The most relevant phenomenon of this past 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 equipment (palm top, mobile PC, mp3 audio
player, digital camera, and so on). Moreover, in the coming years,
portable systems will demand even more nonvolatile 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 Flash memory a
largely utilized, well-consolidated, and mature technology for most
of the nonvolatile memory applications. Today, Flash sales represent
a considerable amount of the overall semiconductor market.
Although in the past different types of Flash cells and architectures
have been proposed, today two of them 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.
This paper will mainly focus on the development of the NOR Flash
memory technology, with the aim of describing both the basic functionality
of the memory cell used so far and the main cell architecture
consolidated today. The NOR cell is basically a floating-gate
MOS transistor, programmed by channel hot electron and erased
by Fowler–Nordheim tunneling. The main reliability issues, such as
charge retention and endurance, will be discussed, together with the
understanding of the basic physical mechanisms responsible. Most
of these considerations are also valid for the NAND cell, since it is
based on the same concept of floating-gate MOS transistor.
Furthermore, an insight into the multilevel approach, where two
bits are stored in the same cell, will be presented. In fact, the exploitation
of the multilevel approach at each technology node allows
the increase of the memory efficiency, almost doubling the density
at the same chip size, enlarging the application range, and reducing
the cost per bit.
Finally, the NOR 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 130-nm technology generations.
The technology development and the consolidated know-how is
expected to sustain the scaling trend down to the 90- and 65-nm
technology nodes as forecasted by the International Technology
Roadmap of Semiconductors. One of the crucial issues to be solved