UW- Madison Geoscience 777. Electron Probe Microanalysis EPMA. Vacuum Systems. 1/16/13. UW- Madison Geology 777. What ’ s the point?.
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Electron Probe MicroanalysisEPMA
We need a high vacuum in the column and chamber to maximize electron-sample interactions (not electron-gas molecules). We need a high vacuum in the gun to prolong the life of the electron source and avoid arcing. We need automatic microprocessor control and integration of vacuum reading, venting, and gun and column settings to avoid catastrophes.
The two main units used to measure pressure (vacuum) are torr and Pascal.
Atmospheric pressure (STD) = 760 torr or 1.01x105 Pascal.
One torr = 133.32 Pascal
One Pascal = 0.0075 torr
An excellent vacuum in the electron microprobe chamber is 4x10-5 Pa (which is 3x10 -7 torr)
Anthony Buonaquisti wrote an excellent article “If you hate vacuum systems, read on” published in Microscopy Today. No one got involved in electron microscopy in order to learn about vacuum science. But the equipment we use responds to poor vacuum practice with poor vacuum quality -- which translates to equipment that doesn’t work well, or doesn’t work at all.
In the following table, he demonstrates the magnitude of the presence of gas molecules, which species dominate at different pressure ranges, the vacuum we achieve with different pumps, and the average distance between molecules colliding with each other (MFP).
Electron microprobes (and SEMs and TEMs) all have similiar pumping systems, being combinations of at least 2 different pump types. To go from atmospheric to moderate vacuum, rough vacuum pumps are utilized. Once the chamber is pumped to a level of ~10-3 torr, high vacuum is acheived via either a diffusion or turbo pump. Some instruments (e.g. Cameca) include additional (“differential”) pumping for the gun, via an ion pump.
Gas molecules from the volume being pumped diffuse into the space between the rotor and chamber case, and are compressed by the rotating rotor until they have a pressure high enough to force upon the exhaust valve. They then exit, through the oil, to the outlet port.
Gas molecules in the pressure range here (from atm down to 10-2 - 10 -3 torr) move via viscous flow.
Oil-sealed rotary-vane rough vacuum pump
Rough vacuum pumps serve several functions: to “rough” out chambers vented to atmosphere, and also to “back” higher vacuum pumps (e.g. diffusion pumps).
Bigelow, Fig 4.1, p. 135
Bigelow suggests this well-known pump might be better called a “vapor jet pump”. High molecular mass oil is heated and moves vertically at 300-400 m/s, compressing against the jets any air molecules that have diffused into the vicinity. The oil molecules and now attached air molecules fall downward, cooling to a liquid against the water-cooled outer jacket. There is thus a build up of air molecules in the lower region, adjacent to the port that is attached to a second pump (e.g. rotary-vane rough vacuum pump), which then remove these air molecules.The pressures (to 10-7 torr) this pump operates at are appropriate for the gas molecules to move by molecular flow (not viscous flow) -- leading to backstreaming of oil vapor (explained later)
Bigelow, Fig 5.1& 2, p. 173
Initial pumping of volumes exposed to the atmosphere proceed through the viscous flow regime, where there are so many gas molecules that their mean free path (MFP) is so short that they collide more readily with each other than with the walls of the tube. They move as a mass in the general direction of low pressure.
When gas pressure drops enough that the MFP is greater than the internal tube diameter, individual gas molecules do not encounter other gas molecules necessarily moving in one direction (to low pressure). Rather, in this molecular flow regime, the flow of
gas in independent of pressure gradient, and depends mainly on tube dimensions and molecule speed (~temperature). In this case, backstreaming of molecules into the high vacuum chamber is possible.
Bigelow, Fig 2,1, p. 31
Backstreaming refers to the movement of gases (including pump oil vapor) from pumps into the vacuum chamber. It can be an important issue with diffusion pumps.
Design of diffusion pumps can make some difference. Placement of a continuous operation cold plate over the diffusion would be the best solution, but it is rarely included in microprobe design.
Oil diffusion pumps have a long history and are considered by many to be less costly and easier to use in a multiple user facility. However, the alternative is the turbo pump.
Turbomolecular pumps use no oil (though they may have greased bearings) and operate like jet engines. Momentum is imparted to gas molecules by disks rotating at very high speeds. Gas molecules randomly entering the entrance collide with the spinning rotor blade, and are propelled toward the pump’s exhaust vent. Turbo pumps can reach 10-7 to 10 -10 torr.
