Masanobu Yoshikawa

Raman spectra of diamondlike amorphous carbon films

Raman spectra of diamondlike amorphous carbon (a‐C) films prepared under atmosphere with various hydrogen gas content have been measured as a function of excitation wavelength. The Raman spectral profiles vary with excitation wavelength depending on electronic absorption spectra associated with π‐π* electronic transitions. Dependence of Raman spectra on excitation wavelength is interpreted in terms of π‐π* resonant Raman scattering from aromatic rings with various sizes rather than polyene chains. The relative intensity of a 1400 cm−1 band against a 1530 cm−1 band is found to decrease with an increase of sp3 content in a‐C films. It is shown that the relative intensity can be used as a parameter for sp3 content.

Characterization of anisotropic stress around Si trenches by polarized Raman spectroscopy

Polarized Raman spectra around trenches formed on (100) silicon wafers have been measured and it has been found that the peak frequency shift varies with the polarization configuration, suggesting that anisotropic stresses occur around the trenches. The different stress components have been calculated by the use of the polarization Raman technique and it was found that the stress distribution of each component approximately agrees with that of each component simulated by a finite element method. Polarized Raman spectroscopy is a powerful technique for the estimation of an anisotropic stress of an electronic silicon device in situ.

Measurement of temperature-dependent stress in copper-filled silicon vias using polarized Raman spectroscopy

An experimental method to determine the temperature dependence of residual stress in three-dimensional (3D) structures was developed using polarized Raman spectroscopy. Stresses of a copper-filled silicon via at three temperatures, 223, 298, and 413 K were derived by measuring the frequency shift of the optical phonons through the backscattering geometry from the cross-section of the structure and assuming non-isotropic biaxial (horizontal and depth) stresses on the cross-section. Both stress components changed from tensile to compressive in almost all areas as the temperature changed from 213 to 413 K. The absolute stress values increased at both low and high temperatures and were smallest at 298 K, which was nearest to the process temperature of copper filling by plating. The main cause of stress is considered to be the difference in the coefficient of thermal expansion between copper and silicon. These results indicate that the temperature dependence of stress of copper-filled vias is affected mainly b...

Stress Characterization of Si by a Scanning Near-Field Optical Raman Microscope with Spatial Resolution and with Penetration Depth at the Nanometer Level, using Resonant Raman Scattering

We have developed a new tapping mode-scanning near-field optical Raman microscope (SNORM) with a caved and pyramidical probe, using resonant Raman scattering and measured the stress distribution of very-large-scale integration (VLSI) standards made of the silicon dioxide film and Si. It has been found that compressive stresses of about 0.69 GPa/cm2 are concentrated on the corner of the area uncovered by silicon dioxide. The SNORM we developed has at least the spatial resolution of less than 250 nm and is a useful tool to measure image of stresses in Si devices within a short time and with a penetration depth of 5 nm.

Characterization of Si nano-polycrystalline films at the nanometer level using resonant Raman scattering

We have measured a resonant Raman scattering from polycrystalline silicon (poly-Si) films with thicknesses of 24–381nm at the depth resolution of approximately 5nm and found that poly-Si films are under compressive stress. The main Raman peak in poly-Si films tends to shift to a higher frequency as the thickness of the underlayer of the silicon dioxide (SiO2) film becomes greater. From this result, it has been considered that the compressive stress becomes larger with an increase in the thickness of the underlayer of the SiO2 film. We have tried to separate the observed Raman shifts into those caused by the crystallite size effect and those caused by stress in the poly-Si films. We did this using the resonant Raman scattering technique and calculation by the phonon confinement model. It has been found that the crystallite size obtained from the Raman measurement roughly agrees with the size obtained from the x-ray measurement. This result suggests that the phonon-dispersion curve does not change significa...

Characterization of Silicon Dioxide Films on a 4H-SiC Si(0001) Face by Fourier Transform Infrared (FT-IR) Spectroscopy and Cathodoluminescence Spectroscopy

We used Fourier transform infrared (FT-IR) spectroscopy to characterize silicon dioxide (SiO2) films on a 4H-SiC(0001) Si face. We found that the peak frequency of the transverse optical (TO) phonon in SiO2 films grown on a 4H-SiC substrate agrees well with that in SiO2 films grown on a Si substrate, whereas the peak frequency of the longitudinal optical (LO) phonon in SiO2 films on a 4H-SiC substrate is red-shifted by approximately 50 cm−1 relative to that in SiO2 films on a Si substrate. We concluded that this red-shift of the LO phonon is mainly caused by a change in inhomogeneity due to a decrease in density in the SiO2 films. Furthermore, cathodoluminescence (CL) spectroscopy results indicated that the channel mobility of the SiC metal-oxide-semiconductor field-effect transistor (MOSFET) decreases roughly in proportion to the increase in the intensity of the CL peak at 460 and 490 nm, which is attributed to the increase in the number of oxygen vacancy centers (OVCs). FT-IR and CL spectroscopies provide us with a large amount of data on OVCs in the SiO2 films on a 4H-SiC substrate.

