IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 8, AUGUST 2006
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MgO-Based Tunnel Junction Material for High-Speed Toggle Magnetic Random Access Memory Renu W. Dave, G. Steiner, J. M. Slaughter, J. J. Sun, B. Craigo, S. Pietambaram, K. Smith, G. Grynkewich, M. DeHerrera, J. Åkerman, and S. Tehrani Technology Solutions Organization, Freescale Semiconductor, Inc. Chandler, AZ 85224 USA We report the first demonstration of a magnetoresistive random access memory (MRAM) circuit incorporating MgO-based magnetic tunnel junction (MTJ) material for higher performance. We compare our results to those of AlOx-based devices, and we discuss the MTJ process optimization and material changes that made the demonstration possible. We present data on key MTJ material attributes for different oxidation processes and free-layer alloys, including resistance distributions, bias dependence, free-layer magnetic properties, interlayer coupling, breakdown voltage, and thermal endurance. A tunneling magnetoresistance (TMR) greater than 230% was achieved with CoFeB free layers and greater than 85% with NiFe free layers. Although the TMR with NiFe is at the low end of our MgO comparison, even this MTJ material enables faster access times, since its TMR is almost double that of a similar structure with an AlOx barrier. Bit-to-bit resistance distributions are somewhat wider for MgO barriers, with sigma about 1.5% compared to about 0.9% for AlOx . The read access time of our 4 Mb toggle MRAM circuit was reduced from 21 ns with AlOx to a circuit-limited 17 ns with MgO. Index Terms—MgO, magnetic random access memory (MRAM), magnetic tunnel junction (MTJ), toggle switching, tunneling magnetoresistance (TMR).
I. INTRODUCTION
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AGNETORESISTIVE random access memory (MRAM) based on magnetic tunnel junction (MTJ) bits has been demonstrated to be competitive with semiconductor memory performance while exhibiting a combination of qualities not found in other memory technologies [1]. For example, MRAM is nonvolatile, has unlimited read and write endurance, and has demonstrated high-speed read and write operations. The large magnetoresistance (MR) and tunable resistance-area product (RA) of MTJ material enables circuit designs with competitive read cycle times. Until recently, such MTJ devices were made with AlO tunnel barriers and produced a MR in the 40%–50% range for ferromagnetic alloys that work well in MRAM. However, in the past year MR values well over 200% have been reported for MgO tunnel barriers. [2]–[4] The high MR signal is thought to be due to a band-structure-related enhancement of the polarization at the interface of MgO and certain ferromagnetic materials. Unfortunately, the CoFe and CoFeB alloys that have been % do not have the magdemonstrated to produce netic properties needed for good toggle switching as required for our MRAM circuits, [5] such as near-zero magnetostriction and low intrinsic uniaxial anisotropy. In addition, there are a number of requirements for MRAM, beyond high MR, that must be fulfilled to have fully functional memories. Many of these properties, such as bit-to-bit variations in resistance and switching, can only be evaluated by integrating the material into a CMOS circuit. The focus of the MTJ material development reported here is on getting improved MR by changing the barrier from AlO to MgO while keeping the other electrical and magnetic properties of the MRAM bits as unchanged as possible. Thus, NiFe (permalloy) was chosen for the free layer alloy due
Digital Object Identifier 10.1109/TMAG.2006.877743