Turbo pumps are nearly free of oil backstreaming (if
certain operating procedures are carefully followed), as the high molecular mass oil vapor is compressed to a ratio > 1040 , versus values of 1010 for nitrogen.
Bigelow, Fig 6.1, p. 229
However, to eliminate ANY possibility of oil backstreaming, oil rough pumps need to be replaced by oil-free pumps. Scroll pumps are one such pump.
A scroll compressor uses 2 interleaving scrolls to pump gases. One of the scrolls is often fixed, while the other orbits eccentrically without rotating, thereby trapping and
pumping or compressing pockets of gas between the scrolls. (wikipedia)
This is a typical vacuum setup, with one high vacuum pump (diffusion or turbo) and one rough pump, and a series of valves.
1) Initial pump down: V1 and V2 closed, V3 open, and chamber, manifold and gun pumped out.
2) When chamber pressure low enough, V3 closes, V2 opens and roughs out the diffusion/turbo pump.
3) When the pressure in the diffusion/turbo pump is low enough, V1 opens and pumps out the chamber and gun to the high operating vacuum.
Not shown is an airlock chamber that would have its own vacuum tubing and valves.
Bigelow, Fig 2,6, p. 42
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Ion pumps are used for ultra high vacuums; they might also be called ‘Getter pumps.’ They consist of short stainless steel cylinders (anodes) between two metal (Ti, or Ti and Ta) plates (cathodes), all sitting within a strong magnetic field parallel to the cylinder axes.
A high voltage is applied between the anodes and cathodes, with the resulting electrons from the cathodes moving in long helical trajectories through the anode tubes, increasing the probability of collision with gas molecules.
Pumping of gas molecules in an ion pump occurs by 4 mechanisms:
1) Surface burial: Ionized gas molecules then accelerate into the cathode, sputtering Ti everywhere. Gas molecules on surfaces are buried by Ti atoms. This is the main pumping mechanism. 2) Chemisorption: Reactive gases (O, N, CO and H) react with fresh Ti to form stable oxides, carbides, nitrides and hydrides. 3) Ion implantation occurs when the electric potential gives some positive gas ions sufficient energy to penetrate the cathodes. 4) Neutral atom implantation: some gas ions strike the cathode and gain
electrons, becoming neutral atoms. If they rebound with enough kinetic energy, they will become buried in the surface of the anode or opposite cathode.
One problem with some ion pumps is “Argon instability”, particularly if there is a leak of P10 detector gas (90% Ar) into the chamber.
Bigelow, Fig 7,1, p. 277
No one gauge can measure pressure from atmosphere to UHV. Different gauges are used to measure vacuum over different pressure segments. There are 3 basic mechanisms:
1) Mechanical use diaphragms that change position due to force of the gas molecules.
2) Gas property gauges measure a bulk property such as thermal conductivity or viscosity.
3) Ionization gauges operate by measuring the current flowing across ionized gas molecules in the gauge.
Left image from Bigelow; Right image from Physics Today advertisement (MKS = company)
These gas property gauges find much usage in our instruments. A thermocouple measures the temperature of a heated wire inside a tube. As the number of gas molecules hitting the wire (and thus conducting heat away from the wire) decreases as the pressure decreases, the temperature of the wire increases. As temperature rises, the voltage generated by the thermocouple increases. This is calibrated and gives a precise reading.
Cold cathode ionization (aka Penning) gauges do not have filaments, and rely on an external event (cosmic ray, radioactive event) to start the action. Once started, the magnetic field constrains the electron to a long helical path with a high probability of of ionizing gas molecules. The current that flows across the gap between anode and cathode is measured with a sensitive microammeter calibrated in pressure units (as less molecules ionized, the current is lower).
Residual gas analyzers are specialized mass spectrometers, used to detect and quantify the gas partial pressures in a vacuum chamber. They may be of the magnetic sector design, or quadrupole design (above). Gas molecules are ionized, and then accelerated into an ion detector, separated by their mass-to-charge ratio (m/z)
RGA spectrum from April 2000; x-axis is M/Z
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Residual Gas Analyzers (RGAs) are rarely utilized in electron microprobes. They are valuable for diagnosing vacuum problems, as well as giving us an appreciation that a ‘vacuum’ is full of gas molecules. The fact that the N (28) peak is 10x the water (18) peak indicates that there is a minor but significant leak of room air into the chamber. Normally immediately after pumpdown, N drops quickly with H2O
being the dominant gas, and then H2O slowly drops too. From prior records of ‘good vacuum’, we can deduce that the atm gases are at least one order of magnitude too high.