TEM 原理及制样

Lecture13:TEM sample preparationContents1TEM sample holders1 2Parts of a TEM holder42.1Types of TEM holder (5)2.2In-situ TEM holders (6)3TEM sample preparation93.1Electrolytic polishing (9)3.2Ion milling technique (12)3.3Cross section sample preparation (17)3.4Replica technique (18)4Focused ion beam TEM sample preparation20 1TEM sample holdersThe specimen holder is used to insert the sample into the TEM from outside for imaging.The sample is loaded onto the TEM stage for imaging.Now, TEM holder and stage are essentially integrated.The TEM holder plays and important role since it allows for introduction of a sample in ambient conditions into an instrument that is held in ually the holder region is pumped separately before the sample is fully introduced in the TEM.A schematic diagram of some of the pumps used in the TEM is showed infigure1.There are2main types of holder in the TEM1.Top loading-the sample has no connection to the outside.After de-livery of the sample to the stage the holder is withdrawn.Top loading holders were used in earlier instrument version,especially for high res-olution since this minimizes external vibrations.A schematic of a top loading holder is shown infigure2.1MM3030:Materials CharacterisationFigure1:Pumping system in a TEM.Taken from Transmission Electron microscopy-Williams and Carter.2MM3030:Materials CharacterisationFigure2:Top entry loader in(A)cross section and(B)top view.Taken from Transmission Electron microscopy-Williams and Carter.3MM3030:Materials CharacterisationFigure3:Schematic of a side entry holder.Taken from Transmission Electron microscopy-Williams and Carter.2.Side loading-these are the most commonly used holders now.Theadvantage of the side loading holders is that it is possible to provide external stimuli(heat,current,load)while imaging.2Parts of a TEM holderThe focus will be on the side loading holder since it is ubiquitous in use.A schematic of the holder is shown infigure3.Various parts of the holder can be identified from thefigure1.O-ring-Provides the mechanical link to the TEM column.The o-ringseparates the portion of the holder in the ambient from the portion in vacuum.Typically a viton o-ring is used and some holders have more than one.The o-rings are usually greased but the excess grease should be removed before inserting in the TEM since this can affect the vacuum.The way in the holder is inserted in the instrument depends on the specific TEM.2.Jewel bearing-This provides the other mechanical link of the holderto the TEM.This is used for mechanical translation of the sample(in the x-y plane).3.The cup-this actually holds the sample.Typical TEM samples are3mm discs though bulk holders are available(not very common).4.Clamping ring-this clamps and holds the sample in place.Both thecup and the clamping ring are exposed to the electron beam and hence4MM3030:Materials CharacterisationFigure4:Some types of side entry holders.From top a rotation holder, heating holder,cooling holder,double-tilt,and single-tilt holder.Taken from Transmission Electron microscopy-Williams and Carter.should be made of a material with low x-rayfluorescence.Be is the material most commonly used.2.1Types of TEM holderThere are various designs for the side entry holder.Some of them are shown infigure4.1.Single tilt holder-most commonly used holder.Tilting around theaxis of the holder is possible.Shown infigure4.2.Quick change holder-this is a single tilt holder with a clamp instead ofeful for quick change of samples.Philips CM12series TEMs have a quick change holder.3.Multiple specimen holder-some holders can hold more than one sample.5MM3030:Materials CharacterisationFigure5:Single tilt holder with2and5specimen cups.Taken from Trans-mission Electron microscopy-Williams and Carter.This is again for multiple sample preparation.These are usually single tilt holders.Multiple tilt holders are shown infigure5.4.Bulk specimen holders-These are used for bulk specimens that cannotbe made into3mm disks.This is not very common.The area of interest should still be thin enough for the electron beam to pass through.5.Double tilt holder-two tilt angles are possible-in-plane and out-of-plane.This is most commonly used for diffraction and imaging.6.Tilt-rotate holder-this allows to select the tilt axis and then rotatethe specimen.7.Low-background holder-the cup and clamping ring are made of Be.Mainly used for x-ray studies(EDAX)since Be has low x-rayfluores-cence.2.2In-situ TEM holdersOne of the advantages of a side loading holder is that it is possible to have signals from the outside to interact with the sample while imaging.This gives added functionality to the TEM and provides for direct structure-property correlations.Some of the in-situ TEM holders