H. Nohira

Tokyo City University · Department of Electrical and Electronic Engineering

Publications

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    ABSTRACT: Effect of W and TiN/W gate metal on the interface quality of La2O3/InGaAs metal–oxide-semiconductor (MOS) interface is investigated. Hard X-ray photoelectron spectroscopy revealed that gate metal greatly affects the oxidation states at La2O3/InGaAs interface after post-metallization annealing (PMA). Results demonstrate that TiN/W gate metal can effectively control the reaction at La2O3/InGaAs interface and also suppress the formation of As, Ga, and In oxides. As a result, superior capacitance–voltage (C–V) characteristics with low interface state density (Dit) of 4.6 × 1011 cm−2/eV (∼0.1 eV from midgap) and leakage current below 10−5 A/cm2 was obtained for TiN/W/La2O3 (10 nm)/InGaAs MOS capacitors. The MOS structure integrity was preserved for annealing temperature up to 620 °C.
    Solid-State Electronics 04/2013; 82:29–33. · 1.48 Impact Factor
  • International Conference on Solid State Devices and Materials, Fukuoka; 01/2013
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    ABSTRACT: Interface reactions of a Ce-oxide layer with Si(100) wafers have been characterized by X-ray photoelectron spectroscopy. The ratio of Ce atoms in Ce3+ states within the Ce-oxide layer has been found to decrease from 47% at as-deposited sample to 26% after annealing. From detailed reaction analysis of valence number transitions of Ce atoms and the creation of SiO2 layer at the interface, the reacted Ce3+ atoms are converted into silicates and Ce4+ with a ratio of 2:1. The energy bandgap of Ce-silicate layer has been determined as 7.67 eV and the valence band offset with respect to Si(100) wafer has been extracted as 4.35 eV.
    Vacuum. 04/2012; 86(10):1513–1516.
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    ABSTRACT: Induced local stress arising from local deformation of top silicon die in the vertically stacked LSI die has been investigated via x-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy (RS). The large positive shift in the core level Si-2s and Si-2p XP spectra for the thinned die revealed that thinned dies were under heavy stress/strain even before stacking. The core level binding energy shift, Eb for Si-1s core level and the relative chemical shift Er for Si in the vertically integrated die system showed that the stacked Si dies were under different stresses in the -bump and the bump-space regions. It was also inferred from the RS results that the stacked 10 m-thick-Si dies were under large tensile strain of >1.5 GPa and a relatively small compressive stress of ~0.5 GPa in the -bump and bump-space region, respectively. Introduction Both the signal propagation delay and the huge power consumption problems are primarily due to the large wiring length in the large scale integration (LSI) systems and the system-in-package (SiP). On the other hand, the concept of three dimensionally (3D) integrated chip (IC) or the 3D-Si integration is more advantageous than the system-on-chip (SoC) technology in many ways such as lower cost, reduced footprints and enhanced electrical performance. The 3D-IC/Si integration can be achieved by vertically stacking several functional chips and connecting them with the through-silicon-vias (TSVs) and microbumps as shown in fig. 1[1]. This vertical stacking of various functional chips thus leads to a decrease in power consumption and an increase in speed due to the short interconnect length. In line with this, Silicon (Si) wafers with thickness less than 20 m are attracting more and more interest especially in three-dimensional (3D) chip stacking in order not only to reduce the TSV length, but also to ease the TSV fabrication and lateral interconnection formation process [2-4]. However, these ultra thin Si dies/wafers are highly susceptible to the induced thermo-mechanical stress caused by various 3D-integration processes. These induced stress become an important issue of concern for the submicron integrated circuits technologies since such stress in a package can induce failures of electronic devices, e.g. die crack, wire break, etc. The preliminary cause for the induced stress in the 3D-LSI Si wafers/dies is the thermal mismatch among different materials involved in a package. Broadly, the thermo-mechanical stress in the bonded 3D-LSIs can be classified into two categories, (i) an interconnect induced stress (by the metals of the TSVs and microbumps) and (ii) Fig. 1: Schematic view of heterogeneously integrated 3D-LSI system. the organic adhesives induced stress (injected in between the different layers of the stacked chips). By and large, the cure temperature of various organic adhesives used in the chip stacking process is well over the 150 ℃. When the whole die cools down, organic adhesives (epoxy material) shrinks more than the Si die/wafer due to the large difference in thermal expansion co-efficient (CTE). This may cause a Fig. 2: Induced local stress in the stacked LSI die caused by the local deformation. ,
    Electronic Components and Technology Conference (ECTC), 2012 IEEE 62nd; 01/2012
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    ABSTRACT: The chemical structures of SiO2/Si interfaces were studied by photoelectron spectroscopy using high-brilliance soft X-ray with photon energy ranging from 500 to 1500 eV at Super Photon ring 8 GeV(SPring-8) and it is able to probe a depth of about 1.2 to 3 nm with energy resolution of 100 meV. On the other hand, high-brilliance hard X-ray with photon energy ranging from 6 to 10 keV is able to probe a depth of about 8.5 to 12.5 nm with energy resolution of 100 meV. Hard photoelectron spectroscopy are particularly useful for studying the composition and the chemical structure of transition layer at high-k dielectric/silicon interface.
