Rapid thermal annealing study of magnetoresistance and ...

APPLIED PHYSICS LETTERS 99, 102502 (2011)

Rapid thermal annealing study of magnetoresistance and perpendicular anisotropy in magnetic tunnel junctions based on MgO and CoFeB Wei-Gang Wang,1,a) Stephen Hageman,1 Mingen Li,1 Sunxiang Huang,1 Xiaoming Kou,2 Xin Fan,2 John Q. Xiao,2,b) and C. L. Chien1,c) 1

Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA

2

(Received 12 April 2011; accepted 4 August 2011; published online 6 September 2011) The tunneling magnetoresistance and perpendicular magnetic anisotropy in CoFeB(1.1-1.2 nm)/ MgO/CoFeB(1.2-1.7 nm) junctions were found to be very sensitively dependent on annealing time. During annealing at a given temperature, decay of magnetoresistance occurs much earlier compared to junctions with in-plane magnetic anisotropy. Through a rapid thermal annealing study, the decrease of magnetoresistance is found to be associated with the degradation of perpendicular anisotropy, instead of impurity diffusion as observed in common in-plane junctions. The origin of the evolution of perpendicular anisotropy as well as possible means to further enhance tunneling C 2011 American Institute of Physics. [doi:10.1063/1.3634026] magnetoresistance is discussed. V Magnetic tunnel junctions (MTJs)1–4 are leading nonvolatile spintronic devices for memory and logic applications. Spin transfer torque (STT) effect is one of the most promising ways to perform writing of MTJs.5 However, the large critical current density for STT switching well in excess of 106 A/cm2 for MTJs with in-plane magnetic anisotropy remains an obstacle. On the other hand, MTJs with perpendicular anisotropy (p-MTJs) are expected to accommodate a lower switching current by eliminating the demagnetization term, and at the same time, provide a faster switching speed due to large anisotropy maintained at reduced thickness.6–8 Therefore, p-MTJs using Co/(Pd, Pt) multilayers or superlattices,9–11 rare-earth transition metal alloys,12,13 and L10-ordered alloys14 have been intensely pursued. In the recent development, high tunneling magnetoresistance (TMR), high thermal stability, and relatively low switching current density have been simultaneously achieved in p-MTJs consisting of CoFeB/MgO/CoFeB with very thin CoFeB electrodes.15 In all types of MTJs based on MgO and CoFeB, thermal annealing after film deposition is essential to achieve a high TMR.16 Most often, one employs conventional thermal annealing (CTA) with ramp time ranging from tens of minutes to hours. In contrast, the ramp time is of the order of seconds in rapid thermal annealing (RTA). It has been demonstrated previously in in-plane MTJs that important information such as dynamic growth of TMR,17,18 fast crystallization of CoFeB,19 evolution of noise spectrum,20,21 and spin dynamics properties22 can be advantageously obtained through RTA study. One intriguing contrast between p-MTJs and their in-plane counterparts is the TMR values. In p-MTJs with CoFeB/MgO/CoFeB, the highest TMR ratio reported has been only 124%,15 whereas TMR up to 600% has been achieved in similar structure with thicker CoFeB layers in MTJs with in-plane anisotropy.23 The lower TMR in p-MTJs is possibly related to the thinner CoFeB and a)

Electronic mail: [email protected]. Electronic mail: [email protected]. c) Electronic mail: [email protected]. b)

