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E345: Techniques and Processes of failure analysis - Coggle Diagram
E345: Techniques and Processes of failure analysis
L1
Why every thing is built on ICs'
1) Reduction of the interconnection parasitic - an IC with multilevel metallization can substantially reduce the overall wiring length.
2) Full utilization of semiconductor wafer's - as devices can be closed-packed within an IC chip.
3) drastic cost reduction in processing cost - as wire bonding is error prone and time-consuming
Semiconductor materials
1) Insulator - very low conductivities
2) Conductors - high conductivities
3) Semiconductors - conductivity in-between insulators and conductors
Reasons for Failure analysis
Integrated chips (ICs) contain hundreds millions of transistors within the microprocessors
Be it expected/sudden, failures have severe business impacts. And need to understand and solve the issues asap
Finding cause of failure is most important as there's hundreds of thousands of transistors
IC manufacturing
Wafer fabrication
Lithography
Etching
Packaging
Wafer Fabrication Flow (Possible steps that can go wrong)
Thin film formation
Forms insulation and metal layers for semiconductors
Impurity Doping
Form regions with controlled conductivity by adding impurities into bulk regions
Photolithography
To define patterns of structure to be formed
Etching
Form patterned profile by removing unwanted material
Measured Attributes - Tools
Film or layer thickness - Secondary Electron Microscopy (SEM), Transmission Electron Microscope (TEM) and Atomic Force Microscopy (AFM)
Morphology & Roughness - Secondary Electron Microscopy (SEM) or Atomic Force Microscopy (AFM)
Film Stoichiometry & Depth Profile - Rutherford Backscattering Spectroscopy (RBS), Automated Electrical Sort (AES) and X-ray Photoelectron Spectroscopy(XPS)
Bulk Impurities (including atmospherics) - >0.5% ( Electrical Discharge System (EDS), Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), Hot Filament Source (HFS)) and <0.5% ( Secondary Ion Mass Spectrometry (SIMS), Glow Discharge Mass Spectrometry (GDMS))
Surface Composition
Elemental: X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES)
Chemical: X-ray Photoelectron Spectroscopy (XPS)
Organic: Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)
Surface Impurities:
Metallic: Total Reflection X-ray Fluorescence (TXRF), Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS), Surface Secondary Ion Mass Spectrometry (SurfaceSIMS), X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES)
Organic: Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)
IC Failure Analysis Techniques
Electrical Characterization Techniques: Four point probe, Hall measurement
To study the I-V, sheet resistance etc
Optical Techniques: Optical microscopy
For external or internal visual inspection
Thermal Imaging Techniques: Liquid Crystal, Infrared Thermography
To locate the hotspot (e.g. due to high current density)
Ion Beam technique - Focused Ion Beam (FIB), Secondary Ion Mass Spectrometer (SIMS)
X-ray Techniques: X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS)
Electron Beam Techniques: Secondary Electron Microscopy (SEM), Auger Spectroscopy, Energy Dispersive X-ray (EDX), Transmission Electron Microscopy (TEM)
Scanning Probe Technique: Scanning Tunnelling Microscopy (STM) and Atomic Force Microscopy (AFM)
L2
Why Failure Analysis
Modern computer chips contains hundreds millions of transistors within its microprocessors.
