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© Digital Integrated Circuits2nd Memories
Digital Integrated Digital Integrated CircuitsCircuitsA Design PerspectiveA Design Perspective
SemiconductorSemiconductorMemoriesMemories
Jan M. RabaeyAnantha ChandrakasanBorivoje Nikolic
December 20, 2002
© Digital Integrated Circuits2nd Memories
Chapter OverviewChapter Overview
Memory ClassificationMemory ArchitecturesThe Memory CorePeripheryReliabilityCase Studies
© Digital Integrated Circuits2nd Memories
Semiconductor Memory ClassificationSemiconductor Memory Classification
Read-Write MemoryNon-VolatileRead-Write
MemoryRead-Only Memory
EPROM
E2PROM
FLASH
RandomAccess
Non-RandomAccess
SRAM
DRAM
Mask-Programmed
Programmable (PROM)
FIFO
Shift Register
CAM
LIFO
© Digital Integrated Circuits2nd Memories
Memory Timing: DefinitionsMemory Timing: Definitions
Write cycleRead access Read access
Read cycle
Write access
Data written
Data valid
DATA
WRITE
READ
© Digital Integrated Circuits2nd Memories
Memory Architecture: DecodersMemory Architecture: Decoders
Word 0
Word 1
Word 2
Word N2 2
Word N2 1
M bits
Storagecell
M bits
S0
S1
S2
SN2 2
SN2 1
A 0
A 1
A K2 1
K 5 log2N
Word 0
Word 1
Word 2
Word N2 2
Word N2 1
Storagecell
S0
Input-Output(M bits)
Intuitive architecture for N x M memoryToo many select signals:
N words == N select signals K = log2NDecoder reduces the number of select signals
Input-Output(M bits)
© Digital Integrated Circuits2nd Memories
ArrayArray--Structured Memory ArchitectureStructured Memory Architecture
Row
Dec
oder
Bit line2L 2 K
Word line
AK
AK1 1
AL 2 1
A0
M.2K
AK2 1
Sense amplifiers / Drivers
Column decoder
Input-Output(M bits)
Storage cell
Problem: ASPECT RATIO or HEIGHT >> WIDTH
Amplify swing torail-to-rail amplitude
Selects appropriateword
© Digital Integrated Circuits2nd Memories
Hierarchical Memory ArchitectureHierarchical Memory Architecture
Advantages:Advantages:1. Shorter wires within blocks1. Shorter wires within blocks2. Block address activates only 1 block => power savings2. Block address activates only 1 block => power savings
Globalamplifier/driver
Controlcircuitry
Global data busBlock selector
Block 0
Rowaddress
Columnaddress
Blockaddress
Block i Block P 2 1
I/O
© Digital Integrated Circuits2nd Memories
Block Diagram of 4 Block Diagram of 4 MbitMbit SRAMSRAMClock
generator
CS, WEbuffer
I/Obuffer
Y-addressbuffer
X -addressbuffer
x1/x4controller
Z -addressbuffer
X -addressbuffer
Predecoder and block selectorBit line load
Transfer gateColumn decoder
Sense amplifier and write driver
[Hirose90]
© Digital Integrated Circuits2nd Memories
ContentsContents--Addressable MemoryAddressable Memory
Add
ress
Dec
oder
Data (64 bits)
I/O B
uffe
rs
Comparand
CAM Array29 words 3 64 bits
Mask
Control Logic R/W Address (9 bits)
Com
man
ds
29 Val
idity
Bits
Prio
rity
Enc
oder
© Digital Integrated Circuits2nd Memories
Memory Timing: ApproachesMemory Timing: Approaches
DRAM TimingMultiplexed Adressing
SRAM TimingSelf-timed
Addressbus
RAS
RAS-CAS timing
Row Address
AddressBus
Address transitioninitiates memory operation
Address
Column Address
CAS
© Digital Integrated Circuits2nd Memories
