Electron Beam MicroAnalysis- Theory and Application

Electron Probe MicroAnalysis -(EPMA)

Energy Dispersive

Spectrometry (EDS)

UofO- Geology 619 Modified from Fournelle, 2006

Review of Wavelength Dispersive Spectrometry (WDS)

What is EDS?Using X-rays to produce electron-hole pairs (total summed

charge which is proportional to incident x-ray energy), which are amplified and then “digitized” by voltage,

displayed as a histogram of the number of x-rays pulses (y axis) versus x-ray energy (x axis). A solid state technique

with unique artifacts.

EDS spectrum for NIST glass K309

(Goldstein et al, Fig. 6.12, p. 356)

Summary• X-rays cause small electric pulses in a solid state detector. Associated electronics produce ‘instantaneously’ a spectrum, i.e. a histogram of count (number, intensity) vs the energy of the X-ray

• Relatively inexpensive; there are probably 50-100 EDS detectors in the world for every 1 WDS (electron microprobe)

• Operator should be aware of the limitations of EDS, mainly the specific spectral artifacts, and the poor spectral resolution for some pairs of elements

Generic EMP/SEM

Electron gun

Column/ Electron optics

Optical microscope

WDS spectrometers

Scanning coilsEDS detector

Vacuum pumps

SE,BSE detectors

Faraday current measurement

There are several types of solid state EDS detectors, the most common (cheapest) being the Si-Li detector. Components: thin window (Be, C, B); SiLi crystal, FET (field effect transistor: initial amp), vacuum, cold finger, preamp, amp and MDA electronics (“multi channel analyzer”).

EDS assemblage

Goldstein et al fig 5.21

EDS Windows Windows allow X-rays to pass and protect detector from light and gases.

Be: The most common EDS detector window has been made of Be foil ~7.6 m (0.3 mil) thick. It allows good transmission of X-rays above ~ 1 keV. It is strong enough to withstand venting to atmospheric pressure, and opaque to optical photons.

Thin - Ultrathin: For transmission of light element X-rays (<1 keV), windows ~0.25 m thick of BN, SiN, diamond or polymer are used. They must use supporting grids to withstand pressure differentials; the grid (e.g., Si or Ni) takes up about 15% of the area, but thin enough that low energy X-rays pass through.

“Windowless”: Here there is no film, and there is a turret that allows swapping with a Be window. Difficult to use as oil or ice can coat the detector surface.

Goldstein Fig. 5.41, p. 318

This plots shows the transmittance of X-rays thru difference types of window material. (Quantum [BN] 0.25 um, diamond 0.4 um). The higher the number, the better

Energy Dispersive System Schematic

How it works: energy gap

X-ray hits the SiLi crystal, producing a specific number of electron-hole pairs proportional to X-ray energy; e.g. one pair for every 3.8* eV, so for incident Fe Ka, 6404 eV, 1685 e-hole pairs are produced. With a bias** applied across the crystal, the holes are swept to one side, the electrons to the other, producing a weak charge. Boron is important acceptor impurity in Si and degrades it (permits thermal excitation: bad); Li is drifted in (donor impurity) to counter its effects.

Goldstein et al, Fig 5.19

A semi-conductor like Si has a fully occupied valence band and largely unfilled conduction band, separated by an energy gap (1.1 eV). Incident energy can raise electrons from the valence to the conduction band.

* 1.1 eV + energy wasted in lattice vibrations, etc**bias: a voltage is applied between 2 points; e.g. one +1500 v, other -1500 v.

How it works: inside the detector

Fig 9.5 Reed; Fig 5.22 Goldstein

X-rays are absorbed by Si, with K shell (photo)electron ejected with hv - Ec (Ec=1.84 keV) energy. This photoelectron then creates electron-hole pairs as it scatters inelastically. The Si atom is unstable and will either emit a characteristic Auger electron or Si ka X-ray. If Auger, it scatters inelastically and produces electron-hole pairs. If Si Ka X-ray,

it can be reabsorbed, in a similar process, or it can be scattered inelastically. In either case, the energy will end up as electron-hole pairs. The result, in sum, is the conversion of all the X-ray’s energy into electron-hole pairs -- with 2 exceptions.

The Monolithic Semiconductor Energy Dispersive X-ray Spectrometer

X-ray detection

ElectronsHoles

X-rays

Au electrode, ~20 nm

Window: Be, BN, C (diamond), or polymer 0.1 - 7 m

Active silicon (intrinsic), 3 mm

- 1000 V

Inactive silicon (n-type), ~100nm

Rear Au electrode, ~20 nm

Si 'dead' layer (p-type), ~100 nm

Al reflective coating, 20 - 50 nm Ice? (pathological defect)

Area10- 60 mm2

X-ray photons with E from ~100 eV to~ 40 keV can be detected, one at a time!

