Noboru Mikami

Model of leakage characteristics of (Ba, Sr)TiO3 thin films

We have investigated the dependence of leakage current and capacitance of Pt/Ba0.5Sr0.5TiO3/Pt capacitors on annealing temperature under high vacuum conditions. It is observed that leakage currents increase asymmetrically for negative and positive bias voltage with increasing annealing temperature. A model of leakage current and capacitance characteristics has been proposed on the assumption of generation of oxygen vacancies by annealing at the interfaces of the dielectric film adjacent to the Pt electrodes. The model predicts the oxygen vacancies of about 1020u2009cm−3.

Novel stacked capacitor technology for 1 Gbit DRAMs with CVD-(Ba,Sr)TiO/sub 3/ thin films on a thick storage node of Ru

Simple stacked cell capacitors for 1 Gbit DRAMs have been constructed with a thick storage node of ruthenium (Ru) and high dielectric constant CVD-(Ba,Sr)TiO/sub 3/ films of equivalent oxide thickness teq=0.50 nm with an excellent step coverage. A storage capacitance of 30 fF/cell was shown to be realized for storage nodes 0.24 /spl mu/m wide, 0.60 /spl mu/m deep, and 0.20 /spl mu/m high, with a charge loss below 19% of the initial stored charge during a pause-refresh time of 10 s in 1 Gbit DRAMs.

Fiber-optic current and voltage sensors using a Bi 12 GeO 20 single crystal

By using Faraday and Pockels effects of a Bi 12 GeO 20 (BGO) single crystal, two kinds of fiber-optic sensors for electric current and voltage were tentatively fabricated. The optimum design of these sensors and their performances were investigated. It was verified that the BGO crystal is very available for either of the current and voltage sensors with regard to their sensitivity and stability. As for the experimental results, the minimum detectable magnetic field of 10-2Oe and the temperature dependence within ±2 percent in the temperature range of -25 to 85°C was obtained for the current sensor, while for the voltage sensor the sensitivity of 10-3V and the temperature dependence within ± 0.5 percent was in the same temperature range.

Temperature dependence of magnetic field sensors using (Cd1−xMnx)Te and a light‐emitting‐diode light source

Verdet constants and optical transmissions of (Cd1−xMnx)Te solid solutions are determined as a function of Mn concentration, temperature, and wavelength. The sensitivity or modulation depth of magnetic field sensors using Faraday rotation in magneto‐optical materials is analyzed using an effective Verdet constant. The effective Verdet constant is defined by a weighted average of normal Verdet constants. The average is over the spectral profile of the light source and is weighted by the intensity of transmitted light through the material and filter. The temperature dependence of the sensitivity in the sensors using (Cd,Mn)Te in various Mn concentrations and light‐emitting‐diode light sources is discussed.

Influence of Ti Sources on Properties of (Ba,Sr)TiO 3 Films Prepared by Liquid Source CVD

We have investigated influences of Ti sources on properties of (Ba,Sr)TiO 3 [BST] films prepared on Pt/SiO 2 /Si substrates by liquid source chemical vapor deposition (liquid source CVD): TiO(DPM) 2 [titanyl bis (dipivaloylmethanato), TiO(C 11 H 19 O 2 ) 2 ], Ti(O-i-Pr) 2 (DPM) 2 [bis (isopropoxy) bis (dipivaloylmethanato) titanium, Ti(o-i-C 3 H 7 )2(C 11 H 19 O 2 ) 2 ], and TTIP [titanium tetraisopropoxide, Ti(O-i-C 3 H 7 ) 4 ]. Improved electrical properties and step coverage were obtained using TiO(DPM) 2 at a substrate temperature T S =420C and a reactor pressure P=1.5Torr as follows: a dielectric constant e=210, equivalent SiO 2 thickness t eq =0.51nm, leakage current density J L =6.6×10∼ −8 A/cm 2 at +1.1V, dielectric loss tan δ=0.007, and coverage of 0.8. However, hillocks appeared on the BST film surface under these conditions, while the film surfaces were relatively smooth using TTIP. The density of these hillocks appeared to be related to the BST(110) peak intensity of the X-ray diffraction pattern.

