Stability Study Of Ultra-thin Perpendicular Magnetically Anisotropic CoFeB Films

With the rapid development of new-generation information technology such as cloud computing,big data,and artificial intelligence,data is growing explosively,and memory,which is the basis of various information storage and applications,has become the foundation and powerful support for the rapid development of information technology.Magnetic non-volatile memory(MRAM)has become the core of next-generation memory research because of its comparable access speed to semiconductor DRAM and its critical non-volatility for information storage.Among them,the perpendicular magnetic anisotropy-magnetic tunnel junction(PMA-MTJ)is the preferred unit for information storage in MRAM.The stability of this PMA-MTJ storage unit and the related influencing factors in the process of preparation and application are the key to reliable information access.Therefore,this study focuses on this core problem and carries out research on the influence of the thickness of the perpendicular anisotropic isomagnetic storage layer,the selection of buffer and overlay nonmagnetic layers,and the annealing temperature on the stability of the memory cell during its preparation and application,as follows.In the study,CoFeB,the current preferred magnetic film material for PMA-MTJ,was used as the core,and a monolayer heavy metal buffer layer/CoFeB/MgO/monolayer heavy metal overlay multilayer film was firstly prepared to carry out the study of the effect of monolayer heavy metal material change on the PMA stability of CoFeB film.The PMA stability of CoFeB/MgO thin films at high temperatures has become a major concern and an urgent problem to be solved because the PMA-MTJ cells are integrated into CMOS circuits that undergo post-processing processes at temperatures higher than270°C.The PMA stability of CoFeB/MgO thin films at high temperatures has become a major concern and an urgent problem to be solved.The results of this study show that the typical metals Ta,W,Mo,and Pt,which are the focus of the current study,have different effects on the PMA stability of the films.Among them,Ta has a general PMA stability performance at high temperature,and the PMA effect of the film is difficult to exist stably at annealing temperature higher than 270°C and working temperature higher than 150°C.The analysis of the microscopic mechanism is mainly due to the diffusion of Ta elements to CoFeB/MgO,which destroys the hybridization of Fe 3d and O 2p orbitals.Mo is the most effective among the monolayer metal layers in enhancing the PMA stability,and its maximum annealing temperature can reach 330°C and maximum working temperature can reach 250°C;while W is the best among the heavy metals in maintaining the PMA stability,and its maximum annealing temperature can reach 330°C and maximum working temperature can reach 200°C;while the single Pt metal layer fails to make the film obtain excellent PMA effect.the good effect of Mo and W on PMA stability may come from its crystalline buffer layer,which inhibits the diffusion of buffer layer metals to the CoFeB/MgO interface.The monolayer Ta and Pt heavy metals have large spin Hall angles and are expected to be applied in the next generation spin-orbit moment nonvolatile magnetic memory SOT-MRAM.However,the memory cell using single layer Ta,Pt as buffer layer cannot obtain excellent PMA stability,so the multilayer composite heavy metal Pt/Ta,Ta/Mo is used in the study to replace the single layer Pt,Ta and CoFeB/Ta(Mo)/CoFeB interpolation structure to achieve the improvement of the stability of CoFeB memory layer based on Ta,Pt heavy metal.The composite buffer layer Ta/Mo can make the highest operating temperature of PMA effect of CoFeB film reach 250°C and the highest annealing temperature tolerated reaches 330°C.The Pt/Ta layer also makes the stability of PMA improved,which can make the film maintain the stability of PMA effect at the operating temperature of 250°C and the highest annealing temperature reaches 200°C.The most significant increase in the maximum annealing temperature tolerated among the intercalated structures is the film with a Mo metal layer inserted in the CoFeB layer,which reaches a maximum of 450°C and its maximum operating temperature also reaches 250°C.This study not only confirms the influence law of different heavy metals on the vertical anisotropy of ultrathin CoFeB,but also lays the foundation for material selection and implementation of heavy metals with both large spin-flow injection efficiency and high PMA stability in SOT-MRAM.