Advanced Methods of Materials Characterization (Lecture 3)

March 14th, 2014

Last week we talked about…

Electromagnetic Spectrum

• Photons are the basic units of all the electromagnetic radiations.

Some Microscopic Terms

• Resolution

• Magnification

• Brightness

• Contrast

• Depth-of-field

Homework: What are the definitions of these terms?

Chapter 1 Scanning Electron Microscopy (SEM)

Question #1:

Why do we need electron microscopy?

Introduction and History

• Electron microscopes were developed due to the limitations of optical microscopes which are limited by the physics of visible light.

• In the early 1930's this theoretical limit had been reached and there was a scientific desire to see the fine details of the interior structures of organic cells (nucleus, mitochondria, etc.).

• This required 10,000x plus magnification which was not possible using current optical microscopes.

Microstructural Features of our Interests

Grain SizeFrom sub-micrometer to the centimeter regimeGrain Shapes

Precipitate Size

Volume Fractions and Distributions of Various Phases

Defects Cracks and voids

What can we see with an SEM?

TopographyThe surface features of an object or "how it looks", its texture;direct relation between these features and materials propertiesMorphologyThe shape and size of the particles making up the object; directrelation between these structures and materials propertiesCompositionThe elements and compounds that the object is composed ofand the relative amounts of them; direct relationship betweencomposition and materials propertiesCrystallographic InformationHow the atoms are arranged in the object; direct relationbetween these arrangements and material properties

Question #2:

How does Scanning Electron Microscope work?

Scanning Electron Microscope

Electron microscopes are scientific instruments that use a beam of energetic electrons to examine objects on a very fine scale.

How SEM works?

- An electron beam is condensed, accelerated, and focused on a specimen by lens;

- The electron beam hits the specimen, producing, among others, secondary and backscattered electrons;

- These electrons are collected by a detector, converted to a voltage, and amplified.

Electromagnetic Lens

Electron-Solid Interactions

Examples of SEM Image

Sources:http://tupian.hudong.com/3960/11.html

Examples of SEM Image

Sources:http://bbs.labscn.com/showphoto.aspx?photoid=2074http://bbs.1718china.com/thread-27547-1-1.html

Examples of SEM Image

Source: http://www.le.ac.uk/bs/em/sem.htm

Examples of SEM Image

Examples of SEM Image

Today let’s look at…

Question #3:

What is the resolution difference between an electron microscope and an optical/light microscope?

Optical Microscopy

Examples of OM Image

Sources:http://www.emeraldinsight.com/journals.htm?articleid=1524385&show=html http://www.eatechnology.com/specialistbusinesses/analytical/materialstesting/opticalmicroscopy

OM vs. SEM: Resolution

Resolution Limits of OM and SEM

Optical Microscopy VS SEM

Max. Magnification

Depth of Field Resolution

OM ~ 4,000x 0.5mm ~ 0.2m

SEM ~ 500,000x 30mm 1.5nm

Combination of higher magnification, larger depth of field, greater resolution, in addition to compositional and crystallographic information makes SEM one of the most heavily used instruments in academic/national lab research areas and industry.

Question #5:

What are the key components of an SEM?

Components of a SEM machine1. Electron optical column consists of:

– electron source to produce electrons– magnetic lenses to de-magnify the beam– magnetic coils to control and modify the beam– apertures to define the beam, prevent electron

spray2. Vacuum systems consists of:

– chamber which “holds” vacuum, pumps to producevacuum– valves to control vacuum, gauges to monitor

vacuum3. Signal Detection & Display consists of:

– detectors which collect the signal– electronics which produce an image from the signal

Components of SEM

Electric guns Electric guns

Thermonic electric gunsThermonic electric guns

Field emission gunsField emission guns

Lens Lens Deflection coilsDeflection coils

Electro-magnetic lensElectro-magnetic lens

Vacuum systemVacuum system

The electron guns should The electron guns should be kept at high vacuum be kept at high vacuum state! Usually better than state! Usually better than 10-7 Torr.10-7 Torr.

The sample chamber work The sample chamber work at medium vacuum status. at medium vacuum status. Better than 10-3 TorrBetter than 10-3 Torr

Signal detecting and processing system

• SE detector// BSE detector

Key parameters for operations of SEM

HV• Accelerating voltages: 1 KeV-30 KeV• The difference in potential between the filament and the

anode. As the voltage is increased, the electrons travel with higher velocity and are more energetic.

• d=0.61 入 /NA

• Reduction in structural details of the specimen surface in SE mode.//Increased electron build up in insulating samples, causing charging artifacts //Increased heating and the possibility of specimen damage

Key parameters for operations of SEM

• WDWorking distance: Working distance is the distance from the bottom of the SEM column to the sample

The shorter the working distance, the smaller the diameter of the beam is at The shorter the working distance, the smaller the diameter of the beam is at the sample surface. So, when possible, the WD is kept at 10mm or smaller for the sample surface. So, when possible, the WD is kept at 10mm or smaller for high resolution imaging.high resolution imaging.

The disadvantage is that focal depth and depth of field is drastically reduced at The disadvantage is that focal depth and depth of field is drastically reduced at small WD.small WD.

Key parameters for operations of SEM

Spot sizeThe size (cross sectional diameter)

that the cone of the beam makes on the surface of the sample affects :

1) the resolution of the image. 2) the number of electrons

generated (therefore the graininess of the image).

