Stress Relaxation of Extremely-Thin-Body Ge-on-Insulator p-Type Metal-Oxide-Semiconductor Field-Effect Transistor Along <100> and <110> Observed By Oil-Immersion Raman Spectroscopy
Authors: Yokogawa Ryo , Maeda Yuiha , Sugawa Sho , Chen Chia-Tsong , Takenaka Mitsuru , Takagi Shinichi , Ogura Atsushi 
Published: February 2024

Background and purpose

Strain techniques and channel materials with high carrier mobility are the principal technology boosters for the realization of high-performance logic devices composed of group IV semiconductors such as silicon (Si). In particular, strain engineering plays an important role in improving the performance of metal-oxide semiconductor field-effect transistors (MOSFETs) from the viewpoint of reducing the effective carrier mass. It is important to evaluate the stress relaxation in group IV semiconductor devices for the optimization of the carrier mobility and the device structure. In this paper, we demonstrated the stress evaluation of extremely-thin-body Ge-on-insulator (ETB GOI) p-type MOSFET along the <110> and <100> longitudinal directions by oil-immersion Raman spectroscopy with the high-numerical-aperture (high-NA) lens to realize precise evaluation of strain states in the Ge channels.

Experiments

Biaxially-strained (001) GOI layer was prepared by applying optimized Ge condensation technique on epitaxially-grown Si/Si0.75Ge0.25/Si-on-insulator substrates with 4-hour slow cooling [1, 2]. After the removal of the thermal oxide, narrow channel patterning was performed by electron-beam (EB) lithography. GOI channel was further thinned down by the cyclic digital etching process (plasma oxidation and wet etching). Surface passivation process was then immediately performed to fabricate GeO
x
and Al2O3 by plasma oxidation and atomic layer deposition at 300oC, respectively. The source-drain regions were formed with EB evaporated Ni through a lift-off process and Al deposition was performed as the back gate electrode.
We used the Raman spectrometer with spectrograph focal length of 2,000 mm. The numbers of grating grooves were 2,400 mm-1 and a visible laser was used as the incident excitation light source whose wavelength was 532 nm. To realize anisotropic stress evaluation assuming the channel region of the ETB GOI MOSFET, we used a high-NA immersion lens for the selective excitation of both LO and TO phonon modes [3]. The high-NA and refractive index n of the oil were approximately 1.4 and 1.5, respectively. Using a high-NA immersion lens, a z-polarized light component can be effectively obtained owing to the aperture angle. Therefore, TO phonons can be excited on the basis of Raman selection rules for (001)-oriented GOI [3, 4].

Results and discussion

Figure 1 shows the channel width dependence of the anisotropic biaxial stress of ETB GOI MOSFET along the <100> longitudinal direction obtained by the oil-immersion Raman spectra. Anisotropic biaxial compressive stress at the GOI channel width direction of the GOI channels was observed even when the GOI channel width is wide (GOI channel width ~ 630 nm). The stress along the GOI channel length (the <100> longitudinal direction) is relatively high. These results are clearly different from the stress relaxation in the ETB GOI MOSFET along the <110> longitudinal direction. In previous studies, a clear stress relaxation at the GOI channel width direction was observed as the GOI channel width becomes narrower of the ETB GOI MOSFET along the <110> longitudinal direction (from channel width ~ 200 nm) [5]. The results observed in this study are generally well correlated with the results obtained in strain relaxation measurements of carbon-doped Si nanowires along <100> by X-ray diffraction with synchrotron radiation [6]. The difference in the strain relaxation mechanism depending on the GOI channel direction may affect the electrical properties, and we consider that it is useful to be physical elucidation of the carrier mobility enhancement.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number 22H00208, Japan. A part of this work was con-ducted at Takeda Sentanchi Supercleanroom, The University of Tokyo, supported by "Nanotechnology Platform Program" of MEXT, Japan, Grant Number JPMXP09F-20-UT-0007.

References

[1] K. -W. Jo et al., Appl. Phys. Lett.
114, 062101 (2019).
[2] C. -T. Chen et al., IEEE Trans. Electron Devices
69, 25 (2022).
[3] R. Yokogawa et al., Jpn. J. Appl. Phys.
57, 106601 (2017).
[4] T. Nuytten et al., APL Mater.
6, 058501 (2018).
[5] R. Yokogawa et al., ECS Trans. 109, 351 (2022).
[6] I. Hirosawa et al., in Extended Abstracts on the 70th Japan Society of Applied Physics (JSAP) Spring Meeting (2023), p. 13-100.

Figure 1