    International Journal of High Speed Electronics and Systems 11/2011; 16(01).
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    ABSTRACT: Mechanical strain/stress and crystal defects are produced in extremely thin wafers (thickness ~10 μm) of 3D-LSIs not only during wafer thinning, but also after wafer bonding using fine-pitch, high-density microbumps and curing. Furthermore, the metal of through-Si via (TSV) and microbump not only becomes the cause of contamination, but also induces strain/stress (due to the difference in the co-efficient of thermal expansion (CTE) between Si and metal) in thinned Si substrate. X-ray photoelectron spectroscopy (XPS) results showed that the crystal quality of Si is highly deteriorated in the ultra-poly ground (UPG) surface after wafer thinning and stress relief. Micro-Raman spectroscopy (μRS) data revealed that a local tensile strain amount to 1.8 GPa was induced by 4×4 μm<sup>2</sup> square sized Si microbumps in 10 μm-thick LSI wafers after bonding and curing. We have noticed that this locally induced strain/stress caused more than 10% change in the ON current of p-MOS transistor. CuSn microbumps have also induced strain/stress at Si wafer surface, and it penetrates deeper for larger bump size and wider for smaller bump pitch.
    Electron Devices Meeting (IEDM), 2010 IEEE International; 01/2010
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    ABSTRACT: Oxygen bonding in La-silicate film with compositional gradient has been characterized by x-ray photoelectron spectroscopy. Based on an analytical model of bridging and nonbridging oxygen, the O 1s spectra arising from La-silicate layer have been deconvoluted with compositionally dependent parameters. For a composition ratio of 1:1 for SiO <sub>2</sub> and LaO <sub>1.5</sub> on the surface of the La-silicate layer, negative binding energy shifts of 0.35 and 0.10 eV for bridging and nonbridging oxygen, respectively, have been found to well interpret the angle-resolved spectra. The method has also been applied to characterize the temperature dependence of interface reactions at La <sub>2</sub> O <sub>3</sub>/ Si with in situ processed Pt electrode. SiO <sub>4</sub> molecules combined with bridging and nonbridging oxygen atoms have been found to form by high temperature annealing. The thickness of the silicate layer of 0.4 nm at as-deposited state has been found grow up to 2.8 nm after 500 ° C annealing. From rough estimation, it has been revealed that 10% of the newly created bridging oxygen atoms by annealing are incorporated into SiO <sub>4</sub> network which contain also nonbridging oxygen atoms.
    Journal of Applied Physics 01/2010; · 2.21 Impact Factor
  • T. Hattori, H. Nohira
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 32(50).
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    ABSTRACT: Chemical bonding states of doped impurities such as B, As, P and Sb in Si were analyzed by soft X-ray photoelectron spectroscopy (SXPES). A step-by-step shallow etching and Hall effect measurements were combined with the SXPES to investigate correlation between chemical bonding state and electrical activation, and to clarify depth profiles of concentration of activated and deactivated impurities in shallow junctions. The study of B doped layer revealed that one chemical bonding state is assined to activated B and the other two states are correlated with deactivated B, which probably form B clusters. On the other hand, two different chemical bonding states were detected for each donor type impurity (As, P and Sb), however, these two states could not be necessarily correlated with the electrically activated and deactivated atoms.