Depth Profiling of Al Ion-Implantation Damage in SiC Crystals by Cathodoluminescence Spectroscopy

Cross-sectional CL measurements have been performed on the cleaved surface of the Al-ion implanted 4H-SiC. The strong L1 luminescence that originates from the DI defect has been observed even in the deep region (~10 μm) where implanted ions do not penetrate. In the implanted layer, CL results show that high-density non-radiative defects remain even after activation annealing. Generation of the DI defect in the deep region is presumably attributed to the diffusion of point defects from the implanted layer.

Abnormal behavior of longitudinal optical phonon in silicon dioxide films on 4H-SiC bulk epitaxial substrate using Fourier transform infrared (FT-IR) spectroscopy.

We report the abnormal behavior of longitudinal optical (LO) phonon in a silicon dioxide (SiO2) film on a 4H-SiC bulk epitaxial substrate using an attenuated total reflection (ATR) technique. The peak frequency of the LO phonon in the ATR spectrum was observed at around 1165 cm−1 and red-shifted by approximately 92 cm−1 relative to that at the grazing incidence (40°), whereas the peak frequency of the transverse optical (TO) phonon in the ATR spectrum agreed well with that at the grazing incidence. Furthermore, the peak frequency of the TO phonon hardly depends on change in the incident angle and thickness, suggesting that the microstructure of the sample is homogeneous within a thickness of 100 nm. On the other hand, we found that the microstructure of the sample was inhomogeneous within a thickness less than 5 nm. Fourier transform infrared (FT-IR) spectroscopy provides us with a large amount of data on microstructures in the SiO2 films on a 4H-SiC substrate.

Cathodoluminescence Microcharacterization of Radiative Recombination Centers in Lifetime-Controlled Insulated Gate Bipolar Transistors

Cross-sectional cathodoluminescence (CL) measurements were applied to the study of electron-irradiated punch-through insulated gate bipolar transistors (IGBTs) to investigate the relationship between radiative recombination centers and electrical characteristics. IGBTs were additionally annealed at temperatures of 200–400 °C for 1 h. As annealing temperature rose, collector–emitter saturation voltage (VCES) decreased and current fall time (tf) increased. The cross-sectional CL measurements showed sharp luminescent peaks at 1018 meV (W or I1), 1040 meV (X or I3), and 790 meV (C) and a broad band at approximately 0.90–1.05 eV. As annealing temperature rose, the intensity of the W line decreased and that of the X line increased, suggesting that small self-interstitial clusters agglomerate and form stable, large self-interstitial clusters reducing the total number of self-interstitial clusters. The C line, which originated from an interstitial oxygen and carbon complex, showed no significant change. We consider that self-interstitial clusters play important roles in the electrical characteristics of lifetime-controlled IGBTs.

Expansion of Stacking Faults by Electron-Beam Irradiation in 4H-SiC Diode Structure

The influence of electron-beam irradiation on defects in 4H-SiC diode structures was investigated by cathodoluminescence (CL) microscopy and spectroscopy. In addition to threading edge and screw dislocations, two types of stacking faults (SFs) were characterized by their emission energy, geometric shape, and the sensitivity of electron-beam irradiation. The SFs at λ = 425 nm (2.92 eV) expand from the surface of basal plane dislocation with line direction [11-20] and change their geometric shape by electron-beam irradiation. The SFs at λ = 471 nm (2.63 eV) are only slightly influenced by electron-beam irradiation. The former corresponds to the Shockley-type SFs previously observed in the degraded p-i-n diodes, and the latter to in-grown SFs with 8H structure. The panchromatic CL images constructed by the sum of monochromatic CL images suggest that there are nonradiative recombination centers in the vicinity of Shockley-type SFs. The nucleation sites and the driving force for SF expansion are discussed.