8t sramセルレイアウト - kobayashilab - kobayashi ... kobe university integrated silicon &...
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Kobe University Integrated Silicon & Software architecture laboratory1
吉本秀輔神戸大学 博士課程1年
E-mail : [email protected]
マルチビットアップセット耐性及びシングルビットアップセット耐性を備えた
8T SRAMセルレイアウト
2Kobe University Integrated Silicon & Software architecture laboratory
Outline
• 背景
• 提案8T SRAM cell layout
• ソフトエラーシミュレーション結果
• 消費電力比較結果
• まとめ
3Kobe University Integrated Silicon & Software architecture laboratory
Outline
• 背景
• 提案8T SRAM cell layout
• ソフトエラーシミュレーション結果
• 消費電力比較結果
• まとめ
4Kobe University Integrated Silicon & Software architecture laboratory
閾値ばらつきの問題
• Systematic variation– Die-to-Die (Inter-die)
• Random variation– Within-Die (Intra-die)
Inter-die
Intra-die
WL
AVth
5Kobe University Integrated Silicon & Software architecture laboratory
SRAMにおける閾値ばらつき
0
100
200
300
400
500
600
700
800
900
1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04
Fre
qu
en
cy
Ion [A]
0
100
200
300
400
500
600
700
800
900
-0.17 -0.11 -0.04 0.02 0.09 0.15 0.22
Fre
qu
en
cy
DVth [V]
Vth variation Ion variation
CMOS 45nm process
Tr. size = minimum
CC, 25degC, VDD=0.5 V
5K Monte calro
x 4.4
x 10-2
Pass gate Tr.
Ion is largely shifted
by the variation
6Kobe University Integrated Silicon & Software architecture laboratory
Y decoder
MC MC MC MC
MC MC MC MC
X d
ec
od
er
Selected
column
Unselected
column
Selected
row
Unselected
row
MC:Memory cell
BL BLN
WL
BL BLN
WL
BL/BLN
WL
VDD GND VDD VDD
Write
current
Disturb
current
0
100
200
300
400
500
600
700
800
900
1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04
Fre
qu
en
cy
Ion [A]
Ion
(Pass gate)
Data hold
failure
Write
failure
低電圧動作を阻害するハーフセレクト問題
Write target cell Unselected cell
7Kobe University Integrated Silicon & Software architecture laboratory
8T + 分割ワード線(Divided WL)構造
MUX
Word,Parity4
BL
WL
MUX MUX
Word4Word1 Word2 Word3
BLMUX
Parity1-4
WL
Bit interleaving
Word,Parity3Word,Parity2Word,Parity1
Divided wordline
Disturbed cellWrite target cell
PL0 PL1
ND0 ND1
NA0 NA1n0 n1
NRD
NRA
WL
RWL
BL BLN RBL
VMC
VSS
8T SRAM cell
8T cellによる読出しディスターブ排除
分割ワード線構造による書込みディスターブ排除
+
低電圧動作が可能となる
8Kobe University Integrated Silicon & Software architecture laboratory
従来8T SRAMセルレイアウトにおける問題
MUX
Word,Parity4
BL
WL
DiffusionNwell Nwell
Poly
n01 n00 n10 n11
Word,Parity3Word,Parity2Word,Parity1
Conventional 8T cell layout
Divided wordline
ラッチのノードが隣接するため,ソフトエラーにより
反転しやすい
ECCによる救済が困難となる
同一ワード内のMultiple
Bit Upset(MBU)が発生しやすい
Latch-0 Latch-1
9Kobe University Integrated Silicon & Software architecture laboratory
Outline
• 背景
• 提案8T SRAM cell layout
• ソフトエラーシミュレーション結果
• 消費電力比較結果
• まとめ
10Kobe University Integrated Silicon & Software architecture laboratory
提案8T SRAM cell layout -1/2-
PL0 PL1
ND0 ND1
NA0 NA1n0 n1
NRD
NRA
WL
RWL
BL BLN RBL
VMC
VSS
Proposed 8T cell layout
NwellNwell
n00 n01
NwellNwell
n10 n11
DiffusionNwell Nwell
Poly
n01 n00 n10 n11
Conventional 8T cell layout8T cell
読出しポートにより,隣接するSRAMセルのラッチ部を分断出来る
1. Transistors of a latch
are PMOS-NMOS-PMOS.
