In WP1, studies were conducted to understand the mechanisms responsible for the dielectric breakdown in magnetic tunnel junctions (MTJs). A key asset of STT-MRAM is their write endurance which is much better than in all other technologies of non-volatile memories (FLASH, 105 cycles; PCRAM, 109 cycles; ReRAM, 1010 10 cycles). Combined with their speed (switching time 1-5ns), and density (sub-20nm cell possible), this makes STT-MRAM a unique candidate for DRAM replacement. However, to achieve the extremely long endurance required for these types of applications close to logic circuits, it was important to understand the mechanisms responsible for early dielectric breakdown in MTJs. The results obtained in HYMAGINE allowed us to point out the key role played by the trapping-detrapping of tunnelling electrons on the breakdown mechanism in MTJs. These trapping sites can have different origins :
- Oxygen or Mg vacancies in the MgO barrier
- Water molecules trapped in the barrier
- Dislocations due to the crystallographic mismatch between Co(Fe) and MgO
- BO formation along the MgO barrier leading to the formation of a BO/MgO interface which can trap electrons due to a difference of electrical polarisability of BO and MgO.
A remarkable result that we obtained within HYMAGINE is that when the barrier is submitted to successive pulses of voltage spaced by a delay Δt between pulses, an increase by 8 orders of magnitude in the endurance is observed for a particular delay between pulses of the order of 70 to 100ns. This particular delay corresponds to the characteristic time that it takes for a trapped electron in a trapping site to escape from the trap. If by properly controlling the growth and oxidation conditions of the tunnel barrier, the trapping sites are eliminated, then an extremely long endurance can be obtained in MTJs. Compared to usual tunnelling through CMOS oxide (as in FLASH memories), a key difference is that direct tunnelling at low voltages takes place in MTJ instead of Fowler-Nordheim tunnelling at large voltages in CMOS oxides. As a result, there is no defect generated in the oxide during the tunnelling in MTJ in contrast to the FLASH-memory case in which electrons lose their excess energy within the oxide, thus creating defects which gradually degrade the oxide quality. Another remarkable results obtained along this line is the observation of a correlation between the 1/f electrical noise in unexercised junctions (as-prepared) and their endurance. Junctions with low 1/f noise amplitude tends to have a long endurance whereas junctions with large 1/f noise amplitude tend to experience early breakdown. This correlation was explained by considering that both phenomena (1/f noise and endurance are related to the density of trapping sites). The observation of this correlation is a very interesting result since it could be used in production as a predictive characterization of the future endurance of STT-MRAM chips.
This study is detailed below: The experiments were conducted on MgO based MTJ of RA in the range 10-30Ω.µm² typical of tunnel barriers used in Thermally Assisted MRAM or STT-MRAM. Fig.1 shows a typical junction on which such endurance characterizations were performed.Fig.1: Typical junction on which the endurance characterizations were performed
More than 3000 junctions were measured during this study since about 50 junctions were exercised till breakdown for each value of voltage amplitude and delay between pulses. The experiments were conducted in accelerated conditions i.e. with bias voltage amplitude in the range 1.2-1.6V about 2 to 3 times larger than typical write voltage in STT-MRAM. The pulses had a constant duration of 30ns with variable delay δt ranging from 1ns to 10µs. Fig.2 shows that upon cycling, no drift in Rmin and Rmax are observed in contrast to PCRAM for instance. The low and high resistance states are quite stable until electrical breakdown occurs. After breakdown, the hysteresis loop loses its characteristic shape (see Fig.2c) probably due to pinhole formation.
Fig.2: Characteristic hysteresis loops before (a) and after (c) breakdown. (d): Evolution of the low-R and high-R resistance states versus pulse number. The pulses were 1.25V in amplitude, 30ns long and 70ns delay between pulses.
The experiments were repeated for each values of voltage pulse amplitude and delay between pulses on 30 to 50 junctions to obtain enough statistics. The obtained cumulative distributions of “dead” junctions were then fitted by a Weibull distribution as illustrated in Fig.3. The shape parameter β and the endurance parameter η were extracted from these fits.
Fig.3: Fit of the cumulative distribution of “dead” junction by Weibull distribution (a). (b) Variation of the endurance parameter η versus amplitude of voltage pulse.
Remarkably, Fig.3b shows that under the used experimental conditions, extremely long endurance exceeding 1030 cycles, could be obtained at applied voltages of 0.5V.
