Hitachi, Renesas Enable Stable Fabrication of Phase-Change Memory

Hitachi Ltd. (NYSE:HIT / TSE:6501) and Renesas Technology Corp. announced the development of a new cell technology that will enable stable fabrication of phase-change memory while maintaining low-power operation performance.

The newly developed cell technology involves forming an interfacial layer of tantalum pent-oxide (Ta2O5) between the plug* that connects to a MOS transistor and the phase-change film, and optimizing the thickness of the interfacial layer. In prototype phase-change memory cells fabricated using this structure, programming operation has been verified with a current of 100 uA at a power supply voltage of 1.5 V. In addition, the excellent adhesion between the Ta2O5 interfacial layer and phase-change film has the potential to provide enhanced stability in memory cell fabrication. The results are being presented at the 2006 International Electron Devices Meeting to be held in San Francisco, U.S.A. from December 11.

Phase-change memory, a type of nonvolatile memory that can be programmed and read, employs Joule heat generated by current and makes use of the difference in the electrical resistance of a film when in an amorphous state** (high resistance) and when in a crystalline state (low resistance). The different electrical resistances represent the data values 1 and 0. Compared with conventional nonvolatile memory types, phase-change memory provides faster programming and read speeds, support for a higher number of rewrite cycles, and lower production cost. It is also well suited to integration, giving it great potential as a next-generation high-integration on-chip nonvolatile memory type.

In conventional phase-change memory it was necessary to heat the phase-change film material to past the melting point during resetting (amorphization). This required a large current of 1 mA or more. Then last year Hitachi and Renesas Technology developed a phase-change film made of oxygen-doped GeSbTe (germanium-antimony-tellurium), and succeeded in fabricating prototype memory cells with lower power requirements. These memory cells require a programming current of only 100 uA when using a power supply voltage of 1.5 V. However, the structure of the cell is such that it is easy for the heat generated in the phase-change film to escape via the plug. This causes the temperature rise during resets to be gradual and was thought to inhibit further reductions in the power requirements. In addition, GeSbTe, the material generally used for the phase-change film, exhibited poor adhesion with the silicon oxide film below it. This raised the need to deal with peeling problems in the phase-change memory fabrication process.

In their efforts to overcome this problem, Hitachi and Renesas Technology have developed a cell technology employing a new configuration. It achieves low-power operation and fabrication stability at the same time.

The newly developed technology uses a cell structure in which an ultrathin Ta2O5interfacial layer is formed between the phase-change film and the plug. This interfacial layer made of Ta2O5 prevents heat diffusion from the phase-change film via the plug. As a result, the temperature rise in the phase-change film is rapid, and the melting point is reached using less power.

In addition, the adhesion between the Ta2O5 interfacial layer and the phase-change film is excellent, increasing the peel strength of the film. Furthermore, by optimizing the method used to form the Ta2O5 layer, it is possible to reduce the resistance variation in a wafer. In this way the issue of GeSbTe peeling during the fabrication process is resolved, and the resistance variation is reduced. The result is greater stability in the memory cell fabrication process.

The phase-change film used in the new memory cells is made of GeSbTe, which is the material in general use for this purpose. The prototype phase-change memory cells employing the new configuration have a verified programming current of 100 uA when using a power supply voltage of 1.5 V. In addition, the resistance variation in a wafer is limited, with a ratio in two digits between the high and low resistance values, and the ability to withstand 100million rewrites has been achieved.

The new technology will promote the realization of the next generation of high-integration on-chip nonvolatile memory and will be expected to lead to new advances in microcontrollers for embedded devices.

About Hitachi
Hitachi America, Ltd., a subsidiary of Hitachi, Ltd., markets and manufactures a broad range of electronics, computer systems and products, and consumer electronics, and provides industrial equipment and services throughout North America. For more information, visit Hitachi, Ltd., (NYSE:HIT)(TOKYO:6501), headquartered in Tokyo, Japan, is a leading global electronics company with approximately 356,000 employees worldwide. Fiscal 2005 (ended March 31, 2006) consolidated sales totaled 9,464 billion yen ($80.9 billion). The company offers a wide range of systems, products and services in market sectors including information systems, electronic devices, power and industrial systems, consumer products, materials and financial services.

About Renesas Technology Corp.
Renesas Technology Corp. is one of the world’s leading semiconductor system solutions providers for mobile, automotive and PC/AV (Audio Visual) markets and the world’s No.1 supplier of microcontrollers. It is also a leading provider of LCD Driver ICs, Smart Card microcontrollers, RF-ICs, High Power Amplifiers, Mixed Signal ICs, System-on-Chip (SoC), System-in-Package (SiP) and more. Established in 2003 as a joint venture between Hitachi, Ltd. (TSE:6501, NYSE:HIT) and Mitsubishi Electric Corporation (TSE:6503), Renesas Technology achieved consolidated revenue of 906 billion JPY in FY2005 (end of March 2006). Renesas Technology is based in Tokyo, Japan and has a global network of manufacturing, design and sales operations in around 20 countries with about 26,200 employees worldwide.

* Plug: A piece of metal material that is embedded in an interlayer connecting hole (through hole).
** Amorphous state: A state of matter in which the atoms and molecules that make up a solid are in an indeterminate arrangement that differs from the regular structure of a crystal. The term noncrystalline is also used to refer to this state.