Introduction
In recent years, with the rapid development of the internet of things (IOT) and electronic technologies, embedded devices such as mobile phones, smart watches, and sports bracelets have become important elements of cloud computing, IOT, and big data analytics. Embedded terminal devices become more usual to the daily life. However, to meet the high storage requirements of these increasingly diverse applications, scientific research personnel face more challenges. Non-volatile memory is more and more popular in the market due to its advantages such as low energy consumption, non-volatile, high density, and low latency. The following is a basic introduction of new non-volatile memory with good future development prospects.
Memory is an important part of computer systems. According to different positions in the storage system, memory can be simply divided into three types: on-chip memory, main memory, and hard disk. Correspondingly, static memory (SRAM), dynamic memory (DRAM), and magnetic disks have become the traditional technologies to realize these storage system. Over the past 40 years, these three technologies have achieved tremendous technical and commercial success. With the exception of magnetic disks, the manufacture of both static and dynamic memory relies on semiconductor integration technology. Although these two technologies are different, they also have a common characteristic: they both use the accumulation and release of charge on semiconductor devices to achieve data storage. For example, in a dynamic memory, the charge is used to represent a logic 1, and the discharge represents logic 0. In static memory, this process is achieved by charging and discharging the parasitic capacitance of the transistor. For example, solid-state storage, which widely used in flash memory, also stores data by capturing and releasing the charge on the floating gate of the transistor.
With the improvement of manufacturing technologies, the size of semiconductor nano devices has continued to shrink, and the charges that can be stored on all of the above traditional memory devices has also decreased, which has brought serious reliability issues:
First, more leakage current; second, a small perturbation of charges will have a relatively greater impact. In addition, the inherent limitations of the charge storage mechanism itself also can’t be avoided, the processing of traditional mainstream memories at the nanoscale and the process disturbances are also very challenging.
From the aspect of functions, static memory and dynamic memory both belong to the volatile memory category. Its characteristic is that when the power supply is off, the data stored in the memory will disappear and cannot be recovered. Especially in the design of dynamic memory, the charge on the capacitor will gradually leak out as the time increases. Therefore, the stored info need to be refreshed periodically. The static memory doesn’t have this issue, but the area of the memory cell is usually large (about 20 times that of the dynamic memory cell), causing serious leakage current.
Flash memory belongs to the non-volatile memory, and the data stored on it will be retained for a long time after the power off. In terms of performance, the first two types of memory read & write speed are on the level of nanoseconds, while the flash memory are on the level of hundreds of microseconds or even milliseconds. For the reading mode, the static memory and the dynamic memory can achieve random storage, for example, any one or several memory cells can be read and written at will. It different for Flash, although its storage density is high, the read operation must be performed in units of pages. Moreover, the content update of the flash memory cannot be achieved by directly overwriting the original content, but must be written to a new erased page. In addition, the maximum times of read and write supported by flash memory is extremely limited, typically between thousands of and millions of times.
Concept and Classification
Concept:
Non-volatile memory refers to the computer memory of the person whose stored data will not disappear when the power is turned off. It is characterized by non-volatile, byte-by-byte access, high storage density, low energy consumption, and fast read and write speed, but the read speed far faster than write, in other words, they are asymmetric in a limited life.
Classification:
According to whether the data in the memory can be rewritten at any time, the traditional non-volatile memory can be divided into two categories: read-only memory (ROM) and Flash memory. New type non-volatile memory compared with traditional non-volatile memory, its device has greatly improved energy consumption, read and write speed, integration density, etc.
At present, the newly developed new non-volatile memory mainly includes four types: dielectric memory (FRAM), magnetic medium memory (MRAM), Ovonic phase change memory (OUM), and polymer memory (PFRAM).
The following highlights four new non-volatile memories.
The technical limitations of traditional memories and the huge challenges brought by the light weight have prompted researchers to look for a new generation of memory devices. People want to find a memory with the following characteristics:
1) Nano level read and write speed of static memory
2) Integrated density with dynamic memory and even flash memory level
3) Flash-like non-volatile memory features
Although such a storage technology has not been fully realized at present, some very promising new storage devices have been developed, and some have even entered the production stage. The four newly developed non-volatile memories are very promising for data processing, because of the limitations of traditional non-volatile memories, it is very likely to replace flash memory in the future.