Size of transistors are getting smaller to pack more transistor into a set area for better efficiency. i.e. compare a computer in 1900s to a computer in 2023
Failures are inevitable due to complexities and rapidly shrinking devices
Occurs due to several points
1) Product development
2) Qualification
3) yield learning
4) Reliability improvement
5) system manufacture
6) field applications
Main points for failure analysis
Sudden/anticipated failures could result in dire business impacts
Understanding failure and take corrective actions quickly
Goal: locate fault site
Challenging to locate a single failed transistor out of the hundreds of millions
Need a systematic and effective approach to locate the failure
Types of Failures
Short Failures
Cracks causes open circuit
Open Failure
Particle causes short circuit
Reasons for open circuit
Failure fault
( Reported fault )
Open-circuit (O/C)
Failure defect ( Analysis report )
Missing interconnects
Corrosion
Mechanical damage/scratch
Open bond
Open metallization
Metallization mircocracks
Local disruption of interconnects
Die cracking
Failure mechanism / Failure cause ( Interpretation )
Mask errors
1) Moisture ( internal gas; poor seal )
2) Contaminants + moisture
Poor processing
/ handling
1) Contaminant present on pad
2) Overpressure in bonding
3) Package stress
4) Al / Au intermetallic formation
5) Materials compatibility ( purple plague )
6) Fatigue failure
7) EM in metallization
8) Processing defects ( Step coverage; lithography )
9) Corrosion
10) Wrong passivation composition
Stress migration in as-deposited metallization
1) Electrical overstress ( EOS )
2) Electrostatic discharge (ESD)
Thermal / Mechanical stress
Failure Analysis Terminology
Failure mode: nature of device malfunction which resulted in the observed failure characteristics
e.g. open circuit, short-circuit, leakage
Failure Defects: nature of fault causing the device malfunction
e.g. micro-cracks, growth
Failure mechanism: physical phenomenon which produced the failure defects
e.g. electro-migration, corrosion
Root-cause: nature of the action which caused the failure mechanism to occur
e.g. poor design, high current density, stress concentration
Failure Analysis Methodology
Step 1: Verify there is a failure
Errors in production testing can cause good IC chips to be falsely identified as failures
Step 2: Characterize the symptoms of the failure
A thorough understanding of the IC failure condition enables the analyst to proceed efficiently and utilize info from similar failures
Step 3: Verify that these symptoms caused the observed failure
There maybe more than one failure mechanism affecting an IC, but only one of these mechanisms is usually responsible for the malfunction being analysed
Step 4; Determine the root-cause of the failure
Involves localization of the failure and determination of the failure mechanism and root cause
Step 5: Suggest corrective action
Corrective actions can be implemented by a team of design, process, reliability and failure analysis engineers to overcome the failure
Step 6: Document the results of the failure analysis
Documentation communicates the result of the analysis and forms a historical record that is invaluable for future failure analysis work
Failure analysis process flow
1) fault localization
identifying the location of fault from a sea of transistors / device
2) De-processing
reversal of wafer fabrication process; films are removed in reverse order of application using etching process
3) Defect localization and characterization
Pin-point the location of defects and gather as much info as possible about the defect and it's location since subsequent steps are irreversible
4) Inspection and physical characterization
Inspection of defects and further characterization is performed to gather info to find out the root-cause
Moore's Law - Device scaling
no. of transistors on a chip will doubled every 18 months
Have to reduce size of transistors to continue increase the number
L3
Measured Attributes
Process and Device Evaluation
Throughout the wafer-fabrication process, many tests and measurements are needed to determine wafer quality and process performance
One process mistake can make wafer completely useless
One "Killer" defect can ruin a die
Good characterization
warns of process that goes out of control & device characterization
essential to analyse circuit performance and conformance for customer specifications
Characterization
Some measurements are direct and some are indirect
Direct measurements
electrical measurements of test wafer and actual devices
Measures direct effect of some process
Physical parameters
Layer thickness and widths, composition and others
