1. Transmission electron microscopy
Specimen Preparation
Summary of Chapter 10 of Transmission Electron Microscopy: A Textbook for Materials Science.
Transmission electron microscopy

2. Introduction Broad subject with several techniques
Preparation technique mustn't affect measurement
Preparation times range from 5 minutes to 2 days
Specimen must be electron transparent
There are several ‘cook books’ dedicated to different methods of preparing different materials.
The way your specimen is prepared should not affect the data you’re getting. This means you should able to prepare the specimen with such a quality that you will get quality data.
There are preparation methods that only work for certain materials. Hence, the preparation time varies depending on your sample.

3. Safety
Liquids used for polishing solutions can be poisonous, corrosive, or explosive
For example, extremely poisonous hydrofluoric acid (HF)
Important to be up-to-date with laboratory safety before preparing the specimen
Safety is extremely important.
You may damage the equipment or even hurt yourself or your workmates.
For example, if you need prepare the specimen into powder form the powder form might be hazardous even though the material is safe and inert in bulk form.
The liquids used for specimen polishing are hazardous. They include hydrogen cyanide, hydrofluoric acid, nitric acid, and perchloric acid.
Before your specimen preparation it is essential you check with your laboratory manager, the reference texts, and the appropriate materials safety data sheets.

4. Before preparing the specimen
Different samples will require different preparation paths
Specimens either self- supported or on a grid
Mechanical stability is crucial
Different sample geometries
Before preparing the specimen you need to understand what are you looking for.
You need to know if mechanical damage needs to be avoided or no chemical changes can occur.
Here is a flow chart that can be used when answering these questions. Depending what you want some methods will be inappropriate.
A self-supporting specimen is one where the whole specimen consists of one material (which may be a composite).
Otherwise specimens are supported on a grid or on copper washer with a single slot.
It is important that the specimen can be handled. If possible, specimen should not be touched. Vacuum tweezers are recommended.
For example, single crystals of GaAs or NiO break easily. That’s why it is an advantage to have the specimen mounted on a grid.
Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY)

5. Self-supporting disks and grids
Grid may contribute to the X-ray spectrum
The best result is achieved with thin specimens
Nominal disk diameter is 3.05 mm
Specimen should be located at the center of a disk
If X-ray analysis is to be performed on the specimen grid may contribute to the signal.
For example, copper peak can be seen in the spectrum.
Manufacturers use disk diameter of 3.05 mm. Thus TEM specimens are referred as 3-mm disks.
No matter the disk size the specimen that is studied should be located in the center of the disk.
Even when tilted the region of interest will stay in the same position.

6. Pre-thinning Initial thinning involves a slice of 100 and 200 mm thick
Cutting the 3-mm disk from the slice
Pre-thinning the disk to a few micrometers
Three parts when you prepare for the final thinning:
Initial thinning to make a slice of material between 100 and 200 mm thick. Cut the 3-mm disk from the slice. Pre-thin the central region from one or both faces of the disk to a few micrometers.
Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY)

7. Final thinning Electropolishing of electrically conducting samples
Specimen is bombarded and sputtered in Ion milling
Ion beam will penetrate the specimen
Ion implantion will occur
For final thinning of the disks can use electropolishing or ion milling. Electropolishing can only be used for electrically conducting samples such as metals and alloys. This method can be fairly quick (from minutes to an hour) and it can produce foils with no mechanical damage. However, the surface chemistry of the sample can be changed. Moreover, electropolishing uses those hazardous chemicals that was mentioned earlier.
With ion milling you bombard your thin TEM specimens with energetic ions or neutral atoms. Ion milling also involves sputtering of material until the film is thin enough for TEM. With ion thinning several variables can be controlled such as are ion energy, beam energy, and beam profile.
Ion thinning is closely related to ion-beam deposition.
Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY)

8. Cross-section specimens (1/2)
Preparation technique in order to study interfaces
Special type of self-supporting disk
Interface is parallel to the electron beam
To study structural and chemical variations close to an interface
Special type of self-supporting disk. This technique is used for studying interfaces.
Samples are cross-sectioned in order to have the electron beam parallel to the interface. Thus one can look at structural and chemical variations close to an interface.
The most widely studied cross-section samples are semiconductor devices which often have multiple layers and therefore have multiple interfaces. However you could use composite materials, samples with surface layers (e.g., oxide-metal interfaces), MBE specimens, quantum-well heterostructures, etc.

9. Cross-section specimens (2/2)
Sample can be cut and glued together
“Sandwich” is sectioned to see the layers
Low-temperature epoxies to be used in gluing
Glued sections can be cut into 3 mm rods
Essentially there are many ways to prepare these kind of samples. Usually the sample is cut and then glued together to produce several layers.
In the picture is shown how the sections are prepared and then glued to form a so-called sandwich.
Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY)

10. Electropolishing Used for a thin sheet of metal
Sheet is cut into approx. 10 mm square and edges are sealed
Exposed ‘window’ is immersed in electrolyte
Surrounded by a cathode and voltage is applied
Viscous layer will build up on the surface
The sheet will thin in the center
Moving on to preparing specimens for grids or washing. This is alternative to the self-supporting disks. Small electron-transparent portions of the specimens are prepared. Electropolishing is used for metals and alloys.
Thin sheet of metal is cut into a square which is approximately 10 mm on the sides. Edges are sealed with polymer lacquer.
One exposed window is left. The metal is immersed in electrolyte. The viscous layer of electrolyte built up at the surface is controlled with the voltage.
After certain time, determined experimentally, the specimen is removed, cleaned, and turned through 180 degrees.
Done correctly you will get a sheet that is thin in the center.

