The advantage of forming diffraction patterns in a crystalline material from areas less than 500 nm in diameter, hereafter defined as microdiffraction, can be readily described. The region of interest, such as a small second-phase particle, a particular grain in a fine-grain alloy, or a portion of the strain field surrounding a crystal defect, is small in volume compared to the surrounding matrix. Any means which reduces the total diffraction-forming volume increases the contribution from the region of interest and results in a diffraction pattern with an improved signal-to-noise ratio. Before the introduction of the STEM electron microscope, the lens configuration used for diffraction was such that spherical aberration of the objective lens prevented diffraction patterns from an area less than 500 nm from being formed. This limitation, and the means for bypassing it with microdiffraction, is worth describing in detail.
KeywordsObjective Lens Select Area Diffraction Diffraction Spot Spherical Aberration Main Beam
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