CMOS Memories

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Digital Integrated
Circuits
A Design Perspective
Jan M. Rabaey
Anantha Chandrakasan
Borivoje Nikolic
Semiconductor
Memories
December 20, 2002
© Digital Integrated Circuits2nd
Memories
Chapter Overview
 Memory Classification
 Memory Architectures
 The Memory Core
 Periphery
 Reliability
 Case Studies
© Digital Integrated Circuits2nd
Memories
Semiconductor Memory Classification
Read-Write Memory
Random
Access
Non-Random
Access
SRAM
FIFO
DRAM
LIFO
Non-Volatile
Read-Write
Memory
Read-Only Memory
EPROM
Mask-Programmed
E2PROM
Programmable (PROM)
FLASH
Shift Register
CAM
© Digital Integrated Circuits2nd
Memories
Memory Architecture: Decoders
M bits
S0
S0
Word 0
S1
Word 1
S2
Word 2
SN 2 2
Nwords
SN 2
M bits
1
Storage
cell
Word 0
A0
Word 1
A1
Word 2
A K2
1
Word N 2 2
Word N 2 1
Decoder
Word N 2
Storage
cell
2
Word N 2 1
K 5 log2N
Input-Output
(M bits)
Intuitive architecture for N x M memory
Too many select signals:
N words == N select signals
© Digital Integrated Circuits2nd
Input-Output
(M bits)
Decoder reduces the number of select signals
K = log2N
Memories
Block Diagram of 4 Mbit SRAM
Clock
generator
Z-address
buffer
X-address
buffer
Predecoder and block selector
Bit line load
128 K Array Block
Subglobal row decoder
SubglobalGlobal
row decoder
row decoder
Block31
30
Block
Block 1
Transfer gate
Column decoder
Local row deco
Sense amplifier and write driver
CS, WE
buffer
© Digital Integrated Circuits2nd
I/O
buffer
x1/x4
controller
Y-address
buffer
[Hirose90]
X -address
buffer
Memories
Read-Only Memory Cells
BL
BL
BL
VDD
WL
WL
WL
1
BL
WL
BL
BL
WL
WL
0
GND
Diode ROM
© Digital Integrated Circuits2nd
MOS ROM 1
MOS ROM 2
Memories
MOS OR ROM
BL[0]
BL[1]
BL[2]
BL[3]
WL[0]
V DD
WL[1]
WL[2]
V DD
WL[3]
V bias
Pull-down loads
© Digital Integrated Circuits2nd
Memories
MOS NOR ROM
V DD
Pull-up devices
WL[0]
GND
WL [1]
WL [2]
GND
WL [3]
BL [0]
© Digital Integrated Circuits2nd
BL [1]
BL [2]
BL [3]
Memories
MOS NOR ROM Layout
Cell (9.5l x 7l)
Programmming using the
Active Layer Only
Polysilicon
Metal1
Diffusion
Metal1 on Diffusion
© Digital Integrated Circuits2nd
Memories
MOS NOR ROM Layout
Cell (11l x 7l)
Programmming using
the Contact Layer Only
Polysilicon
Metal1
Diffusion
Metal1 on Diffusion
© Digital Integrated Circuits2nd
Memories
MOS NAND ROM
V DD
Pull-up devices
BL [0]
BL [1]
BL [2]
BL [3]
WL [0]
WL [1]
WL [2]
WL [3]
All word lines high by default with exception of selected row
© Digital Integrated Circuits2nd
Memories
MOS NAND ROM Layout
Cell (8l x 7l)
Programmming using
the Metal-1 Layer Only
No contact to VDD or GND necessary;
drastically reduced cell size
Loss in performance compared to NOR ROM
Polysilicon
Diffusion
Metal1 on Diffusion
© Digital Integrated Circuits2nd
Memories
NAND ROM Layout
Cell (5l x 6l)
Programmming using
Implants Only
Polysilicon
Threshold-altering
implant
Metal1 on Diffusion
© Digital Integrated Circuits2nd
Memories
Equivalent Transient Model for MOS NOR ROM
V DD
Model for NOR ROM
BL
rword
WL
Cbit
cword

Word line parasitics
 Wire capacitance and gate capacitance
 Wire resistance (polysilicon)

Bit line parasitics
 Resistance not dominant (metal)
 Drain and Gate-Drain capacitance
© Digital Integrated Circuits2nd
Memories
Equivalent Transient Model for MOS NAND ROM
V DD
Model for NAND ROM
BL
CL
r bit
WL
r word
cbit
cword

Word line parasitics
 Similar to NOR ROM

Bit line parasitics
 Resistance of cascaded transistors dominates
 Drain/Source and complete gate capacitance
© Digital Integrated Circuits2nd
Memories
Decreasing Word Line Delay
Driver
WL
Polysilicon word line
Metal word line
(a) Driving the word line from both sides
Metal bypass
WL
K cells
Polysilicon word line
(b) Using a metal bypass
(c) Use silicides
© Digital Integrated Circuits2nd
Memories
Precharged MOS NOR ROM
f
V DD
pre
Precharge devices
WL [0]
GND
WL [1]
WL [2]
GND
WL [3]
BL [0]
BL [1]
BL [2]
BL [3]
PMOS precharge device can be made as large as necessary,
but clock driver becomes harder to design.
