Nonvolatile multilevel memories for digital applications

When thinking of semiconductor memories, it comes naturally to associate st ored bits and memory cells with a one-to-one relationship that, however, is not really a must nor necessarily the most convenient solution for data st orage, since using analog signals and digital-to-analog (D/A) as well as an alog-to digital (A/D) conversions a large number of bits could be memorized in a single cell, although, of course, the use of analog signals presents all the drawback of signal-to-noise ratio that are so well known in electro nics. In fact, the real question in this sense concerns the number of bits used for the A/D and D/A conversions, since the conventional (fully) digita l case can be seen as the simplest realization of a general approach tendin g to infinitely precise analog storage (i.e., an infinite number of stored bits per cell) at the other extreme. Naturally in the real world the confli cting aspects of density (measured in bits per cell) and noise immunity (in a general sense) should be traded off one against the other looking for op timum use of silicon area, of course depending on technology, architectures , circuits and reliability. From this point of view it is obvious that the fully digital approach based on the one-bit one-cell concept does not repre sent necessarily the best solution. Recently, this general question has assumed real and practical significance for nonvolatile memories, since devices storing two bits per cell are now being introduced on the market. At the same time, in a number of research l abs a significant effort is currently being dedicated to the study of the l imits and practical convenience of storage density considering the current state of the art in technology and circuit design. This problem, however pr esents a number of interacting aspects concerning cell concept, programming and reading schemes, and architectures and reliability that are of interes t well beyond the field of nonvolatile memories, because they are ultimatel y dealing with the basic question of analog versus digital signals. In this context, the present paper considers the question of multilevel non volatile memories in all its interacting aspects, analyzing both the curren t state of the art and the future possibilities.