Secondary ion mass spectrometry · Sputter time (sec) Depth profile. 020 40 60 80 100 1 10 100 1000...

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

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Secondary ion mass spectrometrySecondary ion mass spectrometry

0 20 40 60 80 1001

Mass (AMU)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Depth (um)

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ChemicalChemical identificationidentification

Impurity

identification Isotope ratio

64Zn 66Zn67Zn

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Characterization of device structureCharacterization of device structure

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Depth (um)

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Characterization of solar cellCharacterization of solar cell

Characterization•Optimization of processing•Trouble shooting

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

th (µ

P Concentration (cm-3)

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Ion Ion imagingimaging

Na distribution

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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 studies

Material development

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Characteristic featuresCharacteristic features

Quantitative chemical analysis•

High detection sensitivity–

– 1013

atoms/cm3 (ppm-ppb)–

Can measure H

Large dynamic range–

> 5 orders of magnitude

Very high depth resolution–

Resolution of 20 Å

can be obtained

Ion microscopy–

Lateral resolution < 0.5 µm

But,•

Limited to concentration <5%

Samples must be vacuum compatible •

Samples must be partially conductive

Destructive technique

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Comparison to other techniquesComparison to other techniques

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OutlineOutline

Sputtering

Ionization

Diffusion studies

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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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SputteringSputtering

Sigmund P. Theory of Sputtering, Phys. Rev. 184(2), 383 (1969)

Sputtering Yield:number of sputtered atoms per incoming ion

ESUKES

8/3n 383/1ln5.0 S

Sputtering yield:

50.05for 3

keV5.321

2/13/2t

3/2ititiit

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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Dependence of ion

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

0 20 40 60 80 100

Ge content (%)

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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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Example of sputtering yield:

200 µm

0 100 200 300 400 500

th (µ

Width (µm)

Material removed: 1200200 µ3

= 410-8

Current: 200 nASputtering time: 700 sec

Incoming ions: 20010-9A

6.241018

ions/C

700 sec = 9x1014

Sputtering Yield = 2.2 atoms/ion

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Sputtering Sputtering –– Secondary ionsSecondary ions

Energy distribution of secondary ions

0 20 40 60 80 10010

100000

ity (a

Energy (eV)

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IonizationIonization

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

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IonizationIonizationNegative secondary

Positive secondary

/exp YieldIon Negative

/exp YieldIon Positive

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

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64 66 68 70100

M/q (AMU)

Negative modePositive mode

spectrum

of ZnO, Zn peaks.64Zn

(48.6%)66Zn

(27.9%)

(4.1%)

(18.8%)

(0.6%)

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0 20 40 60 80 100

P- - y

Ge concentration (%)

Phosphorus in Si1-x Gex

Limits the quantification procedure:–

SIMS is mainly a tool for measuring small concentrations in a given matrix

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General YieldGeneral 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

is highly dependent on species and matrix

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OutlineOutline

Sputtering

Ionization

Diffusion studies

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The SIMS instrumentThe SIMS instrument

Focused

ion beamMass

Spectrometer

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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•

Measures large molecules

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

amre rm

Lorenz’ force: BvEF qq

Centripetal force:rr

mv rF2

Electrostaticsector analyser

mrmvqvB

Magneticsector analyser

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Secondary ion mass spectrometrySecondary ion mass spectrometry

Brmq E r

sample

detector

ion source

electrostatic sector analyser

Primary beam

0 100 200 300 400 500 6001

Sputter time (sec)

Depth profile

0 20 40 60 80 1001

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 spectrumspectrum

0 20 40 60 80 1001

Mass (AMU)

spectrum

of a ZnO-sample

traces of Li, Na, K, and Cr.

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MassMass InterferenceInterference

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

ity (c

Mass (AMU)

spectrum of graphite

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Mass interferenceMass interference

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

30Si3+ Monitor 11B

75As - 29Si30Si16O

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Energy selectionEnergy selectionelectrostatic sector analyser

Increasing kinetic energy

Energy selection slit

Ejection energy (eV)lo

29Si30Si16O

0 50 100

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MassMass interferenceinterference

Several

ions/ionic

molecules

have similar mass

to charge

ratios:

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

Mass (AMU)

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

Mass (AMU)

30Si1H

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Mass interferenceMass interference

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

30Si3+ Monitor 11B

75As - 29Si30Si16O Energy selection

31P - 30Si1H High mass resolution

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SIMS SIMS –– depth profilingdepth profiling

0 100 200 300 400 500 6001

Sputter time (sec)

Primary beam

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0 100 200 300 400 500 600 700

Sputter time (sec)

CalibrationCalibration of of depthdepth profilesprofiles

”Raw”

phosphorus

profile

0 100 200 300 400 500

th (µ

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

Sputter time (sec)

CalibrationCalibration of of depthdepth profilesprofiles

TCYII ttPt

”Raw”

phosphorus

profile

S: Sensitivity

factor

0 100 200 300 400 500 6000,1

Sputter time (sec)

Concentration calibration

Ion implanted sample:P dose 1e15 P/cm2

sensitivity factor:Relate the intensity to atomic concentration

d Dose

Sensitivity factor:1 count/sec = 3,41015

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Calibration of depth profilesCalibration of depth profiles

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

tratio

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

Sputter time (sec)

”Raw”

phosphorus profile

Calibratedphosphorus profile

Erosion rate:13,3 Å/sec

Sensitivity factor:1 count/sec = 3,41015

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Ion imagingIon imaging

Distribution of given atoms at the surface

Primary

Secondary beam to detector

Intensity recorded as a function of primary beam position

Sample Surface

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Ion Ion imagingimaging

Secondary ion mass spectrometry · Sputter time (sec) Depth profile. 020 40 60 80 100 1 10 100 1000...

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Transcript of Secondary ion mass spectrometry · Sputter time (sec) Depth profile. 020 40 60 80 100 1 10 100 1000...

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