Electronic Design

Nonvolatile Storage Doesn't Require Transistors

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.



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