Electron Microscopes

Dwi Marta Nurjaya

Electron MicroscopyElectron microscope is a type of microscope that uses electrons to illuminate a specimen and create an enlarged image.electron microscope uses electrostatic and electromagnetic lenses in forming the image by controlling the electron beam to focus it at a specific plane relative to the specimen in a manner similar to how a light microscope uses glass lenses to focus light on or through a specimen to form an image.Types of Electron Microscopes:TEM (Transmission EM)SEM (Scanning EM)REM (Reflection EM)STEM (Scanning Transmission EM)STM (Scanning Tunneling M)

*(Source: FEI brochure All you want to know ... Optical ray path with source, condenser (illumination system), specimen, and objective lens Sample is transmitted. (shadow projection) Differences to SEM: Transmission and true optical ray pathComparison of TEM with Slide Projector

*SpecimenObjective lens1st image final image Source Condenser Comparison of TEM with Light Microscope (LM)(Source: Philips brochure Elektronenmikroskopie, p. 6)* c = speed of light

*Size: Maximum 3 mm in diameterThickness: Typically 100 500 nm or even less (HRTEM)BiologyTissue sections on thin carbon/formvar filmParticles (viruses) frozen in thin ice layerMaterialsClassical way: Thin disk thinned to perforation, e.g. by mechanical grinding/polishing and ion thinning, or electro-polishing; electron transparent at edge of holeNowadays: FIB techniquesTEM Specimens

What is SEMScanning electron microscope (SEM) is an EM that designed for direct studying of the surfaces of solid objects.

Advantages of Using SEM over OMMagnification Depth of Field ResolutionOM 4x 1000x 15.5mm 0.19mm ~ 0.2mmSEM10x 3000000x 4mm 0.4mm 1-10nm

The SEM has a large DOF (depth of field)The SEM also produces images of high resolution, which means that closely features can be examined at a high magnificationThe combination of higher magnification, larger depth of field, greater resolution and compositional and crystallographic information makes the SEM one of the most heavily used instruments in research areas and industries.

Advantages of Using SEM over OM

Main ApplicationsTopography The surface features of an object and its texture (hardness, reflectivity etc.) Morphology The shape and size of the particles making up the object (strength, defects in IC and chips...etc.)Composition The elements and compounds that the object is composed of and the relative amounts of them (melting point, reactivity, hardness...etc.) Crystallographic Information How the grains are arranged in the object (conductivity, electrical properties, strength...etc.)

Characteristics of a SEMMAGNIFICATION:For most SEM: 10-300,000x (or more).Useful Magnification: ~20,000xRESOLUTION:Theoretical Limit of Resolution: 50 (5 nm)Practical Resolution: 200 (20 nm)

The resolving power of the SEM depends primarily on the effective beam diameter of the probe (spot size). For two points having a inter-point distance, d, to be resolved, spot size must be smaller than d.

Inside SEM

Inside SEMThe electron beam produced by an electron gun is focused to a point on the sample surface by two condenser lenses. The second condenser lens (sometimes also called as objective lens) focuses the beam to an extraordinarily small diameter of only 10-20 nm.Electrons, either SE or BSE, from the sample surface are detected by a detector and amplified to form images on the screen of a CRT.

Comparison of LM, TEM and SEM

Comparison of OM,TEM and SEM

MicroscopeResolutionMagnificationLight sourceSpecimenBare eyes10m -Visible lightBulk MaterialOptical Microscope0.1-1m 5-2000x Visible light Polished

Scanning electron m.0.01m 100-600,000x electronsBulk materialTransmission electron m.0.1nm1,000-1,000,000xelectronsThin foils

Before SEM there was TEMTEM takes electrons from a source and through condenser and objective electromagnetic lenses, focuses the beam on an area of the sample. If the sample is thin enough for the electrons to travel through, the projector lens will project an interference pattern, (or the image) onto a phosphorus screen below.ADV: extremely high resolution.LIMITATIONS:because the electron beam has to travel through the sample, sample preparation is usually required to make the sample thin enough.since the beam is traveling though the sample, the sample bulk and not the surface is being imaged.

