Phase-change memory

Phase-change memory (also known as PCM, PCME, PRAM, PCRAM, OUM (ovonic unified memory) and C-RAM or CRAM (chalcogenide RAM) is a type of non-volatile random-access memory. PRAMs exploit the unique behaviour of chalcogenide glass. In the older generation of PCM, heat produced by the passage of an electric current through a heating element generally made of TiN was used to either quickly heat and quench the glass, making it amorphous, or to hold it in its crystallization temperature range for some time, thereby switching it to a crystalline state. PCM also has the ability to achieve a number of distinct intermediary states, thereby having the ability to hold multiple bits in a single cell, but the difficulties in programming cells in this way has prevented these capabilities from being implemented in other technologies (most notably flash memory) with the same capability. Phase-change memory (also known as PCM, PCME, PRAM, PCRAM, OUM (ovonic unified memory) and C-RAM or CRAM (chalcogenide RAM) is a type of non-volatile random-access memory. PRAMs exploit the unique behaviour of chalcogenide glass. In the older generation of PCM, heat produced by the passage of an electric current through a heating element generally made of TiN was used to either quickly heat and quench the glass, making it amorphous, or to hold it in its crystallization temperature range for some time, thereby switching it to a crystalline state. PCM also has the ability to achieve a number of distinct intermediary states, thereby having the ability to hold multiple bits in a single cell, but the difficulties in programming cells in this way has prevented these capabilities from being implemented in other technologies (most notably flash memory) with the same capability. Newer PCM technology has been trending in two different directions. One group has been directing a lot of research towards attempting to find viable material alternatives to Ge2Sb2Te5 (GST), with mixed success. Another group has developed the use of a GeTe–Sb2Te3 superlattice to achieve non-thermal phase changes by simply changing the co-ordination state of the Germanium atoms with a laser pulse. This new Interfacial Phase-Change Memory (IPCM) has had many successes and continues to be the site of much active research. Leon Chua has argued that all two-terminal non-volatile-memory devices, including PCM, should be considered memristors. Stan Williams of HP Labs has also argued that PCM should be considered a memristor. However, this terminology has been challenged and the potential applicability of memristor theory to any physically realizable device is open to question. In the 1960s, Calvin Tiebusch of Energy Conversion Devices first explored the properties of chalcogenide glasses as a potential memory technology. In 1969, Charles Sie published a dissertation, at Iowa State University that both described and demonstrated the feasibility of a phase-change-memory device by integrating chalcogenide film with a diode array. A cinematographic study in 1970 established that the phase-change-memory mechanism in chalcogenide glass involves electric-field-induced crystalline filament growth. In the September 1970 issue of Electronics, Gordon Moore, co-founder of Intel, published an article on the technology. However, material quality and power consumption issues prevented commercialization of the technology. More recently, interest and research have resumed as flash and DRAM memory technologies are expected to encounter scaling difficulties as chip lithography shrinks. The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity values. The amorphous, high resistance state represents a binary 0, while the crystalline, low resistance state represents a 1. Chalcogenide is the same material used in re-writable optical media (such as CD-RW and DVD-RW). In those instances, the material's optical properties are manipulated, rather than its electrical resistivity, as chalcogenide's refractive index also changes with the state of the material. Although PRAM has not yet reached the commercialization stage for consumer electronic devices, nearly all prototype devices make use of a chalcogenide alloy of germanium, antimony and tellurium (GeSbTe) called GST. The stoichiometry or Ge:Sb:Te element ratio is 2:2:5. When GST is heated to a high temperature (over 600 °C), its chalcogenide crystallinity is lost. Once cooled, it is frozen into an amorphous glass-like state and its electrical resistance is high. By heating the chalcogenide to a temperature above its crystallization point, but below the melting point, it will transform into a crystalline state with a much lower resistance. The time to complete this phase transition is temperature-dependent. Cooler portions of the chalcogenide take longer to crystallize, and overheated portions may be remelted. A crystallization time scale on the order of 100 ns is commonly used. This is longer than conventional volatile memory devices like modern DRAM, which have a switching time on the order of two nanoseconds. However, a January 2006 Samsung Electronics patent application indicates PRAM may achieve switching times as fast as five nanoseconds. A more recent advance pioneered by Intel and ST Microelectronics allows the material state to be more carefully controlled, allowing it to be transformed into one of four distinct states; the previous amorphic or crystalline states, along with two new partially crystalline ones. Each of these states has different electrical properties that can be measured during reads, allowing a single cell to represent two bits, doubling memory density. PRAM's switching time and inherent scalability make it most appealing. PRAM's temperature sensitivity is perhaps its most notable drawback, one that may require changes in the production process of manufacturers incorporating the technology. Flash memory works by modulating charge (electrons) stored within the gate of a MOS transistor. The gate is constructed with a special 'stack' designed to trap charges (either on a floating gate or in insulator 'traps'). The presence of charge within the gate shifts the transistor's threshold voltage, V t h {displaystyle ,V_{mathrm {th} }} higher or lower, corresponding to a 1 to 0, for instance. Changing the bit's state requires removing the accumulated charge, which demands a relatively large voltage to 'suck' the electrons off the floating gate. This burst of voltage is provided by a charge pump, which takes some time to build up power. General write times for common Flash devices are on the order of 100 µs (for a block of data), about 10,000 times the typical 10 ns read time, for SRAM for example (for a byte).