Holders and Pumps Summary 15.9.2016 Chem-E5225 - Electron Microscopy

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

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 = 10-4 - 10-7 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

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

Exhaust pumps Turbomolecular pumps High speed turbine (20000-50000 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

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

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

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

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

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

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

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

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

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

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

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

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

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