Advanced magnetic tunnel junctions based on CoFeB/MgO interfacial perpendicular anisotropy

Transcription

1 SEMATECH 8th International Symposium on Advanced Gate Stack Technology, Session 4: Spin Transfer Torque Memory Bolton Landing, NY 20, October 2011 Advanced magnetic tunnel junctions based on CoFeB/MgO interfacial perpendicular anisotropy S. Ikeda 1,2 M. Yamanouchi 1, H. Sato 1, K. Miura 3,1,2, K. Mizunuma 2, H. Yamamoto 3, R. Koizumi 2, H. D. Gan 1, S. Kanai 2, J. Hayakawa 3, F. Matsukura 1,2, H. Ohno 1,2 1 Center for Spintronics Integrated Systems (CSIS), Tohoku Univ. 2 Laboratory for Nanoelectronics and Spintronics, RIEC, Tohoku Univ. 3 Central Research Laboratory, Hitachi, Ltd. Acknowledgement: This work was supported by the project "Research and Development of Ultra-low Power Spintronics-based VLSIs under the FIRST Program of JSPS. 1

2 Spintronics devices (Expectation as low power consumption devices) Magnetoresistive RAMs (MRAMs) SPin transfer torque RAMs (SPRAMs) Recording Free Barrier PRAM Bit line Nonvolatility Reference Pinned 10 High speed operation 10 FeRAM MTJ Word line Virtually unlimited Flash Gate Source Drain 10 write endurance 5 1n 10n 100n 1µ 10µ Logic-in-memory architecture Reduction of leak current (static power) Reduction of interconnection delay Mochizuki et al., IEICE Transactions on Fundamentals, E88-A (2005) Matsunaga et al., Appl. Phys. Express, 1 (2008) ; ibid., 2 (2009) Sekikawa et al., IEDM 2008, Suzuki et al., VLSI Technology 2009 Ohno et al., IEDM 2010 Matsunaga et al. VLSI Circuit 2011 Number of write cycles SRAM/DRAM (Volatile) MRAM/SPRAM Memory Processor core Write time (s) MTJ CMOS 2

3 MRAM development 1G Capacity (bits/chip) 100M 10M 1M 100k 10k 1k IBM IBM-Infineon Freescale Sony Freescale(Motorola) Toshiba- NEC Samsung Everspin Toshiba-NEC Cypress ITRI-TSMC-NTHU Sony Toshiba NEC TDK Hitachi-Tohoku Univ. NTHU-ITRI Everspin MRAM (demo) MRAM (product) SPRAM (i-mtj) SPRAM (p-mtj) Hynix-Grandis Magnetic field 32Mb SPRAM VLSI Circuit 2009 Takemura et al., IEEE J. Solid-State Circuits 45 (2010) 869. Spin transfer torque Control Circuits Pre-Decoder 128kb Array SA & Write MTJ cell MgO barrier Word Driver/Dec. Y-Dec. Recording layer pinned layer Anti-ferromagnetic layer MTJ cells Cross section SEM image of SPRAM cell CMOS-Tr. M4 M3 M2 M Year Ikeda et al., IEEE Trans. Electron Devices, 54 (2007) 991. Lower electrode 50 nm 2Mb SPRAM ISSCC 2007 Kawahara et al., IEEE J. Solid-State Circuits 43 (2008)

4 Nonvolatile circuits for logic-in-memory MTJ-Based Nonvolatile Full Adder Matsunaga, Hanyu et al., Appl. Phys. Express 1 (2008) MTJ-Based Nonvolatile Lookup-Table Circuit Chip Suzuki, Hanyu,et al., VLSI Circuit Symposium Matsunaga,... Hanyu et al., Appl. Phys. Express 2 (2009) MTJ-Based Nonvolatile Ternary Content- Addressable Memory (TCAM) cell Row Dec. Column Dec. 2-kb TCAM cell array 174 µm Sense Amp. 2kb-nonvolatile TCAM chip Matsunaga, Hanyu,et al., VLSI Circuit Symposium