Fig.4 shows the variation of the endurance parameter for given amplitudes of the pulses versus the delay between pulses. Remarkably, when the barrier is submitted to successive pulses of same polarity, a dramatic increase in endurance by up to 10 orders of magnitude (!) is observed for an intermediate delay between pulses of the order of 70 to 100ns. This increase is observed for unipolar pulses of both positive and negative polarity. In contrast, it completely disappears when pulses of alternating polarity are applied to the MTJ.
Fig.4: Endurance parameter versus delay between pulses for pulses 30ns long of various amplitude. (a) Comparison of unipolar pulses (red) and pulses of alternating polarities (black), (b) pulses of negative polarity (c) pulses of positive polarity.
These results were interpreted in terms of trapping/detrapping of tunnelling electrons at trapping sites in the tunnel barrier. The trapping/detrapping of electrons in the barrier is responsible for a very large stress of electrostatic origin on the oxide barrier. Indeed, as illustrated In Fig.5, every time an electron gets trapped in the tunnel barrier, a screening positive charge appears in the magnetic metallic electrodes to screen the electrostatic field from the trapped electron. Due to the close proximity between the trapped electrons and the screening charge (a few angstroms), the electrostatic force is quite strong and convert into a very strong compressive stress.
Fig.5: Illustration of the compressive stress generated by a trapped electrons in the tunnel barrier and its corresponding screening charge in the metallic electrode.
The observation of Fig.4 can then be understood as follows: The endurance of the barrier is strongly reduced when the density of trapped electrons in the barrier is high which corresponds to the situation where the voltage pulses are applied with very short delay between them, much shorter than the escape time from the traps (Δt<<100ns). The endurance is also reduced when the delay between pulses is much longer than the escape time (Δt>>100ns) because the barrier is then exposed to an alternating stress which favors the metallic mobility through the barrier and therefore the percolation path formation. Between these two regimes, the observed dramatic increase in endurance results from an optimum trade-off between a moderate average density of trapped charges in the barrier and a moderate time modulation of this density at each voltage pulse. A charge trapping-detrapping model was developed based on this physical picture which showed good coherence with experimental results. This study clearly demonstrated the key role played by electron trapping sites in the tunnel barrier. These traps can be pre-existing dislocations resulting from the lattice mismatch between CoFe and MgO (of the order of 4.3%), interstitial defects, vacancies (O or Mg), absorbed water molecules (MgO is highly hydrophilic) or local inhomogeneities in the oxide polarisability for instance if some BO has formed next to the MgO barrier during the annealing of the MTJ.
Another remarkable results obtained in this study is the observation of a correlation between the 1/f electrical noise of as-prepared MTJ and their endurance as illustrated in Fig.6. In oxides used in CMOS microelectronics, 1/f noise measurements are often used to characterize the presence of traps. Each trap generates a telegraph noise with its own duty cycle. The averaging over a large number of telegraph fluctuators having a random distribution of duty cycles yields a 1/f electrical noise. Since our endurance study demonstrated the key role played by electrical traps on the MTJ endurance, we decided to characterize the low frequency 1/f noise of electrical origin in these MTJs. The 1/f noise in MTJs is usually described using the empirical Hooge formula (Eq. 1) where α is the Hooge-like parameter, γ is the exponent of the 1/f noise, R and A are the junction resistance and area, respectively, and I is the bias current. We measured the electrical noise (not the magnetic one) by applying a large enough magnetic field to avoid magnetic fluctuations in the magnetic electrodes. The Hooge parameter α characterizes the overall amplitude of the electrical noise.
Fig.6 shows the 1/f noise spectra measured on a series of 60 junctions. The endurance of these 60 junctions was subsequently measured. Fig.6 clearly shows a correlation between the junctions’ endurance and their respective noise amplitude characterized by the Hooge parameter.
Fig.6: (left): 1/f noise density versus frequency for a set of about 60 junctions. (Right): endurance versus Hooge parameter for this set of 60 junctions showing a correlation between high endurance and low-noise
Quite interestingly, the observation of this correlation between noise and endurance provides a predictive characterization tool to evaluate a priori the endurance of MTJs in STT-MRAM chips.
