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0Increased boron content for wider process tolerance in perpendicular MTJs
J. P. Pellegren, M. Furuta, V. Sundar, Y. Liu, J.-G. Zhu, and V. Sokalski
Citation: AIP Advances 7, 055901 (2017); doi: 10.1063/1.4972855 View online: http://dx.doi.org/10.1063/1.4972855 View Table of Contents: http://aip.scitation.org/toc/adv/7/5 Published by the American Institute of Physics
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Increased boron content for wider process tolerance in perpendicular MTJs
J. P. Pellegren,1 M. Furuta,2 V. Sundar,2 Y. Liu,1 J.-G. Zhu,2 and V. Sokalski1
1 Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
2Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
(Presented 3 November 2016; received 23 September 2016; accepted 28 September 2016; published online 22 December 2016)
Perpendicular CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) prepared from (Co25Fe75)i_;tBx alloys are found to have better annealing stability when made with 30at% boron relative to a more typical 20at% boron. A comparison of film-level properties shows that perpendicular magnetic anisotropy (PMA) increases significantly for 30at%B, while the range of electrode thicknesses that maintain a perpendicular easy axis also increases. Because capping layer interdiffusion has been previously suggested to play a role in the breakdown of PMA with annealing temperature, we have isolated its effect by studying the annealing process of thin Ta/CoFeB(2nm)/Ta trilayers. Through analysis of the decrease in Curie temperature during annealing, we can infer that higher boron content indeed suppresses growth of the intermixed CoFeB-Ta dead layer. For device structures and processing conditions where interdiffusion is a limiting factor, increasing boron content is shown to result in substantially improved tunneling magnetoresistance (TMR). © 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). (http://dx.doi.org/10.1063/L4972855]
I. INTRODUCTION
As a primary candidate for use in spintronic applications, the CoFeB/MgO material system has been studied extensively since its introduction.1,2 CoFeB alloys were broadly chosen based on a boron concentration expected to minimize the thermodynamic driving force for crystallization, which occurs near 20at%B.3 Alloys at this concentration have been found to optimize the TMR vs. annealing temperature behavior of in-plane MTJs where CoFeB electrodes are typically in the range of 3-5nm thick.4 With the widespread transition towards perpendicular MTJs where much thinner layers are required,5 annealing stability has become a critical challenge and necessitates that alloy selection be revisited in order to leverage secondary effects beyond amorphization during growth.
Understanding annealing stability in MTJs requires a brief description of the functions of the layer adjacent to the electrode, called the capping layer. It is well supported6 that a primary role of the capping layer is to absorb boron from the electrode as it is driven out during crystallization of FeCo. In addition, despite the fact that PMA originates at the CoFeB/MgO interface,7 the capping layer also has a critical impact on the observed anisotropy8-10 that is not well understood. Tantalum is one of a few capping layer materials that produces electrodes with significant PMA and we have used it exclusively in this study. It has been demonstrated that at high temperature Ta interdiffuses with the CoFeB layer and this process has been linked to the breakdown of magnetic properties that is observed at high annealing temperature.11 We seek to understand the interplay of diffusion of Ta and B in our structures during the annealing process in order to explain observed device behavior.
While there is a large body of research dedicated to directly mapping the distribution of various elements, including boron and tantalum, in the MTJ structure,12-16 these experiments are difficult both to conduct and to analyze for devices with very thin electrodes. In previous work,17 we adapted techniques used in the analysis of bulk amorphous and nanocrystalline magnetic materials18 to thin
2158-3226/2017/7(5)/055901/5
7, 055901-1
film systems and we have made use of these indirect methods throughout the current study. These measurements take advantage of the fact that the magnetic properties of the films of interest are often very sensitive to changes in composition and atomic ordering.
