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© Digital Integrated Circuits2nd Wires
The WireThe Wire
Digital Integrated CircuitsDigital Integrated CircuitsA Design PerspectiveA Design PerspectiveJan M. Rabaey et al.
Adapted from Chapter 4 of
Copyright 2003 Prentice Hall/Pearson
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© Digital Integrated Circuits2nd Wires
The WireThe Wire
transmitters receivers
schematics physical
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© Digital Integrated Circuits2nd Wires
Interconnect Impact on ChipInterconnect Impact on Chip
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© Digital Integrated Circuits2nd Wires
Wire ModelsWire Models
All-inclusive model Capacitance-only
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© Digital Integrated Circuits2nd Wires
Impact of Interconnect Impact of Interconnect ParasiticsParasitics
Interconnect parasiticsreduce reliabilityaffect performance and power consumption
Classes of parasiticsCapacitiveResistiveInductive
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© Digital Integrated Circuits2nd Wires
10 100 1,000 10,000 100,000Length (u)
No
of n
ets
(Log
Sca
le)
Pentium Pro (R)Pentium(R) IIPentium (MMX)Pentium (R)Pentium (R) II
Nature of InterconnectNature of Interconnect
Local Interconnect
Global Interconnect
SLocal = STechnologySGlobal = SDie
Sour
ce: I
ntel
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© Digital Integrated Circuits2nd Wires
INTERCONNECTINTERCONNECT
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© Digital Integrated Circuits2nd Wires
Capacitance of Wire InterconnectCapacitance of Wire Interconnect
VDD VDD
VinVout
M1
M2
M3
M4Cdb2
Cdb1
Cgd12
Cw
Cg4
Cg3
Vout2
Fanout
Interconnect
VoutVin
CLSimplified
Model
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© Digital Integrated Circuits2nd Wires
Capacitance: The Parallel Plate ModelCapacitance: The Parallel Plate Model
Dielectric
Substrate
L
W
H
tdi
Electrical-field lines
Current flow
WLt
cdi
diint
ε=
LLCwire SSS
SS 1=
⋅=
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© Digital Integrated Circuits2nd Wires
PermittivityPermittivity
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© Digital Integrated Circuits2nd Wires
Fringing CapacitanceFringing Capacitance
W - H/2H
+
(a)
(b)
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© Digital Integrated Circuits2nd Wires
Fringing versus Parallel PlateFringing versus Parallel Plate
(from [Bakoglu89])
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© Digital Integrated Circuits2nd Wires
InterwireInterwire CapacitanceCapacitance
fringing parallel
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© Digital Integrated Circuits2nd Wires
Impact of Impact of InterwireInterwire CapacitanceCapacitance
(from [Bakoglu89])
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© Digital Integrated Circuits2nd Wires
Wiring Capacitances (0.25 Wiring Capacitances (0.25 µµm CMOS)m CMOS)
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© Digital Integrated Circuits2nd Wires
INTERCONNECTINTERCONNECT
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© Digital Integrated Circuits2nd Wires
Wire Resistance Wire Resistance
W
LH
R = ρH W
L
Sheet ResistanceRo
R1 R2
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© Digital Integrated Circuits2nd Wires
Interconnect Resistance Interconnect Resistance
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Dealing with ResistanceDealing with Resistance
Selective Technology ScalingUse Better Interconnect Materials
reduce average wire-lengthe.g. copper, silicides
More Interconnect Layersreduce average wire-length
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PolycidePolycide Gate MOSFETGate MOSFET
n+n+
SiO2
PolySilicon
Silicide
p
Silicides: WSi 2, TiSi2, PtSi2 and TaSi
Conductivity: 8-10 times better than Poly
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Sheet ResistanceSheet Resistance
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Modern InterconnectModern Interconnect
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© Digital Integrated Circuits2nd Wires
Example: Intel 0.25 micron ProcessExample: Intel 0.25 micron Process
5 metal layersTi/Al - Cu/Ti/TiNPolysilicon dielectric
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InterconnectInterconnectModelingModeling
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© Digital Integrated Circuits2nd Wires
The Lumped ModelThe Lumped Model
Vout
Drivercwire
VinClumped
Rdriver Vout
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© Digital Integrated Circuits2nd Wires
The Lumped RCThe Lumped RC--ModelModelThe Elmore DelayThe Elmore Delay
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© Digital Integrated Circuits2nd Wires
The The EllmoreEllmore DelayDelayRC ChainRC Chain
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Wire ModelWire Model
Assume: Wire modeled by N equal-length segments
For large values of N:
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The Distributed RCThe Distributed RC--lineline
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StepStep--response of RC wire as a response of RC wire as a function of time and spacefunction of time and space
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
0.5
1
1.5
2
2.5
time (nsec)
volta
ge (
V)
x= L/10
x = L/4
x = L/2
x= L
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RCRC--ModelsModels
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Driving an RCDriving an RC--lineline
Vin
Rs Vout(rw,cw,L)
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Design Rules of ThumbDesign Rules of Thumb
rc delays should only be considered when tpRC >> tpgate of the driving gate
Lcrit >> √ tpgate/0.38rcIt is not the case for local circuitsInterconnect delay becomes important for lone wires that can happened in data buses and clock networks
© MJIrwin, PSU, 2000