7/22/2019 3020 Lecture 10 - Eds Wds Fib
1/65
Energy Dispersive X-ray Spectroscopy
EDS, EDX, EDAX, etc.
Detect X-rays emitted by incident electrons Determine compositional information at high spatial
resolution
7/22/2019 3020 Lecture 10 - Eds Wds Fib
2/65
Bremsstrahlung X-rays
Incident e- undergo
Coulombic repulsion with
charged atomic species inthe sample, e- energy is
lost
Conservation of energy and
momentum requires thatphotons are produced
X-ray photons due to
braking (Bremsstrahlung)
gives rise to an energycontinuum
( )
E
EEZiI opcontinuum
=
Gives rise to background in EDS
measurements, must be removedbefore quantification
7/22/2019 3020 Lecture 10 - Eds Wds Fib
3/65
Inelastic Scattering - X-ray emission
After the incoming e- beam excites inner shell e- in the sample, theexcited e- decay back to a ground state
In order to obey energy conservation, they emit an X-ray The X-ray energy is dependent upon the elements present within the sample
Can quantify composition by measuring these energies (EDX)
May also emit an Auger e-
Most X-rays generated are re-absorbed by the sample (will discusslater in quantitative analysis)
X-ray energies are labeled by the type of transition (see graphprevious page):
decay of L to K shells: K decay of M to K shells: K decay of M to L shells: L etc.
K1 = K to LII
K3 = K to LI, but is not an allowed transition (see table in book CD)
7/22/2019 3020 Lecture 10 - Eds Wds Fib
4/65
Inelastic Scattering Characteristic X-ray
Formation
Bohr model - electronsaround an atom willequilibrate in certain energylevels around an atom
Energy levels are dependentupon the nuclear mass andthe number of electrons
7/22/2019 3020 Lecture 10 - Eds Wds Fib
5/65
More Characteristic X-rays
Excited e- decay time ~ 1ps
The probability of a transition is given by:
Incident electron energy (in practice, we want an accelerating voltage 1.5xthe energy of the highest X-ray peak)
Critical Ionization Energy (Ec) a certain E is required in order to excite a
characteristic X-ray (greater than X-ray E) in general, as Z decreases, Ec
decreases - table of all Ec available in table on book CD Cross section for ionization
Fluorescence Yield
Ratio of X-rays produced to the number of shell ionizations
Increases with Z, resets with each X-ray line (K, L, M)
)log(1051.62
20 Uc
UE
bnQ s
c
ss=
7/22/2019 3020 Lecture 10 - Eds Wds Fib
6/65
X-ray Detection
How detect the energy
dispersion of the emitted
X-rays?
Similar to the p-n BSE
detector
X-rays hit a
semiconductor, produceelectron-hole pairs
The number of pairs
produced is proportional to
the energy of the X-rays Si(Li) detector diffusion
produced p-n junction that
has intrinsic region ~mm
range under reverse bias
Si(Li) detectors: must be kept cold (77K) to
stop E field induced diffusion and reduce thermal
noise
Other detectors: HPGe, proportional counters,
microcalorimeters, silicon drift detectors
7/22/2019 3020 Lecture 10 - Eds Wds Fib
7/65
X-ray detection
The number of e-
/hopairs created
equals the X-rayenergy divided by the
energy needed per
charge (3.8eV/charge
in Si(Li) detectors)
ex: for a 5keV X-
ray, 1316 carriers are
produced, which
results in a charge of2x10-16C for most
detectors
How do we measure
such a small charge?