What we measure: Defects
solid materials contains large amounts of imperfections
Common Si wafer defects
point defects
Examples: lattice vacancies, self-interstitial atoms, substitutional impurity atoms, and interstitial impurity atoms
Associated with one or two atomic positions, including vacancies, self-interstitials (host atoms that occupy interstitial sites) and impurities
Interstitial solid solutions from for relatively small impurity atoms that occupy interstitial sites among the host atoms
For substitutional solid solution, impurity substitute for hose atoms
Linear & interfacial defects
Examples: Edge dislocation, screw dislocation, An extra portion of a plane of atoms, or half-plane, the edge of which terminates within the crystal
Dislocation is a linear or one-dimensional defect around which some of the atoms are misaligned
Boundaries that have two dimensions and normally separates regions of the material that have different crystal structures and/or crystallographic orientations
Planar defects
Examples: Gain boundaries, stacking faults and external surfaces
Volume bulk defects
Examples: Pores, cracks
Typical dimension & microscopy (Defects)
Microscopes with different resolutions are used to view defects of particular sizes
Subatomic particles: 1X10-14 m to 1X10-11m
Atom / ion diameter: 1X10^-11m to 1X10^-9m
Unit cell edge lengths: 1X10^-9m to 1X10^8m
Dislocations (width): 1X10^-8m to 1X10^-7m
Second phase particles: 1X10^-8m to 1X10^-4m
Grains: 1X10^-8m to 1X10^-2m
Macrostructural features (porosity, voids, cracks): 1X10^-5m to 1X10^-1 m
Typical dimension & microscopy (Microscope)
Scanning probe microscopes: 1X10^-11m to 1X10^-7m
Transmission electron microscopes: 1X10^-10m to 1X10^-4m
Scanning electron microscopes: 1X10^-9m to 1X10^-3m
Optical microscopes: 1X10^-7m to 1X10^-3m
Naked eye: 1X10^-4m to 1X10^-1m
What we measure - impurity
The electrical conductivity of the semiconducting materials is not as high as that of the metals
Electrical properties of these materials are extremely sensitive to the presence of even minute concentration of impurities
Intrinsic semiconductor are those in which the electrical behaviour is based on the electronic structure inherent in the pure material
When the electrical characteristics are dictated by impurity atoms, the semiconductor is said to be Extrinsic
Extrinsic Semiconductors
Extrinsic semiconductors (both N and P type) are produced from materials that are initially of extremely high purity
Controlled concentrations of specific donors or acceptors are then intentionally added
Termed as doping
In extrinsic semiconductors, large numbers of charge carriers (either electrons or holes, depending on the impurity type) are created at room temperature, by the available thermal energy
Increase of impurities for such carries increase the conductivity in doped material
Impurity Measurement Techniques
Undesired / unintended doping (such as contaminants or change of composition) would cause drastic charge in electrical performance of devices
What we measure - Resistivity
One of the most important electrical characteristic of a solid material is the ease with which it transmits an electric current. Ohm's law relates the current to the applied voltage as shows
V = IR
p = (RA) / I
R = resistance of material through which current is passing
I = Distance between the two points at which the voltage is measured
A = Cross-sectional area perpendicular to the direction of the current
p = Resistivity (Independent or sample geometry). The units for p are ohm-meters
Resistivity (p)
Important for starting material as well as for semiconductor devices because it contributes to the device series resistance, capacitance, threshold voltage and other paraeters
The wafer resistivity is usually modified locally during device processing by diffusion and / or ion implantation
Four point probe + Sheet resistance
Four point probe
Commonly used to measure semiconductor resistivity
Absolute measurement without recourse to calibrated standards
measures bulk or thin film specimens
Consists of four equally spaced metal tips with finite radius
High impedance current source is supplied though outer two probes
Voltmeter measures the voltage across the inner two probes
Sheet resistance
Resistance between two ends is
R = p( L / A ) = p( L / W
t ) = ( p / t )
( L / W ) ohms
L/W has no units, p/t unit is ohms. To distinguish between R and p/t is called sheet resistance (ohms/square)
R = Rsh ( L / W ) ohms
Pro of using sheet resistance: When build square features on the wafer, ever the size of the features ( for example 1
1, 2
2, 3*3) are different, the resistance will be the same of these square features.
