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Examining Everspin's MRAM technology

Posted: 13 Jul 2012 ?? ?Print Version ?Bookmark and Share

Keywords:MRAM? non-volatile memory? Solid state drives? Magnetic Tunnel Junction?

Consumers continue to long for the promise of speed in our electronic gadget startup times. Solid state drives (SSDs) that have been introduced to the market have taken a serious bite out of the wait time involved for a device to go from zero to ready. SSDs currently rely on non-volatile memory (NVM) in the form of flash and, although an order of magnitude faster than conventional hard drives, these drives still incur a noticeable wait time on startup. The use of flash in these drives highlights how read times for flash memory still pale when compared to ROM or DRAM.

In the mid 2000s, magnetoresistive random-access memory, or MRAM, emerged as a promising new NVM technology with high reliability, near infinite lifetime, low power consumption, large operating temperature range and, perhaps most importantly, fast read times comparable to DRAM. Several companies are engaged in research into the technology with the hope of producing a competitive NVM to rival other memory types in density and speeds. Everspin has emerged as one of the first companies to produce an MRAM product that has found commercial acceptance.

Magnetic Tunnel Junction (MTJ) is the key element to store information in MRAM. Conventional MTJ consists of two ferromagnetic layers separated by a thin tunnel dielectric. Information is stored as directions of magnetization vectors in the magnetic layers. The magnetic vector in one layer is magnetically fixed or pinned, while the magnetization of the other layer is free to switch between the same and the opposite directions that are called "parallel" and "antiparallel" states, respectively. When a small bias voltage is applied between the fixed and free layers, a tunneling current then flows through the thin intervening dielectric layer. In response to parallel and antiparallel states, the magnetic memory element represents two different resistances. Accordingly, a detection of change in resistance allows MRAM to provide information stored in the magnetic memory element.

Figure 1: X-Ray of the Everspin MRAM package.

The MTJ structure is integrated into the interconnect portion of an otherwise typical CMOS integrated circuit. During write operation, the selected MTJ is situated between the selected write wordline and selected write bitline. Currents forced in the selected word and bit write lines create magnetic fields in the vicinity of these wires. The vector sum of the fields at the selected MTJ must be sufficient to switch its state. However, the field generated by the selected word or bit write line alone must be small enough that it never switches the state of the so-called half selected MTJs that lie along the selected word and bit write lines.

In 2006, UBM TechInsights analyzed a Freescale MRAM device that had a die size of about 26 mm2 and a density of 4 megabits (Mb). Compared to other NVM technologies, the die efficiency seemed low due to an architecture requiring rather large overheads. Today, Everspin Technologies (a company with its roots in Freescale) is the manufacturer of a 16 Mb MRAM device with a die size of only about 58 mm2, in effect quadrupling the density while only doubling the die size. An obvious method for increasing the density is through process shrink.

After opening up the 16 Mb package, however, it quickly became apparent that the increase in density is not through an advanced process node; both the 16Mb and the 4 Mb devices share a majority of lithographic processes and more specifically the same cell size. It is likely that a smaller process would lead to the half-select issues described above. Instead, Everspin redesigned the architecture and eliminated one of the backend interconnect layers, relying on improved overhead circuitry to minimize the real estate required to write, read and erase data.

Figure 2: Die Photo of the Everspin MRAM.

Everspin claimed a 300 percent increase in MRAM shipments in 2011, citing 250 new design wins, including companies such as Dell, LSI and BMW. Spin-torque, the next generation technology for MRAM, promises even faster read/write cycles with improved densities. Is it possible that MRAM could one day replace flash for use in SSDs and reduce the boot up time even less? We'll know soon enough, as the SSD market grows exponentially, so will the demand for a faster memory.

- Mark Chambers
??Senior Process Analyst
??UBM TechInsights





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