Seiichi Iwata

Electron spectroscopic analysis of the SiO2/Si system and correlation with metal–oxide–semiconductor device characteristics

ESCA (electron spectroscopy for chemical analysis) measurement results on thin SiO2/Si samples are examined comprehensively, critically, and in detail to show that it is possible to correlate these results with MOS (metal–oxide–semiconductor) device characteristics such as flatband (threshold) voltage, oxide breakdown field, mobile‐ion density, hole and electron trap density, and hot‐carrier lifetime. Up to now, much effort has been made to detect SiOx phases at SiO2/Si interfaces since they are thought to have a significant effect on MOS device characteristics. However, correlating the SiOx phases with device characteristics is difficult and involves overcoming two problems. First, the chemical state is difficult to determine exactly due to x‐ray irradiation effects. Second, the amount of defects and impurities which influence device characteristics is usually below the ESCA detection limit (1012–1013 cm−2) in device‐quality SiO2/Si samples. Investigation of the first problem led to the conclusion that i...

SiSiO2 interface characterization by ESCA

Abstract The concentration profiles of oxide films on Si have been studied by using ESCA and ellipsometry. To avoid artifacts from ion milling or chemical thinning, our analyses have been performed on grown oxides up to 5 nm thick on Si. The accurate values of the escape depths obtained from ESCA and the ellipsometric measurements in this study were used to analyze the intensity data for Si 2 p and O 1s photoelectrons. Furthermore, the chemical states of Si atoms have been analyzed. The oxide films were found to be composed mostly of stoichiometric SiO 2 with a very thin (about 0.3 nm) layer of SiO at the SiSiO 2 interface, although the composition of oxide films appeared to be Sirich in appearance for thicknesses less than about 3 nm.

A new tungsten gate process for VLSI applications

In spite of the growing demand for MOS gates and interconnections of higher conductivity, the refractory metal gate process has not received as much attention as those using silicides because it is incompatible with the Si-gate process. The metal gate cannot withstand oxidizing annealing ambients, and source-drain formation by ion implantation is difficult because of the channeling of doping ions through the gate metal during ion implantation. In a new process developed for use in MOS VLSI fabrication, tungsten (W) is used as a gate metal because degradation of SiO2by annealing the metal/SiO2/Si structure at around 1000°C can be minimized if the metal is W. Metal oxidation is prevented by using a H2/H2O ambient for this annealing, which also allows Si to be oxidized in the same ambient. The channeling mentioned above is stopped by forming a thin layer of PSG or WOxon the W. This gate process is believed to be a step forward toward the desired compatibility.

Si‐SiO2 interface characterization from angular dependence of x‐ray photoelectron spectra

The Si‐SiO2 interface is studied by using ESCA. The sample is a very thin oxide film (0.91 nm thick) on Si substrate. The angular dependence of the Si2p spectrum is measured. The Si2p spectrum is composed of SiSi2p photoelectrons emitted from the Si substrate, Six2p photoelectrons emitted from the transition region at the Si‐SiO2 interface, and SiSiO22p photoelectrons emitted from SiO2 film.It is concluded that the chemical shift for Si atoms in the transition region is 1.6 eV (this value is about one‐half of that of SiO2), and that the thickness of the transition region is 0.2–0.3 nm.

The influence of internal stresses in tungsten-gate electrodes on the degradation of MOSFET characteristics caused by hot carriers

The characteristic degradation of MOSFETs with tungsten-gate electrodes caused by hot carriers is shown to be enhanced by internal stresses in gate electrodes. These stresses introduce strains in silicon substrates under the edges of gate electrodes, which increases the number of surface states at the Si-SiO2interfaces. As a result, these internal stresses enhance the degradation of MOSFET characteristics due to hot carriers. A new technique for reducing the strains induced in the region under the gate electrodes is presented. With this technique (namely, annealing before patterning tungsten films for gate electrodes), the degradation of tungsten-gate MOSFETs can be decreased to a level compatible with that of conventional silicon gate MOSFETs.