SDD Backsurface

Central anode,80 m diameter

Resistor bridge

Ring electrodes

Silicon Drift Detector (SDD)

300 m (400 um for TEM)SDDs are thin!

SDDs have a complexback surface electrodestructure.

Active area 5 mm2 to 100 mm2

X-rays

The anode of an SDDis ~ 0.005 mm2 for a 50 mm2 detector, about1/10,000 the area of EDS

X- RAY DETECTORXX-- RAY DETECTORRAY DETECTOR

l High resistivity (n-type) silicon

l p+ electrodes on front and back reverse biased produces potential valley

l Radial applied bias on back-side tilts the valley toward n+ anode

Anode

R ladderSteering electrodes

Back

Internal field efficiently collects over entire detector area and brings charge to central anode

20 m

Raney Nickel AlloyE0 = 20 keV 10 nATC = 500 ns (188 eV at MnK)128x128 10 ms per pixel Mapping 185 sec

H

I

Al

Al

Ni

Fe

Al Fe Ni

PhasesAl 99.5 Ni 0.5Al 71.2 Ni 24.6 Fe 4.2 Al 60 Ni 40 “I”Al 46.5 Ni 53.5 “H”

Artifacts: Si-escape peak; Si internal fluorescence peak

Fig 5.22 Goldstein et al

There are 2 exceptions to the previous neat explanation of how the Si(Li) detector works.Si-escape peaks are artifacts that occur in a small % of cases, where the Si ka X-ray generated in the capture of the original X-ray escapes out of the detector (red in figure). Since this X-ray removes 1.74 keV of energy, the signal generated (electron-hole pairs) by the incident X-ray will be 1.74 keV LOW. This will produce a small peak on the EDS spectrum 1.74 keV below the characteristic X-ray peak. Another artifact is the Si internal fluorescence peak, which occurs if an incident X-ray is absorbed in the Si “dead” layer (green

region). This region is “dead” to production of electron-hole pairs, but Si ka X-rays can be produced here which then end up in the “live” part of the detector, and result in a small Si ka EDS peak.

Artifacts: Si-escape peaks; Si internal fluorescence peak;

extraneous peaks

Goldstein et al Fig 5.39,p. 316

The figure shows a real spectrum of a sample of pure Ti metal -- but there are 7 peaks besides the Ti K and K. At 1.74 keV below each, are the respective escape peaks (blue arrows). Also present is a Si internal fluorescence peak (green arrow). The Fe and Cu peaks are from excitation of metal in chamber or sample holder by BSE or Ti X-rays. Note the sharp drop in the background intensity on the high side of the Ti K peak (= Ti K absorption edge, red arrow). (2 Ti K and Ti K+K explained shortly.)

Note the scale of the spectrum: the Ti

Kmax is 1.3 million counts. These effects are generally weak, but evident when you are looking for minor elements.

Question: Do all characteristic X-rays have Si-escape peaks in a

Si(Li) detector? Why or Why Not?

Hint 1: Sr La does not, but Os Ma does

Hint 2: Look up the characteristic energies of each

Hint 3: Look up the absorption edge (critical excitation) energy of Si Ka

Hint 4: Compare the numbers in 2 to number in 3. Which one will greater than the one in #3?

Would a Si Ka x-ray produced in the sample, which then makes its way thru the vacuum to the EDS detector, have enough energy to knock out the inner shell (K) electron of the Si detector crystal?

Signal processingSi(Li) detector has no internal gain*; for Ca Ka photon with ~1000 e-hole pairs, the charge is only ~10-16 Coulomb (weak!) We need low noise, high gain amplification. Best is multi-stage, with a preamp (FET, field effect transistor) immediately adjacent to the detector

crystal. The detector and FET are cooled to about 100K with liquid nitrogen (LN) to prevent noise (and prevent diffusion of Li in detector). More signal gain provided then by main amplifier (signal now boosted to 1-10 volts) where also RC (resistor-capacitor) circuits are used to shape the pulse, to maximize signal/noise ratio and minimize pulse overlap at high count rates. Then ADC (analog to digital converter) outputs data to the screen as a spectrum display.

