Short Pulse Reading for STT-RAM

fren@ee.ucla.edu

Background• STT-RAM

– Storage element: MTJ• Represents “0/1” by the configuration of magnetization direction

– Read/Write operations: CMOS circuits

• CMOS and MTJ variability are increasing – Resulting in more stringent constraints on CMOS design

2

Poly

n+ n+

WL1

MTJ

BL

n+

Poly

SL M1

M2

M3

MTJ

M4

WL2

Parallel Anti-parallel

Low RP - ”0” High RAP - ”1”

Ferro-magnetic

layers

• Sense the RMTJ (RAP / RP) through IREAD

– Without disturbing the cell (0% switching prob.)

• Two ways to get 0% switching probability– Low current reading (LCR)– Short pulse reading (SPR)

Write

(2) Short Pulse Reading

(1) Low Current ReadingRead

Read Circuit Design

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• JC scaling will eventually create difficulty for LCR

• How to implement SPR?– What is the circuit structure?

Write

(2) Short Pulse Reading

(1) Low Current Reading

Read

2 ways to get 0% switching prob.

Read Circuit Design

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How to implement SPR?• When can we turn off sensing circuit?

– When a safe read margin (VMTJ-VREF > VOS_latch + NM) is established

– VOS_latch < 15 mV

• How fast that read margin can be established?

• The best SPR circuit should be able to establish the largest read margin with the least time.

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• Current Sensing– Speed is limited by the IMTJ

• VMTJ is fixed, between VMTJ_P and VMTJ_AP

– VMTJ-VREF is limited

#1: Current-Mirror Sense Amp (CMSA)

6[1] D. Gogl, et al., JSSC, Vol. 40, No. 4, Apr. 2005[2] J.P. Kim, et al., VLSI, 2011[3] J. Kim, et al., JVLSI, 2011

• Current Sensing– Speed is limited by the IMTJ

• VMTJ is reverse to VMTJ

– Larger VMTJ-VREF

#2: Split-Path Sense Amp (SPSA)

7[1] S.O. Jing, et al., US Patent, Pub. No. US 2010/0321976 A1

-0.5 0 0.50

100

200

300

400

500

600

700

800

900

1000

Voltage (V)

Fre

quen

cy

SPSA

SMTJ,P

-SREF,P

SMTJ,AP

-SREF,AP

RMP = -29.8mV

RMAP

=12mV

• Body Voltage Sensing– Body-connected load is better

than diode connected load– Speed is no longer limited by

IMTJ

#3: Body-Voltage Sense Amp (BVSA)

8[My proposal]

• VMTJ is reverse to VMTJ

– Even larger VMTJ-VREF

– Benefiting from gain of the sense amp

-0.5 0 0.50

100

200

300

400

500

600

Voltage (V)

Fre

quen

cy

Vmtj-Vref, R/ RP

Vmtj-Vref, R/ RAP

RMP = -268mV

RMAP

=303mV

+3σ -3σ

RMP RMAP

RM Definition• RMP = μ(VMTJ,P − VREF,P) + 3σ(VMTJ,P − VREF,P) should be < 0

• RMAP = μ(VMTJ,AP − VREF,AP) − 3σ(VMTJ,AP − VREF,AP) should be > 0

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We compare 3 sensing circuits at ISO reading current:

•#1: Current-Mirror Sense Amp (CMSA)– Qualcomm design [VLSI’11]

•#2: Split-Path Sense Amp (SPSA)– Qualcomm design [US Patent 2010/0321976 A1]

•#3: Body-Voltage Sense Amp (BVSA)– UCLA proposal

to demonstrate the read margin and speed advantage of our approach

RM and performance Comparison

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Currentsensing

Voltagesensing

Simulation Setup• MTJ

– Size: 40x100 nm

– RA = 9 Ω∙um2, TMR = 110%, Rp = 2.9 kΩ

– Iread,P ~ 50 uA, Iread,ap ~ 30 uA

– 5σ MTJ variation• 1 σRA = 4%, 1 σTMR = 5%

• CMOS– 65-nm– Process Variation

• Chip-to-chip + across chip local variation (ACLV)• Monte Carlo Run # = 5000

– Temp and VDD are kept the same in comparison

• room temp

• VDD = 1V

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IMTJ Distribution

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0 20 40 60 80 100 1200

100

200

300

400

500

600

700

800

900

1000

Current (uA)

Fre

quen

cy

CMSA

IMTJ,P

IMTJ,AP

(IMTJ,P

)=36.5uA(I

MTJ,P)=2.61uA

(IMTJ,AP

)=26.9uA(I

MTJ,AP)=2.08uA

0 20 40 60 80 100 1200

100

200

300

400

500

600

700

800

900

Current (uA)

