Secondary electron detectorelectron strikes scintillator and converted to light pulse - Amplified and displayed

Raster the beam over sample and display at the same time and get image (basically an intensity map)

Scan smaller and smaller areas to increase magnification

Object: Convert radiation into an electrical signal which is then amplified

SelectSecondary electrons

Backscattered electrons

X-rays

Auger electrons

Photons from Cathodoluminescence

Absorbed electron current

Incident beamLight

(cathodoluminescence)

BremsstrahlungSecondary electrons

Backscattered electrons

heat

Elastically scattered electrons

Transmitted electrons

Specimen current

Auger electrons

Characteristic X-rays

Sample

Any of the collected signals can be displayed as an image if you either scan the beam or the specimen stage

5 m

Signal Detection

Electron Detectors

Scintillator – Photomultiplier system (Everhart-Thornley, 1960)

1) Electron strikes scintillator

plastic

Li-glass

CaF2 (Eu)

P47

Photons produced

2) Light conducted by light pipe to photomultiplier

3) Signal passes through quartz window into photomultiplier

4) Photons strike electrodes – emit electrons (photoelectric effect)

5) Electrons cascade through electrode stages

output pulse with 105 – 106 gain

Up to 300V potential to collect secondary electrons

Deflect – does not require line-of-sight geometry

Collection efficiency ~ 50% SE

~ 1-10% BSE

Backscattered Electron Detectors

Usually solid state devices

Annular – thin wafer (Si semiconductor)

Extrinsic p-n junction

p-type = positive charge carriers (holes) dominant

n-type = negative charge carriers (electrons) dominant

Use Li as donor

Use B as acceptor

1) Backscattered electron strikes semiconductor

2) Valence electron promoted to conduction band – free to move

Leaves hole in valence band

3) No bias → recombination

Forward bias → current

~ 3.6 eV expended per electron / hole pair

Current of 2800 electrons flows from detector if 10keV electron enters

4) Amplify signal

5) Display

Energy-filtered electron detectorsIn lens detectors

EsB = Energy selective backscatteruses filtering grid

AsB = Angle selective backscatteruses angle

INLENS SE image from a sectionedsemiconductor. Clearly visible: No BSE contrast!

The same section but seen with the LL-BSE; detected with the INLENS EsB at 1.27 kV

Si

Ti

TiN

Si3N4

Simultaneously acquired In-lens SE (left) and EsB image (right) from a fuel cell showing the outer electrode. We see doped ZrO2 and different phases of Ni-oxide.

Gold particles seen with the In-lens SE and AsB detector. We see surface contrast with the In lens SE and crystalline contrast from single elastic scattered BSE electrons (Mott scattering).

11

Beam deceleration: enhancing resolution and contrast

If Bias=0 (no BD):Landing V = HV

What is beam deceleration?New optics mode enabling high resolution imaging and high surface sensitivity at very low kV

BD specifications:• Landing energy range: 30 keV down to 50 eV• The deceleration (Bias) can be continuously adjusted

by the user

Benefits:• Enhances the resolution • Provides additional contrast options• Greatest benefit at 2kV and below

Bias

HVLanding V

Beam

vCD

Sample

TLD

2-mode final lens

Gold on carbon1kV1.75MX imaging, <0.9nm resolution

Gold on carbon2kV2.8MX imaging, <0.8nm resolution

Deprocessed IC1kV600KX imaging

Pt catalyst nanoparticles2kV1.0MX imaging

Low voltage-high contrast detector with beam deceleration

Through-the-lens detector with beam deceleration

Through-the-lens detector without beam deceleration

Pt sample. Landing energy 2keV, Beam deceleration=4kV.

Image Formation

Scanning

Signals are produced as beam strikes sample at single location

To study an area, must scan either beam or sample stage

For beam scanning, there are 2 pairs of scan coils deflecting the beam in X and Y

located in bore of objective lens

Produce a matrix of points – a map of intensities

Output displayed on screen or collected digitally

Each point on specimen corresponds to point on screen

Scanning is synchronized

Emission characteristics produce contrast in resulting image

Topography

Atomic # differences

Etc.

Magnification

Ratio between size of display screen (or recorded image) and size of area on specimen

M = L / l

L = length of scan line on screen

l = length of scan line on specimen

L is fixed, so magnification changed by

changing area scanned on specimen

Mag Area on Sample

10X 1 cm2

1000X 100 μm2

100,000X 1 μm2 specimenscreen

1X10X

Picture Element

Region on specimen to which beam is addressed and from which information is transferred to screen

High resolution screen spot size ~ 100μm diameter

Corresponding picture element depends on magnification

Picture Element size = 100 μm / magnification

= L / N

L = length of scan line on specimen

N = Number of picture elements along the scan line (lines / frame)

Mag Picture Element Size 10X 10 μm 1000X 0.1 μm100,000X 1.0 nm

True focus: area sampled is smaller than picture element size

If beam sampling area extends to at least 2 picture elements

= blurring = “hollow magnification”

