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SIM

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Secondary ion mass spectrometrySecondary ion mass spectrometry (SIMS)(SIMS)

Lasse

Vines

MEN

A3100 -

SIM

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2

Characterization of solar cellCharacterization of solar cell

Characterization•Optimization of processing•Trouble shooting

1,2

1,0

0,8

0,6

0,4

0,2

0,01E16 1E17 1E18 1E19 1E20

Dep

th (µ

m)

P Concentration (cm-3)

MEN

A3100 -

SIM

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3

Characterization of device structureCharacterization of device structureExample: Integrated circuits –

operational amplifier

741

op-amp

MEN

A3100 -

SIM

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Characterization of device structureCharacterization 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

Atom

ic c

once

ntra

tion

(cm

-3)

Depth (um)

Si07

Ge0.3

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A3100 -

SIM

S

5

Secondary ion mass spectrometrySecondary ion mass spectrometry

0 20 40 60 80 1001

10

100

1000

10000

Cou

nts/

sec

Mass (AMU)

Li

OO

2

K

Zn

ZnO

ZnO2

Na Cr

1014

1015

1016

1017

1018

1019

1020

1021

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

PBAs

Atom

ic c

once

ntra

tion

(cm

-3)

Depth (um)

Si07

Ge0.3

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A3100 -

SIM

S

6

OutlineOutline

•

Characteristic features–

Comparison with other techniques

•

Physical processes–

Sputtering

–

Ionization

•

SIMS instrumentation–

Types of mass spectrometers

–

Measurement modes: Mass spectra, Depth profiling, Ion imaging

•

Examples of applications–

Diffusion in semiconductors

–

Identification of surface contamination

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A3100 -

SIM

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OutlineOutline

•

Characteristic features–

Comparison with other techniques

•

Physical processes–

Sputtering

–

Ionization

•

SIMS instrumentation–

Types of mass spectrometers

–

Measurement modes: Mass spectra, Depth profiling, Ion imaging

•

Examples of applications–

Diffusion in semiconductors

–

Identification of surface contamination

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A3100 -

SIM

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Characteristic featuresCharacteristic features

•

Quantitative chemical analysis•

High detection sensitivity–

1016

– 1013

atoms/cm3 (ppm-ppb)

•

Large dynamic range–

> 5 orders of magnitude

•

Very high depth resolution–

Resolution of 20 Å

can be obtained

•

Ion microscopy–

Lateral resolution < 0.5 µ

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A3100 -

SIM

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Comparison to other techniquesComparison to other techniques

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A3100 -

SIM

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10

OutlineOutline

•

Characteristic features–

Comparison with other techniques

•

Physical processes–

Sputtering

–

Ionization

•

SIMS instrumentation–

Types of mass spectrometers

–

Measurement modes: Mass spectra, Depth profiling, Ion imaging

•

Examples of applications–

Diffusion in semiconductors

–

Identification of surface contamination

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A3100 -

SIM

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11

Ion Ion –– solid interactionsolid interaction

Matrix atom

Impurityatom

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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SIM

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SputteringSputtering

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

ESUKES

( ) ( ) ( ){ }8/3n 383/1ln5.0 ξξξξ ++=S

Sputtering yield:

( ) ( ) [ ]( ) 50.05for 3

keV5.321

it6/5

tiit

2/13/2t

3/2ititiit

≤≤≈

++=

ZZZZK

ZZZZMME

Mi , Zi : Ion mass and atomic numberMt , Zt : Target mass and atomic numberU0 : Surface escape barrier in eVEi : 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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SputteringSputtering

•

Dependence of ion

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SputteringSputtering

•

Example of dependence of target on sputtering yield: (Si1-x

Gex

)

0 20 40 60 80 100

1

2

3

Nor

mal

ized

ion

yiel

d

Ge content (%)

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SIM

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SputteringSputtering

•

Example of sputtering yield:

200 µm

0 100 200 300 400 500

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

Dep

th (µ

m)

Width (µm)

Material removed: 1×200×200 µ3

= 4×10-8

cm3

≈

2×1015

atoms

Current: 200 nASputtering time: 700 sec

Incoming ions: 200×10-9A ×

6.24×1018

ions/C ×

700 sec = 9x1014

ions

Sputtering Yield = 2.2 atoms/ion

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SputteringSputtering

•

Example of 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 Sputtering –– Secondary ionsSecondary ions

•

Energy distribution of secondary ions

0 20 40 60 80 10010

100

1000

10000

100000

28Si4

28Si3

28Si2

Sec

onda

ry in

tens

ity (a

rb. u

nit)

