Nanocrystal Memories: An Evolutionary Approach to Flash Memory Scaling and a Class of Radiation-Tolerant Devices

The Flash memory was conceived as an improvement of the EPROM (Erasable Programmable Read Only Memory) concept invented in 1980s from an initial idea of Frohman-Bentchkowsky [1]. The EPROM memory electrically programmed and erased by ultraviolet (UV) - irradiation became the most important non-volatile memory (NVM) application in the 1980s. The Flash, which owes its name to the fact that the whole memory array can be erased quickly in the same time, introduced the advantages of the electrical erase and the possibility to reprogram the read only memory in situ, with no need of removing it from the system [2,3]. Over the years Flash memory has widely been accepted as the NVM of choice for many applications and today the large majority of NVMs is based on Flash technology. The Flash market has grown in a very fast way due to the large diffusion of portable and low power consumption multi-media applications, which requires an extensive use of NVMs. The continuous scaling of nonvolatile memories has pushed the Flash technology toward its limits [4]. Now, several constraints, mainly due to electrical and reliability reasons, are threatening the future scaling of the Flash technology and new concepts of non-volatile memories have been proposed. NVMs based on natural traps in dielectrics (such as SONOS) or on floating nanocrystals (NCs), artificially embedded in dielectrics, offer an interesting scaling alternative to the Flash with the conventional floating gate, because of several potential advantages associated with the discrete nature of the storage [5-7]. These memories are an evolution of the Flash concept where the monolithic floating gate is supplanted by a number of discrete charge trapping nodes. Because in these discrete storage nodes devices charges are immune to the leakage caused by localized oxide defects, they can allow for a very aggressive scaling of the tunnel oxide and hence of the cell area, by keeping good performance and reliability characteristics. Today, nanocrystal and SONOS memories have found an important field of application in embedded systems where the non-volatile memory is hosted into a logic system. The high interest toward embedded applications is mainly driven by the easiness of process, due to the fact that a very thin storage layer can be implemented in place of the thick poly-silicon floating gate as well as to the possibility of using lower voltages. Some semiconductor companies have announced that they have started production of embedded memories based on nanocrystals. Nanocrystals memory has also shown a higher endurance to high temperature than its counterpart SONOS. Recently the NC memories have shown a promising route toward radiation tolerant application. Actually, as information is stored in discrete centers, they are expected to exhibit a higher tolerance to radiation effects such as total ionizing dose effects (TID) and single event effects (SEE). In the first part of this chapter we will present an overview of the nanocrystal memory technology as candidate to be an alternative to conventional Flash NVMs, by showing a comparison with the mainstream technology. The discussion will be focused on the scalability of the device and on its performances and reliability. In the second part of the chapter we will address the application of NC memories as radiation tolerant devices, such as in military applications, nuclear power stations, nuclear waste disposal sites, high-altitude avionics, medical and space applications. In particular, we will compare NC memories characteristics with the ones of Flash memories.