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

Abstract

Introduction Silicon Germanium (Si1-x Ge x ) is recognized as a material for the next-generation transistor device [1]. In particular, Si1-x Ge x is attractive as a channel material that has higher carrier mobility than Si. Furthermore, strain engineering is also valid for Si1-x Ge 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 Si1-x Ge 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 Si1-x Ge 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 Si1-x Ge x mesa structure with high Ge concentration on Ge substrates depending on nanostructure size by oil-immersion Raman spectroscopy. Experiments Si1-x Ge x films were epitaxially grown on the Ge substrate (Si1-x Ge 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 Si1-x Ge 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 Si1-x Ge x were 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 Si1-x Ge 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 Si1-x Ge 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 Si1-x Ge 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 Si1-x Ge 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.