Non Volatile Memory Technologies: Floating Gate Concept Evolution

摘要

The most relevant phenomenon of this last decade in the field of semiconductor memories has been the explosive growth of the Flash memory market, driven by cellular phones and other types of electronic portable equipments (palm top, mobile PC, mp3 audio player, digital camera and so on). Moreover, in the coming years portable systems will ask even more non volatile memories either with high density and very high writing throughput for data storage application, or with fast random access for code execution in place. The strong consolidated know-how (more than ten years of experience), the flexibility and the cost make the floating gate Flash Memory a largely utilized, well-consolidated and mature technology for most of the non-volatile memory application. Today Flash sales represent a considerable amount of the overall semiconductor market. Nowadays two of the several cell architecture proposed up to now can be considered as industry standard: the common ground NOR Flash that due to its versatility is addressing both the code and data storage segments and the NAND Flash, optimized for the data storage market. The exploitation of the multilevel approach at each technology node allows the increase of the memory efficiency, about doubling the density at the same chip size, widening the application range and reducing the cost per bit. In this paper the main issues related to both NOR and NAND Flash memory technology will be summarized, with the aim of describing both the basic functionality of the memory cell and the main cell architecture today consolidated. Both cells are basically a floating-gate MOS transistor, programmed by channel hot electron (NOR) or by Fowler-Nordheim tunneling (NAND) and erased by Fowler-Nordheim tunnel. The main reliability properties, charge retention and endurance, are presented, together with some comments on the basic physical mechanisms responsible for. A couple of innovative approaches to floating gate cell evolution, namely nanocrystal memory and 3-D cell will be described. Finally the Flash cell scaling issues will be covered, pointing out the main challenges. The Flash cell scaling has been demonstrated to be really possible and to be able to follow the Moore's law down to the 90 nm technology generations. The technology development and the consolidated know-how are expected to sustain the scaling trend down to the 50 nm technology node and below as forecasted by the ITRS roadmap.