Beam convergence effects in the weak-beam imaging of inclined planar defects

When defects are imaged using weak beam techniques it is common to use a higher beam convergence than when they are imaged under strong two beam conditions because of the way specimen drift limits the exposure times that can be used. It is demonstrated that, for a typical illumination system, as the convergence is increased the range of tilt across the probed area is also increased. This can affect the weak beam imaging behaviour of a defect, and the α-fringe contrast of thin twins is examined in this context. The contrast changes in the field of view associated with the local variation in tilt are discussed in relation to the degree to which the relative effects of convergence on α-fringe and thickness fringe contrast can be qualitatively understood kinematically. However, some effects, such as the observed increase in α-fringe contrast at moderate convergence, are more difficult to model but are also potentially of greater concern in the characterization of the differences in contrast to be expected for intrinsic and extrinsic faults as well as for twins.