Electrical Properties and Film Structures of (BaxSr1−x)TiO3 Thin Films by RF Sputtering

Thin films of (Ba x Sr j-x )TiO 3 (BST) deposited by RF sputtering have been studied for the application to the capacitor material of dynamic random access memory (DRAM). The effects of film structures on the dielectric constant and the leakage current were investigated. The films deposited at 550–600°C consist of granular grains, and the films deposited above 650°C contain columnar grains. The dielectric constant is related to the grain size in the direction of thickness. The leakage current increases as the film structure changes from granular to columnar. The low leakage current (less than 10 −8 A/cm 2 at +2 and -2 V) and the small value of equivalent SiO 2 thickness (0.9 nm) were attained in the (Ba 065 Sr 035 ) TiO 3 thin films.

Ferroelectric liquid crystal cell having gray scale for high-speed spatial light modulator

Gray scale was obtained with a ferroelectric liquid crystal (FLC) cell in which the twisted state of FLC molecules is gradually unwound by an applied voltage. The cell has an extinction ratio of 60 at 632. 8 nm and a response time of 300 jtsec. A spatial light modulator (SLM) with 256 x 256 pixels was fabricated by incorporating the FLC cell with an amorphous silicon thin film transistor array. This offers us a promising technology to realize a FLC-SLM with gray scale a fast frame time of 300 psec and a pixel number as large as 1000 x 1000.

(Ba,Sr)TiO3 Films and Process Integration for Dram Capacitor

The memory capacity of the DRAM, a major application of LSI and the technology driving semiconductor devices, has quadrupled every three years. To increase device integration, we must solve not only the engineering problems, such as fine-patterning, microtransistor operation and wiring life, but also the problem of capacitor. The storage unit of a DRAM (cell) consists of a capacitor, which accumulates electric charge, and a switching transistor; accordingly, a certain amount of accumulated electric charge must be ensured to restrict device operation while the cell area is being reduced by miniaturization. To ensure accumulated electric charge sufficient for greater DRAM integration, capacitor dielectric film thickness has been reduced, electrode area has been increased by applying a three-dimensional structure, and capacitor materials have been changed to materials with high dielectric constants (from SiO2 films to so-called ON films obtained by oxidizing Si3N4).

(Ba,Sr)TiO/sub 3/ capacitor technology for Gbit-scale DRAMs

Great efforts have been made for the integration of high dielectric constant (Ba,Sr)TiO/sub 3/ (BST) capacitors into DRAMs. This paper presents the current state of the art in BST capacitor technology for Gbit-scale DRAMs, with emphasis on key technical issues for process integration, including electrode materials, barrier layers, and also BST films themselves. The problems which may remain to be solved are also discussed to realize the goal: a barrier layer on top electrode for back-end processes, reliability of integrated BST capacitors, and further improvement of coverage properties of BST films and cell-plate metals.

ELECTRICALLY CONTROLLED CONTINUOUS TILT OF OPTICAL AXIS IN FERROELECTRIC CHIRAL SMECTIC LIQUID CRYSTAL CELL

A continuous tilt of an apparent optical axis caused by an applied ac voltage in a ferroelectric chiral smectic C liquid crystal cell is observed. The cell under crossed Nicols modulates the incident light with the voltage applied between −10 and 10 V. Dispersions of the optical modulation intensity and the dielectric constant have the same relaxation frequency (2 kHz). Observation of the dielectric behavior of the cell suggests that the relaxation in the optical modulation corresponds to that of the Goldstone mode. The deformation of the splay structure in the liquid crystal cell supposedly causes the continuous tilt of apparent optical axis.