SEM vs TEMG.Cambaz, G.Yushin, Y.Gogotsi, V.Lutsenko, Anisotropic Etching of SiC Whiskers, Nano Letters, 6, 3, p.548, 2006. (cover article)

Limitations of using electronselectrons will not freely travel through air - there are enough molecules in air to easily absorb an electron beam. Therefore, the electron source, lenses, and sample must all be under a vacuumsince electrons are electrically charged, the sample needs to be conductive enough to dissipate this charge.

Electron GunThe electron beam comes from a filament, made of various types of materials. The most common is the Tungsten hairpin gun. This filament is a loop of tungsten which functions as the cathode. A voltage is applied to the loop, causing it to heat up. The anode, which is positive with respect to the filament, forms powerful attractive forces for electrons. This causes electrons to accelerate toward the anode. Some accelerate right by the anode and on down the column, to the sample. Other examples of filaments are Lanthanum Hexaboride filaments and field emission guns.

Hairpin Electron GunThe hairpin shape tungsten filament functions as a cathodea small voltage differential applied across the terminals of the filament causes heating of the wireThe flow of current through the wire is called filament currentElectrons thermionically emitted from the filament are accelerated rapidly towards the anodeMaterial: TungstenOperation temp: 2700 K to 3000 KService life: 25 100 hoursVacuum system: 10-5 Torr

Lanthanum Hexaboride - LaB6A thermionic electron gun consists essentially of a heated wire or compound from which electrons are given enough thermal energy to overcome the work function of the source, combined with an electric potential to give the newly free electrons a direction and velocity.Material: Lanthanum Hexaboride (LaB6)Operation temperature : 1700 - 1850 KService life : 500 hoursVacuum system: 10-6 Torr

Field Emission GunElectron source is not heated to remove electrons and for this reason are often referred to as being "cold" sources.Electrons are then pulled from a very small area of the pointed tip and proceed down the column. Often this is aided by a second anode that lies beneath the first. Acting like an electrostatic lens the two anodes serve to further coalesce and demagnify the beam.The lost electrons are replenished by an electron source attached to the tungsten tip. A primary electron beam generated by a field emission source offers significant advantages over those produced by by thermionic emitters.

Material: Tungten (W)Operation temperature : 300 KService life : over 1000 hVacuum system: 10-9 Torr

Comparison of 3 common electron sources

Why Need a VacuumWhen a SEM is used, the electron-optical column and sample chamber must always be at a vacuum:If the column is in a gas filled environment, electrons will be scattered by gas molecules which would lead to reduction of the beam intensity and stability. Other gas molecules, which could come from the sample or the microscope itself, could form compounds and condense on the sample. This would lower the contrast and obscure detail in the image. all electron source need a vacuum environment to operate

1 atmosphere = 760mm Hg = 760 torr = 1.013bar = 101.3 kPa

Vacuum PumpThere are four types of vacuum pumps that are at least somewhat commonly employed in SEMs:Roughing pump/mechanical pumpDiffusion pumpTurbo Molecular pumpIon pump

Electron Sample InteractionThe interactions may be elastic or inelasticThe elastic interactions:incident electrons and nucleusa large-angle deflection of incident electrons.little energy lossThe inelastic interactions:incident electrons and orbital shell electronsa small-angle deflection of incident electrons.heavy energy loss

SEM Detector

Elastic Vs Inelastic

Elastic Vs InelasticElasticBack-scattered electronsInelasticSecondary electronsBremsstrahlung X-raysCharacteristic X-raysAuger electronsOtherCathodoluminescenceSpecimen current

Electron Sample InteractionSEM Electron - PhotonSecondaryBack scatteredAugerCathodoluminescence

Secondary ElectronProduced by inelastic interactions of high energy electrons with valence (or conduction) electrons of atoms in the specimen, causing the ejection of the electrons from the atoms. These ejected electrons with energy less than 50eV are termed "secondary electrons".Each incident electron can produce several secondary electrons.Production of SE is very topography related. Due to their low energy, only SE that are very near the surface (

Secondary Electron DetectorA conventional secondary electron detector is positioned off to the side of the specimen. A faraday cage (kept at a positive bias) draws in the low energy secondary electrons. The electrons are then accelerated towards a scintillator which is kept at a very high bias in order to accelerate them into the phosphor.

