The “vision” in the field of non-volatile memory devices
The vision in the field of memory technologies is to manufacture the so-called “universal memory cell” which is characterized by the following important properties:
• Non-volatile memory storage
Non-volatile memories retain stored data also without external voltage supply
• High density data storage
Memory phenomena must be used which work also in case of nano-scaled materials
• Fast program / erase times
High processing speeds require the use of fast programming and read-out phenomena
• High endurance and retention
Many read/write cycles (“endurance”) and long-term data storage (“retention”)
No memory technology concept exists today in the field which combines in a unique advantageous way all these properties [1, 2]. In consequence, world wide R&D efforts focus today on number of different approaches of which the technologically most important ones are briefly outlined in the following:
FLASH: Floating gate (FG) Flash cells are nowadays the commercially most successful NVM approach in portable electronic devices [1]. It allows the user to electrically program and erase information. In flash memory, the cell structure is similar to EPROM architectures, but with a much thinner, precisely grown tunnelling oxide between the floating gate and the channel (“source”) of the Si MOSFET (see Fig.1). Flash programming (erase) occurs when electrons are placed (displaced) on (from) the floating gate. Storage of charge on the floating gate allows to alter the threshold voltage (Vt) of the transistor between a low and high value, presenting logic 0 and 1 values of the memory cell.
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Figure 1: A typical flash floating gate (FG) memory cell |
However, the further miniaturization (“scaling”) of FLASH is limited. In especial, the scaling of the tunnel oxide results in severe reliability concerns which are difficult to solve. It is predicted that, within the next decade, FLASH will hit a “hard” scaling limit so that a further increase of storage density can not be achieved.