Surface analysis
1 Introduction and electron interaction
1.1 What is a surface
Surface: the separation between a solid phase and an adjacent phase (gas, liquid or another solid) à not detailed enough
à Example: STM image of copper rods demonstrates atomic-level surface structure (not a smoot surface)
Concept of thickness (better concept): considered as a layer with variable thickness
à few angstroms to millimeters
à Type of material & Its role in a given process
à Real samples are complex and we have diDerent type of surfaceà not always
straightforward to analyze (all kind of diDerent techniques)
The interface = surface between two solid materials
(also something we need to analyze) à make it complex
1.2 Types of information
1) Structural information: atomic arrangement (order, strain, orientation), crystal structure, local environment
à Scanning tunneling microscope (STM) àtopography, atomic resolution
à X-ray diCraction (XRD) à crystal structure, tension and orientation
à X-ray absorption fine structure analysis (EXAFS) à local structure, length of bond
2) Physical properties: film thickness, optical properties, wettability, surface area/porosity, elastic/adhesive properties
à Ellipsometry à thickness of thin films à Brunauer-Emmett-Teller (BET)àsurface of porous materials
à Dynamic contact angle (DCA) à Surface plasmon resonance (SPR)
à Atomic force microscopy (AFM)
3) Chemical information (focus on this)
à The Analysis of the composition: Isotopes, elements, molecules & Qualitative, quantitative
à Distribution analysis: information spatial coordinates of the component (lateral, in depth, both = 3D)
à Structure analysis: determination organization of the components (atoms, functional groups)
à Gathering of information:
- Local probe methods: spatial analysis (chemical information)
à Measure local chemical composition at specific points & Limited area
analysis
- Beam methods (In/out particle methods): Bombardment of this particles will give an
interaction with the matrixs of the surface à the collusion will cause an emission of
particles of the surface which can be detected
à Incident radiation (probe) & Detected radiation (signal)
1.3 Beam method
Beam method characteristics
à All operate under vacuum à when bombarding with particles, don’t want interact with other particles present in air
à Beam diameter determines lateral resolution à with a broad beam we see less details
à Depth resolution (information depth) varies with energy (as it goes through the matrix it loses energy) and angle à depending
on which particles
à Detection limit: can be dependent of
element/molecule and matrix Techniques Detection limit Spatial resolution Element coverage
TEM, AES 100 ppm Nano/microscale Bulk/surface
SEM, Raman 10 ppm Microscale Bulk/surface
XPS 1 ppm Surface/ thin film All elements
TOF-SIMS 1 ppm Surface All elements
ICP-MS 10 ppb Bulk/ surface All elements
APT 1 ppb Nanoscale All elements
,2 Methods based on electron
interaction
2.1 Characteristics of an electron
à Particle properties: Charge of electron: q= -1.6 x 10-19 C/ Rest mass: me = 9.1 x 10-31 kg.
!!
When accelerated over a large potential diDerence (V) à relativistic eDect become important: 𝑚 = "
"#$( )$
#
' '
à Wave properties (de broglie hypothesis): De Broglie wavelength formule: 𝜆 = = !) (with h = the
(
Plankc’s constant, m is mass, v is velocity). For electrons accelerated over potential diDerence V:
#.+
𝜆(/ 𝑛𝑚) =
,-.#/%& - $
2.1.1 Scattering (electron scattering in matter)
What is scattering ? An electron changes direction after interacting with one or several atoms in a material
Terminology: Probability of an electron being scattered: p= Nσdx. Cross section σ (cm2): surface of an atom as it is “seen” by the
)
incident electron . Free path length λ (nm): the average distance the electron travels before being sca9ered 𝜆 =
*+
2.1.1.1 Elastic scattering
Electron scatters without losing energy
à Main type: Rutherford scattering: Coulombic interaction between the incident electron and the nucleus
(plus electron cloud)
à No detectable energy loss; mostly results in forward (small angle) scattering à especially for high atomic
number materials: Mean free path λ dependent on the atomic number (Z) of the atom that sca9ers & Au (Z=97) à
λ=5nm and C (Z=6) à λ= 150nm
2.1.1.2 Inelastic scattering
Incident electron loses a detectable quantity of energy (ΔE) which is transferred to other electrons or
causes secondary eDects (heat, photons, electrons).
