The current mainstream non-volatile memory (NVM) technology based on floating-gate flash technology is expected to become a reference technology in the next few years. However, the inherent technical and physical limitations of flash memory make it difficult to shrink the technology node. In this environment, the industry is trying to invent a better memory technology with new materials and new concepts to replace flash technology, more effectively reduce memory and improve storage performance. This article will analyze new major inorganic material-based non-volatile memory technologies such as ferroelectric memory (FeRAM), magnetoresistive memory (MRAM) and phase change memory (PCM), as well as major ferroelectric or conductive based Innovative memory concept for switching organic materials such as polymers. Finally, we focus on phase change memory technology because it is the most likely to be the next generation of non-volatile memory technology, and we will analyze the main features and the latest developments of phase change memory technology. Solar Energy System Nut Ground Screw
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Foreword
Driven by the fast-growing non-volatile memory (NVM) market, in the past decade, several breakthrough memory technologies have emerged in the world, eliminating industry-standard technologies and expanding the application of flash technology. The widely accepted view in the industry is that if any technology succeeds, it will become a product within the next decade. At present, the industry has conducted feasibility studies on two new types of non-volatile memories, one of which is based on inorganic materials, such as ferroelectric memory (FeRAM), magnetoresistive memory (MRAM) or phase change memory ( PCM), another type of memory technology is based on organic materials, ferroelectric or conductive switching polymers. It is worth noting that this decade is coming to an end. Among these non-volatile memories that replace flash memory, only phase change memory has the ability to enter the vast market, and is regarded as the mainstream memory technology of the next decade.
Non-volatile memory instead of flash
Among the two new types of non-volatile memory technologies that have been investigated so far, the memory technology of organic materials based on ferroelectric or conductive switching polymers is still immature and is in the research and development stage. Some research and development groups working on this type of memory material began to believe that this concept will never become a real product. In fact, adapting these concepts to standard CMOS integration requirements and their manufacturing temperatures requires addressing several seemingly insurmountable challenges. On the other hand, the industry has been investigating the concept of new non-volatile memory based on inorganic materials for a long time, and has released several product prototypes in the past few years.
The concept of FeRAM technology emerged as early as the 1990s. Although there have been many technical problems related to new materials and manufacturing modules during the research process, after ten years of efforts, even if the inherent process shrinks, the technology node is much higher than flash memory, and the ferroelectric memory is now commercialized. . This memory concept still uses ferroelectric materials that can be polarized by an electric field. When the temperature is below the Curie point, the shape of the cube appears to be lattice-deformed, and the ferroelectric body is polarized. When the temperature is above the Curie point, the ferroelectric material becomes a paraelectric phase. So far, the industry has proposed a variety of FeRAM cell structures (as shown in Figure 1), these structures belong to two method systems, one is to integrate ferroelectric materials into a single storage element, that is, within the ferroelectric capacitor ( The method of integrating ferroelectric materials in two-transistor/double-capacitor (2T2C) and single-transistor/single-capacitor (1T1C) components, and the other is to integrate ferroelectric materials into selected components, ie, ferroelectric field effect transistors. . All FeRAM architectures have the advantage of fast access and true access to all memory cells. Today, the main direction of development of FeRAM technology is 64Mb memory of 130nm process. 
Figure 1 – FeRAM Cell Architecture
For many years, magnetic tunnel junction (MTJ) memory cells (shown in Figure 2) have been the main development of MRAM developers. The MTJ consists of a transistor and a resistor (1T/1R). These techniques are a special effect created by the integration of tunnel junctions and magnetoresistive materials: when a magnetic field is applied, the resistance changes. The fast, non-destructive read performance of fetching is a prerequisite for high performance, same read and write times, and low power operation. The main disadvantage of MRAM is that the inherent write current is too high and the technology node is limited. In order to overcome these two constraints, the industry has recently proposed a spin-transfer torque RAM (SPRAM) solution, which is a current-induced switching effect caused by spin-torque switching. Although this innovative approach solves some of the common problems of MRAM to some extent, there are still many challenges waiting for researchers to overcome (for example: self-reading disturbances, write times, cell integration, etc.). Today, MRAM manufacturing is limited to 4Mb. Array 180nm process products. 
