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Design Articles

MRAM—The Future of Non-Volatile Memory?

Does your portable wireless design need the speed of SRAM, the high density of DRAM and the non-volatility of Flash in a single low-power memory? It’s time to look into MRAM.

By David Bondurant, Brad Engel, and Jon Slaughter, EverSpin Technologies, Inc.

Today’s portable electronics have become computationally intensive devices as the user interface has migrated to a fully multimedia experience. To provide the performance required for these applications, the portable electronics designer uses multiple types of memories: a medium-speed random access memory for continuously changing data, a high-speed memory for caching instructions to the CPU, and a slower, nonvolatile memory for long-term information storage when the power is removed. Combining all of these memory types into a single memory has been a long-standing goal of the semiconductor industry.

In the mid-1990’s, research on a new type of memory based on magnetic tunnel junction (MTJ) devices began worldwide. The new memory, MRAM (Magnetoresistive Random Access Memory) used sub-micron MTJ devices integrated with standard CMOS circuitry to form a high-speed, non-volatile memory. Because the data is stored as a magnetic state, MRAM is inherently nonvolatile as well as having unlimited endurance and fast operation.

The only company to date to successfully commercialize MRAM is EverSpin Technologies (recently spun-out of parent Freescale Semiconductor). MRAM has now been on the market for over two years and has proven to be a highly-reliable, high-speed non-volatile memory. Unlike other non-volatile memories, MRAM has infinite cycling endurance with no programming induced damage or wear-out. As a result of this unique property, MRAM has found wide acceptance in the battery-backed SRAM replacement market where continuous writing and long-term data retention are required and valued. Another unique property of MRAM is its ability to maintain its high performance and reliability over an extended temperature range. Current EverSpin MRAM products are available for Extended temperature operation from -40°C to 105°C and have been demonstrated to achieve military and automotive specs from -55°C to 150°C.

MRAM’s non-volatility combined with its high programming speed and low write energy per bit provides portable applications with a unique single memory solution. Because data can be stored very quickly and is always nonvolatile, the memory can be frequently updated and then powered down, allowing the designer to optimize the balance between high-performance and battery life. In addition, the MRAM process integration is compatible with standard CMOS, allowing MRAM to be easily embedded in System-on-Chip applications. This capability provides additional flexibility to the portable electronics designer.

How MRAM works

figure 1
Figure 1: The MTJ device has high and low resistance states
corresponding to the two stable magnetic states.

The heart of an MRAM memory cell is the magnetic tunnel junction (MTJ), a small device having two ferromagnetic layers separated by a thin dielectric layer as shown in Figure 1. It is designed to have two stable magnetic states, one with the layers polarized in the same direction and one having their magnetization directions antiparallel. The resistance of the MTJ is low if they are parallel and high if they are antiparallel. Reading an MRAM bit is very simple: a sense circuit passes a small current through the MTJ to measure if it is in the high or low resistance state.

The two major types of MRAM are different in the way they write the magnetic state, Toggle MRAM uses magnetic fields to write while Spin-Torque MRAM uses a current pulse directly through the MTJ. The MRAM in production today is Toggle MRAM, after the robust “toggle” method of writing the bits. Spin-Torque MRAM is in currently in the development phase and is expected to offer improvements in density and power.

figure 2
Figure 2: A Toggle MRAM cell during the write operation
figure 3
Figure 3: Spin Torque MRAM Cell uses current
through the MTJ to switch the magnetization
direction of the free layer.

The write operation for a Toggle MRAM is accomplished by passing currents through adjacent copper lines, as shown in Figure 2, to generate the specific magnetic field pulses needed to reverse the magnetization state. Only bits at the cross-point of two write lines will receive the correct pulses to be written while other bits in the array will not change state. This write scheme is non-destructive and allows random access of any address in the array.

In Spin-Torque MRAM, the magnetic state is switched by passing a current directly through the MTJ as shown in Figure 3, rather than through the influence of magnetic fields. The spin-torque effect switches the device to the parallel state when the current is passed in one direction, and to the anti-parallel state when the current is reversed. This type of switching eliminates one of the current carrying lines, enabling a more dense architecture with a cell size comparable to DRAM.

MRAM Characteristics

figure 4
Figure 4: MRAM has a unique set of memory characteristics
that allow it to replace multiple memory types.

MRAM has a unique set of attributes, summarized in Figure 4, that allow it to replace multiple memory types in many applications.

The first three MRAM attributes result from the fundamentals of data stored as a magnetic polarization. Nonvolatility is inherent because magnetization does not leak away with time like electric charge and because these small magnetic devices cannot be demagnetized like large magnets. Since magnetic devices have a natural frequency response in the GigaHertz range, fast writes are no problem and MRAM read speeds below 10ns have been demonstrated. And finally, write cycles are unlimited because there is no known wear-out mechanism related to switching a magnetic polarization. No matter how many times the magnetic polarization is reversed it always alternates between the same two stable states.

Modular integration describes the ability to add an MRAM memory module to a technology platform without the need for any changes in standard devices or process steps for that platform. MRAM technology is modular because the magnetic devices are inserted between the metal layers of standard CMOS foundry processes with only four added mask layers. The easy integration combined with its unique attributes makes MRAM an ideal embedded memory for many advanced applications. The MRAM module can replace RAM, ROM, and NVM with a single memory type that can be partitioned as needed by the application software.

The MRAM that EverSpin has in production today has proven to be the most reliable non-volatile memory ever produced with intrinsic reliability suitable for the most demanding applications and demonstrated operation over the extended temperature range from -40 ºC to 150 ºC. This robustness is the result of the magnetic properties described previously as well as the inherent stability of the oxide layer used in the devices.

