Abstract
This presentation will focus on NanoBeam Electron Diffraction and its comparison with Convergent Beam Electron diffraction (CBED) and Dark Field Electron Holography (DFEH). Most experimental results have been obtained in SiGe layers embedded in Si (Fig. 1 ) or incorporated into transistors (Fig. 2 ), but some new results on Si nanowires will also be shown. With NBED, a strain sensitivity of 6 10 -4 on local areas of 3x3 nm 2 can be achieved [1]. NBED has several advantages compared to CBED and DFEH. Our conclusion is that with the recent improvements, NBED has become an efficient, simple to use and versatile technique for strain measurement. The microelectronics industry has been looking for simple tools to quantitatively measure strain in areas at the nanometer scale. In TEM, CBED was first used for that purpose [2] . It consists of acquiring with a small, slightly convergent electron probe a local diffraction pattern (spot diameter much smaller than 1 nm, convergent semi-angle in the order of 10 mrad). The transmitted beam is the most useful as it contains many line features, called for simplicity HOLZ lines (Fig. 1 ), and the precise position of these lines allows the strain to be measured with excellent sensitivity (Δa/a= 2.10 -4 ) in perfect crystals [2] . However, CBED has several limitations. Strain relaxation at interface can create a bending of the planes along the electron beam and this bending produce a broadening of the HOLZ lines [3] . (Fig. 1 c-d ). HOLZ lines in the central diffraction spot only appear for thick samples (thickness greater than 100 nm). So CBED cannot be applied to nanostructures. In order to obtain fine HOLZ lines, the crystal have to be tilted a few degrees (~ 10°) from a low index zone axis, which can lead to shadowing effects. More recently, DFEH has been proposed to measured strain [4] [5] . The accuracy of this interference technique is tremendous (certainly below 2. 10 -4 ), but a reference unstrained area near the region of interest is needed and the sample must be flat with a given thickness (about 100nm). Alternatively, NBED or NBD (NanoBeam Electron Diffraction) has been used [6] . In this case, local diffraction patterns are acquired along a low index axis, such as <110> in silicon devices, with a parallel beam (convergent semi-angle < 0.5 mrad) and a beam size larger than in CBED. At first, the technique was limited to beam sizes larger than 10 nm in diameter and to a strain sensitivity of 10 -3 . By using a modern microscope, in our case a FEI TITAN with a probe Cs-corrector, we were able to reduce the spot diameter size to 2.7 nm with a condenser aperture of 50µm (Fig. 1B ) and improve the sensitivity of the technique down to 6.10 -4 (Fig. 1D ) [3] . Strain maps (Fig. 2 ) were also realized (Fig. 2 ). Applications of NBED is not limited to microelectronics devices. As we will show, any isolated nano-objects, like Si nanowires with a core-shell structure can be studied. The beam shape and the accuracy of NBED will be discussed.
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