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Alternatively, the examined itemcan be reconstructed as a virtual object in the computer, element by element. This is usually achieved through mathematical methods like weighted back-projection or algebraic tools such as the simultaneous iterative reconstruction technique (SIRT). [8] The reconstruction is computationally intensive and has only been used in mainstream laboratories with GPU-equipped computers during the past decade. A full 3D reconstruction of a source/drain defect is shown in Fig. 6. The central source/drain area reveals an enlarged amount of Wand diminished amount of Ge. This 3D data representation is free of any projection effects of 2D elemental maps and is a powerful tool to gain insight into the true 3D nature of modern devices. The resolution of the reconstructed 3D object depends on the resolution of each individual map in the (x, y) direction, and on the tilt angle and number of projections in the beamdirection (z-axis). [9] Resolution is estimated to be 1 nm in the (x, y) direction and 2 nm in the z direction. In order to reduce radiation damage, the electron microscope’s acceleration voltage can be lowered. This has the additional advantage of increasing the sample’s x-ray scattering cross section of atoms, resulting in higher x-ray counts. The lower radiationdamage allows for longer exposure times for each EDXmap, resulting in a higher S/N ratio of the 3D reconstruction of the tomogram. Figure 7 depicts the 3D reconstruction of a failing contact structure in an experimental FinFET structure, where the tomogram was acquired at 80 kV. Tilt range was from - 56° to + 68°, and each of the 32 EDS maps was acquired for fiveminutes. As shown in the center contact in Fig. 6, part of the epitaxial Si (red) is replacedby Ti (yellow).

between two steps (image shift, focus shift, and beam tilt) is automatically adjustedby the software running the TEM during the tomogram acquisition. The recording of each map takes four to eight minutes, which makes obtaining the data a rather time-consuming endeavor (one to four hours). However, with an autonomous, programmable TEM tool, this task can be performed without supervision in off-hours or overnight. After the acquisition, maps for each element are aligned with respect to each other. Then the images are linked together to form a simple movie. By watching the movie, the human brain reconstructs the 3D nature of the imaged object. In many cases, the movie is sufficient to share key details about the object’s 3D structure as well as information about the defect under observation.

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 3

Fig. 5 Example of noise reduction by data processing using principle component analysis on a Mn EELS map.

Fig. 7 3D reconstruction of Hf, Si, Ti, Al, andW for amodern FinFET structure. The central W+Ti contact structure has missing epitaxial Si, resulting in a short to the gate due to Ti overfill. Total field of view is roughly 180 nm.

Fig. 6 Top down view of the 3D reconstruction of a S/D defect in a modern FinFET structure. Gates (TiN and Hf) extend from top right to bottom left; Si fins are not shown.

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