Koichi Takemura

SrTiO3 Thin Film Preparation by Ion Beam Sputtering and Its Dielectric Properties

SrTiO3 thin films have been prepared on Pd-coated sapphire substrates by ion beam sputtering of a SrTiO3 target, and their dielectric properties have been studied. Oxygen flow introduction was necessary to obtain good insulating films. Dielectric constant er values were 190 and 240 for 430°C and 540°C substrate temperatures, respectively. These er values were not dependent on film thickness in the range from 200 nm down to 50 nm. A 53 nm-thick film indicated leakage current density of less than 10-8 A/cm2 at up to 2 V, along with a 230 er value.

RuO2/TiN-based storage electrodes for (Ba, Sr)TiO3 dynamic random access memory capacitors

Sputtered (Ba, Sr)TiO 3 (BST) thin film capacitors have been fabricated with thick RuO 2 /TiN-based storage electrodes and poly-Si contact plugs, and the electrical properties of the storage electrodes have been studied. The electrode height was higher than 450 nm and the contact size was 0.8 x 0.8 μm 2 . Resistance of the storage electrodes including contact plugs can be evaluated from the dispersion observed in capacitance-frequency measurements. TiN oxidation at the RuO 2 /TiN interface and native oxide at the TiN/Si contact contribute to the electrode resistance of RuO 2 /TiN electrodes. With increasing BST deposition temperature, the thickness of oxidized TiN in RuO 2 /TiN electrbdes increases and the electrode resistance increases correspondingly. A Ru layer inserted at the RuO 2 /TiN interface, a TiN/TiSi 2 /Si junction and rapid thermal annealing in N 2 ambient of the TiN layer are effective ways to reduce the resistance of RuO 2 /TiN-based electrodes.

A stacked capacitor technology with ECR plasma MOCVD (Ba,Sr)TiO/sub 3/ and RuO/sub 2//Ru/TiN/TiSi/sub x/ storage nodes for Gb-scale DRAMs

A Gb-scale DRAM stacked capacitor technology with (Ba,Sr)TiO/sub 3/ thin films is described, The four-layer RuO/sub 2//Ru/TiN/TiSi/sub x/, storage node configuration allows 500/spl deg/C processing and fine-patterning down to the 0.20 /spl mu/m size by electron beam lithography and reactive ion etching. Good insulating (Ba/sub 0.4/Sr/sub 0.6/)TiO/sub 3/ (BST) films with an SiO/sub 2/ equivalent thickness of 0.65 nm on the electrode sidewalls and leakage current of 1/spl times/10/sup -/6 A/cm/sup 2/ at 1 V are obtained by ECR plasma MOCVD without any post-deposition annealing, A lateral step coverage of 50% for BST is observed on the 0.2 /spl mu/m size storage node pattern, and the BST thickness on the sidewalls is very uniform, thanks to the ECR downflow plasma. Using this stacked capacitor technology, a sufficient cell capacitance of 25 fF for 1 Gb DRAMs can be achieved in a capacitor area of 0.125 /spl mu/m/sup 2/ with only the 0.3 /spl mu/m high-storage electrodes.

Imprint Characteristics of SrBi2Ta2O9 Thin Films with Modified Sr Composition

Imprint characteristics of SrBi2Ta2O9 (SBT) thin films with modified Sr composition were investigated under elevated temperatures. A significant shift in readout charge was observed after applying unipolar pulses (dynamic imprint test) at above 100°C, while only a small shift was detected even when the capacitors were held at 150°C for up to 104 s without any electrical signals (static imprint test). The charge shift was due to the internal voltage formed by applying the unipolar pulses. Its increase, that is, degradation of dynamic imprint endurance with increasing temperature is attributed to both an increase of the internal voltage and a decrease of the coercive voltage (Vc). There was no significant difference in the internal voltage between Sr deficient and Sr non-deficient SBT. SBT with a Sr deficient composition had a relatively high dynamic imprint endurance compared to the Sr non-deficient films because of higher remanent polarization and larger Vc.

Effect of Ar Gas Pressure on Magnetic Properties of Sputter-Deposited Fe–Cu Alloys

Magnetization and Mossbauer effect measurements were made for Fe1-xCux alloys sputter-deposited on water-cooled substrates under Ar gas pressures, PAr, of 8.9 and 0.23 Pa. The scanning electron micrograph (SEM) shows a columnar structure for PAr=8.9 Pa and a fibrous structure for PAr = 0.23 Pa. A perpendicular magnetic anisotropy is observed and the coercive force, Hc, is large for PAr = 8.9 Pa, while an in-plane magnetic anisotropy is observed and Hc is small for PAr =0.23 Pa. The magnetization and average hyperfine field for PAr = 8.9 Pa are smaller than those for PAr = 0.23 Pa. These differences can be ascribed to the presence of CuFeO2, which is detected in the paramagnetic Mossbauer spectra in the Cu rich alloys formed under PAr = 8.9 Pa.

Magnetic and structural properties of nonequilibrium Fe-Cu and Fe-Ag alloys produced by vapor quenching

Fe-rich Fe-Cu and Fe-Ag alloy films were sputter-deposited on water-cooled substrates under Ar gas at pressures, P Ar , between 1.5 and 80 mTorr. X-ray diffraction, scanning electron micrograph (SEM), Mossbauer effect and magnetization measurements were made on these films. With the increase in P Ar , the single bcc phase is extended for Fe-Ag alloys and is almost independent of P Ar for Fe-Cu alloys. The alloy films sputter-deposited under low P Ar have a fibrous structure in the SEM image and show an in-plane magnetic anisotropy in the Mossbauer spectrum, while those under high P Ar have a columnar structure in the SEM image and show a perpendicular magnetic anisotropy in the Mossbauer spectrum. Especially in bcc and fcc mixed Fe-Cu alloys sputter-deposited under high P Ar , the magnetization curve perpendicular to the film plane is almost the same as that parallel to the film plane.