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The lowdown on manufacturing RRAM

Posted: 04 Nov 2014 ?? ?Print Version ?Bookmark and Share

Keywords:resistive random access memory? RRAM? NAND? 3D? CMOS?

Figure 2 shows an RRAM implementation compared to the current 3D NAND implementation. The advantages and disadvantages to this method are listed in table 2.

Most resistive memories today can use the same equipment set currently used in manufacturing the peripheral CMOS-based circuits, and they can implement the memory element at low temperatures. The thermal budget of the RRAM implementation does not impact the CMOS, and depending on the type of memory, RRAM layers can typically survive the thermal budget up to 16 stacks without showing significant changes to device performance.

Figure 2: An RRAM implementation compared to the current 3D NAND implementation.

With respect to 3D integration, one challenge for resistive memories has been the ability to select a specific cell while suppressing the noise from surrounding cells. This issue can be solved using a "selector" that suppresses the leakage, or sneak-path currents, from other cells. As the selector improves, the number of resistors that can be accessed or controlled using the same access transistor increases.

Table 2: Advantages/disadvantages of an RRAM implementation.

Resistive memory 3D implementation goes hand-in-hand with improving the select devices. There are many options to implement a selector as either a stand-alone element or as an intrinsic cell behaviour. When multiple RRAM cells are addressed by a single transistor (1TnR), the area below the memory array gets freed up and can be used to put peripheral circuitry directly under the memory array. This reduces the die size and cost.

One important aspect of 3D memory implementation with a resistive memory element is there is no need for a memory fab. Current logic fabs can manufacture resistive memories with minimal modifications to their existing equipment. As a result, resistive memory requires no new tools and eliminates the massive capital expenditure, estimated to be twice the cost of current memory fabs, associated with installing tools to support multiple stacked layers and 3D via etch and cleans. This is a significant cost advantage and enables resistive memories to be simple to manufacture.

NAND Flash technology has been serving the storage memory applications market for several decades, creating a dependency that has steadily increased as the technology scaled. However, in recent years, attempts to further scale this technology have exposed profound limitations.

Currently, it is widely accepted that scaling below 25 nm significantly degrades performance and reliability, resulting in overhead complexity and computational power demands from the system controller. System manufacturers, along with NAND Flash manufacturers, have begun the quest for a new storage solution. The most promising storage technology is resistive memory.

This breakthrough memory technology provides distinct advantages over 2D NAND, offering improved performance as it scales down, MLC performance using About the author
Sundar Narayanan is the vice president of technology at Crossbar Inc.

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