sem,tem & afm

Template for Microsoft PowerPointPRESENTED BY – ANAMIKA BANERJEE

SCANNING ELECTRON MICROSCOPY

Here comes your footer

SCANNING ELECTRON MICROSCOPE (SEM)

Von Ardenne

first SEM in 1938 by rasteringthe electron beam.

Zworykin et al. 1942, firstSEM for bulk samples.

1965 first commercial SEM byCambridge ScientificInstruments

CHARACTERISTIC INFORMATIONS

Topography- the surface features of an object or how it looks, its texture.

Morphology – the shape and size of the particles making up the object.

Composition - The elements and compounds that the object is composedof and the relative amount of them.

Crystallographic information – How the atoms are arranged in theobject.

COMPONENTS OF SEM


a source (electron gun) of the electron beamwhich is accelerated down the column .a series of lenses which act to control thediameter of the beam as well as to focus the beamon the specimen;a series of apertures which the beam passesthrough and which affect properties of that beam;an area of beam/specimen interaction thatgenerates several types of signals that can be detectedandprocessed to produce an image or spectra;

all of the above maintained at high vacuumlevels

MENA3100

Electron beam-Sample interactions• The incident electron beam is scattered in the sample, both

elastically and inelastically• This give rise to various signals that we can detect.• Interaction volume increases with increasing acceleration

voltage and decreases with increasing atomic number

Images: Smith College Northampton, Massachusetts

Electron GunsWe want many electrons per timeunit per area (high currentdensity) and as small electronspot as possible

• Thermionic Electron Gun(TEG):electrons are emitted when a solidis heated– W-wire, LaB6-crystal

• Field Emission Guns (FEG): coldguns, a strong electric field isused to extract electronsWith field emission guns we geta smaller spot and higher currentdensities compared to thermionicguns-Single crystal of W, etched to a thin tip

Single crystal of LaB6

Tungsten wire

Field emission tip

MENA3100

Detectors

Secondary electron detector:(Everhart-Thornley)

Backscattered electron detector:(Solid-State Detector)

X-rays: Energy dispersive spectrometer (EDS)

Image: Anders W. B. Skilbred, UiO

Vacuum

• Chemical (corrosion!!) and thermal stability is necessary for a well-functioning filament (gun pressure)

– A field emission gun requires ~ 10-10 Torr

– LaB6: ~ 10-6 Torr

• The signal electrons must travel from the sample to the detector (chamber pressure)

– Vacuum requirements is dependant of the type of detector

HOW THE SEM WORKS?


The SEM uses electrons instead of light to forman image.

A beam of electrons is produced at the top of themicroscope by heating of a metallic filament.

The electron beam follows a vertical path throughthe column of the microscope. It makes its waythrough electromagnetic lenses which focus anddirect the beam down towards the sample.

Once it hits the sample, other electrons areejected from the sample. Detectors collect thesecondary or backscattered electrons, andconvert them to a signal that is sent to a viewingscreen similar to the one in an ordinarytelevision, producing an image.


Incoming electrons

Secondary electrons

Backscatteredelectrons

Auger electrons

X-rays

Cathodo-luminescence

Sample

When the accelerated beam of electrons strike a specimen they

penetrate inside it to depths of about 1 μm and interact bothelastically and inelastically with the solid, forming a limitinginteraction volume from which various types of radiationemerges -


The most common imaging mode collects low-energy (<50 eV) Secondary electrons that areejected from the k-shell of the specimen atomsby inelastic scattering interactions with beamelectrons. Due to their low energy, these electronsoriginate within a few nanometers from the samplesurface

Backscattered electrons (BSE) consist of high-energy electronsoriginating in the electron beam, that are reflected or back-scatteredout of the specimen interaction volume by elasticscattering interactions with specimen atoms. Since heavy elements(high atomic number) backscatter electrons more strongly thanlight elements (low atomic number), and thus appear brighter inthe image.

BSE- DETECTOR

SE - DETECTOR

BSE v/s SE

SE produces higher resolution

images than BSE

Resolution of 1 – 2 nm is

possible


BSE

SE

MENA3100

SUMMARY

• Signals:– Secondary electrons (SE): mainly topography

• Low energy electrons, high resolution

• Surface signal dependent on curvature

– Backscattered electrons (BSE): mainly chemistry• High energy electrons

• “Bulk” signal dependent on atomic number

– X-rays: chemistry• Longer recording times are needed

156 electrons!

