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New direction for DRAM?

Memory-IP (intellectual property) vendor Innovative Silicon (ISi) has licensed its Z-Ram memory to Hynix; this is the first time ISi has licensed the technology for use in commodity DRAM. When adopted, the company says, it will be the first major change in DRAM construction for many years. Z-Ram is a single-transistorcell dynamic-memory technology that uses no capacitor. Today’s DRAM vendors build their products in CMOS, and their devices typically have a bit cell consisting of a single transistor and a capacitor: charge on the capacitor represents a stored bit. Licensees of Z-Ram build the memory on SoI (silicon- on-insulator) substrates. Transistors built on SoI exhibit an inherent charge-storage phenomenon, which in most applications is a nuisance that designers seek to minimise. In Z-Ram, ISi exploits this charge and uses it as the stored data. In today’s DRAM, as dimensions of features on the wafer diminish, manufacturers have had to become ever-more ingenious at constructing capacitors that retain sufficient charge to adequately store a bit. In one construction—the trench capacitor—the capacitor is of very high aspect ratio: it is formed in a long, narrow vertical trench etched in the silicon, a difficult-to-manufacture shape that is many times longer (down into the silicon) than it is wide. Such features are becoming increasingly difficult to scale down further as CMOS process technology advances. The Z-Ram, by contrast— says ISi’s CEO Mark-Eric Jones—is just a transistor and scales as well as the base technology. It therefore leads to smaller bit-cell sizes, and hence smaller die area and overall device cost. It also saves cost by requiring fewer process steps. ISi has introduced Z-Ram in two generations: it positioned the first version for use in embedded DRAM in standard products and ASICs and licensed it to—among others—AMD. The later (Gen-2) version of the technology greatly increased the stored-charge retention time (25 times longer) and the detectable level between a “1” and a “0” (by around ten times) and was, Jones says, the key to making the technology suitable for commodity DRAM. The deal with Hynix is, Jones says, “an eight-figure [dollar] sum” for the licence, with royaltiesto follow on production.

Still, you won’t see a Z-Rambased DRAM on your bench tomorrow. “In an ASIC context it takes a couple of years from licence to product,” Jones says. “For commodity DRAM you can probably add a year to that.” The time is needed, he says, for development work and process tuning. But when you do, it will look like a standard DRAM in all respects, although Jones says, “it will say on the data sheet that it’s based on ISi technology.” Going forward, Jones’ assertion is that this deal ensures the continuing presence of DRAM—in densities that will scale with whatever process technologies in general achieve—as a design resource. You can, he says, be confi dent that affordable highdensity DRAM will be freelyavailable for future designs.

Innovative Silicon, www.z-ram.com.