Metal-oxide resistive RAM has reached close to the mainstream for array size with a two-layer, 32-Gbit nonvolatile memory prototype implemented by engineers from SanDisk Corp. and Toshiba Corp. in a 24-nm manufacturing process. Engineers from the two companies presented the dual-layered memory at the International Solid-State Circuits Conference (ISSCC).

By way of comparison, market-leading NAND flash memory from Micron Technology Inc. (Boise, Idaho) achieves 128-Gbit in a 20-nm process using memory cells that can store multiple bits (see Micron launches dense 128-Gbit NAND flash memory).

Resistive RAMs of various types are being developed by a most of the major memory companies and a number of startups as potential replacements for NAND flash at geometries below 20-nm due to their superior scalability. It is expected by many industry observers that ReRAM will be introduced commercially in a vertically-stacked format to take over when vertically-stacked NAND flash runs out of steam. The SanDisk-Toshiba paper therefore offers an important transition to stacked memory structures.

As well as scalability ReRAM is expected to offer fast access, higher read-write endurance and higher reliability than NAND flash. However, the physics behind resistive RAM in various metal-oxide and other forms is still not completely understood.

The ISSCC paper did not reveal which metal-oxide regime SanDisk/Toshiba is using but reported a bit cell planar size of 24-nm by 24-nm and a total die area of 130.7 square millimeters. The I/O interface is NAND-compatible and the read latency is 40 microseconds and the write latency is 230 microseconds.

The 32-Gbit memory is arranged as two stacked layers of 16-Gbit. A scanning transmission electron microscope (STEM) cross-section shows the stack made up of word line 1, device, bit line, device, word line 2. (See paper 12.1 from ISSCC 2013 proceedings: A 130.7 mm2 two-layer 32-Gbit ReRAM memory device in 24-nm technology by Tz-Yi Liu and Tian Hong Yan et al.).

As well as providing two layers of memory array to improve memory density the two-terminal cell with diode selector is stacked above the supporting circuitry. In this work not only are word and bit lines placed under/over the bit cell but also the array control circuit, sense amplifiers, page buffer, and voltage regulator drivers are placed below the memory array.

By: DocMemory
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