The drive for SSDs: What’s holding back NAND flash?

Hagop Nazarian and Sylvain Dubois, Crossbar -November 26, 2013

The Breakthrough RRAM-Based Storage Solution
An RRAM-based SSD controller is not subject to the many burdens NAND Flash places on a storage system.

In RRAM-based storage controllers, the CPU does not need to manage the L2P tables or process the same level of garbage collection and wear-leveling. With these lower needs, RRAM-based storage controllers will be simplified and will cost less than NAND-based storage controllers.

The differentiating cell level and product level characteristics comparing NAND and RRAM technologies are listed in Table 4 below. These characteristics indicate a-Si RRAM-based storage systems will provide superior performance and reliability in emerging applications requiring both high performance, power or endurance to achieve high capacity, high speed and low cost.

Table 4. At-a-glance comparison of NAND-based and RRAM-based performance at the cell and product level.

A Word on Design & CMOS Compatibility
What makes a-Si RRAM stand out from other emerging technologies is the fact that it is so easy to integrate. Unlike the materials used in many new technologies, the amorphous silicon films used in a-Si RRAM are well characterized, robust and are already used in CMOS fabs. For example, the memory developed by Crossbar, Inc. can be a standalone array, or it can be embedded in the backend on top of CMOS into multiple 3D stacked layers.

Currently, RRAM can be fabricated in back-end-of-line (BEOL) processes, since the operation of most RRAM cells does not rely on transistors. In a typical process base, wafer fabrication (which includes the address and sensing circuitry) would be fabricated in a CMOS foundry, followed by the RRAM memory fabrication in either the same fab or a separate BEOL memory fab. Crossbar Inc. has run several tests to ensure that their product is CMOS compatible and have already demonstrated their memory array on multiple integration schemes using a diverse tool set.

Integration: Integrating a-Si RRAM involves patterning and subtractive etching processes. The process flow has a block repeating in order to achieve the stacking of the memory element. a-Si RRAM integration uses standard process steps and tools commonly available in various fabs.

Over the past decades, NAND has imposed demanding tasks upon the system controller in solid storage devices. These management tasks have increased system complexity, power consumption, transistor gate count and overall storage system development cost.

The breakthrough characteristics of a-Si RRAM technology, such as Crossbar Memory, provide memory devices with high performance specifications and flexible features, such as the ability to rewrite to a memory location without the need to erase a block. Simplified devices simplify storage systems and significantly reduce system controller overhead, empowering the creation of emerging technology for generations to come.

More about authors Hagop Nazarian and Sylvain Dubois.

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