EDS Windows: block stray light, avoid oil and ice
contamination, sometimes seal vacuum
Window Materials: Be (old systems), polymer films (TW,UTW, SUTW, most common in new systems), windowless
7/22/2019 3020 Lecture 10 - Eds Wds Fib
8/65
X-ray detection
The charge is
converted to voltage
by the FET
The voltage pulse is
then shaped and
processed by a set of
amplifiers
Problem: want to
average out noise
(need many pulses),but this must happen
before another X-ray
comes into the
detector and charges
pile up
7/22/2019 3020 Lecture 10 - Eds Wds Fib
9/65
X-ray pulse processing
Solution: use 2
amplifiers
Fast amplifier -
determines when a
pulse is coming in or
not, and if there are 2
overlapping pulses -
discriminator andpile-up inspector
decide whether to
reject or accept the
pulse
Slow amplifier -
actually does the
pulse shaping so the
peak height can be
converted into X-rayenergy
7/22/2019 3020 Lecture 10 - Eds Wds Fib
10/65
Time Constants and Dead Time
Important stuff from all the pulse
processing
Time Constant- time allowed for
the pulse processor to evaluate themagnitude of the pulse
short time constant, more counts per
second, but with decreased energy
resolution
long time constant gives better
resolution, but smaller cout rate
Dead Time - period of time between
when a pulse is processed and the
detector shuts off to when thedetector can measure another pulse
(closely related to the time constant)
7/22/2019 3020 Lecture 10 - Eds Wds Fib
11/65
EDS Spectra
Bremsstrahlung background
Characteristic peaks illustrating the presence ofindividual elements
7/22/2019 3020 Lecture 10 - Eds Wds Fib
12/65
Analyzing EDS Data
What we know:
How X-rays are generated How the X-rays are measured in a detector
How the pulse processor turns the detector signal into
X-ray energies
What we can do with this info:
Analyze what elements are in the samples Quantify how much of each element is in the sample
7/22/2019 3020 Lecture 10 - Eds Wds Fib
13/65
Qualitative EDS
Qualitative EDS - find out what elements are present pick highest peak intensity and highest energy first
match each of the peak lines with the energies of the X-rays
Today - all systems are computerized use software to scan through the element lists and determine what is present by
which peaks line up with what is in your spectrum careful of SUM PEAKS!
*ALL measurable peaks* (K, L, M) must match with the elements you think are
there - just one line ID does not mean it is correct
Problems - many elements have peaks that overlap with otherelements
KMnKCr
KCrKV
KVKTi
K
K
Al, P, S, Cl
SrMOs
LineInterferes withLineElementNote: Dont
forget what
you coated your
samples with!
Au/Pd can
overlap with many
elements
7/22/2019 3020 Lecture 10 - Eds Wds Fib
14/65
Si Escape Peaks
It is probable that an incoming X-ray will
ionize the Si within the detector material,
producing electron/hole pairs, Auger
electrons, and characteristic Si X-rays
Some of these characteristic Si K X-rays
can escape from the detector
This shows itself as an extra peak within
the EDS spectra with an energy equal tothe characteristic X-ray energy minus the
Si K X-ray energy (1.74eV)
The probability of forming escape peaks
increases as the energy of the incoming X-ray approaches 1.74eV (see graph)
Si escape peaks will not occur for X-ray
energies less than the Si K ionization
energy (1.838ev)
7/22/2019 3020 Lecture 10 - Eds Wds Fib
15/65
Detector Resolution
Energy spread of the detector is
dependent upon several factors:
natural line width of the characteristicX-rays
electronic noise (cool with LN2)
pulse processing
abruptness of the p-i-n junction in theSi(Li)
Full width at half maximum
(FWHM) is determined at optimal
conditions and standardized for agiven composition (Mn), time
constant, and dead time
Typical ideal resolutions ~137-
140eV
If the FWHM is known at oneenergy, it can be determined at
other energies by:
FWHM= [2.5(EEref) + FWHMref2]1/ 2
7/22/2019 3020 Lecture 10 - Eds Wds Fib
16/65
Quantitative Analysis
Following qualitative analysis (what elements are present),
we may be able to quantitatively determine whatcomposition is present
What controls X-ray intensity?
Composition (number of X-rays at a certain energy being emittedfrom the sample)
X-rays produced = (ionizations/e-)*(X-rays/ionization)*(atoms/mole)
*(moles/g)(g/cm3
)(cm) Detection process (how many X-rays emitted get measured)
X-rays detected = (X-rays/ionization)*(fraction emitted towards
detector)*(detector efficiency)*(dead time)*(signal processing
efficiency) etc..