At low magnifications we use a

larger spot size than at higher magnifications.

Key parameters for operations of SEM

Magnification-- Magnification is the enlargement of an image, or portion of an image

By reducing the size of the area scanned by the scan coils, the SEM changes the magnification of the image!

SEM #1SEM #2

SEM #3

Safety in an EM LabSafety in an EM Lab

ChemicalsCommon chemicalsHandlingDisposal, waste managementCleaning and exposure

EquipmentRadiationElectrical safetyServicing

Physical and Mechanical Hazards

Primary Hazard Color Code

FlammablesFlammables RedRed Toxics/HealthToxics/Health BlueBlue Reactives/OxidizersReactives/Oxidizers YellowYellow

Contact HazardsContact Hazards White White

GeneralGeneral Gray,Gray, Green,Green, OrangeOrange

Many labs color code bottles to aid in segregated chemical storage. The assignments given above are standard for most labs and are based upon chemical manufacturer’s color code designations. Liquids should also be stored away from solids.

Chemicals found in an EM labChemicals found in an EM lab

Aldehydes (glutaraldehyde, paraformaldehyde)Carcinogenic, allergies, sensitivityMinimize exposure to fumes

Cacodylate salts~50% arsenicCarcinogenic, toxicReadily absorbed through skin (garlic taste)

Osmium tetroxide (osmic acid)Toxic, irritant, volatileSpills reduced to metallic osmium with corn oil or Na Ascorbate powder, then cat litter to pick up.

Acetone and alcoholsUsed as solvents, dehydrants and cleanersFlammable- keep in flammables cabinetToxicChemicals dissolved can penetrate skin

Propylene oxideHighly flammableCarcinogen

Picric acidDried salts explosive

Resin ComponentsCarcinogenicAllergic reactions when using antihistamines

Heavy metal salts (lead and uranium)ToxicCarcinogenic

HandlingHandling

Wear:GlovesLab coatDust maskClosed toe shoes

Work deep in ventilation hood

Measuring:New spatula for each chemicalMinimize dustHave clean-up equipment availableAll spills are hazardous wasteClean after yourself !

DisposalDisposal

Spent, expired, or surplus chemicals

Minimize waste!Use less toxic alternatives if available.Use a minimal amount - avoid large amounts.

Keep Waste in separate containers - avoid mixingSome can be recycled.Easier to keep track of amounts for manifests.Some chemicals are not compatible.

Clean up after yourself!

Clean Up!

Always clean up any spills, messes.

Make a spill kit

Mercury difficult to clean upCan’t wipe or pick upUse a vacuum with trap, not vacuum cleaner

(volatilizes the mercury)

Treat the cleaning materials as hazardous waste-Not into trash can or down sink

Pumps

Oil filters to minimize inhaling

Liquid N2 and compressed gasses

Explosion of tank

SF6 gas – changes to toxic Fluorine if heated above 200

C

X-ray exposure

Physical damagePhysical damage

CutsRazor BladesGlass knives

Symptoms of mercury poisoning include tremors, tunnel vision, loss of balance, slurred speech, and unpredictable emotions.

InhalationMercury bulbs can explode if old and overheated

(usually after 200-300 hours of use).

Fires:Fires:

ExtinguishersExtinguishers

Emergency showers and eye Emergency showers and eye washeswashes

Chapter 2 Transmission Electron Microscopy (TEM)

Electron-solid interactionsElectron-solid interactions

Secondary electrons (SEM)//Backscattered electrons (SEM ） //Auger electrons (AES)//X-rays (EDS)Transmitted electrons (TEM)

What is TEM?

• Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through.

• An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera.

What is TEM used for?

• Morphology

– The size, shape and arrangement of the particles which make up the specimen as well as their relationship to each other on the scale of atomic diameters.

• Crystallographic Information

– The arrangement of atoms in the specimen and their degree of order, detection of atomic-scale defects in areas a few nanometers in diameter

• Compositional Information (if so equipped)

– The elements and compounds the sample is composed of and their relative ratios, in areas a few nanometers in diameter

DATE NAME EVENT

1897 J. J. Thompson Discovers the electron

1924 Louis deBroglie Identifies a wavelength to moving electrons =h/mv where

= wavelength h = Planck's constant

m = mass v = velocity

(For an electron at 60kV  = 0.005 nm)

1926 H. Busch Magnetic or electric fields act as lenses for electrons

1929 E. Ruska Ph.D thesis on magnetic lenses

1931 Knoll & Ruska First electron microscope built

1931 Davisson & Calbrick Properties of electrostatic lenses

1934 Driest & Muller Surpass resolution of the LM

1938 von Borries & Ruska First practical EM (Siemens) - 10 nm resolution

1940 RCA Commercial EM with 2.4 nm resolution

1945    1.0 nm resolution

Brief History of TEM

Overview of a TEM Instrument

Four Main Components:

•Illuminating System

•Specimen Manipulation

System

•Imaging System

•Vacuum System

- A electron beam is focused by 2 condenser lenses, restricted by a condenser aperture;

- The beam strikes a specimen and part of it is transmitted;

- This transmitted portion is focused by objective lens into an image;

- The image is passed down through enlarge lenses and a projector lens, being enlarged all the way;

- The image strikes the phosphor image screen and light is generated, allowing user to see the image.

How TEM works?

Components of TEM

JOEL-2010 TEM