à Examples: Phonon scattering , Plasmon scattering , Inner shell excitation
Several scatter processes whereby electron loses result in a teardrop shaped volume and whereby it
finally comes to a halt : Heavier elements (high Z) reduce the interaction volume
Secondary eCect: an eDect introduced by the incident electron beam which can be detected outside
the specimen à are used for chemical characterization: Electromagnetic radiation & electrons.
Secondary electrons: electrons which escape from surface with E < 50 ev: Possible
primary electrons . More likely electrons from the specimen to which energy has been
transferred. High yield (≥1) à good for imaging
Backscattered electrons: electrons which leave the surface prior to losing their energy :
Higher energy than secondary electrons, fewer in number
2.1.2 Relaxation processes of excited atoms
As a results of scattering, electrons can be knocked out of their orbit à atom in excited state
After a while the hole will be filled by another electron from a higher shell à the atom relaxes and gives oD an
abundance of energy (=photon) à can be detected and gives information about the sample
à Cathodoluminescence: Filling of an outer shell vacancy releases low-energy photons (visible, UV, IR). Hole is
created in a outer shell (Used to image crystals and defects)
2.1.2.1 Characteristic X-rays :
à Vacancies in inner shells are filled by upper-level electrons, emitting photons with energies characteristic for each element:
9'
à Yield dependent on Z: 𝑤 =
'0
𝛥𝐸 = 1
= 𝐸'23'45 − 𝐸67845 à Energy release is high (photons in the X-ray domain) (9 ' .0)
2.1.2.2 Auger electrons
Instead of photon emission, the extra energy is imparted to another electron which is ejected (=Auger electron)
9'
Hole in the inner electron orbits . Energy release is high: electron: 1 − 𝑤 = 1 −
(9 ' .0)
2.1.2.3 Bremsstrahlung
Production of X-rays without emission of electron: Non-characteristic X-ray produced by the decleartion of electrons as they
interact with the nuclear field à incident electron loses all its energy . Forms a continuum, not element specific
, 2.2 Transmitted electrons
2.2.1 interaction of electrons with matter
What happens when electrons hit a sample:
- For bulk material: Electrons penetrate deep into the material. Interaction volume is roughly teardrop-shaped .
Electron comes to a halt (absorption) inside the material. Energy is transferred through various interactions
- For thin films: Most electrons pass through the sample without significant interaction. Fewer
interactions occur. More transmitted electrons escape. Smaller interaction volume compared to bulk
Monta carlo simulation : Used to model electron trajectories through matter. Predict: penetration depth,
interaction volume shape and size, secondary and backscattered electron yields
2.2.2 Transmission electron microscope (TEM)
TEM transmits electrons through a thin sample (100nm). Electrons interact
with internal structure. Used for imaging diDraction and chemical analysis of
thin specimens
2.2.2.1 TEM components and optical path
- Electron source (gun): Generate electrons via thermionic emission or field
emission: Accelerates electrons to high energy (40 – 3000kV)
1) Triode source / gun
- Thermionic emission: Heat tungsten filament cathode (low work function) up to
2700-2900 K. Electrons gain thermal energy to escape à electrons gain the energy to
escape (overcome the work function). Apply a potential diDerence between filament
and anode (tens to hundreds of kV)
- Wehnelt cylinder / cap control of the beam diameter: Characteristics: Low
brightness (=beam current density per unit of solid angle expressed in A m-2 sr-1). LaB6
gives higher brightness (unstable and need to change it a lot à takes a lot of time)
2) Field emission gun (FEG)
- Principle: Creates very strong electric field through a fine tungsten tip (~0.4 µm). Electrons escape via
quantum tunneling eDect. Acceleration voltage
- Characteristics: Much higher brightness (factor 100-1000). No heating required
- Components: Fine tungsten tip (cathode): extremely sharp point. Anode : high positive voltage. Extractor
electrode: focuses tunneled electrons
- Condenser lenses (1ste & 2nd): Shapes and focuses the electron beam: Controls beam
size on specimen (how many electrons reach the sample). Controls beam current intensity. Adjustable
convergence angle
à Electrons focusing (electromagnetic lenses):
- Comparable with focusing photons with quartz, lenses, … (not possible for electronsà
electromagnetic lenses)
- Electromagnetic lenses: Work on the fact that electron that moves through homogametic magnetic
field where it will undergo a force. The Lawrence force: F= e(vB). Consist of iron shells with a coil inside with current that we can
increase or decrease to change the magnetic field: The electrons that move away from the center will feel
the Lawrence force à undergo a helical movement and focus in the center
- Perpendicular to the direction of the velocity and the magnetic field
- Condenser aperture: Restrict electron beam divergence: Influences the convergence angle.