Energy (eV)

28Si

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IonizationIonization

•

Ion yield: 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

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IonizationIonizationNegative secondary

Positive secondary

( )( )( )( )vAC

vEC i

/exp YieldIon Negative

/exp YieldIon Positive

−−∝

−−∝−

+

ϕ

ϕ

EiA

C±: Constantsv: velocity perpendicular to surfaceϕ: work function

(Cs)

(O)

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IonizationIonization

64 66 68 70100

101

102

103

104

105

106

107

Sec

onda

ry In

tens

ity(c

ps)

M/q (AMU)

Negative modePositive 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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IonizationIonization

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 20 40 60 80 100

Nor

mal

ized

P- - y

ield

Ge concentration (%)

Phosphorus in Si1-x Gex

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General YieldGeneral Yield

•

Measured intensity It for a specific target atom

[ ] TCYII ttPt γ=

IP : Primary ion current Y : Sputtering yield

(number of sputtered particles per impinging primary ion)

[Ct ]: Concentration of species t γt : Secondary ion formation and

survival probability (ionization efficiency)

T: Instrument transmission function

γt

is highly dependent on species and matrix

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OutlineOutline

•

Characteristic features–

Comparison with other techniques

•

Physical processes–

Sputtering

–

Ionization

•

SIMS instrumentation–

Types of mass spectrometers

–

Measurement modes: Mass spectra, Depth profiling, Ion imaging

•

Examples of applications–

Diffusion in semiconductors

–

Identification of surface contamination

MEN

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SIM

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The SIMS instrumentThe SIMS instrument

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The SIMS instrumentThe SIMS instrument

•

Instruments are usually classified by the type of mass spectrometer:–

Time of Flight•

Simultaneous detection of many elements

•

High transmission

–

Quadrupole•

Low impact energy

–

Magnetic Sector•

High mass resolution

•

High transmission•

Low detection limit

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Time of Time of FlightFlight SIMSSIMS

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QuadrupoleQuadrupole SIMSSIMS

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Magnetic sector Magnetic sector -- mass mass spectrometer spectrometer

sample

detector

ion source

electrostatic sector analyser

magnetic sector analyser

Primary beam

Seco

ndar

y be

amre rm

E0B

Lorenz’ force: ( )BvEF ×+= qq

Centripetal force:rr

mv rF2

−=

2

e0 r

mvqE =

Electrostaticsector analyser

2

mrmvqvB =

Magneticsector analyser

( ) 2

0e

m

ErBr

qm

=

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SIM

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Secondary ion mass spectrometrySecondary ion mass spectrometry

( )2m

0 e

Brmq E r

=

sample

detector

ion source

electrostatic sector analyser

magnetic sector analyser

Primary beam

Seco

ndar

y be

am

re rm

E0B

0 100 200 300 400 500 6001

10

100

1000

10000

Inte

nsity

(cou

nts/

sec)

Sputter time (sec)

Depth profile

0 20 40 60 80 1001

10

100

1000

10000

Cou

nts/

sec

Mass (AMU)

Mass spectrum

Ion image

20 µm

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InstrumentationInstrumentationion sources

sample chamber

electrostatic

sector

analysermagnetic

sector

analyser

detectors

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MassMass InterferenceInterference

11,5 12,0 12,5 13,0 13,5

100

101

102

103

104

105

106

Sec

onda

ry in

tens

ity (c

ps)

Mass (AMU)

Mass

spectrum of graphite

91.6%

8.4%

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Mass interferenceMass interference

•

Several ions/ionic molecules have similar mass to charge ratios:

10B -

30Si3+ Monitor 11B

75As - 29Si30Si16O

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Energy selectionEnergy selectionelectrostatic sector analyser

Seco

ndar

y be

am

re

E0

2

e0 r

mvqE =

Increasing kinetic energy

Energy selection slit

Ejection energy (eV)lo

g (

ion

in

ten

sity

)

75As

29Si30Si16O

0 50 100

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MassMass interferenceinterference

•

Several

ions/ionic

molecules

have similar mass

to charge

ratios:

10B -

30Si3+ Monitor 11B

75As - 29Si30Si16O Energy selection

31P - 30Si1H

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HighHigh massmass resolutionresolution

30,85 30,90 30,95 31,00 31,05 31,10

100

1000

10000

Inte

nsity

(cou

nts/

sec)

Mass (AMU)

magnetic sector analyser

rm

B

2

mrmvqvB =

Exit slit

Discriminating

between

31P and 30Si1H:

