lecture 2 sem empa
TRANSCRIPT
Scanning Electron Microscopy (SEM)
Electron Probe Microanalysis (EPMA)
Analytical Methods in Materials Science 2009
Per Eklund (based on original lecture by Hans Högberg)
Outline
• Practical aspects• Basic principle• Imaging with SEM• Determination of elemental
composition (EMPA/EDS/WDS)
Overview
• SEM is easy to use• Routinely used in both research and industry• And not just in materials science – geology,
archaeology, forensics, biology, …• Image interpretation is intuitive and simple
(unlike TEM)• SEM (1950s) is a much younger technique than
TEM (1930s)
Practical aspects
• SEM is easy to use, but to learn how to use it WELL you need:
• practice• practice, • and practice!!!• But it doesn’t hurt to know a bit about the theory
as well…
Principles of SEM
• Sample information obtained from the interaction between a primary electron beam and a material.
• The better focusing and the smaller wavelength of electrons (compared to light) enables: Higher magnifications >100000xBetter resolution 10 - 50 Å LOM ∼
3000 Å
Increased depth of field 2 μm -1 mm LOM ∼ μmThe ability of an optical device to keep out-of-plane features in relatively good focus.
• Technique allows for sample imaging (morphology topography) determination of elemental composition, electronic properties (cathodoluminescence)…..
Electron interaction with matter
• Auger electrons from the surface region <50 Å, carry elemental and chemical information (compareAES)
• Secondary electrons formed by excitation of the surface elements at 50 –100 Å depth, EK <50 eV sensitive to topography
• Backscattered electrons generated in the whole volume, high energy50 eV< EK >E E-beam , compositionand crystallographic information
• Visible light(cathodoluminescence)electronical/optical properties, band gap
E-beam
Surface
Secondary e-
Auger e-
Backscattered e-
Characteristic X-rays
Visible light…
Excitation volume
The electron excitation results in ejection of electrons and photons, which can be collected, detected and evaluated.
Instrumentation SEM
• The specimen is scanned by an incident electron beam and the electrons and/or photons emitted from the surface are collected, detected and analysed.
• The lenses in the microscope condenses and demagnfies the electron beam to form a focused spot on the surface.
• Probe sizes ∼30- 60 Å.• Analysis requires high vacuum
(HV) conditions. (Exception: environmental SEM, low-vacuum SEM)
• Specimen must be electrically condutive.Schematic diagram of a
scanning electron microscope
PumpsSpecimen Detector
TV screen
E-gun
Lens system
Scan coil
Final lens
Primary electron source
• The source should exhbit:low energy spreadminimise the chromatic aberrationhigh brightnesslow workfunctionresonable lifetime
• Electron beam generated by either:filament techniqueLaB6 crystalField emission (cold or hot)most common today
• Electron gun operated at0 to 30 keV (60 keV)
acceleration voltage
The performance of the electron source is important for the final result in SEM analysis
W-filament, common in ”old” instruments, relatively low brightness, probe size ∼
6 nm Φ=4.5 eV,
lifetime ∼50 h
LaB6 crystal, much higher brightness, Φ=2.7 eV, lifetime ∼300 h, more sensitive
Field emitter, in new instruments excellent brightness, probe size ∼3 nm, lifetime ∼100 h or more, UHV conditions
© FEI Company
Electromagnetic lenses SEM
• The lenses in a SEM instrument use electrostatic or magnetic fields to influence the electron tragectories of the primary beam.(LOM refraction or reflection)
• Electrons are forced to move in the center of the lens to minimize rotation around the optical axis.spherical aberration
• The lens system aims to form a focused point on the specimen
• Spot size defines the resolution small spot yields high resolution
• Apertures to adjust beam divergence, i.e. the mutual repulsion between electrons moving together.
Electro-Magnetic lens
Lens system SEM
• The condenser lens controls the beam size (amount of electrons in the column) Increasing the size of the beam yields a better signal to noise (S/N ratio), but the larger beam diameter results in a lower resolution.