à 1ste and 2nd condenser lens: Creates reduced image of electrons source.
à Condenser aperture: restricts convergence angle β à influences brightnes & coherence
Metal plat with holes in it with different sizes à decides how many electrons will hit sample
- Specimen holder: Holds thin sample in path of electron beam
- Objective lens: Forms the first intermediate image and diDraction pattern (enlargement and imaging); Can switch between
imaging mode and diDraction mode. Enlargement 500 -1000 à Function = imaging
- Intermediate lenses and projector lens: Further enlargement à 3 of 4 lenses (enlargement up to x 1.000.000)
- Field aperture: choice of a limited analysis area
- Fluorescent screen: Converts electron image to visible light: Allows real-time observation
- CCD camera/ binocular viewer: records images or diDraction patterns: Digital or analog detection
1 Introduction and electron interaction
1.1 What is a surface
Surface: the separation between a solid phase and an adjacent phase (gas, liquid or another solid) à not detailed enough
à Example: STM image of copper rods demonstrates atomic-level surface structure (not a smoot surface)
Concept of thickness (better concept): considered as a layer with variable thickness
à few angstroms to millimeters
à Type of material & Its role in a given process
à Real samples are complex and we have diDerent type of surfaceà not always
straightforward to analyze (all kind of diDerent techniques)
The interface = surface between two solid materials
(also something we need to analyze) à make it complex
1.2 Types of information
1) Structural information: atomic arrangement (order, strain, orientation), crystal structure, local environment
à Scanning tunneling microscope (STM) àtopography, atomic resolution
à X-ray diCraction (XRD) à crystal structure, tension and orientation
à X-ray absorption fine structure analysis (EXAFS) à local structure, length of bond
2) Physical properties: film thickness, optical properties, wettability, surface area/porosity, elastic/adhesive properties
à Ellipsometry à thickness of thin films à Brunauer-Emmett-Teller (BET)àsurface of porous materials
à Dynamic contact angle (DCA) à Surface plasmon resonance (SPR)
à Atomic force microscopy (AFM)
3) Chemical information (focus on this)
à The Analysis of the composition: Isotopes, elements, molecules & Qualitative, quantitative
à Distribution analysis: information spatial coordinates of the component (lateral, in depth, both = 3D)
à Structure analysis: determination organization of the components (atoms, functional groups)
à Gathering of information:
- Local probe methods: spatial analysis (chemical information)
à Measure local chemical composition at specific points & Limited area
analysis
- Beam methods (In/out particle methods): Bombardment of this particles will give an
interaction with the matrixs of the surface à the collusion will cause an emission of
particles of the surface which can be detected
à Incident radiation (probe) & Detected radiation (signal)
1.3 Beam method
Beam method characteristics
à All operate under vacuum à when bombarding with particles, don’t want interact with other particles present in air
à Beam diameter determines lateral resolution à with a broad beam we see less details
à Depth resolution (information depth) varies with energy (as it goes through the matrix it loses energy) and angle à depending
on which particles
à Detection limit: can be dependent of
element/molecule and matrix Techniques Detection limit Spatial resolution Element coverage
TEM, AES 100 ppm Nano/microscale Bulk/surface
SEM, Raman 10 ppm Microscale Bulk/surface
XPS 1 ppm Surface/ thin film All elements
TOF-SIMS 1 ppm Surface All elements
ICP-MS 10 ppb Bulk/ surface All elements
APT 1 ppb Nanoscale All elements
,2 Methods based on electron
interaction
2.1 Characteristics of an electron
à Particle properties: Charge of electron: q= -1.6 x 10-19 C/ Rest mass: me = 9.1 x 10-31 kg.