ReadRead--Only Memory CellsOnly Memory Cells
WL
BL
WL
BL
1WL
BL
WL
BL
WL
BL
0
VDD
WL
BL
GND
Diode ROM MOS ROM 1 MOS ROM 2
© Digital Integrated Circuits2nd Memories
MOS OR ROMMOS OR ROM
WL[0]
VDD
BL[0]
WL[1]
WL[2]
WL[3]
Vbias
BL[1]
Pull-down loads
BL[2] BL[3]
VDD
© Digital Integrated Circuits2nd Memories
MOS NOR ROMMOS NOR ROM
WL[0]
GND
BL [0]
WL [1]
WL [2]
WL [3]
VDD
BL [1]
Pull-up devices
BL [2] BL [3]
GND
© Digital Integrated Circuits2nd Memories
MOS NOR ROM LayoutMOS NOR ROM Layout
Programmming using theActive Layer Only
Polysilicon
Metal1
Diffusion
Metal1 on Diffusion
Cell (9.5λ x 7λ)
© Digital Integrated Circuits2nd Memories
MOS NOR ROM LayoutMOS NOR ROM Layout
Polysilicon
Metal1
Diffusion
Metal1 on Diffusion
Cell (11λ x 7λ)
Programmming usingthe Contact Layer Only
© Digital Integrated Circuits2nd Memories
MOS NAND ROMMOS NAND ROM
All word lines high by default with exception of selected row
WL [0]
WL [1]
WL [2]
WL [3]
VDD
Pull-up devices
BL[3]BL[2]BL[1]BL [0]
© Digital Integrated Circuits2nd Memories
MOS NAND ROM LayoutMOS NAND ROM Layout
No contact to VDD or GND necessary;
Loss in performance compared to NOR ROMdrastically reduced cell size
Polysilicon
Diffusion
Metal1 on Diffusion
Cell (8λ x 7λ)
Programmming usingthe Metal-1 Layer Only
© Digital Integrated Circuits2nd Memories
NAND ROM LayoutNAND ROM LayoutCell (5λ x 6λ)
Polysilicon
Threshold-alteringimplant
Metal1 on Diffusion
Programmming usingImplants Only
© Digital Integrated Circuits2nd Memories
Equivalent Transient Model for MOS NOR ROMEquivalent Transient Model for MOS NOR ROM
Word line parasiticsWire capacitance and gate capacitanceWire resistance (polysilicon)
Bit line parasiticsResistance not dominant (metal)Drain and Gate-Drain capacitance
Model for NOR ROM VDD
Cbit
rword
cword
WL
BL
© Digital Integrated Circuits2nd Memories
Equivalent Transient Model for MOS NAND ROMEquivalent Transient Model for MOS NAND ROM
Word line parasiticsSimilar to NOR ROM
Bit line parasiticsResistance of cascaded transistors dominatesDrain/Source and complete gate capacitance
Model for NAND ROMVDD
CL
rword
cword
cbit
rbit
WL
BL
© Digital Integrated Circuits2nd Memories
Decreasing Word Line DelayDecreasing Word Line Delay
Metal bypass
Polysilicon word lineK cells
Polysilicon word lineWLDriver
(b) Using a metal bypass
(a) Driving the word line from both sides
Metal word line
WL
(c) Use silicides
© Digital Integrated Circuits2nd Memories
PrechargedPrecharged MOS NOR ROMMOS NOR ROM
PMOS precharge device can be made as large as necessary,but clock driver becomes harder to design.
WL [0]
GND
BL [0]
WL [1]
WL [2]
WL [3]
VDD
BL [1]
Precharge devices
BL [2] BL [3]
GND
pref
© Digital Integrated Circuits2nd Memories
NonNon--Volatile MemoriesVolatile MemoriesThe FloatingThe Floating--gate transistor (FAMOS)gate transistor (FAMOS)
Floating gate
Source
Substrate
Gate
Drain
n+ n+_p
tox
tox
Device cross-section Schematic symbol
G
S
D
© Digital Integrated Circuits2nd Memories
FloatingFloating--Gate Transistor ProgrammingGate Transistor Programming
0 V
2 5 V 0 V
DS
Removing programming voltage leaves charge trapped
5 V
2 2.5 V 5 V
DS
Programming results inhigher VT.
