MRAM, a major breakthrough by TSMC

In recent years, non-volatile memory (NVM) technology has been experiencing rapid development. With the rise of emerging applications such as artificial intelligence, autonomous driving, and the Internet of Things, the traditional storage system is facing multiple challenges in terms of speed, energy consumption, and stability.

To achieve a balance among "speed", "energy - saving", and "stability", various types of new - generation memories (such as ReRAM, PCM, FeRAM, MRAM, etc.) have entered the R & D and verification stages, aiming to stand out in the "post - DRAM era". Against this backdrop, magnetoresistive random - access memory (MRAM) is considered one of the most promising general - purpose storage solutions due to its combination of high speed, low power consumption, and non - volatility.

It is reported that a multinational research team from institutions such as National Yang Ming Chiao Tung University in Taiwan, China, TSMC, and the Industrial Technology Research Institute has made a major breakthrough in MRAM technology. They successfully developed a spin - orbit torque magnetoresistive random - access memory (SOT - MRAM) based on β - phase tungsten material, achieving remarkable performance indicators: it only takes 1 nanosecond to complete data switching, the data retention time exceeds 10 years, and the tunneling magnetoresistance ratio is as high as 146%. This achievement, published in the journal Nature Electronics, paves the way for the industrial application of next - generation high - speed and low - power storage technologies.

The Need for Storage Technology Transformation

Current computing systems rely on a storage hierarchy consisting of SRAM, DRAM, and flash memory. However, as the technology node breaks through the 10 - nanometer mark, these traditional charge - based storage technologies are facing severe challenges: limited scalability, difficulty in performance improvement, intensified read - write interference problems, and reduced reliability. Especially in the context of the rapid development of artificial intelligence and edge computing, higher requirements are placed on memory - it should have the high - speed response capability of DRAM, the non - volatility of flash memory, and significantly reduce power consumption.

Against this background, emerging non - volatile storage technologies have emerged. In addition to SOT - MRAM, they also include spin - transfer torque magnetic random - access memory (STT - MRAM), phase - change memory (PCM), resistive random - access memory (RRAM), and ferroelectric random - access memory (FeRAM). These technologies all feature non - volatility, low latency, and low power consumption, and can be integrated with existing CMOS semiconductor processes, providing the possibility for the development of new computing architectures.

In comparison, the latency of DRAM is about 14 milliseconds, the read latency of 3D TLC NAND ranges from 50 to 100 microseconds, while the switching speed of the new SOT - MRAM reaches the 1 - nanosecond level, almost comparable to SRAM, and it also retains the advantage of non - volatility - meaning that data will not be lost even when the power is off.

The Unique Advantages of SOT - MRAM

The reason why SOT - MRAM has attracted much attention lies in its unique working principle and technical advantages. It uses materials with strong spin - orbit coupling to generate spin - orbit torque (SOT) to achieve the magnetization reversal of nanomagnets in the magnetic tunnel junction, thereby completing data writing and erasing.

Compared with other storage technologies, SOT - MRAM has three core advantages:

• High - speed writing: Through the spin - orbit torque effect, magnetization reversal can be completed within nanoseconds, which is much faster than the traditional magnetic - field - driven method.
• High energy efficiency: The three - terminal structure design completely separates the read and write current paths, effectively solving the durability problem and the magnetic tunnel junction resistance limitation faced by STT - MRAM, and significantly reducing energy consumption.
• High reliability: Since the read and write operations are independent of each other, the durability of the device is greatly improved, and it can withstand more read - write cycles. At the same time, it has excellent long - term data retention ability.

It is these advantages that make SOT - MRAM expected to replace SRAM at the cache level and become the core storage component of the new - generation computing system.

Overcoming Key Technical Challenges

Although the theoretical advantages of SOT - MRAM are obvious, to achieve industrial application, a key technical bottleneck must be solved: the thermal stability problem of spin - orbit coupling materials.

Tungsten is an ideal candidate material for SOT - MRAM due to its strong spin - orbit coupling characteristics. Especially, tungsten in the A15 structure (β - phase) has a spin Hall angle of up to - 0.4 to - 0.6, with excellent spin - orbit torque efficiency. However, β - phase tungsten is in a metastable state. Under the common heat - treatment conditions in semiconductor manufacturing (usually at 400°C for several hours), it will transform into thermodynamically stable α - phase tungsten. This phase transition is fatal - the spin Hall angle of α - phase tungsten is only about - 0.01, and the spin - orbit torque reversal efficiency is greatly reduced, causing serious degradation of device performance.

The research team's breakthrough solution is to insert an ultra - thin cobalt layer into the tungsten layer to form a composite structure. Specifically, they divided the 6.6 - nanometer - thick tungsten layer into four segments, and inserted a cobalt layer only 0.14 nanometers thick between each segment - this thickness is less than the single - atomic layer of cobalt, so the cobalt is discontinuously distributed. This ingenious design plays a dual role: the cobalt layer acts as a diffusion barrier to suppress atomic diffusion within the tungsten layer; the mixing effect between cobalt and tungsten consumes the thermal budget, thus delaying the occurrence of the phase transition.