7/22/2019 3020 Lecture 10 - Eds Wds Fib
17/65
More Quantitative Analysis
Best way to elminate all of those efficiency factors, etc.is to do Standards Basedanalyses
take a sample of known composition (preferrably close to onethat you are trying to measure) and collect spectra using goodcollection statistics (30% dead time, long collection times,etc.) and standard beam conditions (the same gun bias, spot
size, collection area, etc. you are going to use on your sample) calculate the composition of your sample from:
ZAF = correction for Z (atomic #), A (absorption) and F(fluorescence)
Ci
Cst
ZAF(Ii)
ZAF(Ist)
7/22/2019 3020 Lecture 10 - Eds Wds Fib
18/65
ZAF Correction Factors
Z - larger atoms, more BSE (with relation to standard)
A - X-rays must escape the surface without being reabsorbed
dependent upon the (z) distribution of x-ray production with depth into sample
F - fluorescence - X-ray energy from one element is enough to fluoresce an X-rayfrom a neighboring atom, typically zero for
7/22/2019 3020 Lecture 10 - Eds Wds Fib
19/65
More ZAF
(z) distribution is dependent
upon accelerating voltage!
7/22/2019 3020 Lecture 10 - Eds Wds Fib
20/65
Special Topics - Quant EDS of Thin
Films Thin Film Correction
Model with Monte Carlo
Need to know substrate
comp.
Errors large if thickness of
film unknown High spatial resolution if no
substrate (STEM)
Can also use back to bulkanalysis - use low kV such
that interaction volume is
similar to film thickness
7/22/2019 3020 Lecture 10 - Eds Wds Fib
21/65
Special Topics - Quant EDS of Particles
Particle Analyses
Particle size vs. interaction
volume Alters take off angles
Measure many particles tolook for inhomogeneities /
irregularities in spectra
7/22/2019 3020 Lecture 10 - Eds Wds Fib
22/65
Special Topics - Quant EDS
Surface Roughness Artifacts
Minimize by tilting sample towards
detector
Low Z Analyses
Use lowest possible U to avoid
large A factors and deal with
detection efficiency problems
Use windowless detectors (can
make results suspect)
z corrections important
Quantitative analysis very difficult
7/22/2019 3020 Lecture 10 - Eds Wds Fib
23/65
EDS Summary
Measure composition qualitatively using software, making sure alllines match up
Measure standard composition and ZAF correction factors
Measure sample, quantitatively determine composition usingcorrection factors
Quantitative musts:
choose standards and conditions carefully be aware of ZAF corrections - if ZAF moves significantly away from 1, bad data
Good things about EDS Fast spectra collection
Easy to operate Small, low cost system
Bad things about EDS Spectral artifacts
Poor resolution
7/22/2019 3020 Lecture 10 - Eds Wds Fib
24/65
WDS Intro
Wavelength Dispersive Spectroscopy (WDS)
Used in electron probe microanalysis (EPMA)
Use crystal of known crystallography and takeadvantage of Braggs law to increase resolution
7/22/2019 3020 Lecture 10 - Eds Wds Fib
25/65
WDS Analysis System
7/22/2019 3020 Lecture 10 - Eds Wds Fib
26/65
WDS Crystals
7/22/2019 3020 Lecture 10 - Eds Wds Fib
27/65
WDS Analysis
Advantages
High resolution if take off angle remains same Low Z analysis
Lower background
Disadvantages Slow (need many counts)
Large detector can limit imaging resolution
Mechanical movements needed
7/22/2019 3020 Lecture 10 - Eds Wds Fib
28/65
EDS / WDS Sampling
How determine composition of your sample? BSE imaging can illustrate possible compositional changes in the
specimen Composition from a specific position: spot mode
Composition from set of specific positions: multipoint spectrumcollection
Composition from an area: area scan mode Composition from a boundary: line scan mode
Composition from an array of specific positions: mapping(spectral imaging, dot mapping, X-ray mapping, etc)
Allows for determination of phase distributions
Allows for determination of minor phases
Quantitative mapping possible
7/22/2019 3020 Lecture 10 - Eds Wds Fib
29/65
EDS Mapping - Setup
Qualitative mapping
Max count rate using high beam currents, analytical WD, tilt sample towards detector
Small time constant, low dwell times at each pixel
Ex: 1000cps, 512 x 512 bitmap, 30% deadtime
1 sec dwell time: 375,000 sec, 6240 min, 104 hours!
