STT-RAM: SURVEY OF CURRENT AND EMERGING TRENDS

SARANSH PARASHAR

NEED FOR NEW MEMORY TECHNOLOGY

• Critical applications now-a-days are more data-centric and less compute-centric.

• Memory performance becoming a bottleneck due to increasing amount of data.

• Compute speed can still be increased but processor speed is limited by memory speed.

• Clock speed cannot be increased above certain limit to avoid heating.

• Need of an alternate memory technology which can replace existing technologies in memory hierarchy.

REVOLUTION IN MEMORY

• Researchers proposed STT-RAM (Spin Transfer Torque Random Access Memory).

• It can be used as ‘Universal Memory’.

• It has potential to replace all memories in existing memory hierarchy.

• It has characteristics of every level in memory hierarchy viz. high performance high density high capacity high endurance low power consumption non-volatility future scalability

LEADING DEVELOPERS AND RESEARCHERS OF STT-RAM

• Grandis Inc. is the key inventor and pioneer in STT-RAM technology.

• Hitachi and Tohoku University demonstrated a 32-Mbit STT-RAM in June 2009.

• In 2011, Qualcomm presented a 1 Mbit Embedded STT-MRAM, manufactured in TSMC's 45 nm LP technology at the Symposium on VLSI Circuits.

ARCHITECTURE AND DESIGN OF STT-RAM CELL

• MTJ (Magnetic Tunnel Junction) is the basic storage element of the cell.

• MTJ structure: Two ferromagnetic layers viz. Hard layer and Free layer Hard layer has fixed magnetic orientation. Free layer has variable magnetic orientation. Data is stored as relative magnetic orientation of these two layers.

• Physics: Two physical phenomenon describe the functionality of STT-RAM. Tunneling magnetoresistance (TMR) effect for reading and the spin-transfer torque (STT) effect for writing. TMR enables magnetic state of the Free layer to be sensed and hence the stored information to be read. STT enables electrons/current flowing through MTJ to change the relative orientation

STT-RAM SCHEMATIC, SINGLE CELL SCHEMATIC AND ITS SI SUBSTRATE REPRESENTATION

STT-RAM SCHEMATIC, SINGLE CELL SCHEMATIC AND ITS SI SUBSTRATE REPRESENTATION

• MTJ is connected between the Bit line and drain of the access/selection transistor.

• MTJ is represented as resistance.

• It exhibits low resistance when magnetic orientation between Hard Layer and Free layer is parallel (P).

• It exhibits high resistance when magnetic orientation between Hard Layer and Free layer is anti-parallel (AP).

WORKING OF STT-RAM CELL

• The relative magnetic orientation is changed by passing spin-polarized current through MTJ.

• Current transfers spin angular momentum between the magnetic layers.

• This results in torque on magnetization of Free layer changing its relative magnetic orientation w.r.t. that of Hard layer.

• Depending on the direction of current, the two layers are aligned in parallel or anti-parallel position.

WRITING ‘1’ AND ‘0’

STT-RAM AND OTHER MEMORY TECHNOLOGIES

CHARACTERISTICS OF STT-RAM

• STT-RAM has all the characteristics of a universal memory:Non-volatileHighly scalableLow power consumptionSRAM read/write speedUnlimited endurance (>10^16)DRAM & Flash density (6 F2)Multi-level cell capability

CHALLENGES FACED

• Two main challenges:-1. Stochastic nature of MTJ:

Transient behavior is non-deterministic due to random thermal kicks acting on its magnetization during switching activity. Thermal stability factor (D) is the measure of its stable transient behavior. D = (Hk*Ms^2*Ar*t^2)/(kB*T)

Where, Ms = Saturation magnetization, Ar = Area of MTJ, Hk = Uniaxial anisotropy, t = thickness of free layer

2. High write energy: Intrinsic/switching/write current density (Jc0) can give the measure of write energy. Jc0 = (2* α *Ar*Ms*t*e)(Hk + 2πMs)/(η*Ϧ)

Where, η = spin transfer efficiency, α = damping constant, e = electronic charge

• To maintain D with the high Ms, the switching current increases.

• For reliable memory operation, low switching current with high value of D is required.

CURRENT TRENDS

• Clinton W. Smullen, et al demonstrated a scheme to reduce write energy of STT-RAM by relaxing its non-volatility for using it as fast and energy-efficient cache. Reduction in planar area of MTJ caused reduction in retention time and hence non-volatility.

• In another paper, Clinton W. Smullen, et al, proposed a scheme to reduce write energy by tuning the saturation magnetization ‘Ms’ and thickness of Free layer ‘t’.

• In the same paper, Clinton W. Smullen, et al, proposed another approach wherein the difference between write energy for write 0 and write 1 is considered and the minimum of two is used write STT-RAM cell. 7% average reduction in total write energy is achieved.

• Vidyabhushan Mohan, et al, considered room temperature as parameter to adjust the retention time and hence reduce write energy. 70% reduction in energy-delay product is supposed to be achieved.

CURRENT TRENDS

• Zhou, Ping, et al. proposed early write termination (EWT) scheme to improve the energy efficiency of a STT-RAM cache. In this, the content of cache is read out first before doing a write operation, compared with the incoming bits and only the different bits are wrote back. 80% reduction in write energy and 33% reduction in total energy is achieved .

• Yazdanshenas, Sadegh, et al. proposed coding last level STT-RAM cache wherein the frequent data in the cache lines was coded to avoid unnecessary update of significant part of the word. 12% reduction in ‘write in the cache lines’ is supposed to be achieved as compared to the EWT technique.

CURRENT TRENDS

• Jog, Adwait, et al. cache revive technique to calculate retention time. Some cache blocks retain data even after completion of retention time. The retention time is so chosen that will minimize the number of unrefreshed cache blocks. This technique would increase the performance and energy by 18% and 60%, respectively.

• Yue Zhang1,2, et al, used stochastic nature of MTJ as a knob to maintain the thermal stability. Multi level cell structure was designed and programmed such that cells change their states randomly but correctly.

• Kultursay,E, et al, evaluated the performance of STT-RAM as a main memory and showed that the performance and energy can be significantly improved by using partial write and row buffer write bypass. 60% reduction in main memory energy can be achieved as compared to DRAM.

CURRENT TRENDS

• Park, H., et al, provide an insight into limited density of STT-RAM due to access transistor width and proposed a cell structure with multiple MTJs per access transistor. Limited amount of sharing is possible but considerable increment in density is achieved.

• Clinton W. Smullen, et al, deals with new class of in-plane MTJ structures with high partial perpendicular anisotropy for simulating and modelling memory systems. Perpendicular anisotropy provide faster switching and higher density compared to in-plane anisotropy.

• Grandis Inc. explored dual MTJ structures with two barrier layers and obtained reduced Jc0 with maintained thermal stability.

FUTURE PROSPECTS

• Promising candidate for replacing SRAM, DRAM and MRAM in cache and main memory.

• Excellent scalability feature allows the researchers to evaluate its performance in multicore processing systems and parallel high performance computing systems.

• Compact and simple cell structure provide higher packaging density thus decreasing the cost per unit. Possible to implement multi-level cell structure using MTJ further increasing the cells per unit area.

• High endurance implies increased lifetime of memory.

FUTURE PROSPECTS

• Standalone STT-RAM and Embedded STT-RAM product roadmap:

Mohamad T. Krounbi, S. Watts, D. Apalkov, X. Tang, K. MoonV. Nikitin , A. Ong, V. Nikitin, E. Chen, “Status and Challenges for Non-Volatile Spin-Transfer Torque RAM (STT-RAM) ppt,” International Symposium on Advanced Gate Stack TechnologyAlbany, NYSeptember 29 –October 1 , 2010.

CONCLUSION

• STT-RAM is capable of being called as ‘Universal Memory’.

• maintaining a balance between its two parameters viz. thermal stability and write current density is a prime challenge before it can be used as a Universal Memory.

• Novel techniques have been proposed and are being invented which could help improve its performance and energy efficiency.

THANK YOU!