Analysis of Microsystems Summer 2010

Dr. Oswald Pruckerprucker@imtek.de

http://www.imtek.de/cpi/vl-am.php

Part 6: Chemical composition of surfaces: XPS, SIMS, FTIR

Contents

X-ray photoelectron spectroscopy (XPS, ESCA)Secondary Ion Mass Spectroscopy (SIMS)Fourier Transform Infrared Spectroscopy

Question: What elements or molecular structures

are present at the surface ?

XPS – X-ray photoelectron spectroscopy

What is XPS?- General TheoryHow can we identify elements and compounds?Instrumentation for XPSExamples of materials analysis with XPS

X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a widely used technique to investigate the chemical composition of surfaces.

XPS – The Experiment

X-rays

X-ray penetration depth ~1µm.

Electrons are excited in the entire volume.

Irradiated area ~ 1 cm2

Electrons are collected from a small area < 1 mm2

sampling depth: 10 nm

Detector

Sample

XPS – The result

1000 800 600 400 200 0

x 1 x 10

3s

Zn3pZnAuger

Zn2p

3s 3d3pCu

O1sCu Auger

Cu2p

Inte

nsitä

t

Bindungsenergie (eV)

C1s

XPS – The Experiment

X-rays

X-ray penetration depth ~1µm.

Electrons are excited in the entire volume.

Irradiated area ~ 1 cm2

Electrons are collected from a small area < 1 mm2

sampling depth: 10 nm

Detector

Sample

Questions

What is needed to perform this experiment ?What do the X-rays do with the surface ?How are the electrons generated ?Why do we only sample from near surface areas ?How are the electrons analyzed ?What do I learn from all that ?Is the angle at which the detector is operated important ?

XPS – Prerequisites

Solid samples (no liquids or liquid layers)Ultra high vacuum (< 10-6 mbar)Stability against x-rays

Metals, alloys, oxides, ceramics, organic solids, polymers, biological samples

Photoelectric effect with x-rays:

X-ray Photoelectron Spectroscopy(XPS, „Photoemission“)

Determination of the binding energy:

|En| = hν - Ekin

Element specific !!

Evac=0

En

hν

Ekin = hν - |En|

Electron Spectroscopy for Chemical Analysis: ESCA

XPS – Principle

E

Evac=0

continuum

1s K

2s L1

2p1/2 L2

2p3/2 L3

3s M1

K L M

Outer, partially filled levels:„Valence electrons“, EB ~ 10 eVresponsible for chemical bonds

Inner filled levels:Core electronsEB ~ 100 eV…100 keVchemically inactive,important for XP spectra !

XPS – Principle

1s K

2s L1

2p1/2 L2

3s M1

1 eV

10 eV

100 eV

1 keV

10 keV

100 keV

electron bindingenergies

E

Evac=0

2p3/2

XPS – Principle

XPS – The result

1000 800 600 400 200 0

x 1 x 10

3s

Zn3pZnAuger

Zn2p

3s 3d3pCu

O1sCu Auger

Cu2p

Inte

nsitä

t

Bindungsenergie (eV)

C1s

XPS – Auger electrons

X-ray expells photoelectron

Electron from upper level fills this hole andtransfers the energy difference to another outerelectron

This electron is ejected

Kinetic energy does not depend on x-rayenergy

The XPS instrument measures the kineticenergy of all collected electrons. The electron

signal includes contributions from bothphotoelectron and Auger electron lines.

XPS - satellites

During exit the photoelectrontransfers some energy to furtherelectrons that may or may not leavethe atom.

The reduction of the photoelectronkinetic energy shows up asadditional peak at apparent „lowerbinding energy“

XPS – Sampling depth

XPS is very surface sensitive: only the upper 3 – 10 nm are sampled!

Electrons are generated in the entire irradiated volume: 1mm2 wide and about 1 µm deepElectrons generated within sample are strongly inelasticallyscattered and will not leave the sampleRule of thumb: Sampling depth = 3x mean free pathway

XPS – Sampling depth

The mean free pathway, λ

Depends strongly on the kinetic energy of the electronsLargely independent of the type of element of originXPS: Ekin ~ 2 keV, λ ~ 2-3 nmSampling depth: ~3 λ

[but there are ways to play around with that]

λ

Ekin

XPS – more chem: oxidation states

C1s

Very valuable source foradditional information !!!