11. Ultramicrotomy Slices less than 100 nm thick
Used for biological samples and polymers
Recently used also for crystalline materials
Leaves chemistry of the sample unchanged
Fractures and/or deforms the sample
Used mainly for soft materials such as biological or polymers. A piece is cut of from the specimen using a knife (for example, glass).
For harder materials such as crystalline diamond blade is used.
This blade will cut the soft samples but hard/brittle specimens are likely to experience controlled fracture.
Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY)

12. Grinding and Crushing
Brittle materials are easily prepared by crushing
Crushing done in a clean pestle and mortar in an inter liquid
Liquid can be ultrasonically stirred
Drop of the liquid placed on a holey carbon film on a grid
Liquid will evaporate leaving particles on the support film
Brittle materials such as ceramics and minerals are prepared using grinding ad crushing technique.
This is usually done in an inert liquid. This liquid can be sonicated and allowed to settle.
The liquid is then evaporated and samples are left on a film. If it is difficult to distributed the particles to a grip the crushed material can be mixed into epoxy and ultramicrotome the epoxy.

13. Replication and extraction
Direct replication used to study fracture surfaces or surface topography
Carbon film is evaporated on the surface of interest
Underlying film is etched away
Alternatively, soft plastic can be pressed on the surface
Plastic replicate is then pulled off and coated with carbon
Plastic is dissolved and carbon replicate is picked on a support grid
Carbon film is evaporated on the surface. Underlying film is then etched with acid.
Also a plastic can be used to replicate the surface. Plastic is softened, pressed on the surface and allowed to harden.
The plastic replica is then taken off and coated with carbon. The plastic is dissolved and carbon replica is placed on a support grid.
This extraction replication allows the extract a particle from the surrounding matrix. Thus one can analyse the extracted phase alone without interference from the surrounding matrix.

14. Cleaving Oldest technique
Adhesive tape is attached on both sides of the sample
Two pieces of tape is pulled apart
Special variation: small-angle cleaving technique (SACT)
Mechanical cleaving is what is used for example to produce graphene from graphite.
Classical technique uses tape on both sides of the sample which is pulled apart. This is repeated until the sample is thin enough for TEM.
Shade of the sample is indication of the thickness.
Special variation is SACT. A crack is propagated through the sample. This crack is along a plane that is not a natural fracture plane. After this another crack is created that is shallowly inclined to the first fracture surface.
Thus is technique is used for crystalline samples, for example Si, that are coated with thin films or glass.

15. The 90 degree wedge Used for compound semiconductors such as GaAs
Semiconductors that are grown with a (001) surface
Cleaved on planes that are perpendicular to growth surface
Used for specimens that are grown with (001) surface. These can be easily cleaved on the planes that are perpendicular to the (001) plane.
Useful for compound semiconductors such as GaAs.
Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY)

16. Lithography and etching
Technique used in microelectronics
Preferential area is masked
Other parts of the substrate is chemically etched
This is technique that is used in microengineering and especially microelectronics. Where area that is preferred is masked with using lithography and rest of the surface is then etch away chemically. Etchants are chosen depending on the material in question. This technique is used for, for example, compound semiconductors where one layer works as an etch stop.

17. Focused-ion beam (FIB)
Prices are going down, becoming more readily available
FIB an SEM with a built-in ion mill
Well-controlled beam of Ga ions
Polish the specimen thin for TEM
The prices of the instrument are going down. It is becoming more readily available.
An ion beam is used to prepare the specimen. Ga ions are used often.

18. Specimen storing
Periods up to 1 month: dry, inert atmosphere, and inert container
For long-term storing: no gelatin capsules for delicate materials, no slotted grid-holders for anything that might deform
Always use vacuum tweezers
Old specimens can be cleaned by ion polishing or chemical cleaning
Specimens should be used as soon as possible. Storing is not recommended.
However, storing the specimen under a month is still feasible. It required optimal conditions though.
Most often than not the specimen must be kept dry and in an inert container and atmosphere that it does not react.
You can see why it is not recommended to store the specimens for a long time. Long-term storage can kept even more complicated.
However, specimens can always be ion polished or chemically cleaned after storing.

19. Conclusions Most tedious aspect of TEM work Thinner is always better
Quality of data directly proportional to quality of specimen
Need to find method that works best for your material
Safety is important
It is important to know your specimen before the preparation.
You need to choose the right method and do it carefully and safely. The various aspects and preparation methods has been listed.
Thus it is important to choose the correct method. For this to happen you need to know your material and what you want out of it.

20. Reference
Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY)