Advanced Macro-Model with Pulse-Width Dependent Switching Characteristic for Spin-Transfer-Torque based Magnetic-Tunnel-Junction Elements

Abstract

Spin transfer torque (STT) switching in magnetic tunnel junction (MTJ) is one of the most promising magnetic random access memory (MRAM) technologies because it allows for small dimension, high speed, low programming current and low power dissipation [1]. On the other hand, an STT-based MTJ suffers from write error problems and asymmetric writing current problems because the same current path is used for both reading and writing. An accurate simulation model of an STT element for a circuit simulator such as HSPICE is essential to design MRAM circuits with STT-MTJ elements. There have been several works to model basic characteristics of conventional or STT-based MTJ [2],[3],[4]. These models, however, do not fully represent the behavior that the minimum current density for switching depends on the pulse-width of applied current. The prior models of an STT-based MTJ focused on the modeling of asymmetrical tunneling magneto resistance (TMR). In these models an MTJ switches its magnetic polarity as soon as the write current density exceeds certain threshold even though the duration of the current is very short. If a short current pulse, such as noise, flows through the STT-MTJ, the prior models may produce wrong results by switching the state of resistance when the resistance does not switch in a real situation. That may result in false self-read disturbances appearing in circuit simulations due to short noise currents. Also, the switching may occur earlier in a simulation than in a real situation, which makes timing simulation inaccurate. In this paper, we present an advanced macro model of an STT-MTJ that can accurately represent the characteristics of the STT-MTJ as a circuit element such that the minimum switching current density depends on the pulse-width of the current through it. As HSPICE, the most popular circuit-level simulator, has limitations to directly describe the timing behavior of an STT-MTJ, an imaginary circuit elements are adopted in our macro-model. This work is the first attempt to consider the pulse-width of the current flowing through the STT-MTJ in modeling the switching behavior.;Spin transfer torque (STT)-magnetic tunnel junction (MTJ)은 고집적성과 빠른 speed, cell 정보의 비휘발성, 낮은 소비전력, 무한대의 기록/재생이 가능한 특징을 이유로 magnetic random access memory (MRAM)의 차세대 기술로 각광받고 있다. STT-MRAM과 STT-MTJ를 이용한 회로들의 정확한 simulation을 위해서는 STT-MTJ의 특성을 simulation 상으로 잘 재현해 줄 수 있는 macro-model이 필수적으로 필요하다. 특히 STT-MTJ는 아주 작은 전류 밀도로 동작시키며, MTJ에 write하는 경우와 read하는 경우에 그 경로가 같다. 전류의 크기나 펄스 폭 조절을 정확하게 하지 못하면 write를 하려고 했으나 write가 되지 않을 수 있고, read 동작을 수행하다가 write가 되는 write error가 발생할 수 있다. 이러한 현상을 방지하고 실제 STT-MTJ의 동작을 정확하게 예상하기 위해서는 전류의 방향, 크기뿐만 아니라 전류의 펄스 폭도 고려된 STT-MTJ 용 macro-model이 필요하다. 본 논문은 STT-MTJ의 전류의 pulse-width에 따른 switching 동작 특성을 반영하는 macro-model을 제안한다. 본 macro-model은 HSPICE simulation 상 STT-MTJ의 switching 특성 및 tunneling magneto resistance (TMR)특성을 잘 재현할 수 있도록 구성되었다. 특히 전류의 크기와 전류의 펄스 폭에 따른 switching threshold 변화의 모델링이 가능하며, 전류의 glitch로 인한 simulation의 오동작을 방지할 수 있다. 또한 온도에 따른 TMR 변화와 switching threshold 변화를 R-I loop로 재현할 수 있다.