Tomio Iwasaki

Molecular dynamics analysis of adhesion strength of interfaces between thin films

We have developed a molecular-dynamics technique for determining the adhesion strength of the interfaces between different materials. This technique evaluates the adhesion strength by calculating the adhesive fracture energy defined as the difference between the total potential energy of the material-connected state and that of the material-separated state. The extended Tersoff-type potential is applied to calculate the adhesive fracture energy of metal/dielectric interfaces as well as metal/metal interfaces. We used the technique to determine the adhesion strength of the interfaces between ULSI-interconnect materials (Al and Cu) and diffusion-barrier materials (TiN and W). It was also applied to determine the adhesion strength of interfaces between the interconnect materials and a dielectric material (SiO 2 ). Because the adhesion strength determined by this technique agrees well with that measured by scratch testing, this technique is considered to be effective for determining the adhesion strength.

Ta2O5 Interfacial Layer between GST and W Plug enabling Low Power Operation of Phase Change Memories

A novel memory cell for phase-change memories (PCMs) that enables low-power operation has been developed. Power (i.e., current and voltage) for the cell is significantly reduced by inserting a very thin Ta2O5 film between GeSbTe (GST) and a W plug. The Ta2O5 interfacial layer works not only as a heat insulator enabling effective heat generation in GST but also as an adhesion layer between GST and SiO2 underneath. Nonetheless, sufficient current flows through the interfacial layer due to direct tunneling. A low programming power of 1.5 V/100 muA can therefore be obtained even on a W plug with a diameter of 180 nm fabricated using standard 0.13-mum CMOS technology. In addition, the uniformity and repeatability of cell resistance are excellent because of the inherently stable Ta2O5 film properties

Doped In-Ge-Te Phase Change Memory Featuring Stable Operation and Good Data Retention

We have fabricated a phase change memory using doped In-Ge-Te to improve the data retention required for industrial and automotive use. This chalcogenide features higher thermal stability as well as denser texture and improved adhesion. The memory cell using doped In-Ge-Te provided a larger read margin and better data retention than conventional Ge2Sb2Tes, and we demonstrated 10-year retention at temperatures above 150degC, which is the highest temperature ever reported.

Oxygen-doped gesbte phase-change memory cells featuring 1.5 V/100-/spl mu/A standard 0.13/spl mu/m CMOS operations

We demonstrated the operation of phase-change memory cells that enabled 1.5-V/100-muA programming through a tungsten-bottom-electrode contact with a diameter of 180 nm. This is the lowest power ever reported. This was achieved with oxygen-doped GeSbTe, and resulted from the high electric resistance of the germanium oxides in this material. Germanium oxides were also estimated to restrain the growth of crystal in GeSbTe, and our cells maintained a 10-year thermal lifetime at 100 degC

Slowing single-stranded DNA translocation through a solid-state nanopore by decreasing the nanopore diameter

To slow the translocation of single-stranded DNA (ssDNA) through a solid-state nanopore, a nanopore was narrowed, and the effect of the narrowing on the DNA translocation speed was investigated. In order to accurately measure the speed, long (5.3 kb) ssDNA (namely, ss-poly(dA)) with uniform length (±0.4 kb) was synthesized. The diameters of nanopores fabricated by a transmission electron microscope were controlled by atomic-layer deposition. Reducing the nanopore diameter from 4.5 to 2.3 nm slowed down the translocation of ssDNA by more than 16 times (to 0.18 μs base(-1)) when 300 mV was applied across the nanopore. It is speculated that the interaction between the nanopore and the ssDNA dominates the translocation speed. Unexpectedly, the translocation speed of ssDNA through the 4.5 nm nanopore is more than two orders of magnitude higher than that of double-stranded DNA (dsDNA) through a nanopore of almost the same size. The cause of such a faster translocation of ssDNA can be explained by the weaker drag force inside the nanopore. Moreover, the measured translocation speeds of ssDNA and dsDNA agree well with those calculated by molecular-dynamics (MD) simulation. The MD simulation predicted that reducing the nanopore diameter to almost the same as that of ssDNA (i.e. 1.4 nm) decreases the translocation speed (to 1.4 μs base(-1)). Narrowing the nanopore is thus an effective approach for accomplishing nanopore DNA sequencing.

Optimization of interatomic potential for Si/SiO2 system based on force matching

Abstract In order to analyze the behavior of Si and SiO2, which are basic materials for the large-scale-integrated circuit technology, the molecular dynamics simulation has been widely used. For the simulation, it is necessary to select accurate interatomic potential function, which reproduces the force acting on each atom. However, the validity of potential functions proposed has not been discussed in terms of the force. In this study, the force calculated by the original Tersoff potential is compared with that obtained by an ab initio calculation in some snapshots of Si/SiO2 systems at the temperature of 300 K. It clarifies that the potential does not properly reproduce the force. Then, optimizing the parameters in the Tersoff potential on the basis of the force acting on each atom obtained by the ab initio calculation (force matching method), two modified functions are proposed. One is “Potential A” obtained by optimizing the parameters for the Si crystal and the β-cristobalite SiO2 crystal. The “Potential A” correctly reproduces not only the forces but also the lattice constants. However, it is not effective for Si/SiO2 interface. The “Potential B” obtained by optimizing the parameters for a Si/SiO2 interface model as well as the Si and the β-cristobalite SiO2 crystals. The forces and lattice constants are successfully reproduced for the interface as well. Moreover, the “Potential B” gives a good result for the β-quartz SiO2, which is not used for the optimization. This implies the versatility of the proposed function.

