R. Nagai

256-Mb DRAM circuit technologies characterized by low power and high fabrication yield for file applications are described. The newly proposed and developed circuits are a self-reverse-biasing circuit for word drivers and decoders to suppress the subthreshold current to 3% of the conventional scheme, and a subarray-replacement redundancy technique that doubles chip yield and consequently reduces manufacturing costs. An experimental 256-Mb DRAM has been designed and fabricated by combining the proposed circuit techniques and a 0.25- mu m phase-shift optical lithography, and its basic operations are verified. A 0.72- mu m/sup 2/ double-cylindrical recessed stacked-capacitor (RSTC) cell is used to ensure a storage capacitance of 25 fF/cell. A typical access time under a 2-V power supply voltage was 70 ns. With the proper device characteristics, the simulated performances of the 256-Mb DRAM operating with a 1.5-V power supply voltage are a data-retention current of 53 mu A and an access time of 48 ns.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The hot carrier instability and the related device characteristics of Leff= 1µm MOSFETs with Lightly Doped Drain (LDD) structure is evaluated in detail. For the n- dose below 1E13/cm2, a new type of IBB and IG increase was found when the gate bias, VG, was increased toward and over the drain bias, VD, and related new modes of hot carrier instability were confirmed. The instability for the lower VG stress is attributed to the charge build-up at the n- drain region, while the instability for the larger VG stress is attributed to the oxide degradation at both source and drain regions. The device characteristics and the mechanism of instability for n→= 1E13/cm2 are similar to those of conventional devices. It is shown that the instability inherent to the LDD structure can be suppressed by optimizing the n-dose. Thereby, it is important that the lateral electric field peak remains under the gate.

With the arrival of the multimedia era, high-data-rate memory LSIs are becoming increasingly important to keep up with high-speed CPUs, graphics processors, and other consumers of stored data. Video editing and replaying of high-definition television in particular require a high bandwidth. This paper presents two circuit technologies for a synchronously operating high-data-rate 1 Gb DRAM: a distributed-column-control architecture reducing the burst-mode cycle time, and a ringing-canceling output buffer ensuring reliable high-speed data transfer.

The authors describe 256-Mb DRAM (dynamic random access memory) technologies for file applications. A subthreshold-current limiting scheme for word drivers is shown. The scheme uses a pMOS switching transistor between the wordline voltage and the driver transistor common-source terminal. The subthreshold current of a 256-Mb DRAM is reduced to 3% by applying this scheme to word drivers and decoders, and the total data-retention current is less than that of a 64-Mb DRAM. A redundancy technique is shown which features subarray-by-subarray replacement instead of the conventional line-by-line replacement. To evaluate the circuit technologies described here, an experimental 256-Mb DRAM was fabricated using 0.25- mu m CMOS technology with phase-shift lithography. It uses a 0.72- mu m/sup 2/ RSTC cell with a storage capacitance of 25 fF and operates on a voltage of 1.5-3.6 V.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>