Magnetoresistive Random Access Memory
In contrast to conventional storage technologies such as DRAM or SRAM , the information is stored not with electrical but with magnetic charge elements, i.e. the properties of certain materials are used that change their electrical resistance under the influence of magnetic fields. In principle, different mechanisms of action can be used:
- Anisotropic magnetoresistance (ger .: Anisotropic Magnetoresistance AMR)
- Giant magnetoresistance (ger .: giant magnetoresistance , GMR)
- Magnetic tunnel resistance (ger .: tunneling magnetoresistance , TMR)
The latter is currently the preferred technique for developing magnetoresistive RAMs.
A toggle write MRAM cell ( TW MRAM cell) is based on a field effect transistor . There is a magnetic tunnel contact (MTK) above . The spin magnetization of the variable ferromagnetic layer of the MTK is controlled by the magnetic field of the write line. Depending on the direction in which the current flows through the write line, the direction of the spin magnetization of the variable ferromagnetic layer changes.
Vertical transport MRAM
In Vertical Transport MRAM ( V-MRAM ) is an alternative design of TW-MRAM, in which a vertical line is used to umzumagnetisieren the MTK. The primary purpose of the design is to minimize interference from the write line to neighboring components in order to enable a higher component density on the chip.
In thermal-assisted switching MRAM ( TAS-MRAM ) is an alternative form of MRAM, MRAM with which phase-change RAM connects. The MTK is heated up in order to facilitate the remagnetization. The cell is then operated in a colder state.
Spin Transfer Torque MRAM
With spin transfer torque MRAM ( STT-MRAM ), also referred to as ST-MRAM or SPRAM , the spin magnetization of the ferromagnetic layer of the MTK is controlled directly via the source line, using spin-polarized electrons. The spin moment of the electrons creates a moment in the ferromagnetic layer, whereupon it changes its spin orientation.
In comparison to TW-MRAM cells, STT-MRAM cells have a more compact design, which enables higher storage capacities of the MRAM chips. Also, especially with small structure sizes, smaller currents than with TW-MRAM are necessary. The disadvantage is that the spin coherence of the controlling electrons must be preserved.
The advantage of MRAM technology is that it is non-volatile , which means that the chips retain their stored data even after the power supply has been switched off. This enables electronic devices such as B. computers can be realized, which are ready for operation immediately after switching on and do not first have to load the data necessary for operation from a permanent memory, such as a hard drive , into the main memory. In contrast to established non-volatile memory technologies such as Flash , MRAMs, such as conventional DRAM / SRAM, can be written to practically infinitely. Write and read access times will be in the range from DRAM to SRAM. MRAM is intended to combine the advantages of the various established memory technologies and thereby have the potential for so-called “universal memory” , which could replace DRAM, SRAM, EEPROM and Flash.
The individual layers of the magnetic tunnel contacts are about 1 nm and below only a few atomic layers thick. Individual manufacturing defects, often atom-sized, can lead to a short circuit and thus to failure of the cell. If, on the other hand, the layers are made too thick, the required TMR effect does not occur. This leads to a high failure rate compared to other technologies such as DRAM, SRAM and Flash memory.
Another disadvantage is the somewhat more complex control electronics, as the spin direction must be taken into account. Changing the spin alignment of the ferromagnetic layer is also more energy-intensive than with DRAM and SRAM cells. However, since the cell can maintain its condition for several months and therefore does not need regular refreshment of the storage condition, it is still more energy-efficient overall.
Currently (as of 2017) Everspin Technologies is the only commercial supplier of MRAM memory chips. Until 2012, Everspin used the TMR technology in the so-called toggle-write variant. The magnetic bit is set by the magnetic field of two external write lines, at the crossing point of which the magnetic field is added and thus reaches a strength with which the magnetic polarization of the cell is changed. In 2013, so-called spin-torque modules were introduced. Here the polarization of the cell is changed using a current that flows through the cell. The spin-torque technology enables the production of MRAM in smaller structure sizes and is therefore considered to have a promising future.
Almost all other major memory manufacturers such as Samsung and Hynix have announced that they will invest in MRAM development and production. So far, however, these manufacturers have not presented a finished product and have largely stopped development.
Due to the high price, MRAM is primarily used in industrial systems to prevent critical data loss. Typical applications are programmable logic controllers (PLC), POS / electronic cash , GPS trackers or as a cache in server systems. MRAMs are also increasingly used in the aerospace industry due to their high radiation resistance. MRAM memories were first used in gaming machines to replace battery-backed SRAM memories.
- In 1989, IBM scientists made a series of key discoveries about the “ GMR effect ” in thin film structures.
- In 2000, IBM and Infineon founded the Joint MRAM Development Program .
- In 2002 NVE announced a technology exchange with Cypress Semiconductor .
- In the summer of 2003 a 128 kibit MRAM chip was presented, which was manufactured using 0.18 micrometer technology.
- In June 2004, Infineon presented the first 16- Mibit -MRAM component, also using 0.18 µm technology.
- At the end of 2004, Freescale Semiconductor (formerly Motorola Semiconductor) started delivering 4-Mibit prototypes (0.18 µm).
The series production of the MRAM was announced by various companies ( IBM , Infineon , Motorola ) for the years 2004/2005. Many well-known companies have either withdrawn completely from this branch due to problems in the mass production of the chips or have postponed series production until the end of the decade.
- In 2008, after years of research and development and a long sampling phase at Freescale, series production of the 4-Mibit MRAM MR2A16A begins. This memory chip is very expensive compared to SDR or DDR SDRAMs at around 25 US dollars, which severely limits its field of application. A read / write cycle takes 35 ns , many times longer than with SDRAM or even newer RAM technologies.
- At the end of 2008, Freescales MRAM technology will now be produced and sold by Everspin Technologies as part of a management buyout . Various products are available, which differ both in total memory size (256 KiB to 16 MiB ) and in memory word width (8 bit or 16 bit).
- In 2012, Everspin released an ST-MRAM chip with 64 Mibit storage capacity.
- In 2013 Buffalo Technologies was the first company to manufacture a SATA Ⅲ SSD with Everspin's ST-RAM as cache.
- In 2016, Everspin continued to produce only ST-MRAMs. Since production is carried out in an older factory of the service provider Globalfoundries , as well as due to the larger memory cells compared to NAND, these MRAM chips only offer a storage capacity of 256 Mibit.
- For 2017, Everspin expects an increase in capacity to 1 gbit per ST-MRAM chip.
Due to the high production costs, which are estimated to be 50 times those of NAND flash, and the low storage density, MRAM has so far only been suitable for niche applications.
- MRAM community with information and news . (English language web portal)
- electronics360.globalspec.com: Group Explores Nanoscale SST-MRAM Technology. 22. May 2013
- Christof Windeck: New approaches in MRAM development Heise online, November 7, 2007
- PTB: MRAMs save data without electricity - and now also extremely quickly. March 8, 2011, accessed February 20, 2016 .
Footnotes and individual references
- Christof Windeck: IBM and Infineon announce advances in MRAM . In: Heise-Online. June 10, 2003. Retrieved August 26, 2009.
- Christof Windeck: VLSI Symposium: Many new (M) RAM types . In: Heise-Online. June 15, 2004, Retrieved August 26, 2009.
- Everspin Technologies - MRAM product range ( Memento of the original from August 7, 2013 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- Lutz Labs: Super-fast SSDs with ST-MRAM. In: Heise Online . August 12, 2016, accessed March 1, 2017 .