*gain = electronic multiplication of signal intensity

The first signals in the EDS detector

Goldstein et al (1992), p. 297

The set of electron-hole pairs produced by the impact of the X-ray on the Si(Li) detector produces a tiny charge (~10-16 C), very quickly (~150x 10-9 sec).The FET(preamplifier) changes the charge (capacitance) into a tiny voltage (millivolts). These steps are shown in the first half of (a) to the right. The output of the FET is shown below at (b) where the x axis is time and y is voltage. The “jump” represents the presence of a voltage proportional to the number of electron-hole pairs generated by each X-ray, so Photon 2’s jump is of a higher energy than Photon 1’s jump which is higher than Photon 3’s jump. At a certain point the FET reaches the limit of the number of

charges it can hold, and then there is a reset or zeroing back to some baseline where it starts over. Following this are electronics to shape the voltage into a pulse that can be counted.

Processing Time and Pulse Pileup Rejection

Goldstein et al (1992), Fig. 5.24 and 5.25, p. 300

The user can ‘tweak’ the time constant (T.C.) which sets the time allocated in the electronics to process each pulse (x-ray). In the top figure, a short T.C. (1 s) permits each pulse to be counted correctly. A longer T.C. (10 s) means the “gate” is open longer and a second pulse can enter and be incorrectly added; this is “pileup” and causes distorted spectra. Therefore, circuits are added (#4, bottom figure) to sense when pileup occurs and to ignore that pulse.

Dead Time

Goldstein et al (1992), Fig. 5.25 and 5.29, p. 300 and 303

“Deadtime” is the period during which the detector is “busy” and cannot accept/process pulses. This can introduce error unless it is accounted for, either by extending counting time, or correcting for it in the software. In most systems, the user sets the “live time” which is the time during which counts are actually counted, and the “real time” is automatically determined by the electronics or software.

Optimal deadtime is in the 30-35% range. This optimizes both user/machine time and moderate to high throughput of counts.

80%

40%

60%

Deadtiim

e

Detector performance: peak resolution (FWHM)

Goldstein et al, Fig 5.34, p. 311

The characteristic X-rays generated in the specimens are very close to lines, i.e. only a few eV wide at most. However, the conversion of X-ray to a pulse in the detector has several variables (imperfections) that broaden the peak to between maybe 135-200 eV, depending upon the type of detector and how well maintained it is. The narrowness of the peak is measured by the width of the peak at one half the maximum intensity of the peak -- this is what is termed the FWHM.

In EDS detectors, it is usually measured at the Mn K position, with values of 160 eV and below. Modern (2005) one are quoted at <130 eV.

Why Mn Ka for EDS resolution?

EDS companies (their engineers mainly) do not want to have to carry around an SEM or EMP to be able to test, repair and calibrate an EDS system. Instead they carry a small 1” diameter x 2” long tube that fits over the end of the EDS “snout”. Inside it is an Fe-55 isotope source (half life 2.7 yr) which emits an intense x-ray at 5.985 keV which is only a few eV different than Mn Ka.

Spectral processing:background correction

Goldstein et al Fig. 7.1,2, p. 367

The characteristic X-rays that we need to quantify “ride” atop the continuum, and the continuum contribution to the characteristic counts must be subtracted.

(Top) Linear interpolation (B-D) will be in error due to the abrupt drop of continuum at the Cr K-absorption edge (5.989 keV). B-C is possible but critically dependent upon having good spectral resolution (<160 eV). A-B would be preferable.

(Below) Doing background fit of a complex stainless steel.

Spectral processing:background modeling or filtering

Correcting for the background is done by either of 2 methods: developing a physical model for the continuum, or using signal/noise filtering.

Modeling is based upon Kramers Law: there is a function describing the continuum at each energy level, that is a function of mean atomic number, and measured “detector response”.

Background Modeling

Goldstein et al Fig 7.4, p. 372

The spectrum of Kakanui hornblende (top left), with superimposed calculated (modeled) background, based upon Kramers Law.

Bottom shows after the background has been subtracted. Cu is artifact (stray X-rays). Mn is actually present at <700 ppm.

Background Filtering

Theoretically Fourier analysis will separate out the low frequency continuum signal and high frequency ‘noise’ from the medium frequency characteristic peaks; however, there is overlap and the result is a poor fit. A better filter is the “top hat filter”, where no assumptions are made about the spectrum, and only the mathematical aspects of signal vs noise are considered.

Top Hat Filtering

Reed Fig 12.7 p. 174,Goldstein et al Fig 7.6, p. 374

This filter (top right) moves across the EDS spectrum (with an optimally defined window, ~ 2 FWHM* Mn Ka;~320 eV), and assigns a new value for the center channel based upon subtracting the values in the left and right channel from the center (value hk chosen to total area =0). Thus, in the simple spectrum (bottom right), the center channel (+), when the left and right channels are subtracted, leaves a value ~0.