Publications associated with WP1 :
Charge trapping-detrapping mechanism of barrier breakdown in MgO magnetic tunnel junctions
Amara-Dababi, S., R.C. Sousa, M. Chshiev, H. Béa, J. Alvarez-Hérault, L. Lombard, I.L. Prejbeanu, K. Mackay and B. Dieny
Applied Physics Letters 99 (2011) 083501
Modelling of time dependent dielectric barrier breakdown mechanisms in MgO-based magnetic tunnel junctions
Amara-Dababi, S., H. Béa, R.C. Sousa, K. Mackay and B. Dieny
Journal of Physics D: Applied Physics 45 (2012) 295002
Barrier breakdown mechanisms in MgO-based magnetic tunnel junctions and correlation with low-frequency noise
Amara, S., R.C. Sousa, H. Béa, C. Baraduc and B. Dieny
IEEE Transactions on Magnetics 48 (2012) 4340
Barrier breakdown mechanisms in MgO-based magnetic tunnel junctions under pulsed conditions and correlation with low-frequency noise
Amara-Dababi, S., H. Béa, R.C. Sousa, C. Baraduc, K. Mackay and B. Dieny
Proceedings of Spie Conference, Spintronics V 8461 (2012) 84610Ghttp://dx.doi.org/10.1117/12.930191
Correlation between write endurance and electrical low frequency noise in MgO-based magnetic tunnel junctions
Amara-Dababi, S., H. Béa, R.C. Sousa, C. Baraduc and B. Dieny
Applied Physics Letters 102 (2013) 052404http://dx.doi.org/10.1063/1.4788816
Barrier breakdown mechanisms in MgO-based magnetic tunnel junctions under pulsed conditions
Amara, S., H. Béa, R.C. Sousa and B. Dieny
Proceedings of the 4th IEEE International Memory Workshop (2012) 6213653
Breakdown mechanisms in MgO-based magnetic tunnel junctions and correlation with low-frequency noise
Amara-Dababi, S., R.C. Sousa, H. Béa, C. Baraduc, K. Mackay and B. Dieny
Microelectronics Reliability 53 (2013) 1239http://dx.doi.org/10.1016/j.microre…
First-principles investigation of the very large perpendicular magnetic anisotropy at Fe/MgO and Co/MgO interfaces
Yang H.X., M. Chshiev, B. Dieny, J.H. Lee, A. Manchon and K.H. Shin
Physical Review B 84 (2011) 054401
Enhancement of perpendicular magnetic anisotropy thanks to Pt insertions in synthetic antiferromagnets
Bandiera, S., R.C. Sousa, S. Auffret, B. Rodmacq and B. Dieny
Applied Physics Letters 101 (2012) 072410
Enhancement of perpendicular magnetic anisotropy through reduction of Co-Pt interdiffusion in (Co/Pt) multilayers
Bandiera, S., R.C. Sousa, B. Rodmacq and B. Dieny
Applied Physics Letters 100 (2012) 142410
Heating asymmetry induced by tunneling current flow in magnetic tunnel junctions
Gapihan, E., J. Hérault, R.C. Sousa, Y. Dahmane, B. Dieny, L. Vila, I.L. Prejbeanu, C. Ducruet, C. Portemont, K. Mackay and J.-P. Nozières
Applied Physics Letters 100 (2012) 202410
Temperature stability of (Pt/Co)3/IrMn multilayers
Lechevallier, L., A. Zarefy, F. Letellier, R. Lardé, D. Blavette, J.-M. Le Breton, V. Baltz, B. Rodmacq and B. Dieny
Journal of Applied Physics 112 (2012) 043904
Effects of sputter-deposition-induced and post-deposition thermally-activated intermixing on the exchange bias properties of [Pt/Co]x3/(Pt)/IrMn films
Letellier, F., V. Baltz, L. Lechevallier, R. Lardé, J.-F. Jacquot, B. Rodmacq, J.-M. Le Breton and B. Dieny,
Journal of Physics D: Applied Physics 45 (2012) 275001
Large exchange bias field in (Pt/Co)3/IrMn/Co trilayers with ultrathin IrMn layers
Moritz, J., G. Vinai and B. Dieny
IEEE Magnetics Letters 3 (2012) 4000204
Anisotropic bimodal distribution of blocking temperature with multiferroic BiFeO3 epitaxial thin films
Safeer, C.K., M. Chamfrault, J. Allibe, C. Carretero, C. Deranlot, E. Jacquet, J.-F. Jacquot, M. Bibes, A. Barthélémy, B. Dieny, H. Béa and V. Baltz
Applied Physics Letters 100 (2012) 072402
Co/Ni multilayers with perpendicular anisotropy for spintronic device applications
You, L., R.C. Sousa, S. Bandiera, B. Rodmacq and B. Dieny