!!
When accelerated over a large potential diDerence (V) à relativistic eDect become important: 𝑚 = "
"#$( )$
#
' '
à Wave properties (de broglie hypothesis): De Broglie wavelength formule: 𝜆 = = !) (with h = the
(
Plankc’s constant, m is mass, v is velocity). For electrons accelerated over potential diDerence V:
#.+
𝜆(/ 𝑛𝑚) =
,-.#/%& - $
2.1.1 Scattering (electron scattering in matter)
What is scattering ? An electron changes direction after interacting with one or several atoms in a material
Terminology: Probability of an electron being scattered: p= Nσdx. Cross section σ (cm2): surface of an atom as it is “seen” by the
)
incident electron . Free path length λ (nm): the average distance the electron travels before being sca9ered 𝜆 =
*+
2.1.1.1 Elastic scattering
Electron scatters without losing energy
à Main type: Rutherford scattering: Coulombic interaction between the incident electron and the nucleus
(plus electron cloud)
à No detectable energy loss; mostly results in forward (small angle) scattering à especially for high atomic
number materials: Mean free path λ dependent on the atomic number (Z) of the atom that sca9ers & Au (Z=97) à
λ=5nm and C (Z=6) à λ= 150nm
2.1.1.2 Inelastic scattering
Incident electron loses a detectable quantity of energy (ΔE) which is transferred to other electrons or
causes secondary eDects (heat, photons, electrons).
à Examples: Phonon scattering , Plasmon scattering , Inner shell excitation
Several scatter processes whereby electron loses result in a teardrop shaped volume and whereby it
finally comes to a halt : Heavier elements (high Z) reduce the interaction volume
Secondary eCect: an eDect introduced by the incident electron beam which can be detected outside
the specimen à are used for chemical characterization: Electromagnetic radiation & electrons.
Secondary electrons: electrons which escape from surface with E < 50 ev: Possible
primary electrons . More likely electrons from the specimen to which energy has been
transferred. High yield (≥1) à good for imaging
Backscattered electrons: electrons which leave the surface prior to losing their energy :
Higher energy than secondary electrons, fewer in number
2.1.2 Relaxation processes of excited atoms
As a results of scattering, electrons can be knocked out of their orbit à atom in excited state
After a while the hole will be filled by another electron from a higher shell à the atom relaxes and gives oD an
abundance of energy (=photon) à can be detected and gives information about the sample
à Cathodoluminescence: Filling of an outer shell vacancy releases low-energy photons (visible, UV, IR). Hole is
created in a outer shell (Used to image crystals and defects)
2.1.2.1 Characteristic X-rays :
à Vacancies in inner shells are filled by upper-level electrons, emitting photons with energies characteristic for each element:
9'
à Yield dependent on Z: 𝑤 =
'0
𝛥𝐸 = 1
= 𝐸'23'45 − 𝐸67845 à Energy release is high (photons in the X-ray domain) (9 ' .0)
2.1.2.2 Auger electrons
Instead of photon emission, the extra energy is imparted to another electron which is ejected (=Auger electron)
9'
Hole in the inner electron orbits . Energy release is high: electron: 1 − 𝑤 = 1 −
(9 ' .0)
2.1.2.3 Bremsstrahlung
Production of X-rays without emission of electron: Non-characteristic X-ray produced by the decleartion of electrons as they
interact with the nuclear field à incident electron loses all its energy . Forms a continuum, not element specific
, 2.2 Transmitted electrons
2.2.1 interaction of electrons with matter
What happens when electrons hit a sample:
- For bulk material: Electrons penetrate deep into the material. Interaction volume is roughly teardrop-shaped .