Four different C1s signals from this trifluoroacetic acid ethylester ???

Binding energy of core electron is effected by the chemical environment (oxidation state)Higher oxidation state shows up leads to a higher binding energy of ‘residual electrons’Chemical shift

XPS – Sampling depth

Modification of detector angle offers depth profile informationExample: SiO2 layer on silicon wafer

Further possibility: depth profiling by combination with ion gunsputtering

XPS – quantitative analysis

Quantitative analysis is possible by comparing the signal either to results fromcalibration measurements or (for homogenous samples) by direct comparison ofcomponent signals:

B

A

B

A

II

nn

=

XPS – How are the electrons analyzed?

Principle components of a XPS

X-ray source: MgKa 1253,6 eV or AlKa 1486,6 eV

Concentric Hemispherical Analyser“ (CHA)

electron detector

Separation of electrons of different kineticenergy via electric field

sample

Analyzer

XPS – examples

150 145 140 135 130Binding Energy (eV)

PbO2

Pb3O4

500 400 300 200 100 0Binding Energy (eV)

O

Pb

Pb

Pb

N

Ca

C

NaCl

XPS analysis showed that the pigment used on the mummy wrapping was Pb3O4 rather than Fe2O3

Egyptian Mummy 2nd Century ADWorld Heritage MuseumUniversity of Illinois

Pigment used in artwork on mummy

XPS-application: analysis of carbon fiber- polymer composite material

Woven carbon fiber composite

XPS analysis identifies the functional groups present on composite surface. Chemical nature of fiber-polymer interface will influence its properties.

-C-C-

-C-O

-C=O

-300 -295 -290 -285 -280Binding energy (eV)

N(E

)/E

XPS – examples

XPS - conclusions

Only suitable for samples stable in UHV

Very surface sensitive

Information on deeper layers e.g. by angle-resolvedmeasurements or by combination with sputtering

Counts elements present at the surface

Quantitative measurements are possible

SIMS – Secondary Ion Mass Spectroscopy

Developed by NASA in the 1960s to investigate the composition of moon rocks. SIMS: used to determine the composition of organic and inorganic solids at the outer 5 nm of a sample.Spatial or depth profiles of elemental or molecular concentrations can be generated.Common: detection of impurities or trace elements

SIMS – principle

ultra high vacuumA beam of primary ions or neutral particles impacts the surface with energies of 3-20 keV.Impact causes a collision cascade amongst surface atoms and between 0.1 and 10 atoms are usually ejected. This process is termed sputtering. The sputter yield depends on the nature of the analyte

SIMS – principle

Secondary ions amount to about 10% of all ejected species.Secondary ions can then be analyzed using mass spectroscopy.

SIMS – apparatus

1. Cesium ion source2. Duoplasmatron3. Electrostatic lens4. Sample5. Electrostatic sector - ion energy analyser6. Electromagnet - mass analyser7. Electron multiplier / Faraday cup8. Channel-plate / Fluorescent screen - ion image detector

SIMS – modes

Static SIMS: used to determine surface concentrations of elements and molecules without significantly altering the analyte.

Dynamic SIMS involves the use of a much higher energy primary beam (larger amp beam current). It is used to generate sample depth profiles.

Imaging SIMS like static SIMS does not alter the analyte appreciably. This mode is used to generate images or maps of analytes based upon concentrations of one secondary ion representing either an element or molecule.

Low ion flux / few primary ions

Sputters away app. only a tenth of an atomic monolayer.

Ar+, Xe+, Ar, and Xe

Long analysis times: >15 minutes.

detection of organic molecules.

SIMS – static mode

SIMS – static mode

Example: structural information from a PMMA surface (plexiglass)a: positive and b: negativ ionsMasses can be correlated with typical fragments from the polymer“more than element counting”

SIMS – dynamic mode

higher ion flux causes notable removal of material from the surface: Sputtering

The beam typically consists of O2

+ or Cs+ ions and has a diameter of less than 10 μm.

The experiment time is typically less than a second.

Ion yield changes with time as primary particles build up on the analyte effecting the ejection and path of secondary ions.

