Download Design and Testing of Semiconductor Memories

Semiconductor Memory Design
(SRAM & DRAM)
Kaushik Saha
Contact: [email protected], mobile-98110-64398
STMicroelectronics
Understanding the Memory Trade
The memory market is the most
- Volatile
- Cost Competitive
- Innovative
in the IC trade
Supply
Demand
Memory
market
Technical Change
2
Classification of Memories
RWMemory
Random Access
SRAM (Static)
DRAM (Dynamic)
Non-Random
Access
FIFO (Queue)
LIFO (Stack)
SR (Shift
Register)
CAM (Content
Addressable)
NVRWM
ROM
EPROM
EEPROM
Mask
Programmed
FLASH
PROM (Fuse
Programmed)
3
Feature Comparison Between Memory
Types
4
Memory selection : cost and
performance
DRAM, EPROM
- Merit : cheap, high density
- Demerit : low speed, high power
SRAM
- Merit : high speed or low power
- Demerit : expensive, low density
Large memory with cost pressure :
- DRAM
Large memory with very fast speed :
- SRAM or
- DRAM main + SRAM cache
Back-up main for no data loss when power failure
- SRAM with battery back-up
- EEPROM
5
Trends in Storage Technology
Generation
Increasing die size
factor 1.5 per generation
Combined with reducing cell size
factor 2.6 per generation
*MB=Mbytes
6
The Need for Innovation in Memory
Industry
The learning rate (viz. the constant b) is the
highest for the memory industry
- Because prices drop most steeply among all ICs
• Due to the nature of demand + supply
- Yet margins must the maintained
Techniques must be applied to reduce production
cost
Often, memories are the launch vehicles for a
technology node
- Leads to volatile nature of prices
7
Memory Hierarchy of a Modern Computer System
By taking advantage of the principle of locality:
- Present the user with as much memory as is available in the cheapest
technology.
- Provide access at the speed offered by the fastest technology.
Processor
Control
Registers
Speed (ns):
1s
Size (bytes): 100s
On-Chip
Cache
Datapath
Second
Level
Cache
(SRAM)
Main
Memory
(DRAM)
10s
100s
Ks
Ms
Secondary
Storage
(Disk)
Tertiary
Storage
(Tape)
10,000,000s 10,000,000,000s
(10s ms)
(10s sec)
Gs
Ts
8
How is the hierarchy managed?
Registers <-> Memory
- by compiler (programmer?)
cache <-> memory
- by the hardware
memory <-> disks
- by the hardware and operating system (virtual memory)
- by the programmer (files)
9
Memory Hierarchy Technology
Random Access:
- “Random” is good: access time is the same for all locations
- DRAM: Dynamic Random Access Memory
•
•
High density, low power, cheap, slow
Dynamic: need to be “refreshed” regularly
- SRAM: Static Random Access Memory
•
•
Low density, high power, expensive, fast
Static: content will last “forever”(until lose power)
“Not-so-random” Access Technology:
- Access time varies from location to location and from time to time
- Examples: Disk, CDROM
10
Main Memory Background
Performance of Main Memory:
- Latency: Cache Miss Penalty
•
•
Access Time: time between request and word arrives
Cycle Time: time between requests
- Bandwidth: I/O & Large Block Miss Penalty (L2)
Main Memory is DRAM : Dynamic Random Access Memory
- Dynamic since needs to be refreshed periodically Addresses divided into 2
halves (Memory as a 2D matrix):
•
•
RAS or Row Access Strobe
CAS or Column Access Strobe
Cache uses SRAM : Static Random Access Memory
- No refresh (6 transistors/bit vs. 1 transistor)
Size: DRAM/SRAM 4-8
Cost/Cycle time: SRAM/DRAM 8-16
11
Memory Interfaces
Address i/ps
- Maybe latched with strobe signals
Write Enable (/WE)
- To choose between read / write
- To control writing of new data to memory
Chip Select (/CS)
- To choose between memory chips / banks on system
Output Enable (/OE)
- To control o/p buffer in read circuitry
Data i/os
- For large memories data i/p and o/p muxed on same pins,
• selected with /WE
Refresh signals
12
Memory - Basic Organization
S0
Word 0
• N words
S1
S2
Word 1
Word 2
Single
Storage
Cell
• M bits per word
• N select lines
• 1:N decoder
• very inefficient design
SN-2
Word N-2
SN-1
Word N-1
• difficult to place and route
M bit output word
13
Memory - Real Organization
Array of N x K words
S0
row 0
C of M bit words
row 1
C of M bit words
row 2
C of M bit words
row N-2
C of M bit words
row N-1
Row
Decoder
SR-1
Log2C
Address
Lines
N=R*C
C of M bit words
------------- rows R------------
Log2R
Address
Lines
------------- columns ------------ KxM
- - - - KxM bits - - - -
Column Select
M bit data word
14
Array-Structured Memory Architecture
Problem: ASPECT RATIO or HEIGHT >> WIDTH
AK
AK+1
AL-1
Bit Line
Storage Cell
Row Decoder
2L-K
Word Line
M.2K
Sense Amplifiers / Drivers
A0
Column Decoder
AK -1
Amplify swing to
rail-to-rail amplitude
Selects appropriate
word
Input-Output
(M bits)
15
Hierarchical Memory Architecture
Row
Address
Column
Address
Block
Address
Global Data Bus
Control
Circuitry
Block Selector
Global
Amplifier/Driver
I/O
Advantages:
1. Shorter wires within blocks
2. Block address activates only 1 block => power savings
16
Memory - Organization and Cell
Design Issues
aspect ratio (height : width) should be relative square
- Row / Column organisation (matrix)
- R = log2(N_rows); C = log2(N_columns)
- R + C = N (N_address_bits)
number of rows should be power of 2
- number of bits in a row
sense amplifiers to amplify the voltage from each
memory cell
1 -> 2R row decoder
1 -> 2C column decoder
- implement M of the column decoders (M bits, one per
bit)
• M = output word width
17
Semiconductor Manufacturing Process
18
Basic Micro Technology
19
Semiconductor Manufacturing Process
Fundamental Processing Steps
1.Silicon Manufacturing
a) Czochralski method.