• The objective lens focuses the beam into a spot on the sample. Necessary to have an image in proper focus
• The scan coils enables deflection of the beam by varying the potential between the plates. The beam can be scanned across the sample.
Electron gun
Condenser lens
Sample
Objective/ final lens
Scan coil
y-platesx-
plat
es
Specimen properties in SEM
Samples should be:
• Vacuum compatible(low vapor pressure)solid material etc. organic/biological samples possibleafter pre-treatment
• Electrically condutive• Charge-up effects on non-conductive
samples compensated by:reduced probe currentreduced acceleration voltagecoating by a thin metal film (Au, Pt)
Image obtained from fly
Image obtained from carrot
Imaging modes in SEM
Secondary electrons
Backscattered electrons
Absorbed specimen
current
Cathodo- luminescence
Topographic
Voltage
Magnetic and electric field
Topographic
Crystallographic
Composition
Topographic
Composition
Electronic
Optic
The electrons and photons generated in the excitation volume carry different types of information from the
analysed specimen.
Imaging using secondary electrons
• High-resolution imaging of fine surface morphology(resolution about 3.5 nm).
• Sensitive to the orientation of different surface features (edges etc.) creates an image contrast ideal for evaluating the sample's surface topography.
• Three-dimensional appearance of the specimen image, due to the large depth of field and the scanning mode applied in SEM.
Secondary electrons are produced when the incident electrons from the beam interacts with the atoms in the surface region of specimen. The impact (collision cascade) causes a path change for the incident electron and an ionization of several specimen atoms. The ejected electrons leave the atom with a very small kinetic energy (<50eV)
LOM picture
SEM picture
Pict
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from
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Secondary electron emission
( )[ ]αα cos1)(* −∝ Cei
Incident electron beam
High yieldLow yield
No yield
Yield proportional to
Secondary electron emission
Secondary electron emission depends on topography
topographic imaging
Depth of field
22dtanD
=⎟⎠⎞
⎜⎝⎛ α
D
d
d
αImage plane
Magnification Resolution Depth of fieldSEM LOM
20 5 μm 1 mm 5 μm100 1 μm 200 μm 2 μm200 500 nm 100 μm 0.7 μm
1000 100 nm 20 μm5000 20 nm 4 μm
10000 10 nm 2 μm
Depth of field represents that distance along the microscope axis over which the specimen can be displayed without blurring the
image.
Electron beam
Depth of field
Magnification Resolution Depth of fieldSEM LOM
20 5 μm 1 mm 5 μm100 1 μm 200 μm 2 μm200 500 nm 100 μm 0.7 μm
1000 100 nm 20 μm5000 20 nm 4 μm
10000 10 nm 2 μm
Depth of field in SEM >> depth of field in LOM
Imaging using backscattered electrons
• Backscattered electron imaging provides elemental composition variation, as well as surface topography. (resolution about 5.5 nm)
• Yield is proportional to the atomic number (Z contrast) and depends on the beam energy and incidence angle.
• The larger escape depth compared to secondary electrons results in less good surface topography in imaging.
• Mapping of individual elements• Detectors for combining
topography and composition signals.
Backscattered electrons (BSE) are high-energy electron produced by the elastic collision of the incident electron beam with the electron cone of the sample
atoms.
Polyphase garnets (Na, K, Al, Mg, Fe minerals), due to the Z contrast in the BSE image the Fe-
rich compositions appear brighter.
Backscattered electrons: topographic or compositional
imaging
Imaging using backscattered electrons
Cathodoluminescence in SEM
• In CL the focused electron beam is used and to excite a small region of the sample.
(volume depends on the beam energy)
• The light emitted is evaluated and used to collect a spectrum.
(element/compound specific emission lines)
• By changing the energy of the beam it is possible to perform depth profiling of the optical properties.
Cathodoluminescence (CL) is the light emission associated with the excitation of materials by an electron beam. Method usually applied to semiconductor
materials, but also other classes of materials (e.g. in geology).
