In a recently published article in IEEE Electron Devices Magazine the authors, I was one of them, looked at the impact of external magnetic fields on spin tunnel torque magnetic random-access memories, STT-MRAM. The findings of this application note are important as they indicate that STT-MRAM can replace NOR flash and volatile memories for embedded electronic applications, even as the feature size of the electronics continue to shrink. For these applications, STT-MRAM must not be very susceptible to errors due to ambient magnetic fields.
NOR flash is used for code storage in most embedded electronic devices and NOR flash in embedded electronics can’t scale below 28nm. In addition, static random-access memory, SRAM, lithographic feature scaling has slowed and SRAM memory cells are quite large because they generally need five transistors to retain the charge that creates the memory. MRAM and other emerging non-volatile memories only need a single transistor selector.
Thus, both NOR flash and SRAM are reaching density limits and a new memory technology, particularly a non-volatile memory, that can replace these memories could provide scaling advantages, more memory in a given space and if nonvolatile, allow more lower power operations and thus conserve power, particularly important for battery powered applications. Major semiconductor foundries, including TSMC and Samsung, are now offering MRAM as well as other non-volatile memories as a substitute for embedded NOR flash and to replace slower SRAM caches.
The maximum field tolerance of these MRAM devices is between a few tens of milliTesla, mT, to 100mT, depending upon the MRAM target application. 10mT in SI units is 100G in CGS units of magnetic field. The accepted standard from the International Electrotechnical Commission, IEC, IEC-61000-4-8 regulates the magnitude of magnetic fields that devices can encounter during regular operation, with a maximum value of 1.26mT, or 12.6G. The earth’s magnetic field is about 0.05mT, or 0.5G.
Magnetic measurements in household and work environments demonstrate measurements below 0.1mT, or 1G, at distances greater than 16cm, or 6.3in, from various appliances and tools. The image below from the article, attributed to HPROBE, shows magnetic fields measured at various places in a car. The measurements show less than 1mT for every measurement except directly on a magnetic holder for a phone. Note that magnetic fields drop off rapidly with distance from a source.
This application note says that based on the magnitude of commonly encountered external magnetic fields, the magnetic immunity of STT-MRAM is sufficient for most uses once the chip is mounted on a printed circuit board (PCB) or inserted in its working environment. This statement is supported by the experience acquired during 60 years of use of magnetic hard disk drives (HDDs), including 20 years of HDDs with readers comprising magnetic tunnel junctions (MTJs), 20+ years of use of magnetic field sensors as position encoders in the automotive industry, and 15+ years of use of MRAM.
However, during chip handling, caution does need to be exercised to avoid exposing the chip to external magnetic fields. This can be accomplished with simple precautions. Once an MRAM chip is mounted on a PC board or inserted in its working environment, normal ambient magnetic fields should have no effect. In extreme magnetic environments, the magnetic sensitivity can be controlled by sufficient separation from the field source or methods such as magnetic shielding.
An application note published in the IEEE Electron Devices Magazine shows that STT-MRAM memories should be able to achieve high densities with simple precautions in normal ambient magnetic fields.