Figure 2 – MRAM Cell Architecture Using the MTJ 1T1R Method
Phase change memory
PCM is the best flash replacement technology that can cover different non-volatile memory applications to meet both high performance and high density applications. The PCM utilizes a temperature change to cause a phase transition of the chalcogenide alloy (Ge2Sb2Te5). The basic unit structure consists of a transistor and a resistor (1T/1R), which uses the Joule heating effect caused by the current (Fig. 3-a) to write the unit by detecting the change in resistance between the amorphous phase and the polycrystalline phase. Read the storage unit. Although this technology dates back to the 1970s, it has only recently been re-attempted to use it in non-volatile memories (the commercial success of optoelectronic storage devices using phase-change alloys has also led to the discovery of better performance). Excellent phase change material structure research activities), phase change memory proves its ability to achieve manufacturing maturity. We compared phase change memory with other mature nonvolatile memory technologies in the table later in this article. Combining the advantages of both non-volatile memory and DRAM memory, the new features of PCM are attractive for new applications and a continuous and groundbreaking memory technology. From an application point of view, PCM can be used in all memory systems, and is particularly suitable for memory systems of consumer electronics, computers, and communication three-in-one electronic devices. In particular, in a wireless system, the PCM can be used as a code execution memory; the PCM can be used as a rewritable read-only memory, storing all data structures except the data structure with the highest processing frequency, and storing frequently accessed in the solid state storage subsystem. Pages; save data elements that are easier to manage when processing data immediately; computer platforms can take advantage of their non-volatility. 
Figure 3a - Schematic diagram of self-heating of a written memory cell of a PCM prototype structure
The technical development route of PCM is shown in Figure 4. The industry developed the first chip test carrier using the 180nm technology node and verified the feasibility of the technology. The BJT selected unit is selected for high performance and high density memory because the cell size can be "5F2 (where F is the minimum half pitch of the memory cell). Although the cell size is large ("20F2"), the integrated memory only needs to be Adding a small number of masks to the logic process is a significant cost advantage, so MOS-selected cells are suitable for system-on-chip or embedded applications. 
Figure 4 - Roadmap for PCM Technology Development
Intel and STMicroelectronics have developed a 128Mb 90nm phase change memory called Alverstone, which is now commercialized. Another 45nm 1Gb PCM product is now in advanced development, with a unit size of 5.5F2 (Figure 3-b). 
Figure 3 b-section of the b-45nm technology PCM array
PCM technology development will go along different routes in parallel. The mainstream development route will be to use BJT selected units, along the lithography development route, to shrink the existing technology architecture, providing the smallest unit size. In addition to the widely used Ge2Sb2Te5, the use of new sulfur-based alloys is another important area of ​​research, as this may open up entirely new applications; alloys with extremely fast crystallization rates or higher crystallization temperatures will be more attractive.
In terms of memory architecture, one of the main lines of research will be to implement more than one memory stack using a true array of intersections. Different solutions have been proposed in the industry by integrating a pn junction or Schottky diode as a component in the post process. Sulfur-based materials are particularly suitable for this stacking method because after the chalcogenide material is stacked, its phase change characteristics (like Ge2Sb2Te5) can constitute a memory element, while its electronic switching characteristics (like OTS) constitute a selection element (Fig. 5). In this case, the cell size can reach 4F2, and the bit size is a fraction of the cell size, depending on the number of stacks. This technique is suitable for high-density memories, especially for memory applications. 
Figure 5 - A fully integrated crosspoint PCM array with CMOS technology
In short, the existing technology maturity, technology node reduction capabilities, a wider range of applications, and new materials and new architecture can further expand the scope of application, all of which will pave the way for phase change memory technology to play an important role in the memory market in the next decade. The road 