2. The 8T cell alignment
pattern is slide type
(NOT symmetrically).
Read
port
Read
port
11Kobe University Integrated Silicon & Software architecture laboratory
提案8T SRAM cell layout -2/2-
PL0 PL1
ND0 ND1
NA0 NA1n0 n1
NRD
NRA
WL
RWL
BL BLN RBL
VMC
VSS
Proposed 8T cell layout
NwellNwell
n00 n01
NwellNwell
n10 n11
DiffusionNwell Nwell
Poly
n01 n00 n10 n11
Conventional 8T cell layout8T cell
コモンモードによるSEU耐性向上が期待出来る
NMOS is more sensitive to
SEU than PMOS (x 3.5-4.5)
The NMOSes are adjacent
in the proposed cell
12Kobe University Integrated Silicon & Software architecture laboratory
Outline
• 背景
• 提案8T SRAM cell layout
• ソフトエラーシミュレーション結果
• 消費電力比較結果
• まとめ
13Kobe University Integrated Silicon & Software architecture laboratory
Soft-error simulator (TFIT)
Technology :
65nm generic CMOS
SPICE simulator :
HSPICE B-2008.09
SPICE model :
PTM 65nm CMOS
Built-in module :
Neutron-particle-induced
MBU simulation
Neutron MBU module :
Normalized at NY sea level
supported by iRoC technology
14Kobe University Integrated Silicon & Software architecture laboratory
Neutron MBU simulation (従来8T)-1/3-
0
20
40
60
80
100
120
1
To
tal M
BU
FIT
Conv. 8T SRAM
@ 0.9 V
45.20% 10.40%
2.46%
20.90%
6.21% 0.59%
2 bits MBU (89.40%)
3 bits MBU (16.15%)
MBU in a same word
4 bits / 5 bits MBU
同一ワード内における,2bit以上のMBUが問題となる
MBU in the same word
15Kobe University Integrated Silicon & Software architecture laboratory
0
20
40
60
80
100
120
1 2 3 4
MB
U F
IT
MBU in a same word MBU in different words
Conv. 8T
w/o ECC w/ ECC w/o ECC w/ ECC
Prop. 8T
-66.76%
-96.27%
-90.70%
DiffusionNwell Nwell
Poly
n01 n00 n10 n11
NwellNwell
n00 n01
NwellNwell
n10 n11
MBU in the same word
Conv. 8T
Prop. 8T
Neutron MBU simulation @ 0.9 V -2/3-
提案レイアウトを用いることにより,ワード線方向のMBUを90.70%削減可能.結果,1bit correcting ECC
により96.27%のMBUを救済可能.
