Scroll pump (scroll compressor) is an oil-free vacuum pump used for obtaining #prevacuum with a level of 0.1 mbar. The #scrollpump is based on two scrolls, that are placed insede the working chamber. One of these scrolls is fixed and the other one mobile. Once the gas enters the system through the inlet, it gets carried to the outlet in the middle by the mobile orbiting scroll. This is possible due to the fact that during the mobile scrolls movement a wide gap is locally kept between the two scrolls and the gap moves to the middle, carrying the gas in it. As the volume of the gap decreases at the outlet, the gas is pushed out of the system. Due to wear the orbiting scroll needs to be replaced about twice a year.
The #cryopump is a high vacuum pump that is based on the adsorption of gas on cold surfaces. The cooler the surface – the better the pumping! For that purpose liquid helium is often used for cooling the system parts as it is extremely cold, having a temperature only four degrees above absolute zero. The pumping is done in multiple stages, where each stage has a different temperature. The „warmest“ part is the first stage, that has a temperature of 100 degrees Kelvin (or lower) and that causes water to condensate on it. During the #condensation of water also some other gases may also be trapped under the water and that process is called #cryotrapping. However, in order to effectively pump gases such as nitrogen, even colder surfaces (less than 17 degrees Kelvin) are required. In this stage the metal may also be coated with porous activated charcoal and that allows to adsorb even smaller gas molecules such as helium and even hydrogen (this is called #cryosorption ). So basically the pumping of gases with this #vacuumpump is based on three processes: condensation, cryotrapping and cryosorption.
The titanium sublimation pump is a vacuum pump used as a part of vacuum systems in order to briefly improve the level of #vacuum. The working principle is relatively simple. A pulsing current passes through a titanium filament, causing it to sublimate (goes directly from solid phase to gas phase). The fresh titanium chemically reacts with gas in the vacuum chamber, creating a solid product that deposits on the chamber walls. As the walls are also coated with highly reactive freshly deposited titanium, they may also chemically bind gas molecules that interact with them. Some gases may not chemically bind with titanium but can still be physically trapped under the titanium atoms on the chambers walls.
The ion pump is a high vacuum pump that is based on ionizing the gas. When the gas that enters the #ionpump it always has some ions in it and those are pulled towards the titanium cathode. These ions are then trapped between the titanium atoms and they may also chemically react with titanium forming solid titanium nitride or titanium oxide, depending on the gas. In addition electrons and titanium atoms are emitted from the cathode when bombarded by the ionized gas molecules. These electrons are then accelerated towards the steel anode as the potential between the anode and cathode is thousands of volts. A magnetic field caused by the magnets makes the electrons move spirally so they spend more time in the open and more likely hit gas molecules to ionize them. The generated gas ions move again towards the cathode and kick out even more electrons and titanium atoms. Some of the kicked out titanium atoms also deposit on the anode and in this process bury gas molecules under them. The pump may work for several years if used in high vacuum environment and if the amount of pumped gas is not large. At some point however the cathodes need to be replaced.
Turbomolecular pump ( #TMP ) is a popular high vacuum pump, that is widely used in many vacuum systems (electron microscopes for example) as it is clean, fast and efficient for maintaining high vacuum over a long period of time. Inside the pump there are rotor blades that rotate with a speed up to 90 000 rotations per minute and stator blades that are stationary. If a gas molecule enters the pump then it is hit by the rotor blades that are tilted at a certain angle. The kinetic energy from the blade is transfered to the gas molecule and causes it to move down and hit a stator blade that is also tilted at a certain angle, causing the molecule to „bounce“ down, where it meets the next rotor blade. Eventually the gas molecules reach the bottom of the turbomolecular pump where they are removed with a backing pump (prevacuum pump). The turbomolecular pump usually also needs a pre-vacuum before it can work efficiently and this can be done with a pre-vacuum pump (for example a scroll compressor pump). Once the pressure is low enough, the #turbomolecularpump starts working and the rotation speed of the rotor blades is gradually increased.
The diffusion pump is an oil-based vacuum pump, that is used for obtaining different levels of vacuum – even high vacuum of 10 powered -9 Pascal. In this system the oil is boiled by a heating with a heater. Next, the jets of vaporized oil grab the gas molecules in the chamber and transport them to the colder chambers walls where oil condensates and moves down. At this stage the captured gas molecules are released and removed from the system with a pre-vacuum pump. The #diffusionpump has very high pumping speeds and it can even be used for the pumping of corrosive gases. The downside however, is the possibility of contaminating the vacuum system with oil and therefore oil traps are highly recommended.
