Strain techniques are one of the principal technology boosters for the realization of high-performance logic devices composed of group IV semiconductors such as Si, Ge and their alloy. Recently, it is gradually difficult to measure strain states as miniaturization proceeds to the nanometer scale. Raman spectroscopy is one of the most powerful techniques for phonon measurements and can perform strain evaluation with high spatial resolution nondestructively [1]. However, conventional Raman spectroscopy is limited in strain information based on the Raman polarization selection rules [1].
In this paper, we introduce oil-immersion Raman spectroscopy with the high-numerical-aperture (high-NA) lens to realize precise evaluation of strain states (for example, anisotropic strain) in group IV semiconductor devices.
2. Oil-immersion Raman spectroscopy
In conventional Raman spectroscopy, it is impossible to excite transverse optical (TO) phonon mode due to Raman polarization selection rules under the backscattering geometry from the (001) group IV semiconductor surface. In other words, a uniaxial or isotropic biaxial stress measurements in group IV semiconductors can only be performed by exciting only the longitudinal optical (LO) phonon mode under a conventional (001) backscattering configuration.
To realize anisotropic stress evaluation assuming the channel region of the group-IV semiconductor MOSFET, we used a high-NA immersion lens for the selective excitation of both LO and TO phonon modes. 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 group IV semiconductors [2, 3].
3. Results and discussion
As an example of oil-immersion Raman measurements results, we evaluated the strain states of extremely-thin-body Ge-on-insulator (ETB GOI) MOSFET channels fabricated by Ge condensation [4, 5] by oil-immersion Raman spectra. We observed that the Raman peak shifted toward a higher wavenumber, indicating that high compressive strain in the GOI channels. In addition, a clear splitting of LO and TO phonon modes was confirmed, implying the anisotropic biaxial strain states induced in the ETB GOI channel.
Figure 1 shows the channel width (Wch) dependence of the anisotropic biaxial stress for ETB GOI MOSFET (channel thickness: 10.6 nm) obtained by the oil-immersion Raman spectroscopy. As shown in Fig. 1, the stress relaxation along the GOI channels was observed as the GOI channel width gets narrower. In other words, the uniaxial stress is applied into channels by narrow GOI channel patterning. These behaviors are in good agreement with the previous reports such as strained-SOI [6] and SiGe nano patterns [3][7].
In conclusion, oil-immersion Raman spectroscopy is one of the most powerful strain measurements in Group IV semiconductor device structures.
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.
References
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