Evaluation of Anisotropic Biaxial Stress in Si1-XGex/Ge Mesa-Structure by Oil-Immersion Raman Spectroscopy

Introduction

                 Silicon Germanium (Si_1-xGe_x) is recognized as a material for the next-generation transistor device [1]. In particular, Si_1-xGe_x is attractive as a channel material that has higher carrier mobility than Si. Furthermore, strain engineering is also valid for Si_1-xGe_x similar to the Si to improve transistor performance. Nowadays, complicated nanostructures, such as FinFET, has used in the devices. Thus, the strain (stress) states in Si_1-xGe_x nanostructures should be complicated because of hetero junction and nanoscalling processes [2]. In the nanostructure devices, accurate evaluation of anisotropic biaxial stress states is indispensable. In our previous report, we demonstrated the evaluation of the anisotropic biaxial stress states in various nanostructures including strained Si_1-xGe_x with low Ge concentration on the Si substrate by oil-immersion Raman spectroscopy [3,4]. In this study, we investigated the biaxial stress states in Si_1-xGe_xmesa structure with high Ge concentration on Ge substrates depending on nanostructure size by oil-immersion Raman spectroscopy.

Experiments

                 Si_1-xGe_x films were epitaxially grown on the Ge substrate (Si_1-xGe_x/Ge) whose Ge concentration were approximately x = 0.76, 0.85, and 0.92. The film thicknesses of the samples were 50, 76, and 35 nm, respectively. Furthermore, the various size mesa structures were fabricated by electron beam lithography and reactive ion etching. Figure 1(a) and (b) show the schematic of the Si_1-xGe_x mesa structure and cross-sectional TEM image (x = 0.76). The sample widths (Ws) were 1.0, 0.5, 0.2, 0.1, and 0.05 μm, while the length (L) was fixed at 3.0 μm. In the oil-immersion Raman measurements, the anisotropic biaxial stresses in the strained Si_1-xGe_xwere evaluated. The numerical aperture was 1.4 and the refraction index in the atmosphere was 1.5. The excitation light source wavelength and the spectroscope focal length were 532 nm and 2,000 mm, respectively.

Results and Discussion

                 Figure 2 shows Raman spectra from the Si_1-xGe_x mesa structures with various Ge concentration, where the width of W = 1.0 μm. These spectra were calibrated by the Ge substrate peak at 300 cm^-1. From the result, the peaks of Si_1-xGe_x with lower Ge concentration were observed at lower wavenumber side. The peak shift was caused by the combination of decreasing Ge concentration and increasing tensile stress. Figure 3 shows the biaxial stresses σ_xx and σ_yy in the Si_1-xGe_x with x = 0.76. Here, the variation of σ_xx and σ_yy were clearly observed. The stress variation was depending on the width of mesa structure (W). As a result, σ_yy was more relaxed than σ_xx with decreasing of the width. Furthermore, σ_yy was dramatically decreased at less than W = 0.2 μm, while σ_xx remains almost constant through W = 1.0, 0.5, 0.2 μm. This result demonstrates that the biaxial stress state was clearly depend on the Si_1-xGe_x mesa structure W width. We believe that it is important to evaluate anisotropic biaxial stress states. Therefore, oil-immersion Raman spectroscopy is useful for the stress engineering.

Acknowledgement

                 This study was partially supported by the Japan Society for the promotion of Science through a Grant-in-Aid for Scientific Research B (No. 24360125) and Funding program for World-Leading innovative R&D on Science and Technology.

References

[1] K. Usuda et al., Semicond. Sci. Tecnol. 22,S227 (2007).

[2] M. Tomita et al., Jpn. J. Appl. Phys. 52,04CA06 (2013).

[3] D. Kosemura et al., Jpn. J. Appl. Phys. 52,04CA05 (2013).

[4] K. Usuda et al., Solid-State Electronics 83, 46 (2013).