II. EXPERIMENTAL
Film structures were deposited with DC (for CoFeB, Ta, and Pt) and RF (for MgO) magnetron sputtering using one of two different sputtering systems. For the devices shown in Fig. 1 and the MgO-containing electrodes in Fig. 2b, films were sputtered in a confocal AJA system with base pressure less than 5 x 10-9 Torr and a working pressure between 2-10 mTorr Ar. MgO was sputtered from a face-to-face configuration using a 3" target while metallic materials were sputtered at an angle of 10° from 2" targets. The films shown in Fig. 2a and Fig. 3 were prepared in a high-throughput system with base pressure less than 2 x 10-7 Torr and a working pressure of 2.5 mTorr Ar. Devices were patterned with e-beam lithography and pillar sizes were measured by SEM analysis. Magnetization measurements were performed in a Quantum Design PPMS with a vibrating sample magnetometry (VSM) oven option and a noise floor of <2juemu. Constant heating experiments were conducted at
0 20 50 100 200
RA (Q*nm2)
-2 -1 0 1 2
H (kOe)
j (MA/cm )
FIG. 1. TMR vs. RA product for 100nm diameter MTJs with varying MgO thickness (0.7-1nm) that were annealed at 300°C after patterning (a). Full stack structure with thickness in nm is Si-SiO2/Ta(10)/CoFeB(1-1.2)/MgO(0.7-1)/CoFeB(1.2-1.4)/Ta(3)/Pt(12)/Ta(2). Representative RH (b) and STT-switching (c) behavior for a 1ms current pulse showing a critical current on the order of 106A/cm2.
0.8 1.0 1.2 1.4 1.6 1.8 2.0
Thickness (nm)
Temperature (°C)
FIG. 2. Effective anisotropy vs. CoFeB thickness of Ta(5)/CoFeB(t)/MgO(10) films (a). Saturation magnetization vs. temperature of Ta(5)/CoFeB(5)/MgO(2)/Ta(5) with a ramp rate of 5°C/min (b).
0 100 200 300 400 Temperature (°C)
300 320 340 360 380 400
Tmax(°C)
FIG. 3. Ms vs. T reversal curves for Ta(5)/CoFeB(2)/Ta(5) trilayers (a). For each sample, the temperature is increased to Tmax at 5°C/min then immediately reduced at the same rate. Cooling curves are plotted with symbols while the heating curve for Tmax=400°C is plotted as a faded line. Sample Curie temperature after anneal vs. maximum temperature as extracted from the cooling curves (b).
ramp rates of 5°C/min and feature positions were previously found to be insensitive to ramp rates in the range of 1-10°C/min.
III. RESULTS AND DISCUSSION
Our perpendicular Co17.5Fe52.5B30/MgO/Co17.5Fe52.5B30 MTJs show initial properties comparable to reported values using 20at%B alloys and have annealing stability up to 350°C. A peak TMR of around 90% is obtained for devices with RA product down to 50Q * jum2 (Fig. 1a). Devices with the same structure using Co20Fe60B20 are only stable up to a 300° C anneal and have reduced TMR compared to their boron-rich counterparts. This contrasts with the reported behavior of in-plane devices4 where higher boron content has shown lower TMR. The negative impact of boron concentration in this case was attributed to the presence of excess boron at the FeCo/MgO interface which curbs the spin-filtering effect of MgO.
To understand the observed benefit of increased boron, we have compared film-level anisotropy properties of the two compositions (Fig. 2a). At low annealing temperature, both 20% and 30% boron have a thickness window where the electrode has a perpendicular easy axis that starts at around 0.7nm, in agreement with previous reports. Below this window, the films are found to be magnetically dead after anneal while above it the electrodes are in-plane due to the demagnetizing field dominating interfacial PMA. The effective perpendicular anisotropy observed for 30% B is significantly higher than that for 20%, which leads to a larger perpendicular thickness window for the 30% case. One explanation for the increased anisotropy is a reduction in Ms for the 30% case which lessens the demagnetizing field that favors in-plane alignment. This description is consistent with our results at electrode thicknesses near 2nm, as the tangent lines for the Keff * t vs. t behavior converge to nearly the same intercept, therefore indicating similar values for interfacial PMA. It is critical to note that for thinner electrodes which are more representative of actual devices, the anisotropy behavior diverges from this linear trend. When annealing temperature is increased, thin 20% B electrodes which were within the perpendicular thickness range at low annealing temperature become magnetically dead
while corresponding 30% boron electrodes show a reduction in PMA but remain perpendicular for a reduced thickness range. This behavior is consistent with the observed device stability and is less readily explained by bulk Ms differences, as the magnetization of the 20% boron layers are dropping to zero and should have small demagnetizing fields. We suggest that interfacial anisotropy is decaying at lower temperatures in the 20% B electrodes and turn to measurements taken during annealing in order to provide insight as to the cause of this decay.
