Six hot tech trends emerge in high-speed memory

EE Times is keeping tabs on the latest developments in high-speed memory. Aside from DDR4, there are more emerging technologies that exhibit the highest potential for boosting the performance of servers, systems, and SoCs.

Do you know how fast the latest DDR4 memories are, or how many transistors make up a high speed SRAM cell? Here are six of the latest technologies, from DDR4 to embedded DRAM and various SRAM cells, well as the latest technology from memory specialist Rambus.

DDR4

The latest high speed dynamic memory technology is DDR4, set to reach the market in volume later this year. The specification started in 2005 in order to deliver faster clock frequencies, to provide higher data rates of 4266MT/s (up from 2133Mtransfers/s for DDR3), and to support lower voltages of 1.05V to 1.2V in new chipsets. The new standard also includes Connectivity Test, a type of JTAG boundary scan that boosts device and module manufacturing testing by enabling early fault detection in order to reduce time spent on debugging and improve system reliability. Samsung started production of 4G DDR4 devices on 20nm in September 2013, while Micron Technologies, which now includes Elpida, is shipping 8G and 4G DDR4 devices.

DDR3

The older high-speed memory technology is still thriving and moving from 30nm to 20nm to save power, give it a new lease of life, and cover the gap before DDR4 ships in high volume. DDR3 is currently the mainstream PC technology with voltages of 1.2V to 1.5V and data transfers up to 2133Mtransfers/s.

Embedded DRAM

The interface to the memory banks is often the limiting factor for high-speed performance, and that can be avoided by integrating the memory directly alongside the logic. This is where embedded DRAM (eDRAM) comes in. This technology can deliver memory access speeds that match the processor interface, eliminating the need for memory banks. Normally every bit in a commodity DRAM is refreshed every 64ms or longer, but embedded DRAM allows a much more frequent refresh spec as low as 2ms, but this depends on the memory controller that is used. The issue has traditionally been delivering reliable DRAM capacitors in leading edge process technologies, although the smaller capacitors allow for faster operation. TSMC and Renesas have also used eDRAM in the chips they make for the gaming systems, specifically the Microsoft Xbox and the Nintendo Wii. According to an analysis by Chipworks, these use a conventional form of memory stack with polysilicon wine-glass-shaped capacitors. TSMC uses a cell-under-bit stack where the bitline is above the capacitors, while Renesas relies on a cell-over-bit (COB) structure with the bitline below. Intel also uses a COB stack, but the company builds an MIM capacitor in the metal-dielectric stack using a cavity formed in the lower metal level dielectrics. The part is fabbed in Intel's 9-metal, 22nm process.

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