Electron comes to a halt (absorption) inside the material. Energy is transferred through various interactions
- For thin films: Most electrons pass through the sample without significant interaction. Fewer
interactions occur. More transmitted electrons escape. Smaller interaction volume compared to bulk
Monta carlo simulation : Used to model electron trajectories through matter. Predict: penetration depth,
interaction volume shape and size, secondary and backscattered electron yields
2.2.2 Transmission electron microscope (TEM)
TEM transmits electrons through a thin sample (100nm). Electrons interact
with internal structure. Used for imaging diDraction and chemical analysis of
thin specimens
2.2.2.1 TEM components and optical path
- Electron source (gun): Generate electrons via thermionic emission or field
emission: Accelerates electrons to high energy (40 – 3000kV)
1) Triode source / gun
- Thermionic emission: Heat tungsten filament cathode (low work function) up to
2700-2900 K. Electrons gain thermal energy to escape à electrons gain the energy to
escape (overcome the work function). Apply a potential diDerence between filament
and anode (tens to hundreds of kV)
- Wehnelt cylinder / cap control of the beam diameter: Characteristics: Low
brightness (=beam current density per unit of solid angle expressed in A m-2 sr-1). LaB6
gives higher brightness (unstable and need to change it a lot à takes a lot of time)
2) Field emission gun (FEG)
- Principle: Creates very strong electric field through a fine tungsten tip (~0.4 µm). Electrons escape via
quantum tunneling eDect. Acceleration voltage
- Characteristics: Much higher brightness (factor 100-1000). No heating required
- Components: Fine tungsten tip (cathode): extremely sharp point. Anode : high positive voltage. Extractor
electrode: focuses tunneled electrons
- Condenser lenses (1ste & 2nd): Shapes and focuses the electron beam: Controls beam
size on specimen (how many electrons reach the sample). Controls beam current intensity. Adjustable
convergence angle
à Electrons focusing (electromagnetic lenses):
- Comparable with focusing photons with quartz, lenses, … (not possible for electronsà
electromagnetic lenses)
- Electromagnetic lenses: Work on the fact that electron that moves through homogametic magnetic
field where it will undergo a force. The Lawrence force: F= e(vB). Consist of iron shells with a coil inside with current that we can
increase or decrease to change the magnetic field: The electrons that move away from the center will feel
the Lawrence force à undergo a helical movement and focus in the center
- Perpendicular to the direction of the velocity and the magnetic field
- Condenser aperture: Restrict electron beam divergence: Influences the convergence angle.
à 1ste and 2nd condenser lens: Creates reduced image of electrons source.
à Condenser aperture: restricts convergence angle β à influences brightnes & coherence
Metal plat with holes in it with different sizes à decides how many electrons will hit sample
- Specimen holder: Holds thin sample in path of electron beam
- Objective lens: Forms the first intermediate image and diDraction pattern (enlargement and imaging); Can switch between
imaging mode and diDraction mode. Enlargement 500 -1000 à Function = imaging
- Intermediate lenses and projector lens: Further enlargement à 3 of 4 lenses (enlargement up to x 1.000.000)
- Field aperture: choice of a limited analysis area
- Fluorescent screen: Converts electron image to visible light: Allows real-time observation
- CCD camera/ binocular viewer: records images or diDraction patterns: Digital or analog detection
