Strain Analysis of Si Nanolayers by Ultra-Small-Angle Grazing Incidence X-ray Diffraction


Hiroo Omi and Tomoaki Kawamura
Materials Science Laboratory

 Nanoscale ultra-thin Si films exhibit quantum size effects when their thickness becomes smaller than several nanometers. In order to control nanoscale silicon devices based on the quantum size effect, it is essential to achieve high-quality silicon nanolayers without strain distribution. Ultrathin silicon nanolayers are usually fabricated by thermal oxidation and HF etchback processes on silicon-on-insulator wafers. In this method, the silicon layers are subjected to strain due to the difference in the thermal expansion coefficients between Si and SiO2 during the thermal silicon oxidation. As a result, the strain significantly increases as the silicon nanolayers become thinner, as has been observed by X-ray diffraction and Raman spectroscopy. However, with these conventional apparatuses, we can only detect information about the average lattice strain in the silicon nanolayers; it is difficult to detect information about the localized strains existing on the surface of the thin silicon nanolayers or at the Si/SiO2 interfaces.
 We developed a new apparatus for grazing incidence X-ray diffraction (GIXD) using the syncrotron radiation source at SPring-8 (Fig. 1) and established a method for evaluating such spatially localized strain on a surface. The new apparatus can be used at the incident angle below the critical angle of Si (0.18°). Consequently, for the first time, we succeeded in detecting small strain localized on the surface of silicon nanolayers with this apparatus.
 We probed in depth the strain on the surface of the Si nanolayers by changing the incident angle of GIXD. The diffraction patterns obtained at the incidence angles of 0.01° and 0.1° are originated from 2 nm and 6 nm in depth from the lattice of surface region (Fig. 2(a)). From the intensity analysis based on the two-layered-strain model (two layers with different states of strain), we found that the surface region of the silicon nanolayers has finite strain domains and that the degree of the strain is on order of 10-4 (Fig. 2(b)). Moreover, by applying this method to a thermally annealed sample, we found that a high annealing temperature of 1000℃ is required in order to obtain uniform silicon nanolayers on which strain does not localize.

[1] H. Omi, et al., Appl. Phys. Lett. 86 (2005) 263112.

Fig. 1. Schematics of ultra-small-angle GIXD.
Fig. 2. (a) Si(220) Bragg diffraction, (b) Two layer strain model.

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