Applied Physics Letters 100 (2012) 172411
Benefit of inserting a (Cu/Pt) intermixing dual barrier for the blocking temperature distribution of exchange-biased Co/(Cu/Pt)/IrMn stacks
Akmaldinov, K., S. Auffret, I. Joumard, B. Dieny and V. Baltz
Applied Physics Letters 103 (2013) 042415
Spontaneous anomalous and spin Hall effects due to spin-orbit scattering of evanescent wave functions in magnetic tunnel junctions
Vedyayev, A., N. Ryzhanova, N. Strelkov and B. Dieny
Physical Review Letters 110 (2013) 247204
Influence of a Ta spacer on the magnetic and transport properties of perpendicular magnetic tunnel junctions
Cuchet, L., B. Rodmacq, S. Auffret, R.C. Sousa, C. Ducruet and B. Dieny
Appl. Phys. Lett. 103, 052402 (2013)
Effect of a Cu spacer between Co and Pt layers on the structural and magnetic properties of (Co/Cu/Pt)5/Pt-type multilayers
Bandiera, S., R.C. Sousa, B. Rodmacq, L. Lechevallier and B. Dieny
Journal of Physics D: Applied Physics 46 (2013) 485003
Competition between CoOx and CoPt phases in Pt/Co/AlOx semi-tunnel junctions
Garad, H.M., L. Ortega, A.Y. Ramos, Y. Joly, A. Fettar, S. Auffret, B. Rodmacq, B. Dieny, O. Proux and A.I. Erko
Journal of Applied Physics 114 (2013) 053508
Anatomy of perpendicular magnetic anisotropy in Fe/MgO magnetic tunnel junctions: First-principles insight
Hallal, A., H.X. Yang, B. Dieny and M. Chshiev
Physical Review B 88 (2013) 184423
Enhanced blocking temperature in (Pt/Co)3/IrMn/Co and (Pd/Co)3/IrMn/Co trilayers with ultrathin IrMn layer
Vinai, G., J. Moritz, S. Bandiera, I.L. Prejbeanu and B. Dieny
Journal of Physics D: Applied Physics 46 (2013) 322001
Magnetic properties of patterned arrays of exchange-biased IrMn/Co square dots
Vinai, G., J. Moritz, G. Gaudin, J. Vogel, M. Bonfim, F. Lançon, I.L. Prejbeanu, K. Mackay and B. Dieny
Journal of Physics D: Applied Physics 46 (2013) 345308
Mixing antiferromagnets to tune NiFe-[IrMn/FeMn] interfacial spin-glasses, grains thermal stability, and related exchange bias properties
Akmaldinov, K., C. Ducruet, C. Portemont, I. Joumard, I.L. Prejbeanu, B. Dieny and V. Baltz
Journal of Applied Physics 115 (2014) 17B718
Breakdown mechanisms in MgO-based magnetic tunnel junctions and correlation with low frequency noise
Amara-Dababi, S. ; Sousa, R.C. ; Bea, H. ; Baraduc, C. ; Mackay, K. ; Dieny, B.
Proceedings of the IEEE International Reliability Physics Symposium (2014)
Double barrier magnetic tunnel junctions with write/read mode select layer
Clement, P.-Y. ; Baraduc, C. ; Chshiev, M. ; Dieny, B. ; Vila, L. ; Ducruet, C.
Proceedings of the 6th International Memory Workshop
Focused Kerr measurements on patterned arrays of exchange-biased square dots
Vinai, G. ; Moritz, J. ; Gaudin, G. ; Vogel, J. ; Prejbenau, I.L. ; Dieny, B.
EPJ WEB of Conferences 75 (2014) 05003
Influence of magnetic electrodes thicknesses on the transport properties of magnetic tunnel junctions with perpendicular anisotropy
Cuchet, L. ; Rodmacq, B. ; Auffret, S. ; Sousa, R.C. ; Dieny, B.
Applied Physics Letters 105 (2014) 052408http://scitation.aip.org/docserver/…
Impurity-induced enhancement of perpendicular magnetic anisotropy in Fe/MgO tunnel junctions
Hallal, A. ; Dieny, B. ; Chshiev, M.
Physical Review B 90 (2014) 064422
Large exchange bias enhancement in (Pt(or Pd)/Co)/IrMn/Co trilayers with ultrathin IrMn thanks to interfacial Cu dusting
Vinai, G. ; Moritz, J. ; Bandiera, S. ; Prejbeanu, I.L. ; Dieny, B.
Applied Physics Letters 104 (2014) 162401
IrMn microstructural effects on exchange bias variability in patterned arrays of IrMn/Co square dots
Vinai, G. ; Moritz, J. ; Gaudin, G. ; Vogel, J. ; Prejbeanu, I.L. ; Dieny, B.
Journal of Physics D: Applied Physics 47 (2014) 195302