Toshiyuki Iwabuchi

Crystallization of Sr0.7Bi2.3Ta2O9+α Thin Films by Chemical Liquid Deposition

By chemical liquid deposition in which an alkoxide with a carbon number of 4 or smaller was used as raw material, a Sr0.7Bi2.3Ta2O9+α (SBTO) thin film was fabricated for use as a ferroelectric memory device for the purpose of decreasing the temperature of crystallization and improving surface morphology. The crystallization process was also examined. Crystallization began when the film was heat treated in oxygen at 650° C. When it was heat treated at higher than 700° C, it showed ferroelectric properties, and the squareness (remanent polarization/saturation polarization) of its hysteresis loop was improved at 800° C. A film heat treated at temperature 650° C was a cluster of fine particles, and a film heat treated at 800° C was a cluster of large particles. A film heat treated at 700° C was a mixture of fine particles and large particles. Therefore, it is concluded that the alkoxide with a carbon number of 4 or smaller as raw material enable the lowering of the heat-treatment temperature and improvement of the surface morphology of SBTO thin films.

Characterization of SiO2/Si(100) interface structure of ultrathin SiO2 films using spatially resolved electron energy loss spectroscopy

High‐resolution transmission electron microscopy (HRTEM) with 0.3‐nm resolution, scanning transmission electron microscopy using transmission electron diffraction and electron energy loss spectroscopy (EELS) with 0.7‐nm resolution have been employed to characterize the SiO2/Si(100) interface structure of ultrathin (5–12 nm) SiO2 films grown by rapid thermal oxidation (RTO) at 1100 °C. In the HRTEM image, the SiO2/Si(100) interface of ultrathin SiO2 film grown by RTO in dry oxygen ambient seems to be uniform. The TED patterns also showed that the atomic structure abruptly changes from the crystalline silicon to amorphous oxide network across the SiO2/Si interface. The EELS plasmon‐loss spectra showed that for 5‐nm‐thick SiO2 an intermediate oxide composition indicating a plasmon energy between Si (17.0 eV) and SiO2 (22.5 eV) is present, whereas it disappears for 12‐nm‐thick SiO2 film.

Process dependence of the SiO2/Si(100)interface trap density of ultrathin SiO2 films

The interface trap density of states Dit of ultrathin SiO2 film has been investigated as a function of process variables. The process parameters used were oxidation temperature (1000–1200 °C), oxide thickness (4–10 nm), and annealing condition, including ambient (deuterium, argon, and in vacuum), temperature (500 and 900 °C), and time (10–120 s). Analysis of as‐grown SiO2 films showed that the Dit decreases with increasing oxidation temperature and/or oxide thickness. With annealing in argon or in vacuum at 900 °C, the Dit decreases exponentially at the initial stage, then starts to increase linearly with increasing time. Similar behavior was observed for 900 °C deuterium annealing. Deuterium annealing at 500 °C was more effective in the reduction of the Dit, whereas argon annealing at the same temperature did not affect the density at all. A possible model for annealing kinetics is proposed to explain the experimental results.

Role of SiN Bond Formed by N2O-Oxynitridation for Improving Dielectric Properties of Ultrathin SiO2 Films

Dielectric and physical properties of N2O-oxynitrided (RTON) ultrathin (5-10 nm) SiO2 films formed by in situ multiple rapid thermal processing have been investigated. In high-field stressing under positive and negative polarity biases, the RTON SiO2 films showed a much lower electron-trap generation rate, a lower field-induced leakage current and a higher charge-to-breakdown value, as compared to those of pure SiO2 and rapid thermally NH3-nitrided (RTN) SiO2 films. Fourier transform-infrared reflection attenuated total reflectance (FT-IR ATR) and X-ray photoelectron spectroscopy (XPS) results showed that strong Si-N bonds are formed at the SiO2/Si interface by RTON, whereas a large number of N-H and Si-H bonds, which act as electron traps, are also generated by RTN.

EFFECT OF ANNEALING METHOD TO CRYSTALIZE ON SR0.9BI2.3TA2O9+ THIN FILM PROPERTIES FORMED FROM ALKOXIDE SOLUTION

Sr0.9Bi2.3Ta2O9+α (SBT) thin films were formed from a mixed alkoxide solution and were heat-treated by various annealing methods. The SBT thin film heat-treated with the standard temperature rising at a rate of 10°C/min (standard thermal annealing: STA) showed higher squareness (effective remanent polarization/effective saturation polarization) and effective remanent polarization values than those of the film heat-treated with a rapid temperature rise at a rate of 125°C/s (rapid thermal annealing: RTA). This difference was large in the region of low applied voltage. The low values of the SBT film with RTA treatment were caused by lower crystal continuity in the direction of film thickness than that of the SBT film with STA treatment. The SBT film with RTA treatment showed a higher leakage current density for the entire applied voltage region than that of the film with STA treatment. The higher leakage current density was caused by fine nonstoichiometric (Bi-rich) particles which exist at grain boundaries of large grains. Therefore, it was concluded that the high squareness ratio and low leakage current density are obtained in SBT film which has only one grain in the film thickness direction and does not have nonstoichiometry fine particles around grain boundaries.