to its excellent and well-know magnetic properties that are ideal for toggle switching. Since the optimum anneal temperature of 350 C is nearly 100 C higher than typical anneal temperatures for AlO -based material, the thermal endurance of a typical NiFe-based synthetic antiferromagnet (SAF) structure for the free layer may be insufficient. A modified NiFe SAF free layer was developed and shown to have sufficient thermal endurance to survive the required anneal. Several different MgO growth processes and post anneals were evaluated and optimized to 85% with the NiFe free layer, and up to 230% achieve with CoFeB free layers. Using these materials and the best of the four MgO processes, we report here the first demonstration of a functioning MRAM circuit that employs MgO-based MTJ material for higher read performance. The read performance of this 4 Mb toggle MRAM circuit [6] was evaluated for both MgO-based MTJ material and conventional AlO -based material. The details of analog read tests on fully integrated 4 Mb arrays are reported. II. EXPERIMENTAL DETAILS The MTJ stacks were typical PtMn bottom-pinned structures with a CoFe/Ru/CoFeB pinned SAF, having various tunnel barriers and free layers as described in the following sections. The metallic layers of the MTJ stack are formed by magnetron sputtering in a multi-target system with low deposition rates (0.1–0.5 /s) and precise timing for accurate layer thickness control. Four different processes were evaluated for the fabrication of MgO tunnel barrier: plasma oxidation of a Mg thin film, oxidation of a Mg film with an oxygen radical source, reactive sputtering of Mg in an Ar/O environment, and RF sputtering of an MgO target. For each process, we spent considerable effort on process optimization to be sure that the comparisons were based on data that was reasonably representative of the best that process could offer. Many dozens of samples were made for each process over a range of barrier thicknesses, and characterized in blanket-film form for basic MR and magnetic properties.
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 8, AUGUST 2006
TABLE I SUMMARY OF ADVANTAGES AND DISADVANTAGES OF DIFFERENT MgO TUNNEL BARRIER PROCESSES AND COMPARISON TO AlO TUNNEL BARRIER
For the most promising processes, further blanket-film studies were carried out with different fixed and free layer alloys. The best stacks were characterized in patterned-bit form and finally two were integrated into our 4 Mb CMOS MRAM circuit. The optimized MgO processes used for the results presented here are as follows: 1) deposition of a metallic Mg film, 12–16 thick, followed by oxidation in an RF-produced plasma of Ar : O2 with a 2 : 1 ratio at a pressure of 1 mTorr; 2) deposition of a metallic Mg film, 12–16 thick, followed by oxidation with an oxygen radical source at a pressure of 9 mTorr; 3) reactive sputtering of Mg in an Ar : O2 (5 : 3 ratio) atmosphere at a pressure of 0.5 mTorr; and 4) RF sputtering of an MgO target in a 2 mTorr Ar atmosphere. The deposition steps for processes 1–3 were carried out by Ar sputtering mTorr) in a multi-target magnetron deposition system ( using wafer rotation to obtain good uniformity across 200 mm wafers with 164 mm-diameter targets. Process 4 was developed in a different multi-target magnetron deposition system that uses a scanning substrate table for uniformity with 250 mm-diameter targets. Note that this final process involved only RF deposition of MgO, without the insertion of a metallic Mg layer shown, in other work, to improve MR in low-RA material [7]. The deposition rates were calibrated by X-ray reflectivity on thick films. After deposition, a 2-h anneal in a 1 T field was used to improve the tunnel barriers and set the PtMn pinning. Different anneal temperatures for MgO-based material were evaluated. Films were annealed in vacuum in the range of 265 C to 400 C. The best MRs were achieved with 350 C anneal process, which is consistent with prior work.
