With manufacturers striving to extract more performance from electronic devices, there is an accelerating trend towards alternative types of memory. Paul Stevens considers magnetoresistive random access memory (MRAM) and some of this technology's variants.
Ten years ago a 32MB Compactflash memory card would have been considered adequate for a digital camera, but today a digital camera with limited camcorder capability could benefit from one of the many relatively low-cost memory cards with a capacity of 32GB. This requirement for a 1000-fold increase in memory capacity is not atypical; in other applications, from computers to industrial controllers, designers are demanding memory that is both of a higher capacity and faster.
One of the alternatives to conventional non-volatile memory is magnetoresistive random access memory (MRAM), in which data is stored by means of magnetic storage elements instead of as electric charges or current flows. A variation of MRAM is spin-transfer torque RAM (STT-RAM), which relies on spin-aligned electrons. While the different variations of MRAM technology are, compared with conventional memory, still in the early days of commercialisation, proponents of MRAM believe this technology offers an attractive combination of faster operation, reduced power consumption and, unlike flash RAM that can degrade when it is written to repeatedly, the potential for an indefinitely long life without degradation. Furthermore, while static random-access memory (SRAM), dynamic random-access memory (DRAM), electrically erasable programmable read-only memory (EEPROM) and flash memory all have slightly different characteristics, it is thought that MRAM could potentially replace all of these diverse types.
In June 2011, Crocus Technology, a developer of magnetic semiconductors, unveiled its Magnetic-Logic-Unit (MLU) architecture, which it described as a scalable evolution of its Thermally Assisted Switching (TAS) technology for advanced memory and logic. Claiming this to be a first for the industry, Crocus said its innovative MLU architecture had the potential to create new applications in high-density data storage, secure commerce and communications, high-performance network processing, and high-temperature automotive and industrial applications.
Most magnetic memories are based on arrays of memory cells, each of which contains two magnetic layers. The first, often called the reference layer, is always magnetised in one direction, while the second, called the storage layer, is magnetised in either the same direction as the reference layer to store a '1' or the opposite direction to store a '0'. However, Crocus' MLU architecture can be configured with fixed magnetisation to implement a traditional logical NOR (not Or) function, with floating magnetisation to implement a logical NAND (not And) function, or with driven magnetisation to implement a logical XOR (eXclusive Or) function.
In high-density memory applications, the MLU architecture enables NAND configurations to be implemented in magnetic memory; this was previously possible only with flash memory technology. According to Crocus, MLU NAND memory can be two to four times denser than conventional magnetic memory, with the added benefit of fully random access. Crocus also offers an MLU XOR, called Match-In-Place, which implements ultra-secure compare and encryption functions that are claimed to make smart cards, identity cards, SIM cards and near-field communications (NFC) devices tamper-proof. Match-In-Place also implements the search and compare functions required in network routing applications and high-performance computing, and can achieve up to 50 times the density of conventional complementary metal-oxide-semiconductor (CMOS) hardware search processors. In addition, MLU is capable of operating at temperatures of up to 200°C, making it suitable for use in automotive and industrial electronics.
Dr Bertrand F Cambou, the executive chairman of Crocus Technology, states: "MLU has the potential to replace SRAM, DRAM, NAND, NOR and OTP (one-time programmable) in many standalone and embedded memory products. Because MLU's NOR, NAND and XOR capabilities are built on a single wafer manufacturing process with different design architectures, they can be easily integrated into system-on-chip (SOC) implementations."
Indeed, Crocus announced at the end of 2011 that it had formed an alliance with Starchip, a company focused on designing and qualifying secure semiconductor products for mass production, to develop next-generation SOC products. Crocus and Starchip will work together to embed MLU memory and logic functions within next-generation secure processor-based architectures.
Everspin Technologies, which is a venture-funded spin-out from Freescale Semiconductor, is relatively advanced in manufacturing MRAM devices. At the start of 2012, the company reported that shipments in 2011 were three times those achieved in 2010. Everspin says it is now supplying more than 300 active customers, in three major markets, with over 100 different MRAM products for in excess of 100 applications. As well as its discrete MRAM products, Everspin began shipments of embedded MRAM products in 2011, with an initial volume of more than two million units.
Jim Handy, an analyst with Objective Analysis, comments: "MRAM has gained acceptance as a superior alternative to non-volatile SRAM for RAID (redundant array of independent discs) controllers, allowing Everspin to capitalise on its unique position as a high-volume MRAM supplier. The company's impressive progress this past year is proof of OEMs' (original equipment manufacturers') increased interest and readiness to use MRAM in diverse applications."
Building on its track record in the industrial, energy, and automotive and transportation markets, Everspin achieved its significant growth in 2011 through its activities in the enterprise storage, server and networking segment. A number of leading vendors in this segment have turned to MRAM technology for critical data storage in RAID systems, servers and routers, which require reliable, enduring, fast non-volatile memory to capture meta-data that must be preserved reliably in the event of a power failure.
According to Everspin, MRAM enhances data centre and networking fault recovery capabilities to reduce system downtime. Compared with alternative non-volatile RAM technologies, MRAM also simplifies design by eliminating the need for external components such as resistors, capacitors, batteries or super-capacitors.
MRAM devices have been available for use in harsh operating conditions for several years. In October 2009, e2v claimed to be the first to market with MRAM devices for extended-reliability applications, with the EV2A16A (Fig.1). This is an extended-reliability version of the MR2A16A from Everspin Technologies, offering fast read and write cycle times (35ns), with non-volatility and unlimited read/write endurance (over 20 years for data retention), for demanding applications in the defence, avionics and industrial markets. The 4MB, small form-factor EV2A16A boasts an operating temperature range of -5 to +125°C. To simplify its integration with existing devices, the MRAM module is SRAM-compatible so that existing SRAM controllers may be used without the need for any redesign.
Of course, MRAM also offers benefits in less harsh operating environments, such as solid-state hard disk drives (SSDs) for desktop and laptop computers. In October 2011, Micron Technology, which manufactures SSDs, memory modules and display products, announced a three-year collaboration with Singapore's A*Star Data Storage Institute (DSI) for the development of high-density STT-MRAM.
Current commercial solid state drives (SSDs) use NAND flash memory rather than the spinning hard disc found in conventional disc drives. Demand for SSDs has been increasing rapidly due to their speed and durability, especially in applications such as laptops and mobile devices that are more likely to be subjected to physical shock and vibration than static computers.
However, as the memory industry continues to scale NAND flash memory, it sees issues such as limited endurance and high write power. The industry is therefore looking to alternative non-volatile memory technologies such as STT-MRAM.
For design engineers seeking to upgrade existing electronic products or develop higher-performance, higher-reliability, lower-power, next-generation products, the different variants of MRAM offer significant potential.
Extreme applications for MRAM
To illustrate the breadth of applications for which MRAM is suitable, consider the Everspin AEC-Q100 MRAM products that stores critical calibration data onboard the BMW S 1000 RR Motorrad Motorsport Super Bike, which races in the World Superbike series (Fig.2). Prior to each race, the bike's engine control unit (ECU) is fine-tuned for peak performance and optimised for the rider, track and race conditions. The 4 MB MRAM chip stores adjustable engine parameters relating to, for example, the bike's fuel injection, ignition, braking and acceleration. Because the memory is always non-volatile, a power loss after the data has been written to the MRAM does not affect data integrity.