20 V
10 V 5 V 20 V
DS
Avalanche injection
© Digital Integrated Circuits2nd Memories
A A ““ProgrammableProgrammable--ThresholdThreshold”” TransistorTransistor
“ 0” -state “ 1” -state
DVT
VWL VGS
“ ON ”
“ OFF”
ID
© Digital Integrated Circuits2nd Memories
FLOTOX EEPROMFLOTOX EEPROMFloating gate
Source
Substratep
Gate
Drain
n1 n1
FLOTOX transistor Fowler-NordheimI-V characteristic
20–30 nm
10 nm
-10 V10 V
I
VGD
© Digital Integrated Circuits2nd Memories
EEPROM CellEEPROM Cell
WL
BL
VDD
Absolute threshold controlis hardUnprogrammed transistor might be depletion
2 transistor cell
© Digital Integrated Circuits2nd Memories
Flash EEPROMFlash EEPROM
Control gate
erasure
p-substrate
Floating gate
Thin tunneling oxide
n1 source n1 drainprogramming
Many other options …
© Digital Integrated Circuits2nd Memories
CrossCross--sections of NVM cellssections of NVM cells
EPROMFlashCourtesy Intel
© Digital Integrated Circuits2nd Memories
Basic Operations in a NOR Flash MemoryBasic Operations in a NOR Flash Memory――EraseErase
S D
12 VG
cell arrayBL 0 BL 1
open open
WL 0
WL 1
0 V
0 V
12 V
© Digital Integrated Circuits2nd Memories
Basic Operations in a NOR Flash MemoryBasic Operations in a NOR Flash Memory――WriteWrite
S D
12 V
6 VG
BL 0 BL 1
6 V 0 V
WL 0
WL 1
12 V
0 V
0 V
© Digital Integrated Circuits2nd Memories
Basic Operations in a NOR Flash MemoryBasic Operations in a NOR Flash Memory――ReadRead
5 V
1 VG
S D
BL 0 BL 1
1 V 0 V
WL 0
WL 1
5 V
0 V
0 V
© Digital Integrated Circuits2nd Memories
NAND Flash MemoryNAND Flash Memory
Unit Cell
Word line(poly)
Source line(Diff. Layer)
Courtesy Toshiba
GateONO
FGGateOxide
© Digital Integrated Circuits2nd Memories
NAND Flash MemoryNAND Flash Memory
Word linesSelect transistor
Bit line contact Source line contact
Active area
STI
Courtesy Toshiba
© Digital Integrated Circuits2nd Memories
Characteristics of StateCharacteristics of State--ofof--thethe--art NVMart NVM
© Digital Integrated Circuits2nd Memories
ReadRead--Write Memories (RAM)Write Memories (RAM)STATIC (SRAM)
DYNAMIC (DRAM)
Data stored as long as supply is appliedLarge (6 transistors/cell)FastDifferential
Periodic refresh requiredSmall (1-3 transistors/cell)SlowerSingle Ended
© Digital Integrated Circuits2nd Memories
66--transistor CMOS SRAM Cell transistor CMOS SRAM Cell
WL
BL
VDD
M5M6
M4
M1
M2
M3
BL
© Digital Integrated Circuits2nd Memories
CMOS SRAM Analysis (Read)CMOS SRAM Analysis (Read)WL
BL
VDD
M 5M 6
M 4
M1 VDDVDD VDD
BL
Q = 1Q = 0
Cbit Cbit
© Digital Integrated Circuits2nd Memories
CMOS SRAM Analysis (Read)CMOS SRAM Analysis (Read)
00
0.2
0.4
0.6
0.8
1
1.2
0.5 1 1.2 1.5 2Cell Ratio (CR)
2.5 3
V o l t a g e r i s e [ V ]
Vol
tage
Ris
e (V
)
© Digital Integrated Circuits2nd Memories
CMOS SRAM Analysis (Write) CMOS SRAM Analysis (Write)
BL = 1 BL = 0
Q = 0Q = 1
M1
M4
M5
M6
VDD
VDD
WL
© Digital Integrated Circuits2nd Memories
CMOS SRAM Analysis (Write)CMOS SRAM Analysis (Write)
© Digital Integrated Circuits2nd Memories
6T6T--SRAM SRAM —— Layout Layout VDD
GND
WL
BLBL
M1 M3
M4M2
M5 M6
© Digital Integrated Circuits2nd Memories
ResistanceResistance--load SRAM Cellload SRAM Cell
