electron microscopy · scanning electron microscopy is used for inspecting topographies of...

ELECTRON MICROSCOPY

DR. HARSH MOHANDEPARTMENT OF PHYSICS

M.L.N. COLLEGEYAMUNA NAGAR

(HARYANA)

Scanning Electron Microscopy (SEM)

Scanning electron Microscope (SEM):Principle, Instrumentation, Electron optics, Magnification, Application,

• type of electron microscope capable of producing high-resolution images of a sample surface.

• due to the manner in which the image is created, SEM images have a characteristic 3D appearance and are useful for judging the surface structure of the sample.

Resolution • depends on the size of the electron spot, which in turn

depends on the magnetic electron-optical system which produces the scanning beam.

• is not high enough to image individual atoms, as is possible in the TEM … so that, it is 1-20 nm

Scanning Electron Microscopy (SEM)

Scanning Electron Microscope– a Totally Different Imaging Concept

• Instead of using the full-field image, a point-to-point measurement strategy is used.

• High energy electron beam is used to excite the

specimen and the signals are collected and analyzed

so that an image can be constructed.

• The signals carry topological, chemical and crystallographic information, respectively, of the samples surface.

Advantages of Using SEM over OM

Magnification Depth of Field Resolution

OM 4x – 1000x 15.5mm – 0.19mm ~ 0.2mm

SEM 10x – 3000000x 4mm – 0.4mm 1-10nm

The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time and produces an image that is a good representation of the three-dimensional sample. The SEM also produces images of high resolution, which means that closely features can be examined at a high magnification.

The 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, especially in semiconductor industry.

Scanning electron microscopy is used for inspecting topographies of specimens at very high magnifications using a piece of equipment called the scanning electron microscope. SEM magnifications can go to more than 300,000 X but most semiconductor manufacturing applications require magnifications of less than 3,000 X only. SEM inspection is often used in the analysis of die/package cracks and fracture surfaces, bond failures, and physical defects on the die or package surface.During SEM inspection, a beam of electrons is focused on a spot volume of the specimen, resulting in the transfer of energy to the spot. These bombarding electrons, also referred to as primary electrons, dislodge electrons from the specimen itself. The dislodged electrons, also known as secondary electrons, are attracted and collected by a positively biased grid or detector, and then translated into a signal. To produce the SEM image, the electron beam is swept across the area being inspected, producing many such signals. These signals are then amplified, analyzed, and translated into images of the topography being inspected. Finally, the image is shown on a CRT.

Scanning Electron Microscopy (SEM)

Electron-specimen interaction

Electron Beam and Specimen Interactions

Electron/Specimen InteractionsSources of Image Information

(1-50KeV)

Electron Beam Induced Current (EBIC)

Scanning Electron Microscopy (SEM)

• The energy of the primary electrons determines the quantity of secondary electrons collected during inspection. The emission of secondary electrons from the specimen increases as the energy of the primary electron beam increases, until a certain limit is reached. Beyond this limit, the collected secondary electrons diminish as the energy of the primary beam is increased, because the primary beam is already activating electrons deep below the surface of the specimen. Electrons coming from such depths usually recombine before reaching the surface for emission.

• • Aside from secondary electrons, the primary electron beam results

in the emission of backscattered (or reflected) electrons from the specimen. Backscattered electrons possess more energy than secondary electrons, and have a definite direction. As such, they can not be collected by a secondary electron detector, unless the detector is directly in their path of travel. All emissions above 50 eV are considered to be backscattered electrons.

• Backscattered electron imaging is useful in distinguishing one material from another, since the yield of the collected backscattered electrons increases monotonically with the specimen's atomic number. Backscatter imaging can distinguish elements with atomic number differences of at least 3, i.e., materials with atomic number differences of at least 3 would appear with good contrast on the image. For example, inspecting the remaining Au on an Al bond pad after its Au ball bond has lifted off would be easier using backscatter imaging, since the Au islets would stand out from the Al background.

• • A SEM may be equipped with an EDX analysis system to enable it

to perform compositional analysis on specimens. EDX analysis is useful in identifying materials and contaminants, as well as estimating their relative concentrations on the surface of the specimen.

Scanning Electron Microscopy (SEM)

Principles of SEM

Magnification? Resolution?

Image Formation in SEM

beame-

Beam is scanned over specimen in a raster pattern in synchronization with beam in CRT. Intensity at A on CRT is proportional to signal detected from A on specimen and signal is modulated by amplifier.

A

A

Detector

Amplifier

10cm

10cm

M= C/x

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The Scanning Electron Microscope

• (SEM) bombards a specimen with a beam of electrons instead of light

• Produces a highly magnified image from 100x to 100,0000

• Depth of focus 300X better than optical systems at similar magnification

• Bombardment of the specimen’s surface with electrons– Produces x-ray emissions– Characterize elements present in the material under

investigation

• An electron gun produces a beam of electrons that scans the surface of a whole specimen.

