Electron Probe Microanalysis EPMA Electron Optical Column UW- Madison Geology 777.

Electron Probe MicroanalysisEPMA

Electron Optical Column

UW- Madison Geology 777

What’s the point?We need to create a focused column of electrons to impact our specimen, to create the signals we

want to measure. This process is identical for both scanning electron microscope (SEM) and

electron microprobe (EMP). We use conventional terminology, from light optics, to describe many

similar features here.


Key points• Source of electrons: various electron guns; W in particular. We want high, stable current with small beam diameter.• Lenses are used to focus the beam and adjust the current• Current regulation and measurement essential• Beam can be either fixed (point for quant. analysis) or scanning (for images)• Optical microscope essential to position sample (stage) height, Z axis (= X-ray focus)• Vacuum system essential


Generic EMP/SEM

Electron gun

Column/ Electron optics

Optical microscope

WDS spectrometers

Scanning coilsEDS detector

Vacuum pumps

SE,BSE detectors

Faraday current measurement


Electron GunsSeveral possible electron sources: most common is the W filament, thermoionic type. A W wire is heated by ~2 amps of current, emitting electrons at ~2700 K – the thermal energy permits electrons to overcome the work-function energy barrier of the material.

Another thermoionic source is LaB6, which has added benefits (“brighter”, smaller beam) but it is more expensive and fragile. Both have very good (~1%) beam stability, compared to a different variety of sources, the field emission guns, which are “brighter” and have much smaller beams (great for high resolution SEM images) but lower beam stability and require ultra high vacuum.


W filament:biased Wehnelt CapCurrent (~2 A) flows thru the thin W filament, releasing electrons by thermoionic emission. There is an HV potential (E0) between the filament (cathode) and the anode below it, e.g. 15 keV. The electrons are focused by the Wehnelt or grid cap, which has a negative potential (~ -400 V), producing the first electron cross over.

Goldstein Fig 2.4, p. 27

First electron

cross-over


SX50 Gun and Wehnelt

Wehnelt diameter (below) is ~20

mm


W filament:

W filament is ~125 m diameter wire, bent into hairpin, spotwelded to posts. W has low work function (4.5 eV) and high melting T (3643 K), permitting high working temperature. Accidental overheating will cause quick failure (top right). Under normal usage, the filament will slowly ablate W, thinning down to ultimate failure (uncertain why offset). With care/luck, a filament may last 6-9

months, though 1-2 month life is not uncommon. Goldstein Fig 2.8, p. 33


Some electron units/values

Brightness is a measure of the current emitted/unit area of source/unit solid area of beam (not used in daily activities)

Baseball

HV: speed of the ball

curr: size of the ball

Water through hose

HV: water pressure

curr: size of the stream of water

High voltage and Current - Analogies


Saturation

“Saturation” is the optimization of 1) current stability (on the plateau) and 2) filament life (minimal heating). The Operating or Saturation point is at the “knee” of the plot. On the SX51, “HEAT” is the variable, with saturation usually between 228 and 200, with new filaments at the upper value, and gradually declining as the filament ages (thins). These are unitless values (0-255 scale)

Goldstein et al Fig 2.5, p. 278

Producing minimum beam diameter

(Goldstein et al, 1992, p. 49)

Similar to light optics (though inverted: reducing image size):

d0 is the demagnified gun (filament) crossover--typically 10-50 um, then after first condenser lens, it is further demagnified to crossover d1. After C2 and objective lens, the final spot is 1 nm-1um.

Light Optics Electron Optics

1/f = 1/p + 1/q


Column: focusing the electrons


Simple iron electromagnet: a current through a coil induces a magnetic field, which causes a response in the direction of electrons passing through the field.

Rotationally symmetric electron lens: beam electrons are focused, as they are imparted with radial forces by the magnetic field, causing them to curve toward the optic axis and cross it.

Condenser & Objective Lenses:working distance


Left: shorter working distance (~q2), greater convergence (2): smaller depth of field, smaller spot (d2), thus higher spatial resolution. Right: longer WD, smaller convergence: larger depth of field, larger spot, decreased resolution.