Static power dissipation -- Want RL largeBit lines precharged to VDD to address tp problem
M3
RL RL
VDD
WL
Q Q
M1 M2
M4
BL BL
© Digital Integrated Circuits2nd Memories
SRAM CharacteristicsSRAM Characteristics
© Digital Integrated Circuits2nd Memories
33--Transistor DRAM CellTransistor DRAM Cell
No constraints on device ratiosReads are non-destructiveValue stored at node X when writing a “1” = VWWL-VTn
WWL
BL1
M1 X
M3
M2
CS
BL2
RWL
V DD
VDD 2 VT
DVV DD 2 VTBL 2
BL 1
X
RWL
WWL
© Digital Integrated Circuits2nd Memories
3T3T--DRAM DRAM —— LayoutLayout
BL2 BL1 GND
RWL
WWL
M3
M2
M1
© Digital Integrated Circuits2nd Memories
11--Transistor DRAM CellTransistor DRAM Cell
Write: CS is charged or discharged by asserting WL and BL.Read: Charge redistribution takes places between bit line and storage capacitance
Voltage swing is small; typically around 250 mV.
M1
CS
WL
BL
CBL
VDD 2 VT
WL
X
sensing
BL
GND
Write 1 Read 1
VDD
VDD /2 VDD /2
ΔV BL VPRE– VBIT VPRE–CS
CS CBL+------------= =V
© Digital Integrated Circuits2nd Memories
DRAM Cell ObservationsDRAM Cell Observations1T DRAM requires a sense amplifier for each bit line, due
to charge redistribution read-out.DRAM memory cells are single ended in contrast to
SRAM cells.The read-out of the 1T DRAM cell is destructive; read
and refresh operations are necessary for correct operation.
Unlike 3T cell, 1T cell requires presence of an extra capacitance that must be explicitly included in the design.
When writing a “1” into a DRAM cell, a threshold voltage is lost. This charge loss can be circumvented by bootstrapping the word lines to a higher value than VDD
© Digital Integrated Circuits2nd Memories
Sense Amp OperationSense Amp Operation
DV(1)
V(1)
V(0)t
VPRE
VBL
Sense amp activatedWord line activated
© Digital Integrated Circuits2nd Memories
11--T DRAM CellT DRAM Cell
Uses Polysilicon-Diffusion CapacitanceExpensive in Area
M1 wordline
Diffusedbit line
Polysilicongate
Polysiliconplate
Capacitor
Cross-section Layout
Metal word line
Poly
SiO2
Field Oxiden+ n+
Inversion layerinduced byplate bias
Poly
© Digital Integrated Circuits2nd Memories
SEM of polySEM of poly--diffusion capacitor 1Tdiffusion capacitor 1T--DRAMDRAM
© Digital Integrated Circuits2nd Memories
Advanced 1T DRAM CellsAdvanced 1T DRAM Cells
Cell Plate Si
Capacitor Insulator
Storage Node Poly
2nd Field Oxide
Refilling Poly
Si Substrate
Trench Cell Stacked-capacitor Cell
Capacitor dielectric layerCell plateWord line
Insulating Layer
IsolationTransfer gateStorage electrode
© Digital Integrated Circuits2nd Memories
Static CAM Memory CellStatic CAM Memory Cell
• • • • • •
CAM
BitWord
Bit
••• CAM
Bit Bit
CAM
Word
Wired-NOR Match Line
Match M1
M2
M7M6
M4 M5M8 M9
M3int
SWord
••• CAM
Bit Bit
S
© Digital Integrated Circuits2nd Memories
CAM in Cache MemoryCAM in Cache Memory
A d d r e s s D e c o d e r
H i t L o g i c
CAM
ARRAY
Input Drivers
Tag HitAddress
SRAM
ARRAY
Sense Amps / Input Drivers
DataR/W
© Digital Integrated Circuits2nd Memories
PeripheryPeriphery
DecodersSense AmplifiersInput/Output BuffersControl / Timing Circuitry