Secondary Electron DetectorA PMT works by converting the incoming photons into electrons which are then drawn to dynodes kept at a positive bias. The dynodes are made of material with a low work function and thus give up excess electrons for every electron that strikes them. The result multiplies the signal contained in each photon produced by the scintillator. A typical scintillator/PMT secondary-electron detector, often named an Everhart-Thornley (ET) detector after its two inventors

Backscattered electrons (BSE)FormationCaused when incident electrons collide with an atom in a specimen that is nearly normal to the path of the incident beam.Incident electron is scattered backward (reflected).UseImaging and diffraction analysis in the SEM.Production varies with atomic number (Z).Higher Z elements appear brighter than lower Z elements.Differentiate parts of specimen having different atomic numberBackscattered electrons are not as numerous as others. However, they generally carry higher energies than other types of electrons

EBSD (Electron Back Scattered Detector)The most common detector used is called a surface barrier detector. It sits above the sample, below the objective lens. BSE which strike it are detected.When a BSE electron strikes the detector, electrons in the material move from valence to conduction band.The electrons are now free to move in the conduction band or drop back into the valence band.If a potential is applied, the e- and e+ can be separated, collected, and the current measured. The strength of the current is proportional to the number of BSE that hit the detector.

QBSD (Quadrant Back Scattered Detector)The most common design is a four quadrant solid state detector that is positioned directly above the specimen

Characteristic X-RayWhen the incident beam bounces through the sample creating secondary electrons, it leaves thousands of the sample atoms with holes in the electron shells where the secondary electrons used to be.If these "holes" are in inner shells, the atoms are not in a stable state. To stabilize the atoms, electrons from outer shells will drop into the inner shells, however, because the outer shells are at a higher energy state, to do this the atom must lose some energy in the form of X-ray

How EDX works ?X-rays emitted from the sample atoms are characteristic in energy (wavelength) not only the element of the parent atom, but also which shells lost electrons and which shells replaced them.Example: IronIf innermost shell (the K shell) electron of an iron atom is replaced by an L shell electron, a 6400 eV K alpha X-ray is emitted from the sample Or, if the innermost shell (the K shell) electron of an iron atom is replaced by an M shell electron, a 7057 eV K beta X-ray is emitted from the sampleOr, if the L shell electron of an iron atom is replaced by an M shell electron, a 704 eV L alpha X-ray is emitted from the sampleEDX Spectrum of Iron would have three peaks; An L alpha at 704 eV, a K alpha at 6400 eV, and a K Beta at 7057 eV.

How to Quantify elements by EDXElement Atomic % Weight %Al 2.588 1.203Si 4.247 2.056Ti0.365 0.301Cr 21.793 19.529Mn 0.229 0.216Fe 3.931 3.783Ni 8.337 59.013Nb .261 5.221Mo .249 8.677

Mapping the elementscombination of thenickel, lead, and tinmaps.By placing dots on the screen when an X-ray count of the particular element is received, an image is formed that mimics the SEM image, except the contrast is formed by the elemental X-ray emission.

MagnificationAn image is obtained by taking the signal from the sample and transferring it to a CRT screen. By decreasing the size of the scanned area (from which we get the signal), magnification is produced.

Magnification is determined by taking the ratio of the lengths of the scans: Mag. = L / I

Magnification

ResolutionResolution is the ability to resolve two closely spaced points. While you may have to be at a high magnification to see small features, resolution is NOT the same as magnification.One way to improve resolution is by reducing the size of theelectron beam that strikes the sample:

dmin = 1.29Cs1/43/4[7.92 (iT/Jc)x109 + 1]3/8at low current: dmin = 1.29Cs1/43/4Jc = current density of the source, = electron wavelengthCs = spherical aberration, i = current, T = temperature.

Resolution

Improving Resolutionreducing spot size by using different types of filament.using optimum accelerating voltages.changing the angle of tilt (i.e. emissive area hence number of SEs).using high beam currentshort working distanceusing secondary electron (SE) image, as the optimum resolutions are:5 nm for SE; 25 nm for BSE and 2 m for X-ray mapping.long exposure times, leading to more electrons being detected.reducing lens aberrations to minimum

Effect of Spot SizeA) using smallest spot,B) using a slightly larger size of spotC) using a large size of spot

Effects of Accelerating VoltageHigher voltage generates shorter wavelength of electrons better resolution.Higher voltage causes an increase of the volume of electrons/specimen interactions worse resolution

Effects of Accelerating VoltageMore signal (brighter) 3.0 KeV 20.0 KeV

Effects of Accelerating Voltage3.0 KeV 20.0 KeVBut (sometimes) reduced resolution

Depth of FieldDepth field of the SEM is the greatest among microscopes:20 mm at 10X5 m at 10,000XLarge depth of field is a great advantage for keeping in focus all parts of a rough topography, but a compromise must be made between field depth and resolutionMeans to improve depth of field:using large spot size.using long working distance.using small final aperture size.It is noted here that the machine variables for larger depth of field would cause a reduce in the resolution power of the SEM.

