Variability and Design of SRAM in Scaled and Emerging

C. T. Chuang and P. Su, 04/2010

Variability and Design of SRAM in Scaled and Emerging Technologies

Ching-Te Chuang and Pin Su

Department of Electronics Engineering and

Institute of ElectronicsNational Chiao Tung University

Hsinchu, Taiwan

C. T. Chuang, 10/2008

Part-1: Prof. Ching-Te Chuang

SRAM Design Challenges 6T SRAM Cell and Circuit Techniques Alternative SRAM Cells & Low-Voltage Operation

Part-2: Prof. Pin Su

FinFET vs. Bulk SRAM Analytical FinFET/UTBSOI Subthreshold SRAM Framework SNM of FinFET 6T SRAM Cells Impact of Surface Orientation

Outline

Scaling, Read Disturb & Cell Stability Margin

(H. Pilo, ISSCC, 2005)

Conflicting Read/Write Requirements

2( / )

~( / )

AX AX AXREAD

PD PD PD

I W LV

( / )~

( / )p PUPPUP

TRIPPD PD PD

‘0’‘1’

3( / )

~( / )

AX AX AXWRITE

PUP p PUP

I W LT

To facilitate Read and minimize Read-disturb (VREAD) Strong PD NMOS and weak AX NMOS (small 2)

To improve Writability (Write margin) Strong AX NMOS and weak PUP PMOS (large 3)

Half-Select Disturb

BL BLBR BR

COL0 COL1

‘1’ ‘1’ ‘1’ ‘0’

‘0’

‘1’Selected for WRITE

Disturb failure can occur

BL BLBR BR

COL0 COL1

‘1’ ‘1’ ‘1’ ‘0’

‘0’

‘1’Selected for WRITE

Disturb failure can occur

During a Read or Write operation, half-selected cells on theselected word-line are actually experiencing “Read” operation Disturb similar to Read-disturb

6T SRAM Cell and Circuit Techniques

Thin cell layout Hierarchical bit-lines with short local bit-lines Dual supply Large signal domino-like sensing Unclamped or weakly-clamped local bit-lines Power-gating with header/footer Adaptive Read/Write supply Dynamic Read/Write supply Floating power-line Write Capacitive coupling

Read-assist and Write-assist circuits Suppressed Wordl-Line Negative Bit-Line Self-Adjust/Repair

Exploit device structure (e.g. Asymmetrical Transistor)

Example: IBM Power7TM Dcache

(J. Pille et al., ISSCC, 2010)

Dual supply Hierarchical bit-lines with short local bit-lines Unclamped local bit-lines Large signal domino-like sensing

Suppressed Word-Line: 1

(K. Nii et al., ISSC, 01/2008)

(K. Nii et al., Symp. VLSI Circuits., 2008 )

Split-R to mitigate sheet resistance variation

(Y.Fujimura et al.,ISSCC, 2010)

(S. Mukhopadhyay et al., ISCAS, 2008 )

Methods for Reducing Write Failure

Two boosting capacitorsconnected directly toeach and every BL pair

Negative pulse also coupled into the BL that held at “High”

Cross-coupled P1 & P2provide a push-pull toreduce the disturbance

NBL timing subject to PVTvariation and VT scatter

Transient Negative Bit-Line: 1

(D. P. Wang et al., Int’l SOC Conf., 2007 )

Single boosting capacitorshared by BL pair

DI determines the transient negative pulse passed to BL or BLB

Timing of the negativepulse is derived withdelay chains and several control signals

NBL timing subject to PVTvariation and VT scatter

Transient NBL action initiated by low-going BL

Timing of NBL pulse muchmore tolerant to variation

Extra inverter needed foreach and every BL

Device count and areaoverhead

(K. Nii et al., Symp. VLSI Circuits., 2008 )

(Y.Fujimura et al.,ISSCC, 2010)

Bit-line capacitance monitor tracks BL discharging time and controls the amount of charges stored in boosting capacitor

Constant NBL level for SRAM compiler applications

Self-Adjust/Repair SRAM

(N. Mojumder et al., VLSI Test Symp., 04/2008 & IEEE T-VLSI, 01/2010)

C. T. Chuang and P. Su, 04/2010(J. J. Kim et al., IEEE EDL, 08/2009)

Asymmetrical Transistor in 6T SRAM: 1

C. T. Chuang and P. Su, 04/2010(K. Nii et al et al., ISSCC, 2010)

Asymmetrical Transistor in 6T SRAM: 2

6T vs 8T SRAM: Cell Size Scaling

(Y. Morita et al., Symp VLSI Circuits, 2007)