© Digital Integrated Circuits2nd Memories
Row DecodersRow DecodersCollection of 2M complex logic gatesOrganized in regular and dense fashion
(N)AND Decoder
NOR Decoder
© Digital Integrated Circuits2nd Memories
Hierarchical DecodersHierarchical Decoders
• • •
• • •
A2A2
A2A3
WL 0
A2A3A2A3A2A3
A3 A3A0A0
A0A1A0A1A0A1A0A1
A1 A1
WL 1
Multi-stage implementation improves performance
NAND decoder usingNAND decoder using22--input preinput pre--decodersdecoders
© Digital Integrated Circuits2nd Memories
Dynamic DecodersDynamic Decoders
Precharge devices
VDD φ
GND
WL3
WL2
WL1
WL0
A0A0
GND
A1A1φ
WL3
A0A0 A1A1
WL 2
WL 1
WL 0
VDD
VDD
VDD
VDD
2-input NOR decoder 2-input NAND decoder
© Digital Integrated Circuits2nd Memories
44--input passinput pass--transistor based column transistor based column decoderdecoder
2 - i n p u t N O R d e c o d e r
Advantages: speed (tpd does not add to overall memory access time)Only one extra transistor in signal path
Disadvantage: Large transistor count
A0S0
BL 0 BL 1 BL 2 BL 3
A1
S1
S2
S3
D
© Digital Integrated Circuits2nd Memories
44--toto--1 tree based column decoder1 tree based column decoder
Number of devices drastically reducedDelay increases quadratically with # of sections; prohibitive for large decoders
buffersprogressive sizingcombination of tree and pass transistor approaches
Solutions:
BL 0 BL 1 BL 2 BL 3
D
A 0
A 0
A1
A 1
© Digital Integrated Circuits2nd Memories
Decoder for circular shiftDecoder for circular shift--registerregister
V DD
V DD
R
WL 0
V DD
f
ff
f
V DD
R
WL 1
V DD
f
ff
f
V DD
R
WL 2
V DD
f
ff
f• • •
© Digital Integrated Circuits2nd Memories
Sense AmplifiersSense Amplifiers
tpC ΔV⋅
Iav----------------=
make ΔV as smallas possible
smalllarge
Idea: Use Sense Amplifer
outputinput
s.a.smalltransition
© Digital Integrated Circuits2nd Memories
Differential Sense AmplifierDifferential Sense Amplifier
Directly applicable toSRAMs
M 4
M 1
M 5
M 3
M 2
VDD
bitbit
SE
Outy
© Digital Integrated Circuits2nd Memories
Differential Sensing Differential Sensing ―― SRAMSRAMVDD
VDD
VDD
VDD
BLEQ
Diff.SenseAmp
(a) SRAM sensing scheme (b) two stage differential amplifier
SRAM cell i
WL i
2xx
VDD
Output
BL
PC
M3
M1
M5
M2
M4
x
SE
SE
SE
Output
SE
x2x 2x
y
y
2y
© Digital Integrated Circuits2nd Memories
LatchLatch--Based Sense Amplifier (DRAM)Based Sense Amplifier (DRAM)
Initialized in its meta-stable point with EQOnce adequate voltage gap created, sense amp enabled with SEPositive feedback quickly forces output to a stable operating point.
EQ
VDD
BL BL
SE
SE
© Digital Integrated Circuits2nd Memories
ChargeCharge--Redistribution AmplifierRedistribution Amplifier
0.5
1.0
1.5
2.0
2.5
0.00.0 1.00 2.00
time (nsec)
V
Vin
Vref 5 3V
VL
VS
3.00
Concept
M 2 M 3
M 1VL VS
Vref
CsmallClarge
Transient Response
© Digital Integrated Circuits2nd Memories
ChargeCharge--Redistribution AmplifierRedistribution Amplifier――EPROMEPROM
SE
VDD
BL
Out
Cout
Ccol
CBLM1
M2
M3
M4
WLC
Load
Cascodedevice
Columndecoder
EPROMarrayWL
Vcasc
© Digital Integrated Circuits2nd Memories
SingleSingle--toto--Differential ConversionDifferential Conversion
How to make a good Vref?