Electrical measurements were done on blanket films using the current-in-plane tunneling technique [8] as well on bits patterned with standard lithography techniques. Finally, MgO-based MTJ material was integrated with CMOS to study the circuit performance and compared to AlO -based MTJ. CoFeB was used for the fixed layer and different alloys were evaluated for the SAF free layers, including a variety of CoFeB alloys and permalloy (NiFe). III. RESULTS AND DISCUSSION Each of the tunnel barrier processes studied has advantages and disadvantages, as summarized qualitatively in Table I. A summary of the main findings for each process follows. The plasma oxidation used in Process 1 was much more aggressive than the radical oxidation of Process 2. Although this process oxidizes Al too fast for a controllable AlO process, it worked well for MgO. Tunnel barriers with RA in our desired m were repeatably produced with Mg in the range of 1–10 k 12–16 range, and oxidation times of few tens of seconds. The % maximum MR values obtained with this process, with CoFeB electrodes, were somewhat lower than with Process 3, but the consistency of this process offset that disadvantage for use in integrated devices. Process 2 worked well in many respects but was found to be unsuitable for our resistance range due to the inability of the oxygen radical process to fully oxidize a Mg layer with the thickness needed to reach our RA range. This oxidation process easily produces AlO tunnel barriers with the right resistance, using Al layers in the 7–9 range. However, to get k m with Mg, the barrier process had to be repeated three
DAVE et al.: MgO-BASED TUNNEL JUNCTION MATERIAL FOR HIGH-SPEED TOGGLE MRAM
Fig. 1. Magnetoresistance ratio (MR) of MgO-based magnetic tunnel junctions with CoFeB (B = 28%) free layers as a function of resistance area product (RA), measured on blanket films, for MgO fabricated with Process 3 (reactive sputtering) and Process 4 (RF sputtering from MgO target). The actual resistance of tunnel junction device is the resistance area product (RA) divided by the area of the patterned bit. RA was controlled by varying the thickness of the tunnel barrier as described in the text.
times to build up sufficient thickness, with of Mg for each layer. The large thickness required to reach the RA target appears to be due to a combination of incomplete oxidation and roughness. SAF free layers deposited on these triple barriers had poor AF coupling, possibly also due to high surface roughness. These barriers exhibited good MR values for CoFe-based free layers but were lower than the others for NiFe free layers, again indicating problems with the quality of the top interface. Process 3 resulted in tunnel junctions with the highest MR . For good results it was necessary to devalues, Å, layer of Mg before starting the reactive depoposit a thin, sition. Unfortunately, the MR rises and falls rapidly as this layer is adjusted from zero to 5 Å, resulting in a difficult to control process. Run-to-run control of RA was also challenging, probably because of sensitivity to small changes in the multi-target deposition chamber environment. Reproducibility of results became worse at lower RA and MR values trended lower. In developing Process 4, various mixtures of Ar and O were used for RF sputtering from the MgO target. The best results, MR comparable to process 3, were obtained with pure Ar even though this presumably results in a somewhat oxygen deficient MgO barrier. The main difficulty with this process was in producing good quality junctions with layers thin enough to be in our RA range. As shown in Fig. 1, the MR dropped monotonim range. cally with decreasing RA throughout the 1–1000 k At even lower RA the MR drop accelerated and the barriers began to have problems with shorting, probably due to discontinuous initial growth. Although not suitable for our application, this process could be used for applications requiring higher RA values. Fig. 2 shows MR of MgO-based magnetic tunnel junctions with NiFe free layers as a function of post-deposition anneal temperatures for a range of tunnel junction resistance measured on blanket films. The MgO process involved depositing 16 Å thick Mg film by magnetron sputtering and then oxidizing it by plasma oxidation. The best MR was obtained after 350 C anneal, which is consistent with prior work. The MR deteriorated with the 400 C anneal.
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Fig. 2. Magnetoresistance ratio (MR) of Process-1 MgO-based magnetic tunnel junctions with NiFe free layers as a function of post-deposition anneal temperatures for a range of tunnel junction resistance-area product (RA) measured on blanket films. The 350 C anneal resulted in MR values, 90%, approximately double that obtained with similar AlO junctions.
Fig. 3. Magnetoresistance ratio (MR) of MgO-based magnetic tunnel junctions with CoFeB free layers as a function of B content for two different post-deposition anneal temperatures measured on blanket films. In addition to generally higher MR, the 350 C anneal appears to change the composition dependence of the effective polarization of the interfaces.
