Alex Beck’sScanning Electron Microscope

Basic Manual

LN2 DEWAR

EDS

IR camera

Variable Aperture for fine tuning electron column

Table of Contents

Page #

1. Cover

2. Table of Contents

3. Sample Stage

4. Determining Atomic Number

5. Field of Focus

6. Vacuum Systems

7. Electron Gun

8. Column Environment

9. Beam Sample Interaction

10. Electron Emission

11. Backscattered and Auger Electrons

12. SEM images

13. Energy Dispersive Spectrometer

14. EDS signal processing

15. SEM mapping

16. Preparing Sample

17. Photography

18. Quantitative EDS analysis

MOTORIZED PROGRAMABLE SAMPLE STAGEX-move Left to right on ScreenY-move Top to bottom on ScreenZ-move Vertical Motion up and down

Controls Working Distance. 41/2 mm minimum

Tilt Tilts toward ET Detector onlyRotate 360 degrees on sample axis

Once Door is shut, activate ruffing pump to remove majority of air.Activate Turbomolecular / oil diffusion pump to remove more air.Once air pressure inside SEM falls below 10^-5 torr, turn on electron stream, and begin analysis.

3 SEPARATE INSTRUMENTS for determining atomic number:

Scanning Electron MicroscopeSEM

Base Platform for determining atomic number:

:

Energy Dispersive X-ray Spectrometer EDS

Wave Dispersive X-ray Spectrometer WDS:

nλ = 2 d Sin Θ

Sin -1 λ Char = Θ [set Detector]

2 d

The SEM is for Imaging

Surface Features and TopographySE

Depth of Field Focus

Composition: At. # and ShadowBSE Cathodlumniescence Chemical Variation

The EDS is for Chemical Analysis

Qualitative and Quantitative Analyses Point Analysis Line Analysis

Area Analysis

Elemental Mapping of Area

WHY SEM WORKS BETTER THAN OPTICAL MICROSCOPE

Wavelength of Visible Light:

Violet 400 nm to Red 750 nm wavelength

DeBroglie Equation: λ = h/mv h = Planck’s Constant

m = mass v = velocity

For an electron wavelength this becomes:

λ = 1.23/Voltage

At 10 Kv acceleration voltage: 0.0123 nm wavelength

At 30 Kv acceleration voltage: 0.0071 nm wavelength

Optical field of focus: SEM field of focus:

SEM - Sub Component SystemsVacuum Systems

Electron Gun

Column Environment

Chamber Environment

Stage

Detectors

Beam - Sample Interaction

Software

SEM VACUUM SYSTEMS

Mechanical Pump: [ 10 -3 torr] 760 mm Hg

Roughing Pump

Backing Pump

Vapor Pump: Oil Diffusion Pump 10 –5 10 –6 torr

Turbo Molecular Pump

10 -7 torr

The Electron Gun:

Supplies electrons under variable acceleration voltage [ 1000 to 30,000 volts]

Wehnelt Assembage

Wehnelt Cylinder

Filament

Wehnelt Grid

Anode Plate

Filament Saturation

Filament Current

Ele

ctro

n E

mis

sion

False Peak

THE COLUMN ENVIRONMENT

Condensing Lenses (Electromagnetic)

Controls “Spot Size” [Probe Current]

Final Lens (Electromagnetic)

Apertures: Fixed and Variable

Scanning Coils: Electromagnetic

Stigmator Coils: Electromagnetic fixes Astigmatism in beam shape

Magnetic Lenses can become fixed on certain settings and must be degaussed, every once in a while to eliminate hysteresis memory, which can cause beam misalignment and blurry images. This effect is most pronounced when beam properties are changed, and magnetic field is realigned.

BEAM - SAM PLE INTERACTION

Secondary Electrons (< 50 volts) SE Inelastic Collisions - 95% of electron population

Back Scatter Electrons (5 - 30 Kev) BSE

Elastic Collisions - 5% of electron Population

Characteristic X-rays: Kalpha, K beta, etc.

