Electronic Design

Novel Denser Memories Make Data Rates Sing

"Memories are made of this." That classic line from an old tune is certainly apropos, but maybe we should tweak it to read "memories are made of these." The latest developments in memory technology run the gamut of material technologies, from silicon to silicon-on-insulator to carbon nanotubes to magnetoresistive, ferrorelectric, and phase-change materials, and who knows what else.

In their quest to develop the ideal memory chip-a low-cost device that reads and writes data at high speed, holds an unlimited amount of data, and can retain data when there's no power-researchers are exploring many avenues.

While advanced research is ongoing, current memories are getting denser and faster. Improved interfaces, smaller features, improved cell designs, and enhanced architectures are all playing their parts. In the DRAM arena, one new avenue of research is the fabrication of DRAMs that do away with the physical capacitors by using silicon-on-insulator, which brings its floating body effect to the mix.

A few of the companies currently studying the approach include Innovative Silicon, Philips, and Toshiba. They estimate that by eliminating the capacitors, the DRAMs will be almost twice as dense as today's memories. However, it will take a few more years to commercialize the technology.

In the meantime, the first-generation double-data rate (DDR) synchronous DRAMs are starting to cede to the second-generation versions that will offer 1-Gbit densities. The DDR2 DRAMs will feature data-transfer rates that pick up at the high end of DDR1's capabilities-433 MHz-and then push those rates to 533, 667, and eventually 800 MHz.

Accelerating Data Transfers
Still higher-speed DRAMs that can transfer 8 bits/clock are expected later this year from Toshiba. Coming in 256- and 512-Mbit densities, the DRAMs (based on the XDR differential interface developed by Rambus) will initially be able to transfer data at rates of 3.2 to 4 Gbits/s using eight differential, 0.2-V, signals. An enhanced version of the interface, slated for sampling in late 2005, ups the data rate to between 4.8 and 6.4 Gbits/s.

But DDR interfaces aren't standing still. The third-generation, DDR3, will be finalized later this year, with first samples expected in late 2006. During this time, DRAM densities also may increase to 2 Gbits/chip. A presentation by Samsung Electronics Co., scheduled for next month's IEEE International Solid State Circuits Conference (ISSCC) in San Francisco, will detail the first prototype of a 2-Gbit DDR2 SDRAM that can transfer data at 800 Mbits/s.

By leveraging shrinking features and new technologies, nonvolatile flash memories are achieving unheralded density levels. As next month's ISSCC, two presentations will detail prototypes of 8-Gbit NAND flash memories based on process rules as small as 63 nm. Developed by Samsung and the joint team of SanDisk and Toshiba, both will employ multilevel storage cells that hold two bits. Don't expect these mammoth devices for a while, though. It may take a year or more to develop production-worthy versions.

Additional developments in flash include a novel architecture created by Samsung called OneNAND, which combines controllers for both NAND and NOR operation. On one chip, Samsung combines a high-performance NAND controller, a NOR flash interface, a single-level-cell NAND flash array, and up to 5 kbytes of internal buffer-RAM. The chip attains a sustained read performance of 31 Mbytes/s and a program performance up to 7 Mbytes/s.

Although flash memories can deliver lots of bits, the read and write times are very asymmetrical, with read in the tens to hundreds of nanoseconds and writes in the millisecond range. New nonvolatile memory technologies, such as magnitoresistive RAMs, ferroelectric RAMs, and memories based on phase-change materials or carbon nanotubes, show promise as an alternative to flash when symmetrical performance is needed or larger numbers of write cycles must be handled.

Competing with flash memory for portable storage, the latest generation of 1.8-in. and sub-1-in. hard-disk drives offers capacities of 2 to 20 Gbytes. Over the next year or so, expect capacities to continue to increase. The sub-1-in. form factor will pack 3 to 6 Gbytes, with drives from Cornice, Hitachi, and Toshiba. The 1.8-in. form factor is expected to hit 80 to 100 Gbytes, from vendors such as Fujitsu and Toshiba. These drives should trigger a new generation of video-based cell phones and portable information products.

Memory Tech Trends
Higher-Performance, Denser DRAMs
Designers will see the first prototypes of 2-Gbit DDR2 SDRAMs and production samples of 512-Mbit DDR2 SDRAMs that operate at 533 MHz.

New Interfaces Deliver More Bandwidth
The first DRAMs based on the extreme data rate (XDR) interface will offer data bandwidths of 4.8 to 6.4 Gbits/s per port. Future versions promise data rates of 8 Gbits/s in 2006.

Flash Memories Go Mammoth
Advances in process technology will enable companies to demonstrate NAND-based flash memory chips capable of storing 8 Gbits on one chip.

Fully Buffered DIMMs Really Pack It In
Memory modules for high-end systems and servers will move to a fully buffered design. It will allow for much larger memory banks, eliminating the bus loading limits that restricted the number of modules able to plug into a bus.

Novel Memory Approaches Proliferate
Efforts to craft denser memories or create the ideal memory chip are taking shape. Novel technologies and new memory cells are based on SOI body effects for capacitor-less DRAMs and carbon-nanotube, magnetoresistive, and phase-change materials for nonvolatile storage cells.

Dieting Disk Drives
Disk drives continue to shrink as portable media players and cell phones demand gigabytes of storage. Sub-1-in. drives with 2 to 4 Gbytes of storage will go into mass production this year. Drives capable of holding 10 Gbytes are now on the drawing boards.

TAGS: Toshiba
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