b) Wafer Manufacturing
c) Crystal structure
2.Photolithography
a) Photoresists
b) Photomask and Reticles
c) Patterning
20
Lithography Requirements
21
Excimer Laser DUV & EUV lithography
Power o/p
Pulse Rate
NovaLine Laser – Lambda Physik
22
Dry or Plasma Etching
23
Dry or Plasma Etching
24
Dry or Plasma Etching
Combination of chemical and physical etching – Reactive
Ion Etching (RIE)
Directional etching due to ion assistance.
In RIE processes the wafers sit on the powered electrode.
This placement sets up a negative bias on the wafer which accelerates
positively charge ions toward the surface.
These ions enhance the chemical etching mechanisms and allow
anisotropic etching.
Wet etches are simpler, but dry etches provide better line width
control since it is anisotropic.
25
Dry Etching
Reactive Ion Etching- RIE
26
CMOS fabrication sequence
4.2 Local oxidation of silicon
(LOCOS)
-
-
-
Paulo
Moreira
The photoresist mask is removed
The SiO2/SiN layers will now act as
masks
The thick field oxide is then grown
by:
• exposing the surface of the
wafer to a flow of oxygen-rich
gas
The oxide grows in both the vertical
and lateral directions
This results in a active area smaller
than patterned
patterned active area
Field oxide (FOX)
n-well
active area after LOCOS
Technology
p-type
27
LOCOS: Local Oxidation
28
Advanced CMOS processes
Shallow trench isolation
n+ and p+-doped polysilicon gates (low threshold)
source-drain extensions LDD (hot-electron effects)
Self-aligned silicide (spacers)
Non-uniform channel doping (short-channel effects)
n+ poly
Silicide
p+ poly
Oxide spacer
n+
p-doping
p+
n+
n-doping
p+
n-well
Shallow-trench isolation
Source-drain
extension
p-type substrate
Paulo
Moreira
Technology
29
Process enhancements
Up to eight metal levels in modern processes
Copper for metal levels 2 and higher
Stacked contacts and vias
Chemical Metal Polishing for technologies with several
metal levels
For analog applications some processes offer:
- capacitors
- resistors
- bipolar transistors (BiCMOS)
Paulo
Moreira
Technology
30
Metalisation
Metal deposited first, followed by photoresist
Then metal etched away to leave pattern, gaps filled with SiO2
31
Electroplating Based Damascene Process Sequence
Pre-clean
IMP barrier + Copper
25 nm
10-20 nm
Electroplating
CMP
+ 100-200 nm
Simple, Low-cost, Hybrid, Robust Fill Solution
32
33
34
Example CMOS SRAM Process
0.7u n-channel min gate length, 0.6u Leff
1.0u FOX isolation using SiNiO2 masking
0.25u N+ to P+ spacing
Thin epi material to suppress latchup
Twin well to suppress parasitic channel through field transistors
LDD struct for n & p transistors to suppress hot carrier effects
Buried contacts to overlying metal or underlying gates
Metal salicide to reduce poly resistivity
2 metals to reduce die area
Planarisation after all major process steps
- To reduce step coverage problems
• on contact cut fills
• Large oxide depositions
35
SRAM Application Areas
Main memory in high performance small system
Main memory in low power consumption system
Simpler and less expensive system if without a
cache
Battery back-up
Battery operated system
36
SRAM Performance vs Application
Families
37
Typical Application Scenarios
SRAM
MMU
FPU
BIU
CORE
L1
16KB
L2
256KB
ALU
DRAM
64MB
I/O
PCI
ISA
i586 based PC
Hand phone and Cache
38
Market View by Application
39
Overview of SRAM Types
SRAMs
Asynchronous
Low Speed
Medium Speed
High Speed
Synchronous
Flow Through / Pipelined
Zero Bus Turnaround
Double Data Rate
Dual Port
Interleaved / Linear Burst
Special
CAM / Cache Tag
FIFO
Multiport
40
SRAM Array
SL0
Array Organization
• common bit precharge lines
• need sense amplifier
SL1
SL2
41
Logic Diagram of a Typical SRAM
A0-AN
CS!