FRAM
a. Intoduction
FRAM is the non-volatile memory technology in the new generation. In terms of performance, it consumes low energy and can store data for a long time although there is power failure. It combines the characteristics of high read-write speed of RAM and long-term data storage of ROM. Embedded FRAM in the non-volatile memory situation of radiation-resistant and low-power has great significance. It can be embedded in the chip in a more direct way and has better performance than any other alternative chips. In terms of manufacturing and technology, FRAM is easier to reduce size than flash memory due to the advanced nodes (65 nm or smaller), and does not require the use of very thin oxides or high voltages.
b. Limitation
When FRAM reaches a certain times of read and write, FRAM cells will lose their durability, and the FRAM yield problem caused by array size restrictions and further improvements in storage density and reliability still need to be resolved.
c. Application
FRAM is a non-volatile memory that combines the advantages of low power consumption, high speed, long service life, and anti-radiation. It is promising in RF 1C card, fast startup memory, and system chip of cache and aerospace.
d. Commercial progress
From the point of international respect, well-known American company Ramtorn, which developed the first 4K bit commercial ferroelectric memory in 1993; after 1998, Ramtorn focused on product research and development, and handed over all production to semiconductor manufacturers. With the time goes by, Ramtorn represents the highest level of PZT-based commercial ferroelectric memory. Many other countries started late in the field of ferroelectric research and mainly based on scientific research. For example, the main work of some countries is still the preparation of ferroelectric thin films.
MRAM
a. Introduction
MRAM is a non-volatile memory. For the performance, the write speed of MRAM is extremely fast, almost 1000 times that of flash memory, and 20 times that of FRAM. And it has unlimited read and write times, also it can switch on and off instantly and extend the battery life of portable computers. In terms of manufacturing and technology, the circuit of MRAM is simpler than ordinary memories, and only one readout circuit is needed for chip access. In addition, MRAM is easier to integrate (only 5 photomask layers are needed in the entire process), and there is no need to redesign at the transistor level of flash memory. All other core technologies used in the design can remain the same and work consistently.
From 1986 to 1988, Albert Fert and Peter Grünberg discovered that nano-multilayer films composed of alternating ferromagnetic and non-magnetic metal layers made of molecular beam epitaxy have a much larger size than AMR, which is named as giant magnetoresistance (GMR). GMR is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers, but it was difficult to put into practice. Soon, further research by Parkin team found that the (ferromagnetic / non-magnetic metal / ferromagnetic) three-layer film made by sputtering technology has a much larger giant magnetic resistance at room temperature than a single-layer ferromagnetic metal. For the spin valve, opened the way for practical use of GMR.
The discovery and research of GMR had led to the realization of high-sensitivity read heads in high-density disks, and promoted the development of the entire modern hard disk industry. The earliest application of spin valve sensors in hard disk read heads was in the IBM Deskstar 16GP Titan, which was released in 1997 and has a storage capacity of 16.8GB. In 2007, Hitachi introduced the Deskstar 7K1000, the first 1TB hard drive.
b. Limitation
MRAM is much higher than flash memory in the production costs.
c. Application
With the advantages of low power consumption, high-speed reading, high integration, radiation resistance and unlimited rewrites, MRAM is used in storage, industrial automation, gaming, energy management, communications, consumption electronic,transportation and avionics fields. In addition, the IOT and big data analytics are gradually emerging,ubiquitous sensor terminals need to collect massive amounts of data, in order to save storage power consumption, MRAM and STT-MRAM have become the better choices for their relatively good performance.
d. Commercial progress
In 2006, Freescale launched the world’s first commercial MRAM product with a capacity of 4Mb. Judging from the current product specifications and development status, the use of MRAM is still limited to some specific markets. From the perspective of cost and capacity, it cannot compete with NAND flash memory with a maximum capacity of 8Gb and DRAM with 512Mb capacity. However, with Samsung, Intel, TSMC and Global Foundry and other integrated circuit leaders strengthening investment in R & D and related production lines, STT-MRAM is gradually begin mass production, partially replacing SRAM and DRAM products and becoming one of the mainstream memories.
OUM
a. Introduction
Phase change memory is a kind of memory that realizes information storage through material phase change. It is the non-volatile and large-capacity storage technology advocated by Intel, the world’s number one semiconductor chip manufacturer. In terms of performance, it has a long read and write operation life and is easier to integrate than flash memory. OUM memory cells are extremely dense, and read operations is more safer than other memories. Low energy consumption, requiring very low power to operate. In addition, OUM unit can write about 1 billion times, which makes it an ideal alternative to large-capacity memory in portable devices. From the aspect of manufacturing process, compared with the integration of existing logic circuits, its storage unit is only 1/3 of MRAM and FRAM, and its production cost is lower than other new memories.
Crystalline Phase and Amorphous Phase Change
Although phase change memory is often categorized as “new memory”, the concept of “phase change” has introduced over 50 years. In 1962, the phase transition of As-Te glass was discovered. In 1968, Stanford Robert Ovshinsky described in an article that certain semiconductor materials can rapidly switch between two different states of resistance and conductivity under the action of an electric field (on the order of 10μs), he utilized chain structures, cross links, polymeric concepts, and divalent structural bonding with a huge number of unbonded lone pairs to achieve what is now referred to as the “Ovshinsky Effect”, an effect that turns special types of glassy, thin films into semiconductors upon application of low voltage.
This discovery directly led to a large number of subsequent studies on the phase transition of thin films based on tellurium-arsenide-germanium-silicon alloy materials or sulfur-based glasses. In 1970, Nevill and Gordon Moore demonstrated the world’s first 256-bit phase change memory, and Moore was later known for putting forward the famous “Moore’s Law” about the number of transistors in a dense integrated circuit doubles about every two years and served as the co-founder of Fairchild Semiconductor and CEO of Intel. After that, research on semiconductor memories based on phase change materials has gradually slowed down due to issues such as materials and power consumption, but phase change materials have been used very successfully in rewritable optical discs such as CD-RW / DVD-RW.
b. Limitation
The read and write speed and frequency of OUM are not as good as FRAM and MRAM, and how to maintain its driving temperature stably is also a big technical problem.
c. Application
Phase change memory is suitable for wired and wireless communication equipment, consumer electronics, PC and other embedded applications due to its fast read and write speed, strong upgrade capabilities, and low power consumption. For example, it is used in the aerospace embedded system and used in smart meters to further integrate its storage architecture.
d. Commercial progress
Phase change memory, as one of the most promising new memories, can be embedded at all levels of the memory architecture. Because of the similarity between phase change memory and dynamic memory, especially its lower power consumption and scalability, it has been considered as the best substitute for dynamic memory. But phase change memory also has disadvantages. The first is its limited times of erases and writes (usually only 107 to 108). If the number of erasing and writing exceeds this limit, the life of the memory cell will end, and the device can no longer be used. The second disadvantage is the limited write speed. The write speed of phase change memory is 6-10 times slower than dynamic memory.
Write Operation of Phase Change Memory Unit
Nevertheless, phase change memory still has good applications in some fields. Several related studies have proposed various methods to overcome these shortcomings. For example, an architecture adjustment is used to compensate for the loss caused by the performance of writing, which can greatly reduce power consumption, thereby accelerating the commercialization of phase change memory as the main memory of a computer.
In addition, the multi-level cell technology has been successfully implemented on phase change memory. In the design of a multi-level cell phase change memory, 2N resistive states are used to represent N digits, respectively, in other words, in a 2-bit multi-level cell phase change memory, 00, 01, 10, and 11 can be represented by four different resistance values, respectively. In the specific design, the resistance of the phase change material can be changed by adjusting the amplitude and time of the writing current / voltage.
PFRAM
a. Introduction
PFRAM is a plastic, polymer-based, and non-volatile memory. In terms of performance, PFRAM has advantages such as good stability and low power consumption. From manufacturing process, high density can be obtained through three-dimensional stacking technology, which is easy to make and has extremely low manufacturing costs, only about 10% of NOR-type flash memory.
b. Limitation
PFRAM has a limited read and write operation life and its reads are destructive.
c. Application
The final product of (PFRAM will be an all-organic storage system, which will be suitable for personal computers, handheld computers, digital cameras, mobile phones, handheld radios and communication devices, GPS systems, audio, video, game background program and other important fields.
d. Commercial progress
PFRAM develops slowly in commercial use, and Intel is in a leading position. In 2014, Intel recruited JonKrueger (architecture and software engineer) to work for its polymer memory group and greatly promote the development of multi-layer plastic memory, finally their work is close to the software development stage, which indicates that this memory technology will accelerate to the market.
Memory technology will continue to improve to meet different applications. On the one hand, The new type memories will create a new market and enter various application markets, on the other hand, it involves new materials and research concepts, it will be difficult to become the mainstream of the market in a long time. However, in the aerospace, industrial automation, embedded cache of system chip and other sub-application areas, the new non-volatile memory will gradually transfer its technological breakthroughs to market penetration and achieve rapid development.
With the advent of the 5G era, the development of application markets such as the IOT, artificial intelligence, and smart cities, and the urgent need for diverse memory requirements, coupled with traditional memory market change, new type memories will play an increasingly important role in the market.
