Research Of Error Patterns In MLC NAND Flash

Flash-based solid state disk(SSD) is widely used in storage solution nowadays to improve the development of the whole information technology because of its attractive features, such as high performance, resistance to physical shock, low energy consumption, and so on. Flash is used as medium in SSD in the form of chips, and MLC NAND Flash chips are most widely used. In recent years, in order to meet the requirement of high capacity and low cost, flash chip manufacturers keep scaling down to smaller process technology nodes, which increases the probability of flash errors and reduces the reliability of SSD. The research of error patterns in MLC NAND flash is the key to solve the problem, and is very important for the future development of SSD.All the research work is conducted on the basis of the data storage principle, organization, and operation mechanisms of flash. Three error patterns, including write interference error, write error, and read interference error, are proposed. A hardware test platform which can support the direct access to flash chips and gain raw data from physical medium is devised and built. A test software is also developed. Corresponding to each error pattern, an access pattern is designed. The experiments are performed on the test software and hardware platform, and raw bit errors are collected for further analysis of the corresponding error pattern.Through experiments, some conclusions are drawn as follows.(1) Assigning physical pages with different policies while programming causes different raw bit errors. Random programming causes large quantities of raw bit errors, in-wordline-order programming causes a few raw bit errors, and in-page-order programming causes few raw bit errors.(2)It is more probable for raw bit errors to occur in an MSB(Most Significant Bit) page than in the LSB(Least Significant Bit) page in the in the same wordline, and there may occur more raw bit errors in the MSB page.(3)Other pages in a block may encounter raw bit errors because of the read operation of a page in the same block. And it is possible for the raw bit errors to increase as the read count increases.