WE_L
2 N words
x M bit
SRAM
OE_L
M
D
Write Enable is usually active low (WE_L)
Din and Dout are combined to save pins:
- A new control signal, output enable (OE_L) is needed
- WE_L = 0, OE_L = 1
• D serves as the data input pin
- WE_L = 1, OE_L = 0
• D is the data output pin
- Both WE_L = 1, OE_L = 1
• Result is unknown. Don’t do that!!!
42
Simple 4x4 SRAM Memory
read
precharge
enable
2 bit width
M=2
A1
R=2
N_rows = 2R = 4
C=1
A2
c
N_columns = 2 x M = 4
N=R+C=3
Array size =
N_rows x N_columns = 16
bit line precharge
WL[0]
BL !BL
WL[1]
->
WL[2]
WL[3]
A0
A0!
clocking and
control ->
Column Decoder
sense amplifiers
WE! , OE!
write circuitry
43
Basic Memory Read Cycle
System selects memory with /CS=L
System presents correct address (A0-AN)
System turns o/p buffers on with /OE=L
System tri-states previous data sources within a
permissible time limit (tOLZ or tCLZ)
System must wait minimum time of tAA, tAC or tOE
to get correct data
44
Basic Memory Write Cycle
System presents correct address (A0-AN)
System selects memory with /CS=L
System waits a minimum time equal to internal setup time
of new addresses (tAS)
System enables writing with /WE=L
System waits for minimum time to disable o/p driver (twz)
System inputs data and waits minimum time (tDW) for data
to be written in core, then turns off write (/WE=H)
45
Memory Timing: Definitions
Read Cycle
READ
Read Access
Read Access
Write Cycle
WRITE
Write Access
Data Valid
DATA
Data Written
46
Memory Timing: Approaches
MSB
Address
Bus
LSB
Row Address Column Address
Address
Bus
RAS
Address
Address transition
initiates memory operation
CAS
RAS-CAS timing
DRAM Timing
Multiplexed Adressing
SRAM Timing
Self-timed
47
The system level view of Async SRAMs
48
The system level view of synch SRAMs
49
Typical Async SRAM Timing
A
N
WE_L
2 N words
x M bit
SRAM
OE_L
M
Write Timing:
D
Data In
D
Read Timing:
High Z
Data Out
Data Out
Junk
A
Write Address
Read Address
Read Address
OE_L
WE_L
Write
Hold Time
Read Access
Time
Read Access
Time
Write Setup Time
50
SRAM Read Timing (typical)
tAA (access time for address): how long it takes to
get stable output after a change in address.
tACS (access time for chip select): how long it takes
to get stable output after CS is asserted.
tOE (output enable time): how long it takes for the
three-state output buffers to leave the highimpedance state when OE and CS are both
asserted.
tOZ (output-disable time): how long it takes for the
three-state output buffers to enter highimpedance state after OE or CS are negated.
tOH (output-hold time): how long the output data
remains valid after a change to the address
inputs.
51
SRAM Read Timing (typical)
stable
ADDR
stable
stable
 tAA
Max(tAA, tACS)
CS_L
tOH
tACS
OE_L
tAA
DOUT
tOZ
valid
tOE
tOZ
valid
tOE
valid
WE_L = HIGH
52
SRAM Architecture and Read Timings
tOH
tAA
tACS
tOZ
tOE
53
SRAM write cycle timing
/WE controlled
/CS controlled
54
SRAM Architecture and Write Timings
Setup time = tDW
tDH
Write driver
tWP-tDW
55
SRAM Architecture
56
SRAM Cell Design
Memory array typically needs to store lots of bits
- Need to optimize cell design for area and performance
- Peripheral circuits can be complex
• Smaller compared to the array (60-70% area in array, 30-40%
in periphery)
Memory cell design
- 6T cell full CMOS
- 4T cell with high resistance poly load
- TFT load cell
57
Anatomy of the SRAM Cell
->
Write:
•set bit lines to new data value
•b’ = opposite of b
•raise word line to “high”
•sets cell to new state
•May need to flip old state
Read:
•set bit lines high
•set word line high
•see which bit line goes low
58
SRAM Cell Operating Principle
•Inverter Amplifies
•Negative gain
•Slope < –1 in middle
•Saturates at ends
• Inverter Pair Amplifies
•Positive gain
•Slope > 1 in middle
