Motohiro Tomita

Evaluation of Strained-Silicon by Electron Backscattering Pattern Measurement: Comparison Study with UV-Raman Measurement and Edge Force Model Calculation

We demonstrate the results of strain (stress) evaluation obtained from electron backscattering pattern (EBSP) measurement for samples of a strained Si-on-insulator (SSOI) and a Si substrate with a patterned SiN film. Two-dimensional stress distributions were obtained in 40×40 µm2 areas of the SSOI. The biaxial stress state was also obtained in the SSOI. Furthermore, clear cross-hatch contrast was observed, especially in the distribution of shear stress Sxy, in contrast to with the other distributions of normal stress Sxx and Syy. One- and two-dimensional stress distributions in the Si substrate with the patterned SiN film were also obtained from EBSP measurement. Moreover, the results were compared with those of UV-Raman measurement and edge force model calculation, and were found to have a good correlation with each other. EBSP measurement was used to measure the complicated biaxial stress including the shear stress in a sample with a 150-nm-wide space pattern. We can conclude that EBSP measurement is a useful method for precisely measuring stress with high spatial resolution.

Evaluation of Anisotropic Strain Relaxation in Strained Silicon-on-Insulator Nanostructure by Oil-Immersion Raman Spectroscopy

Precise stress measurements have been desired in order to apply strained Si substrates to next-generation transistors. Oil-immersion Raman spectroscopy enables the evaluation of the anisotropic stress state in the strained Si layer of the strained Si substrate even under (001)-oriented Si backscattering geometry. First, we found that the phonon deformation potentials (PDPs) reported by Anastassakis et al. in 1990 was the most valid among the three sets of PDP previous reported. Using these PDPs, the precise Raman measurements of biaxial stress in strained Si-on-insulator (SSOI) nanostructures were performed. The biaxial stresses σxx and σyy decreased with the decrease in SSOI width and length, which was consistent with the finite element method calculation.

Stress Measurements in Si and SiGe by Liquid-Immersion Raman Spectroscopy

Strained Si technology is important for engineering field-effect transistors (FETs) [1,2]. There are two types of the strained Si technologies. One is so-called global strained Si technology. Another is so-called local strained Si technology. The former is the technology of using a strained Si substrate which has a several-dozen-nanometers-thick strained Si layer at the top of the substrate [3-5]. The strained Si layer is obtained by growing Si on SiGe, therefore, large tensile strain with biaxial isotropy can be induced in Si. The isotropic biaxial tensile strain in Si allows for performance improvements for both of nand ptype FETs. Homogeneous strain distribution can be obtained under the critical thickness of the strained Si layer [6].

A scalable Si-based micro thermoelectric generator

A new device architecture of micro thermoelectric generator (μ-TEG) is proposed. The μ-TEG utilizes silicon nanowires as the thermoelectric (TE) material, and it can be fabricated by the CMOS-compatible process. It is driven by an “evanescent thermal field” exuding around a heat flow perpendicular to the substrate. We demonstrate experimentally that the TE power increases in the shorter TE leg lengths. The results show that the TE power density is scalable by miniaturizing and integrating the proposed structure.

Super-Resolution Raman Spectroscopy by Digital Image Processing

We demonstrate the results of a strain (stress) evaluation obtained from Raman spectroscopy measurements with the super-resolution method (the so-called super-resolution Raman spectroscopy) for a Si substrate with a patterned SiN film (serving as a strained Si sample). To improve the spatial resolution of Raman spectroscopy, we used the super-resolution method and a high-numerical-aperture immersion lens. Additionally, we estimated the spatial resolution by an edge force model (EFM) calculation. One- and two-dimensional stress distributions in the Si substrate with the patterned SiN film were obtained by super-resolution Raman spectroscopy. The results from both super-resolution Raman spectroscopy and the EFM calculation were compared and were found to correlate well. The best spatial resolution, 70 nm, was achieved by super-resolution Raman measurements with an oil immersion lens. We conclude that super-resolution Raman spectroscopy is a useful method for evaluating stress in miniaturized state-of-the-art transistors, and we believe that the super-resolution method will soon be a requisite technique.

Measurement of Anisotropic Biaxial Stresses in Si1-xGex/Si Mesa Structures by Oil-Immersion Raman Spectroscopy

Anisotropic biaxial stress states in Si1-xGex/Si mesa structures were evaluated by oil-immersion Raman spectroscopy. Using a high-numerical-aperture lens, the electrical field component perpendicular to the surface, i.e., z-polarization, can be obtained. The z-polarization enables the excitation of the forbidden optical phonon mode, i.e., the transverse optical (TO) phonon mode, even under the backscattering geometry from (001)-oriented diamond-type crystals. The anisotropic biaxial stress evaluation of Si1-xGex was considered difficult compared with that of Si, because many unknown parameters exist for Si1-xGex, e.g., phonon deformation potentials (PDPs), the Ge concentration x, and the factor of Raman shift on x. In this study, PDPs and the Ge concentration in Si1-xGex were investigated in detail. As a result, using precise PDPs and x, a clear dependence of anisotropic biaxial stress states in Si1-xGex on the mesa structure shape was observed.