5 Technology issues toward realization of nonvolatile VLISs To realize nonvolatile VLISs (MRAM and logic-in-memory) using the leading edge technology node, there are still issues to be addressed. ISSUES High output (TMR ratio>100%) Low switching current (I C < F μa) + Scalability Thermal stability for nonvolatility (E/k B T > 40) Annealing stability in back-end-of-line process (T a > 350 o C) MTJs have to satisfy these requirements with scalability at the same time. In order to satisfy these requirements, we focus on the MTJs with perpendicular anisotropy electrodes. 5

6 トンネル磁気抵抗比 TMR ratio (%) (%) Progress of MTJs %@RT(1144%@5K) Tohoku Univ. & Hitachi MgO-barrier MTJs CanonANELVA & AIST IBM AIST AlO x -barrier MTJs Tohoku Univ Year 年 AIST AIST CoFeB MgO CoFeB In-plane anisotropy pseudo-sv Cr/Au Ru (5) CoFeB(6) MgO(2.1) CoFeB(5 ) Ru(10) bcc (001) rock salt (001) bcc(001) Perpendicular anisotropy Cr/Au pseudo-sv Cr/Au Ru (5) CoFeB(1.7) MgO(0.9) CoFeB(1 ) Ru(10) S. Ikeda et. al., Appl. Phys. Lett. 93, (2008). Nat. Mat., 9, 721 (2010) SiO 2 /Si sub. SiO 2 /Si sub. 6

7 M-H curves of CoFeB/MgO stack samples with different t CoFeB MgO(1) CoFeB(t CoFeB ) Ru(10) SiO 2 /Si sub. T a =300 o C, 4 koe, 1h M (T) t CoFeB = 2.0 nm T a =300 o C t CoFeB = 1.3 nm T a =300 o C µ 0 H s_in-plane = 0.34 T M s = 1.58 T K = 2.1x10 5 J/m 3 in-plane out-of-plane µ 0 H (T) In-plane anisotropy Perpendicular anisotropy S. Ikeda et al., Nat. Mat. 9 (2010)

8 t CoFeB dependence of Kt CoFeB in CoFeB/MgO stack MgO(1) CoFeB(t CoFeB ) Ru(10) SiO 2 /Si sub. T a = 300 o C K t CoFeB (mj/m 2 ) Perpendicular in-plane VSM FMR t CoFeB (nm) Kt CoFeB = (K b -M S2 /2µ 0 ) t CoFeB +K i From the y-intercept, K i = 1.3 mj/m 2. The CoFeB/MgO interfacial anisotropy K i is dominant in the perpendicular anisotropy because K b is negligible. S. Ikeda et al., Nat. Mat. 9 (2010)

9 t MgO dependence of M s and K Saturation magnetization M s and effective magnetic anisotropy energy density K are evaluated from the out-of-plane M-H curves. M s (T) K (10 5 Jm -3 ) t MgO = 0.6 nm t MgO (nm) Mg O 1 monolayer of MgO is ~0.21 nm 0.42 nm More than 3 monolayers of MgO is required to stabilize the perpendicular anisotropy induced by CoFeB-MgO interface. M. Yamanouchi et al., J. Appl. Phys. 109 (2011) 07C712. 9

10 Possible factor of the perpendicular anisotropy in CoFeB/ MgO stack 3d z 2 (m=0) : in-plane anisotropy O 2 p z Fe anti-bonding 3d z 2 Fe 3dz 2 (m=0) O 2p Z σ-coupling bonding K. Nakamura et al., Phys. Rev. B, 81 (2010) R. Shimabukuro et al., Physica E 42 (2010) d z 2 band of Fe is pushed up above the Fermi energy. The contribution of 3d z 2 orbital of Fe becomes small, resulting in appearance of perpendicular anisotropy. 10

11 TMR properties in CoFeB-MgO p-mtjs Al 2 O 3 V - Cr/Au Ru (5) CoFeB(1.5 ) MgO(0.9) CoFeB(0.9 ) Ru(10) SiO 2 /Si sub. T a = 300 o C V + R (kω) Antiparallel Parallel µ 0 H (T) (nm) Clear hysteresis -> top and bottom CoFeB electrodes have perpendicular anisotropy. 40 nm S. Ikeda et al., Nat. Mat. 9 (2010)