Metrology an Inspection
General term applied to the measurement physical surface features
Separated into 3 main classifications by application
Critical Dimension (CD) measurement
Overlay measurements
Particle and pattern defect detection
Thin film parameter measurement i.e. resistivity, thickness and stress
Common measured attributes
surface morphology: Thin Film thickness
Defects: Rsistivity
Cause: Impurity / contaminants concentration
L4
Film thickness measurement
Total thickness of the sample
Thickness of variously doped semiconductor layers
Thickness of external layers of foreign materials such as dielectric or metal films deposited on a semiconductor surface
Thickness typically range from a few millimetres to a few nanometres and thus require several different techniques in order to provide reasonable sensitivity over such as extended range
Optical Techniques
widely used as:
applicable to opaque and transparent films
generally yields thickness values of high accuracy
measurements are rapidly performed, frequently non-destructive, and utilize relatively in-expensive equipment
Interferometry and ellipsometry are basic optical methods for determining film thickness
Both interferometry and ellipsometry, incident light impinges on the film surface and a reflected portion is measured
Interferometry
If light is light is irradiated onto a wafer some beams of light are reflected on top of it and some penetrate into the film. The latter will be reflected on bottom of this layer or penetrate into another layer beneath and so on
These multiple-reflection light waves boost and weaken each other according to their phase difference
The phase difference of each multiple-reflection light is determined by the light wavelength and optical path length (i.e. the distance that light moves back and forth in the thin film multiplied by the film's refractive index).
Optical path difference is related to film thickness
The colour of the film is affected by the thickness and its refractive index so the film colour is merely a rough guide to thickness
Colour chart (Not applicable if the film thickness < half wavelength of the shortest visible light
t = to ( no / nf )
to: Thickness read from the chart
no: Reflective index of chart material
nf: Refractive index of measured film
Full colour chart refer to CA2 WS L4 Q2
Ellipsometer
non-destructive and non-contact
requires neither sample preparation nor special measurement environment.
Used to determine dielectric properties (complex index of refraction) and thickness of thin transparent films (a few Armstrong to a few um)
Measured attributes
film thickness
refractive index
Spectroscopic ellipsometer combined with variable angle of incidence is used
alloy ratio
crystallinity
Interfacial roughness
What happens if the film is not transparent visible light?
Using longer wavelength
e.g. infrared light, provided the layer which may not transparent to
What happens if the film is highly absorbed to the light source?
cannot use ellipsometer for example: metal
Working Principle
Measures change in polarization state of light reflected from the surface of a sample
Arrangement of optical components between the source and detector defines the type of ellipsometer being used. 3 basic designs of ellipsometer are null, polarization modulation and rotating element ellipsometer, but all three are approximately the same work.
Refer to CA2 slide 32
Light
Property
Light can be described as an electromagnetic wave travelling through space. For purposes of ellipsometry, it is adequate to discuss the wave's electric field behaviour in space and time, also known as polarization
When the light has completely random orientation and phase, it's considered unpolarized
For ellipsometry, however we are interested in the kind of electric field follows a specific path and traces out a distinct shape at any point. This is known as polarized light
Polarization
When two orthogonal light waves are in-phase, the resulting light will be linearly polarized The relative amplitudes determine the resulting orientation
If the orthogonal waves are 90 degree out-of-phase and equal in amplitude, the resultant light is circularly polarized
The most common polarization is "elliptical", one that combines orthogonal wave of arbitrary amplitude and phase. This is where ellipsometer gets it name
Mechanical Techniques: Surface Profiler
Takes measurements electromechanically by moving the sample beneath a diamond-tipped stylus
High-precision stage moves a sample beneath the stylus according to a user programmed scan length, speed and stylus force
The stylus is mechanically coupled to LVDT ( Linear Variable Different Transformer )
Surface variations cause stylus to be translated vertically. Electrical signals corresponding to stylus movement are produced as the core position of the LVDT changes.
The LVDT scales an AC reference signal proportional to position change
After A/D conversion, the digitized signals from performing a single scan are stored in computer memory.
Thin film thickness can be measured by sensing the mechanical movement of a stylus as it traces the topography of a film-substrate step. The step can be made by masking part of the area during thin film deposition.