Tensor Evaluation of Anisotropic Stress Relaxation in Mesa-Shaped SiGe Layer on Si Substrate by Electron Back-Scattering Pattern Measurement: Comparison between Raman Measurement and Finite Element Method Simulation

A strained SiGe layer will be used in next-generation transistors to improve device performance along with device scaling. However, the stress relaxation of the SiGe layer may be inevitable in nanodevices, because the SiGe layer is processed into a nanostructure. In this study, we evaluated the anisotropic stress relaxation in mesa-shaped strained SiGe layers on a Si substrate by electron backscattering pattern (EBSP) measurement. Moreover, we compared the results of EBSP measurement with those of anisotropic Raman measurement and finite element method (FEM) simulation. As a result, the anisotropic stress relaxation obtained by Raman spectroscopy was confirmed by EBSP measurement. Additionally, we obtained a good correlation between the results of EBSP measurement and FEM simulation. The σxx and σyy stresses were markedly relaxed and the σzz and σxz stresses were concentrated at the SiGe layer edges. These stresses were mostly relaxed in the distance range from the SiGe layer edges to 200 nm. Therefore, in a SiGe nanostructure with a scale of less than 200 nm, stress relaxation is inevitable. The results of EBSP and Raman measurements, and FEM simulation show a common tendency. We believe that EBSP measurement is useful for the evaluation of stress tensors and is complementary to Raman measurement.

Evaluation of Strained Silicon by Electron Back Scattering Pattern Compared with Raman Measurement and Edge Force Model Calculation

Global and local strained-Si samples, namely strained-Si on insulator (SSOI) wafer and a Si substrate with a patterned SiN film were each evaluated by electron backscattering pattern (EBSP). In the EBSP measurements for SSOI, biaxial tensile stresses (biaxial tensile strains and compressive strain perpendicular to the surface) were obtained, whose values were consistent with those obtained by UV-Raman spectroscopy. One-dimensional stress distributions in the Si substrate with the patterned SiN film were obtained by EBSP, UV-Raman spectroscopy with a deconvolution method, and edge force model calculation. The results were well consistent with each other. EBSP allows us to measure stress and strain in the patterned SiN sample with 150-nm wide space. Furthermore, anisotropic biaxial stress including shear stress was also obtained by EBSP.

Evaluation of controlled strain in silicon nanowire by UV Raman spectroscopy

The evaluation of strain states in silicon nanowires (Si NWs) is important not only for the surrounding gate field-effect transistors but also for the thermoelectric Si NW devices to optimize their electric and thermoelectric performance characteristics. The strain states in Si NWs formed by different oxidation processes were evaluated by UV Raman spectroscopy. We confirmed that a higher tensile strain was induced by the partial presence of a tetraethyl orthosilicate (TEOS) SiO2 layer prior to the thermal oxidation. Furthermore, in order to measure biaxial stress states in Si NWs accurately, we performed water-immersion Raman spectroscopy. It was confirmed that the anisotropic biaxial stresses in the Si NWs along the length and width directions were compressive and tensile states, respectively. The Si NW with a TEOS SiO2 layer on top had a larger strain than the Si NW surrounded only by thermal SiO2.

Miniaturized planar Si-nanowire micro-thermoelectric generator using exuded thermal field for power generation

Abstract For harvesting energy from waste heat, the power generation densities and fabrication costs of thermoelectric generators (TEGs) are considered more important than their conversion efficiency because waste heat energy is essentially obtained free of charge. In this study, we propose a miniaturized planar Si-nanowire micro-thermoelectric generator (SiNW-μTEG) architecture, which could be simply fabricated using the complementary metal–oxide–semiconductor–compatible process. Compared with the conventional nanowire μTEGs, this SiNW-μTEG features the use of an exuded thermal field for power generation. Thus, there is no need to etch away the substrate to form suspended SiNWs, which leads to a low fabrication cost and well-protected SiNWs. We experimentally demonstrate that the power generation density of the SiNW-μTEGs was enhanced by four orders of magnitude when the SiNWs were shortened from 280 to 8 μm. Furthermore, we reduced the parasitic thermal resistance, which becomes significant in the shortened SiNW-μTEGs, by optimizing the fabrication process of AlN films as a thermally conductive layer. As a result, the power generation density of the SiNW-μTEGs was enhanced by an order of magnitude for reactive sputtering as compared to non-reactive sputtering process. A power density of 27.9 nW/cm2 has been achieved. By measuring the thermal conductivities of the two AlN films, we found that the reduction in the parasitic thermal resistance was caused by an increase in the thermal conductivity of the AlN film and a decrease in the thermal boundary resistance.