Considerations for Choosing the Right EDS Detector Size for Analyzing Metallic Materials

Energy Dispersive (EDS) technology and scanning electron microscope (SEM) go hand in hand. Electron Microscopy provides the imaging while EDS provides the “chemistry”. They play a key role in the development, production, and failure analysis of metals and advanced metallic materials, as they provide a look at the material composition.

An EDS detector is used to separate the characteristic X-rays of different elements into an energy spectrum, and then utilizes software to analyze the spectrum and determine specific elements within a sample. Certain EDS detector technology enables the user to distinguish overlapping peaks in transition metals even when using low voltages, where L- and M-lines are prone to energy conflicts. This technology makes high spectral resolution analysis accessible to the everyday microscopist.

There are many features to consider as to which EDS detector should be used with the electron microscope. Quantitative and qualitative approaches provide distinctive inputs to the analysis while the instrument options provide others. There are different mapping routines and various shapes (round, oval) and sizes (16mm, 19mm, 25mm, etc.) of tubes. There are many active areas that can range anywhere from 10mm² to 150mm². Resolution is varied but has improved over the years, as have acquisition times. So what do you need to consider before choosing the EDS detector and which specifications are relevant?

There are three main drivers in specifying an EDS detector and microanalysis system:

Energy resolution
Sensitivity
Post-processing algorithms

When it comes to energy resolution, speed matters. Faster acquisition equals worse resolution. Detectors are specified at slow rates. Longer integration times result in superior energy resolution; shorter integration times are required for high count rates. Sometimes poor resolution is just fine, while other times even really good resolution will never be good enough. Also keep in mind that the low energy part of the spectrum is affected more dramatically than the moderate to higher part of the spectrum. EDS detectors are routinely specified @ 2,000 – 3,000 cps.

Regarding sensitivity, EDS detectors often carry a light element sensitivity specification termed as “Sensitive to”. The detector system absorbs X-rays. There is a window between the SEM chamber and the crystal and a thin metal layer on the detector crystal is used to avoid cathodoluminescence (the emission of photons of characteristic wavelengths from a material that is under high-energy electron bombardment). Note that some detectors use Nitrogen (N₂) backfill.

Light element detection is not possible with a window and has challenges well beyond window technology. In addition variable pressure mode, as well as other factors, has a major impact on light element detection. In fact, the only good light element quantitative analysis requires full standards. As indicated, the technology for light element detection exists today, but you need to specifically plan for it. So before choosing your detector, you need to decide if light element detection or mapping is important to your application.

Lastly, algorithms and post-processing techniques are important to your decision. Peak deconvolution, background subtraction, and matrix correction algorithms are critical to high quality mapping of a sample. Phase mapping is even more powerful than element mapping algorithms.

Here is a screenshot of a qualitative analysis of transition metal, noting the active area and sensitivity:

Considering these three main factors of energy resolution, light element/low energy, and algorithms and post processing techniques, your decision comes down to knowing your application. When is a small area better than large? When is large better than small? Do I need both? You need to ask yourself these questions because the EDS choice depends on your answers to these questions.

You can get more details and specifics about EDS detector considerations in this slide presentation. In it, I compare 100mm²/60mm² vs 30mm²/10mm² EDS detector sizes, and provide spectra, mapping, and analysis for various samples.

Access the presentation.