•Saturates at ends
59
Bistable Element
Stability
 Require Vin = V2
 Stable at endpoints
recover from pertubation

Metastable in middle
Fall out when perturbed
Ball on Ramp Analogy
60
SRAM Cell technologies
Bipolar ECL : NPN with dual emitter
NMOS load
A) Enhancement : additional load
gate bias
B) Depletion : no additional load
gate bias
High Load
Resistance (4T)
Full CMOS
(6T)
Thin Film
Transistors
61
6T & 4T cell Implementation
6T Bistable Latch
High resistance poly
4T Bistable Latch
62
Reading a Cell
Icell
DV = Icell * t
----Cb
Sense Amplifier
63
Writing a Cell
0 -> 1
1 -> 0
64
Bistable Element
Stability
 Require Vin = V2
 Stable at endpoints
recover from pertubation

Metastable in middle
Fall out when perturbed
Ball on Ramp Analogy
65
Cell Static Noise Margin
Cell state may be disturbed by
•DC
•Layout pattern offset
•Process mismatches
•non-uniformity of implantation
•gate pattern size errors
•AC
•Alpha particles
•Crosstalk
•Voltage supply ripple
•Thermal noise
SNM = Maximum Value of Vn
Without flipping cell state
66
SNM: Butterfly Curves
1
SNM
2
2
SNM
1
2
1
1
2
67
SNM for Poly Load Cell
68
6T Cell LayoutN Well
B-
B+
Connection
VDD
PMOS
Pull Up
Q/
Q
NMOS
Pull Down
GND
SEL
SEL MOSFET
Substrate
Connection
69
6T SRAM Array Layout
70
Another 6T Cell Layout
Stick Diagram
bit
bit’
VDD
T
T
T
T
T
T
word
Gnd
GND and contact shared
with cell to left
These four contacts shared
with (mirrored) cell below
2 Metal Layer Process
71
6T Array Layout (2x2)
Stick Diagram
bit
Gnd
VDD
bit’
bit
Gnd
bit’
VDD
word
word
VDD
72
6T Cell Full Layout
Transistor sizing
- M2 (pMOS) 4:3
- M1 (nMOS) 6:2
- M3 (nMOS) 4:2
M2
All boundaries
shared
38l H x 28l W
Reduced cap on bit
lines
M1
M3
73
6T Cell – Example Layout & Abutment
Vdd
Vdd
Vdd
T4
T3
T4
T2
T3
Vdd
Vss
T3
T4
T1 T1
Vss Vss
T5
T6
Vdd
T6
T5
T5
T6
T6
T2
Vss
Vss
T5
T1
T2
Vss Vss
T3
Vdd
T4
B’ B
T3
B’
B
Vss
Vdd
Vdd
T4
T2
T6 B
T1 T1
T5
T2
Vss
4 x 4 array
2 x 2 abutment
74
6T and 4T Cell Layouts
R1
BIT
T6
Vdd
GND
T4
VDD
T5
R2 BIT!
T3
Q
Q
T1
T4
T3T2
T1
Word
Line
T2
BL
GND
WL
BL
75
6T - 4T Cell Comparison
6T cell
- Merits
• Faster
• Better Noise Immunity
• Low standby current
- Demerits
• Large size due to 6
transistors
4T cell
- Merits
• Smaller cell, only 4
transistors
• HR Poly stacked above
transistors
- Demerits
• Additional process step
due to HR poly
• Poor noise immunity
• Large standby current
• Thermal instability
76
Row Decode
Transistor Level View of Core
Precharge
Column Decode
Sense Amp
77
SRAM, Putting it all together
2n rows, 2m * k columns
n + m address lines, k bits data width
78
Hierarchical Array Architecture
Subblocks
1 / output bit
Select 1 column / subblock
Row
Address
Column
Address
Block
Address
Global Data Bus
Control
Circuitry
Block Selector
Global
Amplifier/Driver
I/O
Advantages:
1. Shorter wires within blocks
2. Block address activates only 1 block => power savings
1 sense amp / subblock
79
Standalone SRAM Floorplan Example
80
Divided bit-line structure
81
SRAM Partitioning
Partitioned Bitline
82
SRAM Partitioning
Divided Wordline Arch
83
Partioning summary
Partioning involves a trade off between area,
power and speed
For high speed designs, use short blocks(e.g 64
rows x 128 columns )
- Keep local bitline heights small
For low power designs use tall narrow blocks (e.g
256 rows x 64 columns)
- Keep the number of columns same as the access
width to minimize wasted power
84
Redundancy
Row
Address
Redundant
rows
Fuse
:
Bank
Memory
Array
Column Decoder
Row Decoder
Redundant
columns
Column
Address
85
Periphery
• Decoders
• Sense Amplifiers
• Input/Output Buffers
• Control / Timing Circuitry
86
Asynchronous & Synchronous SRAMs
87
Address Transition Detection
Provides Clock for Asynch RAMs
VDD
A0
DELAY
td
A1
DELAY
td
AN-1
DELAY
td
ATD
ATD
...
