Last Updated 7/6/09

Standard Operating Procedure EDAX EDS System and Genesis Software

This document is intended as a guide to the operation of the EDAX EDS System and Genesis Software by certified users. It provides details on obtaining and analyzing an x-ray spectrum. This guide is not intended for novice users.

*Please fill out the logsheet before beginning using the microscope.

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1. EDAX System Overview The Quanta 200 SEM is equipped with an Energy Dispersive Spectroscopy (EDS) system that detects the x-rays emitted from a sample during electron imaging. The system consists of three main components: an x-ray detector, separated from the SEM chamber by a very thin polymer window; pulse processing circuitry, which determines the energy of the detected x-rays; and analyzer equipment, which interprets the x-ray data and displays it on a computer screen. Characteristic x-rays can be generated in a sample when electrons of a minimum kinetic energy are incident on it. The incident electrons excite the atoms in the sample, and as the atoms relax, electrons move from outer electron shells to inner ones. During this transition the electrons give off energy, in the form of photons. The x-ray detector is housed inside a metal tube, which is inserted into the SEM chamber so that the detector is very close to the final aperture of the SEM column, and pointed at the surface of the specimen. Thus, some of the emitted X-rays will pass through the thin polymer window onto the surface of the detector. The detector is a semiconductor made of silicon doped with lithium. X-rays striking the semiconductor create an electrical charge within the semiconductor. This charge is then analyzed to determine the x-ray energy and the number of x-rays being emitted. A cryostat, consisting of a liquid nitrogen-filled dewar, is mounted on the side of the detector housing. Through thermal contact, the liquid nitrogen (LN2) keeps the detector cold, around 90K. The electronic noise from the circuit at room temperature is so high that no usable signal can be detected. The low temperature of the LN2 reduces the thermal noise to acceptable levels. Additionally, a voltage is applied to the detector during operation, and at room temperature, the lithium in the detector is mobile enough that it can diffuse through the silicon in response to the potential difference, destroying the detector. The LN2 prevents this from happening. 2. Start up Procedure 1. It is assumed that a sample has been properly loaded into the microscope, that the

accelerating voltage is on, and that a region of interest has been located. 2. For x-ray analysis, as a general rule, select an accelerating voltage of at least 2x the

highest x-ray peak energy expected. For example, if iron is the highest energy peak at 6.39, the minimum kV should be about 14 or 15.

3. The sample must be located at a working distance of approximately 10mm. The EDS

detector is positioned such that 9.8 to 9.9 mm is the optimal position (see Figure 1). If the sample is above or below this position the number of x-rays hitting the detector will be greatly reduced.

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Figure 1. Example of the change in x-ray signal (CPS) with working distance

4. The EDS system is controlled by means of a Windows-based user interface (UI)

program, called Genesis. This program is located on the EDAX EDS Computer. Double click on the Genesis shortcut on the desktop to open the program. The program window is shown in Figure 2.

Figure 2. Genesis Main Window. The yellow circles identify the location of three important EDS parameters: CPS, DT and TC.

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3. Set up EDS Parameters 1. There are two important parameters that must be optimized. At the bottom left of the

screen are two fields: CPS (counts per second) and DT (dead time). 2. Generally, to change the CPS, change the spot size in the SEM. As a general rule, the

CPS should be at least 900. For quantitative work, a higher count rate (more throughput) is better, to increase the signal to background ratio.

3. Generally, to change the DT, adjust the amp time. DT should be between 20% and

40%, preferable at around 35% As a rule of thumb, use an Amp Time of 25.6 or 51.2 µs. Selecting auto for the Amp Time will take the current count rate and set it so that dead time is between 20 and 40%. The 20-40% range is recommended, as within this range sum peaks will be mostly negligible.

4. The IR camera on the SEM uses an infrared LED light. This light will overload the

detector if quad 4 is not paused. If DT is 100%, double check that you have paused the camera.

5. Once the CPS and dead time are adjusted, use the buttons on the right hand side of the

screen to collect and analyze the spectrum. 4. Collect A Spectrum 1. The buttons for collecting and analyzing the spectrum are in the Spectrum Panel on the

right hand side of the program window. The panel layout is shown in Figure 3.

Spectrum collection panel -

Peak Identification panel -

Background panel -

Quantification panel -

Figure 3. The Main Spectrum Panel

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2. Click on Clear to remove any old peaks. Click Collect to begin collecting a new spectrum. To stop collection, click Collect again.

3. For a fixed collection time, refer to the Preset drop down menu at the right-hand side of

the toolbar. Select a spectrum collection time, in seconds, either from the presets or by typing a value in the box.

4. In the adjacent drop down menu, select a time option: clock time, live time or ROI

Counts. Clock time refers to actual time passed; live time refers to the actual time the detector is active. To use ROI Counts, ROIs must first be created. Refer to the EDAX manual for more detail. The live time setting is preferable for repeated measurements.