Better solutions: decrease dwell time (lower total counts) or use smaller energy range for
analysis
Ex: 0.02sec dwell time: 7,500 sec, 125 min, 2 hours
Long scan times: need drift correction
Quantitative mapping Need sufficient counts and energy resolution to do background subtraction, ZAF
corrections
Usually sacrifice spatial resolution to prevent long scans
7/22/2019 3020 Lecture 10 - Eds Wds Fib
30/65
Data Recall (all images courtesy EDAX)
Produce line
profiles from
spectral images
Recall EDS
data from
each pixel(voxel)
Produce
element
maps
7/22/2019 3020 Lecture 10 - Eds Wds Fib
31/65
EDS Mapping
Digital Dot Mapping (spectra at each pixel)
Window around energy region of interest
Displaying data:
Grayscale - intensity of peaks given grayscale value
(need for scaling the data)
Color - intensity again scaled to give bright / dark areas,different colors given to different elements
Modern EDS systems allow for determination and display of
composition at each pixel or along line profiles
7/22/2019 3020 Lecture 10 - Eds Wds Fib
32/65
Quant Map - ZAF Corrected
Pb M
S K
Pb M ZAFPb L ZAF
BSE S K ZAF
7/22/2019 3020 Lecture 10 - Eds Wds Fib
33/65
Live Spectral Mapping
Frame averaging gives gradually better EDS data at each pixel,
displays live maps either elementally or as overlay
7/22/2019 3020 Lecture 10 - Eds Wds Fib
34/65
Electron Energy Loss Spectroscopy
EELS
Typically used in the TEM / STEM
Can be done in parallel and serial modes Measures the amount of energy lost by an electron after
it passes through a sample of some thickness
7/22/2019 3020 Lecture 10 - Eds Wds Fib
35/65
EELS
Bend electron beam around hemispherical
analyzer to separate e- of different energies in
space Can also select a certain window of e- energies to
image with (energy filtered TEM, EFTEM or GIF)
Make maps of composition without STEM
7/22/2019 3020 Lecture 10 - Eds Wds Fib
36/65
EELS
Information gathered from EELS
Characteristic energy loss gives elemental composition,
complementary to EDS Zero loss peak intensity gives sample thickness if mean
free path length is known
Plasmon losses give information about majority carriermobility
Ability to map electronic conductivities, etc.
At high energy resolution, can get information about
bandgap Need specialized electron source
7/22/2019 3020 Lecture 10 - Eds Wds Fib
37/65
FIB
Focused Ion Beam
Controlled milling / machining using ion beams
7/22/2019 3020 Lecture 10 - Eds Wds Fib
38/65
Ion - Solid Interactions
Elastic / Inelastic collisions
Sputtering: Elastic collisions,
momentum transferred fromincident ions into the targetatoms, forming secondary ions(SI)
aka backsputtering
SIs can be measured by its ownplate detector, gives rise todifferent contrast than SEs, similarto BSE
Inelastic scattering: producesSE, phonons, plasmons, etc. Nuclear
Electronic
7/22/2019 3020 Lecture 10 - Eds Wds Fib
39/65
Ion - Solid Interaction Ranges
R = range
Rp = projected range
Xs = projected rangealong a vector normal
to the surface
Rr = radial range
7/22/2019 3020 Lecture 10 - Eds Wds Fib
40/65
Collision Cascades
Collision cascade: moving sea of particles within a solidunder ion bombardment
3 regimes: I. Single knock-on (M1M2, Eo is high)
Majority of atoms within the spike volume move during thecollision cascade
7/22/2019 3020 Lecture 10 - Eds Wds Fib
41/65
More Ion - Solid Interactions
Modeling energy loss in amorphous solids Universal screening function leads to the stopping power of the
target material Take into account both conservation of momentum and the
interatomic potential
Best for MSE scientists to model with SRIM calculations (MonteCarlo, www.srim.org)