Fre

quen

cy

SPSA

IMTJ,P

IMTJ,AP

(IMTJ,P

)=49.1uA(I

MTJ,P)=3.19uA

(IMTJ,AP

)=31.4uA(I

MTJ,AP)=2.27uA

0 20 40 60 80 100 1200

100

200

300

400

500

600

700

800

900

1000

Current (uA)

Fre

quen

cy

BVSA

IMTJ,P

IMTJ,AP

(IMTJ,P

)=53.3uA(I

MTJ,P)=3.16uA

(IMTJ,AP

)=33.5uA(I

MTJ,AP)=2.17uA

(uA) CMSA SPSA BVSA

μ (IMTJ,P) 36.5 49.1 53.5

σ (IMTJ,P) 2.61 3.19 3.16

μ (IMTJ,AP) 26.9 31.4 33.5

σ (IMTJ,AP) 2.08 2.27 2.17

CMSA

BVSA

SPSA

SMTJ − SREF Distribution

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VDD

VBIASN VBIASN

VMTJVREF

SREFSMTJVBIASP VBIASP

-0.5 0 0.50

50

100

150

200

250

300

350

400

Voltage (V)

Fre

quen

cy

BVSA

SMTJ,P

-SREF,P

SMTJ,AP

-SREF,AP

RMP = -38.3mV

RMAP

=74.7mV

-0.5 0 0.50

100

200

300

400

500

600

700

800

900

1000

Voltage (V)

Fre

quen

cySPSA

SMTJ,P

-SREF,P

SMTJ,AP

-SREF,AP

RMP = -29.8mV

RMAP

=12mV

BVSA

SPSA

VMTJ and VREF Distribution

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0 0.2 0.4 0.6 0.8 10

100

200

300

400

500

600

700

800

900

Voltage (V)

Fre

quen

cyCMSA

VMTJ,P

VREF,P

VMTJ,AP

VREF,AP

RMP = -234mV

RMAP

=277mV

0 0.2 0.4 0.6 0.8 10

100

200

300

400

500

600

700

800

900

1000

Voltage (V)

Fre

quen

cy

SPSA

VMTJ,P

VREF,P

VMTJ,AP

VREF,AP

RMP = -574mV

RMAP

=382mV

0 0.2 0.4 0.6 0.8 10

200

400

600

800

1000

1200

1400

Voltage (V)

Fre

quen

cy

BVSA

VMTJ,P

VREF,P

VMTJ,AP

VREF,AP

RMP = -806mV

RMAP

=680mV

After VMTJ and VREF are settledAfter VMTJ and VREF are settled

CMSA

BVSA

SPSA

VMTJ − VREF Distribution and RM

(mV) CMSA SPSA BVSA

RMP −268 −596 −829

RMAP 303 432 696

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-1 -0.5 0 0.5 10

100

200

300

400

500

600

Voltage (V)

Fre

quen

cyCMSA

VMTJ,P

-VREF,P

VMTJ,AP

-VREF,APRM

P = -268mV

RMAP

=303mV

-1 -0.5 0 0.5 10

100

200

300

400

500

600

Voltage (V)

Fre

quen

cy

SPSA

VMTJ,P

-VREF,P

VMTJ,AP

-VREF,AP

RMP = -596mV

RMAP

=432mV

-1 -0.5 0 0.5 10

100

200

300

400

500

600

Voltage (V)

Fre

quen

cy

BVSA

VMTJ,P

-VREF,P

VMTJ,AP

-VREF,AP

RMP = -829mV

RMAP

=696mV

After VMTJ and VREF are settledAfter VMTJ and VREF are settled

CMSA

BVSA

SPSA

Write

Read

RM vs. Sensing Time (Pulse Width)

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RM (mV)

CMSA SPSA BVSA

100 2.17 1.94 0.60

200 3.54 2.80 0.67

300 N/A 3.84 0.74

400 N/A 5.78 0.82

500 N/A N/A 0.93

600 N/A N/A 1.25

650 N/A N/A 1.58

700 N/A N/A N/A

Sensing time (ns) required to achieve a given RM

Sensing time (ns) required to achieve a given RM

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-1000

-800

-600

-400

-200

0

200

400

600

800

Sensing Time (ns)

Rea

d M

argi

n (m

V)

CMSA, R/ RP

CMSA, R/ RAP

SPSA, R/ RP

SPSA, R/ RAP

BVSA, R/ RP

BVSA, R/ RAP

Summary and Conclusions

Methodology:•Proposed body-voltage sense amp (BVSA) reading circuit is compared with two existing current-sense reading circuits. Read margin and sensing time are compared at the same reading current.

Observations:•Our circuit shows the biggest read margin

– > 400 mV improvement as compared CMSA– > 250 mV improvement as compared to SPSA

•Our circuit achieves high read margin with much shorter pulse width (sensing time)

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