Image

Detector

Electron gun288 electrons!


The SEM image is a 2-D intensity map in the analog ordigital domain. Each image pixel on the displaycorresponds to a point on the sample, which is proportional tothe signal intensity captured by the detector at each specificpoint.

Unlike optical TEM no true image exists in the SEM. It isnot possible to place a film anywhere in the SEM and recordan image.

The image is generated and displayed electronically. Theimages in the SEM are formed by electronic synthesis, nooptical transformation takes place, and no real of virtualoptical images are produced in the SEM.

MAGNIFICATION IN THE SEM

• No optical transformation is responsible for image magnification in the SEM.

• Magnification in the SEM depends only on the excitation of the scan coils which determines the focus of the beam.

• The magnification of the SEM image is changed by adjusting the length of the scan on the specimen (Lspec) for a constant length of scan on the monitor (Lmon), which gives the linear magnification of the image (M)

• M = Lmon/Lspec


MENA3100

Some Comments on RESOLUTION• Best resolution that can be obtained when size of the

electron spot is complimentary to the sample surface– The introduction of FEG has dramatically improved the

resolution of SEM’s

• The volume from which the signal electrons are formeddefines the resolution– SE image has higher resolution than a BSE image

• Scanning speed:– a weak signal requires slow speed to improve signal-to-noise

ratio

– when doing a slow scan drift in the electron beam can affectthe accuracy of the analysis

SAMPLE REQUIREMENTS

• Since the SEM is operated under high which means that liquidsand materials containing water and other volatile componentscannot be studied directly. Also fine powder samples need to befixed firmly to a specimen holder substrate so that they will notcontaminate the SEM specimen chamber.

• Non-conductive materials need to be attached to a conductivespecimen holder and coated with a thin conductive film bysputtering or evaporation. Typical coating materials are Au, Pt,Pd, their alloys, as well as carbon.

• There are special types of SEM instruments such as VPSEMand ESEM that can operate at higher specimen chamberpressures thus allowing for non-conductive materials or even wetspecimens to be studied.


MENA3100

WHY ESEM?

• To image challenging samples such as:

– insulating samples

– vacuum-sensitive samples (e.g. biological samples)

– irradiation-sensitive samples (e.g. thin organic films)

– “wet” samples (oily, dirty, greasy)

• To study and image chemical and physical processes in-situ such as:

– mechanical stress-testing

– oxidation of metals

– hydration/dehydration

TUNNELING ELECTRON MICROSCOPY


TUNNELING ELECTRON MICROSCOPE (TEM)

The first electron microscope wasbuilt 1932 by the German physicistErnst Ruska, who was awarded theNobel Prize in 1986 for its invention.

the first commercial TEM in 1939.

1nm resolution

Typical accel. volt. = 100-400 kV (some instruments - 1-3 MV)

BASIC PRINCIPLESThe design of a transmission electronmicroscope (TEM) is analogous to that of anoptical microscope. In a TEM high-energy(>100 kV) electrons are used instead of photonsand electromagnetic lenses instead of glasslenses. The electron beam passes an electron-transparent sample and a magnified image isformed using a set of lenses. This image isprojected onto a fluorescent screen or a CCDcamera. Whereas the use of visible light limitsthe lateral resolution in an optical microscope toa few tenths of a micrometer, the much smallerwavelength of electrons allows for a resolution of0.2 nm in a TEM.

Condenser system :(lenses & apertures)for controlling illumination on specimen

Objective lens system: image-forming lens - limits resolution; aperture - controls imaging conditions

Projector lens system: magnifies image or diffraction pattern onto final screen

Intermediate lens: transmitting or magnifying the enlarge image.

INSTRUMENT COMPONENTS

SAMPLE

Incoming electrons

Secondary electrons

Auger electrons

X-rays

Cathodo-luminescence

Inelasticallyscattered electrons

Inelasticallyscattered electronsunscattered

electrons

Backscatteredelectrons

IMAGING

Image contrast is obtained by interaction of theelectron beam with the sample. In the resulting TEMimage denser areas and areas containing heavierelements appear darker due to scattering of theelectrons in the sample. In addition, scattering fromcrystal planes introduces diffraction contrast. Thiscontrast depends on the orientation of a crystallinearea in the sample with respect to the electron beam.