No additional information gained by increasing magnification

Depth of Field

Determined by distance where beam broadening exceeds one picture element

Beam broadening due to divergence angle

Depth of field

Plane of focusD

Region of image in effective focus

Long working distance

Sample surface

Short working distance Insert smaller objective aperture to improve D

Depth of Field (D)

Aperture radius( μm)

Mag. 100 200 600

10X 4 mm 2 mm 670 μm

1000X 40 μm 20 μm 6.7 μm

100,000X 0.4 μm 0.2 μm 0.067 μm

Must choose between two modes of operation

1) High resolution = short working distance

2) High depth-of-field = long working distance and / or small aperture

Compared to light microscopes at the same magnification

SEM 10 – 100 X greater depth-of-field

Contrast origins

Compositional differences

Different emitted current intensities for scanned areas of different average atomic #

BSE intensity is a function of Z

1) Regions of high average Z appear bright relative of low Z areas

2) The greater the Z difference = greater obtainable contrast

3) High Z = high η, so z contrast not as high for adjacent pairs of elements higher in periodic chart

Electron Backscatter

Backscattering more efficient with heavier elements

Can get qualitative estimate of average atomic number of target

Image will reveal different phases

Brighter = higher average Z

Topography

Backscattered electrons

If ET detector not biased, or negatively biased

If no SEs are detected, then only those BSEs scattered directly into detector will be counted (line-of-sight geometry)

Those surfaces facing detector will be bright

As if viewing specimen with light source in direction of detector

Topography

Secondary + Backscattered electrons

ET detector positively biased

Collect secondary electrons emitted from all surfaces, more where incidence angle is high

Entire surface appears illuminated

Always some contribution of BSEs

high Z areas

surfaces oriented toward detector

Solid-state detection system - application of the p-n junction diode

Take p-type Si

Apply Li to surface

Diffuses to form p-n junction

Apply reverse bias at high temp (room temp)

expands intrinsic region

Must keep cold (LN2 = 77K) or Li will diffuse

++++++

Depletion width W

Space-charge layers

Direction of built-in field

-------

p n

X-Ray spectrometryEDS: Energy dispersive spectrometry

Inelastically scattered – absorbed

Number of charges created:

N = E / Є

E = photon energy

Є = 3.8 eV for Si

5 KeV photon →

1300 electrons (2 X 10-16 C)

4) Potential sweeps electrons and holes apart

-500 to -1500 V

1) After passing through isolation / protection window (Be, BN, C, etc.) X-ray absorbed (photoelectric absorption) by Si

2) Inner shell ionization of Si → electron ejected with energy = 1.84 eVPhotoelectron creates electron-hole pairs (elevating electrons to the conduction

band)

3) Relaxation of the Si back to the ground state → SiK X-ray or Auger electron

To preamplifier

X-rays

p-type region (dead layer ~ 0.1μm)

Li-drifted, intrinsic region

n-type region

Gold contact surface (~2000Å)

Gold contact surface (~200Å)

Electrons

holes

X-Ray spectrometryEDS: Energy dispersive spectrometry

6) Leads to output pulse (convert charge to voltage in preamplifier)

→ linear amplifier

7) Sort by voltage in a multichannel analyzer

→ voltage histogram

EDS

Resolution ~ 150 eV

If separation < 50eV, very difficult to resolve

If looking for a minor element in the presence of major elements, need even more separation (200eV or more)

Fe – Co

Ti – V

Cr – Mn

Pb – S

Ba – Ti

Si – Sr

W - Si

EDS detector

Silicon Drift Detector (SDD)

Conventional diode = homogeneous electric field between layers

SDD = radially gradient potential field in active volume

Electrons guided toward center readout node

Can process very high count rates (up to 1,000,000 cps)

No LN2 cooling

Wavelength Dispersive Spectrometry (WDS)

Bragg Law:

θ

nλ = 2d sinθ

d

At certain θ, rays will be in phase,

otherwise out of phase = destructive interferencecambridgephysics.com – Bragg’s Law demonstration

d is known - solve for λ by changing θ

Move crystal and detector to select different X-ray lines

Si Kα

S Kα

Cl Kα

Ti Kα

Gd Lαsample

Crystal monochromator

Proportional counter Maintain Bragg condition = motion of

crystal and detector along circumference of circle (Rowland circle)

Spectrometer focusing geometry

Curve crystal to improve collection efficiency

Crystal bent to 2R Crystal bent to 2R, then ground to R – All rays have same angle of incidence and focus to detector

VLPET

Only small areas of the sample will be “in focus” for vertical spectrometers

In focus region = elongate ellipsoid on sample

For vertical spectrometers –

Shortest axis of focus ellipsoid coincides with stage Z (parallel to electron optic axis)