• Secondary electrons emitted from the specimen produce the image.

Scanning Electron Microscopy (SEM)

Figure 3.9b

Beam passes down the microscope column

Electron beam now tends to diverge

But is converged by electromagnetic lenses

Cross section of electromagnetic lenses

Electron beam produced here

Sample

Diagram of Scanning Electron Microscope or SEMin cross section - the electrons are in green

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Scanning Electron Microscope

SEM components

What is SEM

Scanning electron microscope (SEM) is a microscope that uses electrons rather than light to form an image. There are many advantages to using the SEM instead of a OM.

The SEM is designed for direct studying of the surfaces of solid objects

Cost: $0.8-2.4M

Column

SampleChamber

TV Screens

A Look Inside the ColumnColumn

A more detailed

look inside

Source: L. Reimer, “Scanning Electron Microscope”, 2nd Ed., Springer-Verlag, 1998, p.2

Electron Gun

e- beam

α

Cathode Ray Tube (CRT) accelerates electrons towards the phosphor coated screen where they produce flashes of light upon hitting the phosphor. Deflection coilsDeflection coils create a scan pattern forming an image in a point by point manner

Color CRT?

Image Magnification

Example of a series of increasing magnification (spherical lead particles imaged in SE mode)

How an Electron Beam is Produced?

• Electron guns are used to produce a fine, controlled beam of electrons which are then focused at the specimen surface.

• The electron guns may either be thermionic gun or field-emission gun

Electron beam Source

W or LaB6 FilamentThermionic or Field Emission Gun

Thermionic Emission Gun

• A tungsten filament heated by DC to approximately 2700K or LaB6 rod heated to around 2000K

• A vacuum of 10-3 Pa (10-4 Pa for LaB6) is needed to prevent oxidation of the filament

• Electrons “boil off” from the tip of the filament

• Electrons are accelerated by an acceleration voltage of 1-50kV

-

+

Source of Electrons

T: ~1500oCThermionic Gun

W and LaB6 Cold- and thermal FEG

Electron Gun PropertiesSource Brightness Stability(%) Size Energy spread Vacuum W 3X105 ~1 50µm 3.0(eV) 10-5 (τ ) LaB6 3x106 ~2 5µm 1.5 10-6

C-FEG 109 ~5 5nm 0.3 10-10

T-FEG 109 <1 20nm 0.7 10-9

(5-50µm)

E: >10MV/cm

(5nm)

Filament

W

Brightness – beam current density per unit solid angle

Electron Gun

W hairpinLaB6 crystal

FEG

Thermionic Sources

Increasing the filament current will increase the beam current but only to the point of saturation at which point an increase in the filament current will only shorten the life of the emitter

Beam spot image at different stage of heating

Magnetic Lenses

• Condenser lens – focusingdetermines the beam current which impinges on the sample.

• Objective lens – final probe forming determines the final spot size of the electron beam, i.e., the resolution of a SEM.

Electromagnetic Lenses

An electromagnetic lens is essentially soft iron core wrapped in wire

As we increase the current in the wire we increase the strength of the magnetic field

Recall the right hand rule electron will move in a helical path spiralling towards the centre of the magnetic field

Electromagnetic lens

Why Need a Vacuum?

When a SEM is used, the electron-optical column and sample chamber must always be at a vacuum.

1. 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.

2. 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.

The Condenser Lens

• For a thermionic gun, the diameter of the first cross-over point ~20-50µm

• If we want to focus the beam to a size < 10 nm on the specimen surface, the magnification should be ~1/5000, which is not easily attained with one lens (say, the objective lens) only.

• Therefore, condenser lenses are added to demagnify the cross-over points.

The objective lens

The objective lens aperture

Aperture in SEM: either to limit the amount of electrons or enhance contrast

How Is Electron Beam Focused?A magnetic lens is a solenoid designed to produce a specific magnetic flux distribution.

p

q

Magnetic lens(solenoid)

Lens formula: 1/f = 1/p + 1/q

M = q/pDemagnification:

(Beam diameter)

F = -e(v x B)

f ∝ Bo2

f can be adjusted by changing Bo, i.e., changing the current through coil.

The Condenser Lens

Demagnification:

M = f/L

C1 controls the spot size

C2 changes the convergence of the beam

Condenser-lens system

The condenser aperture must be centered

The Objective Lens

• The objective lens controls the final focus of the electron beam by changing the magnetic field strength

• The cross-over image is finally demagnified to an ~10nm beam spot which carries a beam current of approximately 10-9-10-

10-12 A.

