Evaluation of Anisotropic Biaxial Stress in Thin Strained-SiGe Layer Using Surface Enhanced Raman Spectroscopy

Abstract

Introduction Silicon germanium (SiGe) is recognized as a high mobility channel material for the next-generation transistors in place of Si. Nowadays, various future devices using SiGe are being developed [1]. The accurate evaluation of strain (stress) is essential to realize future devices using SiGe. However, the evaluation of stress in thin SiGe layer is difficult because of poor signal embedded in those from huge amount of thick Si substrate [2]. We have previously reported that relatively weak TO phonon excitation in strained SiGe by oil-immersion Raman spectroscopy [3]. We also reported that the evaluation of anisotropic biaxial stress in strained Si on relaxed SiGe layer by exciting longitudinal optical (LO) and transversal optical (TO) phonons using surface enhanced Raman spectroscopy (SERS) [4]. In this study, the SERS measurements were performed to excite the TO phonon in strained SiGe layer more efficiently. Experiment The thin SiGe layer on Si substrate with approximately 30% Ge concentration was used as the sample. The thickness of the SiGe layer was approximately 38 nm. Undecane (C11H24) was used for the solvent of Ag nanoparticles. After spin-coating the Ag nanoparticles on the SiGe surface, the sample was sintered at 190 ›C for 5 min. The average diameter of Ag particles was approximately 30 nm (nominal value). Oil-immersion lens was used to enhance the SERS effect. The numerical aperture (NA) was 1.4 and the refraction index n was 1.5. On the other hand, in the conventional measurement, NA and n were 0.7 and 1.0, respectively. The wavelength of the excitation light source and the focal length of the spectroscope were 532 nm and 2,000 mm, respectively. Results and Discussion Fig. 1 shows the SEM image of the SiGe surface with the Ag nanoparticles. Fig. 2 shows Raman spectra from strained SiGe on the Si substrate coated (w/ SERS) and uncoated (w/o SERS) by the Ag nanoparticles. These spectra were normalized by the Si substrate peak at 520 cm-1. The relative intensity of the SiGe peak was dramatically enhanced and the peak position of SiGe shifted toward a lower wavenumber. Furthermore, the enhanced Raman spectrum was clearly attributed to not only the oil-immersion measurement but also the SERS effect. Fig. 3 shows the peak of strained SiGe obtained by the SERS measurement after the subtraction of the Si substrate peak from the raw data. Both TO and LO phonon components were clearly extracted in fig. 3. From these Raman shift values, the anisotropic biaxial stress can be calculated by the relationship between stress and Raman with the use of phonon deformation potentials (PDPs) and elastic compliances [4,5,6]. As a result, σxx = -2.13 and σyy= -2.21 GPa were obtained. From the results, it was confirmed that the TO phonon was able to be efficiently excited by the SERS. We concluded that the shift on the low wavenumber side was definitely attributed to the TO phonon excitation. The TO/LO phonon excitation allows us to evaluate anisotropic biaxial stress in thin strained-SiGe. 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.