Anisotropic Biaxial Strain Evaluation in Carbon-Doped Silicon Using Water-Immersion Raman Spectroscopy

Abstract

Background and purpose Carbon-doped silicon (Si:C) is used in the source and drain region as a stressor to the channel of n-type MOSFET [1]. However, there are few reports directly evaluate the strain states of Si:C. Furthermore, it has not been evaluated the relaxation of the strain state in Si:C after microfabrication. In this study, we evaluated the anisotropic biaxial strain relaxation of Si:C caused by microfabrication, using water-immersion Raman spectroscopy. Experimental method Si:C films were grown on (001) Si substrate by molecular beam epitaxy. The film thicknesses were 43 and 37 nm, with C concentration of 0.600% and 1.06%, confirmed by cross-sectional TEM and SIMS measurement, respectively. Then the Si:C films were processed into patterns by electron-beam lithography and dry etching. Fig. 1 shows the schematic of the patterned Si:C structure. The pattern width (W) were varied as 1000, 500, 200, and 100 nm, while the pattern length (L) was fixed at 5 μm. The Raman shifts of the LO and TO phonon modes are essential for the anisotropic biaxial strain evaluation. We adopted the water-immersion Raman spectroscopy with appropriate polarizer to obtain both the LO and TO phonon mode Raman shifts from the patterned Si:C [2]. The numerical aperture of the objective lens and the refractive index of pure water were 1.2 and 1.3, respectively. The oblique light component induced by the high-NA lens can effectively excite TO phonon even under the forbidden backscattering geometry and enable us to evaluate anisotropic strain state. The wavelength of the excitation light source and the focal length of the spectrometer were 355 nm and 2000 mm, respectively. Here, the wave number resolution was 0.1 cm−1. Results and Discussion Figure 2 shows the W dependence of LO/TO Raman shifts in the patterned Si:C. As shown in Fig. 2, the Raman shift from the patterned Si:C shifts to the higher wavenumber side as W decreases. From Fig. 2, it is confirmed that the tensile strain is originally induced in the film and relaxed as the pattern width decreases. Furthermore, since the Raman peaks shift significantly at W ≦ 200 nm, it can be considered that the strain is relaxed considerably at 100 nm from the both pattern edges. It is clear that strain relaxation by microfabrication depends on the pattern size. Fig. 2 shows the same tendency as in the patterned SSOI [2]. From Fig. 2, since the strain in the patterned Si:C1.06% was more significantly relaxed as the W decreased, it is considered that the larger the strain applied in the film, the easier to be relaxed by microfabrication. The strain in Si:C increases as the C concentration increases, however, even if the C concentration is increased to introduce a huge amount of the strain into Si:C, the residual strain after relaxation due to the microfabrication may be smaller than the lower C concentration Si:C. We can estimate from the LO/TO peak shifts of Si:C0.600% that the strain perpendicular to the pattern length is dominantly relaxed while the strain parallel to the pattern is kept unrelaxed. Therefore, the strain relaxation in the low C concentration Si:C with small applied strain was anisotropic, while the relaxation for highly strained Si:C with higher C concentration sowed more isotropic. IN conclusion, t2his study experimentally revealed the anisotropic strain relaxation in Si:C.