Shohei Terada

Understanding Li-K edge structure and interband transitions in LixCoO2 by electron energy-loss spectroscopy

The authors clarified fine structures of Li-K edge of LiCoO2 reflecting core–hole effects, using monochromated transmission electron microscopy—electron energy-loss spectroscopy (TEM–EELS) and first-principles calculations. Variation of interband transitions into empty Co 3d states hybridized with O 2p states with decrease in x of LixCoO2 was also presented. A reduced peak of interband transitions at 3.2 eV in low-loss EELS spectrum with decrease in x was related to reduction in the original empty Co eg states for LiCoO2 and appearance of empty bands just below the eg band.The authors clarified fine structures of Li-K edge of LiCoO2 reflecting core–hole effects, using monochromated transmission electron microscopy—electron energy-loss spectroscopy (TEM–EELS) and first-principles calculations. Variation of interband transitions into empty Co 3d states hybridized with O 2p states with decrease in x of LixCoO2 was also presented. A reduced peak of interband transitions at 3.2 eV in low-loss EELS spectrum with decrease in x was related to reduction in the original empty Co eg states for LiCoO2 and appearance of empty bands just below the eg band.

Polymorphs Discrimination of Nickel Silicides in Device Structure by Improved Analyses of Low Loss Electron Energy Loss Spectrum

Transmission electron microscope (TEM) with electron energy loss spectroscopy (EELS) is an indispensable tool in developing the nickel silicide structures in transistors of 65 nm node or further. The low loss EELS is a simple method in discriminating the phases of nickel silicides, however it has an energy resolution problem, because of the electrical and mechanical instability of the hardware. This problem can be overcome by proposed energy calibration by standards (ECS) method, where the well-calibrated plasmon loss peak of Si (Ep = 16.6 eV) EELS spectrum and zero loss peak are used as references. For this purpose, the low loss spectra of the nickel silicide and two references can be simultaneously acquired by the newly designed spatially resolved TEM-EELS.

Low-Temperature Bonding of Silver to Aluminum

Direct bonding of silver to aluminum was achieved using silver-oxide particles with silver acetate (CH3COOAg) as a bonding material. When silver acetate was used, it reduced the silver oxide, and the resulting silver powder was sintered at 190 °C. In contrast, when silver oxide alone was used, the reduction and sintering required a temperature of 400 °C. The shear strength of the bonds formed with the silver-oxide/silver-acetate joining medium increased with increasing bonding temperature; for example, 2.5 min holding at 400 °C in air under a bonding pressure of 2.5 MPa yielded an average shear strength of 9 MPa. As confirmed by cross-sectional transmission electron microscopy equipped with electron-energy-loss spectroscopy (TEM-EELS), the bonding of sintered silver to aluminum was achieved through an aluminum-oxide layer formed on the aluminum. This bonding of sintered silver to aluminum oxide at a temperature of 400 °C or less is thought to be due to the large amount of heat (680 J/g) generated by the combustion of CH3 radicals in silver acetate during sintering.

The development and characteristics of a high-speed EELS mapping system for a dedicated STEM

A new EELS (electron energy loss spectroscopy) real-time elemental mapping system has been developed for a dedicated scanning transmission electron microscope (STEM). The previous two-window-based jump-ratio system has been improved by a three-window-based system. It is shown here that the three-window imaging method has less artificial intensity in elemental maps than the two-window-based method. Using the new three-window system, the dependence of spatial resolution on the energy window width was studied experimentally and also compared with TEM-based EELS. Here it is shown experimentally that the spatial resolution of STEM-based EELS is independent of the energy window width in a range from 10 eV to 60 eV.

Chemical Shift of Electron Energy-Loss Near-Edge Structure on the Nitrogen K-Edge and Titanium L 3 -Edge at TiN/Ti Interface

We investigated the chemical shift of the electron energy-loss near-edge structure (ELNES) for the nitrogen K-edge and titanium L3-edge measured from the interface region between a titanium nitride layer and a titanium layer. Both the titanium nitride and titanium layers were prepared by a sputtering method. Elemental analysis for nitride and titanium in the vicinity of the interface region was performed using a standard technique in electron energy-loss spectroscopy. It was demonstrated that both the ELNES of nitrogen K-edge and titanium L3-edge presented the chemical shift, more or less, depending on the composition of TiNx. The experimental findings were interpreted using a first-principles band structure calculation. The chemical shifts of nitrogen K-edge and titanium L3-edge can be used as fingerprinting for readily distinguishing the composition of TiNx.

Phase State Analysis of Nickel Silicides in Complementary Metal–Oxide–Semiconductor Device Using Plasmon Energy Map

Phase states of nickel silicides in a complementary metal–oxide–semiconductor (CMOS) device were investigated using energy-filtering transmission electron microscopy (EF-TEM). Differences in plasmon energy at each location of the device were identified in two dimensions using a plasmon energy map to analyze the phase states of nickel silicides. We determined that the near side of polycrystalline silicon (poly-Si) corresponds to the NiSi phase in the gate electrode and that the contact corresponds to the NiSi2 phase, although determining the different phase states of nickel silicides using the contrast of a TEM image was difficult.

Searching ultimate nanometrology for AlOx thickness in magnetic tunnel junction by analytical electron microscopy and X-ray reflectometry.

Practical analyses of the structures of ultrathin multilayers in tunneling magneto resistance (TMR) and Magnetic Random Access Memory (MRAM) devices have been a challenging task because layers are very thin, just 1-2 nm thick. Particularly, the thinness (approximately 1 nm) and chemical properties of the AlOx barrier layer are critical to its magnetic tunneling property. We focused on evaluating the current TEM analytical methods by measuring the thickness and composition of an AlOx layer using several TEM instruments, that is, a round robin test, and cross-checked the thickness results with an X-ray reflectometry (XRR) method. The thickness measured by using HRTEM, HAADF-STEM, and zero-loss images was 1.1 nm, which agreed with the results from the XRR method. On the other hand, TEM-EELS measurements showed 1.8 nm for an oxygen 2D-EELS image and 3.0 nm for an oxygen spatially resolved EELS image, whereas the STEM-EDS line profile showed 2.5 nm in thickness. However, after improving the TEM-EELS measurements by acquiring time-resolved images, the measured thickness of the AlOx layer was improved from 1.8 nm to 1.4 nm for the oxygen 2D-EELS image and from 3.0 nm to 2.0 nm for the spatially resolved EELS image, respectively. Also the observed thickness from the EDS line profile was improved to 1.4 nm after more careful optimization of the experimental parameters. We found that EELS and EDS of one-dimensional line scans or two-dimensional elemental mapping gave a larger AlOx thickness even though much care was taken. The reasons for larger measured values can be found from several factors such as sample drift, beam damage, probe size, beam delocalization, and multiple scattering for the EDS images, and chromatic aberration, diffraction limit due to the aperture, delocalization, alignment between layered direction in samples, and energy dispersion direction in the EELS instrument for EELS images. In the case of STEM-EDS mapping with focused nanoprobes, it is always necessary to reduce beam damage and sample drift while trying to maintain the signal-to-noise (S/N) ratio as high as possible. Also we confirmed that the time-resolved TEM-EELS acquisition technique improves S/N ratios of elemental maps without blurring the images.