    01/2010;
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    ABSTRACT: The chemical bonding states and electrical characteristics of SrO capped La2O3/CeOx gate dielectric have been examined. Angle-resolved X-ray photoelectron spectroscopy measurement has revealed that Sr atoms diffuse into silicate layer to form SrLa-silicate after annealing. Owing to the incorporation of Sr atoms into silicate layer, a transistor operation with an equivalent oxide thickness (EOT) below 0.5 nm has been demonstrated. A strongly degraded effective electron mobility of 78 cm2/V s at 1 MV/cm has been obtained, which fit well with the general trend in small EOT range below 1 nm. Although process optimization is needed to improve the performance of transistors, Sr capping technique can be useful for EOT scaling.
    Microelectronics Reliability. 01/2010; 50(3):356-359.
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    ABSTRACT: Chemical bonding states of doped impurities, boron (B) and arsenic (As), in silicon (Si) shallow junctions were studied by soft X-ray photoelectron spectroscopy (SXPES). A step-by-step shallow etching and Hall effect measurements were combined with the SXPES to investigate correlation between chemical bonding state and electrical activation of the impurities, and to clarify depth profiles of concentration of activated and deactivated impurities in shallow junctions. The study of B doped layer revealed that one chemical bonding state having the lowest biding energy is assigned to activated B and the other two states are correlated with deactivated B, which probably form B clusters. On the other hand, two different chemical bonding states were detected for As. It was found that the state having higher binding energy was correlated with activated As, while another state was probably correlated with deactivated As cluster.
    01/2010;
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    ABSTRACT: A direct high-k/Si gate stack has been proposed for gate oxide scaling. With LaCe-silicate, an EOT of 0.64 nm with an average dielectric constant (kav) of 17.4 has been obtained and an extremely low gate leakage current (Jg) of 0.65 A/cm2. The flatband voltage (Vfb) can be controlled by the compositional ratio of La in the LaCe-silicate layer. Furthermore, incorporation of Ge atom into the silicate layer can effectively shift the Vfb to positive direction.
    01/2010;
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    ABSTRACT: Wafer thinning and formation of through-Si via (TSV) and metal microbump are key processes in 3D LSI fabrication. However, it might introduce mechanical stress and crystal defects in thinned wafers. In addition, Cu for TSV and microbump might introduce metal contamination in thinned Si substrate. Then the impact of mechanical stress and metal contamination in the thinned Si substrate has been investigated. The remnant stress left after wafer thinning was evaluated by micro-Raman spectroscopy (¿RS) and XPS. It was found that the mechanical stress remained in the back surface of Si substrate after wafer thinning and a part of this mechanical stress appeared in the surface of Si substrate. The metal contamination in such thinned Si substrate has been evaluated by a C-t method. It was found that the carrier generation lifetime was degraded by Cu diffused into Si substrate at relatively low temperature of 200°C. The mechanical stress/strain in the thinned Si substrate after wafer bonding was also evaluated to investigate the influences of metal microbumps to the thinned Si substrate. It was found that the local mechanical stress was generated in the Si substrate surface by the microbumps. This local stress caused a 3% change in the ON current of MOS transistor.
    01/2009;
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    ABSTRACT: Chemical bonding states of boron (B) in shallow P+/N junctions were studied by soft X-ray photoelectron spectroscopy (SXPES). The concentration profiles of B having different binding energies were successfully determined the SXPES combined with the step-by-step etching of Si substrates. The concentration profiles of B having the lowest binding energy can be assigned as activated B, which agreed quite well with those of holes determined by the Hall measurements, while those having the middle and highest binding energies must be attributed to deactivated B. Effects of the spike-RTA and flush lamp annealing (FLA) were compared regarding the concentration profiles of B and UV Raman spectroscopy. Arsenic (As) doped layers were also studied by the X-ray photoelectron spectroscopy and the two different bonding states were revealed for As atoms embedded in Si substrates.
    01/2008;
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    ABSTRACT: We calculate the difference between the valence charges of the ground state and the Slater transition state for several types of Si compounds and Al compounds. We assume the dipole moment induced by core-hole generation at the Slater transition state to be the product of the valence charge difference and the bond length of the compounds. We find that the calculated dipole moment has a strong linear correlation well with (ε-1)/(V+2), which is deduced from experimental values of the optical dielectric constant V.
