References: Oxford Instruments website “Scanning Electron Microscopy and X-Ray Microanalysis” by Joseph Goldstein et al “ Energy Dispersive Spectroscopy on the SEM: A Primer ” by Bob Hafner “Transmission electron microscopy IV Spectrometry” by David B. Williams et al

EDS softwares: INCA (SEM)and EDAX (TEM)

Dr. Yina Guo Materials & Surface Science Institute

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Distances depend strongly on material and beam energy. Note SEM X-ray resolution, typically about 1 mm, TEM X-ray resolution ~ width of beam

Interaction volume for electrons in a bulk sample. Distances are for 20 kV electrons in Cu, for Al multiply by 3.

Interaction volume

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The important point is that the voltage pulse produced is proportional to the energy of the incoming X-ray photon.

Operator control

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Things under your control: • Geometry of the Detector (working distance) • Accelerating voltage • Process time • Duration of signal acquisition (to obtain a statistically significant

number of counts and thus good peak/backgound ratios) • Probe current (changing probe currents will necessitate realignment

of the microscope)

The important point is that the voltage pulse produced is proportional to the energy of the incoming X-ray photon.

Operator control

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The X-Max Silicon Drift Detector Detector size: 50mm2

EDS system in Hitachi-SU70 SEM

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INCA acquisition process

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INCA acquisition process_Microscope Setup

Input rate: This shows the approximate rate of photons striking the detector. As you adjust the microscope beam current, the input rate or the approximate rate of X-rays entering the detector will be in direct proportion to the beam current.

Acquisition rate: This shows how fast the system is accumulating spectrum counts. Spectrum counts determine statistical precision and limits of detection. Since it takes time to measure photon energy, there is a chance that second photon will arrive while the pulse processor is measuring the first. If this happens, the measurement is rejected and the acquisition rate is therefore less than the input rate.

Beam current setting

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INCA acquisition process_Microscope Setup Process time: The signal (voltage step) from the preamplifier is transformed into a voltage pulse that is suitable for the multi channel analyzer. Shaping and noise reduction of the signal are achieved by digital computation. The noise on the voltage ramp from the detector is effectively filtered out by averaging the signal. The time over which the waveform is averaged is called the process time (Tp). Tp is under control of the operator. The longer the Tp, the lower the noise. If noise is minimized, the resolution of the peak displayed in the spectrum is improved, and it becomes easier to separate or resolve, from another peak that is close in energy. However, there is a trade-off between the process time that is used, and the speed at which data can be measured. The longer the process time, the more time is spent measuring each X-ray, and the fewer events that can be measured.

Dead time-time during which pulses are not measured

Deadtime = (1 – Output rate/Input rate) x 100. Deadtimes of 30-60% will tend to maximize output. The operator can and should maximize output rates for a given sample and process time by controlling probe current/spot size.

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INCA acquisition process_Quant Optimization

• For a given X-ray pulse the X-ray energy it is assigned to depends on the gain of the amplifier which can drift over time.

• Thus the system must be calibrated by acquiring a spectrum from a known element such as Co or Cu.

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Microscope setup

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INCA acquisition process_Acquisition setup

EDS Setting

WDS Setting

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INCA acquisition process_Confirm elements

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INCA acquisition process_Confirm elements

1. Ensure that the incident beam energy is high enough. Beam energy > 2 × Highest peak energy. 2. Ensure that the spectrum is reliable. Repeat the spectrum from the same area, ensure there are enough counts 3. Use prior knowledge of the sample to know which elements are likely to be present and which are unlikely to be present. 4. Confirm elements by looking for other peaks from that element. 5. Work from high energy to low energy identifying peaks. At high energy, there are fewer peaks - and it is easier to resolve neighbouring peaks.

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INCA acquisition process_Quant setup

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INCA acquisition process_Spetrum comparison

EDS line scan analysis (Aghada)

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MX (VX) Vanadium

Chromium

Iron

M= Nb or V, X = C and/or N*

Czyrska-Filemonowicz, A., Zielinska-Lipiec, A., Ennis, P.J. (2006) 'Modified 9% Cr Steel for Advanced Power Generation: Microstructure and Properties' Journal of Advancements in Materials and Manufacturing Engineering, 19(2), 43-48.

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Wt% MoL V K CrK FeK NbK At% MoL V K CrK FeK NbK

multipoint1 13.49 0.63 65.3 20 0.58 multipoint1 7.93 0.7 70.82 20.2 0.35 M23C6

multipoint2 13.68 0.61 62.57 22.59 0.54 multipoint2 8.06 0.68 68.05 22.88 0.33 M23C6

multipoint3 11.31 0.45 63.88 23.86 0.5 multipoint3 6.59 0.49 68.72 23.9 0.3 M23C6

multipoint4 8.31 69.2 11.55 0.94 10 multipoint4 4.83 75.82 12.4 0.94 6.01 V-rich MX

multipoint5 3.06 73.84 16.97 1.07 5.07 multipoint5 1.69 77.04 17.34 1.02 2.9 V-rich MX

multipoint6 2.69 71.87 15.88 4.13 5.43 multipoint6 1.49 75.18 16.27 3.94 3.11 V-rich MX

multipoint7 13.27 0.6 64.87 20.47 0.79 multipoint7 7.8 0.66 70.38 20.68 0.48 M23C6

EDAX-EDS example 1_Multi-point analysis

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EDAX-EDS example 2_Elemental mapping

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