M-Systems Flash Disk Pioneers Flash Memory manual Comparing Binary and MLC Flash Technologies

Implementing MLC NAND Flash for Cost-Effective, High-Capacity Memory

of NOR flash and achieving barely adequate reliability, but it has serious limitations: its performance is far slower than standard NOR flash.

NAND flash appeared to be the ideal media for data storage, due to its high-speed erase and write, high density (thus high capacity) and small size, as compared with NOR and AND devices. Based on these promising characteristics, Toshiba chose NAND flash as the basis on which to implement MLC technology. Toshiba’s first MLC NAND product, just introduced in December 2002, offers up to a

50 percent decrease in die size compared to standard NAND, and about a 70 percent decrease in size, compared with competing NOR MLC products.

However, NAND flash itself is not a perfect media. It contains a large number of randomly scattered bad blocks, requires on-the-fly error correction, and uses a non-standard I/O interface, making it difficult to integrate. These limitations are dramatically worsened in MLC NAND, along with a slower programming time (compared to standard NAND) and a different software interface. The combination of these characteristics makes MLC NAND all but unusable as a stand-alone local data storage solution.

M-Systems’ x2 technology, selected by Toshiba to enable their MLC NAND technology, implements reliability, performance and media management enhancements to perfect MLC NAND - without the need for a full scale controller (e.g., ATA or SCSI). The combination of MLC NAND and x2 technology in Mobile DiskOnChip G3 brings smartphones, STBs and other embedded systems the most cost-effective flash disk.

Comparing Binary and MLC Flash Technologies

Basic Flash Technology

Figure 1 shows the basic structure of a flash memory cell, which is similar to a standard MOS transistor. However, unlike a standard transistor, a flash cell must be able to retain charge after power removal in order to permanently store data. To accomplish this, a layer called the floating gate is added between the substrate and the select gate. The floating gate is isolated from the substrate and the select gate by layers of oxide.

A transistor can be biased (voltage can be applied to the source, drain, gate and substrate) to optionally conduct a current between its source and drain. The voltage level at which the transistor conducts is called its threshold voltage (VTh). The transistor conducts only if the voltage between the select gate and source (VGS) is larger than VTh. Adding/Removing charge to/from the floating gate modifies the VTh. To determine if the floating gate is charged, two conditions must be met: a specific VGS must be applied to the cell and the circuit must be capable of sensing if the transistor is conducting. These are the basic elements needed to implement flash data storage.

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