are1.Heating holder-for heating samples ually a resistive heateris used with temperatures as high as1000◦C possible.Care should be6MM3030:Materials CharacterisationFigure6:Bulk specimen holder.Taken from Transmission Electron mi-croscopy-Williams and Carter.taken not to weld the sample to the holder.A heating holder is shown infigure4.2.Cooling holder-holders can go up to liquid nitrogen(77K)or heliumtemperatures(4K).This is mainly used for biological samples,where beam damage is minimized by cooling the sample.A holder is shown infigure4.3.Cryo-transfer holder-cooling holder with cryo-transfer so that thesample is never heated to room temperature.4.Straining holder-holder is used for applying external strain to theTEM ually piezoelectric actuators are used for precise strain application.They can be combined with other in-situ holders,like heating holder,likefigure5.EBIC and CL holders-these can supply electrical current to the sam-ple.There are a number of in-situ holders developed by specific research groups. For example,in-situ thermal evaporation is possible in the TEM.Similarly dynamic TEM holders are available for driving in-situ reactions at high heat-ing rates combined with diffraction measurement.7MM3030:Materials CharacterisationFigure7:Strain holder with heating.Taken from Transmission Electron microscopy-Williams and Carter.8MM3030:Materials Characterisation 3TEM sample preparationSample preparation is an important part of TEM analysis.There are two main criteria for TEM sample preparation1.The sample should be electron transparent.If not the whole sampleat least the ROI should be thin.Typical thickness values for metallic samples should ually,100nm is a upper limit for the sample thickness.2.The sample should be mechanically robust for handling.Usually,the sample to be investigated is directly prepared from the under-lying specimen.Sometimes for bulk specimens replica samples are prepared. This is mainly for studying surface topographies and precipitates.TEM sam-ples are either self-supported or mounted on a grid for analysis.Copper grids are the most commonly used,though for high temperature work Mo grids are used.For nanoparticles and thinfilms a-Cfilm is used as support.a-C has low contrast in the TEM and will not obscure the contrast arising from the specimen.Some typical TEM grids are shown infigure8.The TEM preparation technique depends on the type of sample and also on the property of interest.Mechanical damage during sample preparation might be of an issue for studying dislocation densities in a specimen.Simi-larly cross sectional TEM preparation might be necessary for some samples, which can be a time consuming process.For most samples we need to pre-thin the sample to an initial thickness of100-200µm.Cut a3mm disk from the sample(for bulk specimens).Thin the central disk to a fewµm and then thin further(by different techniques)till sample is electron transparent. Sample sizes in the different stages of TEM preparation are shown infigure 9.3.1Electrolytic polishingElectrolytic polishing is used for conducting samples like metals/alloys in order to produce samples that are electron transparent.The initially sheet thickness can be around a few hundredµm.This can be prepared by rolling or grinding bulk specimens.Similarly,metal coatings on substrates can be peeled offand used for thefinal thinning.Thin discs can also be cut from bulk specimens.This process is called coring.Some of the different equip-ment for coring is shown infigure10.There will be some mechanical damage at the surface due to coring but these can be removed during polishing. These discs are thinned by electrolytic polishing.The most common elec-9MM3030:Materials CharacterisationFigure8:Typical TEM grids.Taken from Transmission Electron microscopy -Williams and Carter.Figure9:Stages in the TEM sample preparation from a bulk specimen. Taken from TEM sample preparation-Sridhara Rao et al,Microscopy:Sci-ence,Technology,Applications and Education.10MM3030:Materials CharacterisationFigure10:Different coring tools(A)mechanical punch(B)Abrasive-slurry disc cutter(B)Ultrasonic cutter(D)Spark-erosion cutter.Taken from Trans-mission Electron microscopy-Williams and Carter.11MM3030:Materials Characterisation trolytic polishing technique is the window technique.The sample is made the anode and a thin stainless sheet is made the cathode.The sample is immersed in the electrolyte,which is usually cooled by water or liquid nitro-gen.Perchloric acid is usually used as the electrolyte.The sample edges are covered by lacquer to expose a’window’,hence the name.The experimental setup and the hole generation are shown infigure11.When a current is applied the material is dissolved from the anode(sample) and deposits