As a result, in a TEM image of a sample consisting ofrandomly oriented crystals each crystal will have itsown grey-level. In this way one can distinguishbetween different materials, as well as imageindividual crystals an crystal defects. Because of thehigh resolution of the TEM, atomic arrangements incrystalline structures can be imaged in large detail

High resolution TEMimage of a multi-walled carbonnanowire. The wireconsists of segments,bounded by innersegment boundaries(From 2008Koninklijke PhilipsElectronics N.V.).

RESOLUTION IN TEM

• In a TEM, a monochromatic beam of electrons is acceleratedthrough a potential of 40 to 100 kilovolts (kV) and passedthrough a strong magnetic field that acts as a lens. Theresolution of a modern TEM is about 0.2 nm.

• More recently, advances in aberration corrector design have beenable to reduce spherical aberrations and to achieve resolutionbelow 0.5 Ångströms at magnifications above 50 million times.

• rth= 0.61λ/β β= semi-collection angle of magnifying lens

λ= electron wavelength

4/14/367.0 sCr Best attained resolution ~0.07 nmNature (2006)

Cs = spherical aberration

Electron Energy Loss Spectroscopy (EELS)

When travelling through the sample the electrons may loseenergy due to (multiple) inelastic scattering events. Theamount of energy that is transferred from the incident electronto the sample is dependent on the composition of the sample.Because the primary beam of electrons has one well-definedenergy, the spectrum of the electrons that have passed thesample contains chemical information on the irradiated area.

Quantification of the spectrum enables determination of (local)concentrations of elements. The fine structure in the EELSspectra provides information on the chemical binding of theatoms involved.

TOMOGRAPHYTEM tomography involves the acquisition of a large series ofimages at many tilt angles of the sample with respect to theelectron beam. In analogy to the CT scanner used in medicaldiagnostics, the acquired tilt-series is reconstructed into a 3-Drepresentation. This technique is especially useful in case ofstudies on 3-dimensionally shaped objects, such as layers insmall pores and 3-D shapes of small objects

The 3-dimensional reconstruction of the morphology of a part of a GaP-GaAs hetero-structured nanowire with 40 nm diameter.

SAMPLE PREPARATION IN TEM

TEM foil specimens were prepared by

mechanical dimpling down to 20 μm,followed by argon ion milling operatingat an accelerating voltage of 5 kV and 10°incidence angle, with a liquid nitrogencooling stage to avoid sample heating andmicrostructural changes associated withthe annealing effect.

For TEM observations, thin samples arerequired due to the important absorptionof the electrons in the material. Highacceleration voltage reduces theabsorption effects but can causeradiation damage (estimated at 170 kVfor Al). At these acceleration tensions, amaximum thickness of 60 nm is

required for TEM

CONTRASTS: Electrons that go through a sample

DIFFRACTION CONTRASTFormed by incident electrons that arescattered by the atoms of thespecimen elastically. These electronscan then be collated using magneticlenses to form a pattern of spots; eachspot corresponds to a specific atomicspacing (a plane). This pattern canthen yield information about theorientation, atomic arrangementsand phases present in the area beingexamined.

BRIGHT FIELD CONTRASTformed directly by occlusionand absorption of electrons inthe sample. Thicker regions ofthe sample, or regions with ahigher atomic number willappear dark, whilst regionswith no sample in the beampath will appear bright hencethe term "bright field".

LIMITATIONS OF THE TEM

• Sampling---0.3mm3 of materials: The higher the resolution the

smaller the analyzed volume becomes. Drawing conclusions from a singleobservation or even single sample is dangerous and can lead tocompletely false interpretations

• Interpreting transmission images---2D images of 3D specimens, viewed in transmission, no depth-sensitivity.

• Electron beam damage and safety---particularly in polymer and ceramics: The high energy of the electron beam utilized in electron microscopy causes damage by ionization, radiolysis, and heating

• Specimen preparation---”thin” below 100nm

ADVANCES IN TEM

CRYO-TEMUsing dedicated equipment,

it is possible to freeze 0.1 μmthick water films and studythese films at -170˚C in theTEM. This enables imagingof the natural shape oforganic bilayer structures.Also, agglomerationprocesses in a dispersion canbe studied. In addition, theapplication of cryogenicconditions facilitatesstudies of beam-sensitivesamples.