Condenser Lenses:adjusting beam current


Probe current (Faraday cup current, e.g. 20 nA) is adjusted by increasing or decreasing the strength of the condenser lens(es): a) weaker condenser lens gives smaller convergence 1 so more electrons go thru aperture. Thus higher current with larger probe (d2) and decreased spatial resolution (2). b) is converse case, for low current situation.


Probe diameter

Reed 1993, Fig 4.11, p. 46

“What is the beam/probe size”? I would suggest this is a philosophical question: what is the theoretical size of the beam--before it enters the specimen?-- a question of limited importance in EPMA For that hypothetical question, Reed provides a ballpark estimate (for ~5 nA of beam current, the minimum diameter is 0.2 m.

(D is demagnification, is beam semi-angle).

However, in the real world, the actual “interaction volume” (due to electron scattering) and thus size of analysis volume is larger, as you can appreciate from your Monte Carlo simulations.

Probe current: monitoring and stabilization

EPMA requires precise measurement of X-ray counts. X-ray count intensity is a function of many things, but here we focus on electron dosage. If we get 100 counts for 10 nA of probe (or beam or Farady) current, then we get 200 counts for 20 nA, etc.

Therefore, it is essential that we 1) measure precisely the electron dosage for each and every measurement, and 2) attempt to minimize any drift in electron dosage over the period of our analytical session. The first relates to monitoring, the second to beam regulation.


Probe current monitoring

Goldstein et al 1992, Fig 2.25, p. 65

Electron beam intensity must be measured, to be able to relate each measurement to those before and after (i.e. to the standards and other unknowns). This is done with a Faraday cup, where the beam is focused tightly within the center of a small aperture over a drilled out piece of graphite (or metal painted with carbon). Current flowing out is measured.

Why graphite? Because it absorbs almost all of the incident electrons, with no backscattered electrons lost.


Probe current monitoring

Goldstein et al 1992, Fig 2.25, p. 65

Modern electron microprobes have built in, automatic, Faraday cups. This is a small cup that sits just outside of the central axis of the column, and can be swung in to intercept the beam upon automated control. This is typically done at the end of each measurement on both standards and unknowns, and using these values, the measured X-ray counts are normalized to a nominal value (e.g. 1 nA, or actual nominal value like 20 nA)

In older instruments, this automation was not implemented. An alternative solution would be to create a homemade Faraday cup and mount it with samples, and move the stage to it to do the measurement, or measure absorbed current on another reference material (e.g. brass).


Probe current regulation

Reed 1993, Fig 4.12, p. 47

Optimally, the beam current should remain as constant as possible, particularly over the duration of each measurement (depends upon number of elements, etc, but most are 45-120 seconds). This is accomplished in a feedback loop with the condenser lenses, where a beam regulation aperture measures the electrons captured on a well defined area (red area on bottom aperture), where larger

aperture above it provides ‘shading’ and eliminates ‘excess’ electrons (green).


Scanning Coils The primary mission of the electron microprobe is to focus the beam on a spot and measure X-rays there. However, it was early recognized that being able to scan (deflect) the beam had two advantages: X-rays could be produced without moving the stage, and electron images could be used to both identify spots for quantification, and for documentation (e.g. BSE images of multiphase samples).

Scanning requires 1) deflection coils and 2) display system (CRT) with preferably 3) digital capture ability.

Reed 1993, Fig 2.3, p. 18


SX51 specs


Optical Microscope An essential part of an electron microprobe is an optical microscope. The reason is that we need to consistently verify that all standards and specimens sit at the precise same height (Z position). This is because they must all be in “X-ray spectrometer focus”, which shortly you will find described as the “Rowland circle”. Mounting of specimens relative to an absolute height is problematic, for a variety of reasons (difficult to mount samples perfectly flat, and the fact that we use different holders and shuttles manufactured to different tolerances, together with different screw tightings by operators.)


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Electron Probe Microanalysis EPMA Electron Optical Column UW- Madison Geology 777.

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