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

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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

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irch,

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lm P

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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