5. Peak Identification

1. To adjust the view of the spectrum, use the expand ( <> or ◊ ), contract (>< or ∨) and

home keys from the toolbar. The toolbar and these controls are shown in Figure 4. The spectrum display may also be adjusted by clicking and dragging with the mouse.

Figure 4. The toolbar and scale adjustment shortcuts. 2. Click Clear All in the spectrum collection panel to remove any old peak labels. 3. Click on the Peak ID button in the Spectrum Panel for automatic peak identification.

The primary peaks identified by the program will be labeled on the spectrum. 4. Click HPD to display a theoretical spectrum, based on the identified peaks and the

collection parameters, on top of the collected spectrum. HPD is used to confirm if the correct elements have been identified.

5. For manual peak ID, click on the ◊ symbol to expand the panel. The expanded view is

shown in Figure 5. There are several ways to manually identify peaks. 6. Known elements can be entered in the text box. Press enter to display the spectrum

peaks for that element on top of the collected spectrum. Click Add to add the element to the Elements list.

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Figure 5. Expanded view of the peak identification panel

7. With the mouse, click on a peak of interest. A list of Possible elements will be displayed. The most likely peaks are at the top of the list. Scroll through the list and select an element to display the spectrum peaks on top of the collected spectrum. Click Add to add the element to the Elements list.

8. To remove an element from the Elements list, highlight it and click Delete. 9. The Z+ and Z- buttons can be used to scroll through the elements. 10. Check or uncheck the Alpha lines only, elem, shell, and trans boxes to change how

peak labels are displayed.

11. Click on the ◊ symbols to hide the panel. 12. To view the original spectrum without overlays, right click with the mouse cursor in the

spectrum window.

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6. Quantitative Analysis The quantification models make three important assumptions:

• The sample is smooth and polished. • The sample is homogeneous. • The sample is infinitely thick to the electron beam.

If any one of these assumptions is not correct, then the quantification may not be accurate. More complex modeling may be required to reduce error. 1. Click the Bkg button in the Spectrum Panel to view the background. 2. For standardless quantification results, click the Quantify button. The results will use

an automatic background subtraction and will be normalized to 100 percent. Only the elements labeled will be quantified.

3. To save the quant information, copy the information, then save in the Notepad

application (shortcut is located at the bottom of the Windows desktop. 4. A semi-standardless method of quantification is available, but is beyond the scope of

this guide. See the EDAX Genesis manual or a knowledgeable person for more information.

7. Saving the Spectrum 1. The spectrum can be saved by clicking the Save button at the bottom of the Spectrum

Panel. If you use .spc as the extension, all the spectrum information will be saved and can be reanalyzed at a later date. It is recommended that you save all work using this format.

2. To save just an image of the spectrum, save using the .bmp extension. 3. A shortcut to the default save location, C:\Documents and Settings\supervisor\My

Documents\My Edax\EDS spectra, is on the Windows desktop.

8. Shut-down Procedure

1. When done, exit the program. You will get an error (XtnSrvr Module), which you can

ignore. Click Don’t Send, and turn off the computer screen.

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9. Microscope Control

1. The SEM can also be controlled from within Genesis. To access the Microscope Control system, click on the column/beam icon at the far right of the toolbar, as shown in Figure 6. Most basic SEM functions can be controlled through this interface.

Figure 6. SEM column control from within Genesis.

2. Click on the spectrum icon next to the column control icon to return to the Spectrum control system.

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10. Additional Notes on Elemental Analysis with EDS For the most accurate results, you will need to do your analysis in two steps: qualitative and quantitative. In identifying what elements are present, you would like to prevent any peak overlaps. The best spectral resolution is obtained with a longer time constant, as seen from the amp time and resolution data in Table 1 below. Dead time will also change, so remember to adjust CPS as needed.

Amp Time (µs)

Resolution (eV)

Dead Time (%)

102.4 129 85 51.2 130 48 25.6 132 27 12.8 144 13 6.4 155 8 3.2 177 4 1.6 208 2

Table 1. Spectral Resolution at various time constants (measured May 2008)

A longer amp time is preferred when determining qualitative composition of an unknown sample. However, it is also preferable to stay within the ~20-40% DT range. You can increase the amp so DT goes up to 67%. This will give the maximum throughput, but sum peaks will be significant and can easily be mistaken for real peaks. The relationship between the counts in the x-ray peak also becomes non-linear and even decreases for DT above 35%, as shown in Figure 7.

Figure 7. Number of counts stored in the Mn Ka peak in 10 seconds (amp time 12.8us).

