Reliability Research And Optimization Of Magnetic Memory Based On STT

As we all know,there are some limitations in current mainstream memory technologies.For example,although SRAM and DRAM are fast,they are volatile.NAND flash memory is non-volatile,but slow and has limited endurance.Therefore,a type of memory that combines the advantages of these three storage technologies is needed.Spin-transfer Torque Magnetic Random Access Memory(STT-MRAM)is a good choice,as it not only combines the advantages of these three storage technologies but also has potential applications in emerging technologies such as artificial intelligence and the Internet of Things.However,during the research of STT-MRAM,a "balloon" phenomenon was discovered in the Write Error Rate(WER)curve of the device array.This phenomenon can cause the write voltage of STT-MRAM to be set too high,resulting in a decrease in the device’s erasable write cycles.Therefore,the focus of this study is on how to eliminate the balloon phenomenon.The specific research content is as follows:(1)The stability of intermediate states that appear during the flipping process of the free layer was studied using Monte Carlo simulation.It was concluded that the unstable intermediate state is the main cause of the balloon phenomenon.The position,quantity,size,write voltage pulse width,current density,and exchange strength of the defects in the free layer were verified using OOMMF software,and the simulation results showed that: 1)internal defects in the free layer are the main cause of intermediate states,and the more internal defects there are,the more likely intermediate states will occur;2)the larger the volume and quantity of internal defects in the free layer,the more likely intermediate states will occur;3)the more defects included in the domain wall after the write voltage is removed,the longer and more stable the intermediate state will be;4)intermediate states are most likely to occur in a certain range of current densities;5)when the exchange strength is greater than a certain value,it can offset the increase in the thermal stability factor,thus eliminating the intermediate state.(2)Based on the simulation results,experiments were designed to study the WER curve and electrical and magnetic parameters of devices under different diameters,test temperatures,write voltage pulse widths,and stack structures.It was found that the optimal solution to solve the "balloon effect" was to adjust the stack structure of the magnetic tunnel junctions.The free layer of the magnetic tunnel junctions in the devices consisted of three layers: free layer 1/spacer layer/free layer 2.This paper proposes to change the Co/Fe ratio in free layer 2 and the thickness of the spacer layer to eliminate the "balloon effect" in the WER curve.The two optimization methods corresponded to a decrease in the WER width of the device array from 0.36 to 0.24 and 0.18,respectively.Based on a spacer layer thickness of t+0.8 (?),it was proposed to increase the thickness of free layer 2 by 1 (?) to improve the thermal stability of the devices.The average thermal stability factor of the device array increased from 52.18 to 60.84 at 100℃,and the proportion of devices greater than 54.1increased to 100%.Then,the Co content in free layer 1 was moderately increased to increase the TMR value of the devices by 7%,which in turn improved the STT efficiency of the devices.The device’s flip efficiency reached a maximum of 60.9 through the change rate of the flipping voltage,while the WER width was 0.16 and the thermal stability factor was 64.4.Finally,through a series of optimizations,the "balloon effect" of the device array was greatly improved,and the data retention capability,erase/write endurance,and energy efficiency of the device array were greatly increased.