chemical and physical characterization s-69.4123 postgraduate course in electron physics i p...

Chemical and Physical Characterization

S-69.4123 Postgraduate Course in Electron Physics I P

16.11.2011

Introduction

Probing with – Electron beams

– Ion beams

– X-rays

Measurands – Imaging

– Composition

– Impurities

– Crystal structure

– Thickness

Outline

Introduction

Electron beam techniques

Ion beam techniques

X-ray techniques

Conclusions

Loosely bound electrons kicked out from the sample (E < 50eV)

High-E beam -> several SE:s for each incident e-

Secondary electrons

Electrons from electron gun

The beam is focused on the sample

Secondary electrons ejected from the sample

Scanning electron microscope (SEM)

Raster scan over the sample

Secondary electrons from each spot

-> intensity for each spot

-> image


Electron wavelength

10 kV acceleration voltage ->

(compare to optical λ ≈ 400 nm)

(Rayleigh: )

However:

Practical resolution ~1 nm (SEM in Micronova)1

1: http://www.speciation.net/Database/Instruments/Carl-Zeiss-AG/SUPRA-40-;i666


Low-E electrons escape only from surface

X-rays from larger area

Z+ -> depth –

E+ -> depth +

SEM signals and scattering

Auger electron spectroscopy (AES)

Auger electrons:

Characteristic energies -> element identification

Low energy (30-3000 eV)-> surface probing

Auger electron spectroscopy (AES)

Scanning -> resolution ~10nm

Chemical analysis: – Detectable elements Z = 3 and up

– Detection limit 0.1 – 1%

– Chemical information (Si vs SiO2)

SEM, Electron microprobe

X-ray generation:

Similar to Auger


X-ray energies characteristic for elements

Detection limit 102 – 104 ppm

Detection: – X-rays create electron-hole pairs in a detector crystal

– Which are detected and counted

– Nehp ~ X-ray energy


Detector types: – Fast: Energy-dispersive spectrometer (EDS)

– Accurate: Wavelength-dispersive spectrometer (WDS)

Energy – element identificationIntensity – density

Transmission electron microscopy (TEM)

Electron gun

Focused on a thin sample

Electrons pass through -> scattering in the sample-> image or diffraction pattern

Atomic scale resolution

Diffraction pattern –> crystal structure and direction

Also electron microprobe available

EELS for accurate analysis

TEM Images: Nature nanotechnology [1748-3387] Caroff, P v:2008 vol:4 iss:1 s:50

Transmission electron microscopy (TEM)

Electron microscopy

Advantages – High resolution

– High depth of field in SEM

– Analysis tools integrable

– Crystalline structure in TEM

Drawbacks– Sample charging –> insulating samples difficult to probe

– TEM sample preparation (sample thickness ~≤200 nm)

– Beam damage especially in TEM

– Vacuum required

Secondary Ion Mass Spectrometry (SIMS)

Sample is bombarded with an ion beam

Sputtering

Fraction of sputtered material ionized

Measured by mass spectrometer

Counts for mass/charge ratios

Possible overlap (e.g. N, O, H, C + molecules often present)

Sputtering -> depth profiling – Initially distorted by sputtering yield


Two common modes: – Static: surface probed for complete mass spectrum

– Dynamic: one mass/charge ratio is probed in a depth scan (sputtering ~10µm/h)

Static scan: Surface and interface analysis [0142-2421] Ogaki, R v:2008 vol:40 iss:8 s:1202 Depth profile: Applied physics letters [0003-6951] Zolper, J C v:1996 vol:68 iss:14 s:1945


All elements detectable

Detection limit 1014 – 1018 cm-3 (~0.1 – 100 ppm)-> the most sensitive beam technique

Depth profiling

Lateral resolution 0.5 – 100 µm, depth 5 - 10 nm

Cons: destructive, high vacuum needed, crater wall effects, preferential sputtering, knock-on effects...