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interdiffusion problem during conventional hour-long annealing. Meanwhile, it is believed that the perpendicular magnetic anisotropy (PMA) is governed by interfacial bounding states between Co(Fe) and oxygen,15,24 which are known to experience a more drastic change in annealing.25 Therefore, it is imperative to investigate the evolution of both tunneling magnetoresistance and perpendicular anisotropy in these p-MTJs. We have undertaken such studies. The structure of the MTJs is Si/SiO2/Ta(7 nm)/Ru(15 nm)/Ta(7 nm)/Co40Fe40B20(1.1-1.2 nm)/MgO(1-2 nm)/ Co40Fe40B20(1.2-1.7 nm)/Ta(10 nm)/Ru(24 nm). After the fabrication of the multilayers, MTJs in circular shapes with radius ranging from R ¼ 2.5 to 50 lm were patterned by standard microfabrication process. The MTJs were then annealed by either a RTA or a CTA system. The rate for ramp-up and ramp-down in the RTA system is up to 40 C/s, while for the CTA system, the ramp-up rate is 5 C/s and the ramp-down rate is less than 2 C/s. The unexpected dependence of TMR on annealing time of a series of MTJs with the essential structure of CoFeB (1.1 nm)/MgO(1.8 nm)/CoFeB (1.2-1.7 nm) is shown in Fig. 1. These samples were on the same wafer and were annealed using a CTA system for 40 min and 80 min at 300 C. The TMR in the MTJs annealed for 40 min is about 95% and shows little variation among different MTJs. Surprisingly, the TMR drastically decreases to only 20-40% after annealing for a longer period of 80 min and exhibits a much larger sampleto-sample variation. This is in sharp contrast to the MTJs with in-plane anisotropy, where the atomic interdiffusion is essentially negligible at 300 C and TMR would continue to increase even after annealing for a few hundred hours.18 This result indicates that certain properties of the p-MTJs behave very differently as opposed to those of in-plane MTJs. We have carried out a RTA study to elucidate this problem. The MTJs were annealed successively for different durations at 340 C with representative TMR curves shown in Fig. 2. The as-prepared MTJ shows only a few percent of TMR. The bell-shaped curve of TMR ¼ 8% indicates that the equilibrium magnetic easy axis is not in the perpendicular orientation. Upon annealing for 3 min at 340 C, the

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FIG. 1. (Color online) TMR vs. top CoFeB layer thickness after annealed at 300 C for 40 min and 80 min using a CTA system.

TMR quickly increased to 111%, with the establishment of perpendicular easy axis. However, the TMR begins to decrease after annealing for 6 min. The antiparallel state of the junction is totally lost after annealing for 10 min. Finally, TMR drastically decreases to about 20% after annealing for 23 min, accompanied by the loss of the perpendicular easy axis. These results demonstrate the delicacy of the PMA essential for p-MTJ, its establishment and disappearance at a relative low annealing temperature of 340 C. The evolution of TMR during annealing in MTJ with inplane anisotropy was previously modeled by a competition between crystallization, interdiffusion, and defects reduction processes.18 The initial growth of TMR is due to the crystallization of amorphous CoFeB and the decay of TMR during prolonged annealing is caused by impurity diffusion. If the same activation energy of 150 kJ/mol obtained in in-plane MTJs with thicker CoFeB layers19 still holds here for thinner CoFeB layers, the crystallization of CoFeB would be far from complete after only 23 min anneal at 340 C. Thus, the rapid decrease of TMR must be due to interdiffusion and/or a degradation of the PMA in the p-MTJ. To gain further insight, the evolution of the parallel and antiparallel resistances (RP and RAP) as well as the value of the TMR is shown in Fig. 3(a). For symmetry conserving coherent tunneling in MgO based MTJs, the conductance in the parallel channel is mostly con-

FIG. 2. TMR curves of the p-MTJ after annealed for different amount of time at 340 C.

Appl. Phys. Lett. 99, 102502 (2011)