While Toggle MRAM will continue to improve and scale to higher densities, Spin-Torque MRAM is in the R&D phase with the goal of providing a significant increase in density, lower cost, and lower power. The spin-torque effect begins to be practical for dimensions below 100 nm and the required current decreases as the area of the bit decreases. This and other factors make 65 nm the approximate entry point for Spin-Torque MRAM with smaller technology nodes becoming increasingly attractive.

Operating Spin-Torque MRAM arrays and reliable spin-torque switching have already been demonstrated. For example, EverSpin has demonstrated 65nm-scale bits cycled to an equivalent of over 1012 cycles without degradation of the bit parameters (Figure 5). While a number of challenges remain to be overcome before this type of MRAM will be ready for production, the results to date indicate that this technology could challenge all memory types at future lithography nodes.

Advantages for Portable Applications

figure 5
Figure 5: ST MRAM Endurance not degraded by cycling

The current EverSpin 1, 2, and 4Mb MRAM products have been designed for high performance with 35 ns read/write cycle time and SRAM interface timing. Typical line-powered applications include computer storage systems & servers, industrial automation & robotic, energy management, transportation, and military/avionics systems. However, a number of applications take advantage of MRAM’s ability to be rapidly cycle power to provide zero power standby current. One current MRAM wireless application is a wireless battery-powered sensor that is periodically powered up to take measurements and then powered down into an ultra-low standby current state with MRAM powered-off. Other examples are contactless RF ID tag devices for factory automation and transportation that are powered from an RF field that with the MRAM off between transactions.

We expect MRAM applications in battery-powered and wireless applications to increase significantly as the first serial SPI MRAM products reach the market next year. The products are compatible with industry standard serial EEPROM, Flash, and FRAM products but offer sleep mode standby currents as low as 3 uA, high serial clock rates, no write delays, unlimited endurance, and superior high temperature data retention. These products will be directly compatible with most ultra low power MCU products.

The modular nature of MRAM makes it easily embedded for replacement of both embedded Flash and SRAM with a single memory type. It integrates with CMOS logic processes with much less process complexity than either embedded Flash or DRAM, and is added at the end of the process to minimize impact on underlying mixed signal circuits.

Looking farther into the future, successful development of Spin-Torque MRAM technology would enable density and price points comparable to NOR Flash and low-power DRAM. Portable systems would benefit by the ability of Spin-Torque MRAM to provide DRAM read/write speed, unlimited endurance, low active current and standby current, and the zero power down current, while operating at a native 1.2 volt power supply level without charge pumps. Such a stand-alone memory would reduce system chip count while increasing software flexibility to partition memory as either data or program storage dynamically. Embedded Spin-Torque MRAM would improve system operation by reducing leakage currents, simplifying block power down, and improving system flexibility to partition programs and data dynamically.

Competitive Landscape and Outlook

Today’s low-power DRAM, NOR Flash, and Flash technologies are encountering increasingly difficult challenges as processes are scaled to 45 nm and below. A number of new memory technologies are competing to replace the incumbent commodity memory technologies. Ferroelectric RAM and Phase Change Memory technologies have been proposed as the replacements for wireless applications, but have faced limitations in read/write speed, write endurance, and long term data retention. Because of the proven manufacturability and high-reliability of MRAM, Spin-Torque MRAM has emerged as a more attractive alternative for advanced nodes. Spin-Torque MRAM has the potential to provide the superior speed, unlimited endurance, and long term data retention that the market needs, along with high density, low cost, and low power. It is competitive with both DRAM and NOR Flash cell size at 65 nm while providing lower write power than DRAM or SRAM.

 

 

MRAM
(180 nm)

MRAM (65nm)

Spin-Torque MRAM (65 nm)

Flash (65 nm)

DRAM (65 nm)

SRAM (65 nm)

Cell Size (um2)

1.25

0.16

0.04

0.04

0.03

0.3

Read Time

35 ns

10 ns

10 ns

10 - 50 ns

10 ns

1 ns

Program Time

5 ns

5 ns

10 ns

0.1-100 ms

10 ns

1 ns

Program Energy/bit

150 pJ

100 pJ

1 pJ

10,000 pJ

5 pJ

5 pJ

Endurance

> 1015

> 1015

>1015

105 write

>1015

>1015

 

Non-volatility

YES

YES

YES

YES

NO

NO

Table 1 – Comparison of Advanced Memories

Conclusions

Wireless and portable applications will benefit from a memory that combines the fast speed of SRAM, high density of DRAM, and the non-volatility of Flash in a single low-power memory that could simplify both chipset and standalone memory hierarchies. A number of technologies are competing to fill this need. With MRAM in production and Spin-Torque MRAM in development, this integrated magnetic technology is one of the leading candidates to replace DRAM & Flash in wireless systems.

Current MRAM products demonstrate a uniquely useful set of attributes – fast read/write speed, unlimited endurance, long term data retention without power, and zero power standby current with power off. MRAM is currently being used in selected wireless applications and the first chips with embedded MRAM are expected to enter the market soon. Intensive R&D efforts are focused on Spin-Torque MRAM with the goal of achieving density comparable to DRAM and NOR Flash but with lower write power and all the other advantages of MRAM.

EverSpin Technologies, Inc.
Chandler, AZ
(719) 661-7889
www.everspin.com

This article originally appeared in the July, 2008 issue of Portable Design. Reprinted with permission.

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