Vacuum can be understood as space from where matter (for example air) has been removed. It naturally exists in outer space but for certain applications, like materials characterization techniques, it needs to be achieved artificially. The desired level of vacuum is obtained with the help of a suitable vacuum pump. For example low vacuum (low quality vacuum with higher pressure) can be generated with a diffusion pump, scroll compressor pump, rotary vane pump, diaphragm pump or a sorption pump. High vacuum (high quality vacuum with very low pressures) however, can be obtained with high vacuum pumps such as the turbomolecular pump, ion pump, titanium sublimation pump and cryopump. The level of vacuum is measured with devices called vacuum gauges (vacuum meters) like the thermocouple gauge, pirani gauge, penning ionization gauge and the quadrupole mass spectrometer (analyzer). The working principle of vacuum pumps and vacuum gauges is explained with 3D animations in the video lecture above.
The scanning electron microscope is a powerful tool used by many scientists for studying and developing materials. This device uses electrons instead of photons and that allows the operator to visualize even nano-scaled surface details. Therefore it is very useful for also studying corrosion processes and creating efficient thin coatings which have a thickness of less than a micrometer.
In the video above you will see the working principle of this microscope and how it is operated.
The electrons are generated in the electron gun on top of the microscope. There are different source types but one of the oldest used is a tungsten filament that is heated over 2400 degrees celsius by passing a current through it. At this temperature electrons emit from the filament and are pulled down the column with the help of an anode. The operator selects the voltage that accelerates the electrons. It can be up to 30 kV in conventional scanning electron microscopes. The accelerated electrons are scattered and need to be focused – this is done with the help of multiple focusing lenses. These lenses also tune the amount of electrons that reach the sample. For example by broadening the electron beam, the electrons are absorbed by the column and dont reach the sample. In order to scan across the sample row by row, the electron beam needs to be moved in such a manner. This is achieved with the help of scanning coils that are placed below the focusing lenses. The final focusing of the beam is done with objective lenses located below the scanning coils. In order to get an image of the substrate the surface is scanned row by row with the electron beam. In each spot signals like secondary electrons, backscattered electrons or characteristic X-Rays are generated. By detecting the signal emitted from each scanned spot, an image of the surface is generated. So for example by detecting secondary electrons a secondary electron image is generated.
Secondary electrons are generated when the primary beam electrons kick out electrons from the substrate atoms. In that process primary beam electrons lose energy as it is transfered to the secondary electrons. These secondary electrons generaly have low energy and escape only from near the surface, giving a good image of the substrates topography. The electrons are collected with a special detector that has a positively charged collector for pulling the negatively charged electrons towards it.
Backscattered electrons are primary beam electrons that are scattered back in a similar direction as they interact with the substrate atoms nucleous. In that process they dont lose much energy and can be emitted even from deep layers of the substrate. Therefore they carry the bulk information of the sample and generated images are not completely topographic. The electron yield strongly depends on the atomic number of the substrate. For example regions on substrate with higher atomic number appear brighter on the image as more electrons are backscattered in these regions.
Characteristic X-Rays are generated when sample atoms exited by primary beam electrons are undergoing a relaxation process where the inner shell vacant spot is filled with an outer shell electron. The emitted radiations energy depends strictly on the atoms number and on the electrons binding energies that are involved in this process. Therefore this signal can be used to detect different elements and do quantitative element analysis.
For a real SEM study the sample first needs to be prepared – it has to be dry, conductive and with a suitable size. Non-conductive samples can be coated with a thin (couple nanometers) layer of metal (Au, Pt..), which allows SEM studies as they conduct away the heat and negative charge caused by electrons. The sample is moved into the microscope through an airlock or the main chamber door and placed on a stage with a special holder. The stage can move in any desired direction (x,y,z, rotate, tilt). The sample is then moved under the electron column to a suitable working distance. After focusing and other adjustments the first electron image can be taken.
Element microanalysis can be done in several ways but first an electron image of the sample is obtained. The most common way to do element analysis is selecting a characteristic spot on the sample and then bombard it with the narrow (couple nm wide) electron beam. As a result X-Rays are emitted from the couple micrometer wide interaction volume. This gives information from a very localized area but in some cases an average composition of the material is needed. For that a larger area (lets say 50 x 50 microns) is bombarded by electrons and the X-Rays emitted from that area are detected. It is also possible to map the distribution of elements on a larger area by scanning across the surface with the electrons row by row and collect X-Rays from each scanned point. Based on the detected X-Rays an image is created that shows the distribution of certain elements.
In order to see what is inside a material a focused ion beam (FIB) is used to make a cross-section or a thin lamella. The cross-sections or lamella are then studied with electrons at a suitable angle by tilting it with the help of the stage.
Scanning electron microscopes generally work in a high vacuum in order to prevent surface contamination and electron or x-ray interactions with the gas in the chamber which would affect the quality of the image. However there are also specially designed environmental scanning electron microscopes (ESEM-s), that work in low vacuum. The ESEM can therefore even be used to study living cells or bacteria.