Factors limiting accuracy of stylus measurements:
Stylus penetration and scratching of films:
Occurs on very soft films
Substrate roughness:
Excessive noise is introduced into the measurement as a result and this creates uncertainty in the position of the step especially when the film thickness is rather small
Thin Film Stress
Stresses are generally present in thin films. Such residual or internal stresses directly affect a variety of phenomena including:
1) Adhesion
2) Generation of crystalline defects
3) Formation of surface such as hillocks
Stress tends to increase with thickness, promoting film peeling, stress is a prime limitation to growth of very thick films
Substrate deformation and distortion also necessarily arise from stress in the overlaying films. In IC's even the slightest bowing of silicon wafer presents significant problems
Common issues in aluminium lines, stress contributes to reliability failure such as electro-migration
Cracks
voids
Tensile Stress
Develops when growing film is shorter than oxide
Compatibility, requires both film and substrate have same length
Film is constrained and stretches while substrate contracts
Compressive stress
Compressive stress develops when film is longer than substrate
Sometimes, tensile stresses are large causing film fracture. Similarly, exceedingly high compressive stresses can cause film wrinkling and loss of adhesion
L5
Simple Lens
Thin lens is a lens with a thickness that is negligible compared to the focal length of the lens
Distance along the optical axis between the two surfaces of the lens
Can be +ve or -ve
+ve lens converge light
-ve lens diverge light
Properties of light
Visible light, used in light microscopy
Is a form of electromagnetic radiation contained in discrete units i.e photons
Determine amount of energy associated with proton at a specific wavelength is E = hv
Energy, frequency and wavelength
E = hc / λ
λ = the wavelength (m)
E = energy (eV)
h = Planck's constant ( 6.63 X10^-34 js = 4.14X10^-15 eVs )
c = speed of light
Refraction & Reflection
Refraction
Occurs when light bends
Due to change in speed of light as light passes through different mediums
Reflection
Diffuse reflection
Occurs when light is reflected off rough surface
Mirror-like reflection
Occurs when light is reflected off a smooth surface
Diffraction
Spreading light that occurs when a beam of light contacts an object
In microscope
Light from illuminator is diffracted by the image plane
collected by the objective lens and focused in the image plane
Wave constructively and destructively interfere to form a contrast image
Constructive Interference
Amplitude of the final waves is a sum of amplitude of 2 identical wavs that are in phase
destructive interference
Waves may annihilate each other when they are out of phase with each other by λ / 2
Airy disk
Central diffraction spot that contains
84% of the light from point source
result of passing an extended Electromagnetic wave front through a small aperture.
Diameter of the disk:
D = 1.22λ / NA
D = diameter of the disk
λ = wavelength
NA = numerical aperture
Rayleigh Criterion & Resolution Limit
2 adjacent object points are resolved when the airy disk of 1 point coincides with the first diffraction minimum of the other point in the image
Rayleigh set the limit for the smallest resolvable distance between 2 points as D = 1.22λ / NA
Rayleigh Resolution Limit
Numerical Aperture, NA, of the lens is the light collecting ability of the lens entering the objective form a foxed object distance
Related to the refractive index of a material that represents the optical density between materials like space between objective lens and specimen. (optical density - the speed of propagation of light rays)
NA = n * sinθ
θ = half of collection angle of objective
NA = Numerical aperture
n = refractive index of the lens working medium (refractive index of air = 1)
To improve resolution, n or sineθ needs to increase
Resolution VS Magnification
Magnification = how much bigger a sample appears under the microscope than in real life
Resolution = ability to distinguish between 2 points on an image at the smallest separation
Function of microscope = Enhance resolution & enlarges view of object that cant be seen by the human eye
Enlargement of image not necessary means higher/increased resolution. Increase resolution = image size increase not resolution
Commonly used contrast technique
Differences in light intensity between the specimen and the adjacent background relative to the overall background
Contrast mechanism
Bright field: Produce a dark image against a brighter background
Dark field: Produce a bright image against a dark background
Phase contrast: Enhance contrast for object when there is small phase shifts in the light passing through a transparent speciment
Chromatic & Spherical Aberrations
Chromatic Aberrations:
Occurs as lens refracts light differently depending on the wavelength
refer to CA3 slide 20
Solution: Use a compound lenses that have different colour dispersing properties
Spherical Aberrations:
Due to spherical surfaces of lens
refer to CA3 slide 20
Solution: Use a combination of +ve and -ve lenses with different thickness
L6
Electron Microscopy
Used due to shrinking size of transistors
Advance microscopy techniques with higher resolution (e.g. SEM, TEM, AFM) are needed.
