Elpida Memory, Inc announced the successful development of a new polymetal gate process for high-speed DRAM. The new process " refines Elpida's low-resistance polymetal technology applied for 1 Gigabit DDR2 SDRAM " achieving higher transistor performance and improved data retention time essential for obtaining higher yields and improved long-term memory cell reliability. This new technology will allow Elpida to develop more advanced high-speed DRAM products that are both higher-performance and cost-effective.
Elpida's New Polymetal Gate Technology Scaling of a design rule is causing a rapid decrease in the size of the memory cells that store data. However, the threshold voltage of the memory cell transistor must be held at a certain level regardless of its size, resulting in a higher concentration of impurities in the substrate. Doping concentration increase causes a higher electric field in the memory cell, leading to problems such as degraded data retention due to leakage of electric charge and loss of long-term cell reliability. These problems, in turn, cause a drop in production yield. In addition, to achieve high-speed DRAM such as DDR2, the performance of the transistors used in the memory cells and the peripheral circuits that drive the memory cells must be enhanced.
Elpida's new polymetal gate technology improves the operating speed, data retention time and long-term reliability by enhancing transistor performance in both the memory cells and their peripheral circuits.
Elpida's new polymetal gate technology offers the following features:
1. Improved memory cell transistors
By using a gate polysilicon doped with the opposite conductivity type, the impurity concentration of the substrate can be reduced while leaving the transistor threshold voltage level unchanged. The electric field in the memory cell can, therefore, be moderated allowing for enhanced data retention and long-term transistor reliability. A drop in the parasitic resistance also contributes to a 40% increase in the drive current of the transistors, allowing for higher-speed operation.
2. Improved performance of peripheral cell transistors Until now, DRAMs have been manufactured using a specific high-temperature heating process which made it impossible to use the same type of high-performance transistors as those used in microprocessors. However, in Elpida's new polymetal gate technology, the temperature of the heating process was lowered and the gate insulating film was changed from an oxide film to an oxynitride film, allowing microprocessor-type transistors to be used for the peripheral circuits, leading to improved transistor performance.
Elpida can now develop high-performance DRAM at a lower cost by incorporating these two new enhancements into its original, low-resistance polymetal gate process. Elpida has also confirmed the operation of 512 Megabit DDR2 SDRAM (at 667 Mbps) using the new technology in application trials, and it will incorporate the technology into high-speed DRAM products slated for mass production during 2004. Elpida gave a presentation on its new polymetal gate technology enhancements at the 2003 International Electron Devices Meeting (IEDM) on December 8, 2003 in Washington DC.