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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 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

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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