Keitaro Imai

Leakage Current Mechanism of Amorphous and Polycrystalline Ta[sub 2]O[sub 5] Films Grown by Chemical Vapor Deposition

The leakage current mechanism of the chemical vapor deposition Ta{sub 2}O{sub 5} film has been investigated. In the case of an as-deposited amorphous film, the presence of impurities such as carbon and hydrogen remaining in the film leads to the Poole-Frenkel type leakage current. The oxidation of these impurities results in a reduction in leakage current. O{sub 2} plasma is especially effective for oxidizing impurities, leading to a drastic reduction of the leakage current. However, O{sub 2} plasma cannot reduce the leakage current of the Ta{sub 2}O{sub 5} film crystallized at 700 C. This leakage current is not due to C and H, but rather to Si penetrated into the Ta{sub 2}O{sub 5} film from the underlying poly-Si electrode. Therefore, the amorphous Ta{sub 2}O{sub 5} film treated by O{sub 2} plasma is most suitable for use in stacked capacitors.

Density relaxation of silicon dioxide on (100) silicon during thermal annealing

Measurements of the refractive index of thermally grown oxide after annealing reveal that the density relaxation of the oxide is well described by a stretched exponential decay function. The experiments of two‐step oxidation show that oxygen diffusivity in the oxide exponentially decreases with the oxide density. The relation between the oxide density and the refractive index is well expressed by a simple power relation rather than the Lorenz–Lorenz formula.

Ultrathin Ta2 O 5 Film Capacitor with Ru Bottom Electrode

The characteristics of the ultrathin Ta 2 O 5 film capacitors with Ru bottom electrodes have been investigated. Ru films are deposited on n-Si by sputtering in 10% O 2 /Ar ambient, for the bottom electrode, and Ta 2 O 5 films are deposited by chemical vapor deposition using Ta(OC 2 H 5 ) 5 and O 2 . By O 2 plasma treatment at 400°C after N 2 thermal treatment at 700°C, excellent properties, are obtained such that the effective SiO 2 film thickness is 0.68 nm for 6 nm thick Ta 2 O 5 film and the leakage current is less than 1 × 10 8 A/cm 2 between the range of -2.1 and +1.8 V. The Ta 2 O 5 film with a Ru bottom electrode is one of the most suitable structures for Gbit dynamic random access memory capacitors.

Novel Pb(Ti, Zr)O3(PZT) Crystallization Technique Using Flash Lamp for Ferroelectric RAM (FeRAM) Embedded LSIs and One Transistor Type FeRAM Devices

A novel method of ferroelectric capacitor formation for Ferroelectrie random access memory (FeRAM) embedded LSIs and one-transistor-type FeRAMs has been developed. Amorphous Pb(Ti, Zr)O3(PZT) films were successfully transformed to the perovskite phase by a flash lamp technique with a crystallization time of 1.2 ms at a substrate temperature of 350°C. A flash lamp energy of 27 J/cm2 was sufficient to form a ferroelectric crystal structure due to rapid thermal effects with little heat diffusion in the depth direction. This technique enabled PZT film crystallization in Pt/PZT/Pt structures on multi-Al wiring layers. Granular PZT grains were observed on Pt, Ru and RuO2 electrodes, which indicates that crystal growth begins from the film surfaces. Ferroelectric property was verified by the process at 350°C maximum temperature. PZT films were also crystallized directly on SiO2. This is useful for the fabrication of embedded FeRAM devices and 1Tr-type FeRAMs. The flash lamp process was found to have great potential for application to dielectric film formation, annealing processes and so on.

