1. Holders and Pumps Summary
Chem-E Electron Microscopy

2. Why to care about pumps and holders
TEM requires high vacuum (HV) or ultra high vacuum (UHV)
The pressure has to be at least 10-5 Pa
The microscope column has to be contamination-free to achieve accurate results
Otherwise excellent results can be ruined with a poor sample holder and the vacuum around it
Hydrocarbons and water vapor are most common contaminats
Holders and Pumps - Summary, Antti Myllynen

3. Vacuum
Vacuum in TEM is usually generated with a combination of exhaust pumps and trapping pumps
Exhaust pumps remove air from the microscope
Trapping pumps trap air until the pump is turned off
High vacuum = Pa
Ultra high vacuum = 10-7 Pa and lower
Modern vacuum systems in TEM’s are generally:
Clean enough
Fully automated
Transparent to the user
Holders and Pumps - Summary, Antti Myllynen

4. Exhaust pumps Roughing pumps Mechanical rotary pumps Can reach 10-1 Pa
Reliable and cheap, but may transmit vibrations to TEM
Diffusion pumps
Oil vapor jets remove air molecules which are extracted by the mechanical pump
Can reach 10-9 Pa
Reliable and cheap, but could insert contaminations to the microscope due to oil
Holders and Pumps - Summary, Antti Myllynen

5. Exhaust pumps Turbomolecular pumps
High speed turbine ( RPM) exhaust gases from the microscope
Can reach UHV region
Not so reliable due to high speeds and could also transmit vibrations to the TEM
Holders and Pumps - Summary, Antti Myllynen

6. Trapping pumps Ion pumps
Are mainly used to maintain HV or UHV and are started only after a pressure of10-3 Pa is reached
Ionization processes are used to remove air
Usually Ti-ions are used
Cryogenic pumps
Liquid N2 is used to cool molecular sieves with high surface area
This cold surface removes air molecules
Can reach 10-4 Pa
Holders and Pumps - Summary, Antti Myllynen

7. Whole vacuum system
The whole system is generally a combination of different exhaust and trapping pumps
For example, roughing pump, diffusion pump and ion pumps can be used in a single system
First, roughing pump is used to create low vacuum
Next, diffusion pump is started
Finally, also ion pumps are started
Holders and Pumps - Summary, Antti Myllynen

8. Leak detection Small leaks are always present
Pumping is done all the time to maintain vacuum
Large leaks cannot be compensated by pumping
The leak can be found by placing a mass spectometer to the pumping line of the microscope and by releasing helium gas close to the suspected leak
Small He-atoms are easily sucked through the leak into the microscope  Easy to register with mass spectrometer
Most common point of failure is the stage airlock because it is used very often
Holders and Pumps - Summary, Antti Myllynen

9. Repairing a leak
The O-ring seal has to be replaced if the leak is in the airlock
After repairing the leak, the microscope column has to be ”baked”
The internal surfaces of the microscope are heated up to at least 100 °C
A temperature of 150 – 200 °C is required for UHV-TEM
Baking is done to boil off any residual water vapor and hydrocarbons that may have entered the column while maintenance
Check manufacturers manual before baking because some TEM accessories, such as XEDS and EELS systems can be damaged by high temperatures
Holders and Pumps - Summary, Antti Myllynen

10. Contaminations
Even though modern TEMS are generally clean enough, contamintaions can be present due to inserting the sample inside the microscope
Hydrocarbons and water vapor are the most common contaminations in TEM
Hydrocarbons are cracked under e-beam and may be deposited to sample surface
Partial pressure of hydrocarbon should be less than 10-9 Pa
Holders and Pumps - Summary, Antti Myllynen

11. Contaminations Contaminations may also come from the operator
Cleaning the airlock is critical to minimize contamintations
Cleaning can be done by several means which depend on the sample:
Heating the specimen to over 100 °C
Cooling the specimen with liquid N2
Pre-pumping the airlock with oil-free pump
Plasma cleaning the specimen holder
Contaminations may also come from the operator
Do not touch anything that goes inside the microscope
Use latex gloves always and do not even breathe on the sample
Holders and Pumps - Summary, Antti Myllynen

12. Sample holder
Is used to hold and move the sample laterally and vertically
Other possibilities are rotating, tilting, heating, cooling, straining and biasing the sample
Possible problems with holders are transmitting vibrations, drift and contaminations to the specimen
A holder can also be a source of x-rays which could degrade analysis
Holders and Pumps - Summary, Antti Myllynen

13. Sample holder Two different designs have been used Side-entry holder
A rod with a motor attached to tilt and/or rotate the sample
Commonly used today because more versatile
Top-entry holder
A cartridge wich is loaded into the TEM
Generally replaced by side-entry holders, because they preclude XEDS analysis in TEM
The specimen in TEM can generally by either 2.3 or 3.05 mm in diameter
Holders and Pumps - Summary, Antti Myllynen

14. Tilt and rotate holders
Single-tilt holder
Tilting around one axis
Quick change holder
Also one axis tilt
Doesn’t hold the specimen very strongly
Don’t use for magnetic species
Multiple-specimen holder
5-sample single-tilt holder or
2-sample double-tilt holder
Holders and Pumps - Summary, Antti Myllynen

15. Tilt and rotate holders
Bulk-specimen holder
Used for surface imaging and diffraction of larger samples, usually ~ 10 mm x 5 mm
Double-tilt holder
Tilt-rotate holder
Low-backgroung holder
Cup and clamping ring are made of Be to minimize the generation of x-rays
Tomography holder
Allows to tilt the sample through a full 360°
Holders and Pumps - Summary, Antti Myllynen

16. In-situ holders
Special holders to change the specimen while observed in TEM
Heating holder
Can heat specimen up to 1300 °C
Make sure that the specimen doesn’t form a eutectic alloy with the holder material
Cooling holder
Either liquid N2 or liquid-He temperatures
Single or double tilt
Minimizes surface-borne contaminations, but can also work as a cryo-pump which would actually attracts contaminations
Holders and Pumps - Summary, Antti Myllynen

17. In-situ holders Cryo-transfer holder
Some liquids, latex emulsions and tissue in general requires cryogenic temperatures
This holder enables transfer cold specimens into the TEM without condensating water vapor
Straining holder
Applies a load to the sample
Dislocations, cracks, etc. can be monitored with TEM
Probing holder
Can be used to ”poke” the specimen while observing the effect with TEM
Holders and Pumps - Summary, Antti Myllynen

18. In-situ holders EBIC and CL holders
Electrical feed-through by applying a bias across the sample surface
Can be used for example to monitor charge recombination in semiconductors
Holders and Pumps - Summary, Antti Myllynen