tributed by electrons with D1 characteristics.26 If the reduction of TMR is caused by the added impurity scattering of the D1 band electrons due to interdiffusion, RP would increase sharply as observed previously.18 However, in the present case, the decrease of TMR is clearly associated with the decrease of RAP and not the increase of RP, indicating that interdiffusion is not the main cause for the decrease of TMR. As RAP directly depends on the antiparallel alignment of the two CoFeB layers as shown in Figure 2, the decrease of RAP is the direct consequence of losing the antiparallel plateau caused by reduced PMA in longer annealing. This is more evidently shown by the measured coercivity in Fig. 3(b). In order to confirm this evolution of PMA during annealing, the anisotropy field of the top CoFeB electrode was measured by a vibrating sample magnetometer (VSM) after annealing for different time durations. This result is presented in Fig. 3(b). It shows basically the same feature: a very rapid increase at the beginning, followed by a sharp decreasing once the annealing is longer than a few minutes. This is the reason why the coercivity is reduced and a welldefined AP plateau cannot be maintained. The anomalous Hall effect (AHE) measurement of the bottom electrode showing in the inset of Figure 3(b) exhibits essentially the same dependence. Therefore, the decay of the TMR after its peak value is due to the decrease of PMA instead of atomic interdiffusion which would only occur after much longer annealing at 340 C.18 It has been reported that in Pt/Co/AlOx (MgO) structure, the PMA depends very sensitively on the oxidation states of Co. As a result, the optimal oxidation of Al or Mg gives rise to a strong PMA of the Co layer, whereas the under-oxidized or over-oxidized Al or MgO lead to no or weaker PMA, respectively.27,28 Annealing also introduces PMA in the SiO2/Co/Pt structure.29 Despite the unclear role of Pt layer, these observations are in agreement with first principle calculations, where the PMA in a Fe/MgO bilayer is generated by the hybridization between 3d orbitals of Fe and 2p orbitals of oxygen.24

FIG. 3. (Color online) (a) Evolution of TMR, RP, and RAP for the p-MTJ showing in Figure 2. (b) Evolution of coercivity for top (black square) and bottom (red dot) electrodes in the same junction and the evolution of anisotropy field (blue triangle) for the top CoFeB film measured by VSM. Inset shows the AHE measurement of the bottom electrode on a patterned Hall bar after annealing for different time durations.


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tradeoff between the crystallization of the amorphous CoFeB and the decay of the PMA. These results also demonstrate that the PMA strength in this system can be controllably tuned by post-growth thermal annealing. The authors would like to thank Dorrie Bright for the help during the initial part of this research. This work is supported by NSF DMR05-20491 and DOE DE-FG02-07ER46374. 1

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FIG. 4. (Color online) Evolution of TMR with annealing time at different temperatures, by either a RTA or CTA process. Inset shows 117% TMR from one of the MTJs.

Within this context, our experimental observations can now be understood through the change of the oxidation states of CoFeB during annealing. Besides the normal factors of crystallization, interdiffusion and defect reduction,18 one has to include the dynamic change of PMA in order to describe the evolution of TMR during the annealing of these p-MTJs. Initially, PMA is very weak, presumably because of the slight oxidation of CoFeB due to the ion bombardment when MgO is deposited. After annealing for an optimal length of time, any possible interfacial CoOx or FeOx are reduced and the proper p-d hybridization that results in PMA is maximized.24,25 During further anneal, TMR continues to increase due to the increased portion of crystallized CoFeB. However, the best window for optimized Co–O and Fe–O bonding has passed;28 therefore, the PMA begins to deteriorate. At a critical point, the antiparallel state can no longer be well defined because of the decay of PMA, which leads to the decrease of TMR. Such decay of PMA in our MTJs shows up only after a few minutes of annealing at temperature higher than 300 C. The evolution of TMR for temperatures ranging from 240 C to 380 C is shown in Fig. 4 with MTJs annealed by the RTA and the CTA processes. These p-MTJs have the structure of CoFeB (1.2 nm)/MgO(1.2-2 nm)/CoFeB (1.7 nm). The highest TMR achieved in this study is close to 120%, by either 30 min CTA at 300 C or 3 min RTA at 340 C. It is worth mentioning that the slightly thicker CoFeB bottom electrodes here compared to those in Fig. 1 (1.2 nm vs. 1.1 nm) appear to be helpful in stabilizing PMA during annealing, resulting in the increase of TMR by about 20% at 300 C. This indicates a suitable increase of the CoFeB thickness within the PMA range or exchange-biasing CoFeB could also help to further improve the TMR ratio. In summary, we have presented the first annealing-timeresolved study on the p-MTJs based on CoFeB/MgO/CoFeB. Our research suggests that the PMA in this system has very sensitive dependence on high temperature annealing. The decay of PMA during prolonged annealing leads to the incomplete antiparallel alignment, which results in the decay of TMR. Therefore, in order to achieve higher TMR, the annealing condition must be carefully chosen to maintain a
