TEM Sample Preparations 100

8/4/2019 TEM Sample Preparations 100

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SAMPLE PREPARATION FORTRANSMISSION ELECTRONMICROSCOPE

ByEngr.Nafis-ul-Haque(MM-10)

Course code: MM-535Department of Materials EngineeringNED University of Engineering and Technology, Karachi


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Refrences ASM Metal Handbook Vol 10

http:/bama.ua.edu/~hsmithso/index.shtml

G. Thomas and M.J. Goringe, Transmission ElectronMicroscopy of Materials, John Wiley & Sons, 1979

Goodhew, P.J., Humphreys F.J, Electron Microscopy andAnalysis, 2nd ed., Taylor & Francis, 1988.

Loretto, M.H., Electron Beam Analysis of Materials, 2nd ed.,Chapman and Hall, 1984.

Williams, D.B. and Carter, C. B., Transmission ElectronMicroscopy, Plenum, 1996

Brandon, D. and Kaplan, W.D., MicrostructuralCharacterisation of Materials, Wiley, 1999 or latest edition.

Wachtman, J.B., Characterization of Materials,Butterworths-Heinemann, 2000 or latest edition


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Form: Solids (metals, ceramics, minerals, polymers,biological, and so on)

Size: An approximately 5-m-thick, 3-mm-diam disk

Preparation: Bulk specimens must be sectioned andelectrothinned or ion milled to produce regions thatpermit transmission of the electron beam.

Powdered samples are often dispersed on a thincarbon substrate.

AIM OF THE SAMPLE


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The Ideal TEM Sample is

Representative

Thin

Stable

Clean

Flat

Parallel-sided

Easily handled

Conductive

Free from segregation

Self-supporting/or alternatively be able

to support on various types of grids

THE IDEAL TEM SAMPLE


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Common Techniques forInitial Preparation


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1. General Sample Preparation

2. Mechanical TEM Sample Preparation

3. Replica TEM Sample Preparation

TEM SAMPLE PREPARATION METHODS


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Dimpling

Dimpling is based on the mechanical action of abrasion. The friction of grains of amaterialharder than the object to be polished generates a dimple. Abrasion takes placein the presence of a fluid, which acts both as a lubricant and as a cooling agent.

Electropolishing / Chemical polishingThis technique is only suitable for conducting materialssuch as metalsand semi conductors. Whetherfragile or ductile, specimen must be ofbulktype, compactand generally single phased. For somemultiphased materialsand in the presence of precipitates or segregations the appropriate polishing

conditions can remain hardto define.

Ultramicrotomy

Sectioning a sample with the cutting edge of a diamond knife. The fracture is initiated by

the knife and propagates parallel to the surface without disruption of the sectioned slice.Cleaving

This technique is used for the limited materials and is relatively non-specificFIB/ Ion Milling

It is common to thin specimens to electron transparency by bombarding them withenergetic ions

1. GENERAL TEM SAMPLE PREPARATIONMETHODS


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Initial Thining

Disk Cutting Desk punch

Pre-thining Mechanical pre-thining

Tripod polishing

Sandwich Technique

Final Thining

Electropolishing Ion milling

ultramicrotomy

SATGES FOR GENERAL TEM SAMPLEPREPARATION

Sample mounting


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DISK CUTTING

Starting materials is ground or sliced (cleaved) to slabs about200 um in thickness Then the disc can be cut using ultrasonic disc cutter or the

disc punch

Ultrasonic

Disc CutterThe UltrasonicDisc Cutter isideal for cuttingTEM disks frombrittle materialssuch as ceramicsandsemiconductors. Disc Punch

(mechanical)

The Disc Punch is ideal for

cutting TEM disks from Metals

Sheet placedin punch

Use an abrasive slurry of eitherboron nitride or silicon carbide

INITIAL THINNING

Ultrasonic Disc Cutter


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MECHANICAL PRE-THINING

DIMPLING The Dimpler provides with the easiest and

most reliable means to produce manydifferent types of samples for TEM and canbe used on ceramics, many semiconductors,carbons, carbon composites, oxides,borides, silicides, glasses and many others.

When prethinning, the Dimpler produces anultra-high area for successful, artifact-freeion thinning, while maintaining a greateredge thickness to help prevent breakagewhile handling.

The thickness achieved will depend on thematerial being thinned; however, hardspecimens typically can be dimpled to lessthan 5 mm with a 100 mm thick.

Concave dimple

Model 656 Dimple Grinder


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DIMPLER GRINDER

Grinding wheel provides thincenter and durable rim

Pressure, speed, and depth of

grinding can be selected bycontrols

Stop at several m thickness

Dimple grinding of 3 mm discs is usually preparative to another more precisemethod of thinning, such as ion milling, chemical or electropolishing.

Model 980 Precision Dimple Grinder

MECHANICAL PRE-THINING CONT..


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TRIPOD POLISHING

Top view of thetripod polisher

Front view of thetripod polisher

Bottom view of thetripod polisher

It consists in two main tools: a polishing machine and a specimen holdermaintained by the operator. The low speed rotary polisher is fitted witha glass plate as a support for the abrasive disc. The glass or Pyrexspecimen holder is adjusted by means of three micrometer screws(tripod). Polishing is generally carried out using a lubricant (water or

other solvent).

MECHANICAL PRE-THINING CONT..


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SANDWICH TECHNIQUE

Multilayer prepared as a sandwichprior to tripod polishing (1.5-1.7mm wide).

