Materials research benefits from a number of analytical techniques that can be used in combination with an electron microscope, including energy-dispersive x-ray spectroscopy (EDS), electron-backscatter diffraction (EBSD), and wavelength-dispersive spectrometry (WDS). A multi-capability system can resolve data ambiguities created by a system acting alone--the challenge is how to optimize the performance of one technology without compromising the performance of another. By making design tradeoffs, our group developed an electron microscope accessory that integrates all three techniques in one package for simultaneous data acquisition and processing.
A map obtained using automated EBSD scanning. The color delineates the phase identified using EBSD data alone.
Because of the nature of the signals being collected, each system requires a dedicated detector. In all three technologies, maximizing the signal necessitates physically close coupling between the signal source and the detector. The EDS unit's cryogenically cooled silicon-lithium detector element sits at the tip of a long 'cold finger' adjacent to the sample, affording the greatest possible solid angle for collection. Likewise, we located the phosphor screen of the EBSD detector very close to the sample to allow collection of diffraction patterns for a large solid angle around the sample.
In order to simultaneously collect EDS and EBSD data, detector geometry is critical. EBSD requires highly tilted samples (about 70°). Both detectors must have a line of sight to the tilted sample surface, which is more difficult for some microscope port configurations than others. The objective is to optimize the design so that the signal obtained at a single working distance is as strong as possible for both detectors.
One important objective for EBSD detector design is to maximize the solid angle of the signal collected. We can achieve this by either increasing the detector area or bringing a smaller detector in very close to the sample. Without careful design, both approaches can prevent x-rays from reaching the EDS detector. The advantage is that a larger detector mounted sufficiently far from the sample can increase signal-to-noise ratio (SNR) because noise drops off faster with distance than intensity signal. In addition, separating the detector from the samples minimizes the danger of collision with the sample/stage.
Adding a WDS unit to the combined EDS/EBSD system augments its capabilities, but adds additional challenges. Unfortunately, the WDS components are difficult to mount close to the sample because of their physical size and complexity. We started with a compact WDS design that is compatible with multiple systems. In microscopes with large chambers, the long beam path between the sample and the spectrometer can introduce significant 1/d 2 losses. To mitigate this, we mounted a grazing-incidence, x-ray focusing device inside the chamber, within 20 mm of the sample, to capture a large solid angle. The graze angles for such optics are wavelength (energy) dependent, so it was necessary to develop three nested, contoured cones to guide the radiation diverging from the sample into a parallel beam that returns to the externally mounted spectrometer.
Within the spectrometer, five configurable analyzing crystals receive the radiation while sweeping through a range of incident angles intended to satisfy the Bragg law for the energy of interest. Each crystal (or synthetic multilayer) permits diffraction of a specific range of energies; typical configurations allow for complete coverage of the energies transferred by the collection collimator, with a typical energy resolution from 5 eV to about 25 eV.
Our design group optimized the performance of each tool without compromising the performance of the others by creating detailed 3-D models of the microscope's stage, chamber, pole pieces, and beam ports and then running ray-tracing simulations of all possible beam paths. The resultant design minimizes integration problems while simplifying serviceability and ease of installation. Integrating the hardware and software of all three analytical techniques together provides the scientist with a tool to obtain increased insight into the relationships between the chemical and crystallographic aspects of material microstructures. oe
Del Redfern is materials characterization product manager for EDAX Inc., Mahwah, NJ.