Tuning Optoelectrical Properties of ZnO Nanorods with Excitonic Defects via Submerged Illumination

When applied in optoelectronic devices, a ZnO semiconductor dominantly absorbs or emits ultraviolet light because of its direct electron transition through a wide energy bandgap. On the contrary, crystal defects and nanostructure morphology are the chief key factors for indirect, interband transitions of ZnO optoelectronic devices in the visible light range. By ultraviolet illumination in ultrapure water, we demonstrate here a conceptually unique approach to tune the shape of ZnO nanorods from tapered to capped-end via apical surface morphology control. We show that oxygen vacancy point defects activated by excitonic effects near the tip-edge of a nanorod serve as an optoelectrical hotspot for the light-driven formation and tunability of the optoelectrical properties. A double increase of electron energy absorption on near band edge energy of ZnO was observed near the tip-edge of the tapered nanorod. The optoelectrical hotspot explanation rivals that of conventional electrostatics, impurity control, and alkaline pH control-associated mechanisms. Thus, it highlights a new perspective to understanding light-driven nanorod formation in pure neutral water.

Correlation Between Whisker Initiation and Compressive Stress in Electrodeposited Tin–Copper Coating on Copper Leadframes

To evaluate the contribution of coating stress to whisker initiation from IC package leads, the stress distribution in the coating was investigated by finite-element analysis (FEA). Two different leadframe samples, which were composed of the same tin-copper coating on two different copper-leadframe materials, namely, copper-iron (hereafter, CUFE; corresponding to CDA number C19400) and copper-chromium (CUCR; CDA number C18045), were used to examine the whisker-initiation behavior on the coating surfaces. The two samples showed significantly different tendencies of whisker initiation from the coating. That is, after long-term storage at room temperature, no whisker initiation was observed on the coating on the CUCR sample, whereas long whiskers (with a maximum length of more than 200 μm) were formed from the coating on the CUFE sample. The FEA calculation on the leadframe samples revealed that the coatings had a two-directional stress gradient, namely, one gradient toward the surface and another toward the base leadframe material. It also indicated a difference between the stress distributions in the two samples. The gradient of normal stress on the coatings grain boundaries (GBs), toward the surface of the CUFE sample, was found to be larger than that in the CUCR sample. This result implies that the tin-atom flux along a GB in the coating on the CUFE sample was larger than that on the CUCR sample because the atom flux along the GB was proportional to the stress gradient. It agrees with the above-mentioned whisker-initiation behaviors in the samples. We thus conclude that in the CUFE sample, a whisker initiates either from a surface grain immediately on top of a GB or from surface grains located on both sides of the same GB. To confirm this conclusion, the correlation between the tin-diffusion sites and whisker formation sites was investigated. Simulation of atom diffusion by molecular dynamics indicated that the dominant tin-diffusion site is a GB when compressive stress is applied in the direction normal to the GB. Investigation of the correlation between the whisker roots and coating microstructures of the CUFE sample showed that the whisker roots were located on top of GB intersections in the coating. These results indicate that whisker-initiation sites are correlated with dominant tin-diffusion sites and that each whisker initiates either from a surface grain located immediately on top of a GB or from surface grains located on both sides of the same GB.

Molecular-Dynamics Analysis of Interfacial Diffusion Between High-Permittivity Gate Dielectrics And Silicon Substrates

Interfacial oxygen diffusion from high-permittivity gate dielectrics (ZrO 2 and HfO 2 ) into Si substrates in ultra-large-scale integrated circuits must be suppressed to prevent the formation of interfacial layers between the gate dielectrics and the Si substrates. Oxygen diffusion was analyzed by using a molecular dynamics technique that includes many-body interactions and charge transfer between different elements. The analysis results showed that the addition of Ti is effective in suppressing interfacial oxygen diffusion. The results also showed that the diffusion at the ZrO 2 /Si(111) and HfO 2 /Si(111) interfaces is much more suppressed than the diffusion at the ZrO 2 /Si(001) and HfO 2 /Si(001) interfaces.

A pathway of nanocrystallite fabrication by photo-assisted growth in pure water

We report a new production pathway for a variety of metal oxide nanocrystallites via submerged illumination in water: submerged photosynthesis of crystallites (SPSC). Similar to the growth of green plants by photosynthesis, nanocrystallites shaped as nanoflowers and nanorods are hereby shown to grow at the protruded surfaces via illumination in pure, neutral water. The process is photocatalytic, accompanied with hydroxyl radical generation via water splitting; hydrogen gas is generated in some cases, which indicates potential for application in green technologies. Together with the aid of ab initio calculation, it turns out that the nanobumped surface, as well as aqueous ambience and illumination are essential for the SPSC method. Therefore, SPSC is a surfactant-free, low-temperature technique for metal oxide nanocrystallites fabrication.