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STT-RAM Circuit Design. MTJ Specs (Update), MTJ Sharing. I-STT MTJ Specs (Jianping). UPDATED SPECS R P ≈ 744 Ω TMR ≈ 136% AP→P: 630 μ A Max (breakdown current) 387 μ A for 3ns switching 330 μ A for 5ns switching P→AP: 1.5mA Max (breakdown current) Need more device measurements

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Stt ram circuit design

STT-RAM Circuit Design

MTJ Specs (Update),

MTJ Sharing


I stt mtj specs jianping
I-STT MTJ Specs (Jianping)

  • UPDATED SPECS

    • RP ≈ 744Ω

    • TMR ≈ 136%

    • AP→P:

      • 630μA Max (breakdown current)

      • 387μA for 3ns switching

      • 330μA for 5ns switching

    • P→AP:

      • 1.5mA Max (breakdown current)

      • Need more device measurements

    • IWRITE(P→AP)/IWRITE(AP→P): 1.5-2

    • 1ns read pulse (P→AP) with 1% chance of write: 220μA

      • AP→P might be better


Maximum write currents thick oxide
Maximum Write Currents (Thick Oxide)

  • Thin Oxide: 1.25nm

    • Max VDD = 1.0V

    • LMIN = 50nm

  • Medium Oxide: 2.2nm

    • Max VDD = 1.5V

    • LMIN = 100nm

    • Per μm width: IMAX,MEDIUM/IMAX,LVT = 93%

  • Thick Oxide: 5.2nm

    • Max VDD = 3.3V

    • LMIN = 230nm

    • Per μm width: IMAX,THICK/IMAX,LVT = 73%


Mtj sharing

MTJ Sharing

TMR Degradation

(Reading)


Tmr degradation
TMR Degradation

SL

BL<M>

BL<2>

BL<1>

Parallel Resistance (R||)

degrades TMR

WL<2>

WL<N>

WL<1>

Parasitic Parallel Resistance

MTJ1,1

MTJ1,2

MTJ1,M

MTJN,M

MTJ2,1

MTJ2,2

MTJ2,M

MTJN,2

MTJN,1


Effective r p and r ap
Effective RP and RAP

  • Worst case TMR’: largest RP’ and smallest RAP’

  • Largest RP’:

  • Smallest RAP’


Effective tmr
Effective TMR

  • Putting it all together:

    Example 1kbit Arrays:

  • TMR = 120%, M = 2, N = 16, 32-bit words: TMR’ = 4.8%

  • TMR = 120%, M = 2, N = 8, 64-bit words: TMR’ = 9.8%

  • TMR = 120%, M = 2, N = 4, 128-bit words: TMR’ = 20.7%


Monte carlo simulations m 2 3
Monte Carlo Simulations (M = 2, 3)

  • ERROR IN MATLAB CODE used to generate last week’s Monte Carlo plots → Not simulating the intended cases!

    • Fixed and reran simulations → results not so good

  • Ran multiple simulations with and w/o random variations in RP and TMR (based on worst case from Jianping)

    • Extracted worst case TMR’ and TMR’ for 10% read error

  • For 128-bit words, with bit read error = 10.0%:

    • # error correcting bits = 36 (1/5 word)

      • Probability of a word error: 1 in 6,788 reads

    • # error correcting bits = 32 (1/4 word)

      • Probability of a word error: 1 in 3.59x106 reads

    • # error correcting bits = 43 (1/3 word)

      • Probability of a word error: 1 in 13.2x1012 reads



Monte carlo simulations m 2 n 4
Monte Carlo Simulations: M = 2, N = 4

  • TMR = 120%

  • RP = 500Ω

  • 25k Simulations

  • TMR’

    • Worst Case = 20.7%

    • ~10% Read Error = 30.0%

  • TMR = 120%, 3σ = ±12%

  • RP = 500Ω, 3σ = ±50Ω

  • 25k Simulations

  • TMR’

    • Worst Case = 3.7%

    • ~10% Read Error = 25.4%


Monte carlo simulations m 2 n 8
Monte Carlo Simulations: M = 2, N = 8

  • TMR = 120%

  • RP = 500Ω

  • 25k Simulations

  • TMR’

    • Worst Case = 9.8%

    • ~10% Read Error = 14.6%

  • TMR = 120%, 3σ = ±12%

  • RP = 500Ω, 3σ = ±50Ω

  • 25k Simulations

  • TMR’

