Features
ED is a technique for examining the crystal structure from the diffraction pattern obtained by irradiating a sample with an electron beam. Crystallographic information of the substance is then obtained.
In the case of a transmission electron microscope (TEM, shown in Fig. 1), the electron diffraction pattern consists of regularly spaced spots for a single crystal, concentric rings for a polycrystal, and broad rings for an amorphous sample.
The regions of interest in transmission electron microscopy can be examined.Together with an elemental analysis by the EDX method, it is possible to identify both the substances and their crystallinity.
Application Examples
Substances such as silicon, compound semiconductors, oxide semiconductors, and metals, can be evaluated with respect to the following properties:
- Degree of crystallinity
- Crystal structure identification
- Orientation of single and poly crystals
Principle
When the angle of the incident electron beam satisfies the Bragg diffraction condition with respect to the lattice plane of the crystal,the electrons are elastically scattered as shown in Fig. 2 (λ is the de Broglie wavelength of the electrons). Electrons that do not satisfy the Bragg condition are scattered and interfere with each other, whereas the beam of diffracted electrons have higher intensities in specific directions, thereby forming a spot pattern.
By analysing the positional relationship between the transmitted light and multiple diffraction spots, information on the crystal structure can be obtained. If the half diffraction angle, θ, is sufficiently small, the distance to the detector is L, and the distance from the transmitted light to the diffraction spot is R, R/L can be approximated as:
Data examples
Single crystal Si
When single-crystal, such as Si, is measured, a pattern of diffraction spots according to the symmetry of the crystal is obtained.The spots correspond to the Fourier transform of the crystal lattice (reciprocal lattice space) with the centre spot having zero spatial frequency,hence corresponding to undiffracted electrons transmitted though the sample. The distance between any spot and the centre determines the corresponding crystal plane spacing, and the angle between two straight lines connecting spots with the centre determines the corresponding crystal plan orientation (see Fig. 4).
Polycrystalline silicon
Because crystals of all orientations exist in the measurement area, they form concentric patterns (divide rings) rather than spots. The lattice plane spacings are obtained from the radii of the rings.
Quartz glass
An amorphous sample, such as glass, has no crystal structure so no clear diffraction spots are
observed. Instead, blurred concentric halo patterns are formed (see Fig. 4).
Data format of deliverables
- High-resolution JPEG files using MST’s download service.
- High-resolution MPT* images.
*MPT is an MST proprietary file format which allows length analysis using MST’s Image Measuring Tool (available upon request).
Specifications
Items for enquiries
- Purpose and content of measurement
- Sample information
- Number of samples, and availability of preliminary samples
- Location of the point/area subject to analysis
- Layer structure (composition) and film thickness
- Particle size and density in the case of powders
- Handling instructions
- Preferred due date for preliminary analysis report
- Due date for delivery of final report
Caution
- Like TEM, thinning of the sample by focused ion beam etching or milling is necessary. In some cases, however, thinning may prove difficult.
- Sample processing and observation may alter or deform the sample.
- Hydrocarbon and other substances may adsorb to the sample during analysis.
- Magnetic particles can damage the electron microscope if they are dispersed in the chamber. Therefore, magnetic powders are not allowed as samples.
［ED］電子回折法の分析事例はこちらからご覧ください。