Packing more memory into silicon.

Packing more memory into silicon

One measure of progress in microelectronics is the numberof bits of data that can be stored in a single computer memory chip. Only seven years ago, that number was 64,000 bits. This year, manufacturers are just starting to produce commercial quantities of "dynamic random-access memory' chips that can each hold a million pieces of data (SN: 3/2/85, p.135). But researchers already have the next target in sight. At last month's International Solid State Circuits Conference in New York City, Japan's Nippon Telegraph & Telephone Corp. (NTT) grabbed the spotlight with a prototype design for a 16-megabit memory chip. Such a chip would have room for the equivalent of about 64 pages of newspaper text.

NTT's experimental memory chip crams about 40 millioncircuit elements into a space about half the size of a postage stamp. Its features are so small that it would take more than a hundred of the chip's 0.7-micron circuit lines to match the width of a human hair. To save space, some of the circuit elements lie in microscopic trenches etched in the silicon and others are stacked one on top of the other. Retrieving a bit of data from one of the chip's storage cells takes about 80 billionths of a second.

Meanwhile, engineers at the IBM Corp. in Essex Junction, Vt.,have developed a high-speed 4-megabit chip (see photo) with circuit lines as narrow as those on the NTT chip. Although it has only a quarter of the NTT chip's storage capacity, the IBM memory can be fabricated on the same production line now being used to manufacture IBM's 1-megabit chips. This demonstrates, say IBM researchers, that their chip will likely be relatively easy to manufacture on a large scale.

Each storage cell in a dynamic random-access memory chipconsists of one capacitor and one transistor. Data are stored as the presence or absence of electric charge on a capacitor's surface. In the IBM design, each capacitor is created by etching a deep, narrow hole into the silicon, coating the hole's side walls with an insulator and then filling the hole with an electrically conducting material. A bit stored in one cell can be retrieved in only 65 nanoseconds. Like the NTT chip, the IBM operates on a single 3.3-volt power supply instead of the 5-volt supply required by earlier memory chips.