Characteristics of (Ba, Sr)TiO3 Capacitors with Textured Ru Bottom Electrode

Ru films were fabricated by dc magnetron sputtering in an Ar/O2 mixture ambient in order to examine the Ru films as electrodes of Ba0.5Sr0.5TiO3 (BST) thin-film capacitors. The 100-nm-thick Ru film deposited on Si at 450?C at an O2/(Ar+O2) flow ratio of 40% at 0.5 kW was textured along c-axis. The full-width at half maximum (FWHM) of 3.14? was obtained for the Ru (002) diffraction peak in an X-ray diffraction (XRD) pattern. BST films deposited on the Ru bottom electrode were also textured along (110). The relative dielectric constant of BST films increased with a decrease in the FWHM of BST (110). The relationship between electrical properties of Ru/BST/Ru capacitors and the orientation of the Ru bottom electrode and BST films was also investigated.

Si-SiO2 interface structures: chemical shifts in Si 2p photoelectron spectra

From XPS depth profiling of silicon oxide films formed on (100), (110) and (111) surfaces prepared by various oxidation processes, the effect of crystallographic orientation on the chemical shift was found, and the effects of oxidation temperature, oxidation atmosphere and annealing on the chemical shifts were found to be small. These results imply that chemical shifts are weakly affected by the change in the Si-O-Si bond angle near the Si-SiO2 interface.

Ru Electrode Deposited by Sputtering in Ar/O2 Mixture Ambient.

Ru films are fabricated by dc magnetron sputtering in an Ar/O2 ambient, as the bottom electrodes of Ba0.5Sr0.5TiO3 thin film capacitors. The Ru films deposited on Si in an Ar/O2 mixture ambient show low resistivity and low film stress and do not form Ru2Si3 following thermal processing even at 700°C. It becomes clear that there exist very thin amorphous and crystalline layers composed of Ru, Si and O between the Ru films and Si in the case of Ru films deposited in an Ar/O2 ambient. A low contact resistance at the Ru/n+-Si interface is obtained after annealing at 700°C. An effective SiO2 film thickness of 0.42 nm is obtained for an actual Ba0.5Sr0.5TiO3 film thickness of 42 nm with a leakage current of less than 1×10-8 A/cm2 in the range between -1.5 V and +1.8 V for a Ru/Ba0.5Sr0.5TiO3/Ru/n+-Si capacitor without a barrier metal layer.

Structural studies of ultrathin silicon oxides and their interfaces by XPS

Abstract From XPS depth profiling of silicon oxide films formed on (100), (110) and (111) surfaces prepared by various oxidation processes, the orientation dependent structure of ultrathin silicon oxide films, the weak stress relaxation effect on the chemical shift and the orientation dependent oxidation-induced chemical shifts were studied.

Composition Control of Barium Strontium Titanate Thin Films Prepared by Chemical Vapor Deposition

Composition control of barium strontium titanate thin films prepared by chemical vapor deposition using Ba(THD)2, Sr(THD)2, and Ti(THD)2(i-OC3H7)2 (THD: 2,2,6,6-tetramethyl-3,5-heptanedionate) has been studied by investigating the effects of deposition temperature and the supply rates of metal sources. Although there were complicated correlations among the deposition rates of Ba, Sr, and Ti under kinetically limited conditions, the deposition rate ratio of Ba/Sr was linearly related to the partial pressure ratio of Ba(THD)2/Sr(THD)2, and the ratio of Ti/(Ba+Sr) spontaneously approaches unity. In order to explain these characteristics, we proposed a model wherein the THD complex of Ba or Sr reacts with the THD complex of Ti and generates a dimer such as (THD)2Ba–O–Ti(THD)2, and this dimer becomes the dominant precursor for chemical vapor deposition (CVD) under kinetically limited conditions.

Time-Dependent Leakage Current of BaSrTiO3 Film under High Temperature Bias Stress

Time-dependent leakage currents are investigated under high voltage stress to BaSrTiO3 thin film at high temperatures. Under the application of negative gate stress, the leakage current increases once and decreases after reaching the maximum point. Under the application of positive gate stress, the leakage current steeply increases after a given time. Based on current-voltage characteristics observed following the application of positive and negative stresses and current-time characteristics at low voltage, the degradation mechanism is discussed.