Electron Beam MicroAnalysis- Theory and ApplicationElectron Probe MicroAnalysis -(EPMA)Electrons: Instrumentation and Theory of Electron Solid InteractionsUofO- Geology 619

Electron Microprobe InstrumentationWhat Makes a Microprobe?

High Stability Electron SourceFocussing WDS X-ray OpticsHigh Precision StageReflected Light OpticsBeam Current (Faraday Cup)

Electron Microprobe Instrumentation - contdVacuum: Whats the point?Column and chamber vacuum to maximize electron-sample interactions (not electron-gas molecules). Gun vacuum to prolong the life of the electron source and avoid arcing. Automatic microprocessor control and integration of vacuum reading, venting, and gun and column settings to avoid catastrophes.

Electron Microprobe Instrumentation - contdUnits 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)

Electron Microprobe Instrumentation - contdPumpsElectron 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.

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).

Oil 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)

Molecular flow vs viscous flow 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.

BackstreamingMovement 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.

Turbo 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 pumps exhaust vent. Turbo pumps can reach 10-7 to 10 -10 torr. Turbo pumps are nearly free of oil backstreaming (ifcertain operating procedures are carefully followed), as the high molecular mass oil vapor is compressed to a ratio > 1040 , versus values of 1010 for nitrogen.

SEM vacuum setupThis 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.

Ion pump-1 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.

Ion pump-2Bigelow, Fig 7,1, p. 277 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 gainelectrons, 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.

Measuring VacuumNo single gauge can measure pressure from atmosphere to UHV. Different gauges are used to measure vacuum over different pressure segments. 1) Mechanical use diaphragms that change position due to force of the gas molecules. 2) Gas property gauges measure a bulk property such asas thermal conductivity or viscosity. 3) Ionization gauges operate by measuring the current flowing across ionized gas molecules in the gauge

Thermocouple Gauges 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.

Ionization Gauges 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).

Electron-Solid Interactions

Beam PenetrationBeam penetration decreases with Z Beam penetration increases with energyElectron range ~ inelastic processesElectron scattering (aspect) ~ elastic processes

Elastic and inelastic scattering E0 = accelerating voltage (of electrons emitted from gun); usually 5-25 keV(a) Elastic Interactions Backscattering of electrons (BSE)(b) Inelastic InteractionsPlasmon excitation (in metals, loosely bound outer-shell electrons are excited)Phonon excitation (lattice oscillations, i.e., heating)Secondary electron excitation (SE)Inner-shell ionization (Auger electrons, X-rays)Bremsstrahlung (continuum) x-ray generationCathodoluminescence radiation (non-metal valence shell phenomenon)

Scattering lexicon Cross section: a measure of the probability that an event of a certain kind will occur, e.g. K-shell cross section. Q = N/nint, where N=events of certain type/vol (sites/cm3), ni=number incident particles/unit area (particles/cm2), and nt=number target sites/vol (sites/cm3). Q has units of cm2 and is thought of as an effective size which the atom presents as a target to incident particle. The Q for elastic scattering is ~10-17 cm2 and for K-shell ionization is ~10-20 cm2.Mean free path: average distance an electron travels within a specimen between events of a specific type. MFP=A/(NArQ) where A is atomic wt (g/mol), NA is Avogadros number, r is density (g/cm3).

Backscatter Electron ProductionElastic process

Backscatter Electron DetectionA solid-state (semi-conductor) backscattered electron detector (a) is energized by incident high energy electrons (~90% E0), wherein electron-hole pairs are generated and swept to opposite poles by an applied bias voltage. This charge is collected and input into an amplifier. (b) It is positioned directly above the specimen, surrounding the opening through the polepiece.In-Lens and Energy Selective BSE

Secondary Electron ProductionHigh keV beam electronFew eV secondary electronVirtually identical keVSE imaging: the signal is from the top 5 nm in metals, and the top 50 nm in insulators. Thus, fine scale surface features are imaged. The detector is located to one side, so there is a shadow effect one side is brighter than the opposite.Pollen

Secondary ElectronsInelastic scattering of HV beam electron can promote loosely bound electrons from valence to conduction band in semiconductor or insulator with enough energy to move thru the solid (in metals, promotion from conduction-band directly). Backscattered electrons can also produce secondary electrons.a) Complete energy distribution of electrons emitted from target. Region I and II are BSE, Region III secondary. b) Secondary electron energy distribution, measured (points) and modeled (lines)By definition, these secondary electrons are

Characteristics of Secondary and BSE ElectronsEnergy distribution of all electrons emitted from specimen under keV electron bombardment:N(E/E0)SEs (eV)SEs are VERY low energy electrons!E/E0BSEs (keV)Cat fleaSE: TopographicBSE: Compositional

Cathodo-luminescence:Auger electron spectra of silver with an incident beam energy of 1 keV. Derivative and integral spectra are compared (after Goldstein et al. 1981)

Zircon CL image from a ultra high pressure rock (China)

CL image of showing growth zones in a zircon

X-ray Generation

Fluorescent Yield:

Electron Range:

At very low energies, the electrons do not have a very efficient way of losing energy to the crystal lattice, so the mean free path is long.

At very high energies, the electrons are moving so fast that they literally "zip by" the other atoms so fast that the electrons of the stationary atoms do not have time to respond (or get excited).

At intermediate energies (on the order of 50 eV), the incident electrons can very easily lose energy by creating electronic excitations or ionization events in the solid, and the mean free path is very short.

Mean Free Path (electrons)

It all ends up as heat eventually!

Material

k

T oK (15 keV, 0.02 uA, 2 um)

T oK (20 keV, 0.05 uA, 2 um)

obsidian glass

0.014

51

171

zircon

0.042

17

57

quartz

0.10

7.2

24

calcite

0.05

14

48

mica

0.006

120

400

iron metal

0.80

0.9

3

epoxy

0.002

360

1200