Diff.S.A.Cell
2xx
Output
WL
Vref
BL
12
© Digital Integrated Circuits2nd Memories
Open Open bitlinebitline architecture with architecture with dummy cellsdummy cells
CS CS CS CS
BLL
L L 1 L 0 R0
CS
R1
CS
L
… …
BLR
V DD
SE
SE
EQ
Dummy cell Dummy cell
© Digital Integrated Circuits2nd Memories
DRAM Read Process with Dummy CellDRAM Read Process with Dummy Cell3
2
1
00 1 2 3
V
BL
BL
t (ns)
reading 03
2
1
00 1 2 3
V
SE
EQ WL
t (ns)
control signals
3
2
1
00 1 2 3
V
BL
BL
t (ns)
reading 1
© Digital Integrated Circuits2nd Memories
Voltage RegulatorVoltage Regulator
-
+
VDD
VREF
Vbias
Mdrive
VDL
Mdrive
VDL
VREF
Equivalent Model
© Digital Integrated Circuits2nd Memories
Charge PumpCharge Pump
CLK
VDD
A BM1
M2Vload
Cload
Cpump
2VDD 2 VT
VDD 2 VT
0 V
VB
Vload
0 V
© Digital Integrated Circuits2nd Memories
DRAM TimingDRAM Timing
© Digital Integrated Circuits2nd Memories
RDRAM ArchitectureRDRAM Architecture
memoryarray
m u x / d e m u x
n e t w o r k
Databus
Clocks
Column
Rowdemux packet dec.
packet dec.
Bus
k k3 l
demux
© Digital Integrated Circuits2nd Memories
Address Transition DetectionAddress Transition Detection
DELAYtdA0
DELAYtdA1
DELAYtdAN2 1
VDD
ATD ATD
…
© Digital Integrated Circuits2nd Memories
Reliability and YieldReliability and Yield
© Digital Integrated Circuits2nd Memories
Sensing Parameters in DRAMSensing Parameters in DRAM
From [Itoh01]
4K
10
100
1000
64K 1M 16M 256M 4G 64GMemory Capacity (bits /chip)
C
D
,
Q
S
,
C
S
,
V
D D
,
V
s m a x
CD(1F)
CS(1F)
Q S(1C)
V smax (mv)
VDD (V)Q S 5 CS V DD /2V smax 5 Q S / (CS 1 CD)
© Digital Integrated Circuits2nd Memories
Noise Sources in 1T Noise Sources in 1T DRamDRam
Ccross
electrode
a-particles
leakage CS
WL
BL substrate Adjacent BLCWBL
© Digital Integrated Circuits2nd Memories
Open BitOpen Bit--line Architecture line Architecture ——Cross CouplingCross Coupling
SenseAmplifierC
WL 1
BL
CBL
CWBL CWBL
CC
WL 0
CCBL
C C
WL D WL D WL 0 WL 1
BL
EQ
© Digital Integrated Circuits2nd Memories
FoldedFolded--BitlineBitline ArchitectureArchitecture
SenseAmplifier
C
WL 1
CWBL
CWBL
C
WL 0 WL 0 WL D
CC
WL 1
CC
WL D
BL CBL
BL CBL
EQ
x
x
y
y
…
© Digital Integrated Circuits2nd Memories
TransposedTransposed--BitlineBitline ArchitectureArchitecture
SA
Ccross
(a) Straightforward bit-line routing
(b) Transposed bit-line architecture
BL 9
BLBL
BL 99
SA
CcrossBL 9
BLBL
BL 99
© Digital Integrated Circuits2nd Memories
AlphaAlpha--particles (or Neutrons)particles (or Neutrons)
1 Particle ~ 1 Million Carriers
WL
BL
V DD
n1
a-particle
SiO 21
11
11
12
22
22
2
© Digital Integrated Circuits2nd Memories
YieldYield
Yield curves at different stages of process maturity(from [Veendrick92])
© Digital Integrated Circuits2nd Memories
RedundancyRedundancy
MemoryArray
R o w D e c o d e r
Redundantrows
Redundantcolumns
RowAddress
Column Decoder ColumnAddress
FuseBank:
© Digital Integrated Circuits2nd Memories
ErrorError--Correcting CodesCorrecting CodesExample: Hamming Codes
with
e.g. B3 Wrong
