Comparison of CBED and dark-field holography for strain mapping in nanostructures and devices

A comparison is presented of the results obtained by two transmission electron microscopy (TEM) techniques, CBED and the new electron holographic technique HoloDark, on two selected structures in which the Si crystal has been stressed by two different means. First, we focus on a real transistor in which the Si channel is stressed by recessed SiGe source and drain, then, we report on results obtained on a flash memory structure in which the Si crystal is stressed by the presence of a Si₃N₄ liner grown on the Si surface. In homogenous media and highly symmetric structures, the CBED and HoloDark techniques give similar results for the measurement of deformations with about the same precision, in the 10^-4 range. However, there are situations in which the HoloDark technique appears clearly superior. These include highly anisotropic media in which the relaxation of thin stressed samples may render the CBED technique inoperative. HoloDark offers a full mapping of the 2D strain tensor, including rotation and shear and gradients, without recourse to extensive modeling and simulation. HoloDark proves to be a technique of choice for strain measurements in real devices.