© Digital Integrated Circuits2nd
Memories
Non-Volatile Memories
The Floating-gate transistor (FAMOS)
Floating gate
Gate
Source
D
Drain
G
tox
tox
n+
p
n+_
S
Substrate
Device cross-section
© Digital Integrated Circuits2nd
Schematic symbol
Memories
Floating-Gate Transistor Programming
20 V
10 V
S
5V
0V
20 V
D
Avalanche injection
© Digital Integrated Circuits2nd
2 5V
S
5V
0V
D
Removing programming
voltage leaves charge trapped
2 2.5 V
S
5V
D
Programming results in
higher V T .
Memories
FLOTOX EEPROM
Gate
Floating gate
I
Drain
Source
20–30 nm
V GD
-10 V
10 V
n1
n1
Substrate
p
10 nm
FLOTOX transistor
© Digital Integrated Circuits2nd
Fowler-Nordheim
I-V characteristic
Memories
EEPROM Cell
BL
WL
VDD
© Digital Integrated Circuits2nd
Absolute threshold control
is hard
Unprogrammed transistor
might be depletion
 2 transistor cell
Memories
Flash EEPROM
Control gate
Floating gate
erasure
n 1 source
Thin tunneling oxide
programming
n 1 drain
p-substrate
Many other options …
© Digital Integrated Circuits2nd
Memories
Cross-sections of NVM cells
Flash
© Digital Integrated Circuits2nd
EPROM
Courtesy Intel
Memories
NAND Flash Memory
Select transistor
Word lines
Active area
STI
Bit line contact
© Digital Integrated Circuits2nd
Source line contact
Courtesy Toshiba
Memories
Characteristics of State-of-the-art NVM
© Digital Integrated Circuits2nd
Memories
Read-Write Memories (RAM)
 STATIC (SRAM)
Data stored as long as supply is applied
Large (6 transistors/cell)
Fast
Differential
 DYNAMIC (DRAM)
Periodic refresh required
Small (1-3 transistors/cell)
Slower
Single Ended
© Digital Integrated Circuits2nd
Memories
6-transistor CMOS SRAM Cell
WL
V DD
M2
M5
Q
M1
BL
© Digital Integrated Circuits2nd
M4
Q
M6
M3
BL
Memories
CMOS SRAM Analysis (Read)
WL
V DD
M4
BL
Q= 0
M5
V DD
Cbit
© Digital Integrated Circuits2nd
M1
Q= 1
V DD
BL
M6
V DD
Cbit
Memories
CMOS SRAM Analysis (Read)
1.2
Voltage Rise (V)
1
0.8
0.6
0.4
0.2
Voltage
rise [V]
0
0
© Digital Integrated Circuits2nd
0.5
1 1.2 1.5 2
Cell Ratio (CR)
2.5
3
Memories
CMOS SRAM Analysis (Write)
WL
V DD
M4
M5
Q= 1
M1
BL = 1
© Digital Integrated Circuits2nd
M6
Q= 0
V DD
BL = 0
Memories
CMOS SRAM Analysis (Write)
© Digital Integrated Circuits2nd
Memories
6T-SRAM — Layout
VDD
M2
M4
Q
Q
M1
M3
GND
M5
BL
© Digital Integrated Circuits2nd
M6
WL
BL
Memories
Resistance-load SRAM Cell
WL
V DD
RL
M3
BL
RL
Q
Q
M1
M2
M4
BL
Static power dissipation -- Want R L large
Bit lines precharged to V DD to address t p problem
© Digital Integrated Circuits2nd
Memories
SRAM Characteristics
© Digital Integrated Circuits2nd
Memories
3-Transistor DRAM Cell
BL 1
BL 2
WWL
WWL
RWL
M3
X
M1
CS
M2
RWL
V DD 2 V T
X
BL 1
BL 2
V DD
DV
V DD 2 V T
No constraints on device ratios
Reads are non-destructive
Value stored at node X when writing a “1” = V WWL-VTn
© Digital Integrated Circuits2nd
Memories
3T-DRAM — Layout
BL2
BL1
GND
RWL
M3
M2
WWL
M1
© Digital Integrated Circuits2nd
Memories
Sense Amp Operation
V BL
V(1)
V PRE
D V(1)
V(0)
Sense amp activated
Word line activated
© Digital Integrated Circuits2nd
t
Memories
1-T DRAM Cell
Capacitor
M 1 word
line
Metal word line
