equal thickness fringe

When one diffracted wave is matched to the Bragg condition in a wedge-shaped crystalline specimen with a constant orientation, both the bright-filed image and dark-field image of this specimen exhibit periodic fringes depending on the change of the thickness. This pattern is termed (equal) thickness fringes. Two beams whose wavelengths are slightly different are formed in the transmitted wave and the diffracted wave due to the two-beam dynamical diffraction effect. They interfere with each other to cause beats or thickness fringes. The thickness change between the neighboring fringes (extinction distance) is proportional to the reciprocal of the crystal structure factor of the diffracted wave. The fringes are used for the estimation of the specimen thickness. The composition change of a multilayer semiconductor film can be measured from the difference of the extinction distances because the value of the crystal structure factor depends on the composition of each layer.



Bright-field image of an AlCu alloy taken at an accelerating voltage of 200 kV, where 200 reflection is excited.
The AlCu specimen is wedge-shaped and its thickness increases toward a far direction (upward in the image) from the vacuum side (left bottom of the image). Equal thickness fringes exhibiting thickness changes are seen.

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