Bremstrahlung [X-ray Continuum - Breaking Radiation]

Auger Electrons : Low Energy, top 1nm

Cathodluminescence: : visible light

Heat Volume of Excitation

Monte Carlo simulation of interaction volume with changing beam current

10Kv

20Kv

30 Kv

Electron Population

EVERHART - THORNLEY DETECTOR Secondary Electrons Imaging 1960

BACK SCATTER DETECTOR - Centarus DetectorBACK SCATTER DETECTOR - Centarus Detector

Annular Shape

Solid state scintillation detector

Directly above Sample and just below the Pole Piece

Responds to electrons with several Kev and higher.

Images appear without angular light perspective.

Quad Detectors can correct this artificially

Responds to Atomic Number

Auger Electrons

Low energy

Only come from around top 1nm

Require Auger Electron detector, somewhat new and expensive

BSE Image BSE Image

SE ImageSE ImageBSE Image

3.8 ev per hole-e

is organic and very fragile, requires uniform pressure on both of its sides.

Energy Dispersive Spectrometer - EDS

Artifact Signals - Signal Processing

Summation Peaks:Summation Peaks: Produced by extremely high count rates. Small peak appears at exactly twice the ev value of a major peak. Two counts come in so fast that they are counted as one by the pulse Processor (50 nsec recovery)

Example: Al = 1.49 Kev. 2 Al = 2.98 Kev

Silicon Escape PeakSilicon Escape Peak : An incoming photoelectron causes the Si detector to emit a characterisitic 1.74 Kev Kalpha X-ray. If this X-ray escapes the system before being absorbed, it will take that energy away, and a small peak will appear at exactly 1.74 Kev LESS than some major peakZAF CORRECTIONS

Z = Atomic Number: Larger size atoms will produce more X-rays than smaller

A = Absorbence of specific X- rays generated by one element by another.

F = Fluorescence: X-ray emission induced by emission of other atoms.

EDS MAPPING, with user defined color coding.

Takes a long time to scan maps even at lower resolutions.

SE BSE

CLCL

MAP MAP

Preparing Sample

Samples can be placed into SEM naked, but are normally mounted in Epoxy.

Any Sample to be placed in SEM must either be conducting all the way through or be sputtered with an extremely thin conductive coating, usually gold or carbon. This is important as negation of conduction through or around sample results in build up of charge which repels other electrons and disrupts any analysis.

Samples are normally prepared in a cylindrical epoxy mold.

•Too make mold place sample at bottom of container and pour in epoxy solution.

•Once epoxy has dried, remove mold from container.

•To grind mold down to sample use a very low grit sand paper, of for example 240. Use Buchler 1,000 Minimet Polishing, or equivalent.

•After sample is exposed wash sample and hands thoroughly to make sure no grains survive to a finer grit.

•Use an ultrasonic cleaner for approximately 10 minutes to shake out any further grains.

•Grind mold at higher grit ex. 320, then repeat above two steps. Apply even higher grit ex. 600, then repeat above two steps.

•Grind mold with 6 micrometer diamond solution apply same ultra sonic cleaning.

•Polish mold with .05 micrometer diamond solution, and wash.

•Just look at that mirror shine as you attach the mold to a mound with conducting carbon strips which both hold the mold in place and provide conduction to ground.

•Use sputter coater to coat mold with conductor.

•Place sample in SEM and begin analysis.

Photography

Keep voltage around 20 kV

Low magnification images usually have magnification less than 5,000X, with less concern for beam sample interactions.

Long working distance normally around 20-30mm.

Moderate to large spot size 200-300 notwithstanding.

Higher magnification up to 100,000X, lessens depth of field and vice versa.

Resolution is vital and should always be maximized. These are a few ways to increase resolution:

Short working distance, 10-15mm NO LESS.

Small spot size 100 or less.

Small variable aperture.

Use of Stigmater very important.

Quantitative EDS data analysis

For optimal EDS analysis sample should be highly polished.

25V, 15 mm working distance, 400 spot size.

Standardize beam to Cu standard that you place on sample mount.

Processing time 5- 6 (slow), or 3-4 (fast)

Dead time is when EDS is overwhelmed with data. Dead time must occupy less than 1/3 of time, or else recalibrate voltage and spot size.

Aztec software user-friendly quick and easy, however lacks finer intricacies and extra functionality.

Inca software is more professional and accurate and has a wide assortment of advanced features including color coded elemental mapping.