Ryuichi Sugie

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.

Impact of back-grinding-induced damage on Si wafer thinning for three-dimensional integration

Ultrathin wafers, which enable the low-aspect-ratio through-silicon vias to be formed easily, are indispensable for bumpless three-dimensional (3D) stacking. To clarify thinning-induced damage in detail, atomic-level defects occurring during wafer thinning and due to mechanical stress at microregions of the fracture surface have been studied. Such damage was evaluated by µ-Raman spectroscopy, laser microscopy, transmission electron microscopy, and positron annihilation spectroscopy. Coarse (#320 grit) grinding causes a roughly 500 MPa compressive stress, resulting in the formation of a less than 5 µm defect layer. Fine (#2000 grit) grinding enables the formation of a plane surface and reduces the stress to 100–200 MPa. However, a damaged layer of 200 nm still remains and an almost 100-nm-thick layer of vacancy-type defects exists. After chemical mechanical polishing (CMP), a stress-free surface was obtained and no defects were found except atomic-level vacancies, which were detected in a layer of 4 nm thickness after 1 µm CMP.

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.

RAMAN SCATTERING OF INTERFACE MODES IN ZNTE-CDSE SUPERLATTICES

Raman spectra have been measured for ZnTe/CdSe superlattices grown by hot wall epitaxy. A mechanical vibrational interface phonon (MVIF) mode localized at the Zn–Se interface is distinctly observed in addition to quasiconfined longitudinal optic (LO) modes. The relative intensity of the MVIF mode is increased as the period of the superlattice becomes short. Raman spectral profiles calculated by use of a linear chain model and a bond polarizability model explain this behavior qualitatively. The quasiconfined LO modes show resonant enhancement for excitations at the band gap energies of ZnTe and CdSe. The effect of atomic diffusion on the interfacial structure has been examined in thermally annealed superlattices by Raman measurement. It is shown that Raman scattering of the interface mode provides information about the interdiffusion of atoms and the sharpness of the heterointerfaces.

Photoreflectance and Resonant Raman Scattering of Above-Barrier Transitions in a GaAs/AlGaAs Superlattice

We have investigated the energy states above the barrier potential (above-barrier states) in a (GaAs)10/(Al0.3Ga0.7As)10 superlattice using photoreflectance and resonant-Raman scattering spectroscopies. The energies of the photoreflectance signals observed in the above-barrier region higher than the band-gap energy of Al0.3Ga0.7As just agree with the peak energies of the resonant-Raman profiles of the longitudinal-optical phonons. From the miniband energies calculated by an effective-mass approximation, we conclude that the photoreflectance signals and the resonant Raman profiles are caused by the transitions associated with the above-barrier states at the mini-Brillouin-zone edge.

Characterization of stress distribution in ultra-thinned DRAM wafer

Impact of backside thinning damages and topside device structures on the elastic stress distributions in ultra-thinned Si substrates were studied using μ-Raman spectroscopy and TEM observations. The compressive and tensile stresses due to the backside damages and the top-side device structures, respectively, are in equilibrium. The variations in elastic stress depend on the topside device structures such as shallow trench isolations (STIs) and memory-cell transistors, and to a lesser extent on the backside damages. Even for DRAM samples thinner than 4 microns, the elastic deformations underneath STIs and memory-cell transistors areas are considered to be no leakage current degradations, because the relation between retention time and pass rate shows little difference before and after thinning.

Characterization of process-induced defects in SiC MOSFETs by cross-sectional cathodoluminescence

Cross-sectional cathodoluminescence (CL) and scanning capacitance microscopy (SCM) measurements were carried out for silicon carbide (SiC) metal–oxide–semiconductor field-effect transistors (MOSFETs) to investigate process-induced defects. The DI defect-related line at 426 nm and a broad luminescence at approximately around 430–470 nm, which were produced by ion implantation, were observed in addition to the near-band-edge emission. CL images showed that the densities of nonradiative recombination and DI centers were high near the source region. Moreover, DI centers existed even in the n-drift region located 10 µm from the surface. These results indicate that many types of defects diffuse and interact with each other during annealing even in the area where dopant atoms are not implanted. The annealing process not only activates dopant atoms but also induces the diffusion of unstable native defects and transforms their structure into more thermally stable defects such as DI centers.

Influence of wafer thinning process on backside damage in 3D integration

Ultra-thinning less than 10 microns of Si wafer is expected to realize small TSV feature which provides low aspect ratio and coupling capacitance. However, a detail of residual surface damage during thinning is unrevealed. In this paper, subsurface damage following wafer thinning from the back of 300 mm wafers using three different types of thinning process was investigated by means of Raman spectroscopy, XTEM, and Positron annihilation analysis, respectively. A coarse grinding generates significant rough subsurface ranged several micron and damage layer including amorphous and plastic-deformed Si along grinding topography. Fine grinding, second step of thinning, reduced those surface roughness and almost removed after thinning at least removal of 50 microns. However, plastic-deformed subsurface layer with a thickness of 100 to 200 nm are still remained which leaves an inside elastic stress layer ranging up to about 10 microns in depth. Chemical-Mechanical Polishing (CMP) process as a final step of thinning enables to remove residual damages such as structural defects and lattice strains after 1-5 microns thick polishing while vacancy-type defects only remain.