17-11-2012, 01:49 PM
PHASE CHANGE MEMORY
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Abstract :
Phase Change Memory is a promising memory technology that has recently experienced a re-surgence of interest. PCM employs a reversible phase change phenomenon to store information through a resistance change in different phases of a material. Advances in memory technology and pioneering work conducted by Numonyx has moved the technology to the forefront of the memory industry R&D activity. PCM offers a combination of some of the best attributes of NOR flash, NAND flash, EEPROM and RAM in a single memory device. These capabilities uniquely combined with the potential for lower memory subsystem costs could poten¬tially create new applications and memory architectures in a wide range of systems.This paper describes the basic technology and capabilities of PCM.
Introduction: -
In computing, the term Memory refers to the physical devices used to store programs (sequences of instructions) or data on a temporary or permanent basis for use in a computer or other digital electronic devices [3]. Memories are classified on the basis of function ability, access patterns and the nature of the storage mechanism [1]. Two types of memories are: Read-Only (ROM) and Read-Write (RWM) Memories. As the name suggests RWM have the feature of providing both read and write function. These RWM memories belong to the category called volatile memories, in which the data is lost when power is turned off. It is desirable to use memory devices that will retain the information even after the power is temporarily interrupted [2]. Semiconductor memories, have also been developed with this characteristic and are known as Non-Volatile Memories (NVM). Read–only memories (ROM) belongs to the category of NVMs.These memories are used as secondary storage. The most widely used primary storage is volatile form of random access memory (RAM). NVM have certain limitations of which it cannot be used as primary storage. The data in ROM cannot be modified, it can only be read. The most recent entry in the field are memory modules that can be classified as Non Volatile, yet offer both read and write functionality and comes in the class of non-volatile read-write memories(NVRWM) [1]. It includes EPROM (erasable programmable read-only memory), EEPROM (electrically erasable programmable read only memory), flash memory.
Advantages of Non-Volatile Memories:
• Information Retained even after supply voltage is off
• Low Cost
• High Density
• Low Power Consumption
• Low voltage power supply operation
• Operation over wide temp range
• Fast access
The first category of NVM consists of ROM also known as the masked ROMs( M-ROMs) , in which the data are permanently written during the manufacturing and cannot be altered by user. At manufacturing time, the truth table is provided by user. The manufacturer converts the truth table into appropriate ROM mask pattern, which implements the required logic function.
Another category of NVM is user-programmable ROM in which data can be entered by user. The first example of this type is programmable ROMs (PROMs) in which the data can be written only once; that’s why are also called OTPs (One-time programmable). The PROMs are manufactured with fusible links (usually made of nichrome, polysilicon, or titanium-tungsten) which can be blown by the user to provide connecting paths to the memory storage elements.
History and Background :
In 1950s and 1960s, Dr. Stanford Ovshinsky began researching the properties of a class of amorphous materials [5]. Amorphous materials are those materials which do not have definite crystalline structure. By 1968, he reported that certain glasses have the property of reversibly change in resistivity, upon the change in phase. By 1970, he with his wife Dr. Iris Ovshinsky founded, Energy Conversion Devices (ECD), published the results of a collaboration with confounded of Intel-Gordon Moore. The September 28, 1970 issue of Electronics featured the world’s first PCM, a 256 bit semiconductor device. In 2005, ST Microelectronics (ST) and Intel agreed to co-develop a 90mm PCM technology.
Since first work in 1970, much progress has been done in semiconductor manufacturing technology, enables the practical development of PCM. During that time period, phase change materials begin used in rewriteable CDs and DVDs. Now a days, most DVD-RAMs use the exact same alloy used in Numoyx PCM development.
Theory of Operation:
The operating principle of PCM is based on the reversible change between the two phases i.e. amorphous and crystalline of phase change materials. These two phases of chalcogenide material have different electrical resistivity and this also forms the basis by which the data are stored. As shown in fig. , in the amorphous phase, the material is highly disordered i.e. there is absence of regular order to the crystalline lattice [5]. This phase has high resistivity and high reflectivity and used to represent a binary 0. In contrast, in the polycrystalline phase, the material has regular crystalline structure and has low resistivity and reflectivity. This phase is used to represent a binary 1.