16Kobe University Integrated Silicon & Software architecture laboratory
Neutron MBU simulation -3/3-
0.9V-1.3Vにおいて,MBU救済率が30%改善
-30%
17Kobe University Integrated Silicon & Software architecture laboratory
3-D TCAD simulation -1/2-
N1 N0
(a)
2.1E+20
eDensity [cm3]
4.1E+16
8.2E+12
1.6E+09
3.2E+05
6.4E+01
N1 N0
(b)
2.1E+20
eDensity [cm3]
4.1E+16
8.2E+12
1.6E+09
3.2E+05
6.4E+01
Prop. 8TConv. 8T
3-D NMOS & SPICE models : PTM 65-nm CMOS
Simulator : Synopsys TCAD
Particle LET : 5.49 MeV
18Kobe University Integrated Silicon & Software architecture laboratory
3-D TCAD simulation -2/2-
0.0E+00
3.0E-05
6.0E-05
9.0E-05
1.2E-04
1.5E-04
-0.8
-0.4
0
0.4
0.8
1.2
1.6
1.0E-12 1.0E-11 1.0E-10 1.0E-09
Cu
rren
t [A]
Vo
lta
ge
[V
]
Time [s]
N1 [V]
N0 [V]
N1 Disturb Current [A]
0.0E+00
3.0E-05
6.0E-05
9.0E-05
1.2E-04
1.5E-04
-0.8
-0.4
0
0.4
0.8
1.2
1.6
1.0E-12 1.0E-11 1.0E-10 1.0E-09
Cu
rren
t [A]V
olt
ag
e [V
]
Time [s]
N1 [V]
N0 [V]
N1 Disturb Current [A]
N0 Disturb Current [A]
(a)
(b)
SEU tolerance
is improved
Conv. 8T
Prop. 8T
Flipped
Not flippedCommon-mode effect 0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.3
Conv. 8T
Prop. 8T
0.9 V
LE
Tth
[MeV
]
+66.47%
19Kobe University Integrated Silicon & Software architecture laboratory
Outline
• 背景
• 提案8T SRAM cell layout
• ソフトエラーシミュレーション結果
• 消費電力比較結果
• まとめ
20Kobe University Integrated Silicon & Software architecture laboratory
最低動作電圧改善
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0.2 0.4 0.6 0.8 1 1.2
BE
R
VDD [V]
Prop. 8T
Conv. 6T
FS corner, Temp.=125°C
# of monte calro: 20K
0.88 V
-0.45 V
0.43 V
PL0 PL1
ND0 ND1
NA0 NA1n0 n1
WL
BL BLN
VMC
VSS
PL0 PL1
ND0 ND1
NA0 NA1n0 n1
NRD
NRA
WL
RWL
BL BLN RBL
VMC
VSS
Prop. 8T
Conv. 6T
21Kobe University Integrated Silicon & Software architecture laboratory
消費電力比較結果
PL0 PL1
ND0 ND1
NA0 NA1n0 n1
WL
BL BLN
VMC
VSS
PL0 PL1
ND0 ND1
NA0 NA1n0 n1
NRD
NRA
WL
RWL
BL BLN RBL
VMC
VSS
Prop. 8T
Conv. 6T
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.2 0.4 0.6 0.8 1 1.2
Po
wer
(R/W
=50/5
0)
[W]
VDD [V]
FF corner, Temp.=125°C
Freq.=10MHz, Cap.=1Mb
-77.2%
0.43 V 0.88 V
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0.2 0.4 0.6 0.8 1 1.2
BE
R
VDD [V]
Prop. 8T
Conv. 6T
-77.21%
22Kobe University Integrated Silicon & Software architecture laboratory
面積オーバヘッド見積もり結果
1.0
1.5
2.0
4 8 16 32 64 128
Are
a o
verh
ead
(SR
AM
macro
basis
)
The number of bits/word (B)
Prop. 8T
Conv. 8T
30Kobe University Integrated Silicon & Software architecture laboratorySELSE 2011
BL
BL
N
VS
S
VM
C
VS
S
RB
L
RWL
WL NA0
ND0PL0
PL1ND1
NA1
NRD
NRA
n00n01
2.570mm
0.5
45m
m
VS
S
VM
C
48.45% area overhead
(64 bits/word)
Prop. 8T
23Kobe University Integrated Silicon & Software architecture laboratory
Outline
• 背景
• 提案8T SRAM cell layout
• ソフトエラーシミュレーション結果
• 消費電力比較結果
• まとめ
24Kobe University Integrated Silicon & Software architecture laboratory
Summary
MBU & SEU耐性を有する8T SRAM cell layoutを提案
90.70% MBU reduction
-0.45V VDDmin improvement
77.21% power reduction
分割ワード線構造の適応範囲を広げ,SRAMの低電圧・低電力化を促進出来る
プロセス・設計コストをかけずに,レイアウトの工夫のみでMBU改善が可能となる