Phase-change memory

Phase -change random access memory ( Abbr: PCRAM or PRAM, in a specific embodiment also " Ovonics Unified Memory ", OUM, or " chalcogenide RAM", C-RAM ) is a novel non-volatile memory in the electronics (2009). The operating principle of the memory, the change in the electrical resistance of the memory material, depending on whether it is in the amorphous (high resistance / RESET state) or in a crystalline (low resistance / SET state) phase. The material used is a chalcogenide alloy ( chalcogen compound) - similar to the material, which also provides the basis of phase change in a CD -RW or DVD - RAM for data storage. The case used material combinations for example consist of germanium, antimony and tellurium (often alloys of the two compounds GeTe and Sb2Te3 ).

Design and function

The basic structure of a PCRAM memory cell resembles a first DRAM: it consists of a select transistor and the resistive phase change of planes element in which takes place the information storage. A plurality of memory cells are - arranged in a matrix - such as the DRAM.

The resistive memory element comprises a metallic top electrode, a bottom metal electrode and between the phase change material.

The change in the unseeded ground state ( "RESET state" ) by amorphization of a portion of the phase change material. To the material by means of a current pulse of higher current (hundreds of micro- amperes ) of lesser duration is heated ( e.g., 50 nanoseconds). After the end of the pulse, the material will cool very quickly - so quickly that it remains in the amorphous state and does not crystallize. To generate this pulse power effective, non-linearity may be used in the current-voltage curve of amorphous chalcogenide: this condition in itself is characterized by a high resistance. However, where the applied voltage is a threshold voltage, the material is again highly conductive (English: dynamic on state).

The change back to the crystalline set state ( "SET state" ) is (up to a few hundred microamps tens ) caused by a prolonged current pulse (eg 100 nanoseconds) lower amperage. Wherein the amorphous material is heated above the crystallization temperature (see glass), and as long as kept at this temperature that nucleation and crystallization begins to take place.

For reading the information, a voltage is applied across the resistive element, which produces such a low power, that the temperature does not reach the necessary level for a phase change in the material. Depending on the state it flows another stream, which is used for reading.

History

Already in the 1920s, the change in the electrical conductivity by a structural change in a chalcogenide ( MoS2) was discovered. In the 1950s we explored the semiconducting properties of crystalline and amorphous chalcogenides. Reversible phase-change materials have been studied with their electrical and optical properties, and then in the 1960's. At the time of the construction of a nonvolatile memory for electronic applications based on this principle has already been proposed.

Then, the phase - change technology, however, was initially developed in terms of their optical application and made ​​commercially viable: for rewritable CDs ( first product in 1990, Matsushita ) and DVDs. Only when in the course of these developments, materials have been discovered that were concerning writing times and currents in regions of interest, also got the phase-change RAM drive development.

In early 2006, Samsung memory chip as the world leader a 256 - megabit prototype and mid-2006, a 512 - megabit prototype as a replacement for flash chips before. Series production started in September 2009.

At the Intel Developer Forum 2007 in Beijing, Intel announced yet for the second half of 2007 at a private PRAM under the code name Alder Stone, with the so-called " ovonic " technology - derived from the discovery of phase-change materials Stanford Ovshinsky - works. The first promised chips have 128 megabits, however, significantly smaller capacity than NOR and NAND devices.

IBM announced in June 2011 that they have produced stable, reliable, high-performance multi-bit PCRAM.

Applications

The change in resistance between the two phases comprises about four orders of magnitude, which allows high Abtastunterschiede (English sensing margins). Since the phase change is (more or less) repeatable very quickly and, the storage principle for random access memory is, that is, as a possible replacement for DRAM and SRAM, and in particular, as it also is not volatile, both phases (amorphous and crystalline) are stable, as an alternative to the flash memory. Since the memory element with respect to the vertical structure in the area of the metallization ( the so-called back -end of line, BEOL ) is placed, and the materials and processes are largely integrated in a CMOS fabrication, the phase change memory is suitable not only for the manufacturing of memory devices, but also for embedded memories (English: embedded memory), realized on the same piece of silicon, such as the logic circuit, which mainly constitutes the block.

At present ( 2009) are in some semiconductor companies test structures and prototypes for memory modules in development. Samsung has started mass production in September 2009 and since April 2010 provides the memory as 512 Mbps The - integrated in an unspecified designated multichip package for mobile devices - from as PRAM. The Micron - Numonyx subsidiary distributes PCRAM since April 2010 under the name " Phase Change Memory" (PCM). The chips are sold in pairs with a size of 128 Mbits.

Development

In comparison to other non-volatile memory in the development stage PCRAM shows similar expected values ​​with respect to performance, long-term durability, and scalability.

The biggest problem of this storage principle is the need for writing Current: In order to be competitive, high storage capacity are indispensable in the same compact size, making it a high packing density of circuit elements, and thus a high degree of miniaturization in PCRAM and similar technologies are needed. Therefore, the MIS transistors used in PCRAM have a channel length of less than a hundred nanometers ( a nanometer is one millionth of a millimeter ) to give the maximum possible current drops to a few hundred microamps.

For this reason, it is worked in various ways to the reduction of the electric current required for writing:

• Phase change material and its doping: the exact composition of the material and by introducing foreign substances - eg nitrogen or tin - the electrical resistance of the material to be increased.
• Size and shape of the bottom electrode: only material in the immediate vicinity of the bottom electrode is phase changed; Therefore, the bottom electrode determines the amount of phase change material, and thus the necessary power for heating. On the one hand electrodes are published that contact the material only selectively laterally in order to have as little aufzuschmelzendes material. On the other hand, the phase change material in so-called sub ​​-resolution vias deposited ( openings in the insulating layer, which are so small that they can no longer be defined by the photolithographic resolution, but, for example by reflow or etch -back techniques need to be further reduced in size).
• Thermal insulation of the storage element: the material is surrounded by thick metal electrodes and poor thermal insulating material as a part of the generated heat energy for melting the material flows away without the desired effect.
• Use of bipolar transistors: a few companies evade the problem by using this powerful transistor technology for the selection transistor. For most phase-change memory applications that would be uninteresting due to the significant additional costs.

Approaches to multi-level storage have been proposed:

• Variation of the conversion volume by varying the programming pulses: different high programming currents down according to the pulse of more or less material of the phase transition. This more than two distinguishable resistive states can hold it - even though the temperature dependence of the resistance, the sensing margin narrows significantly.
• Crystal lattice dependent resistor depending on the doping of the material, it can also come in different crystalline structures (hexagonal and face-centered cubic ) - depending on the temperature in the crystallization phase - present. The two crystal lattices are again distinguished by their electrical resistance - but only an order of magnitude.