Depth of FieldWorking distance and aperture size affect depth of field and resolutionWorking distance is the distance between the surface of the specimen and the front surface of the objective lens, which is around 5-25 mm for most SEMs.Aperture size controls the semi-angle, .The working distance between objective lens and specimen can also control the semi-angle. Thus, smaller aperture and longer working distance larger depth of field

However,The smaller aperture reduces the current of beam worse resolution.The longer working distance increases lens spherical aberration worse resolution.

ds is the spot size. can be expressed as aperture size A and working distance, WD

Depth of FieldThe depth of field is greater at long working distance than that at short working

Depth of Field

ContrastContrast is the ratio of the change in signal between any two points on the specimenand the average signal.

Topographic Contrast: -- mainly given by secondary electron signal, which is sensitive to the surface structure of specimens. Secondary electrons can escape from the surface of most materials at depths about 5 nm (50 ).

Compositional Contrast: -- given by the backscattered electron signal, which change with the differences in atomic number, i.e., nuclear charge of the atoms composing the specimen. The higher the atomic number, the greater the backscattering. In smooth plane surfaces we are dealing entirely with compositional contrast

ContrastMajor Influences on Contrastthe incidence angle of impinging electrons onto the specimen.the collecting angle from the specimen to the positively charged collector.

Machine Variables for Contrast ImprovementReducing working distanceUsing smaller apertureUsing larger electron beam size

Topography Contras

Composition Contrast Because of this dependence of on atomic number, images produced using BSE show characteristic atomic number contrast.

That is, features of high average Z appear brighter than those of low average Z

Contrast ComparisonThe topography of the specimen will also affect the amount of backscatter signal and so backscatter imaging is often carried out on flat polished samples

More information about SEM ImageGo to :http://www.emal.engin.umich.edu/courses/semlectures/iform1.htmlhttp://micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.htmlhttp://education.denniskunkel.com

Specimen PreparationStub

Specimen PreparationSampling: Cutting or Replica formation Caution: surface contamination with dust must be avoid during cutting.

Specimen mounting: using of a conductive cement, gluing the specimen onto a aluminum stub. Caution: contamination of sample surface or the SEM by out gassing of solvent in the adhesive.

Specimen surface treatment:No surface treatment is needed if the surface is conductive or if the insulating layer of the specimen is thin enough for some electrons to penetrate an underlying conductor. For example: fracture surface of a metal or thin plastic coating on a metal surface.

Specimen PreparationA thin layer (~100 ) of conductive coating (gold or gold plus palladium or carbon) is needed, if the interesting surface is an insulator. For example: fracture surfaces of polymer and ceramics.Surfaces of wet specimens are preserved by freeze-drying or use of a cold stage. For example, fibers of wet paper or liquid sample.Morphology of biological and polymeric materials may be observed with specially prepared samples. For example: Generating a fast fracture surfaces, using chemical or plasma etching, etc.

Preparation for biological samplesFixation: to stabilize samples in life like condition and to protect the sample from disruption, dimensional change and loss of materials during subsequent processing.Chemical fixation: primary fixing in aldehyde or mixture of aldehydes, the secondary fixing in osmium tetroxide (OsO4).Cryogenic fixation: ultra rapid freezingDehydration:Solvent drying: in a graded series of solutions containing acetone and ethanol.Critical-point drying: the sample is infiltrated with liquid CO2 in a chamber.As the temperature increases, the pressure increases, finally driving all the liquid into the gas phase, at which the point sample is dry and gas can be exhausted from chamber.Freeze-substitution method: for ultra-rapid freezing samples. Air drying Freeze-drying

Sputtering EquipmentSputtering a thin gold or carbon layer (~ 10 nm) on a specimen surface can improve the conductivity of specimens. Also you need ensure the conductive layer connected to the base of specimen holder.

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