88
Row Decoders
Collection of 2R complex logic gates
organized in a regular, dense fashion
(N)AND decoder 9->512
WL(0) /= !A8!A7!A6!A5!A4!A3!A2!A1!A0
…
WL(511) /= A8A7A6A5A4A3A2A1A0
NOR decoder 9->512
WL(0) = !(A8+A7+A6+A5+A4+A3+A2+A1+A0)
…
WL(511) = !(!A8+!A7+!A6+!A5+!A4+!A3+!A2+!A1+!A0)
89
A NAND decoder using 2-input pre-decoders
WL 1
WL 0
A0A1 A0 A1 A0 A1 A0A 1
A1 A 0
A0
A1
A 2A3 A2 A3 A2 A3 A2 A3
A3 A2
A2
A3
Splitting decoder into two or more logic layers
produces a faster and cheaper implementation
90
Row Decoders (cont’d)
A0/ A1/
A0 A1/
A0/ A1
A0 A1
A2/ A3/
A2 A3/
A2/ A3
A2 A3
R0/
R1/
R2/
… and so forth
A0
A1
A2
A3
91
Dynamic Decoders
Precharge devices
GND
GND
VDD
WL 3
WL 3
VDD
WL 2
VDD
WL 1
WL 2
WL 1
VDD
WL 0
VD D 
A0
A0
A1
A1
Dynamic 2-to-4 NOR decoder
WL 0
A0
A0
A1
A1

2-to-4 MOS dynamic NAND Decoder
Propagation delay is primary concern
92
Dynamic NOR Row Decoder
Vdd
WL0
WL1
WL2
WL3
A0
!A0
A1
!A1
Precharge/
93
Dynamic NAND Row Decoder
WL0
WL1
WL2
WL3
!A0
A0
!A1
A1
Precharge/
Back
94
Decoders
n:2n decoder consists of 2n n-input AND gates
- One needed for each row of memory
- Build AND from NAND or NOR gates
A1
Make devices on address line minimal size
Scale devices on decoder O/P to drive word lines
Static CMOS
Pseudo-nMOS
A0
A1
word0
1
1
8
4
word1
A1
1
word2
A0
1
word3
A0
word0
word
A0
1/2
4
16
A1
1
1
2
8
word
word1
word2
word3
95
Decoder Layout
Decoders must be pitch-matched to SRAM cell
- Requires very skinny gates
A3
A3
A2
A2
A1
A1
A0
A0
VDD
word
GND
NAND gate
buffer inverter
96
Large Decoders
For n > 4, NAND gates become slow
- Break large gates into multiple smaller gates
A3
A2
A1
A0
word0
word1
word2
word3
word15
97
Predecoding
- Group address bits in predecoder
- Saves area
- Same path effort
A3
A2
A1
A0
predecoders
1 of 4 hot
predecoded lines
word0
word1
word2
word3
word15
98
Column Circuitry
Some circuitry is required for each column
- Bitline conditioning
- Sense amplifiers
- Column multiplexing
Need hazard-free reading & writing of RAM cell
Column decoder drives a MUX – the two are
often merged
99
Typical Column Access
100
Pass Transistor Based Column
Decoder
A1
A0
2 input NOR decoder
BL3 !BL3
BL2 !BL2
BL1
!BL1
BL0 !BL0
S3
S2
S1
S0
Data
!Data

Advantage: speed since there is only one extra transistor in
the signal path

Disadvantage: large transistor count
101
Tree Decoder Mux
Column MUX can use pass transistors
- Use nMOS only, precharge outputs
One design is to use k series transistors for 2k:1
mux
- No external decoder logic needed
B0 B1
B2 B3
B4 B5
B6 B7
B0 B1
B2 B3
B4 B5
B6 B7
A0
A0
A1
A1
A2
A2
Y
to sense amps and write circuits
Y
102
Bitline Conditioning
Precharge bitlines high before reads

bit
bit_b
Equalize bitlines to minimize voltage difference
when using sense amplifiers

bit
bit_b
103
Bit Line Precharging
Static Pull-up Precharge
Clocked Precharge
clock
BL
!BL
BL
!BL
equalization transistor - speeds up
equalization of the two bit lines by
allowing the capacitance and pull-up
device of the nondischarged bit line to
assist in precharging the discharged line
104
Sense Amplifier: Why?
Bit line cap significant for large array
- If each cell contributes 2fF,
• for 256 cells, 512fF plus wire cap
- Pull-down resistance is about 15K
- RC = 7.5ns! (assuming DV = Vdd)
Cell pull down
Xtor
resistance
RCDV
t
Vdd
Cannot easily change R, C, or Vdd, but can
change DV i.e. smallest sensed voltage
Cell current
- Can reliably sense DV as small as <50mV
105
Sense Amplifiers
 DV
b
tp = C
---------------I cell
large
make D V as small
as possible
small
Idea: Use Sense Amplifer
small
transition
s.a.
input
output
106
Differential Sensing - SRAM
V DD
V DD
BL
PC
VDD
EQ
V DD
y M3
BL
M4
M1
x
SE
M2
y
x
x
x
M5
SE
WL i
(b) Doubled-ended Current Mirror Amplifier
V DD
SRAM cell i
Diff.
Sense
x
x
Amp
y
y
D
D
y
y
x
x
SE
(a) SRAM sensing scheme.
(c) Cross-Coupled Amplifier
107
Latch-Based Sense Amplifier
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.