Problem: Sputtering depth is not easily determined from sputtering time

SIMS - conclusions

High cost / effort technique (UHV etc.)

Sometimes long measurement times

Yields information on chemical composition (not only elements)

Depth profiling and imaging are possible

Important tool for very surface sensitive questions (e.g. contaminations)

FTIR – Fourier Transform Infrared Spectroscopy

Uses infrared to excite good vibrationVery important tool for structural analysis in Organic ChemsitryNot a strictly surface sensitive technique (only some variants are)

FTIR – theory

light can be described as an electromagentic wave ...

MicrowavesX-rays

λ/[nm] 1 - 10 10 – 400 400 - 800 800 – 1.000.000 > 1.000.000

(Wavelength [µm]) 1 10 100 1000

(Wavenumber [cm-1]) 10.000 1000 100 10

NIR MIR FIR

Most relevant for analysis oforganic compounds: MIR

Mid-Infrared (MIR): λ: 2.5µm – 25µm (4.000 – 400cm-1)

(ν = frequency/speed of light [cm-1])

photon energies: 1 – 15 kcal/mole

not enough energy to excite electrons

induces vibrational excitations of covalently linkedatoms and groups

covalent bonds can be described as stiff springsthat can be stretched and bent

ν⋅⋅= chE

FTIR – theory

vibrational energies are assigned to quantum levels(analogous to electronic states)frequency of vibration ∝ dissociation energy of chemical bond

treating covalent bondsas stiff springs:

m1 m2

FTIR – theory

m1 and m2: mass of respective atoms

f=‚spring constant‘ – proportional to strength ofcovalent bond

Differences in bond strength?

C-N C=N C≡N1.100cm-1 1.660cm-1 2.220cm-1

Differences in mass?

O-H O-C3.600cm-1 1.240cm-1

m1 m2

FTIR – theory

What happens if molecules are placed in an IR beam ?

permanent dipole in an electrical field starts to rotate and vibrate(stretching and bending);

example: water

molecule takes up energy from the electrical field

FTIR – theory

... but not all molecules do have a permanent dipole ?

induced dipole (e.g. CO2)

induced dipole (methylene-group)

FTIR – theory

permanent dipole not required !

FTIR – theory

FTIR – theory

FTIR – theory

The whole molecule vibrates: fingerprint region in FTIR spectra

Bands that are very characteristic for a given molecule:cannot be easily analyzed in detail„porter spectrometer“

FTIR – theory

Techniques and requirements:

Transmissionrequirements: polished Si-wafer, at least 10nm organic adlayer

DRIFTrequirements: solid powder, powder holder with aligned mirrors for amplification

GIRrequirements: reflecting surface (e.g. gold), organic adlayer down to 2nm can bedetected, increase of sensitivity by increase in pathlength through layer

ATR-IRrequirements: special ATR crystal (thick Si-wafer), organic adlayer down to 2nm canbe detected, increase in sensitivity by multiple reflections at interface

shallow angle (5-15°) large area

FTIR – techniques

... the heart of the IR-maschine ...

Michelson Interferometer1. separating electromagnetic waves2. induce phase shift3. bundeling (interfering) electromagnetic

waves

incominglight

fixed mirror

movable mirror

semipermeablemirror

outgoing light beam

black: ‚reference wave‘ (reflected from fixed mirror)pink: ‚altered wave‘ (from movable mirror)red: resulting wave (interference of black and pink wave)

FTIR – instrumentation

Interferogram using3 wavelengths:

symmetric interferogram

large retardations –fading deviations fromaverage value

Interferogram usingmultiple wavelengths:

Measured intensitybecomes a function ofretardation x of the mirror

∫= νπνν dxIxI )2cos()()(

FTIR – techniques

Do some math:

intensity of polychromatic interference:

rewrite integral as (using window function D):

Fouriertransformation („FT“):

∫=B

dxIxI νπνν )2cos()()(

∫−∞

∞+

= dxxDxII )2cos()()()( πννν

informationin [cm]

information in [cm-1]

FFT

∫−∞

∞+

= νπννν dxDIxI )2cos()()()(

FTIR – techniques

measure spectrum

substract background

FTIR – techniques

No vacuum needed volatile substances may be measured

Yields information on chemical composition (groups)

Surface sensitive techniques are available

FTIR – conclusions