    Applied Physics Letters 01/2008; 93. · 3.52 Impact Factor
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    01/2008;
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    ABSTRACT: The chemical states of Fe and Pt in in situ annealed L10 structured FePt nanodots formed by self-assembled nanodot deposition method have been systematically investigated by angle resolved x-ray photoelectron spectroscopy. From the Fe3p and the Pt4f core level x-ray photoelectron (XP) spectra, it is evident that both the Fe and Pt of the nanodots were oxidized in the as-grown state. After the in situ annealing under high vacuum, a peak corresponding to metallic Fe begins to appear, and subsequently the metallic peak fraction increased with the increase in the annealing temperature. In line with this, the peak fraction of the respective oxides is drastically decreased. Irrespective of the annealing temperatures, it is inferred from the intensity of the XP spectrum that the Fe atom of the FePt nanodots is highly prone to oxidation than the Pt atom. Nevertheless, the valence band spectra of the as-grown FePt nanodot film clearly depict the presence of metallic Fe-Pt alloy. We would like to explain the results of the core level and valence band XP spectra as follows: only the peripheral Fe and Pt atoms of the as-formed FePt nanodots are bonded to the oxygen of the cosputtered SiO2 matrix, whereas the metallic core of the as-formed FePt nanodots is always preserved. The very good vacuum ambient during postannealing highly promotes the dissociation of oxygen from the metal oxides via reduction reaction. This results into an increase in the fraction of metallic Fe and Pt at the periphery of FePt nanodots and the formation of high quality SiO2 matrix after annealing. Similar results were also observed for the monatomic W as well as Pt nanodots embedded in SiO2 matrix. Hence, this simple, rather effective method of in situ annealing of metal dots dispersed in an insulating matrix can be readily employed in the fabrication of high-density nanodot memory devices.
    Journal of Applied Physics 01/2008; 104. · 2.21 Impact Factor
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    ABSTRACT: We find a new relationship between the optical dielectric constant of Al dielectric films and their chemical shifts measured by X-ray photoelectron spectroscopy (XPS). We measure the difference between core-level binding energy shift for Al 1s and core-level binding energy shift for Al 2p, DeltaE1s - DeltaE2p, for AlN using high-resolution high-energy synchrotron radiation. We find that DeltaE1s - DeltaE2p correlates well with the optical dielectric constants of Al, AlN, and Al2O3. This is consistent with the case of our previously reported Si compounds. First-principles calculations are performed to determine the mechanism behind the observed correlation.
    Journal of Physics Conference Series 01/2008; 100(1).
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    ABSTRACT: We describe state-of-the-art photoelectron spectroscopy studies of SiO2/Si interfaces that play fundamental roles in metal-oxide-semiconductor (MOS) field-effect transistors. We show comprehensive photoelectron spectra of SiO2/Si interfaces, which were taken from SiO2/Si samples of extremlye high-quality by the high-resolution photoelectron spectroscopy technique with either synchrotron or laboratory X-ray sources. The spectra discussed here include the Si 2p, the Si 1s, the O 2s, and N 1s core-level spectra, and the valence-band spectra. We perform quantitative analysis using selected values of the photoionization cross-section and the electron escape depth, which is governed by both inelastic scattering and elastic scattering in SiO2. On the other hand, we analyze peak energies by considering peak energy shifts that are due to several factors. Atomic structures are discussed in terms of intermediate oxidation states at SiO2/Si(100) interfaces and strained Si–O–Si bonds near the interfaces, while electronic structures are discussed in terms of valence-band offset at the interfaces and dielectric constants near the interfaces. Applications of photoelectron spectroscopy study to advanced oxide formation are also shown in terms of depth profiling of oxynitride films and interface structures of low-temperature oxide.
    Progress in Surface Science 01/2007; 82(1):3-54. · 7.14 Impact Factor
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    ABSTRACT: The authors measure the difference of core-level binding energy shifts for Si 1s and Si 2p , ΔE<sub>1s</sub>-ΔE<sub>2p</sub> , for various Si compounds using high-resolution high-energy synchrotron radiation. They find that the ΔE<sub>1s</sub>-ΔE<sub>2p</sub> values are in very good correlation with the dielectric constant values of the Si compounds. Using this relation, they deduce the local dielectric constant for each of the Si intermediate oxidation states formed at the Si O <sub>2</sub>/ Si interface. The results are in good agreement with values predicted by a first-principles calculation.
    Applied Physics Letters 11/2006; · 3.52 Impact Factor

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