1
1
0
= 3
© Digital Integrated Circuits2nd Memories
Redundancy and Error CorrectionRedundancy and Error Correction
© Digital Integrated Circuits2nd Memories
Sources of Power Dissipation in Sources of Power Dissipation in MemoriesMemories
PERIPHERY
ROWDEC
selected
non-selected
CHIP
COLUMN DEC
nC DE VINT f
mC DE VINT f
CPTVINT f
IDCP
ARRAY
m
n
m(n 2 1)ihld
miact
VDD
VSS
IDD 5 SCiDVif1S IDCP
From [Itoh00]
© Digital Integrated Circuits2nd Memories
Data Retention in SRAMData Retention in SRAM
( A )
1.30u
1.10u
900n
700n
500n
300n
100n
0.00 .600 1.20 1.80
Factor 7
0.13 m CMOSm
0.18 m CMOSm
VDD
I leak
age
SRAM leakage increases with technology scaling
© Digital Integrated Circuits2nd Memories
Suppressing Leakage in SRAMSuppressing Leakage in SRAM
SRAMcell
SRAMcell
SRAMcell
VDD,int
VSS,int
VDDVDD VDDL
sleep
sleep
SRAMcell
SRAMcell
SRAMcell
VDD,int
sleep
low-threshold transistor
Reducing the supply voltageReducing the supply voltageInserting Extra ResistanceInserting Extra Resistance
© Digital Integrated Circuits2nd Memories
Data Retention in DRAMData Retention in DRAM101
100
102 1
102 2
102 3
102 4
102 5
102 6
15M 64M 255M 1G 4G 15G 64G
Capacity (bit)
Cur
rent
(A)
3.3 2.5 2.0 1.5 1.2 1.0 0.8Operating voltage (V)
0.53 0.40 0.32 0.24 0.19 0.16 0.13
Extrapolated threshold voltage at 25 C (V)
I ACT
IAC
IDC
Cycle time : 150 nsT 5 75 C,S5 97 mV/dec.
From [Itoh00]
© Digital Integrated Circuits2nd Memories
Case StudiesCase Studies
Programmable Logic ArraySRAM Flash Memory
© Digital Integrated Circuits2nd Memories
PLA versus ROMPLA versus ROMProgrammable Logic Arraystructured approach to random logic“two level logic implementation”
NOR-NOR (product of sums)NAND-NAND (sum of products)
IDENTICAL TO ROM!
Main differenceROM: fully populatedPLA: one element per minterm
Note: Importance of PLA’s has drastically reduced1. slow2. better software techniques (mutli-level logic
synthesis)But …
© Digital Integrated Circuits2nd Memories
Programmable Logic ArrayProgrammable Logic Array
GND GND GND GND
GND
GND
GND
V DD
V DD
X 0X 0 X 1 f0 f1X 1 X 2X 2
AND-plane OR-plane
Pseudo-NMOS PLA
© Digital Integrated Circuits2nd Memories
Dynamic PLADynamic PLAGND
GNDVDD
VDD
X 0X 0 X 1 f0 f1X 1 X 2X 2
ANDf
ANDf
ORf
ORf
AND-plane OR-plane
© Digital Integrated Circuits2nd Memories
Clock Signal Generation Clock Signal Generation for selffor self--timed dynamic PLAtimed dynamic PLA
f
tpre teval
f AND
f
f AND
f AND
f OR
f OR
(a) Clock signals (b) Timing generation circuitry
Dummy AND row
Dummy AND row
© Digital Integrated Circuits2nd Memories
PLA LayoutPLA LayoutVDD GNDφ
And-Plane Or-Plane
f0 f1x0 x0 x1 x1 x2 x2Pull-up devices Pull-up devices
© Digital Integrated Circuits2nd Memories
4 4 MbitMbit SRAMSRAMHierarchical WordHierarchical Word--line Architectureline Architecture
Global word line
Sub-global word line
Block groupselect
Blockselect
Blockselect
Memory cell
Localword line
Block 0
•••
Localword line
Block 1
•••
Block 2...