High-E ions incident on the sample

Ions collide with sample atoms losing energy

Energy of backscattered ions measured

Energy loss depends on the material

Additional energy loss due to interactions with electrons

Rutherford Backscattering (RBS)

Non-destructive

Determination of – Masses -> elements

– depth distribution (res. ~10nm)

– crystalline structure (ions penetrate deeper between crystal planes)


X-ray fluorescence (XRF)

Same as electron microprobe with e- -> X-ray

Comparison to electrons: + no charging + no vacuum+- larger area +- deeper penetration- no imaging

X-ray fluorescence (XRF)

Surface analysis with total reflection XRF (TXRF)

Small incident angle assures surface probing

XRF sensitivity: 100 ppm or 5x1018 cm-3

X-ray photoelectron spectroscopy (XPS)

High-energy version of photoelectric effect

Like XRF, but ejected electron is measured


Commonly used to inspect alloys


Surface technique – e- escape depth shallow

Elemental + chemical analysis – Measured energy depends on chemical surroundings

Sensitivity ~0.1% or 1019 cm-3

X-ray topography (XRT)

Defect detection:– Take monochromatic X-rays

– Diffract the X-rays from a crystal plane (Bragg)

– Take an image of the diffracted intensity

– See defects and strain

X-ray topography (XRT)

Surface scan:

Through-sample scan:

31

X-ray diffraction (XRD)

Sample tilted over θ-angle

Intensity peaks at diffraction

Structure and composition information

Imaging techniques

Technique Resolution Depth-of-field Damage Cost Comments

Optical microscope

0,25 µm Moderate No Low

SEM ~1 nm Good Organics Medium Charging

TEM < 50 pm[2] Poor Yes High Charging

XRT 1 µm Good No Medium Defect imaging

(AFM) Atomic bonds[3]

Poor No Medium

2: "Lithium Atom Microscopy at Sub-50pm Resolution By R005". JEOL News 45 (1): 2–7. 3: Science 337, 1326 (2012);

Surface elemental / chemical characterization

Technique Smallest element

Lateral resolution

Depth resolution

Detectionlimit cm-3

Information type

Scantime

AES (scan) Z=3 10 nm 2 nm 1019 elem+chem 30min

EMP-EDS Z≈11 1 µm 1 µm 1019 Elemental 30min

EMP-WDS Z≈4 1 µm 1 µm 1018 Elemental 2h

SIMS Z=1 1 µm 1 nm 109 cm-2 Elemental 1h

RBS Z=3 0.1 cm 20 nm 1019 elemental 30min

TXRF Z≈6 0.5cm 5 nm 1010 cm-2 Elemental 30min

XPS Z=3 100 µm 2 nm 1019 Elem+chem 30min

More complete table on book page 677

Depth profile elemental / chemical characterization

Technique Smallest element

Lateral resolution

Depth resolution

Detectionlimit cm-3

Information type

Scantime

Depth by ...

AES (scan) Z=3 10 nm 2 nm 1019 elem+chem 30min Sputtering

SIMS Z=1 1 µm 1-30 nm 1014-1018 Elemental 1h Sputtering

RBS Z=3 0.1 cm 20 nm 1019 elemental 30min Energy scale

XRF-EDS Z≈11 0.1-1cm 1-10µm 1019 Elemental 30min Penetration

XRF-WDS Z≈4 0.1-1cm 1-10µm 1018 Elemental 30min Penetration

XRT None 1-10µm 100 - 500µm

- Cryst. struct. + strain + defect

45min Penetration

More complete table on book page 677

Tool availability in Otaniemi

Eds in nanotalo sem, tem, µnova low-res sem Tool Availability in

AaltoComments Cost per tool

Optical microscope

All over Low

SEM Micronova, Nanotalo

In Nanotalo a ”proper” SEM with EDS

Tens-hundreds €

TEM Nanotalo Different tools available (res. <1Å)

~1 M€

XRD Micronova ~100 k€

Conclusions

Electrons, ions and X-rays give extensive chemical and physical information

Suitable technique depends on application – Needed sensitivity, destructive/non destructive,

contact/noncontanct, conductivity...

37

Elemental / chemical characterization

chemical and physical characterization s-69.4123 postgraduate course in electron physics i p...

Documents

auger slide

electron wavelength

scattering slide

electron physics

xray energy slide

sample electrons

mass spectrometer slide

diffraction pattern