Electron beam microscope is developed to offer a better spatial resolution to overcome the limitation of light microscope due to diffraction of light
Optical Microscopy vs SEM
Optical Microscope (OM):
The smallest details which can be distinguished in a light microscope are approximately 200nm but an unaided human eye can only detect details up to 200um. As a result any magnification > 1000X only makes details bigger by not finer details visible.
Known as 'Empty Magification'
Higher magnification, requires 3 or more stages of magnification that introduces extra aberrations to the systems
Magnification: 5X-1500X, Depth of Field: 0.5mm, Resolution: approxi 0.2um
Scanning Electron Microscopy (SEM)
Capability on order of magnitude better than optical microscope due to the small wavelength of the probed beam
wavelength of visible light is between 400 to 700nm
wavelength of electrons about 3600 eV or 0.02nm
Magnification: 10X-500KX, Depth of Field: up to cm, Resolution 1.5nm
How it works:
(Uses electrons instead of light to form an image)
Beam of electrons is produced at the top of the microscope by heating of a metallic filament
Electron beam follows a vertical path through the column of the microscope
Makes it through electromagnetic lenses which directs and focuses beam down towards sample
When electron beam hits sample, other electrons are ejected from sample and collected by detector
Converted signal is sent to a viewing screen, producing an image
Key components in SEM:
Electron optical column
Vacuum system
Signal detection & display
Depth of Field & Depth of Focus
Depth of Field
Range of distance along the optical axis in which the specimen can move without the image appearing to lose sharpness
Depth of focus
Extend of the region around the image plane in which the image will appear to be sharp
Extent of a zone on the specimen which appears focus at the same time
To improve depth of field:
Decrease the angle of convergence
Increasing the working distance
Decreasing the aperture size
D = (4X10^5
W) / (A
M) um
W = working distance
D = depth of field
d = beam diameter
P = pixel diameter
A = aperture
a = beam convergence angle
m = Magnification
Electron - Solid Interaction
Electron beam strikes a sample, a large no. of signals are generated
Interaction of the primary beam with the sample creates an excitation volume in which the electrons are scattered (Elastic & inelastic scattering)
Elastic: Electrons may be reflected with no loss of energy, absorbed by the specimen, giving rise to secondary electrons with low energy, together with X-rays
e.g. Auger Electron, Backscattered Electron, Secondary Electron, Continuous & Characteristic X-rays
Inelastic: Absorb and give rise to emission of visible light (an effect is cathodoluminescence)
e.g. Auger Electron, Cathoduminescence
Sample Characteristics
SE
Provides info on surface topography. Used in Voltage Contrast & Magnetic Contrast Imaging
BSE
Provides info on atomic no. topography & materal
AE
Surface sensitive & provides info on chemical composition through thickness (EDX, WDX)
Characteristics X-rays
Provides info on chemical composition through thickness
CL
Emitted photons provides electrical info & crystal defects info
SEM Signals - Sample Characteristics
This is sensitive to surface morphology or topography
Secondary Electrons (SE) detectors measure the SE emission intensity as electron beam raster across the sample, especially at the edges where more SE escapes from the sharp edges
Due to low energy (< 50eV), only secondary electrons (SE) from within few nm of surface can escape.
As Secondary Electrons (SE) moves, they will interact with other atomic electrons and loss energy
Due to loss in Kinetic Energy (KE), the avg distance that a SE travels is very small
Most SE are brought to rest within interaction volume
Those created near surface may escape
Topographical Images
Secondary Electrons (SE) & Backscattered Electrons (BSE) coefficient are minimum when the surface of the specimen is perpendicular to the electron beam as there is less possibilities of electrons being scattered out of the surface
Hence SE & BSE electron coefficient varies with tilt of the specimen
BSE is able to get topographical images but as the direction of BSE is more peaked at the forward direction, the yield of electron is also lower.
As electron beam tilts, more electrons are likely to scatter out of the specimen rather than penetrating further into it.
There's relatively lower yield of BSE collected as compared to SE
Rough surface observed with BSE imaging will have more shadows compared to SE imaging.