SiO2
Poly
n+
Field Oxide
n+
Poly
Inversion layer
induced by
plate bias
Cross-section
Diffused
bit line
Polysilicon
gate
Polysilicon
plate
Layout
Uses Polysilicon-Diffusion Capacitance
Expensive in Area
© Digital Integrated Circuits2nd
Memories
SEM of poly-diffusion capacitor 1T-DRAM
© Digital Integrated Circuits2nd
Memories
Advanced 1T DRAM Cells
Word line
Insulating Layer
Cell plate
Capacitor dielectric layer
Cell Plate Si
Capacitor Insulator
Refilling Poly
Transfer gate
Isolation
Storage electrode
Storage Node Poly
Si Substrate
2nd Field Oxide
Trench Cell
© Digital Integrated Circuits2nd
Stacked-capacitor Cell
Memories
Static CAM Memory Cell
Bit
Bit
Bit
Bit
Bit
Word
CAM
Word
•••
•••
CAM
M4
M8
M9
M6
M7
M5
CAM
•••
•••
Bit
Word
CAM
S
M3
Match
int
S
M2
M1
Wired-NOR Match Line
© Digital Integrated Circuits2nd
Memories
CAM in Cache Memory
CAM
SRAM
ARRAY
ARRAY
Hit Logic
Address Decoder
Input Drivers
Address
© Digital Integrated Circuits2nd
Tag
Sense Amps / Input Drivers
Hit
R/W
Data
Memories
Periphery
 Decoders
 Sense Amplifiers
 Input/Output Buffers
 Control / Timing Circuitry
© Digital Integrated Circuits2nd
Memories
Row Decoders
Collection of 2M complex logic gates
Organized in regular and dense fashion
(N)AND Decoder
NOR Decoder
© Digital Integrated Circuits2nd
Memories
Hierarchical Decoders
Multi-stage implementation improves performance
•••
WL 1
WL 0
A 0A 1 A 0A 1 A 0A 1 A 0A 1
A 2A 3 A 2A 3 A 2A 3 A 2A 3
•••
NAND decoder using
2-input pre-decoders
A1 A0
A0
A1
© Digital Integrated Circuits2nd
A3 A2
A2
A3
Memories
Dynamic Decoders
Precharge devices
GND
VDD
GND
WL 3
VDD
WL 3
WL 2
WL 2
VDD
WL 1
WL 1
V DD
WL 0
WL 0
VDD f
A0
A0
A1
A1
2-input NOR decoder
© Digital Integrated Circuits2nd
A0
A0
A1
A1
f
2-input NAND decoder
Memories
4-input pass-transistor based column
decoder
BL
BL
BL
BL
0
A0
1
2
3
S0
S1
S2
A1
S3
2-input NOR decoder
D
Advantages: speed (tpd does not add to overall memory access time)
Only one extra transistor in signal path
Disadvantage: Large transistor count
© Digital Integrated Circuits2nd
Memories
4-to-1 tree based column decoder
BL 0 BL 1 BL 2 BL 3
A0
A0
A1
A1
D
Number of devices drastically reduced
Delay increases quadratically with # of sections; prohibitive for large decoders
Solutions: buffers
progressive sizing
combination of tree and pass transistor approaches
© Digital Integrated Circuits2nd
Memories
Decoder for circular shift-register
V DD
WL
V DD
V DD
WL
0
R
f
f
f
f
R
V DD
V DD
WL
1
f
f
f
f
R
V DD
2
f
f
f
f
• • •
V DD
© Digital Integrated Circuits2nd
Memories
Sense Amplifiers
DV
×
C
tp = ---------------Iav
large
make D V as small
as possible
small
Idea: Use Sense Amplifer
small
transition
s.a.
input
© Digital Integrated Circuits2nd
output
Memories
Differential Sense Amplifier
V DD
M3
M4
y
M1
bit
SE
M2
Out
bit
M5
Directly applicable to
SRAMs
© Digital Integrated Circuits2nd
Memories
Latch-Based Sense Amplifier (DRAM)
EQ
BL
BL
VDD
SE
SE
Initialized in its meta-stable point with EQ
Once adequate voltage gap created, sense amp enabled with SE
Positive feedback quickly forces output to a stable operating point.
