Non-destructive determination of local strain with 100-nanometre spatial resolution

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Abstract
Structure sizes of ∼ 180 nm are now standard in microelectronics, and state-of-the-art fabrication techniques can reduce these to just a few tens of nanometres (ref. 1). But at these length scales, the strain induced at interfaces can locally distort the crystal lattice, which may in turn affect device performance in an unpredictable way. A means of non-destructively characterizing such strain fields with high spatial resolution and sensitivity is therefore highly desirable. One approach is to use Raman spectroscopy2, but this is limited by the intrinsic ∼ 0.5-µm resolution limit of visible light probes. Techniques based on electron-beam diffraction can achieve the desired nanometre-scale resolution. But either they require complex sample preparation procedures3 (which may alter the original strain field) or they are sensitive to distortional (but not dilational) strain within only the top few tens of nanometres of the sample surface4,5. X-rays, on the other hand, have a much greater penetration depth, but have not hitherto achieved strain analysis with sub-micrometre resolution6. Here we describe a magnifying diffraction imaging procedure for X-rays which achieves a spatial resolution of 100 nm in one dimension and a sensitivity of 10-4 for relative lattice variations. We demonstrate the suitability of this procedure for strain analysis by measuring the strain depth profiles beneath oxidized lines on silicon crystals.