具有指数分布读噪声的NOR存储器件的纠错
Error Correction for NOR Memory Devices with Exponentially Distributed Read Noise
摘要：
具有多级存储单元（MLC）的高密度NOR闪存器件由于其较高的本征位出错率（IBER）而遇到可靠性的突破。芯片制造商公司提供两种解决方案来满足输出位出错率（OBER）规范：使用纠错码（ECC）的部分覆盖或单级单元（SLC）中的数据存储，这显著增加了管芯成本。NOR闪存允许以小部分写入信息，因此，由于所需的高冗余（例如，$\sim$ 50%），完全错误保护变得昂贵。这与NAND闪存一次写入大量信息非常不同; NAND ECC只需要$\sim$ 10%的冗余。本文分析了一种适用于单字节或非存储器的新型差错保护方案。该方法不需要任何冗余单元，但假设第5编程级别。该信息被映射到由等于2的最小曼哈顿距离分隔的4维空间中的状态。此代码保留了信息容量：一个字节占用四个存储单元。我们证明了OBER $\sim$ IBER $^{3/2}$缩放定律，其中IBER是针对4级MLC存储器计算的。例如，具有IBER $\sim10^{-9}$的4级MLC（这对于高密度产品是不可接受的）可以转换为OBER $\sim10^{-12}$。我们假设IBER由指数分布的读取噪声决定。这是NOR闪存器件的情况，因为指数尾对于随机电报信号（RTS）噪声和大多数电荷损失、电荷增益和电荷共享数据损失是典型的。
英文摘要：
The scaling of high density NOR Flash memory devices with multi level cell (MLC) hits the reliability break wall because of relatively high intrinsic bit error rate (IBER). The chip maker companies offer two solutions to meet the output bit error rate (OBER) specification: either partial coverage with error correction code (ECC) or data storage in single level cell (SLC) with significant increase of the die cost. The NOR flash memory allows to write information in small portions, therefore the full error protection becomes costly due to high required redundancy, e.g. $\sim$50%. This is very different from the NAND flash memory writing at once large chunks of information; NAND ECC requires just $\sim$10% redundancy. This paper gives an analysis of a novel error protection scheme applicable to NOR storage of one byte. The method does not require any redundant cells, but assumes 5th program level. The information is mapped to states in the 4-dimensional space separated by the minimal Manhattan distance equal 2. This code preserves the information capacity: one byte occupies four memory cells. We demonstrate the OBER $\sim$ IBER$^{3/2}$ scaling law, where IBER is calculated for the 4-level MLC memory. As an example, the 4-level MLC with IBER $\sim10^{-9}$, which is unacceptable for high density products, can be converted to OBER $\sim10^{-12}$. We assume that the IBER is determined by the exponentially distributed read noise. This is the case for NOR Flash memory devices, since the exponential tails are typical for the random telegraph signal (RTS) noise and for most of the charge loss, charge gain, and charge sharing data losses.
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