Influence of crystallographic orientation on local strains in silicon: A combined high-resolution X-ray diffraction and finite element modelling investigation

The processing of Si-based devices induces very high local strains in silicon. This strain field is worth investigating in particular in Shallow Trench Isolation, which is a key step in the production of non-volatile-memories and transistors. In this study we investigate by high high-resolution X-ray diffraction the periodic strain field induced in silicon by the STI process. High-resolution X-ray diffraction is shown to be very sensitive (< 10-4) to local strains and has the distinct advantage of being non-destructive. Investigated samples are periodic arrays of SiO2-filled trenches in single crystal (001) Si. The trenches are etched either along [100] or along [110]. The experiments have been performed on a 4 circles goniometer with a laboratory source. We have recorded Si 004 and asymmetric Si 224 or Si 404 reciprocal space maps in order to extract axial strains in the 3 directions. The trenches array induces a periodic strain field in silicon, which gives rise to distinct satellites in reciprocal space. The intensity of these satellites is related to the strain field. We also performed elastic calculations with a finite element modelling code. Finally structure factor calculations are performed using the displacement field determined from mechanical modelling. We focus here on the effect of silicon elastic anisotropy on the strain field in 580 nm and 200 nm period samples. The influence of line crystallographic orientation ([100] or [110]) on the strain field is evaluated by finite element modelling and compared with experimental ones. The silicon anisotropy has a weak effect on strain and is compensated by process variations due to the line orientation