Yoshifumi Kawamoto

A fully depleted lean-channel transistor (DELTA)-a novel vertical ultrathin SOI MOSFET

A fully depleted lean-channel transistor (DELTA) that has a gate with a vertical ultrathin SOI structure is reported. In the deep submicrometer region, selective oxidation is useful in realizing SOI isolation. It provides high crystalline quality, as good as that of conventional bulk single-crystal devices. Using experiments and three-dimensional simulation, it was shown that the gate structure has effective channel controllability and its vertical ultrathin SOI structure provides superior device characteristics.<<ETX>>

An experimental 1.5-V 64-Mb DRAM

Low-voltage circuit technologies for higher-density dynamic RAMs (DRAMs) and their application to an experimental 64-Mb DRAM with a 1.5-V internal operating voltage are presented. A complementary current sensing scheme is proposed to reduce data transmission delay. A speed improvement of 20 ns was achieved when utilizing a 1.5-V power supply. An accurate and speed-enhanced half-V/sub CC/ voltage generator with a current-mirror amplifier and tri-state buffer is proposed. With it, a response time reduction of about 1.5 decades was realized. A word-line driver with a charge-pump circuit was developed to achieve a high boost ratio. A ratio of about 1.8 was obtained from a power supply voltage as low as 1.0 V. A 1.28 mu m/sup 2/ crown-shaped stacked-capacitor (CROWN) cell was also made to ensure a sufficient storage charge and to minimize data-line interference noise. An experimental 1.5 V 64 Mb DRAM was designed and fabricated with these technologies and 0.3 mu m electron-beam lithography. A typical access time of 70 ns was obtained, and a further reduction of 50 ns is expected based on simulation results. Thus, a high-speed performance, comparable to that of 16-Mb DRAMs, can be achieved with a typical power dissipation of 44 mW, one tenth that of 16-Mb DRAMs. This indicates that a low-voltage battery operation is a promising target for future DRAMs. >

A fully depleted lean-channel transistor (DELTA)-a novel vertical ultra thin SOI MOSFET

A fully depleted lean channel transistor (DELTA) having a gate structure and vertical ultrathin SOI (silicon-on-insulator) structure with selective field oxide is reported. In the deep submicron region, selective oxidation is useful for achieving SOI isolation. It provides a high-quality crystal and a Si-SiO/sub 2/ interface as good as those of conventional bulk single-crystal devices. Using experiments and simulation, it was shown that the gate structure of DELTA has effective channel controllability and its vertical ultrathin (<0.2- mu m) SOI structure provides superior device characteristics, e.g. the reduction of short channel effects, minimized subthreshold swing, and high transconductance. The DELTA layout is consistent with the conventional ULSI circuit layout. Thus, DELTA offers both consistency with conventional MOSFETs and good scalability As a 3-D device. As a result, DELTA provides a promising approach for MOSFET structures of less than 0.1 mu m in size.<<ETX>>

Promising storage capacitor structures with thin Ta/sub 2/O/sub 5/ film for low-power high-density DRAMs

To ensure the required capacitance for low-power DRAMs (dynamic RAMs) beyond 4 Mb, three kinds of capacitor structures are proposed: (a) poly-Si/SiO/sub 2//Ta/sub 2/O/sub 5//SiO/sub 2//poly-Si or poly-Si/Si/sub 3/N/sub 4//Ta/sub 2/O/sub 5//SiO/sub 2//poly-Si (SIS), (b) W/Ta/sub 2/O/sub 5//SiO/sub 2//poly-Si (MIS), and (c) W/Ta/sub 2/O/sub 5/W (MIM). The investigation of time-dependent dielectric breakdown and leakage current characteristics indicates that capacitor dielectrics that have equivalent SiO/sub 2/ thicknesses of 5, 4, and 3 nm can be applied to 3.3-V operated 16-Mb DRAMs having stacked capacitor cells (STCs) by using SIS, MIS, and MIM structures, respectively, and that 3 and 1.5 nm can be applied to 1.5-V operated 64-Mb DRAMs having STCs by using MIS and MIM structures, respectively. This can be accomplished while maintaining a low enough leakage current for favorable refresh characteristics. In addition, all these capacitors show good heat endurance at 950 degrees C for 30 min. Therefore, these capacitors allow the fabrication of low-power high-density DRAMs beyond 4 Mb using conventional fabrication processes at temperatures up to 950 degrees C. Use of the SIS structure confirms the compatability of the fabrication process of a storage capacitor using Ta/sub 2/O/sub 5/ film and the conventional DRAM fabrication processes by successful application to the fabrication process of an experimental memory array with 1.5- mu m*3.6- mu m stacked-capacitor DRAM cells. >