The Objective Lens - Focusing

• By changing the current in the objective lens, the magnetic field strength changes and therefore the focal length of the objective lens is changed.

Out of focus in focus out of focuslens current lens current lens currenttoo strong optimized too weak

Objectivelens

Depth of Field

Detector and sample stage

Electron Detectors and Sample Stage

Objectivelens

Sample stage

Topographical Contrast

Topographic contrast arises because SE generation depend on the angle of incidence between the beam and sample.

Bright

Dark

+200V

e-

lens polepiece

SE

sample

Everhart-ThornleySE Detector

Scintillator

light pipe

Quartzwindow

+10kVFaraday

cage

Photomultiplier tube

PMT

Electron beam – Specimen Interaction. Note the two types of electrons produced.

Electrons from the focused beam interact with the sample to produce a spray of electrons up from the sample. These come in two types – either secondary electrons or backscattered electrons.

As the beam travels across (scans across) the sample the spray of electrons is then collected little by little and forms the image of our sample on a computer screen.

We can look more closely at these two types of electrons because we use them for different purposes.

+

-

Inelastic scattering

+

-

Elastic scattering

Energy of electron from beam is lost to atom

An incoming electron rebounds back out (as a backscattered electron)

A new electron is knocked out (as a secondary electron)

• Secondary Electrons:Source Caused by an incident electron passing "near" an atom in the specimen, near enough to impart some of its energy to a lower energy electron (usually in the K-shell). This causes a slight energy loss and path change in the incident electron and the ionization of the electron in the specimen atom. This ionized electron then leaves the atom with a very small kinetic energy (5eV) and is then termed a "secondary electron". Each incident electron can produce several secondary electrons.UtilizationProduction of secondary electrons is very topography related. Due to their low energy, 5eV, only secondaries that are very near the surface (< 10 nm) can exit the sample and be examined. Any changes in topography in the sample that are larger than this sampling depth will change the yield of secondaries due to collection efficiencies. Collection of these electrons is aided by using a "collector" in conjunction with the secondary electron detector. The collector is a grid or mesh with a +100V potential applied to it which is placed in front of the detector, attracting the negatively charged secondary electrons to it which then pass through the grid-holes and into the detector to be counted.

A 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.

The position of the secondary electron detector also affects signal collection and shadow. An in-lens detector within the column is more efficient at collecting secondary electrons that are generated close to the final lens (i.e. short working distance).

Secondary Electron Detector

Side Mounted In-Lens

What are the differences between these two images?

• Backscattered Electrons: Formation Caused by an incident electron colliding with an atom in the

specimen which is nearly normal to the incident's path. The incident electron is then scattered "backward" 180 degrees. Utilization

The production of backscattered electrons varies directly with the specimen's atomic number. This differing production rates causes higher atomic number elements to appear brighter than lower atomic number elements. This interaction is utilized to differentiate parts of the specimen that have different average atomic number.

The most common design is a four quadrant solid state detector that is positioned directly above the specimen

Backscatter Detector

Example of an image using a scanning electron microscope and secondary electrons

Here the contrast of these grains is all quite similar.We get a three-dimensional image of the surfaces.

Grain containing titanium so it is whiter

Grain containing of silica so it is darker

Example of an image using a scanning electron microscope and backscattered electrons

Here the differing contrast of the grains tells us about composition

So how does this work – telling composition from backscattered electrons?

The higher the atomic number of the atoms the more backscattered electrons are ‘bounced back’ out

This makes the image brighter for the larger atoms

Titanium – Atomic Number 22

Silica – Atomic Number 14

+

-

Inelastic scattering

If the yellow electron falls back again to the inner ring, that is to a lower energy state or valence, then a burst of X-ray energy is given off that equals this loss.

This is a characteristic packet of energy and can tell us what element we are dealing with

Understanding compositional analysis using X-rays and the scanning electron microscope

Backscattered Electrons (BSE)

BSE are produced by elastic interactions of beam electrons with nuclei of atoms in the specimen and they have high energy and large escape depth.BSE yield: η=nBS/nB ~ function of atomic number, ZBSE images show characteristics of atomic number contrast, i.e., high average Z appear brighter than those of low average Z. η increases with tilt.

Primary

BSE image from flat surface of an Al (Z=13) and Cu (Z=29) alloy

Effect of Atomic Number, Z, on BSE and SE Yield

Interaction Volume: I

The incident electrons do not go along astraight line in the specimen, but a zig-zagpath instead.

Monte Carlo simulations of 100 electron trajectories

e-

Interaction Volume: II

The penetration or,more precisely, theinteractionvolumedepends on theAcceleration

voltage(energy of electron)and the atomicnumber of thespecimen.

Escape Volume of Various SignalsEscape Volume of Various Signals

• The incident electrons interact with specimen atoms along their path in the specimen and generate various signals.