12 Junction size dependence of I c0 F = 40 ~ 76nmφ Cr/Au Ru (5) CoFeB(1.5) MgO(0.9) CoFeB(0.9 ) Ru(10) SiO 2 /Si sub. T a =300 o C w/o H ex Critical current, I C0 (µa) I C0 (P to AP) I C0 (AP to P) Junction size 53 nm 44 nm 59 nm 70 nm 76 nm Recording layer volume, V rec (nm 3 ) = S t CoFeB 50 nm I c0 linearly decreases with junction area S (volume V rec ). We confirmed scalability of switching current for spin transfer torque. H. Sato et al., Appl. Phys. Lett. 99 (2011)

13 Junction size dependence of E/k B T Thermal stability factor, E/k B T nm 53 nm 44 nm Junction size 70 nm 76 nm 20 pulse current experiment pulse field experiment previous report Recording layer volume, V rec (nm 3 ) = S t E/k B T maintained almost constant values even though the junction diameter was varied from 40 nm to 80. I C0 reduces in proportion to the volume of the recording layer but the E/k B T values are not affected much by the volume down to 40 nm in diameter. H. Sato et al., Appl. Phys. Lett. 99 (2011)

14 Origin of the junction size dependence of I C0 and E/k B T No clear reduction in E/k B T with different junction size Nucleation type magnetization reversal Nucleation diameter, D n (nm) Effective diameter, D n (nm) Junction size in diameter (nm) Junction area (diameter): S (D) Nucleation diameter: D n Nucleation embryo D n showed almost constant values of 40 nm in diameter, which is almost independent of junction size. I C0 =J C0 S=J C0 (π(d 2 -D n2 )/4)+π(D n /2) 2 ) S E/k B T=K eff π(d n /2) 2 /k B T D n 2 H. Sato et al., Appl. Phys. Lett. 99 (2011)

15 J c0 and E/k B T of MTJs with 40 nm diameter Cr/Au Ru (5) Al 2 O 3 CoFeB(1.7) Al 2 O 3 MgO(0.85) CoFeB(1.0 ) Ru(10) SiO 2 /Si sub. w/o H ex T a =300 o C w/o H ex T a =350 o C 40 nm J c0 T a ( o C) TMR ratio RA (%) (Ωµm 2 ) (MA/cm 2 ) E/k B T J c0 and E/k B T are maintained after annealing at 350 o C. This MTJ system has a back end of the line (BEOL) compatibility. S. Ikeda et al., Nat. Mat. 9, (2010) 721. K. Miura et al., MMM 2010, HC

16 Comparison of MTJs Type Stack structure (nm) Size (nm) MR (%) RA (Ωμm 2 ) J C0 (MA/cm 2 ) I C0 (μa) Δ=E/k B T I C0 /Δ T a ( o C) Ref. i-mtj CoFeB(2)/Ru(0.65)/CoF eb(1.8) SyF 100x200 >130 ~10 2 ~ ~ J. Hayakawa et al., IEEE T-Magn., 44, 1962 (2008) p-mtj L10- FePt(10)/Fe(t)/Mg(0.4)/ MgO(1.5)/L10-FePt(t) Blanket 120 (CIPT) 11.8k M. Yoshizawa et al., IEEE T-Magn., 44, 2573 (2008) p-mtj L10- FePt/CoFeB/MgO(1.5)/ CoFeB/Co based superlattice Blanket 202 (CIPT) H. Yoda et al., Magnetics Jpn. 5, 184 (2010) [in Japanese]. p-mtj [Co/Pt]CoFeB/CoFe/Mg OCoFe/CoFeB/TbFeCo Blanket (CIPT) K. Yakushiji et al., APEX 3, (2010) p-mtj [CoFe/Pd]/CoFeB/MgO/ CoFeB/[CoFe/Pd] 800x800 N 100 (113) 18.7k (20.2k) (325) K. Mizunuma et al., MMM&INTERMAG2010 p-mtj CoFeB (1)/ TbCoFe (3) 130 φ ~ p-mtj L1 0 -alloy φ M. Nakayama et al., APL 103, 07A710 (2008) T. Kishi et al., IEDM 2008 p-mtj Fe based L1 0 (2)/CoFeB (0.5) H.Yoda et al., Magnetics Jpn. 5, 184 (2010) [in Japanese]. p-mtj CoFeB/MgO/ CoFeB 40 φ 113 (124) 16 (18) 3.8 (3.9) 48 (49) 39 (43) 1.23 (1.14) 350 (300) S. Ikeda et al., Nat. Mat. 9 (2010) 721. K. Miura et al., MMM2010,HC-02 These p-mtj technologies will be a promising building block for nonvolatile VLSIs using spin-transfer torque switching. 16