108
Sense Amplifier
bit’
bit
word
sense
clk
isolation transistor
regenerative amplifier
109
Sense Amp Waveforms
1ns / div
bit
200mV
wordline
bit’
wordline
begin precharging bit lines
2.5V
BIT
BIT’
sense clk
sense clk
110
Write Driver Circuits
111
Twisted Bitlines
Sense amplifiers also amplify noise
- Coupling noise is severe in modern processes
- Try to couple equally onto bit and bit_b
- Done by twisting bitlines
b0 b0_b b1 b1_b b2 b2_b b3 b3_b
112
Transposed-Bitline Architecture
BL’
Ccross
BL
SA
BL
BL"
(a) Straightforward bitline routing.
BL’
BL
Ccross
SA
BL
BL"
(b) Transposed bitline architecture.
113
114
DRAM in a nutshell
Based on capacitive (non-regenerative) storage
Highest density (Gb/cm2)
Large external memory (Gb) or embedded DRAM
for image, graphics, multimedia…
Needs periodic refresh -> overhead, slower
115
116
Classical DRAM Organization
(square)
bit (data) lines
r
o
w
d
e
c
o
d
e
r
row
address
Each intersection represents
a 1-T DRAM Cell
RAM Cell
Array
word (row) select
Column Selector &
I/O Circuits
data
Column
Address
Row and Column Address
together:
- Select 1 bit a time
117
DRAM logical organization (4 Mbit)
118
DRAM physical organization (4 Mbit,x16)
119
Memory
Systems
address
n
DRAM
Controller
n/2
Memory
Timing
Controller
DRAM
2^n x 1
chip
w
Bus Drivers
Tc = Tcycle + Tcontroller + Tdriver
120
Logic Diagram of a Typical DRAM
RAS_L
A
9
CAS_L
WE_L
256K x 8
DRAM
OE_L
8
D
Control Signals (RAS_L, CAS_L, WE_L, OE_L) are all active
low
Din and Dout are combined (D):
- WE_L is asserted (Low), OE_L is disasserted (High)
•
D serves as the data input pin
- WE_L is disasserted (High), OE_L is asserted (Low)
•
D is the data output pin
Row and column addresses share the same pins (A)
- RAS_L goes low: Pins A are latched in as row address
- CAS_L goes low: Pins A are latched in as column address
- RAS/CAS edge-sensitive
121
DRAM Operations
Write
- Charge bitline HIGH or LOW and set wordline HIGH
Read
- Bit line is precharged to a voltage halfway
between HIGH and LOW, and then the
word line is set HIGH.
- Depending on the charge in the cap, the
precharged bitline is pulled slightly higher
or lower.
- Sense Amp Detects change
Explains why Cap can’t shrink
Word Line
C
.
.
.
Bit Line
Sense
Amp
- Need to sufficiently drive bitline
- Increase density => increase parasitic
capacitance
122
DRAM Access
1M DRAM = 1024 x 1024 array of bits
10 row address bits
arrive first
Row Access Strobe (RAS)
1024 bits
are read out
10 column address bits
arrive next
Subset of bits
returned to CPU
Column decoder
Column Access Strobe (CAS)
123
DRAM Read Timing
Every DRAM access begins at:
- The assertion of the RAS_L
- 2 ways to read:
RAS_L
CAS_L
A
early or late v. CAS
WE_L
256K x 8
DRAM
9
OE_L
D
8
DRAM Read Cycle Time
RAS_L
CAS_L
A
Row Address
Col Address
Junk
Row Address
Col Address
Junk
WE_L
OE_L
D
High Z
Junk
Data Out
Read Access
Time
Early Read Cycle: OE_L asserted before CAS_L
High Z
Data Out
Output Enable
Delay
Late Read Cycle: OE_L asserted after CAS_L
124
DRAM Write Timing
Every DRAM access begins at:
RAS_L
- The assertion of the RAS_L
- 2 ways to write:
early or late v. CAS
A
CAS_L
WE_L
256K x 8
DRAM
9
OE_L
D
8
DRAM WR Cycle Time
RAS_L
CAS_L
A
Row Address
Col Address
Junk
Row Address
Col Address
Junk
OE_L
WE_L
D
Junk
Data In
WR Access Time
Early Wr Cycle: WE_L asserted before CAS_L
Junk
Data In
Junk
WR Access Time
Late Wr Cycle: WE_L asserted after CAS_L
125
DRAM Performance
A 60 ns (tRAC) DRAM can
- perform a row access only every 110 ns (tRC)
- perform column access (tCAC) in 15 ns, but time between
column accesses is at least 35 ns (tPC).
• In practice, external address delays and turning around buses
make it 40 to 50 ns
These times do not include the time to drive the
addresses off the microprocessor nor the memory
controller overhead.
- Drive parallel DRAMs, external memory controller, bus to
turn around, SIMM module, pins…
- 180 ns to 250 ns latency from processor to memory is
good for a “60 ns” (tRAC) DRAM
126
1-Transistor Memory Cell (DRAM)
row select
Write:
- 1. Drive bit line
- 2.. Select row
Read:
- 1. Precharge bit line
- 2.. Select row
- 3. Cell and bit line share charges
•
bit
Very small voltage changes on the bit line
- 4. Sense (fancy sense amp)
•
Can detect changes of ~1 million electrons
- 5. Write: restore the value
Refresh
- 1. Just do a dummy read to every cell.
