Yoshihiro Sugita

High resolution-high energy x-ray photoelectron spectroscopy using third-generation synchrotron radiation source, and its application to Si-high k insulator systems

High-resolution x-ray photoelectron spectroscopy (XPS) at 6 keV photon energy has been realized utilizing high-flux-density x rays from the third generation high-energy synchrotron radiation facility, SPring-8. The method has been applied to analysis of high-k HfO2/interlayer/Si complementary metal–oxide–semiconductor gate-dielectric structures. With the high energy resolution and high throughput of our system, chemical-state differences were observed in the Si 1s, Hf 3d, and O 1s peaks for as-deposited and annealed samples. The results revealed that a SiOxNy interlayer is more effective in controlling the interface structure than SiO2. Our results show the wide applicability of high resolution XPS with hard x rays from a synchrotron source.

Impact of additional factors in threshold voltage variability of metal/high-k gate stacks and its reduction by controlling crystalline structure and grain size in the metal gates

We have clarified that, in a metal/high-k gate stack, as well as the variability introduced by random dopant fluctuations (RDF), the threshold voltage variability (TVV) is attributable to the crystal structure and grain size in the metal gate. We have successfully eliminated this additional factor by reducing the grain size in the metal gate. We demonstrated that the incorporation of C into TiN metal gates transforms the crystalline film into an amorphous one, effecting a reduction in the TVV in HfSiON pFET devices. We observed that the TVV of C-incorporated TiN devices was dominated by RDF, indicating that the additional factor due to the metal gate had been diminished.

Si Quantum Dot Formation with Low-Pressure Chemical Vapor Deposition

We report a simple technique for fabricating a layer of isolated Si quantum dots. The procedure uses conventional low-pressure chemical vapor deposition (LPCVD) for an extremely short deposition time in the early stage of poly-Si film growth. The layer resulting from a deposition time of 60 s has isolated Si nanocrystals 5–20 nm in diameter and 2–10 nm in height. Optical absorption measurement shows that the Si-nanocrystal spectrum changes drastically and the onset of absorption shifts to higher energies compared to that of bulk Si. This shift can be explained by the energy gap widening caused by quantum size effects. Special attention is paid to the Brownian migration of Si nanocrystals for fabricating Si quantum dots.

Microstructure and optical absorption properties of Si nanocrystals fabricated with low‐pressure chemical‐vapor deposition

We report a simple technique for fabricating a layer of isolated Si quantum dots on SiO2 glass substrates. This technique uses conventional low‐pressure chemical‐vapor deposition for an extremely short deposition time in the early stage of poly‐Si film growth. The layer after a deposition time of 60 s has isolated Si nanocrystals 5–20 nm in diameter and 2–10 nm in height. The measurements of optical absorption coefficient α show that the absorption edge for Si nanocrystals shifts to higher energies compared to that of bulk Si, indicating a widening of the energy gap caused by quantum size effects. The linear relationship (αhν)1/2 against hν suggests that the Si nanocrystal, whose diameter is as small as 10 nm, basically maintains the properties of an indirect band‐gap semiconductor. Special attention must be paid to the Brownian migration of Si nanocrystals for fabricating Si quantum dots.

The role of dissolved oxygen in hot water during dissolving oxides and terminating silicon surfaces with hydrogen

Abstract Evidence of the removal of surface oxides from a partially oxidized Si(111) surface, and the simultaneous termination of the new dangling bonds by hydrogen in hot water, has been obtained. The amount of remaining oxides decreased by lowering the dissolved-oxygen concentration (DOC) from 4 to 0.004 ppm. After immersing in hot water with a DOC of 0.004 ppm for 60 min, almost all of the surface was hydrogen-terminated and only a tiny amount of silanol groups remained. The appearance of vertical dihydrides at step edges showed that the oxidation process on the hydrogen-terminated surface continues, even in water with a DOC of 0.004 ppm.