17 Issue for p-mtjs Bit Information 0 Bit Information 1 Free Reference N S N S Parallel state Low resistance Different polarity =Stable S N N S Antiparallel state High resistance The same polarity =Unstable The thermal stability p =E/k B T of anti-parallel state in p-mtjs becomes low by comparison with parallel state. Slide 17 17

18 Enhancement of thermal stability Conventional structure H s dipole interlayer coupling field P, AP = [1±H s /H c0 ] 2 Free Reference AP E/k B T 1 0 P Stepped structure H s Free Reference E/k B T 1 0 AP P H s decreases by employing step structure with large reference layer. AP increases with decreasing diameter of reference layer. 18

19 Two types of p-mtjs Conventional structure Stepped structure 100 nm F = 100 nm F = 100 nm 100 nm 1.5 nm 0.9 nm 0.9 nm CoFeB MgO CoFeB Free layer Reference layer CoFeB MgO CoFeB 1.5 nm 0.9 nm 0.9 nm F : Feature size 300 nm Slide 19 K. Miura et al., 2011 VLSI Technology, 11B-3. 19

20 Resistance (kω) 4 3 H s in two types of p-mtjs Conventional structure H s Magnetic field (mt) Stepped structure Conventional structure Stepped structure TMR ratio (%) RA (Ωµm 2 ) H s (mt) 22 5 Resistance (kω) 4 3 H s Magnetic field (mt) Slide 20 H s can be reduced by using stepped structure. K. Miura et al., VLSI Technology 2011, 11B-3. 20

21 P and AP in two types of p-mtjs P,AP Probability 1 Conventional structure AP -> P P -> AP Magnetic field (mt) Probability 1 Stepped structure AP -> P P -> AP Magnetic field (mt) Conventional structure Stepped structure P AP AP in stepped structure increases. K. Miura et al., VLSI Technology 2011, 11B-3. 21

22 Cell area (µm 2 ) Cell area of stepped structure SRAM 100F 2 (F: feature size) F = 100 nm CoFeB MgO CoFeB 32 nm 300 nm F = 100 nm nm Technology node (nm) CoFeB MgO CoFeB 200 nm Cell area of 0.09 µm 2 (300nmφ) in the stepped structure corresponds to SRAM cell area at 32 nm technology node. Cell area can be down to 0.04 µm 2 (200nmφ) without degrading the retention time over 10 years, which corresponds to SRAM cell area at 20 nm technology node. K. Miura et al., VLSI Technology 2011, 11B-3. 22

23 Summary We have demonstrated that the critical current I c0 in CoFeB/MgO perpendicular anisotropy MTJs (p-mtjs) can be scaled down with decreasing recording layer volume. The thermal stability factor E/k B T can be maintained at about 40 even though the recording volume was reduced to 40 nm. The magnetization reversal in CoFeB/MgO p-mtjs is dominated by nucleation type magnetization reversal (nucleation diameter ~40 nm). CoFeB/MgO p-mtjs show the high TMR ratio of more than 100%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49 μa at the same time. CoFeB/MgO p-mtj with step structure shows the enhancement of thermal stability in antiparallel state, which achieves thermal stability for data retention time over 10 years. 23