127
DRAM architecture
128
Cell read: refresh is the art
DV  V ' BL -VBL  (VSN
Cs
- VBL )
Cs  Cb
129
Sense Amplifier
130
131
DRAM technological requirements
Unlike SRAM : large Cb must be charged by small sense
FF. This is slow.
- Make Cb small: backbias junction cap., limit blocksize,
- Backbias generator required. Triple well.
Prevent threshold loss in wl pass: VG > Vccs+VTn
- Requires another voltage generator on chip
Requires VTnwl> Vtnlogic and thus thicker oxide than logic
- Better dynamic data retention as there is less
subthreshold loss.
- DRAM Process unlike Logic process!
Must create “large” Cs (10..30fF) in smallest possible area
- (-> 2 poly-> trench cap -> stacked cap)
132
Refreshing Overhead
Leakage :
- junction leakage exponential with temp!
- 2…5 msec @ 800 C
- Decreases noise margin, destroys info
All columns in a selected row are refreshed when read
- Count through all row addresses once per 3 msec.
(no write possible then)
Overhead @ 10nsec read time for 8192*8192=64Mb:
- 8192*1e-8/3e-3= 2.7%
Requires additional refresh counter and I/O control
133
Dummy cells
½ Bitline
½ Bitline
Vdd/2
Vdd/2
precharge
precharge
Wordline
134
Alternative Sensing Strategy
Decreasing Cdummy
Convert to differential sense
- Create a reference in an identical
structure
Data Col BL
Needs
- A method of generating ½ signal
swing of bit line
Operation:
- Dummy cell is ½ C
- active wordline and dummy
wordline on opposite sides of
sense amp.
- Amplify difference
Vdd
DV"1/ 0"   C
1  b Cs 2
Dummy Col BL’
BL’
BL
Dummy Col BL’
1
So Small Cs small swing, Large Cb small swing
Overhead of fabricating C/2
Data Col BL
135
Alternative Sensing Strategy
Increasing Cbitline on Dummy side
Double Bitline
Data
Dummy
SA outputs D and D pre-charged to VDD through Q1, Q2 (Pr=1)
•reference capacitor, Cdummy, connected to a pair of matched bit lines and is at 0V (Pr=0)
•parasitic cap Cp2 on BL’ is ~ 2 Cp1 on BL,
sets up a differential voltage LHS vs. RHS due to rise time difference
SA outputs (D, D) become charged, with a small difference LHS vs. RHS
Regenerative Action of Latch
136
DRAM Memory Systems
address
n
DRAM
Controller
n/2
Memory
Timing
Controller
DRAM
2^n x 1
chip
w
Bus Drivers
Tc = Tcycle + Tcontroller + Tdriver
137
DRAM Performance
Cycle Time
Access Time
Time
DRAM (Read/Write) Cycle
Time >> DRAM (Read/Write)
Access Time
- 2:1; why?
DRAM (Read/Write) Cycle
Time :
- How frequent can you initiate an
access?
DRAM (Read/Write) Access
Time:
- How quickly will you get what you
want once you initiate an access?
DRAM Bandwidth Limitation:
- Limited by Cycle Time
138
Fast Page Mode Operation
Fast Page Mode DRAM
Column
Address
N cols
- N x M “SRAM” to save a row
Row
Address
N rows
After a row is read into the
register
DRAM
- Only CAS is needed to access
other M-bit blocks on that row
N x M “SRAM”
- RAS_L remains asserted while
M-bit Output
CAS_L is toggled
1st M-bit Access
M bits
2nd M-bit
3rd M-bit
4th M-bit
Col Address
Col Address
Col Address
RAS_L
CAS_L
A
Row Address
Col Address
139
Page Mode DRAM Bandwidth Example
Page Mode DRAM Example:
- 16 bits x 1M DRAM chips (4 nos) in 64-bit module (8 MB module)
- 60 ns RAS+CAS access time; 25 ns CAS access time
- Latency to first access=60 ns Latency to subsequent accesses=25
ns
- 110 ns read/write cycle time; 40 ns page mode access time ; 256
words (64 bits each) per page
Bandwidth takes into account 110 ns first cycle, 40 ns for
CAS cycles
Bandwidth for one word = 8 bytes / 110 ns = 69.35 MB/sec
Bandwidth for two words = 16 bytes / (110+40 ns) = 101.73
MB/sec
Peak bandwidth = 8 bytes / 40 ns = 190.73 MB/sec
Maximum sustained bandwidth = (256 words * 8 bytes) / (
110ns + 256*40ns) = 188.71 MB/sec
140
4 Transistor Dynamic Memory
Remove the
PMOS/resistors from the
SRAM memory cell
Value stored on
the drain of M1 and M2
But it is held there only by
the capacitance on those
nodes
Leakage and soft-errors
may destroy value
141
142
First 1T DRAM (4K Density)
Texas Instruments
TMS4030 introduced
1973
NMOS, 1M1P, TTL I/O
1T Cell, Open Bit Line,
Differential Sense Amp
Vdd=12v, Vcc=5v,
Vbb=-3/-5v (Vss=0v)
143
16k DRAM (Double Poly Cell)
MostekMK4116,
introduced 1977
Address multiplex
Page mode
NMOS, 2P1M
Vdd=12v, Vcc=5v, Vbb=5v (Vss=0v)
Vdd-Vt precharge,
dynamic sensing
144
64K DRAM
Internal Vbbgenerator
Boosted Wordline and
Active Restore􀂄
- eliminate Vtloss for ‘1’
x4 pinout
145
256K DRAM
Folded bitline architecture
- Common mode noise to
coupling to B/Ls
- Easy Y-access
NMOS 2P1M
- poly 1 plate
- poly 2 (polycide) -gate, W/L
- metal -B/L
redundancy
146
1M DRAM
Triple poly Planar cell,
3P1M
-
poly1 -gate, W/L
poly2 –plate
poly3 (polycide) -B/L
metal -W/L strap
Vdd/2 bitline reference,
Vdd/2 cell plate
147
On-chip Voltage Generators
Power supplies
- for logic and memory
precharge voltage
- e.g VDD/2 for DRAM Bitline .