Sandwich inserted into

the metal ring.Sandwich maintainedby the transversalbars. Embedding resinappears in red.

To protect the material surface while achieving a cross section. It

also allows doubling or multiplying the observable material quantityin the same object.

The specimen is cut to the desired dimensions. (1.7 mm wide, 5-10mm long and 0.2-1 mm thick). A thin glue film is spread over eachsurface to be protected. A specimen-glue-specimen sandwich is

formed when the two coated surfaces are put together face to face


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SANDWICH TECHNIQUE

Schematic of a sample sandwich. The films sides of two pieces of the original semiconductor wafer are glued together.They are supported by two dummy pieces. The dummy pieces are usually scraps remaining from other wafers and are thesame material as the substrate.


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ELECTROPOLISHING

Polish 1st side of Sample on Diamond Lapping Filmuntil smooth

Glue Sample onto Copper Grid

Polish 2nd side of Sample until 10 m thick

Ion Milling until 100 nm thick

FINAL THINNING


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ELECTROPOLISHING

The most common technique for thinning electrically conductive materialssuch as metal alloys is electropolishing

The principle behind the method is that the specimen is made the anode inan electrolytic cell so that when current is passed through the cell, thesolution dissolves the alloy and deposits it on the cathode

major limitation of electropolishing is that it cannot be used onnonconducting materials such as ceramics and semiconductors

FINAL THINNING CONT


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ION MILLING

It is common to thin specimens to electron transparencyby bombarding them with energetic ions

The ions knock atoms off the surface in a process calledsputtering, gradually thinning the material.

The ions used are accelerated toward the sample withan applied potential ranging from 4 to 6 keV at anglesranging from 58 to 208 from the sample surface, i.e., ata glancing angle

Usually two ion guns are used so that the sample can bethinned from both sides simultaneously, and the sampleis often cooled with liquid nitrogen to avoid excessiveheating by the ion beams

FINAL THINNING CONT


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ION MILLING

A focused ion beam (FIB) microscope is becoming popular for preparing

thin membranes for TEM

A FIB microscope is similar to a scanning electrom microscope (SCANNINGELECTRON MICROSCOPY) except that it uses electrostatic lenses to focusan ion beam on the specimen instead of electromagnetic lenses andelectrons

The ions are typically accelerated toward the specimen at 30 keV so thatrapid sputtering of the sample occurs


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Focused Ion

Beam Milling


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Schematic of sample after ion milling. The areabordering the hole is electron transparent. The holecrosses the film/substrate interface twice on each piece(four times total for the sample).


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MOUNTING THE SAMPLE

Select stubs that are flat on the surfaceand do not have rounded edges. A step milled around the rim of the stub

can protect the samples that project radiallyoutward. Keep the epoxy that holds down thesample to the stub to a minimum. This makesit easier to remove the samples. A spot of epoxy is used as an index and tocheck when epoxy is cured.

Use a Cut-down and a sharp pair oftweezers to break the sample from the stub. Use a normally prepared gridto mount the sample.

2 MECHANICAL TEM SAMPLE


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2.MECHANICAL TEM SAMPLEPREPARATION

CRUSHINGIt is applicable to materials in bulk or fine particles forms. It is essentiallyused to study ceramics, minerals and mixed composite materials. Materials must be fragile

or rendered fragile, very to medium hard.

CRYO-ULTRAMICROTOMYTo make a thin slice of constant thickness (30-150nm) with very soft materials that cannot be cut at room temperature. This technique hasbeen improved by biologists and suits polymer preparation.

TRIPOD POLISHINGPolishing by gentle abrasive rubbing of a specimen with aslight tilt (0.3-0.7) to obtain an electron transparent wedge with smooth and highlypolished surfaces.

ULTRAMICROTOMYSectioning a sample with the cutting edge of a diamond knife.

The fracture is initiated by the knife and propagates parallel to the surface withoutdisruption of the sectioned slice.

WEDGE CLEAVAGEFracture by cleavage along an atomic plane of the material.Cleavage uses the fact that crystals can split along weakly bonded planes. It depends onplane atom density and type of inter atom bonds. Cleavage is also made easier by thegeneration of dislocations at initial stage.


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3. REPLIA TEM PREPARATION

When it is not practical or possible to put certainspecimens into the TEM, the replica techniquemay be employed to examine surface features.

Replicas may be negative (single stage) or positive(double stage) and a number of materials maybe used to make these replicas.

The materials and methods employed in thisexercise are most suitable for non-biologicalmaterials or for hard, dry biological materials.


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TEM Replica Types

Figure: Three replication techniques:

(a) Direct carbon replica

(b) Shadowed Carbon-platimun Replica

(c) Two-stage plastic-carbon replica


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Making Replica

A metallurgical specimen is generally firstpolished flat, in order to make the laterlifting of the replica as easy as possible,and then etched.

The specimens must now be washed and

dried and the replication stage should bestarted as soon as possible to avoid thedeposition of airborne dust which wouldcontaminate the replica.

A thin layer of carbon must now be

coated onto the etched surface.

This is generally achieved by using acarbon arc source in a vacuum coatingunit.


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Materials

Any of the solid material can be analysedin the TEM microscope, the samplepreparation technique remain the same

but the tools and apparatus may sometime change

Because some materials need special care

for preparation like abrasive cutoff shouldnot be used for low melting temperaturematerial, it may alter the internal

strucuture


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