Investigation of fluorine in SiO2 and on Si surface by the 19F(p,αγ)16O reaction, secondary‐ion mass spectrometry, and x‐ray photoelectron spectroscopy

A fluorinated thermal SiO2, grown after HF surface treatment without de‐ionized water rinse, was estimated to contain ∼3×1013 cm−2 of fluorine by the 19F(p,αγ)16O reaction. Secondary‐ion mass spectrometry data indicate that the SiF distribution is peaked at the SiO2/Si interface in the fluorinated oxide. The time‐dependent change of the absolute amount of fluorine on the HF‐treated silicon surface as a function of storage time in air or in vacuum was also investigated by the 19F(p,αγ)16O reaction. The initial number of fluorine atoms on the HF‐treated silicon surface was estimated to be ∼1015 cm−2 before substantial desorption took place. Fluorine atoms desorb from the silicon surface much more rapidly if the sample is stored in air than in vacuum. These results were also supported by the x‐ray photoelectron spectroscopy measurement.

Electron transport across aluminum/ultrathin silicon oxide/phosphorus implanted silicon barriers

Schottky barrier diodes are fabricated on silicon surfaces whose impurity concentrations are controlled by ion implantation techniques. The barriers are produced by electron beam evaporation or sputtering of the metal. The forward characteristics of the diodes show that the forward voltage of Schottky barrier diodes made by sputtering is 50–100 mV larger than those made by electron beam evaporation. Electron spectroscopy for chemical analysis measurements show that, for sputtering, the silicon oxide thickness at the interface between the metal film and silicon substrate is 0.2 nm thicker than for electron beam evaporation. Theoretical and experimental studies of Schottky barrier diodes in which the metal and silicon substrate are separated by a thin layer silicon oxide are reported.

Tungsten gate technology using wet hydrogen oxidation

This report describes two key techniques for using tungsten (W) in place of poly-silicon (Si) or polycide in highly reliable metal gate technology for VLSIs: a new oxidation method, Wet Hydrogen Oxidation (WHO), to allow Si oxidation without oxidizing W, and W gate formation by sputtering using a high-purity W target. The effects of these new techniques on characteristics of MOSFETs were investigated. Gate breakdown voltage was improved after WHO with no unfavorable effects on other device characteristics. Flatband voltage shift due to mobile-ion contamination was greatly reduced using a high-purity W target. Compared with poly-Si gates, the W gate MOS capacitors have higher gate breakdown voltage. W gate technology was applied to NMOS VLSIs, showing potential for megabit level VLSIs because of its low resistance ( < 0.5 Ω/□) and improved compatibility with poly-Si gate technology.

Evaluation of Mobile Ion Contamination in Tungsten-Gate MOS Process Using Triangular Voltage Sweep Method

Mobile ion contamination in tungsten (tU) gate M0S fabrication is studied using. triangul-ar voltage sweep (TVS). It is proved that mobiLe ionic contaminants in Si0r fihn come from MOS gate electrode or photoresist film in W-gate MOS fabrication. It is found that the change in MOS chaiacteristics due to this contamination is related to the sodium (Na) concentration of electrode materj-aL or photoresist film, increased by the electric field induced during ashing in 0r plasma. A W-gatb MOS device free from mobil-e ion contamination i; obtained uJing a lighly purified Ul sputtering target and reduci-ng the electril cal field caused by plasma ashing process.

Characteristics of Tungsten Gate MOSFETs for VLSIs

This paper reports on the evaluation of MOSFETs, fabricated by tungsten gate technology. Stable and controllable threshold voltage and steep tailing are obtained, due to the good interface properties of a Eungsten gate MoS strueture, rn addition, 0.65 V larger work function of tungsten, compared with conventional nr-doped polysilicon, brings about some improvemenL in device characteristics such as a 15-20 Z increase in electron mobility and 0.5 times suppressed. impact ionization at the drain. These results demonstrate the feasibili-ty of tungsten gate MOSFETs as a VLSI element.