The experimental verification is exciting: this composite tungsten structure can maintain phase stability at 400°C for up to 10 hours and can even withstand a high temperature of 700°C for 30 minutes, while traditional single - layer tungsten undergoes a phase transition after only 10 minutes of annealing at 400°C. Through transmission electron microscopy, X - ray diffraction, and nano - diffraction tests at the Taiwan Photon Source, the researchers confirmed the stability of β - phase tungsten.

More importantly, this composite structure not only solves the thermal stability problem but also maintains excellent spin conversion efficiency. Through spin - torque ferromagnetic resonance and harmonic Hall resistance measurements, the team measured the spin Hall conductivity of the composite tungsten film to be about 4500 Ω⁻¹·cm⁻¹ and the damping - like torque efficiency to be about 0.61. These parameters ensure efficient magnetization reversal performance.

Comprehensive Performance Verification

The theoretical breakthrough can only be truly realized through device verification. Based on the composite tungsten film solution, the research team successfully fabricated a 64 - kilobit SOT - MRAM prototype array and completed comprehensive performance tests and verifications under conditions close to actual applications.

In terms of switching speed, the device achieves a spin - orbit torque reversal speed at the 1 - nanosecond level, with performance almost comparable to SRAM and far exceeding that of DRAM and flash memory. Statistical tests on 8000 devices show that their reversal behavior is highly consistent. The intrinsic reversal current density under long - pulse (10 nanoseconds) conditions is only 34.1 megaamperes per square centimeter, demonstrating excellent stability and repeatability.

The data retention ability is also excellent. According to the cumulative distribution function (CDF) estimation, the thermal stability parameter Δ of the device is about 116, meaning that its data retention time can exceed 10 years, fully meeting the strict requirements of non - volatile storage.

In the tunneling magnetoresistance ratio (TMR) test, the device achieved a TMR value as high as 146%, indicating that a high - quality interface is formed between MgO and Co₄₀Fe₄₀B₂₀, providing strong support for a stable read margin and a reliable process window.

In terms of energy - consumption control, the three - terminal structure design realizes the complete independence of read and write operations, fundamentally reducing energy consumption, making it particularly suitable for power - sensitive applications such as edge computing and mobile terminals.

In addition, thanks to the participation of TSMC's research team, the entire design has been optimized for the existing semiconductor back - end process since the project's inception, ensuring excellent process compatibility and paving the way for future large - scale mass production.

It is worth mentioning that the research team also achieved X - type reversal without an external magnetic field. This achievement is due to the symmetry - breaking effect in the composite tungsten material, which not only further simplifies the device structure but also improves the integration level and design flexibility, opening up a new direction for the engineering application of SOT - MRAM.

Opening a New Era of Storage Technology

The significance of this research goes far beyond the technical breakthrough in the laboratory. It points out a new direction for the development of the entire storage industry. Different from many new - generation storage technologies still in the concept - verification stage, the SOT - MRAM based on composite tungsten considers process compatibility and manufacturability from the very beginning of the design. The research team has successfully fabricated a 64 - kilobit array and plans to further expand it to the megabit (Mb) level of integration, while reducing the write energy consumption to the sub - picojoule level per bit.

In the scenarios of artificial intelligence and edge computing, SOT - MRAM also shows unique advantages. The high - frequency data access during AI training and inference is the main source of energy consumption, and SOT - MRAM, with its high - speed, non - volatile, and low - power characteristics, can be used as an on - chip cache for AI accelerators, significantly reducing system energy consumption. In edge devices, its non - volatility means that the device can be quickly started and stopped without data loss, which is particularly beneficial for battery - powered IoT terminals.

At the same time, the emergence of SOT - MRAM may promote the reconstruction of the storage hierarchy. The traditional three - level architecture of "SRAM cache - DRAM main memory - flash external memory" may undergo changes. SOT - MRAM is expected to fill the performance gap between SRAM and DRAM and may even replace one of them in some applications, thereby simplifying the architecture and improving system efficiency.

At the material - science level, the "composite - layer stabilization of metastable phases" strategy proposed in the research is not only applicable to tungsten but also provides new ideas for the phase - stability research of other functional materials. The team plans to further explore new - type oxides and two - dimensional interface materials to improve overall performance and reliability.

More profoundly, this breakthrough may promote innovation in computing architectures. The high - speed and low - power SOT - MRAM makes new - type architectures such as "in - memory computing" more feasible and provides a new path to break through the "memory wall" bottleneck of the traditional von Neumann structure.

Conclusion

Currently, the SOT - MRAM based on composite tungsten solves the thermal stability problem of β - phase tungsten through ingenious material design, achieving a perfect combination of nanosecond - level switching and ultra - long data retention. This is not only an academic achievement but also a core technology reserve for the next - generation computing system.

For the research team, their goal is not only to demonstrate excellent laboratory performance but also to show through system - level verification how MRAM can significantly reduce overall power consumption in practical applications and promote technological innovation in AI, edge computing, and mobile devices. As the integration progresses from the kilobit level to the megabit level, we have reason to expect that this new - type memory will enter our smart devices in the near future, opening a new era of storage technology.

This article is from the WeChat official account "Semiconductor Industry Observation" (ID: icbank), written by Shao Yiqi and published by 36Kr with permission.