Fig. 4. Comparison of magnetoresistance ratio (MR) of MgO-based MRAM cells with CoFeB and NiFe free layers to AlO -based cells with NiFe free layers in a 4 Mb toggle MRAM circuit. This MR was measured with an analog read of the cell, which includes the series resistance effect of the pass transistor, operating at 350 mV bias. The various data points shown for each wafer are measurements on different die across that wafer. Different wafers with the same stack had different oxidation times. MgO-NiFe provides about double, and MgO-CoFeB about four times, the signal of the AlO -NiFe cell.
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Since the optimum anneal temperature of 350 C is nearly 100 C higher than typical anneal temperatures for AlO -based material, the overall thermal endurance of the MTJ had to be
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 8, AUGUST 2006
TABLE II COMPARISON OF IMPORTANT MATERIAL-RELATED ATTRIBUTES FOR MgO-BASED AND AlO -BASED TUNNEL JUNCTIONS. IN BOTH CASES THE FIXED LAYER IS CoFeB AND THE FREE LAYER IS A NiFe-BASED SAF. THE MAGNETORESISTANCE RATIO (MR), BREAKDOWN VOLTAGE (V ), AND HALF-MR BIAS VOLTAGE (V ) WERE MEASURED IN ISOLATED PATTERNED BITS. THE BIT-TO-BIT RESISTANCE STANDARD DEVIATION ( ) WAS MEASURED IN THE INTEGRATED 4 Mb A THICK NiFe FREE LAYER, WHICH IS A MEASURE OF THE NEEL COUPLING BETWEEN THE FIXED ARRAYS. H IS THE HYSTERESIS LOOP OFFSET FIELD FOR A 40 IS THE OPTIMUM TEMPERATURE NEEDED FOR A 2 h FIELD ANNEAL THAT FOLLOWS THE DEPOSITION AND FREE LAYERS ACROSS THE TUNNEL BARRIER. T
improved. Therefore, we had to develop high thermal endurance SAF free layers with good soft magnetic properties. A typical NiFe SAF free layer is composed of two NiFe layers separated 8 Å that provides by a Ru spacer layer, with thickness the antiparallel magnetic coupling. However, such structures undergo a thermally activated failure of the Ru spacer at temperatures between 250 C and 300 C [9]. To make a NiFe SAF with improved thermal endurance, we exploited the weak AF 18 Å region. Thin coupling peak that occurs in the CoFe layers were added on each side of the Ru layer to increase 200 the coupling strength to get a SAF saturation field Oe. These structures show no signs of failure even after several hours at 350 C. As shown in the figure, the highest MR values obtained with this free layer are 90%. While this MR is much lower than the values possible with certain CoFe or CoFeB alloys, it is double the MR achieved with AlO -based tunnel junctions having the same fixed and free layers. Fig. 3 shows magnetoresistance ratio (MR) of MgO-based magnetic tunnel junctions with CoFeB free layers as a function of a range of B content in CoFeB free layer for two different post-deposition anneal temperatures measured on blanket films. This series of free layer alloys was made by co-sputtering from B . Retwo targets to form a series of alloys [Co Fe ] sults after 300 C anneal showed there is an optimum B content of 8%-atomic to obtain the maximum MR, which is just below the crystalline-amorphous transition point. On the low-B, crystalline side of the curve, the MR increases with increasing B content to a maximum. However, once the CoFeB transforms to the amorphous phase, further increase in the B content decreases MR. This trend is very similar to what is seen in AlO junctions: lower MR with crystalline CoFeB compared to amorphous, and slowly decreasing MR with increasing B content above the amorphous transition However, the behavior is dramatically different for material annealed at 350 C. As seen in Fig. 3, the highest MR is obtained with pure CoFe, and the MR is nearly constant, possibly dropping very slowly with increasing B content. Since the polarization with MgO is generally attributed to band structure effects that are sensitive to crystalline