6T cell: Conflicting Read/Write margin requirements Read: Large pull-down NMOS cell Tx & large ratio (pull-down/access width ratio) Write: large access pass-Tx

8T cell: Pull-down NMOS cell Tx can be reduced w/o degrading Read margin Dynamic voltage control enables further scaling

Va for memory cell, Vmax for WWL and RWL 64 Mb 8T SRAM with Vmin = 0.42 V in 90 nm CMOS

8T SRAM: Half-Select Disturb during Write

(Y. Morita et al., Symp VLSI Circuits, 2007)(R. Joshi et al., Symp VLSI Circuits, 2007)

During Write, half-selected cells on the same wordline still experience storage node disturb similar to “Read” disturb in 6T SRAM Especially worse for dual supply SRAM with VMAX applied to WWL

Array architecture approach (L. Chang et al., Symp. VLSI Circuits, 2007) No column select. Floorplan such that all bits in a word are spatial adjacent Constraint: Bits from different words can not be physically interleaved 5.3 GHz, 0.41 V Vmin, 32Kb subarray in 65 nm PD/SOI

Gated Write wordline signal (Byte Write) Local Write wordline “on” only for the selected block 6.6+ GHz, 0.4 V Vmin, dual-supply, 1.2 Mb SRAM in 65 nm PD/SOI

Write-back scheme RWL activated even during Write, all cell data in selected WL read out to D-latches Dataout is then written back to half-selected cells 0.42 V Vmin, 64 Mb SRAM in 90 nm CMOS

Sub-VT 8T SRAM 256Kb, 65 nm CMOS, 25kHz, 2.83/3.96μW (Read/Write)@ Vmin =

0.35V Buffer-foot with gate-boosted foot driver

(N. Verma and A. P. Chandrakasan, ISSCC, 2007)

256Kb 10T Sub-VT SRAM in 65nm CMOS Enable sub-VT Read/Write by lowering leakage from unaccessed cells

For QB=‘0’, Ileak across top RWL NFET ~ 0 For QB=‘1’ Ileak across top RWL NFET ~ 0 with negative VGS

Floating VVDD while boosting WWL by ~ 50-80mV improves V divider 10X, making cell Writable at 0.3V (WL and Write driver operated at 100 mV above VDD;Write is much easier in Sub-VT with exponential dependency of V divider)

475 KHz, 3.28 W @ VDD = 0.4V Leakage reduction

0.6V -> 0.4V: 2.5X ; 0.6V -> 0.3V: 3.8X ; 1.2V -> 0.3V: 60X

(B. H. Calhoun and A. Chandrakasan, ISSCC, 2006)

Sub-VT SRAM w. Data-Independent BL Leakage 480 Kb: 120 KHz, 10.2 A @ 0.2V in 130 nm CMOS Data-independent bit-line leakage

(T. H. Kim et al., ISSCC, 2007)

32 Kb: 166 KHz@0.25V (Leakage: 1.4 uA for 49 Kb@0.25V) in 90 nm CMOS Vcc,min = 0.16V: 500 Hz, 0.123 W @ 0.16V W_WL shared by cells in a column

Writing a cell does not affect Hold stability of other cells along the same WLAllows bit-interleaving for soft-error immunity

Differential Read w. dynamic DCVSL for better stability and leakage tolerance

10T Sub-VT SRAM w. Bit-Interleaving and Differential Read

(I. J. Chang et al., ISSCC, 2008)

C. T. Chuang, 10/2008

Part-1: Prof. Ching-Te Chuang

SRAM Design Challenges 6T SRAM Cell and Circuit Techniques Alternative SRAM Cells & Low-Voltage Operation

Part-2: Prof. Pin Su

FinFET vs. Bulk SRAM Analytical FinFET/UTBSOI Subthreshold SRAM Framework SNM of FinFET 6T SRAM Cells Impact of Surface Orientation

Outline

Ultralow-power applications such as portable devices, implanted medical instruments, and wireless body sensing networks require SRAM operating at low supply voltage below VT.

Degraded functional robustness is a serious concern for subthreshold SRAM.

Conventional Bulk 6T SRAM cell has failed to offer adequate stability in the subthreshold region.