CL λ
5kV
SEM image of a pinhole defect in an AlN film grown on a 6H-SiC substrate
CL image λ=346 nm AlN emission line
CL image λ=496 nm 6H-SiC emission line
5 kV 15 kV
Cou
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yJe
ns B
irch,
Th
inFi
lm P
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iU
Other analysis modes in SEM
Additional techniques are:
• Electron channelling (EC)crystalline materialssymmetry, orientation and lattice information
• Electron Backscattered Diffraction (EBSD)comparable with EC, also called orientation imaging microscopy (OIM)
• Magnetic contrast imagingferromagnetic materials
• Electric contrast imaging
Electron channelling pattern (ECP) from quartz
The SEM instrument can also be used to obtain information regarding the structural, magnetic and electrical properties of the specimen.
Detectors applied in SEM
• Secondary electrons ejected from the specimen are analysed by an Everhart-Thornley detector (E-T).
• The E-T detector consists of a Faraday cage in front of a scintillator, coupled to a light pipe leading to a photomultiplier tube
• The Faraday cage is held at a positive potential of a few hundred volts to collect the low energetic secondary electrons
• In the scintillator the electrons are accelerated to produce light upon impact.
• The photomultiplier produces an output signal related to the total number of electrons collected.
Secondary electrons
SEM chamber with E-T detector and BSE quadropole detector
E-T detector
BSE detector
Detectors applied in SEM
• The high-energy BSE are collected by a silicon diode (solid-state) detector.-Collisions with the semiconductor generates electron-hole pairs. -Number of pairs dependent on the electron energy. -Migration of the holes and electrons creates a potential, which produces a current dependent on the electron flux and the electron energy.
• Qudropole detectors are often used to individually mix topographic and composition information obtained from the BSE.
• A scintillator (modified E-T detector) can used as BSE detector. -No collection potential needed for bakscattered electrons-Collection yield related to scintillator size and specimen proximity
Backscattered electrons
Polepiece
Silicon diode detector
Backscattered electron detector assembly
Influence of acceleration voltage SEM
Acceleration Voltage
Unclear surface structures
More edge effects
More charge-up
More damage
Clear surface structures
Less damage
Less charge-up
Less edge effects
High resolution
Small probe sizeHigh
Low
Low resolution
Larger probe size
The acceleration voltage affects the size, shape and yield (ratio between backscattered and secondary electrons) of the excitation volume.
Influence of probe current/diameter SEM
Probe current
Deteriorated resolution
More damage
High resolution obtainable
Less damage
Smooth image Large
SmallGrainy image
The achievable magnifications and resolutions for a SEM image are highly dependent on the probe diameter (size) and the probe current.
Influence of working distance SEM
Working distance
Smaller depth of field
Low resolution
High resolution Small
LargeGreater depth of field
The distance between the sample surface and the final/objective lens. Changed by raising or lowering the specimen. Distance typically between
a few mm to 30-40 mm.
Influence of objective aperture SEM
Aperture size
Low resolution
Smaller depth of field
High resolution
Greater depth of field
Large current Large
SmallGrainy image
The objective aperture lens regulates the amount of signal nesecessary for forming an image in SEM
Summary so far
Backscattered electrons: topographic or compositional imaging, modest resolution
Secondary electron imaging: topographic imaging only, high
resolution
Electron Probe Microanalysis (EPMA)
• X-rays are produced in the whole excitation volume-characteristic X-rays (elastic)-bremsstrahlung (inelastic)-fluorescence from X-rays
• Yield/ratio depends on acceleration voltage and speciment composition
• Characteristic X-rays are used for -qualitative analysis-quantitative analysis
• Detection of the elements: -Wavelength Dispersive X-Ray Detector (WDS)-Energy Dispersive X-Ray Detector (EDS)-(Gas flow proportional counter)
The electron beam specimen interaction also results in the emission of characteristic X-rays, which can be detected and analysed.