orientation, we hypothesize that the very different behavior seen with the higher anneal temperature is related to the changes in the details of atomic arrangement at the interface between the crystalline MgO and the CoFeB, such as local ordering across the interface. Fig. 4 shows the MR ratio of MRAM cells, which includes the series resistance effect of the pass transistor, operating at 350 mV bias. This MR represents the raw signal available to the sense circuitry to read the memory state. The various data points shown for each wafer are measurements on different die across that wafer. The wafers with MgO barriers and NiFe free layers have MR approximately double that of the similar AlO -based material, and the MgO-CoFeB wafers have approximately four times the MR. Note however, that for the sensing scheme used in this MRAM circuit, the important figure of merit for signal is not MR of the cell, but rather the ratio of MR to the width of the bit-to-bit resistance distribution, MR/ . To have read signal for all the bits in the 4 Mb array, a minimum of approximately 6 separation is required between the midpoint reference and the mean resistance of the bits in both high and is the minimum low resistance states [10]. Thus, requirement to have finite signal for all the bits, and larger ratios provide more useable signal for faster sensing of the memory state. As expected, the switching of the CoFeB bits was not consistent and the switching distributions were wide. These switching problems are likely due to the nonzero magnetostriction of the CoFeB alloy and possibly differences in the patterning compared to the baseline NiFe patterning process. As a result, we focus additional analysis of the CMOS data on the NiFe material. Table II shows a comparison of several important parameters for integrated 4 Mb arrays with AlO /NiFe and MgO/NiFe stacks. As can be seen in the table, the resistance distribution % compared to width is wider for the MgO material, % for AlO . The wider distributions for MgO barriers may be related to the fact that MgO barriers are polycrystalline and AlO barriers are amorphous. One can expect that there may be more local variation in the tunneling current with the poly-
DAVE et al.: MgO-BASED TUNNEL JUNCTION MATERIAL FOR HIGH-SPEED TOGGLE MRAM
Fig. 5. Comparison of access times at 25 C for (a) AlO -based, and (b) MgO-based MTJ material integrated into a 4 Mb toggle MRAM circuit. The strobe time includes all circuit functions needed for the read operation. In (a) a strobe time of 21 ns is required to read error-free with the AlO material; (b) shows an improvement to 17 ns for the MgO material. The 17 ns limit is related to other circuit functions that occur during the strobe time, not to a lack of signal.
crystalline material due to grain-boundary roughness and possibly other inhomogeneities. However, the increase in is more than offset by the increase in MR, resulting an increase in from 28 to 40 at the circuit bias voltage of 350 mV. Thus, we obtain a 42% increase in real signal available to the circuit by replacing the AlO with MgO and using the high-thermal-endurance NiFe SAF free layer. Fig. 5 shows a read-performance comparison of the AlO /NiFe and MgO/NiFe bits. In this test, the time for the read signal to develop is varied by the circuit strobe time. The strobe time includes all circuit functions needed for the read operation. All bits in the 4 Mb array are preset so that the test will fail (read the wrong state) when the signal is read too fast. As the signal is allowed more time to develop, more bits are read correctly until the fail count goes to zero. As shown in Fig. 5(a), a strobe time of 21 ns is required to read error-free with the AlO material, and (b) shows an improvement to 17 ns for the MgO material. The 17 ns limit is related to other circuit functions that occur during the strobe time, not to a lack of signal. Given the higher signal now available, the circuit design could be optimized for MgO to achieve even faster access times. IV. SUMMARY AND CONCLUSIONS Four different processes for growing MgO tunnel barriers were evaluated and compared to an AlO process used for MRAM devices. The best process, based on suitability for a 4 Mb MRAM circuit, was Process 1, Mg deposition followed by plasma oxidation. A modified NiFe SAF free layer, ex, was shown ploiting the AF coupling peak near to have sufficient thermal endurance to survive the required