HAADFAnother way of obtainingcompositional as well asstructural informationusing TEM is HighAngle Annular DarkField (HAADF) imaging.For this application adedicated detector is usedthat only collectselectrons that areelastically scattered overlarge angles by thespecimen. The intensitythat is detected isdependent on the averageatomic number Z

ENERGY FILTERED TEM(EFTEM)A special filter on the TEMallows for selection of a verynarrow window of energiesin the EELS spectrum.Using the correspondingelectrons for imaging,EFTEM is performed. As aresult, a qualitativeelemental map is obtained.EFTEM is the onlychemical analysis procedurein the TEM that does notuse a scanning beam. As aconsequence, it is muchfaster.

SEM V/S TEM• in SEM is based on scattered

electrons

• The scattered electrons in SEMproduced the image of the sampleafter the microscope collects andcounts the scattered electrons.

• SEM focuses on the sample’ssurface and its composition.

• SEM shows the sample bit bybit

• SEM provides a three-dimensional image

• SEM only offers 2 million as amaximum level ofmagnification.

• SEM has 0.4 nanometers.

• TEM is based on transmitted electrons

• In TEM, electrons are directly pointed toward the sample.

• TEM seeks to see what is inside or beyond the surface.

• TEM shows the sample as a whole.

• TEM delivers a two-dimensional picture.

• TEM has up to a 50 million magnification

• The resolution of TEM is 0.5 angstroms

SUMMARY

Each microscope works is very different from another.

SEM scans the surface of the sample by releasing electrons andmaking the electrons bounce or scatter upon impact. Themachine collects the scattered electrons and produces an image.The image is visualized on a television-like screen. On the otherhand, TEM processes the sample by directing an electron beamthrough the sample.

Images are also a point of difference between two tools. SEM images are three-dimensional and are accurate representations while TEM pictures are two-dimensional and might require a little bit of interpretation.

In terms of resolution and magnification, TEM gains more advantages compared to SEM.

The result is seen using a fluorescent screen

ATOMIC FORCE MICROSCOPY

BRIEF HISTORY OF AFM

Atomic force microscopy (AFM) to investigate the electrically non-conductive materials, like proteins.

In 1986, Binnig and Quate demonstrated for the first time the ideas of AFM, which used an ultra-small probe tip at the end of a cantilever (Phys. Rev. Letters, 1986, Vol. 56, p 930).

In 1987, Wickramsinghe et al. developed an AFM setup with a vibrating cantilever technique (J. Appl. Phys. 1987, Vol. 61, p 4723), which used the light-lever mechanism.

AFM : COMPONENTS AND THEIR USES

COMPARISON BETWEEN AFM AND ELECTRONIC MICROSCOPES

• Optical and electron microscopes can easily generate twodimensional images of a sample surface, with a magnification aslarge as 1000X for an optical microscope, and a few hundredsthousands ~100,000X for an electron microscope.

• However, these microscopes cannot measure the vertical dimension (z-direction) of the sample, the height (e.g. particles) or depth (e.g. holes,pits) of the surface features.

• AFM, which uses a sharp tip to probe the surface features by rasterscanning, can image the surface topography with extremely highmagnifications, up to 1,000,000X, comparable or even better thanelectronic microscopes.

• measurement of an AFM is made in three dimensions, the horizontalX-Y plane and the vertical Z dimension. Resolution (magnification)at Z-direction is normally higher than X-Y plane.

ATOMIC INTERACTION AT DIFFERENT TIP-SAMPLE DISTANCES

Repulsion:

• At very small tip-sample distances (a few angstroms) avery strong repulsive force appears between the tip andsample atoms. Its origin is the so-called exchangeinteractions due to the overlap of the electronic orbitals atatomic distances. When this repulsive force is predominant,the tip and sample are considered to be in “contact”.

Attraction (Van der Waals):

• A polarization interaction between atoms: Aninstantaneous polarization of an atom induces apolarization in nearby atoms – and therefore an attractiveinteraction.

Different modes of tip-sample interaction when in contact

Friction:

The cantilever bends laterally due to a friction force between thetip and the sample surfaces.

Adhesion:

• Adhesion can be defined as “the free energy change to separateunit areas of two media from contact to infinity in vacuum orin a third medium”.

• In general, care has to be taken with the term adhesion, since itis also used to define a force - the adhesion force. In addition tothe intrinsic adhesion between tip and sample, there is anotherone from the capillary neck condensing between the tip andwater meniscus --- interference from the huminity.