In-situ Ms vs. T behavior of Ta/CoFeB/MgO during annealing (Fig. 2b) shows that the magnetization irreversibly increases in the range of 250-350°C, which is associated with crystallization of the a-FeCo phase. At higher temperatures, the moment undergoes an irreversible decrease attributed to Ta/CoFeB interdiffusion. Increasing boron content results in a higher temperature for a-FeCo formation, which is reasonable given the purpose of boron in the alloy is to inhibit the crystallization of FeCo. This effect is also consistent with the behavior of in-plane devices which show that in order to see large increases in TMR characteristic of spin filtering, higher annealing temperature is required for higher boron content.4 In previous in-situ anneal studies,17 we have identified decreases in crystallization temperature that accompany a reduction in electrode thickness. This dependence can therefore be offset and perpendicular electrodes can be made to crystallize at the same temperature as in-plane electrodes by tuning boron content. Indeed, one explanation for the crystallization temperature dependence on thickness is that some boron diffuses into the capping layer before crystallization, reducing the effective boron content. This boron depletion would become more significant as the electrode thickness is reduced because there is less total boron in the smaller CoFeB layer.
In addition to the difference in crystallization temperature, the onset of irreversible moment loss also appears to occur later for higher boron content. The moment loss is complicated by moment increases due to crystallization, so in order to isolate interdiffusion we analyze the annealing behavior of thin (<3nm) CoFeB layers sandwiched by 5nm Ta (Fig. 3). In our previous studies on these film systems, we have found that such trilayers show no crystallization features upon annealing and instead irreversibly decrease in moment until they are magnetically dead at room temperature. We have also demonstrated that the Curie temperature (TC) of CoFeB-Ta alloy films is strongly dependent on the Ta content because Ta disrupts the exchange between magnetic elements, as similarly observed in the behavior of related bulk nanocrystalline materials.18 TC can therefore be used as a proxy for the amount of Ta that has intermixed with the CoFeB layer. Boron content on its own appears to produce only minor differences in TC as the magnetization of both compositions drop to zero around 350°, though TC is initially slightly higher for 20% B. When annealing is reversed at temperatures up to 300°C, small increases in Ms are observed at room temperature which may be the result of boron expulsion. At higher reversal temperatures, the TC observed upon cooling is significantly lower than the value for the heating curve, indicating Ta interdiffusion. TC is found to decrease more slowly for 30% boron, a clear indication that higher boron content is suppressing diffusion of Ta into the CoFeB layer.
In summary, we have observed improvements in TMR and the annealing stability of PMA in perpendicular MTJs using electrodes with increased boron content. This effect has been traced to the suppression of capping layer interdiffusion which has been demonstrated through novel Curie temperature analysis. Our results are consistent with a hypothesis that boron expelled from the electrode passivates the capping layer. As a caveat, we note that interdiffusion is likely not a limiting factor in all cases. The devices we have fabricated have relatively thick Ta seed and capping layers that may exacerbate problems related to interdiffusion. In addition to device structure, process differences may affect the propensity for intermixing by altering interface properties, in particular surface roughness. While we do not expect the device performance dependence on boron content to be universal, our results highlight a non-obvious role for boron in a material system where improved understanding is crucial to development of large-scale commercializable magnetoresistive random access memory.
ACKNOWLEDGMENTS
The authors are grateful for financial support from the Samsung Global MRAM Innovation (SGMI) Project and for insightful discussions with Dr. Eugene Chen.
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