Ion Implantation Flux - time rate of flow of energy (ions / cm2 / s)
Fluence - number of particles incident on a specific area (ions /cm2) during a certain time interval
Dose - quantity of ions absorbed by a medium (ions / cm2)
Beam Current / Current Density - time rate of flow (Amps (C/s) orAmps / cm2)
7/22/2019 3020 Lecture 10 - Eds Wds Fib
42/65
More Ion - Solid Interactions
Sputtering - sputteringyield = # ejected particles
per incident ion Depends on target,incident energy, angle ofincidence
In FIB, sputter yield variesbetween 0.1 and 100
Ejected ion energiestypically between 2-5eV
Sputter yield tables
7/22/2019 3020 Lecture 10 - Eds Wds Fib
43/65
Dual beam:
FEGSEM
FIB
EDS
Omniprobe
EBSD
Mounted directlyunder FIB column
Nova 200 Nanolab
7/22/2019 3020 Lecture 10 - Eds Wds Fib
44/65
The FIB Instrument
Similar to SEM
optically Accelerate ions into a
solid sample - but
why? Attachments:
GIS
Liftout / manipulator Dual Beam
EDS, EBSD
7/22/2019 3020 Lecture 10 - Eds Wds Fib
45/65
FIB Column
Optically similar to SEM
Consists of:
Gun (next slide)
Condenser Lens
Beam defining aperture (changes spot size and beam current
alone, no real change in condenser setup)
Beam blanker - Faraday cage which brings beam out of optic
axis and into bulk material
Objective lens - changes focal length
Scan coils
Stigmators
7/22/2019 3020 Lecture 10 - Eds Wds Fib
46/65
FIB Sources
Liquid Metal Ion System (LMIS)
heat Ga metal above melting temperature
Ga flows to a W tip with radius ~2-5m
use field emission to form 2-5nm Ga tip (Taylorcone)
extract Ga+ ions and accelerate them down the
column
Ga flow continuously replenishes source
i
7/22/2019 3020 Lecture 10 - Eds Wds Fib
47/65
FIB Imaging
SE detection
same as SEM (E-T detector)
spatial resolution limited by large interaction volume, aberrations (largeenergy spread for incident beam)
SI detection
can image insulators!!! - as long as charging is not too severe
similar to BSE - only line of sight ions hit the detector Topographical contrast
surface roughness introduces contrast into the electron image
Channeling contrast
ion channeling into individual grains at different orientations givesdifferent backscatter yields, thus contrast
FIB I C l Ch li C
7/22/2019 3020 Lecture 10 - Eds Wds Fib
48/65
FIB Ion Column Channeling Contrast
FIB I i P bl
7/22/2019 3020 Lecture 10 - Eds Wds Fib
49/65
FIB Imaging - Problems
Ion beam can damage / alter the surface you are
trying to image
Sputtering can take the surface of interest away
Beam induced grain growth
Secondary Ga phase formation
FIB Milli
7/22/2019 3020 Lecture 10 - Eds Wds Fib
50/65
FIB Milling
Effects of Z, crystal orientation, incidence angle
Damage (amorphization, theatre curtain)
effects of accelerating voltage, beam current, angle of
incidence
Redeposition
Grain Growth
Ga secondary phase formation
FIB Milli Th t C t i Eff t
7/22/2019 3020 Lecture 10 - Eds Wds Fib
51/65
FIB Milling - Theatre Curtain Effect
FIB Milli R d iti
7/22/2019 3020 Lecture 10 - Eds Wds Fib
52/65
FIB Milling - Redeposition
FIB Milling Effects of Orientation
7/22/2019 3020 Lecture 10 - Eds Wds Fib
53/65
FIB Milling - Effects of Orientation
FIB Milling Parameters
7/22/2019 3020 Lecture 10 - Eds Wds Fib
54/65
FIB Milling - Parameters
Can mill using patterns Rectangle - mill top to bottom, reverse, or side to side
Cross section (swimming pool)
Cleaning cross section (one line at a time progressively towardsendpoint)
Circle or donut (mill top to bottom, reverse, side to side, or annular)
Can also mill using bitmap (24-bit RGB)
R: not used G: 0 - 255 (0 = blanking, 255 = no blanking)
B: dwell time (0 = 100ns, 255 = determined by user)
Only works in 100ns steps
