Evaluation of Anisotropic Biaxial Stress in Extremely-Thin Body (100) Silicon-Germanium-on-Insulator p-Type Metal-Oxide-Semiconductor Field-Effect-Transistor by Oil-Immersion Raman Spectroscopy
Authors: Maeda Yuiha , Yokogawa Ryo , Sugawa Sho , Chen Chia-Tsong , Toprasertpong Kasidit , Takenaka Mitsuru , Takagi Shinichi , Ogura Atsushi 
Published: February 2024

Background and Objective
Silicon germanium (SiGe)-on-insulator (SGOI) p-MOSFETs are expected for obtaining low power consumption and high current drive of CMOS since SiGe has higher hole career mobility than Si and the compressive strain can be induced in the SiGe channel without complicated heterostructures. In addition, the buried oxide between the Si substrate and the SiGe layer realizes suppression of leak current, and an extremely thin body (ETB) SGOI prevents short-channel effects. Strain engineering is important for improving the performance of p-MOSFETs because the hole mobility increases due to the compressive strain in the channel region. In previous studies, we have reported that anisotropic strain in ETB GOI channels of p-MOSFET improved electrical properties [1]. However, there are few reports regarding the detailed stress evaluation in ETB SGOI p-MOSFETs. The strain state has changed complicatedly by the SiGe device process. Therefore, a detailed stress measurement is indispensable to determine career mobility enhancement in the actual channel fabrication. In this study, we evaluated the anisotropic biaxial stress in the channel region of the ETB (100) SGOI p-MOSFETs by oil-immersion Raman spectroscopy.
Experiments We prepared (100) SGOI p-MOSFETs along <110> with a channel thickness of approximately 10 nm by applying the Ge condensation process with slow cooling on the 60 nm-thick Si0.75Ge0.25 films epitaxially grown on SOI substrates with 10 nm-thick superficial Si layer [2]. We used the Ge condensation process that combined oxidation and intermixing annealing in order to suppress strain relaxation in the Ge-rich region [3] and obtained Ge-rich SiGe layer (Ge fraction: from 65 to 70%). For the evaluation of anisotropic biaxial stress in the SiGe channel region, we measured longitudinal optical (LO) and transverse optical (TO) phonon modes of Raman spectra for Ge-Ge vibration mode derived from SiGe by using oil-immersion Raman spectroscopy. In the oil-immersion Raman measurements, the wavelength of the excitation light source was 532 nm, the laser power at the sample surface was less than 1 mW, and the focal length of the spectrometer was 2,000 mm. The numerical aperture of the oil-immersion lens was 1.4, and the reflective index of oil was 1.5. We measured p-MOSFETs with different channel widths to examine the channel width dependence.
Results and Discussion
Figure 1(a) and (b) show the Raman spectra of the LO and TO phonon modes obtained from the SiGe channel region at channel width (Wch) = 738 nm, respectively. We observed that Raman peaks of LO phonon mode shifted toward the higher wavenumber side compared to the Raman peak of strain-free SiGe. These results suggest that the high compressive strain is induced in the SiGe channel.
Figure 1(c) shows the calculated transverse stress (σxx) and longitudinal stress (σyy) of the samples using LO and TO Raman shift for four different channel widths under the assumption of the anisotropic biaxial stress and Ge fraction 68% in the SiGe layer. From Fig. 1(c), we observed transverse stress (σxx) decreases with narrowing channel width. On the other hand, the longitudinal stress (σyy) did not significantly change regardless of channel width. These results suggest that the strain states in the SiGe channel region became closer to the uniaxial strain with narrowing the channel width. These changes are in good agreement with the previous electrical characteristics studies of the GOI p-MOSFETs [1], and it is expected that the compressive strain enhances the hole mobility in the ETB SGOI p-MOSFETs as well.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 22H00208, Japan. A part of this work was conducted at Takeda Sentanchi Supercleanroom, The University of Tokyo, supported by "Nanotechnology Platform Program" of MEXT, Japan, Grant Number JPMXP09F-20-UT-0007.

Reference

[1] C. -T. Chen et al., IEEE Trans. Electron Devices
69, 1 (2022).
[2] C. -T. Chen et al., Proc. IEEE Symp. VLSI Technology (2021).
[3] K. -W. Jo et al., Appl. Phys. Lett.
114, 062101 (2019).

Figure 1