Nonvolatile Storage Doesn't Require Transistors

June 25, 2009
The CMOx nonvolatile memory technology from Unity Semiconductor targets storage-class memory applications. CMOx is based on new materials in the semiconductor process called conductive metal oxides that use the movement of io

The CMOx nonvolatile memory technology from Unity Semiconductor targets storage-class memory applications. CMOx is based on new materials in the semiconductor process called conductive metal oxides that use the movement of ionic charge carriers to store information. With 64-Gbit chip capacity on the horizon, it looks to be a challenger to NAND flash.

The technology employs a multi-layer, multi-level cell (MLC) approach that gives it an inherent fourfold improvement over MLC flash. Each CMOx MLC cell stores 2 bits. The four-layer approach stores 8 bits (Fig. 1). The structure uses a passive cross-point multi-layer memory array that doesn’t require a transistor per cell.

Memory technologies such as phase-change memory (PCM) and magnetoresistive RAM (MRAM) still use a transistor per cell. This tends to prevent their use in cross-point multi-layer chip architectures. The cross-point memory array structure delivers very dense chips.

The CMOx cells have a resistance change element that differs from resistive RAM (RRAM) memory. Unity Semiconductor’s approach isn’t filamentary. Instead, it uses uniform conduction across the device.

In addition to a fourfold density improvement, CMOx should also provide a write speed improvement of five to 10 times compared to MLC NAND flash with better endurance characteristics as well. The technology uses less than 1 µA of write current per cell.

Unity Semiconductor splits the production process into a standard front-end-of-line (FEOL) CMOS base wafer creation and custom backend- of-line (BEOL) memory layer processing (Fig. 2). The FEOL can be fabricated at a CMOS logic foundry with existing production capability and capacity on a trailingedge CMOS (90 nm) process. This approach reduces risk and cost for those producing CMOx chips.

The initial CMOx 64 Gbitdevice is expected to run at 100 MHz with a maximum data rate of 200 Mbytes/s. It should have a sustained write speed of 60 Mbytes/s and sustained read speeds of 100 Mbyte/s. Unity Semiconductor expects to have 64-Gbit products available in 2010.

BILL WONG

UNITY SEMICONDUCTORwww.unitysemi.com

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