BALET

A needle-shaped sample cut out of any bulk material by a focused ion beam can be imaged in the microscope by sample tilting in several different orientations, resulting in a series of single projection images. In a second step, each tilted single-projection image can be back-projected in the computer in either real or reciprocal space to obtain a three-dimensional reconstruction of the original object.

For compact semiconductor structures, the sample is ideally needle-shaped along the tilt axis to best meet the projection requirements of electron tomography. The imaging mode is adapted to the problem, for example the HAADF-STEM (high-angle annular dark-field scanning TEM) method serves a chemically sensitive imaging and reveals compositional variations.

With modern software it is possible to visualize the reconstruction data in many ways and to display it three-dimensionally: the tomogram can be viewed from all spatial directions and by selective sections details inside can be made visible or highlighted for further analysis.

Example: Reconstruction of an epitaxial AlGaAs-GaAs interface to determine both its morphological roughness and the chemical width.