M(31P) = 30.973761M(30Si1H) =30.98160

30,85 30,90 30,95 31,00 31,05 31,10

100

1000

10000

Inte

nsity

(cou

nts/

sec)

Mass (AMU)

31P

30Si1H

ΔΜ/Μ = 4000

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Mass interferenceMass interference

•

Several ions/ionic molecules have similar mass to charge ratios:

10B -

30Si3+ Monitor 11B

75As - 29Si30Si16O Energy selection

31P - 30Si1H High mass resolution

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IsotopesIsotopes

12,96 13,00 13,04

100

101

102

103

104

105

Sec

onda

ry in

tens

ity (c

ps)

Mass (AMU)

11,5 12,0 12,5 13,0 13,5

100

101

102

103

104

105

106

Sec

onda

ry in

tens

ity (c

ps)

Mass (AMU)

11,5 12,0 12,5 13,0 13,5

100

101

102

103

104

105

106

Sec

onda

ry in

tens

ity (c

ps)

Mass (AMU)

Mass

spectrum of graphite

11,5 12,0 12,5 13,0 13,5

100

101

102

103

104

105

106

Sec

onda

ry in

tens

ity (c

ps)

Mass (AMU)

91.6%

8.4%

98.9%

1.1%

13C 12C1H

High

mass

resolution

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MassMass spectrumspectrum

0 20 40 60 80 1001

10

100

1000

10000

Cou

nts/

sec

Mass (AMU)

Li

OO2

K

Zn

ZnO

ZnO2

Na Cr

Mass

spectrum

of a ZnO-sample

with

traces of Li, Na, K, and Cr.

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SIMS SIMS –– depth profilingdepth profiling

0 100 200 300 400 500 6001

10

100

1000

10000

Inte

nsity

(cou

nts/

sec)

Sputter time (sec)

Primary beam

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A3100 -

SIM

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40

0 100 200 300 400 500 600 700

1

10

100

Inte

nsity

(cou

nts/

sec)

Sputter time (sec)

CalibrationCalibration of of depthdepth profilesprofiles

”Raw”

phosphorus

profile

0 100 200 300 400 500

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

Dep

th (µ

m)

Width (µm)

Depth calibration

Sputter time: 700 sec Depth: 9310 Å

Erosion rate:13,3 Å/sec

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0 100 200 300 400 500 600 700

1

10

100

Inte

nsity

(cou

nts/

sec)

Sputter time (sec)

CalibrationCalibration of of depthdepth profilesprofiles

[ ] TCYII ttPt γ=

”Raw”

phosphorus

profile

( )[ ]t1 CS=

S: Sensitivity

factor

0 100 200 300 400 500 6000,1

1

10

100

1000

10000

Inte

nsity

(cou

nts/

sec)

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 = 3,4×1015

P/cm3

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Calibration of depth profilesCalibration of depth profiles

0,0 0,2 0,4 0,6 0,81E14

1E15

1E16

1E17

1E18

P co

ncen

tratio

n (c

m-3)

Depth (µm)0 100 200 300 400 500 600 700

1

10

100

Inte

nsity

(cou

nts/

sec)

Sputter time (sec)

”Raw”

phosphorus profile

Calibratedphosphorus profile

Erosion rate:13,3 Å/sec

Sensitivity factor:1 count/sec = 3,4×1015

P/cm3

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Ion imagingIon 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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Examples of ApplicationsExamples of Applications

•

Depth profiling–

Dopant

diffusion in semiconductors

–

Isotope enriched superstructures

•

Ion imaging–

Characterizing contacts on sample surface

–

Impurity after processing

•

Mass spectrum–

N doping of MOCVD grown ZnO

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Radiation enhanced diffusion of B in SiRadiation enhanced diffusion of B in Si

Irradiation

2 MeV

H+

1 Hour, 580°C

Depth (µm)

B c

once

ntra

tion

(cm

-3)

0.5 1.0 1.5

1015

1016

1017

Fick

model:

DB = 1×10-14 cm2/s

Lévêque

et al. J. Appl. Phys. 89, 5400 (2001)

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Isotope enriched superstructuresIsotope enriched superstructuresAs in-diffusion in a Ge

isotope enriched superstructure

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Ion imagingIon imaging

30 µm

200 µm

Si Cu Na

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Ion Ion imagingimaging

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N doping of MOCVD grown N doping of MOCVD grown ZnOZnO

•

Main impurities:–

C, Al, Si, and Ca

ZnO Al2 O3

•

Profiles of contamination–

Ca is introduced through interface and from the surface

•

New optimized samples have reduced contamination–

Al still present

•

Intentional uniform N doping achieved