High-Density Layer at the SiO2/Si Interface Observed by Difference X-Ray Reflectivity

We have developed a high-accuracy difference X-ray reflectivity (DXR) method using intense synchrotron radiation for the evaluation of ultrathin thermal oxides on Si(100). By carefully analyzing DXR data for gate oxides with thicknesses of 40 A and 70 A grown at 800° C to 1000° C, the existence of a dense ( ~2.4 g/cm3), thin (~10 A) layer at the SiO2/Si interface has been revealed. The thickness of the interfacial layer decreases with increasing oxidation temperature. Oxides grown in O3 or HCl/O2 have a thinner interfacial layer compared to those grown in O2.

High‐precision x‐ray reflectivity study of ultrathin SiO2 on Si

We have developed a high‐precision difference‐x‐ray‐reflectivity technique using intense synchrotron radiation and applied this to evaluate native oxides and ultrathin thermal oxides on Si(100). We have successfully evaluated the density of native oxides. Native oxides formed by HCl and NH4OH solutions have a low density, in contrast to the oxides formed by H2SO4 solution and UV/O3 whose densities are close to those of thermal oxides. By carefully analyzing ultrathin thermally grown oxides with thicknesses of 40 and 70 A grown at 800–1000 °C, we have revealed the existence of a dense (∼2.4 g/cm3), thin (∼10 A) layer at the SiO2/Si interface. Oxides grown in O3 or HCl/O2 have a thinner interfacial layer compared to those grown in O2. We have evaluated the effects of ambient and temperature at oxidation on the interfacial layer and the SiO2 layer.

In Situ Infrared Spectroscopy on the Wet Chemical Oxidation of Hydrogen-Terminated Si Surfaces

In situ infrared spectroscopy was used to observe the early stage of oxidation on hydrogen-terminated Si(100) and (111) in solution. The observation confirmed that the oxidation starts with the insertion reaction of oxygen atoms to the back bonds of the topmost silicon atoms in H2O2 solution. It was found both that the back bond and Si–H bond were oxidized in ozonized water. The dependence of the oxidation reaction of the Si–H bond and its back bond on surface orientation was discussed and the evidence of island growth of the oxide film was shown.

Thermal oxide growth at chemical vapor deposited SiO2/Si interface during annealing evaluated by difference x-ray reflectivity

The x-ray interference technique has been applied to evaluate the structural changes of high temperature grown chemical vapor deposited (CVD) SiO2 film under several post annealing conditions. In annealing above 800 °C in O2 ambient, a thermal oxide growth has been found at the CVD SiO2/Si interface, and its precise thicknesses have been determined. The estimated diffusion coefficient of the oxidant in CVD film was about three times larger compared to that of thermal oxide. A threshold voltage shift in the oxide was found to strongly correlate to the thickness of the thermal oxide rather than to thermal modifications of the CVD SiO2 itself.

Structural fluctuation of SiO2 network at the interface with Si

Abstract We analyzed the density of thermally grownSiO 2 /Siby X-ray reflectivity measurements and local vibration properties by infrared spectroscopy. The macroscopic density varied with growth conditions. A lower growth temperature caused higher film density. We also found a thin (1 nm) and dense (2.35–2.4 g/cm 3 ) transient layer at theSiO 2 /Siinterface. The film density was constant to the thickness direction without the transient layer. On the other hand, IR properties showed characteristic film thickness dependencies. At a film thickness greater than 10 nm, the frequency of the transverse optic (TO) mode of Si-O stretching shifted to the red direction, decreasing with the film thickness; while the frequency of the longitudinal optic (LO) mode is unchanged. This red shift of TO mode has no relation to the film density. At a film thickness of less than 6 nm, we found that both the TO and LO mode shift to the red direction simultaneously. The red shifts gradually increased with decreasing film thickness. This indicates that the SiO 2 network was densified by compressive stress. We assumed that the macroscopic and microscopic fluctuation were related to the oxide growth and formation of a hetero-junction.