Simulation Research On Discrete Charge Trapping Memory

With the restrict of reducing the size of of traditional floating gate memory, we urgent need to develop the next-generation non-volatile memory. At present, there is a very wide range of research about this new type of non-volatile memory, such as ferroelectric memory, resistive memory, nanocrystal memory, charge trapping memory, phase change memory and organics memory. Among these kinds of memories, descrete charge trapping memory is studied widely and CTM(charge trapping memory) is the main research stream because of its compatible process with the current CMOS technology, so CTM has the great potential to become the next-generation non-volatile memory and to occupy the future memory market quickly.When the size of fash memory scales down to very small, there will be serious defects on data retention. The thinner the floating barrier layer, the more dangerous the information disclosure, because that the barrier layer will easily produce defects during many P/E cycles, which will cause storage charges leak at one time. Discrete charge trapping memory, which is using the discrete traps in the charge storage layer to store charges seperately, it theoretically offers a good solution to above problem happened in flash memory with small size, at this condition, even if there is some defect in barrier layer, only a small party of the stored charges near the defect area will leak out, which will not affect the whole stored information state and then this device can work normally. There are three main research orientation on the discrete charge trapping memory. First, to achieve the information discrete storage by discrete structure, such as nanocrystal memory. Second, by using the traps or defects existing in material itself to store information seperately. Third, to combine the above tow methods together.Nanocrystal memory is a kind of non-volatile memory using the nanocrystals as the strage medium, such use of discrete nanocrystals can easily achieve discrete charges storage, which solve the information leakage happened in traditional flash memory when the barrier oxide getting thinner and thinner. Besides, nanocrystal memory can reduce the operating voltages and maintain good data retention. So it is also a strong contender for the next generation of nonvolatile memory, attracting great interest from the world. Although there are many advantages of nanocrystal memory, take the following features for example, good P/E speed, low operating voltages and so on. When the size is getting small, the number of the nanocrystals will get strict limited, because the larger the number of the nanocrystals and the diameter of the nanocrytals, the better the storage performance of the nanocrystal memory will be, obviously, the above phenomenon is a contradiction. And the number of nanocrystals in different devices from the same production process may be very different, which will be a great impact for device uniformity. Therefore, we need to explore a more ideal nonvolatile memory.At the same time, CTM is adopting the traps or defects existing in different charge trapping layer material itseltf to store charges, even in these materials having more or better traps an defects by modification to achieve better storage function. It also can achieve multibits storage, thus the storage density will be improved with the same area, same technology. At present, the main CTM structure SONOS device can at least reach22nm process node.Thus, charge trapping memory is undoubtedly a effective solution to many problems, such as the decline in reliability in the traditional floating memory under small size, breaking the bottleneck of the further reduction of nanocrystal memory size.In summary, this paper has launched a series of simulation studies about discrete charge trapping memory, Firstly, this article briefly describes the storage mechanism, structure and application of flash memory, and then the simple describtion of CTM storage mechanism, structure and research information. In chapter three, the main focus is put on the nanocrytal memory structure simulation and its corresponding electric simulation, which offers the relationship between the nanocrystal memory structure and the storage properties. In chapter four, great attention is put to the material selection guidance for charge trapping layer in CTM, the whole chapter focuse on the performance parameters extraction. A new perspective and method is adopted here to study CTM. MS, which is a material soft for material calculation and analysis, is used to computer charge trapping layer material, in order to offer a scientific method for material selection. Some research about effective defects selection is also done here, to assit the trapping layer material selection. All these work can play a great role in CTM research, which can reduce the CTM research costs, shorten the research cycle and ensure the experiment success. In final chapter, the overall study summarizes the work of this article and gives a outlook.