on the cathode.The rate of dissolution depends on the cur-rent and applied voltage.The I−V characteristics are shown infigure12. Depending on the current and voltage,there are three regimes-etching,pol-ishing,and pitting.The edges are coated so that material removal will start within the window.Once a hole is formed within the window the sample is pulled out.The region around the hole is usually electron transparent and can be mounted on a TEM grid.The problem with this technique is that it is hard to control the location of the hole in the window.A modification to the window technique is the Bollmann technique.A pointed cathode is used to form an initial hole in the location of choice and this is then replaced byflat electrodes to expand the electron transparent re-gion.A variation of the Bollmann technique,which is very commonly used, is the Jet polishing technique.A schematic of the jet polishing technique is shown infigure13.Here the electrolyte is locally sprayed on the sample, using pointed electrodes.The jet helps in polishing from both ends of the sample and hence it is faster.The method also reduces electrolyte use and can produce large electron transparent areas.The electrolyte polishing technique can be used for samples that are conduct-ing-mostly metals and alloys.For non-conducting samples like ceramics, semiconductors,depositedfilms,other TEM sample preparation techniques are needed.3.2Ion milling techniqueFor non-conducting samples usually grinding and polishing steps are used in order to reduce sample thickness.Some an ultramicrotome is used in order to generate thin samples.These can be either electron transparent or can be used as the starting material for further thinning.The schematic of the technique is shown infigure14.For samples,where ultramicrotome cannot be used then a standard tripod polisher is used in order to thin the sample. This produces samples that are a fewµm thick.Thefinal polishing step is done by an ion beam miller.The schematic of the ion beam miller and an actual instrument are shown in figure15.The sample is bombarded by high energy ions or neutral atoms.12MM3030:Materials CharacterisationFigure11:Window polishing technique.Taken from Transmission Electron microscopy-Williams and Carter.13MM3030:Materials CharacterisationFigure12:I−V characteristics during polishing.Taken from Transmission Electron microscopy-Williams and Carter.Figure13:Schematic of the jet polishing technique.Taken from Transmis-sion Electron microscopy-Williams and Carter.14MM3030:Materials CharacterisationFigure14:Schematic of ultramicrotome technique.Taken from Transmission Electron microscopy-Williams and Carter.15MM3030:Materials CharacterisationFigure15:Schematic of ion beam milling technique.Taken from Transmis-sion Electron microscopy-Williams and Carter.16MM3030:Materials CharacterisationFigure16:Penetration vs.thinning as a function of beam incidence.Taken from Transmission Electron microscopy-Williams and Carter.Usually Ar ions are used and they are formed by passing the Ar gas though a high voltage(4-6keV).The sample is held in vacuum and also usually cooled by liquid nitrogen.The ions are incident on the sample and sputter material away.To minimize ion penetration the beam is usually incident at a low angle( 20◦)though if the angle is very small the sputter rate is small.The trade offbetween ion penetration and thinning rate is shown in figure16.Ion beam is highly controlled and a localized process but it is time consuming.Sputter rates are usually a few˚A per second so that creating an electron transparent sample can take hours,especially if the initial thickness is high.3.3Cross section sample preparationCross sectional samples are routinely interrogated in the TEM,especially for studies of interface.The schematic of the cross section process is shown in figure17.Slices from the sample are cut using a diamond slicer.These slices are placed between spacer layers and then glued on to a grid.The slices are glued in such a way that the interface is parallel to the slot in the grid. This sample is then thinned by standard tripod polishing until it is a few µm thick.Thefinal sample is thinned using a ion beam miller to create an electron transparent sample.17MM3030:Materials CharacterisationFigure17:Cross sectional sample preparation.Taken from Transmission Electron microscopy-Williams and Carter.3.4Replica techniqueReplica technique is used for studying bulk specimens which cannot be de-stroyed to prepare electron specimens.It is also useful for studying surface topography features and precipitates though SEM techniques have gradually replaced replica sample preparation.The schematic of the replica technique is shown infigure18.A replica of the sample