DOC PREVIEW
UW-Madison GEOSCI 777 - Electron Probe Microanalysis EPMA

This preview shows page 1-2-20-21 out of 21 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 21 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 21 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 21 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 21 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 21 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

PowerPoint PresentationWhat’s the point?Key pointsUnits of VacuumWhy the fuss?Vacuum Regimes: What’s whatPumps“Rough vacuum” pumpOil Diffusion PumpMolecular flow vs viscous flowBackstreamingTurbo PumpsSEM vacuum setupIon pump-1Ion pump-2Measuring vacuumThermacouple GaugesIonization GaugesRGAA snapshot of the vacuum in our SX51 chamberReferencesVacuum SystemsElectron Probe MicroanalysisEPMA UW- Madison Geology 777What’s the point?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. UW- Madison Geology 777Key points• Description of numbers of air molecules with different pumps• Rough vacuum pumps• Diffusion and turbo pumps• Ion pumps• How vacuum is measuredUW- Madison Geology 777Units of VacuumThe 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 PascalOne Pascal = 0.0075 torrAn excellent vacuum in the electron microprobe chamber is 4x10-5 Pa (which is 3x10 -7 torr)Why the fuss?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).UW- Madison Geology 777Vacuum Regimes: What’s whatWhat # of gasmoleculespresentDominant gasspeciesMean FreePath (betweencollisions)PressureAtmosphericPressure1019 gasmolecules/cm3N>O>H2O MFP=0.1 um 760 torrMechanicalPumping1014 gasmolecules/cm3H2O>N,OMFP = 1 cm10-3 torrDiffusionPumping109 gasmolecules/cm3H2OMFP=105 cm 10-6 torrUltra HighVacuum106 gasmolecules/cm3H, HeMFP=106 m 10-10 torrUW- Madison Geology 777Pumps 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. UW- Madison Geology 777“Rough vacuum” pumpOil-sealed rotary-vane rough vacuum pumpBigelow, Fig 4.1, p. 135Gas 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.Rough vacuum pumps serve several functions: to “rough” out chambers vented to atmosphere, and also to “back” higher vacuum pumps (e.g. diffusion pumps).UW- Madison Geology 777Oil Diffusion PumpBigelow, Fig 5.1& 2, p. 173Bigelow 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)UW- Madison Geology 777Molecular flow vs viscous flowBigelow, Fig 2,1, p. 31 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.UW- Madison Geology 777Backstreaming 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.UW- Madison Geology 777Turbo Pumps 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 (ifBigelow, Fig 6.1, p. 229certain operating procedures are carefully followed), as the high molecular mass oil vapor is compressed to a ratio > 1040 , versus values of 1010 for nitrogen. UW- Madison Geology 777SEM vacuum setupBigelow, Fig 2,6, p. 42This is a typical vacuum setup, with one high vacuum pump (diffusion or turbo) and one rough pump, and a series of


View Full Document

UW-Madison GEOSCI 777 - Electron Probe Microanalysis EPMA

Documents in this Course
Load more
Download Electron Probe Microanalysis EPMA
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Electron Probe Microanalysis EPMA and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Electron Probe Microanalysis EPMA 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?