The development of aberration correction in electron optics has pushed the lateral resolution of transmission electron microscopes (TEM) below one Ångström. However, the spatial resolution of TEM in the z dimension is still very poor. Scanning confocal electron microscopy (SCEM) has been developed to improve the axial resolution in 3D electron imaging and was first demonstrated by [1].
More recently, the development of double-aberration-corrected TEMs has increased the useable range of convergence and collection angles, with the prospect of a commensurate improvement in the depth of field in SCEM. Furthermore, by using inelastically scattered electrons, there is the potential to achieve incoherent confocal imaging modes with improved depth resolution [2, 3, 4], in analogy with confocal fluorescence light microscopy [5]. However, the low energy loss electrons have a large coherence length due to the collective nature of the plasmon excitation and high energy core loss electrons have extremely low excitation probability, leading to a poor signal-to-noise ratio (SNR).
In this work, we develop a 3D confocal imaging technique using coherent elastically scattered electrons. This method exploits the depth sensitivity of electrons that have suffered a specific momentum change, rather than energy change. In the confocal mode, elastically scattered electrons that are not in focus at the confocal plane will incur a lateral shift in that plane, depending upon their change in momentum, providing diffraction contrast that is very sensitive to depth. This strong depth sensitivity is combined with a large SNR made possible by using elastically scattered electrons. Applications to the imaging of 3D engineered nanostructures will be demonstrated.