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Volume 39 (Apr 21, 2005)
Future Memories are Made of This: Embedded DRAM Completes the System on the Chip (2/2)
The capacitor rules
All of the benefits of the UX6D embedded DRAM are possible because NEC Electronics developed clever ways to make capacitors in a standard CMOS process. Figure 2 shows the evolution of these capacitors from one generation to the next. The capacitors are made in a cup shape to maximize their surface area while taking up the smallest possible space on the chip. The cup is made by forming a tiny metal cup, coating it with an insulator (a dielectric), then forming another metal cup inside the first two layers.
Even as these capacitors have shrunk in size to keep up with the shrinking chip features of each new generation, the capacitors' ability to store charge (capacitance) has remained the same. That is a curious result because capacitance depends on a capacitor's surface area. Here, the surface area shrinks, yet the capacitance does not.
The secret of the smaller capacitors lies in the materials used to make them. Capacitance depends on the distance between the conductors as well as the surface area. While the surface area is shrinking, NEC Electronics is using new dielectrics that allow the metal layers to be closer together.
In the UX6D process, NEC Electronics has changed from a tantalum compound to a dielectric based on zirconium-a different form than the cubic zirconium that looks so much like diamond, but no less valuable. NEC Electronics has pioneered the use of zirconium dielectric in DRAM, and the benefits are significant. Without this new material the UX6D capacitors would have to be 85 percent taller, 64 percent bigger in area, or some combination of these expansions (Figure 3). Such size increases are just about out of the question for the UX6D fine-line process.
The success of the UX6D embedded DRAM thus comes down to the use of zirconium dielectric. Best of all, this zirconium material is fully compatible with the CMOS fabrication process. NEC Electronics uses standard CMOS fab equipment and standard process steps, keeping costs to a minimum and ensuring reliability.
More than a pretty structure
The useful features of NEC Electronics embedded DRAM are not just about the capacitor structure. The DRAM is also unusually easy to use in an SoC.
SoC designers must have a way to configure the DRAM blocks without laying out each capacitor cell manually. So if they want a 64-megabit DRAM with a certain bus width, they specify a macrocell with those parameters. A software tool then generates that "macro" automatically.
These macros are usually much larger than anything else on the SoC, and SoCs often include multiple macros. As a result, routing interconnects around the macros can be difficult (Figure 4). NEC Electronics is unusual in enabling routing over the top of the macros. Moreover, while competitors' macros have a fixed orientation, the NEC Electronics macros can be oriented in any way that suits a chip's layout (Figure 5).
Such ease of use is one of many innovative features that have helped make NEC Electronics a volume and performance leader in embedded DRAM. Whether you appreciate high-speed games or long waits between cell-phone battery charges, you might be enjoying the benefits of embedded DRAM in coming years with the UX6D process behind it all.