Crown-shaped stacked-capacitor cell for 1.5-V operation 64-Mb DRAMs

A self-aligned stacked-capacitor cell called the CROWN cell (a crown-shaped stacked-capacitor cell), used for experimental 64-Mb-DRAMs operated at 1.5 V, has been developed using 0.3- mu m electron-beam lithography. This memory cell has an area of 1.28 mu m/sup 2/. The word-line pitch and sense-amplifier pitch of this cell are 0.8 and 1.6 mu m, respectively. In spite of this small cell area, the CROWN cell has a large capacitor surface area of 3.7 mu m/sup 2/ because (1) it has a crown-shaped capacitor electrode, (2) its capacitor is on the data line, and (3) it has a self-aligned memory cell fabrication process and structure. The large capacitor area and a Ta/sub 2/O/sub 5/ film equivalent to a 2.8-nm SiO/sub 2/ film ensure a large storage charge of 33 fC (storage capacitance equals 44 fF) for 1.5-V operation. A small CROWN cell array and a memory test circuit were successfully used to achieve a basic DRAM cell operation. >

The impact of data-line interference noise on DRAM scaling

A kind of data-line (DL) interference noise in a scaled DRAM cell array is found and studied through analysis. The dynamic behavior of cell arrays due to sense-amplifier operation is derived analytically. Analysis shows that the amount of interference noise is more than three times larger than expected from simple data-line coupling. A novel experimental technique for precise noise determination is developed to verify the analysis. Analytical results are in good agreement with the experimental data. It is found that the interference noise plays a dominant role in determining the operating margin of the DRAM and that a novel process or a cell array architecture for minimizing the interference noise is indispensable in 16-Mb DRAM and beyond. >

A 1.5 V DRAM for battery-based applications

The authors report low-power, high-signal-to-noise-ratio (SNR) 16 Mbit DRAM (dynamic RAM) techniques which allow 1.5-V battery operation. To reduce power consumption, the data-line voltage swing is the sum of the threshold voltages for nMOS and pMOS transistors in the sense amplifier. A plate-pulse circuit technique, a three-level word pulse, and a 3.4- mu m/sup 2/ data-line shielded STC cell enhance SNR in the memory array. The main features of the DRAM are compared with those of the SNB (storage-node-boosted) technique and a conventional half-V/sub CC/ circuit technique.<<ETX>>

A new stacked capacitor DRAM cell characterized by a storage capacitor on a bit-line structure

The authors introduce a diagonal active stacked capacitor cell with a highly packed storage node (DASH) for use in a 16-Mb DRAM (dynamic random access memory). This novel cell features a storage capacitor formed above a bit line and the diagonal active area, which provides a large storage capacitance, 35 fF/bit, in a cell size of 3.4 mu m/sup 2/. The average charge retention time measured using an experimental 2-kb array is 30 s at 40 degrees C, indicating that the DASH has a superior potential for application to 16-Mb DRAMs. The memory cell leakage current is controlled to the order of 10/sup -12/ A.<<ETX>>

A 60-ns 16-Mbit CMOS DRAM with a transposed data-line structure

Low-noise, high-speed circuit techniques for high-density DRAMs (dynamic random-access memories), as well as their application to a single 5-V 16-Mb CMOS DRAM with a 3.3-V internal operating voltage for a memory array, are described. It was found that data-line interference noise becomes unacceptably high (more than 25% of the signal) and causes a serious problem in 16-Mb DRAM memory arrays. A transposed data-line structure is proposed to eliminate the noise. Noise suppression below 5% is confirmed using this transposed data-line structure. A current sense amplifier is also proposed to maintain the data-transmission speed in common I/O lines, in spite of a reduced operating voltage and increased parasitic capacitance loading in the memory array. A speed improvement of 10 ns is achieved. Using these circuit techniques, a 16-Mb CMOS DRAM with a typical RAS access time of 60 ns was realized. >

Dual-operating-voltage scheme for a single 5-V 16-Mbit DRAM

A dual-operating-voltage scheme (5 V for peripheral circuits and 3.3 V for the memory array) is shown to be the best approach for a single 5-V 16-Mb DRAM (dynamic random-access memory). This is because the conventional scaling rule cannot apply to DRAM design due to the inherent DRAM word-line boosting feature. A novel internal voltage generator to realize this approach is presented. Its features are the switching of two reference voltages, a driver using a PMOS-load differential amplifier, and the word-line boost based on the regulated voltage, which can ensure a wider memory margin than conventional circuits. This approach is applied to an experimental 16-Mb DRAM. A 0.5% supply-voltage dependency and 30-ns recovery time are achieved. >