Orientation Control of Sr0.7Bi2.3Ta2O9+α Thin Films by Chemical Liquid Deposition

The orientation of Sr 0.7 Bi 2.3 Ta 2 O 9+α (SBT) films, which are layer-type bismuth compounds, was controlled by varying the Sr-source. In this paper, the effect of crystal orientation on film characteristics is described. The crystal orientation of the SBT ferroelectric films did not affect the surface morphology, leakage current or fatigue characteristics, but it did affect the shape of the hysteresis loop (polarization) and the window value of the C-V characteristics when the films were connected to a metal-oxide-semiconductor (MOS) diode. Although a complete c-axis orientation film with a stoichiometry of SrBi 2 Ta 2 O 9 shows no spontaneous polarization in general, the highly c-axis orientated Sr 0.7 Bi 2.3 Ta 2 O 9+α film in this study showed some spontaneous polarization. The polarization values are larger than expected by considering orientation alone. A deviation from stoichiometry resulted in an increase made in the polarization along the c-axis. Therefore, control of the crystal orientation and composition of SBT films is quite an important factor in actual applications.

Kinetics of Rapid Thermal Oxidation of Silicon

We have proposed, for the first time, a practical model for ultrathin (<10 nm) SiO2 film growth in rapid thermal oxidation (RTO) kinetics. The results showed that the overall RTO growth can be well described by the linear-parabolic model proposed by Deal and Grove [J. Appl. Phys. 36 (1965) 3770]. Moreover, the model proposed indicates that in order to fit the experimental data, the oxide growth in a ramp-up process must be included in the linear-parabolic scheme. As a result, we have succeeded in explaining the RTO growth kinetics in the RTO temperature range from 950 to 1200°C and in a wide thickness range from 15 to 250 A without making any special assumptions. The resultant activation energies for the linear rate constant (B/A) and for the parabolic rate constant (B) are 2.0 eV and 1.74 eV, respectively.

Characterization of SiO2/Si(100) interface structure of N2O-oxynitrided ultrathin SiO2 films

The SiO2/Si(100) interface structure of N2O-oxynitrided ultrathin SiO2 film has been investigated. The structure of the SiO2/Si interface, evaluated from the lattice image of high-resolution transmission electron microscopy (HRTEM), seems to be quite uniform, at least ordered within one or two atomic layers. The layer of the half-spacing order of the Si lattice, which consists of one or two atomic layers, is found between the Si and SiO2 layers. Nanoanalysis by transmission electron energy loss spectroscopy (EELS) indicates that nitrogen atoms bonded to Si atoms are present near the SiO2/Si interface. The EELS plasmon loss image showed the accumulation of nitrogen within 1 nm of the interface.

5 nm gate oxide grown by rapid thermal processing for future MOSFETs

Formation of 5-nm-thick SiO2 film has been studied by rapid thermal processing (RTP). The SiO2/Si(100) interface of 5-nm RTP-grown SiO2 film evaluated by high-resolution TEM is quite uniform, at least ordered within one or two atomic layers. The TDDB characteristics have indicated that short-time breakdown failure diminishes with decreasing oxide thickness, drastically below 6 nm in thickness. However, leakage current at low field, which is related to oxide wear-out, is observed in ultrathin RTP-SiO2 film (<6 nm). We have applied 5-nm-thick RTP-SiO2 as a gate oxide of MOSFETs and obtained sufficient subthreshold and transconductance characteristics as device performance.

The Dielectric Reliability of Very Thin SiO2 Films Grown by Rapid Thermal Processing

Very thin SiO2 films, 3–10 nm in thickness, have been formed on silicon by rapid thermal processing (RTP), and their dielectric reliability has been investigated in comparison with furnace-grown oxides. The SiO2 films grown by RTP are superior to furnace oxides on both the dielectric breakdown and the Si-SiO2 interface characteristics. On the RTP SiO2 films, quantum oscillation in MOS tunneling is clearly observed at 300 K. The Si-SiO2 interface roughness is estimated from the Fowler-Nordheim plot. These results indicate that the Si-SiO2 structure is ordered within one monolayer.