Chemical analysis with EDXOn Supra40

How to get good qualityquantitative chemical analyses

- Understanding the involved physical phenomena - Choice of conditions of analyses- Artefacts and misinterpretations - Quantitative mapping- Case of light elements

Nathalie Siredey-Schwaller | 09/01/2014

Basic cares : sample preparation• Sample should be an electric conductor

– Good electric conductivity of mounting– Good electric junction between sample and microscope : use of silver glue. Pay

attention to the release of solvent.– Beware of oxyde layer at the surface of sample / cleanliness of the surface.

• Surface of the sample : analysis depth is only a few microns– Surface should be out of contamination / clean– A precise quantification needs a flat and horizontal surface (phirhoZ method).

• Measuring beam current : with a Cu tape put near the sample.This allows to measure the system factor. Sample spectra could then been compared to

standards spectra.

Consequences of bad-electrical conductivity

After 5 min analysisBefore recording spectrum

Deviation of beam

Duane-Hunt limit should be identical to primary electron energy

Chemical signal changes

• Position of sample

• No object between sample and detector

• Infra-red camera is « off » : it induces noise signal

Conditions for acquiring EDX signal

WD = 8.5 to 9 mm, magnification > 300

EDX detector

Sample

Détecteur

Echantillons

Sas d’entrée

No analyses inside the hole

How to choose voltage and beam aperture ?• Voltage has to be chosen according to chemical elements expectedEnough energy to eject an orbital electron of the element electron re-organization

and emission of characteristic X-rays : K or L, (M) spectraThe efficiency is the best for Eprimary beam = 2*Eionization

– Analysis of light elements with small voltage (better signal/background ratio)– The highest-Z element to measure determines the voltage. – The voltage is more specially chosen according small content amount elements– Massic absorption coefficients for K spectra are the better known. Usual values are15 keV, 20 keV

• Beam current has to be chosen according to detector settings– Detector with high spectral resolution only accepts small intensity signal– Mapping needs good X-ray statistic per points, so high intensity signal

Measuring beam current / intensity of the signal• You have to do it in order to be able to compare your sample to standards.• Measuring System Factor SF• Conditions of measurments : on copper :

– Focus to the good working distance– Tuning of the primary electronic beam : deviation (wooble), stigmatism.– Check the tilt angle (must be 0°and not 70°)– Choose the right device settings for the detector– Calibrate. Check if spectral position of Cu-K peak is OK.

– Once this is done, nor the focus, nor the electron beam should be changed.

Peak maxima match with color element lines

Choice of device settings• Detector has dead time, during which X-ray signal cannot be count. For good

statistics, dead time shall not to exceed 10%• 60 kcps setting = better spectral resolution, higher dead time ( less

acceptable intensity)• 275 kcps setting has less spectral resolution but accepts more signal.

use of 60 kcps (40 keV) setting when peaks are close together, or overlap use 275 kcps (20 keV) when high statistic are needed (mapping)

60 kcps

275 kcps

Conditions of spectra acquisition• Analysed area should be homogeneous• What is a point spectrum ?

– Existence of an emitting volume

rho (g/cm3) E0 (keV) z max (µm)

2 (Al2O3) 15 3.3

5 (Fe3O4) 15 1.3

8 (Fe) 15 0.8

20 (Pb, Au) 15 0.3

Theses phases cannot be analyzed separately

Size of analyzed particle is about 1 micron. At 25 kV, part of the Al matrix actually contributed to the signal.

Acquisition of a spectrum• Acquisition time :

– higher is, better is signal / background ratio. X-ray emission is a statistic phenomenon : relative error is proportionnal to 1/√N.. Better is 106 counts.

– Small as possible if existence of C-contamination of the sample or if sample is a bad electrical conductor.

– When mapping, time spent on one point should be as less as possible use of high beam current

• Identification of elements :– An element is identified by all characteristic X-rays. Color

lines indicate the relative intensities.– All peaks should be explained – Beware of peaks overlapping– Beware of artefacts due to detector saturation : 2 photons

E1, E2 are count together as one photon E=E1+E2. Exists in case of high signals.