More examples refer to CA3 slide 33-35
Due to shape of the interaction volumes & it's relationship to the surface of the specimen, electrons are less likely to be emitted from the specimen when the specimen is perpendicular to the beam.
Imaging using SE allows user to study the surface or topography of a sample, resulting in a clearer contrast especially when the surface is not levelled.
Effect is especially prominent when the surface has irregular edge and corners. Topographical images obtained with SE looks like solid objects viewed under light.
Topography image can be attained with BSE by tilting the beam
Backscattered Electrons Imaging
BSE has poor spatial resolution as BSE are produced from entire upper half of the interaction volume
Are sensitive to atomic number variations
heavy elements reflect more electrons than light elements
BSE images contain compositional contrast used to distinguish different phases
BE comes from sub-surface interaction volumes from hundreds nm to less than several um in diameter.
BSE - Atomic Number
no. of secondary electrons emitted from the specimen for each incident electron is know as Secondary Electron Coefficient (δ)
No. of backscattered electrons emitted from the specimen for each incident electron is known as Backscattered Electron Coefficient (η)
Refer to CA3 slide 37 for diagram, the figure shown is the yield of the backscattered electron is dependent on the atomic no. of the specimen whereas the yield of the secondary electron is not
BSE Penetrates Deeper into the Ionization Pear then Secondary Electrons (SE), it is able to Yield More Compositional Information since the BSE coefficient Varies With The Atomic Number and different atomic number provides Different Compositional Magnitudes.
Transmission Electron Microscopy
A technique used for analysing the morphology, crystallographic structure and even composition of a specimen
Provides much higher spatial resolution than SEM.
SEM & TEM differences
Similarities
Both use a beam of electron directed at the specimen.
(But the way images are produced and magnified are entirely different)
SEM image: electron beam is swept across inspected area, producing many signals that are analysed and translated into images of topography
TEM image: Collects and processed the portion of the primary beam that is transmitted through or to the other side of the specimen, to form an image
Differences
Profile
TEM
Provides info on the Internal Structure of thin specimen but SEM is used to study the Surface, Near Surface or Structure of bulk specimen
Signals are Scattered Of Primary Beam electrons by the atoms of the specimens when they are being Transmitted Through The Specimen
SEM
Signals is a result of the Surface Interaction Of Primary Beam With The Sample. e.g. secondary electrons, back scattered electrons
Specimen
SEM has a big +ve over TEM: Ease of specimen preparation
Conducting specimens, SEM needs no special preparation while insulating materials require a thin coat of metal
Accelerating Voltage
Max Accelerating Voltage for SEM typically at 30kV which is less than TEM at a range of medium voltage (300-400kV) and high voltage (600-3000kV)
SEM's electron gun is smaller and require less isolation
Others
SEM uses fewer lenses since image formation uses the scanning principle. TEM require additional imaging lenses.
Summary
TEM
Passes a beam of electrons through the specimen
Electrons that pass through the specimen are detected by a fluorescent screen on which the image is displayed
Thin sections of specimen are needed for TEM as the electrons have to pass through the specimen for image to be produced
Has best resolution
SEM
Pass beam of electrons over surface of specimen forming a 'scanning' beam
Electrons reflected off the surface of the specimen if it was coating in heavy metals
Thicker structures can be seen under SEM as electrons don't have to pass through sample to form images Providing good 3D images of surface
SEM resolution is lower than TEM
L7
Probe Microscopy
Scanning Probe Microscopy (SPM)
Provides very high resolution images: 2 to 3Å Armstrong lateral resolution,
0.1Å vertical resolution
3-D imaging i possible
A family of SPM known as the Atomic Force Microscope (AFM) can image both conductors as well as insulators.
Another family known as the Scanning Tunneling Microscope (STM) can image electrically conductive samples.
Doesn't Require extensive sample preparation unlike other high resolution microscopy techniques such as the Scanning Electron Microscope (STM) can image electrically conductive sample.
Operates in air, liquid or vacuum
Can measure electrical, magnetic, optical as well as physical properties
Atomic scale manipulations as well as lithography possible