© Digital Integrated Circuits2nd
Memories
Charge-Redistribution Amplifier―
V
EPROM
DD
SE
Load
M4
Out
V casc
M3
Cascode
device
Cout
Ccol
WLC
Column
decoder
M2
BL
WL
© Digital Integrated Circuits2nd
M1
CBL
EPROM
array
Memories
Open bitline architecture with
dummy cells
EQ
L
L1
L0
V DD
R0
R1
L
SE
BLL
CS
…
CS
BLR
CS
Dummy cell
© Digital Integrated Circuits2nd
…
SE
CS
CS
CS
Dummy cell
Memories
DRAM Read Process with Dummy Cell
3
3
2
2
BL
V
1
0
0
BL
V
BL
1
2
1
0
3
BL
0
1
t (ns)
2
3
t (ns)
reading 0
reading 1
3
EQ
WL
2
V
SE
1
0
0
1
2
3
t (ns)
control signals
© Digital Integrated Circuits2nd
Memories
Voltage Regulator
VDD
Mdrive
VDL
VREF
Equivalent Model
Vbias
VREF
+
Mdrive
VDL
© Digital Integrated Circuits2nd
Memories
DRAM Timing
© Digital Integrated Circuits2nd
Memories
RDRAM Architecture
Bus
Clocks
Data
bus
k
k3 l
memory
array
network
mux/demux
Column
Row
© Digital Integrated Circuits2nd
demux
packet dec.
demux
packet dec.
Memories
Address Transition Detection
V DD
A0
DELAY
td
A1
DELAY
td
A N2 1
DELAY
td
© Digital Integrated Circuits2nd
ATD
ATD
…
Memories
Reliability and Yield
© Digital Integrated Circuits2nd
Memories
Sensing Parameters in DRAM
1000
C D(1F)
V smax (mv)
100
smax
V
,
DD
V
,S
10
C
,S
Q
,D
C
C S(1F)
Q S(1C)
V DD (V)
Q S 5 C S V DD / 2
V smax 5 Q S / (C S 1 C D )
4K
64K
1M 16M 256M 4G
Memory Capacity (bits
© Digital Integrated Circuits2nd
64G
/ chip)
From [Itoh01]
Memories
Noise Sources in 1T DRam
BL
substrate Adjacent BL
CWBL
a -particles
WL
leakage
CS
electrode
Ccross
© Digital Integrated Circuits2nd
Memories
Open Bit-line Architecture —Cross Coupling
EQ
WL 1
WL 0
WL
C WBL D
C WBL
WL D
WL 1
WL 0
BL
BL
C BL
C
C
© Digital Integrated Circuits2nd
C
Sense
Amplifier
C BL
C
C
C
Memories
Alpha-particles (or Neutrons)
a -particle
WL
V DD
BL
n1
SiO 2
2
1
2
1
1
2
2
1
2
1
1
2
1 Particle ~ 1 Million Carriers
© Digital Integrated Circuits2nd
Memories
Yield
Yield curves at different stages of process maturity
(from [Veendrick92])
© Digital Integrated Circuits2nd
Memories
Redundancy
Row
Address
Redundant
rows
:
Fuse
Bank
Redundant
columns
Memory
Array
Row Decoder
Column Decoder
© Digital Integrated Circuits2nd
Column
Address
Memories
Error-Correcting Codes
Example: Hamming Codes
e.g. B3 Wrong
with
1
1
=3
0
© Digital Integrated Circuits2nd
Memories
Redundancy and Error Correction
© Digital Integrated Circuits2nd
Memories
Sources of Power Dissipation in
Memories
V DD
I DD 5 S C iD V if1S I DCP
CHIP
nC DE V INT f
m
selected
C PT V INT f
mi act
I DCP
n
ROW
DEC
PERIPHERY
m(n 2 1)i hld
non-selected
ARRAY
mC DE V INT f
COLUMN DEC
V SS
© Digital Integrated Circuits2nd
From [Itoh00]
Memories
Data Retention in SRAM
1.30u
1.10u
0.13 m m CMOS
Ileakage
900n
700n
500n
Factor 7
(A)300n
0.18 m m CMOS
100n
0.00
.600
1.20
1.80
VDD
SRAM leakage increases with technology scaling
© Digital Integrated Circuits2nd
Memories
Suppressing Leakage in SRAM
V DD
V DD
low-threshold transistor
V DDL
sleep
V DD,int
sleep
V DD,int
SRAM
cell
SRAM
cell
sleep
SRAM
cell
SRAM
cell
SRAM
cell
V SS,int
Inserting Extra Resistance
© Digital Integrated Circuits2nd
SRAM
cell
Reducing the supply voltage
Memories
Case Studies
 Programmable
Logic Array
 SRAM
 Flash
Memory
© Digital Integrated Circuits2nd
Memories
PLA versus ROM
 Programmable Logic Array
structured approach to random logic
“two level logic implementation”
NOR-NOR (product of sums)
NAND-NAND (sum of products)
IDENTICAL TO ROM!