FinFET SRAM for Ultralow-Power Applications

Random Mismatch in Bulk Transistors

60 dies in a 300mm wafer measured

Id1,Vth1, β1

Id2,Vth2, β2

matching pair in one die

[J. Kuo et al., IEEE TNANO, March 2010]

-80 -60 -40 -20 0 20 40 60 800

Vth (mV)

Control Strained Gaussian distribution

PFETLgate=54nm

• The impact of Random DopantFluctuation increases with device scaling because the sensitivity of a transistor increases with decreasing channel volume.

0.0 5.0x1017 1.0x1018 1.5x1018 2.0x10180.25

0.55W total=75nm, Leff=25nmtHfO2=2nm, VDS=0.05V

AR=0.5

V th (V

)Doping concentration (cm

Threshold Voltage Sensitivity to Doping

Generic Multi-Gate Structure

[Y. Wu and P. Su, IEEE TNANO, May 2008]AR=2 AR=1 AR=0.5

• Small fin-width (Wfin) enhances the gate control and reduces the Vth dependence on channel doping.

Wfin AR = Hfin/Wfin

BOX(oxide)

source

Single Fin (W=Hfin)

BOX(oxide)

pitchsource

Multiple Fin (W=3Hfin)

FinFET

• The device width for FinFET is quantized (integer multiple of Hfin).• In a minimum-sized FinFET SRAM cell, single-fin devices are used.

FinFET vs. Bulk

• FinFET exhibits steeper subthreshold slope and larger Ion/Ioff ratio than the Bulk counterpart.

0.0 0.2 0.4 0.6 0.8 1.010-14

Vg [V]

Bulk Planar FinFET

Vds=0.05, 1.0V

Identical Id,sat

~160X106

0.0 0.1 0.2 0.3 0.40.0

VL (V)

6T BULK SRAM (=1.5)

6T FinFET SRAM (single-fin, =1.0)

Vdd=0.4V

Lg=25nm, Wfin=7nm, Hfin=20nm, EOT=.65nmVCS

VBLB VBL

VCSVCS

VBLB VBL

FinFET vs. Bulk

• FinFET SRAM cell shows larger Read Static Noise Margin (RSNM) than the Bulk counterpart because of its superior electrostatic integrity.

250 300 350 400

Temperature [K]

w/o self-heating: Bulk SRAM (=1.3) FinFET SRAM

w/i self-heating: Bulk SRAM (=1.3), (=150 W/mK) FinFET SRAM (=15 W/mK)

Vdd = 0.4V

[M.-L. Fan et al., IEEE TED, June 2010]

0.2 0.3 0.4

Vdd [V]

BULK (=1.3) BULK (=1.5) FinFET (=1.0)

Standard 6T SRAM

FinFET vs. Bulk

• FinFET SRAM cell can be more robust than the Bulk counterpart.

0 0.1 0.2 0.3 0.40

-20 0 20 40 60 800

RSNM [mV]

READ failure

0 0.1 0.2 0.3 0.40

30 40 50 60 70 80 900

RSNM [mV]

150 samples

Bulk SRAM with RDF

150 samples

FinFET SRAM with Fin LERVdd=0.4VVdd=0.4V

[M.-L. Fan et al., IEEE TED, June 2010]

Independent-Gate Control Capability of FinFET

FrontGate

BackGate

Tied Gate Independent Gate

Cell (II) : Ying-Yang feedback

Cell (III) : Improved Ying-Yang feedbackCell (IV) : Doubled-WL

WSNM Adaptively controlled

Several 6T FinFET SRAM Cells

[Z. Guo et al., ISLPED 2005]

[O. Thomas et al., ISCAS 2007]

[M. Yamaoka et al., Symp. VLSI Circuits 2004]

Cell (I): standard tied-gate 6T

baxxqN

a 21 2

effieff

effeff

sinsinhsinh1

1 13 sinsinsinhsinh

m n effeffyy z

WmyLkeyke

zyxzxxzyx ,,,,, 321

The drain current can be calculated using the channel potential solution (x,y,z):

exp exp

, ,exp

f in fin

i S Dna

n VkT Vqq N kT q kT q

I dyq x y z

dxdzkT

Analytical FinFET Subthreshold Model

[Y. Wu and P. Su, IEEE TED, Nov. 2008]

BL BLB

VddVdd

BL BLB

SNM = min [ SNML, SNMR]

VL (V)

longest diagonal aa' , bb'

VL (V)

SNMR = bb'/ sqrt(2)

longest diagonal : aa' , bb'

SNML = aa'/ sqrt(2)