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You should strive to correctly identify all the elements present at the qualitative stage for several reasons. During quantitative analysis, you can identify any spurious peaks (sum or escape). Also, if your peaks do not overlap (i.e. are not very close in energy), the resolution does not matter so much, and you can use any time constant. For quantitative analysis, a higher count rate (more throughput) is better, to increase the signal to background ratio. So rerun the analysis using a lower amp time and higher counts, again keeping the DT between 20 and 40% if possible. Note that with a short amp time the detection of the light elements will not be as good as at a higher amp time. If you are unsure of how the spectrum changes as you change the CPS and DT, overlay 2 spectra as you change parameter. For example, try changing DT to see how the sum peaks change. To compare the spectra obtained at 2 different settings, collect and save one spectrum. Then, from the View menu in the toolbar, select Compare. Select Single Overlay, and Swap A, B. Then collect the second spectrum. An example with a nickel–chromium–iron alloy is given below. The first spectrum, shown in black outline, was collected at an amp time of 102.4 µs. Notice that two Cr L peaks and two Ni L peaks are visible. The second, red, spectrum was collected at an amp time of 1.6 µs. At this setting, the two Cr peaks and the two Ni peaks are not distinguishable. In fact, there is no separation between Cr and the Ni peaks at all. Both spectra were collected at a dead time of ~35%, however the CPS for the second spectrum was much higher than for the first. In this case, it would be advisable to use the high amp time and better resolution to identify the elements present. Then, decrease the amp time (and thus increase the CPS) and collect a second spectrum, then using the well-separated Ni K and Cr K peaks for quantification. By the way, the quantification results for the alloy below are 71% Ni, 17% Cr, 10% Fe and 2% Si (atomic fraction).

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11. Mapping 1. Open Genesis.

2. To set up the image parameters, click on the Image tab.

3. On the RHS of the screen, right click near the Collect Image button.

4. Select the image resolution (usually 1024x800).

5. Select the number of strips. This setting is usually 25 for a rastered image. If your

sample exhibits charging, you can use 1 strip.

6. Click on the carats (<>) and select either BSE or SE. This selection primarily affects the image label.

7. The detector (Det) should be set to 1.

8. The gray scale level (S) default values are 0 or black for Smin, and 4095 for Smax. This is equivalent to a grayscale range of 0 (black) to 255 (white).

9. Reads is the number of times the signal at each pixel is read and averaged. 1 read gives a low signal to noise ratio. 16 gives a medium signal.

10. Avoid using autoscale, as this automatic feature does not always provide the best settings.

11. The BSD is better to use for mapping as it provides better contrast between phases and

particles. Whether you use BSD or SE image, the area you will map should be imaged in Quadrant 4 on the SEM (upper left hand quadrant). This is the channel that Genesis reads when it takes control of the microscope.

12. To acquire the maps, click on the Maps/Line tab. 13. Click the Collect e- button to acquire your electron image. 14. Click the Collect Spectrum to acquire a spectrum. Click again to stop acquisition. 15. Click on the carats (<>) to open the spectrum menu.

16. Using either the Element list or the Z list, select the lines you wish to map. These

become the ROI or regions of interest in the map (the range of x-ray energies which the system will map).

17. Close the spectrum menu. 18. Select Maps.

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19. Click the Live checkbox. This sets the acquisition to be done frame by frame, which provides continuous updating of the image. If live is not checked, the data is acquired pixel by pixel.

20. If Live is not checked, you can select Quant mapping. This function removes the

background and does HPD peak fits. Use this if you need to deconvolute the lines when there is overlap, or when you want to remove the background signal.

21. Right click near the Collect Maps button to change the Map collection parameters. 22. Maps Resolution refers to the number of pixels the system takes measurements at.

Pixel size should be similar to the beam size. 128x100 is frequently sufficient. 256x200 may also be used. Any further increases in resolution may be wasted, due to the beam spread in the material.

23. A Dwell time of 1000µs gives good signal to background ratio.

24. The number of ROIs is set automatically depending on the number of elements in the Z

list. 25. Frames sets the number of (256x200 pixel) (1000µs dwell time) frames that are taken

and added to each other. You can select a high number of frames, then stop the acquisition early of you are satisfied with the data.

26. Select use Z list to use the default lines. 27. Select HV off when done if you will leave the instrument unattended while doing a

long measurement.

28. Click the Collect Maps button, set your path and file name, and begin the acquisition. 29. While running, you can select 8 views plus spectrum under the View toolbar. 30. To stop the collection early, click the Collect Maps button again. You can choose to

abort by stopping immediately, or stop at the end of the frame to simply stop the acquisition before the specified number of frames.

31. When the acquisition is finished, select 4, 16 or 36 views from under the View toolbar.

32. Click Auto LUT to artificially brighten the maps, if needed

33. To overlay the maps, select the maps to overlay using the mouse and the keyboard shift

button.

34. Under the Process toolbar, select color, then substitution overlay.

35. The overlay will automatically be saved as a bmp file, along with the image and the maps.

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36. The image can be annotated from Genesis. Open the Edit menu in the toolbar, then

select annotate image. An Image Annotation box will pop up. To add a crosshair, first click on + in the toolbar, then click where you want the crosshair to be located. Finally, check Crosshair in the Image Annotation box.

37. In the toolbar, there is an EDS Multi Point button. This allows multiple maps to be

made on the same image area. Options include spot mode, reduced raster, and freehand. The measurement time (EDS preset time) must be entered before selecting the area to scan on the image.