*FWHM: full width at half maximum.

More Artifacts: Pulse Pile Up

There is a short period of time (0) during each X-ray capture by the EDS detector, when the detector can capture a second X-ray “by mistake”. The electronics cannot distinguish this “sum peak” from a

true single X-ray peak, and “piles” it up with all the other peaks from the elements actually present. For 2 major elements, could be 3 sum peaks; for 3, 6. In reality,you only see 1 or 2 unless you zoom in to the background level. Always consider their possible presence.

Sum Peaks In qualitative analysis of silicates, there are some combinations of element K peaks that fall close to K peaks of elements possibly present, as indicated in the table below:

Sum Peak Element eV separate

Mg + Ca V 6Si + Ca Cr 18Mg + K Ti 57Al + Si K 87

Ca + Ca Ni 91

And More Artifacts

There is always a potential for ‘stray’ X-rays being detected. It thus pays for the EDS operator to understand what the path is for the electron beam and for the X-rays, and know what ‘other’ elements might show up unintentionally.

This is particularly true for EDS associated with TEM, where specimens routinely sit on grids (Cu?) and the high energy (200 keV?) electrons can go through the specimen and hit a metal part of column or chamber, with the resulting X-rays finding a way back to the detector.

And More Artifacts

Another thing: many SEM labs use gold or palladium coating on specimens. These very thin coats will produce definite x-ray peaks!

Family of Pd L lines

Artifical EDS spectrum

Artificial: no background, no artifacts, and assumes EACH element at 100% concentration. Why, then, the two slopes?? Peak intensities of elements from Si to Na decrease, and also from Si to Zn -- why? (Hint: 2 physical phenomena)

Artificial spectrum

Slope down from Si to Na: X-ray energies are increasingly weaker, and are absorbed both within the specimen and by the window. Also fluorescent yield is less for low Z elements and high Z elements “share” more incident electrons with outer shells.

Slope down from Si to Zn: there are less and less X-rays being produced because the accelerating voltage is constant (e.g. 10 keV) and the overvoltage is lower.

The actual spectrum of pure elements, as generated at the point of impact, would be one steady decreasing curve from Na down to Zn, following the red curve superimposed here.

Evolution of EDS spectrum: from the specimen to the monitor - 1

Goldstein et al Fig 5.53 (by R. Bolon) p. 330

The spectrum on our monitor (d) is a result of many things impacting the real spectrum generated within the specimen (a). At instant of generation within the specimen, there is only the Ka, Kb and continuum. An instant later (b), as the X-rays leave the specimen, two things can happen: some of the continuum X-rays above 5.464 keV are absorbed, producing the drop in the continuum there.

Simulation of element (say V) X-ray generation and display

Evolution of EDS spectrum: from the specimen to the monitor - 2

Goldstein et al Fig 5.53 (by R. Bolon) p. 330

Also in (b) the lower energy continuum is absorbed, causing the dropoff in the spectrum there. When the X-rays hit the detector (c), Si fluorescence peaks can result. And after signal processing (d), the display will show peak broadening, sum peaks, Si-escape peaks, further decrease of intensity and low energy noise.

Simulation of element (say V) X-ray generation and display

EDS-WDS comparison

Characteristic EDS WDS

Geometric collection efficiency <2% <0.2%

(solid angle)

Spectral resolution (FWHM) 160 eV and less 2-10 eV

Instantaneous X-ray detection ~ 1 keV thru E0 (window dependant) a few eV

Maximum count rate 10s of thousands cps over whole spectrum tens of thousands cps (one wavelength)

Artifacts sum peaks, Si escape peaks, Si fluor. peak n>1 peaks, Ar escape peaks

Light elements? With windowless or thin window detector With synthetic diffractors ("crystals")

Detection Limits ~1000-5000 ppm, 0.1-.5 wt% <100-500 ppm, <0.01-.05 wt%

Bottom Line Cheaper, quicker but some elements are More expensive, but with much better

too close together to resolve spectral resolution and higher Pk/Bkg,

(eg S Ka, Mo La, Pb Ma) giving lower detection limits.

Further EDS details

There are several modern EDS companies, with most producing very informative brochures that go into the technical details of EDS hardware (and software):

For example: Oxford Instruments <www.osinst.com/ANLPDP174.htm> has a nice technical publication “EDS Hardware Explained” available as a pdf.