Error Correction Codes and Signal Processing in Flash Memory

Abstract

This chapter is to introduce NAND flash channel model, error correction codes (ECC) and signal processing techniques in flash memory. There are several kinds of noise sources in flash memory, such as random-telegraph noise, retention process, inter-cell interference, background pattern noise, and read/program disturb, etc. Such noise sources reduce the storage reliability of flash memory significantly. The continuous bit cost reduction of flash memory devices mainly relies on aggressive technology scaling and multi-level per cell technique. These techniques, however, further deteriorate the storage reliability of flash memory. The typical storage reliability requirement is that non-recoverable bit error rate (BER) must be below 10-15. Such stringent BER requirement makes ECC techniques mandatory to guarantee storage reliability. There are specific requirements on ECC scheme in NOR and NAND flash memory. Since NOR flash is usually used as execute in place (XIP) memory where CPU fetches instructions directly from, the primary concern of ECC application in NOR flash is the decoding latency of ECC decoder, while code rate and error-correcting capability is more concerned in NAND flash. As a result, different ECC techniques are required in different types of flash memory. In this chapter, NAND flash channel is introduced first, and then application of ECC is discussed. Signal processing techniques for cancelling cell-to-cell interference in NAND flash are finally presented.