A Perpendicular Spin Torque Switching based MRAM for the 28 nm Technology Node

Transcription

1 A Perpendicular Spin Torque Switching based MRAM for the 28 nm Technology Node U.K. Klostermann 1, M. Angerbauer 1, U. Grüning 1, F. Kreupl 1, M. Rührig 2, F. Dahmani 3, M. Kund 1, G. Müller 1 1 Qimonda AG 2 Siemens AG 3 Altis Semiconductor <1>

2 Perpendicular Spin Torque (P-ST) based MRAM o A New Concept Outline Assessment for 28 nm Node o Data Retention o Low Switching Currents o Cell to Cell Interaction o Barrier Reliability Cell Layout Read Analysis <2>

3 Magnetic Hysteresis: Cell Resistance R Low Conventional MRAM 0 Magnetic Field H High WRITE: Word/Bit line field used to set magnetic free layer READ: Electrical determination of R by sense amplifiers Anti- Parallel 1 Parallel 0 <3>

4 Spin Torque Select-Based MRAM Bit Line free layer barrier pinned S D magneto resistance resistance change MR [%] [ % ] "1" Set "0" 20 Set "1" "0" switching voltage V c [mv] c [ mv ] Writing is done by a critical select current <4>

5 Perpendicular Anisotropy In-Plane Magnetization Perpendicular Magnetization interface H k shape H k free layer barrier reference layer Perpendicular anisotropy is very high <5>

6 Realization CoFeTb CoFeTb Source: Spin transfer switching in TbCoFe / CoFeB / MgO / CoFeB / TbCoFe magnetoresistive tunneling junctions with perpendicular magnetic anisotropy, M. Nakayama et al., BB-09, 52 nd Magnetism and Magnetic Materials Conference (MMM) in Tampa, Nov Feasibility of concept is demonstrated MTJ stack engineering is important <6>

7 Scalability of Activation Energy E a 1 = Vol M 2 s H MTJ size Anisotropy Material k activation energy E a [k B T ] activation energy E a [ k B T ] P-ST I-ST Perpendicular In-Plane Product Target: 85 k B T MTJ width w [nm] w [ nm ] High anisotropy ensures scaling below 20 nm <7>

8 Scalability of Switching Current switching current current I c I [µa] c [ µa ] I c P-ST I c I-ST I c ~ w I c ~ const MTJ width width w w [nm] [ nm ] In-Plane Perpendicular Absence of demagnetization fields reduces required switching current I c ~ 30 µa <8>

9 field component H (x or z) [ Oe ] p disturb, x or z [ ] Cell to Cell Interaction P-ST I-ST Perpendicular In-Plane distance from center x [F = 28 nm] distance from center x [ F = 28 nm ] Significantly reduced stray field interaction <9>

10 Impact of Interaction on E a activation energy E a [ k B T ] activation energy E a [k B T ] 90 Product Target: 85 k B T In-Plane Perpendicular P-ST I-ST structural cell size [F 2 with F = 28 nm] structural cell size [ F ² with F = 28 nm ] Correct E a by H 1 disturb H k ~1.5 High data retention at dense spacing <10>

11 switching voltage V voltage c or V BD [V] [ V ] Reliability Estimates V c P-ST V c I-ST V BD at Product Life Time In-Plane 0.2 Perpendicular RA RA of of MTJ barrier [Ωµm [ Ωµm² 2 ] ] P-ST allows to use high RA for reliable operation <11>

12 Cell Layout at 28 nm Node 6 28 nm 2F 3F 6 F² layout ensures sufficient current drivability <12>

13 single bit error per 1 access 1E-10 1E-14 1E-18 1E-22 1E-26 1E-30 Read Disturb γ := ratio (read / write) current γ = 0.5 γ = 0.4 γ = temperature T [ C] [ C ] I c ~ 30 µa At I c ~ 30 µa a read current of I r ~ 10 µa (γ ~ 0.3) is feasible without read disturb <13>

14 MTJ Stack Performance Measured magneto resistance (MR) for in-plane systems MR = ( R 1 - R 0 ) / R 0 [%] MR [ % ] RA of RA MTJ of barrier MTJ [Ωµm [ Ωµm² 2 ] ] MR := ( R 1 - R 0 ) / R 0 I-ST demonstrated high MR at low RA P-ST will require similar stack performance <14>

15 Read Circuit WL c Source Line MTJ BL 1.1 V potential select transistor reference current Typical: R 0 = 6 kω R 1 = 12 kω R para = 14 kω CSL READ_EN MBL I_REF voltage compliance for MTJ: controlling MBL potential by V_READ = 0.95 V V_READ V_READ SA_IN EQL SA_REF OUT optimized SA current compliance I r Current compliance avoids read disturb <15>

16 signal [V] [ V ] signal [ [V] V ] Read Operation Simulation 1,2 0,8 0,4 0,0 1,2 0,8 0,4 READ_EN OUT MBL SA_IN SA_REF 0, time [[ns] ] Fast random array read access time ~ 30 ns demonstrated I r read read current current [ [µa] ] <16>

17 Summary Perpendicular Spin Torque has been studied targeting the 28 nm node. Expected benefits are: long data retention (> C) low write current (~ 30 µa) small cell sizes (~ 6 F²) high write endurance and no read disturb Random access speeds are 30 ns for read and 10 ns for write. <17>