Secondary Ion Mass Spectrometry...

1

Secondary Ion Mass Spectrometry

(SIMS)

Thomas Sky

2

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)

3

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

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)

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

Outline

• Basic principle and characteristic features

• Physical processes

– Sputtering

– Ionization

• SIMS instrumentation

– Types of mass spectrometers

– Measurement modes: Mass spectra, Depth profiling, Ion

imaging

7

8

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

9

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

10

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

11

Outline

• Basic principle and characteristic features

• Physical processes

– Sputtering

– Ionization

• SIMS instrumentation

– Types of mass spectrometers

– Measurement modes: Mass spectra, Depth profiling, Ion

imaging

12

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

13

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.

14

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.

15

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)

15

• Sputtering of polycrystalline Fe surface

• The erosion rate is different for the different grains: Sputtering yield vary with the crystal orientation

16

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

17

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

18

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

19

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

20

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)

21

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%)

22

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

23

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

24

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

25

Outline

• Basic principle and characteristic features

• Physical processes

– Sputtering

– Ionization

• SIMS instrumentation

– Types of mass spectrometers

– Measurement modes: Mass spectra, Depth profiling, Ion

imaging

26

The SIMS instrument

Focused ion beam

Mass Spectrometer

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

28

Quadrupole SIMS

29

Time of Flight SIMS

Time Of Flight-SIMS (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!

TOF-SIMS Analysis beam

Sputter beam

Analysis

gun

Extraction

Spectrum

Sputter

gun

33

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

34

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

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

36

Instrumentation

ion sources

sample chamber

electrostatic sector

analyser magnetic sector

analyser

detectors

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

37

Mass spectrum

Mass spectrum of a ZnO-sample with traces of Li, Na, K, and Cr.

2

m

0 e

Brm

q E r

38

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%

39

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

40

Mass interference

• Several ions/ionic molecules have similar mass

to charge ratio:

10B - 30Si3+

2

0e

m

Er

Br

q

m

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%

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

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

44

Mass interference

• Several ions/ionic molecules have similar mass

to charge ratio:

10B - 30Si3+ Monitor 11B

75As - 29Si30Si16O Energy selection

31P - 30Si1H

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

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

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

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+

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

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

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

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

54

Ion imaging

‘Summary’

55