Memory: From DRAMs To Ferroelectrics
The ability to store data in everything from silicon to crystal lattices has opened the door to a wide range of commercial storage products. Potential future storage structures will allow the eventual holographic storage of terabits in small, half-inch cubes of crystalline material. Today's system designers have a wide choice of storage options, from DRAMs and SRAMs on the volatile side, to flash, ferroelectric, and other forthcoming technologies for nonvolatile storage.
SRAM and DRAM densities have been improving at a relentless pace for decades. New deep-submicron processes with features below 0.13 µm are leading to 8- and 16-Mbit SRAMs that will go into production this year. In 2000, 512-Mbit DRAMs entered sampling. Although a number of manufacturers have presented conference papers detailing functional 1- and even 4-Gbit DRAM designs, most are still laboratory curiosities. Just one or two companies are presently sampling 1-Gbit devices to customers.
Novel process technologies are being used and explored for future memory generations. Some 3D approaches stack layers of memory cells one above the other. Also, the use of multiple voltage levels in a memory cell lets each cell hold two or more bits worth of data.
Although stacking technology has been around for years, it has been way too expensive for commercial use. Mainstream chemical-mechanical polishing production technologies are now being used to flatten the wafer's surface. There also is a wafer-bonding/fracturing approach where several layers of memory cells can be formed on top of a control circuit. This allows the creation of a very dense and high-performance DRAM.
Relentless advances in process technology have also characterized nonvolatile storage devices. The UV EPROM endured for a long time, but finally gave way to EEPROM and now to flash memory. Today's flash memories have capacities of 512 Mbits using 1 bit/cell, but the first devices to employ a 2-bit/cell storage scheme for 1-Gbit capacities per chip are around the corner. In addition to their use in computing systems, high-capacity flash memories have found a home in many consumer applications, such as digital film for electronic cameras and digital tape for MP3 music players.
Flash memories have one theoretical limitation—a wearout mechanism that curtails the number of write cycles to about 1 million for the best devices available to date. Ferroelectric-based memory cells eliminate this limitation. But for now, memory capacities of FRAM-based chips are limited to a few megabits. Processing improvements over the next few years promise to boost capacities to hundreds of megabits and perhaps let them compete with DRAMs.
No matter the type of semiconductor memory, it's clear the best is yet to come.
Several companies will sample higher-density SRAMs with 8- and 16-Mbit capacities. These memories will employ 6T memory cells to achieve a low-power standby capability for portable system applications.
Commercial FRAMs with capacities above 4 Mbits will be sampled. These memories will offer the best combination of features—nonvolatile storage and no wearout. That will allow system designers to treat them like nonvolatile DRAMs.
In the latter half of this decade, laser-based holographic data storage crystals will be able to store terabytes of data in crystal lattices. This scheme could provide almost unlimited amounts of offline nonvolatile storage.
See associated timeline.