SNM = min [ SNML, SNMR]

Analytical Subthreshold Drain Current Model

Butterfly Curve by Solving Storage Nodes

Static Noise Margin Calculation

Design Analysis for Various Subthreshold SRAM Cells

Analytical FinFET/UTBSOI Subthreshold SRAM Framework

[V. P.-H. Hu et al., IEEE TED, Sep. 2009]

0.0 0.1 0.2 0.3 0.40.0

VL [V]

TCAD model

Nominal CellLeff=25nmWfin=7nmHfin=20nmVdd=0.4V

Verification – Scalability & Efficiency

0.00 0.05 0.10 0.15 0.200.00

Leff=25nmWfin=7nmHfin=20nmVdd=0.2V

VL [V]

TCAD model

0.0 0.1 0.2 0.3 0.40.0

Leff=20nmWfin=5nmHfin=15nmVdd=0.4V

VL [V]

TCAD model

Scaled cell

Lowered Vdd

Nominal cell

[M.-L. Fan et al., IEEE SOI Conf., Oct. 2009]

• Analytical approach not only shows better efficiency than TCAD mixed-mode simulation, but also enables an assessment for sensitivity to process variations.

• Cell (II), (III) and (IV) show significant RSNM improvement as compared with cell (I).

• The percentage improvement in RSNM is larger for Subthresholdthan Superthreshold.

• R/W WL voltage control is effective to improve the cell stability of subthreshold SRAM.

Asymmetrical cell (V)Double word-line structure(IV)

Improved Ying-Yang feedback (III)Ying-Yang feedback(II)

Standard tied-gate 6T(I)

RSNM Comparison of 6T FinFET SRAM Cells

Subthreshold (Vdd=0.4V) Superthreshold (Vdd=1.0V)

(I) (II) (III) (IV) (V)

75% 71% 75%

62% 56% 62%

[M.-L. Fan et al., VLSI-TSA, April 2010]

• Cell (II) and (III) fail to WRITE at Vdd =0.4V. Boosted WL technique is needed to restore the Write-ability.

• The Write-ability of Cell (IV) is not degraded.

0.40 0.45 0.50 0.55 0.60

VWL=172mV

V]WL voltage [V]

Cell (II) Cell (III)

VWL=133mV

(V)(IV)(III)(II)(I)write1 write0

-131%-220%

V] Subthreshold (Vdd=0.4V) Superthreshold(Vdd=1.0V)

WSNM Comparison of 6T FinFET SRAM Cells

[M.-L. Fan et al., VLSI-TSA, April 2010]

Stability of Double-WL FinFET SRAM Cell

• Local random variation (Fin LER) together with global process variations (process corners) are considered by Monte Carlo simulations.

• Double word-line SRAM cell with adequate nominal RSNM can provide sufficient / ratio in RSNM for various process corners (TT = 10.9, FS = 6.4, SF = 13, SS = 14.2, FF = 7.2) at Vdd=0.4V.

Cell (IV) : Doubled-WL

[O. Thomas et al., ISCAS 2007]

101214

FFSSFS SF

0 0.1 0.2 0.3 0.40

0.4150 samples

Worst case:FNSP corner

Vdd=0.4V±20% device parameter deviations (Leff and Wfin) are assumed.

Impact of Surface Orientation on FinFET SRAM Cell

• For FinFET, the conventional sidewall conducting (110) surface orientation can be rotated by layout to improve the cell stability.

All (110) NMOS (100)

0 3 6 9 12 15-0.05

(111)(110)

tch [nm]

Si-NMOS

Leff = 25nmVDS=0.05V

symbols: simulation lines: model

Leff = 25nmtch = 4nmVGS = 0.1V

(111)(110)

(100)0.0

(110,100) (110,110)(100,110)

(PD,PG)

PU (110) PU (100)

(100,100)

Vdd=0.4V

(100,110)(110,110)(100,100)

(PD,PG)

PU(110) PU(100)

(110,100)

Vdd=0.4V

0 3 6 9 12 15-0.05

(111)(110)

tch [nm]

Si-NMOS

Leff = 25nmVDS=0.05V

symbols: simulation lines: model

Leff = 25nmtch = 4nmVGS = 0.1V

(111)(110)

With superior electrostatic integrity, suppressed random dopant fluctuation, independent-gate control capability and surface-orientation optimization, FinFET SRAM cell can be more robust than the Bulk counterpart.

Conclusion

Variability and Design of SRAM in Scaled and Emerging

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