surface is prepared using a plastic mold.The mold is then removed from the surface and the surface of the specimen is replicated by the surface of the plastic.A thinfilm of carbon or metal like Cr,Pt is evaporated on the surface of the plastic.Sometimes the evaporation is done from an oblique angle,shadow evaporation,to en-hance the contrast.The plastic is removed by dissolving in a suitable solvent and thefilm is thenfloated on to a grid for analysis.A variation of the replica technique is the extraction replica technique.As the name implies,it is used to extract and study precipitates embedded in a matrix.The schematic of the process is shown infigure19.The bulk sample is etched in order to expose the precipitates.A Cfilm is then evaporated on the sample and then etching is continued until the bulk material is removed leaving behind the precipitates.A TEM image of precipitates removed by the extraction technique is shown infigure20.18MM3030:Materials CharacterisationFigure18:Replica technique for sample preparation.Taken from Transmis-sion Electron microscopy-Williams and Carter.Figure19:Extraction replica technique for sample preparation.Taken from Transmission Electron microscopy-Williams and Carter.19MM3030:Materials CharacterisationFigure20:TEM image of sample prepared by extraction replica technique. Taken from Transmission Electron microscopy-Williams and Carter.4Focused ion beam TEM sample prepara-tionFocused ion beam(FIB)instrument along with SEM is becoming increas-ingly important in TEM sample preparation.The advantage of FIB-SEM is that it is possible to prepare TEM specimens from precise locations in the sample.This is not possible in other techniques since there is always some uncertainty from where thefinal TEM sample will be obtained.One area where the FIB is used extensively is in the microelectronics industry for evaluation of defects in parts of the micro-chip.Here the area to be investi-gated is of the order of tens of nm s,so that conventional TEM preparation techniques cannot be used.The schematic of the2beam FIB-SEM is shown infigure21.The FIB can be considered as an ion beam miller except that Ga ions are used instead of Ar ions in a conventional ion beam miller.The Ga ions are originated from a liquid metal source(Ga melting point is30◦C).The beam is accelerated and scanned over the specific portion of the specimen for milling.Modern FIB-SEM also have gas-injection systems(GIS)for Pt deposition and a mi-cromanipulator(Omniprobe)for removing the specimen from the sample and attaching to the TEM grid.The steps involved in sample preparation are1.Identification of the ROI.2.Deposition of a Pt layer on top of this region.Pt deposition is done20MM3030:Materials CharacterisationFigure21:Schematic of the2beam FIB-SEM.Taken from Transmission Electron microscopy-Williams and Carter.using the GIS.The Pt deposition is done over a region fewµm long and wide and a few nm thick.3.The Ga ions are used to ion mill the region around the Pt layer to createa trench.Sample can be manipulated in-situ with three dimensionaltranslation and rotation possible.The Pt layer is also cut from below to create a lamellar.4.The Omniprobe is used to lift this lamellar from the sample and thenattach it to the TEM grid.The Pt metal is used to weld the lamellar to the grid.5.Thefinal cleaning of the lamellar is done using the Ga ions.Finalthinning to electron transparency is also done.The various stages in making the lamellar are shown infigure22.The entire TEM preparation is done in-situ and the SEM is used for imaging the sample preparation.The FIB can also be used for imaging.The FIB-SEM instru-ment and the electron and ion beam column is shown infigure23.The TEM sample before lift-offand thefinal sample attached to the grid is shown in figure24.Thefinal sample has dimensions of the order ofµm but its thick-ness is of the order of nm,so that it is electron transparent.The FIB-SEM technique is the most versatile of the TEM sample preparation techniques since it can be used for both conducting and insulating samples.It also al-lows for the preparation of TEM samples from specific areas of the sample. But sample preparation and manipulation using the FIB-SEM is technically challenging.21MM3030:Materials CharacterisationFigure22:Stages in creation of the lamella from a specimen are shown. Taken from Transmission Electron microscopy-Williams and Carter.22MM3030:Materials CharacterisationFigure23:Dual beam FIB-SEM instrument with the close-up of the elec-tron and ion beam column.Taken from Transmission Electron microscopy-Williams and Carter.23MM3030:Materials CharacterisationFigure24:TEM lamellar before lift-offand thefinal sample attached to the TEM grid.From P.Swaminathan-unpublished.24。