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350 C anneal, and it had better switching properties than the CoFeB-based free layers evaluated. The MRAM cells using MgO and the NiFe SAF had double the MR compared to , resulting in a reduction of AlO -NiFe, and 42% larger the read cycle time from 21 ns to a circuit-limited 17 ns. This is the first demonstration of a functioning MRAM circuit that employs MgO-based MTJ material for higher read performance. To get the full benefit of the high MR values possible with MgO, more process work should be directed toward improving the bit-to-bit resistance and switching uniformity, and MRAM circuit designs need to be optimized to take full advantage of the increased signal. In addition, the reliability of MgO devices has yet to be established to the extent of the heavily studied AlO devices. REFERENCES [1] S. Tehrani, J. M. Slaughter, N. DeHerrera, B. N. Engel, N. D. Rizzo, J. Salter, M. Durlam, R. W. Dave, J. Janesky, and G. Grynkewich, “Magnetoresistive random access memory using magnetic tunnel junctions,” Proc. IEEE, vol. 91, no. 5, p. 703, May 2003. [2] S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, and S.-H. Yang, “Giant tunneling magnetoresistance at room temperature with MgO (100) tunnel barriers,” Nature Mater., vol. 3, pp. 862–867, 2004. [3] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, “Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions,” Nature Mater., vol. 3, pp. 868–871, 2004. [4] D. D. Djayaprawira, K. Tsunekawa, M. Nagai, H. Maehara, S. Yamagata, and N. Watanabe, “230% room-temperature magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions,” Appl. Phys. Lett., vol. 86, no. 092 502, 2005. [5] B. N. Engel, J. Åkerman, B. Butcher, R. W. Dave, M. DeHerrera, M. Durlam, G. Grynkewich, J. Janesky, S. V. Pietambaram, N. D. Rizzo, J. M. Slaughter, K. Smith, J. J. Sun, and S. Tehrani, “A 4-Mb toggle MRAM based on a novel bit and switching method,” IEEE Trans. Magn., vol. 41, no. 1, Jan. 2005. [6] M. Durlam, D. Addie, J. Akerman, B. Butcher, P. Brown, J. Chan, M. DeHerrera, B. N. Engel, B. Feil, G. Grynkewich, J. Janesky, M. Johnson, K. Kyler, J. Molla, J. Martin, K. Nagel, J. Ren, N. D. Rizzo, T. Rodriguez, L. Savtchenko, J. Salter, J. M. Slaughter, K. Smith, J. J. Sun, M. Lien, K. Papworth, P. Shah, W. Qin, R. Williams, L. Wise, and S. Tehrani, “A 0.18 m 4Mb toggling MRAM,” in IEDM 2003 Proc., vol. 34, Dec. 2003. [7] K. Tsunekawa, D. D. Djayaprawira, M. Nagai, H. Maehara, S. Yamagata, and N. Watanabe, “Giant tunneling magnetoresistance effect in low-resistance CoFeB/MgO (001)/CoFeB magnetic tunnel junctions for read-head applications,” Appl. Phys. Lett., vol. 87, no. 072 503, 2005. [8] D. C. Worledge and P. L. Trouilloud, “Magnetoresistance measurement of unpatterned magnetic tunnel junction wafers by current-in-plane tunneling,” Appl. Phys. Lett., vol. 83, no. 1, pp. 84–86, 2003. [9] S. V. Pietambaram, J. Janesky, R. W. Dave, J. J. Sun, G. Steiner, and J. M. Slaughter, “Exchange coupling control and thermal endurance of synthetic antiferromagnet structures for MRAM,” IEEE Trans. Magn., vol. 40, no. 4, pp. 2619–2621, Jul. 2004. [10] J. Åkerman, M. DeHerrera, M. Durlam, B. Engel, J. Janesky, F. Mancoff, J. Slaughter, and S. Tehrani, Magnetoelectronics, M. Johnson, Ed. London, U.K.: Elsevier, 2004.
Manuscript received February 17, 2006; revised May 17, 2006. Corresponding author: R. Dave (e-mail:
[email protected]).