 Main difference
ROM: fully populated
PLA: one element per minterm
Note: Importance of PLA’s has drastically reduced
1. slow
2. better software techniques (mutli-level logic
synthesis)
But …
© Digital Integrated Circuits2nd
Memories
Programmable Logic Array
Pseudo-NMOS PLA
GND
GND
GND
V DD
GND
GND
GND
GND
V DD
X0
X0
X1
AND-plane
© Digital Integrated Circuits2nd
X1
X2
X2
f0
f1
OR-plane
Memories
Dynamic PLA
f AND
V DD
GND
f
OR
f
OR
f AND
V DD
X0
X0
X1
X1
AND-plane
© Digital Integrated Circuits2nd
X2
X2
f0
f 1 GND
OR-plane
Memories
Clock Signal Generation
for self-timed dynamic PLA
f
f
f
f
AND
Dummy AND row
f
OR
AND
tpre teval
f
Dummy AND row
f
AND
OR
(a) Clock signals
© Digital Integrated Circuits2nd
(b) Timing generation circuitry
Memories
PLA Layout
And-Plane
VDD
x0 x0 x1 x1 x2 x2
Pull-up devices
© Digital Integrated Circuits2nd
Or-Plane
f
GND
f0 f1
Pull-up devices
Memories
Bit-line Circuitry
Block
select
Bit-line
load
ATD
BEQ
Local WL
Memory cell
B /T
B /T
CD
CD
CD
I/O
I/O line
I/O
Sense amplifier
© Digital Integrated Circuits2nd
Memories
Sense Amplifier (and Waveforms)
Address
I /O
I /O
ATD
SEQ
Block
select
ATD
BS
SA
BS
BEQ
Vdd
I/O Lines
GND
SA
SEQ
SEQ
SEQ
SEQ
SEQ
Vdd
DATA
Dei
SA, SA
GND
DATA
BS
Data-cut
© Digital Integrated Circuits2nd
Memories
1Gbit NAND Flash Memory
Charge pump
2kB Page buffer & cache
10.7mm
2
125mm
32 word lines
x 1024 blocks
16896 bit lines
11.7mm
© Digital Integrated Circuits2nd
From [Nakamura02]
Memories
125mm2 1Gbit NAND Flash Memory









Technology
0.13m p-sub CMOS triple-well
1poly, 1polycide, 1W, 2Al
Cell size
0.077m2
Chip size
125.2mm2
Organization 2112 x 8b x 64 page x 1k block
Power supply 2.7V-3.6V
Cycle time
50ns
Read time
25s
Program time 200s / page
Erase time
2ms / block
© Digital Integrated Circuits2nd
From [Nakamura02]
Memories
Semiconductor Memory Trends
(up to the 90’s)
Memory Size as a function of time: x 4 every three years
© Digital Integrated Circuits2nd
Memories
Semiconductor Memory Trends
(updated)
© Digital Integrated Circuits2nd
From [Itoh01]
Memories
Trends in Memory Cell Area
© Digital Integrated Circuits2nd
From [Itoh01]
Memories
Semiconductor Memory Trends
Technology feature size for different SRAM generations
© Digital Integrated Circuits2nd
Memories
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