Electromagnetic interactions between tip and sample

• Electrostatic interaction: Caused by both the localized charges and the polarization of the substrate due to the potential difference between the tip and the sample. It has been used to study the electrostatic properties of samples such as charges on insulator surfaces or ferroelectric domains.

• Magnetic interaction: Caused by magnetic dipoles both on the tip and the sample.

AFM IMAGING MODES

Contact mode (left): the deflection of cantilever is kept constant. Non-contact mode (right): the tip is oscillated at the resonance frequency and the amplitude of the oscillation is kept constant. Tapping mode: somewhere between the contact and non-contact mode.

CONTACT MODE

Contact mode AFM consists of raster-scanning theprobe (or sample) while monitoring the change incantilever deflection with the split photodiodedetector. A feedback loop maintains a constantcantilever deflection by vertically moving thescanner to maintain a constant photo-detectordifference signal. The distance scanner movesvertically at each x, y data point is stored by thecomputer to form the topographic image of thesample surface. This feedback loop maintains aconstant force during imaging.

Two contact scanning modes: Constant Height and Constant Force

Constant-force mode Constant-height mode

http://www.ntmdt.com/spm-principles/view/constant-force-mode

http://www.ntmdt.com/spm-principles/view/contact-scanning-capacitance-microscopy-constant-height-mode

Constant-force scan v/s constant-height scan

Constant-force• Advantages:

– Large vertical range– Constant force (can

be optimized to the minimum)

• Disadvantages:– Requires feedback

control– Slow response

Constant-height

• Advantages:– Simple structure (no

feedback control)

– Fast response

• Disadvantages:– Limited vertical

range (cantileverbending anddetector dynamicrange)

– Varied force

Tapping Mode AFM consists of oscillating the cantilever at itsresonance frequency (typically ~300kHz) and lightly“tapping” the tip on the surface during scanning.

A feedback loop maintains a constant oscillation amplitudeby moving the scanner vertically at every x,y data point.Recording this movement forms the topographical image.

The advantage of Tapping Mode over contact mode is that iteliminates the lateral, shear forces present in contact mode.This enables Tapping Mode to image soft, fragile, andadhesive surfaces without damaging them, which can be adrawback of contact mode AFM.

http://www.ntmdt.com/spm-principles/view/semicontact-mode

Comparison between the three scanning modes: damage to the sample

Contact mode imaging (left) is heavily influenced byfrictional and adhesive forces, and can damage samplesand distort image data.

Non-contact imaging (center) generally provides lowresolution and can also be hampered by the contaminant(e.g., water) layer which can interfere with oscillation.

Tapping Mode imaging (right) takes advantages of thetwo above. It eliminates frictional forces byintermittently contacting the surface and oscillatingwith sufficient amplitude to prevent the tip from beingtrapped by adhesive meniscus forces from thecontaminant layer.

Imaging by contact and non-contact (tapping) mode

AFM IN LIQUID ENVIRONMENT

One extraordinary feature of AFM is to work in liquid environment. A keypoint for liquid AFM is a transparent solid (usually glass) surface, which,together with the solid sample surface, retains the liquid environmentwhilst maintains stable optical paths for the laser beams. An optional O-ring can be used to form a sealed liquid cell. Otherwise, the system can alsowork in an “open cell” fashion.

Advanced imaging techniques of AFM

Contact-Mode scanning

• Lateral force microscope (LFM) --- measures lateral deflections, shows surface friction.

• Force modulation microscope (FMM) ---detecting surface stiffness or elasticity;

Tapping-Mode scanning

• Phase mode imaging --- detecting surface structure or elasticity property.

AFM v/s SEM

Compared with Scanning Electron Microscope, AFM providesextraordinary topographic contrast direct height measurementsand un-obscured views of surface features (no coating isnecessary).

SEM is conducted in a vacuum environment, and AFM is conducted in an ambient or fluid environment

Si covered with GaP

SEM IMAGE AFM IMAGE

AFM v/s TEM

• Compared with two dimensional Transmission Electron Microscopes, three dimensional AFM images are obtained.

• No expensive sample preparation in AFM is required as compared to TEM and yield far more complete information than the two dimensional profiles available from cross-sectioned samples.

SUMMARY

sem,tem & afm

Science