Stage focus extremely important

Light optical system = very short depth of field

Advantageous for focusing X-ray optics

MonochromatorsUse different crystals (or synthetic multilayers) with different d-spacings to get different ranges in wavelength

Smaller d = shorter λ detection and higher spectral resolution

synthetic crystals

pseudocrystals (e.g., stearate films on mica)

layered synthetic microstructures (multilayers) - LSM

“crystal” 2d(Å)LIF Lithium flouride 4.0

PET Pentaery thritol 8.7

TAP (TlAP) Thallium acid phthalate25.76

Ge Germanium 6.532

LAU Lead laurate 70.0

STE Lead stearate 100.4

MYR Lead myristate 79.0

RAP Rubidium acid phthalate 26.1

CER Lead cerotate 137.0

LSM W / Si W / C 45

60

80

90

98

Lowest Z diffracted Resolution Count Rates

Kα LαLIF K In high medium

LLIF high high

PET Al Kr medium high

LPET medium very high

VLPET medium ultra-high

TAP O V low medium

LTAP low high

STE B low medium

LSM Be low very high

1 5 10 50 100

Wavelength (Å)

LIFPET

TAPSTE

Resolution can be improved somewhat with use of collimating slits

Accelerating voltageMonochromator

(“crystal”)

Spectrometer number

Diffraction order

K lines also available on PET

K lines also available on LIFCr, Mn, Fe usually prefer LIF for high spectral resolution

Crystal Comparison

0

100

200

300

400

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

length / PET

Inte

nsi

ty (

cps/

nA

)

PbMa

UMb

PET

LPET

VLPET

Detectors for WDS analysisUsually gas filled counter tubes

1) X-ray enters tube and ionizes counter gas (Xe, Ar)2) eject photoelectron3) photoelectron ionizes other gas atoms4) Released electrons attracted to + potential on anode

wire – causes secondary ionizations and increases total charge collected

5) Collect charge and convert to output pulse – the energy of this pulse will be proportional to the energy of the X-ray - → count

Gas proportional counters

Use Ar, Xe, Kr…

1-3 kV on anode wire

windows

Be

Mylar

Formvar

Polypropylene

“softer” X-rays = thinner windows

Can be sealed, or gas - flow.

Low energy detection: low pressure flow (Ar – 10%CH4 = P-10)

Higher energy : sealed Xe (low partial pressure Xe + CH4) or high pressure P-10

For P-10

28 eV absorbed / electron – ion pair created

MnKα = 5.895 KeV

210 electrons directly created

Increase signal by increasing bias and # of secondary ionizations = gas amplification factor

Gas type

Shift P-10 peak to lower λ by increasing pressure

High pressure

Low pressure

Xe, low pressure

X-ray pulse must be processed by electronics resulting in dead timeAnother X-ray may enter during this time = not counted

Correct for (usually a few microseconds)

N = N’ / (1 – Τ N’)

T = dead time

N’ = measured count rate

N = actual count rate

raw

Pulse Height Analysis

Used to separate energies of overlapping lines (recall: nλ = 2d sinθ)

Variables: bias

baseline

window

Al in chromite FeCr2O4

λ Al Kα = 8.339 Å

λ Cr KβIV = 8.34 Å

E Al Kα = 1.487 KeV

E Cr KβIV = 5.946 KeV

Apatite

λ P Kα = 6.157 Å

λ Ca KβII = 6.179 Å

E P Kα = 2.013 KeV

E Ca KβII = 4.012 KeV

baseline

In integral mode the pulse height analyzer accepts all counts above the baseline

In differential mode, an energy acceptance window is employed to select a particular line

In some cases, the overlap in energy and wavelength is impossible to resolve – must use overlap corrections

V in ilmenite (FeTiO3)

V Kα = 2.5036 Å

Ti Kβ = 2.51399 Å

Sr in feldspar

Sr Lα = 6.8629 Å

Si Kβ = 6.753 Å generally use TAP at this wavelength

baseline

Pb M3-N4

Pb M

Pb M

WDS – background measurement

S sK

S K

S KS K absorption edge

Increasing spectrometer efficiency

WDS – background measurement

Comparison of EDS and WDS

LaPO4

Comparison of EDS and WDS

Comparison of EDS and WDS

Comparison of EDS and WDS

Comparison of EDS and WDS

La Lα1,2

Comparison of EDS and WDS

La Lα1,2

Th interferences on U-M region

Th absorption edges significant for high Th monazite

Brabantite

ThO2

Monazite (LIF monochromator) in wavelength region of NdL

EDS spectrum

EDS vs. WDS

WDS EDS

Element range ≥4 ≥10 (Be) (≥ 4 thin window)

Resolution to 5eV ~150eV

Instant range = eV resolution entire range

Max. count rate 50,000 cps <2000cps (SDD ~ 1,000,000)

Data collection time minutes minutes

Artifacts rare lots

Sensitivity at least 10X EDS

Pk/bkg vs. voltage