If user defines 500ns dwell time, only 5 milling levels allowed If user defines 500s dwell time, 5000 levels allowed (better
depth control
Can ultimately define depth by changing the number of passes
More FIB Milling Parameters
7/22/2019 3020 Lecture 10 - Eds Wds Fib
55/65
More FIB Milling Parameters
Computer analyzes milling data pixel by pixel
Max number of points = 1Million
Pitch (x,y) = size of mill set in UI / #pixels in image
Pattern resolution = HFW / pixels in length
Overlap = resolution / spot size (given by selected beam
current aperture)
Decreasing overlap (necessary at low beam
currents) changes milling rate and possibly
structure
FIB TEM Specimen Preparation
7/22/2019 3020 Lecture 10 - Eds Wds Fib
56/65
FIB TEM Specimen Preparation
Geometries
H bar or Swimming Pools
Wedge polishing / FIB finishing
Omniprobe Wedge FIB and total liftout technique
Plan-view
In-situ and Ex-situ liftout Artifacts
Ga implantation / contamination - reduce with low kV
cleaning Theatre curtain / Amorphization - prevent with Pt
Spotting
FIB TEM Specimen Prep
7/22/2019 3020 Lecture 10 - Eds Wds Fib
57/65
FIB TEM Specimen Prep
In-situ Liftout
7/22/2019 3020 Lecture 10 - Eds Wds Fib
58/65
In-situ Liftout
Dual Beam FIB
7/22/2019 3020 Lecture 10 - Eds Wds Fib
59/65
Dual Beam FIB
Allows for 3-Dreconstructionsfrom serial sections
IMOD (U.Coloradosoftware)
AMIRA(Mercurysoftware,tgs.com)
EDS - TEM prep
EBSD - TEM prep
X-C SEMpreparation
FIB SIMS
FIB Auger
Courtesy: IMOD
Other Ion Systems in the FIB
7/22/2019 3020 Lecture 10 - Eds Wds Fib
60/65
Other Ion Systems in the FIB
Gas Injection Systems (GIS), i.e., Deposition (e-beam, i-beam)
Allow for deposition / etching of structures using gases other than the Ga beam
precursor chemical is heated and injected into the beam path of the electrons / ions via along needle
precursor is converted into metallic species
Methylcyclopentadienyl Pt trimethyl - (CH3)3(CH3C5H4)Pt
Tungsten Hexacarbonyl, W(CO)6
Trimethylamine Alane, (CH3)3NAlH3
TEOS, TMOS + H2O - SiO2 Phenanthrene, pyrene, napthalene for C deposition
On roadmap: Au, Pd, Co, Fe
Enhanced Etching chemistries
gaseous chemical is injected into the Ga beam path, enhances the etch rate by reacting
with the target material Halogens used most often (XeF2, Cl2, Br2, I2) for Si, SiO2, Al, W
also H2O for PMMA and polyimide
Delineation etch (2,2,2-trifluroroacetamide) removes insulator, leaving Si intact
CuRx - reduces Cu channeling / preferential sputtering
Selective C mill - removes organics and plastics, leaving inorganics intact
GIS Deposition with e- and ions
7/22/2019 3020 Lecture 10 - Eds Wds Fib
61/65
GIS Deposition with e and ions
e- beam deposition
1. Lower dep rate (~0.2)
2. No Ga contamination
3. No surface damage
4. C and porosity content
increased5. Smaller structures
possible
Ion Beam Deposition
1. High dep rate
2. Ga contamination
3. Residual surface
damage
4. Effects of beam taillimit minimum feature
size
3-D Nanofabrication
7/22/2019 3020 Lecture 10 - Eds Wds Fib
62/65
3 D Nanofabrication
Use GIS
deposition to
produce nanoscale3-D structures
Usefulness:
connect lines in IC
devices
lithography mask
repair
MEMS
Milling cap layers
3-D Nanofabrication
7/22/2019 3020 Lecture 10 - Eds Wds Fib
63/65
3 Na o ab ca o
3-D Etches
7/22/2019 3020 Lecture 10 - Eds Wds Fib
64/65
Useful for
variable etchingrate materials
and devices
FIBIX
7/22/2019 3020 Lecture 10 - Eds Wds Fib
65/65
X-ray spectroscopy induced by Ga beam
No Bremsstrahlung X-rays, greatly reduced background
and higher resolution at low Es
Very sensitive at low Es
No high E X-rays detectable
NeednA to get decent spectra
Sample will not last long in spot mode at these high
beam currents