Sidebar: Shrinking Size, Growing Challenges

Current memories face several challenges. For example, a Dynamic Random Access Memory cell consists of one transistor and one capacitor. The transistor charges the capacitor, and a current is applied to it to detect its state (0 or 1). But as intercircuit distances in memory chips shrink from 90 nanometers today to 65nm or smaller in the next five years, the charge the capacitor and the charge it can hold will become smaller and more difficult to detect.

"To retain the same amount of charge, you have to increase the height of the capacitor. This causes all sorts of process problems," says Subodh Toprani, CEO of ZettaCore Inc. in Englewood, Colo. To get around that, manufacturers must create deep trenches in or build stacks on the silicon. "Imagine trenches with an aspect ratio of 100:1. The [fabrication] challenges are enormous," he says.

And as the number of transistors on a chip increase and circuit sizes shrink, power consumption and heat rise. DRAM also requires more power than other types of memory because cells must be continuously refreshed up to 10 times per second to retain data.

By contrast, SRAM is static -- the data is retained without refreshes for as long as power is on. Static RAM is much faster than DRAM but has trouble with atmospheric alpha particle radiation, which can reset bits, and so requires specialized error detection and correction circuitry. In addition, chip density is limited to 64Mbit or 128Mbit versus 256Mbit to 1Gbit for DRAM. The lower density limits its use in some applications, such as processor cache.

As with DRAM, the move to smaller processes is creating problems with voltage leaking, in which transistors consume power even when they aren't switching.

Meanwhile, flash memory is nonvolatile, offers high densities and is relatively inexpensive. But it's also the slowest memory, it's power-hungry, and it wears out after about 100,000 writes. Much of the current research is centered around making a better alternative to flash.

Copyright © 2005 IDG Communications, Inc.

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