X-ray emission
Bremsstrahlung
Characteristic emission
Detectors in EPMA
EDS• Solid-state detector• Records X-ray energies• Requires no focusing of X-rays• Limited energy resolution (~100 eV)• Light element analysis difficult• Allows smaller electron probes, but
poor accuracy at low concentrations• Simple design• Complete elemental spectrum
displayed• Operates at cryogenic temperatures
(77 K)• VERY EASY TO USE• Not so easy to use WELL!
WDS• Crystal spectrometer• Records X-ray wavelenghts• Spatial separation of X-ray lines• High resolution (~1 eV)• Light element analysis with proper
choice of crystal• High sensitivity for trace elements• Space consuming• Analysis of one element at a time• Peak and background measured
separately• Slow, complicated
Comparision between Energy Dispersive X-Ray Detector (EDS) and Wavelength Dispersive X-Ray Detector (WDS)
WDS detector in EPMA
• X-ray diffraction separate the different X-ray energies wavelengths
• Requires a set of crystals with different lattice spacing to cover all the element in the peroidic chart
• Focusing requires moving detector and crystal
use much more space than EDS detectors.
• WDS resolution is far superior to the EDS resolution
applied in careful analytical workPrinciples of the of the Wavelength
Dispersive spectrometer
Crystals applied in WDS
4-7Ni-C multilayer
4-7W-Si multilayer
4-72.5-11.9Cerotate (CER)
6-94.0-13.5Laurate (LAU)5-82.5-8.5Stearate (STE)
11-140.26-1.5Gypsum
11-140.2-1.08Ortho-phtalate rubidium hydrogen (RAP)
11-140.34-2.5Ortho-phtalate postassium hydrogen (KAP)
12-210.18-1.03Ammonia dihydrogen phosphate (ADP)
14-220.14-0.83Pentaerythriol (PET)14-220.14-0.83Ethylene diamine tartrate (EDT)
16-370.09-0.53Sodium Chloride (NaCl)
16-340.11-0.60Germanium (Ge)19-350.1-0.38Lithium fluoride (LiF)
Atomic number range for Kα
radiationWavelengthRange (nm)
Crystal
EDS detector in EPMA
• Soild-state detector-semiconductor crystal Si(Li)-protected from contamination by a Be-window or thin polymer (Mylar) window-cooled in i LN2 to minimize the thermionic creation of charge carriers-front and back of crystal kept at high potential difference-X-rays generates electron-hole pairs -electron-hole pairs exhibit a characteristic creation energy- the total number of charge carriers created is proportional to the incident X-ray energy
EDS detector
Qualitative EDS analysis
• EDS spectra contain the characteristic emissions (K, L, M) from all elements (Z>4) in the specimen
• Spectrum displayed as intensity as a function of energy
• Light elements difficult to detect if you have a Be window-mylar-window normal in modern systems -windowless analysis possible
• Spectrum treatment somewhat difficult-separation between characteristic peaks and bremsstrahlung-separation of diferent elements and emissions-light elements difficult to quantify
EDS spectrum with intensity as a function of energy
Quantitative analysis EPMA
• The measured intensity (I) is converted to composition of each element (A) (weight or atomic percentage)
• Analysis technique use references of known composition• Calculated from
• Correction of the atomic number (Z) due to different probabilities for backscattering
(scales with the atomic number)• Correction of the absorption (A),
depends on the shape and size of the excitation volume(acceleration voltage and specimen composition)
• Correction of the secondary fluorescence (F) small effectdepend on the acceleration voltage
AA
A CZAFII )(' )(
=
Summary EMPA• Quantitative analysis with EDS
difficult in many cases:-lack of suitable standards-element separation in spectrum-light elements difficult to quantify-secondary fluorescence strong for elements that are nearest neighbors (Z<21, e.g. Al and Si) or next nearest neighbors (Z>21) in the periodic table
• Quantitative anlysis works well for many ”normal” materials, IF YOU HAVE SUITABLE STANDARDS!!!
• Superb method for qualitative analysis mapping – are elements present or not, and where are they (mapping)
SEM picture and Ni and C elemental maps from a BCN film grown on Ni