Hitachi's Multi-Level AG-AND Flash Memory for Gigabit Flash Chips

2001 will probably be remembered in tech circles as the Year of Flash Memory. This year, we've seen everything from tiny flash cards that hold gobs of memory to solid-state storage devices the size of a pen to network attached flash devices. Hitachi Semiconductor thinks it can improve the capacity and speed of flash chips with its new Assist-Gate AND-type (AG-AND) flash memory cell. The proprietary cell design uses gate-assisted field isolation and hot-electron injection to achieve high-speed programming, with a possible write speed of 10 MByte/s operating in a multi-level memory cell configuration. The AG-AND flash cell is built on a 0.13-micron process and has an area of 0.052 micron2 (on a per-bit basis with 2-bit/cell operation) which is more than 50% smaller than conventional AND flash cells built on a 0.18-micron process. Hitachi says the AG-AND cell design will form the basis of gigabit (Gb) generation flash memory chips that can record large volumes of digital data at high speed. To achieve a smaller cell size and improve performance, Hitachi researchers changed the cell structure from the conventional SGI (shallow groove isolation) to a gate-assisted field isolation architecture. In the AG-AND flash cell, cell isolation assist gates are packed closely with alternating self-aligned floating gates to achieve the small cell area. To increase the writing speed for the AG-AND flash memory cell, the research team changed the programming method from Fowler-Nordheim (F-N) tunneling to source-side hot electron injection for high-speed, low-current parallel writing. In the commercial development of 0.13-micron process products, a chip design with a four-bank configuration can provide writing speed of up to 10 MB/s as a discrete device or in a small form factor flash card. At this speed, Hitachi says that one hour of CD-quality MP3 music (around 64 megaBytes) can be written to the flash card in around 6 seconds.