Startup wants to replace NAND and DRAM with silver RRAMs

August 05, 2013

Bernard Cole-August 05, 2013

Targeting what it thinks is a new category of very high capacity and high-performance non-volatile memory, Crossbar Inc. has unveiled what it thinks is a revolutionary silver-based Resistive RAM technology.

The company has fabricated and tested numerous prototypes of a working memory array at several commercial fabs in various configurations embedded in SoCs for use in mobile and embedded designs.

Based on work by Professor Wei Lu at the University of Michigan , the new RRAM uses a silver-ion based technology as a replacement for traditional non-volatile memory. It incorporates a CMOS access controller monolithically with a memory array and is one of several alternative ReRAM offerings being researched by the industry. Most of these are trying to achieve equivalent or exceed the performance to NAND flash memory but can be scaled beyond the current 15 nanometer, two-dimensional limit for NAND flash.

The company is currently negotiating with a number of the major semiconductor fabrication firms to transfer its silicon IP to their CMOS processes, in preparation for offering the technology as licensable IP for use in processor based designs. As the technology matures, it expects to offer the technology in end user products similar to the way NAND flash is now marketed.

According to George Minassian, chief executive officer, Crossbar, Inc., the new RRAM technology will eventually be capable of storing up to one terabyte (TB) of data on a single 200 square millimenter IC chip, enabling massive amounts of information, far beyond the capabilities of most current DRAM and flash alternatives.

The memory density is possible, he said, through the use of a simple three layer structure completely compatible with current CMOS technology (Figure 1). “The base structure is nothing more than a non-metallic bottom electrode, an amorphous silicon switching medium and a metallic top electrode,” he said.

“The resistance switching mechanism is based on the formation of a filament in the switching material when a voltage is applied between the two electrodes. This simple and very scalable memory cell structure enables an entirely new class of RRAM, which can be easily incorporated into the back end of line of any standard CMOS manufacturing fab.”

Figure 1. The memory technology stores bits as changes in resistance caused by formation of nanometer-thick filaments in a highly resistive switching medium held between two plates.

As shown above, a positive voltage applied at the top electrode drives ions into the switching medium, creating a 6nm filament that “stores” an on state as a reduced resistance through the switching medium, which is highly resistive. A negative charge applied at the top plate pulls ions back to the top plate and resetting the cell to an "off" state.

Already fabricated in devices by five different foundries to prove out the CMOS-compatible technology, RRAM elements can be fabricated as three-dimensional stacks on top of the CMOS base layers (Figure 2), allowing integration of up to eight memory stacks using available fabrication processes, the equivalent of about a terabyte of memory on a single chip - enabling devices with a terabyte of RRAM on-chip.

Figure 2. A Crossbar memory stack on CMOS base layers.

“Non-volatile memory is ubiquitous today, as the storage technology at the heart of the $1.1 trillion electronic market - from tablets and USB sticks to enterprise storage systems,” said Minassian. “And yet today's non-volatile memory technologies are running out of steam, hitting significant barriers as they scale to smaller manufacturing processes.”

Removing that barrier, Crossbar's technology, he said, will deliver 20x faster write performance; 20x lower power consumption; and 10x the endurance at half the die size, compared to today's best-in-class NAND Flash memory.

The new RRAM is based on resistive RAM (RRAM) patents from the University of Michigan and Crossbar has filed an additional 100 unique patents, with 30 already issued, relating to the development, commercialization and manufacturing of RRAM technology.

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