1T1C 1Mbit FRAM Prototype
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Tokyo, February 9, 2001--- Fujitsu Limited has developed new redundant circuit technology and externally controllable reference circuit technology suitable for mass production of megabit-class ferroelectric random access memory (FRAM®) using 0.5-micron process. The developments mark an important step toward realizing mass-production of high-capacity FRAM able to meet growing demand for high-capacity, multifunction devices in such areas as smart cards.
This technology was announced at the recently concluded ISSCC 2001 (International Solid-State Circuits Conference) held in San Francisco, California.
Background
As a non-volatile memory - which doesn't lose data even when power source is cut - with read and write speeds equivalent to those of SRAM and DRAM, FRAM has attracted considerable attention. However, to ensure reliability, FRAM devices are currently designed with a two transistors, two capacitors (2T2C) structure that has inherently constrained the speed of development for higher density devices. Development of single transistor, single capacitor (1T1C) FRAM architecture has been seen as a way to resolve these structural problems. Yet, because the signal level of stored information that can be detected in the 1T1C structure is only about half the level possible with the 2T2C structure, practical efforts utilize this structure require guaranteeing a stable operating margin and improving yield. Accordingly, improvements have had to be made in both process and circuit design technologies.
Fujitsu's newly devised circuit technologies comprise new methods for saving defective cells: one is a new redundancy technology for replacing defective cells, while the other is an externally controllable reference circuit technology for detecting cells with operating margin defects. Highlights follow.
New Technology Highlights
1) Redundant circuit technology
FRAM devices to date have employed a laser fuse-type fuse ROM to store information regarding the positions of the defective cells, but Fujitsu's new redundant circuit technology enables such information to be stored on the FRAM itself. Furthermore, taking advantage of the special characteristics of FRAM, Fujitsu has devised a selection control circuit that enables selection between ordinary memory domain and redundant memory domain. This method has the following advantages over the current laser fuse method:
- Redundant circuit functions can be incorporated without increasing the number of manufacturing processes.
- The bit storage density of FRAM is about 35 times that of fuse ROM, so redundant circuits can be laid out in a small surface area at a lower cost.
- There is no need for the laser cutting process to store defect information and the laser apparatus used for that purpose, thus lowering manufacturing costs.
- Since FRAM can store more defect information than fuse ROM, the salvage rate is higher, leading to improved yield.
- Since writing and verifying defect information can be performed in the same way as writing and reading on other ordinary cells, it is also easy to mount FRAM on hybrid ICs.
Using the technology described above, Fujitsu believes it can achieve a bit defect salvage rate about 500 times higher than with existing technology.
2) Externally controllable reference circuits
In order to select cells without an operating margin, Fujitsu made provision for changing the reference voltage from the outside while the wafer is being tested. This function is used to select cells with small operating margins so that they too can be salvaged with the redundancy function. This allows further increases in yield and greater reliability for the 1T1C system.
Furthermore, to evaluate the process improvements, Fujitsu conducted rewrite fatigue characteristic and data retention characteristic tests. As a result, it was able to confirm that a stable operating margin was maintained after ten billion rewrites and also about 10 years retention. This shows that Fujitsu will be able to produce 1T1C products with reliability equivalent to that of 2T2C products.
Fujitsu's newly developed FRAM uses 0.5-micron process technology, but it can also be produced using next-generation 0.35-micron technology. Based on this achievement, Fujitsu plans to aggressively pursue the commercialization of 1T1C FRAM products.
Fujitsu has been working on FRAM development with Ramtron International of the US since 1996. These results described above represent one fruit of this cooperative venture.
* FRAM is a registered trademark of Ramtron International Co. All other company/product names mentioned may be trademarks or registered trademarks of their respective holders and are used for identification purposes only.
About Fujitsu
Fujitsu Limited (TSE: 6702) is a leading provider of Internet- based information technology solutions for the global marketplace. Comprising over 500 group companies and affiliates worldwide -- including ICL, Amdahl and DMR Consulting -- it had consolidated revenues of 5.26 trillion yen ($49.6 billion) in the fiscal year ended March 31, 2000. Fujitsu's pace-setting technologies, world-class computing and telecommunications platforms, and global corps of over 60,000 systems and services experts make it uniquely positioned to unleash the infinite possibilities of the Internet to help its customers succeed. Altogether, the Fujitsu Group has 188,000 employees and operations in over 100 countries.
Internet: http://www.fujitsu.com/