Double-photon peak

Surface C, O contamination peaks

Overlapping of peaks

Quantitative analysis of a spectrumTwo possible methods :

• P/B ZAF method, without standards

• Phirho(Z) method, with standards. Better in case of special conditions– Open library with the same device settings– Load the method of analysis– Quantify

• Check quantitative analysis– Built spectrum should be identical to real spectrum– Total unnormalized massic sum should be equal to 100 %

• Quite good quality analysis if result is between 96% and 104%• If result is less then 100% : maybe some elements have been forgotten

P/B ZAF method• ZAF :

– Z = « atomic number » effectIonization cross section, ability to stop the electrons according to the material.– A = absorptionDepends on X-ray energy, density, depth of emission– F = fluorescence effect between elements

One photon emitted by element A is absorbed by element B, inducing fluorescence. Photon from A is lost, one photon from B is observed.

• Intensity of X-ray characteristics peaks is compared to intensity of background no standards are needed

(z) method• The function (Z) is calculated. This function describesemission of photons X, for a given material. It depends on :

–Density of the material –Depth Z, from where the emission occurs

Z and A are simultaneously determined. It is then obtained curves, describing the emitted and emergent signals, according to the depth.

• Fluorescence between elements is a phenomenon distinct from emission. Its calculation is similar to the previous case (P/B ZAF).

• Analyze with standards : For each elemental X-ray characteristic peak, sample signal is compared

to standard signal (known composition or pure standard). “k” is a corrective term from Z.A.F. corrections. “k” depends of amounts of

all the elements existing in the sample and is only obtained after an iterative process.

dards

sample

dards

sample

II

kCC

tantan

)(tan puredards

sampleratio I

Ik

Total sum is almost 100%

Perfect match between real spectrum and built oneExcept for these energies (C contamination)

Loading of the standards

- Choice of the method : triangle near « quantify ». - May be loaded from recorded files, may be created, may be saved.

• One special point may be choose (beware of emitting volume !)

The objects

Indicates the intensity of signal = number of counts per second.Use this indication to determine the minimum analysis time on each point

• Lines may be analyzed– Acquisition :

• Choose the number of points in the line. Distance between points should be higher than size of emitting volume.

• Thanks to the global spectrum, determination of all the chemical elements.• Determination of the measuring time. According to the signal intensity. At least egal to

number of points x 20 000 counts.• Possibility of increasing width of the line

– Quantification : choose the appropriate method. – Saving : in the project or in a file .txt, readable by Excel.

• The aim is to record a mapping of a sample area. This mapping may be subsequently analyzed.

• Acquiring a mapping :– Use of a high beam current in order to reduce analysis time. Consequently, use

appropriate detector settings (130 kcps, 275 kcps)– Determination of the total points number– Determination of the total analysis time= nber of points * 20 000 counts– Select the chemical elements to display.– To begin, click on « Acquire » (parameters = right-hand triangle). To stop, click on the

same button. The mapping is realized with a lot of quick scanning. This allows to reduce C contamination, charge effect. The mapping becomes more and more precise with time: signal / background ratio increases.

• Save the mapping : in a separate file : .rtb or .bcf, usually quite big . In this file are recorded the full spectra of each point.

• Quantitative analysis of mapping : Qmap– May be time-consuming, so to be done as a post-process.– Select the quantification method (triangle near « Qmap »). – To begin : click on « Qmap ». – Results may be displayed on maps, false color maps, or recorded on “.txt” files (1 for

each elements) readable by Excel

The « hypermap » object

• This mapping was realized on K-Si peak energy

• This is not a Si-content mapping: at this energy, some others peaks overlap

• Also beware, in qualitative mapping, of variation in background signal

• A quantitative mapping is mandatory

Qualitative mapping before quantitative analysis

N1 = Number of counts / second

N2 = Number of points / lineMinimum = 100 points

S = Size of the image = HxV

Time of the mapping, for 20 000 counts per points is (in seconds) :N3 x 20 000 / N1

N2 = should be chosen to be H / dd = distance between two points (1,2 or 3 microns).N3 = (H/d) x (V/d) is the total number of points in the mapping

• Preliminary considerations :Detector should have a good spectral resolutionCharacteristics X-rays of light elements have small energy. In this range, there is C contamination,

usually of high number of X-ray characteristics peaks, bad transmission of the detector window. In order to increase signal/background ratio, primary electron energy should be small, which is usually incompatible with the other chemical elements

• The best method is to measure out by difference, once all other chemical elements have been analyzed.

This needs a careful analyzing of the other elements– PhirhoZ method with standards– Very good standards (at least 1 million counts standards)– Very good statistics of the spectrum (at least 1 million counts standards)– Check on an area not containing light element that total unnormalized sum is 100±1 %.

Select « measure out by difference » inside the method

Chemical analysis including one light element