    • Worst Case = -4.1%

    • ~10% Read Error = 12.4%


Monte carlo simulations m 2 n 16
Monte Carlo Simulations: M = 2, N = 16

  • TMR = 120%

  • RP = 500Ω

  • 25k Simulations

  • TMR’

    • Worst Case = 5.1%

    • ~10% Read Error = 8.5%

  • TMR = 120%, 3σ = ±12%

  • RP = 500Ω, 3σ = ±50Ω

  • 25k Simulations

  • TMR’

    • Worst Case = -10.3%

    • ~10% Read Error = 4.7%



Monte carlo simulations m 3 n 4
Monte Carlo Simulations: M = 3, N = 4

  • TMR = 120%

  • RP = 500Ω

  • 25k Simulations

  • TMR’

    • Worst Case = 0.0%

    • ~10% Read Error = 15.6%

  • TMR = 120%, 3σ = ±12%

  • RP = 500Ω, 3σ = ±50Ω

  • 25k Simulations

  • TMR’

    • Worst Case = -10.1%

    • ~10% Read Error = 15.9%


Monte carlo simulations m 3 n 8
Monte Carlo Simulations: M = 3, N = 8

  • TMR = 120%

  • RP = 500Ω

  • 25k Simulations

  • TMR’

    • Worst Case = -11.2%

    • ~10% Read Error = -0.6%

  • TMR = 120%, 3σ = ±12%

  • RP = 500Ω, 3σ = ±50Ω

  • 25k Simulations

  • TMR’

    • Worst Case = -16.7%

    • ~10% Read Error = -1.0%


Monte carlo simulations m 3 n 16
Monte Carlo Simulations: M = 3, N = 16

  • TMR = 120%

  • RP = 500Ω

  • 25k Simulations

  • TMR’

    • Worst Case = -15.9%

    • ~10% Read Error = -11.2%

  • TMR = 120%, 3σ = ±12%

  • RP = 500Ω, 3σ = ±50Ω

  • 25k Simulations

  • TMR’

    • Worst Case = -23.4%

    • ~10% Read Error = -11.4%


Mtj sharing1

MTJ Sharing

Device IREAD/IWRITE Requirements

(Writing)


Defining i read max i write min
Defining IREAD,MAX & IWRITE,MIN

  • IREAD,MAX: The maximum read current such that the probability of flipping the MTJ is less than some ε(i.e. ε = 0.1% → IREAD,MAX = 200μA)

  • IWRITE,MIN: The minimum write current such that the probability of failing to flip the MTJ is less than some ξ(i.e. ξ = 0.1% → IWRITE,MIN = 600μA)


I read i write for 1t 2mtj 1t 3mtj
IREAD/IWRITE for 1T-2MTJ & 1T-3MTJ

BL<1>

  • Example: 1T-2MTJ architecture

WL<2>

WL<1>

BL<2>

IWRITE,MIN

α∙IREAD,MAX


I read i write for 1t 2mtj
IREAD/IWRITE for 1T-2MTJ

RP Case 1:

RAP Case 1:

RP Case 2:

RAP Case 2:

“1”

“1”

“1”

“1”

“0”

“0”

“0”

“0”

RAP

RP

RAP

RP

RP

RP

RP

RAP

RP

RAP

RP

RAP

RP

RP

RAP

RP


I read i write vs tmr for 1t 2mtj
IREAD/IWRITE vs. TMR for 1T-2MTJ


I read i write for 1t 3mtj
IREAD/IWRITE for 1T-3MTJ

RP Case 1:

RAP Case 1:

RP Case 2:

RAP Case 1:

RP

RP

“1”

“0”

RAP

RP

RAP

RP

RP

“1”

“0”

RP

RP

RP

RAP

RAP

RP

RAP

“1”

“1”

“0”

“0”

RP

RP

RAP

RAP

RP

RP


I read i write vs tmr for 1t 3mtj
IREAD/IWRITE vs. TMR for 1T-3MTJ


Summary
SUMMARY

  • TMR Degradation (READING)

    • M = 2:

      • Not as good as previously thought

      • Read circuit need to work for 25-30% TMR

        • 10-12% for more wordlines

    • M = 3: not really possible (negative TMR)

  • IREAD/IWRITE (WRITING)

    • For TMR = 120%, χ = 1.5-2:

      • M = 2: IREAD/IWRITE > 0.36-0.43

      • M = 3: IREAD/IWRITE > 0.42-0.49


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