Kazuma Takeuchi

Biaxial stress evaluation in GeSn film epitaxially grown on Ge substrate by oil-immersion Raman spectroscopy

GeSn is being paid much attention as a next-generation channel material. In this work, we performed the excitation of forbidden transverse optical (TO) phonons from strained GeSn, as well as longitudinal optical (LO) phonons, under the backscattering geometry from the (001) surface by oil-immersion Raman spectroscopy. Using the obtained LO/TO phonons, we derived the phonon deformation potentials (PDPs), which play an important role in the stress evaluation, of the strained Ge1− x Sn x for the first time. The results suggest that PDPs are almost constant for the Ge1− x Sn x (x < 0.032). Biaxial stress calculated using the derived PDPs reasonably indicated the isotropic states.

Examination of phonon deformation potentials for accurate strain measurements in silicon–germanium alloys with the whole composition range by Raman spectroscopy

The phonon deformation potentials (PDPs), p and q, of Si1−xGex with the whole range of the Ge concentration x were examined in detail in pursuit of accurate strain measurements by Raman spectroscopy. An oil-immersion Raman technique was adopted to extract the PDPs of Si1−xGex, in which a complex sample preparation process or a stress-introduction device is not necessary. The strain-shift coefficients bLO and bTO, which can be calculated using the obtained PDPs, were compared with the values in the literature, and we suggested which values were best for application to accurate strain measurements. Ab initio calculation was also performed to understand the behavior of the PDPs throughout the whole range of x in Si1−xGex.

First Demonstration of X-ray Mirrors Using Focused Ion Beam

We report on novel X-ray mirrors fabricated with a focused ion beam for future astronomical missions. We fabricated a test sample from a silicon wafer by forming six slits whose sidewalls were used as X-ray reflection surfaces. The six slits were designed with a size of 25 × 300 × 170 µm3 and with different inclination angles of 0 and ±1°. We examined X-ray reflection using three slits with different inclination angles at Al Kα 1.49 keV. Consequently, we demonstrated X-ray reflection from all the three slits. All the sidewalls have multiangular components with a microroughness of ~1 nm rms. ~30–45% of the total surface area is effective for X-ray reflection. We confirmed that the inclination angles are consistent with the designed values.

Origin of additional broad peaks in Raman spectra from thin germanium-rich silicon–germanium films

Additional broad peaks in Raman spectra from thin Ge-rich SiGe films were examined in detail. The broad peak on the low-wavenumber side of first-order optical phonon was also present for pure Ge, which indicates that the localized mode is not the reason for the broad peaks. Furthermore, other factors, e.g., strain, defect, phonon confinement effect, Fano effect, and fluorescence from oil, were excluded from the origin of the broad peaks. We assigned the broad peaks to surface optical phonon modes. The dependence of this surface mode on Ge concentration in Ge-rich SiGe was also investigated and the behavior was discussed.

X-ray evaluation of high-verticality sidewalls fabricated by deep reactive ion etching

We report the fabrication and characterization of high-verticality sidewalls by deep reactive ion etching (DRIE). We quantitatively evaluated the verticality of the sidewalls with a width of 20 µm and a depth of 300 µm by using an X-ray beam (1.49 keV). To the best of our knowledge, we succeeded in constraining the verticality and smoothness of the DRIE-fabricated sidewalls with the highest accuracy. The verticality of the sidewalls against the wafer surface was estimated from the shifts of the X-ray focus to be 8.7 ± 3.2 arcmin on average within the wafer, while the resolution of the X-ray focus was 21.1 ± 2.7 arcmin in half-power diameter. Although the verticality and resolution require further improvements, we verified that the X-ray imaging technique is valid for quantifying the sidewall properties.

Pattern-dependent anisotropic stress evaluation in SiGe epitaxially grown on a Si substrate with selective Ar+ion implantation using oil-immersion Raman spectroscopy

Biaxial stress states in SiGe stripes on Si substrates fabricated by a novel selective Ar+ ion implantation technique were evaluated by oil-immersion Raman spectroscopy. The oil-immersion technique is appropriate for the measurement of strain induced in nanostructure devices, because it has a higher spatial resolution than conventional Raman spectroscopy and can evaluate anisotropic stress states owing to the excitation of multiple optical phonon modes. Results indicate that quasi-uniaxial stress states exist in SiGe layers in unimplanted Si areas, which depends on the stripe-width ratio of implanted and unimplanted areas, and that quasi-uniaxial stress states are successfully induced in SiGe by the present technique, which can be considered as the channel materials of high-performance transistors.

Anisotropic strain evaluation induced in group IV materials using liquid-immersion Raman spectroscopy

We evaluated anisotropic biaxial strain induced in silicon channel region of metal-oxide-semiconductor field effect transistor (MOSFET) as well as novel channel or source/drain materials such as silicon germanium (SiGe) and germanium tin (GeSn) using liquid-immersion Raman spectroscopy. Uniaxial stress in Si channel region predicted by the simulation was well reproduced by Raman measurement. For the evaluation of SiGe and GeSn, the phonon deformation potentials (PDPs) were derived for the first time, because they are indispensable to obtain the biaxial strain. The PDPs of SiGe indicate clear Ge concentration dependence, which is decreasing with Ge concentration, while the PDPs of GeSn with less than 3.2% Sn concentration exhibit almost constant. Using the derived PDPs, we obtained the anisotropic biaxial strains introduced in the finite patterned SiGe precisely.