMU)
45
Mass interference
• Several ions/ionic molecules have similar mass
to charge ratio:
10B
75As
31P
-
-
30Si3+
29Si30Si16O
30Si1H
Monitor 11B
Energy selection
High mass resolution
46
Isotopes
98.9%
6
5
10
10
3
2
10
1
10
0
10
11,5
8.4%
5
10
4
10
12,0
12,5
13,0
Mass (AMU)
3
2
10
1
10
0
10
13,5
5
10
10
Secondary intensity (cps)
4
10
10
91.6%
6
Secondary intensity (cps)
Secondary intensity (cps)
10
11,5
4
10
High mass
1.1%
resolution
3
10
13C
2
10
1
10
12,0
12,5
12
13,0C
1H
13,5
Mass (AMU)
Mass spectrum of
graphite
0
10
12,96
13,00
Mass (AMU)
13,04
47
SIMS – depth profiling
Primary beam
Intensity (counts/sec)
m  Brm 

q
E0 re
2
10000
1000
100
10
1
0
100
200
300
400
500
600
Sputter time (sec)
48
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+
Calibration of depth profiles
Depth calibration
0,0
Depth (µm)
Intensity (counts/sec)
”Raw” phosphorus profile
100
-0,2
-0,4
-0,6
-0,8
10
-1,0
0
1
100
200
300
400
500
Width (µm)
0
100 200 300 400 500 600 700
Sputter time (sec)
Sputter time: 700 sec
Depth: 9310 Å
Erosion rate: 13,3 Å/sec
50
Calibration of depth profiles
Concentration calibration
Ion implanted sample:
P dose 1e15 P/cm2
Intensity (counts/sec)
Intensity (counts/sec)
”Raw” phosphorus profile
100
10
1
0
100
10
1
0,1
0
100 200 300 400 500 600
sensitivity factor:
Relate the intensity to atomic concentration
Sputter time (sec)
 S C 
S: Sensitivity factor
1000
Sputter time (sec)
100 200 300 400 500 600 700
I t  I PY Ct  tT  1
10000
t
S
Dose
 I x  dx
Sensitivity factor:
1 count/sec = 3,41015 P/cm3
51
Calibration of depth profiles
1E18
-3
Intensity (counts/sec)
P concentration (cm )
Calibrated
phosphorus profile
”Raw” phosphorus profile
100
10
1
0
100 200 300 400 500 600 700
Sputter time (sec)
Erosion rate:
13,3 Å/sec
1E17
1E16
1E15
1E14
0,0
0,2
0,4
0,6
0,8
Depth (µm)
Sensitivity factor:
1 count/sec = 3,41015 P/cm3
52
Ion imaging
Intensity recorded as a
function of primary beam
position
Primary beam
Secondary
beam to
detector
Distribution of given atoms at
the surface
Sample Surface
53
Ion imaging
54
‘Summary’
55