backgate bias
- reduce leakage
WL select overdrive
(DRAM)
148
Charge Pump Operating Principle
Vin
~ +Vin
+Vin
Charge Phase
Vin
dV
+Vin
Discharge Phase
dV
Vo
Vin = dV – Vin + dV +Vo
Vo = 2*Vin + 2*dV ~ 2*Vin
149
Voltage Booster for WL
Cf
CL
d
Vhi
Vhi
dV
Vcf(0) ~ Vhi
VGG=Vhi
+
VGG ~ Vhi + Vhi
CL
Cf
Vcf ~ Vhi
150
Backgate bias generation
Use charge pump
Backgate bias:
Increases Vt -> reduces leakage
• reduces Cj of nMOST when applied to p-well
(triple well process!),
151
smaller Cj -> smaller Cb → larger readout ΔV
Vdd / 2 Generation
2v
1v
1.5v
0.5v
~1v
0.5v
1
v
0.5v
1v
Vtn = |Vtp|~0.5v
uN = 2 uP
152
4M DRAM
3D stacked or trench cell
CMOS 4P1M
x16 introduced
Self Refresh
Build cell in vertical
dimension -shrink area
while maintaining 30fF cell
capacitance
153
154
Stacked-Capacitor Cells
Poly plate
Hitachi 64Mbit DRAM Cross Section
Samsung 64Mbit DRAM Cross Section
155
Evolution of DRAM cell structures
156
Buried Strap Trench Cell
157
Process Flow of
BEST Cell DRAM
· Array Buried N-Well
· Storage Trench Formation
· Node Dielectric (6nm TEQ.)
· Buried Strap Formation
· Shallow Trench Isolation
Formation
· N- and P-Well Implants
· Gate Oxidation (8nm)
· Gate Conductor (N+ poly /
WSi)
· Junction Implants
· Insulator Deposition and
Planarization
· Contact formation
· Bitline (Metal 0) formation
· Via 1 / Metal 1 formation
· Via 2 / Metal 2 formation
158
Shallow Trench Isolation -> Replaces LOCOS isolation -> saves area by eliminating Bird’s Beak
BEST cell Dimensions
Deep Trench etch with
very high aspect ratio
159
256K DRAM
Folded bitline architecture
- Common mode noise to
coupling to B/Ls
- Easy Y-access
NMOS 2P1M
- poly 1 plate
- poly 2 (polycide) -gate, W/L
- metal -B/L
redundancy
160
161
162
Transposed-Bitline Architecture
BL’
Ccross
BL
SA
BL
BL"
(a) Straightforward bitline routing.
BL’
BL
Ccross
SA
BL
BL"
(b) Transposed bitline architecture.
163
Cell Array and Circuits
1 Transistor 1 Capacitor Cell
- Array Example
Major Circuits
- ・Sense amplifier
- ・Dynamic Row Decoder
- ・Wordline Driver
Other interesting circuits ・
-
Data bus amplifier ・
Voltage Regulator
Reference generator
Redundancy technique
High speed I/O circuits
164
Standard DRAM Array Design Example
165
WL direction
Column predecode
(row)
64K cells
(256x256)
1M cells =
Global WL decode + drivers
64Kx16
Local WL
Decode
166
BL direction (col)
DRAM Array Example (cont’d)
2048
256x256
64
256
512K Array Nmat=16 ( 256 WL x 2048 SA)
Interleaved S／A & Hierarchical Row Decoder/Driver
(shared bit lines are not shown)
167
168
169
170
Standard DRAM Design Feature
・Heavy dependence on technology
・The row circuits are fully different from SRAM.
・Almost always analogue circuit design
・CAD:
- Spice-like circuits simulator
- Fully handcrafted layout
171