•••
© Digital Integrated Circuits2nd Memories
BitBit--line Circuitryline CircuitryBit-lineload
Blockselect ATD
BEQ
Local WL
Memory cell
I/O lineI/O
B /T
CD
Sense amplifier
CD CD
I/O
B /T
© Digital Integrated Circuits2nd Memories
Sense Amplifier (and Waveforms)Sense Amplifier (and Waveforms)
BS
I /O I /O
DATA
Blockselect ATD
BSSA SA
BS
SEQ
SEQ
SEQ
SEQSEQ
Dei
I/O Lines
Address
Data-cut
ATD
BEQ
SEQ
DATA
Vdd
GND
SA, SAVdd
GND
© Digital Integrated Circuits2nd Memories
1 1 GbitGbit Flash MemoryFlash Memory
Sense Latches(10241 32) 3 8
Data Caches(10241 32) 3 8
Sense Latches(10241 32) 3 8
Data Caches(10241 32) 3 8
Wor
d Li
ne D
river
Wor
d Li
ne D
river
Wor
d Li
ne D
river
Wor
d Li
ne D
river
512Mb Memory Array 512Mb Memory ArrayBL0 BL1 ····· BL16895 BL16996 BL16897··· BL33791
SGDWL31
WL0SGS
Block0
BLT0Block1023
Block0
Block1023
Bit Line Control CircuitBLT1
I/O I/O
From [Nakamura02]
© Digital Integrated Circuits2nd Memories
Writing Flash MemoryWriting Flash MemoryN
umbe
r of m
emor
y ce
lls
0V 1V 2VVt of memory cells
Verify level 5 0.8 V Word-line level 5 4.5 V
3V 4V
Result of 4 timesprogram
R e a d l e v e l ( 4 . 5 V )
N u m b e r o f c e l l s
1000V 1V 2V
Vt of memory cells
3V 4V
102
104
106
108
Evolution of thresholds Final Distribution
From [Nakamura02]
© Digital Integrated Circuits2nd Memories
125125mmmm22 1Gbit NAND Flash Memory1Gbit NAND Flash Memory
10.7
mm
11.7mm
2kB
Pag
e bu
ffer &
cac
heC
harg
e pu
mp
16896 bit lines
32 word lines x 1024 blocks
From [Nakamura02]
© Digital Integrated Circuits2nd Memories
125125mmmm22 1Gbit NAND Flash Memory1Gbit NAND Flash MemoryTechnology 0.13μm p-sub CMOS triple-well
1poly, 1polycide, 1W, 2AlCell size 0.077μm2Chip size 125.2mm2Organization 2112 x 8b x 64 page x 1k blockPower supply 2.7V-3.6VCycle time 50nsRead time 25μsProgram time 200μs / pageErase time 2ms / block
Technology 0.13μm p-sub CMOS triple-well1poly, 1polycide, 1W, 2Al
Cell size 0.077μm2Chip size 125.2mm2Organization 2112 x 8b x 64 page x 1k blockPower supply 2.7V-3.6VCycle time 50nsRead time 25μsProgram time 200μs / pageErase time 2ms / block
From [Nakamura02]
© Digital Integrated Circuits2nd Memories
Semiconductor Memory TrendsSemiconductor Memory Trends(up to the 90(up to the 90’’s)s)
Memory Size as a function of time: x 4 every three years
© Digital Integrated Circuits2nd Memories
Semiconductor Memory TrendsSemiconductor Memory Trends(updated)(updated)
From [Itoh01]
© Digital Integrated Circuits2nd Memories
Trends in Memory Cell AreaTrends in Memory Cell Area
From [Itoh01]
© Digital Integrated Circuits2nd Memories
Semiconductor Memory TrendsSemiconductor Memory Trends
Technology feature size for different SRAM generations