Electron holography is an interferometric technique conducted in a transmission electron microscope (TEM) which allows the reconstruction of the phase and the amplitude of the electron wave. It is used to map electrostatic, magnetic and strain fields at the nanometer scale.
There are many different optical configurations for electron holography [1]. The most widespread is the “off-axis” configuration where a Möllenstedt-Düker biprism is placed after the specimen to interfere an object wave and a reference wave. For strain mapping [2], the beams diffracted by the substrate are interfered with the beams diffracted by the epitaxial layer. The electron hologram is then recorded on a CCD camera and a phase image is numerically reconstructed. A strain map is then obtained by differentiation of the phase in the direction of the chosen reciprocal lattice vector.
In this article, we developed an alternative dark-field holographic configuration that uses a biprism located before the specimen. This configuration is called differential-phase contrast (DPC), based on the work of McCartney et al. [3]. Two incident beams with a slightly different angle are created by the pre-specimen biprism. The hologram is then recorded in a defocused plane where beams diffracted by slightly distant regions of the specimen interfere. One advantage of this technique is that the DPC phase reconstructed from the hologram is directly proportional to the strain. Another advantage is that the reference region does not need to be as big as the region of interest. The enclosed image shows a hologram and a strain map obtained on a strained transistor with recessed SiGe source/drain. The SiGe source/drain are used to compress the Si channel and increase the mobility of the charge carriers [4].
[1] JM Cowley, Ultramicroscopy 41, 4 (1992), p. 335-348.
[2] M Hÿtch et al., Nature 453, 7198 (2008), p. 1086-1089.
[3] MR McCartney et al., Ultramicroscopy 65, 3-4 (1996), p. 179-186.
[4] SE Thompson et al., Electron Device Letters, IEEE 25, 4 (2004), p. 191-193.
Reference
T. Denneulin, F. Houdellier, M. Hÿtch, “Differential phase-contrast dark-field electron holography for strain mapping”, Ultramicroscopy, Volume 160, January 2016, Pages 98-109.
DOI : 10.1016/j.ultramic.2015.10.002
Contact
Dr. Thibaud Denneulin : thibaud.denneulin chez cemes.fr