On-line measurement of specimen thickness by Convergent Beam Electron Diffraction

Abstract

Convergent Beam Electron Diffraction (CBED) thickness measurement is the easiest and most accurate way of determining the thickness of crystalline materials. The method was described by Kelly et al. The specimen thickness can be calculated from a few measurements on a recorded diffraction pattern in a matter of minutes (by hand) or seconds (by a computer program).For thickness measurement a CBED pattern is needed that contains a two-beam diffracting condition, with a dark Kikuchi line going through the centre of the Bright-Field disc and the corresponding bright Kikuchi line through the centre of a Dark-Field disc. Parallel to the bright Kikuchi line, the Dark-Field disc contains a number of fringes (Fig. 1) whose distance from the Kikuchi line varies with specimen thickness. The data needed for a measurement are the electron wavelength, the d-spacing dhkl of the diffraction used, the distance 2θB between the Bright-Field disc and Dark-Field disc in the CBED pattern, and the distances Δθi between the dark thickness fringes and the bright Kikuchi line in the Dark-Field disc (Fig. 2).