J.H. Yi

Highly manufacturable high density phase change memory of 64Mb and beyond

Highly manufacturable 64Mbit PRAM has been successfully fabricated using N-doped Ge/sub 2/Sb/sub 2/Te/sub 5/ (GST) and optimal GST etching process. Using those technologies, it was possible to achieve the low writing current of 0.6 mA and clear separation between SET and RESET resistance distributions. The 64Mb PRAM was designed to support commercial NOR flash memory compatible interfaces. Therefore, the fabricated chip was tested under the mobile application platform and its functionality and reliability has been evaluated by operation temperature dependency, disturbance, endurance, and retention. Finally, it was clearly demonstrated that high density PRAM can be fabricated in the product level with strong reliability to produce new nonvolatile memory markets.

Full integration and reliability evaluation of phase-change RAM based on 0.24 /spl mu/m-CMOS technologies

We have fully integrated a nonvolatile random access memory by successfully incorporating a reversibly phase-changeable chalcogenide memory element with MOS transistor. As well as basic characteristics of the memory operation, we have also observed reliable performances of the device on hot temperature operation, endurance against repetitive phase transition, writing imprint, reading disturbance and data retention.

Novel cell structure of PRAM with thin metal layer inserted GeSbTe

We have developed a novel cell structure of PRAM with metal interlayer. This novel structure has been proposed to solve the over-programming fail. We have examined the cause of over-programming by simulation of the phase transition of chalcogenide and successfully demonstrated reliable cell operation of this novel structure in writing current level, crystallization speed, and endurance. It can be explained by a model in which the metal interlayer is a local heat sink and the top GST layer is a thermal insulator.

Writing current reduction for high-density phase-change RAM

By developing a chalcogenide memory element that can be operated at low writing current, we have demonstrated the possibility of high-density phase-change random access memory. We have investigated the phase transition behaviors as a function of various process factors including contact size, cell size and thickness, doping concentration in chalcogenide material and cell structure. As a result, we have observed that the writing current is reduced down to 0.7 mA.

Phase-change chalcogenide nonvolatile RAM completely based on CMOS technology

We have integrated a phase-change chalcogenide random access memory, completely based on 0.24 /spl mu/m-CMOS technologies. A twin cell and BL clamping circuits are introduced to enlarge fabrication tolerance and to reduce cell perturbation during reading operation. To draw back current as much as possible, Co salicidation is also applied to transistor formation. By constructing a simple cell structure with Ge/sub 2/Sb/sub 2/Te/sub 5/, we have observed reliable phase-transitions by driving current through MOS transistors. With 100 ns-writing pulses of 2 mA for RESET and 0.6 mA for SET, the device operates successfully with a considerable sensing signal at reading voltage of as low as 0.2 V.

CMOS device scaling beyond 100 nm

CMOS device scaling beyond 100 nm has been investigated. Issues on scaling and previous works to solve them were reviewed. Super steep retrograde channel formation using selective epitaxial growth of undoped silicon effectively suppressed short channel effect and improved transconductance. The stack gate dielectrics of oxynitride and nitride suppressed boron penetration and improved drive currents. Transistor characteristics and reliability issues on gate oxide scaling were investigated in the regime of large gate leakage currents. High performance CMOS transistors of L/sub gate/=70 nm and T/sub ox/=1.4 nm were fabricated, which showed current drives of 860 /spl mu/A//spl mu/m (NMOS) and 350 /spl mu/A//spl mu/m (PMOS) at I/sub off/=10 nA//spl mu/m and V/sub dd/=1.2 V.

On the gate oxide scaling of high performance CMOS transistors

The gate oxide scalability of high performance CMOS transistor has been investigated. In terms of gate leakage, the T/sub ox/ can be scaled down to at least 8 /spl Aring/ with I/sub G/ not exceeding I/sub off/ limit suggested by ITRS. To reduce boron penetration, remote-plasma-nitridation (RPN) oxides were studied. Devices with RPN oxides showed excellent resistance against boron penetration, improved hole mobility, reduced gate leakage, and improved transistor performance. The gate oxide scalability can be extended using the RPN process.

Scalable Two-Transistor Memory (STTM)

A novel memory device called Scalable Two-Transistor Memory (STTM) has been developed. STTM is a floating gate device with the writing mechanism of direct tunneling through the multiple tunnel junction (MTJ). STTM has potential advantages of scalability, high density, high speed, long data retention, low voltage operation, low power consumption, and good endurability. We have fabricated and successfully demonstrated the memory cell operation of the STTM for the first time. The STTM unit cell fabricated using 0.16 /spl mu/m silicon processing showed the writing speed of /spl sim/100 ns and the data retention time of /spl sim/200 sec. with the operation voltages of -5/spl sim/5 V. Also, we developed a novel architecture for the high-density STTM cell array with an unit cell size of 4F/sup 2/ and a process scheme to fabricate it.