• Owing to the difference in energy of these signals, their ‘penetration depths’ are different

• Therefore different signal observable on the specimen surface comes from different parts of the interaction volume

• The volume responsible for the respective signal is called the escape volume of that signal.

If the diameter of primary electron beam is ~5nm- Dimensions of escape zone of

Escape Volumes of Various Signals

•Secondary electron: diameter~10nm; depth~10nm

•Backscattered electron: diameter~1µm; depth~1µm

•X-ray: from the whole interaction volume, i.e., ~5µm in diameter and depth

Electron Interaction Volume

5µm

a b

a.Schematic illustration of electron beam interaction in Ni

b.Electron interaction volume in polymethylmethacrylate (plastic-a low Z matrix) is indirectly revealed by etching

Pear shape

Magnification

The magnification is simply the ratio of the length of the scan C on the Cathode Ray Tube (CRT) to the length of the scan x on the specimen. For a CRT screen that is 10 cm square:

M= C/x = 10cm/xIncreasing M is achieved by decreasing x.

M x M x 100 1 mm 10000 10 µm 1000 100 µm 100000 1 µm

Low MLarge x40µm

High Msmall x7µm

2500x 15000x1.2µm

e-

x

Resolution LimitationsUltimate resolution obtainable in an SEM image can be limited by:

1. Electron Optical limitationsDiffraction: dd=1.22λ/α for a 20-keV beam, λ =0.0087nm and α=5x10-3 dd=2.1nmChromatic and spherical aberrations: dmin=1.29λ3/4 Cs

1/4

A SEM fitted with an FEG has an achievable resolution of ~1.0nm at 30 kV

due to smaller Cs (~20mm) and λ.

2. Specimen Contrast Limitations

Contrast dmin

1.0 2.3nm 0.5 4.6nm 0.1 23nm

0.01 230nm

3. Sampling Volume Limitations (Escape volume)

How Fine Can We See with SEM?

• If we can scan an area with width 10 nm (10,000,000×) we may actually see atoms!! But, can we?

• Image on the CRT consists of spots called pixels (e.g. your PC screen displays 1024×768 pixels of ~0.25mm pitch) which are the basic units in the image.

• You cannot have details finer than one pixel!

Resolution of Images: I• Assume that there the screen can display 1000

pixels/(raster line), then you can imagine that there are 1000 pixels on each raster line on the specimen.

• The resolution is the pixel diameter on specimen surface.

P=D/Mag = 100um/Mag

P-pixel diameter on specimen surfaceD-pixel diameter on CRT, Mag-magnification

Mag P(µm) Mag P(nm)10x 10 10kx 10 1kx 0.1 100kx 1

• The optimum condition for imaging is when the escape volume of the signal concerned equals to the pixel size.

Resolution of Images: II

• Signal will be weak if escape volume, which depends on beam size, is smaller than pixel size, but the resolution is still achieved. (Image is ‘noisy’)

Resolution of Images: III

Resolution of Images: IV• Signal from different pixel will overlap

if escape volume is larger than the pixel size. The image will appeared out of focus (Resolution decreased)

Resolution of Images: V

Pixel diameter on Specimen

Magnification µm nm

10 10 10000

100 1 1000

1000 0.1 100

10000 0.01 10

100000 0.001 1

In extremely good SEM, resolution can be a few nm. The limit is set by the electron probe size, which in turn depends on the quality of the objective lens and electron gun.

Depth of Field

D = (µm)AM

4x105W

To increase D

Decrease aperture size, ADecrease magnification, MIncrease working distance, W (mm)

Depth of Field

Image Contrast

Image contrast, Cis defined by

SA-SB ∆SC= ________ = ____

SA SA

SA, SB Represent signals generated from two points, e.g., A and B, in the scanned area.

In order to detect objects of small size and low contrast in an SEM it is necessary to use a high beam current and a slow scan speed (i.e., improve signal to noise ratio).

SE-topographic and BSE-atomic number contrast

SE Images

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Scanning Electron Microscope

The Scanning Electron Microscope is analogous to the stereo binocular light microscope because it looks at surfaces rather than through the specimen.

Main Applications• Topography

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.)

SE Images - Topographic Contrast

The debris shown here is an oxide fiber got stuck at a semiconductor device detected by SEM

1µm

Defect in a semiconductor device Molybdenumtrioxide crystals

BSE Image – Atomic Number Contrast

BSE atomic number contrast image showing a niobium-rich intermetallic phase (bright contrast) dispersed in an alumina matrix (dark contrast).

Z (Nb) = 41, Z (Al) = 13 and Z(O) = 8Alumina-Al2O3

2µm

Can you see the difference?

TEM SEMLIGHT

We learn more from mistakes than successes…