© Digital Integrated Circuits 2nd Inverter Impact of Interconnect Interconnection Fundamental...

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© Digital Integrated Circuits2nd Inverter

Impact of InterconnectImpact of Interconnect

Interconnection Fundamental limitation of Digital

Technology at all scales Classes of parasitics:

– Capacitive– Resistive– Inductive (Impact usually package/board level)

© Digital Integrated Circuits2nd Inverter

Interconnect Impact on ChipInterconnect Impact on Chip

© Digital Integrated Circuits2nd Inverter

10 100 1,000 10,000 100,000

Length (u)

No

of

ne

ts(L

og

Sc

ale

)

Pentium Pro (R)

Pentium(R) II

Pentium (MMX)

Pentium (R)

Pentium (R) II

Nature of InterconnectNature of Interconnect

Local Interconnect

Global Interconnect

SLocal = STechnology

SGlobal = SDie

So

urc

e:

Inte

l

© Digital Integrated Circuits2nd Inverter

INTERCONNECTINTERCONNECTCapacitanceCapacitance

© Digital Integrated Circuits2nd Inverter

Capacitance of Wire InterconnectCapacitance of Wire Interconnect

VDD VDD

Vin Vout

M1

M2

M3

M4Cdb2

Cdb1

Cgd12

Cw

Cg4

Cg3

Vout2

Fanout

Interconnect

VoutVin

CLSimplified

Model

© Digital Integrated Circuits2nd Inverter

Capacitance: The Parallel Plate ModelCapacitance: The Parallel Plate Model

Dielectric

Substrate

L

W

H

tdi

Electrical-field lines

Current flow

WLt

cdi

diint

LLCwire SSS

SS

1

© Digital Integrated Circuits2nd Inverter

PermittivityPermittivity

© Digital Integrated Circuits2nd Inverter

Fringing CapacitanceFringing Capacitance

W - H/2H

+

(a)

(b)

© Digital Integrated Circuits2nd Inverter

Fringing versus Parallel PlateFringing versus Parallel Plate

(from [Bakoglu89])

© Digital Integrated Circuits2nd Inverter

Interwire CapacitanceInterwire Capacitance

fringing parallel

© Digital Integrated Circuits2nd Inverter

Impact of Interwire CapacitanceImpact of Interwire Capacitance

(from [Bakoglu89])

© Digital Integrated Circuits2nd Inverter

Wiring Capacitances (0.5 Wiring Capacitances (0.5 m CMOS)m CMOS)

Layer N+ P+ Poly Poly2 M1 M2 M3

Sub 420 730 87 -- 32 16 10 aF/m2

Ndif 2450 aF/m2

Pdif 2360 aF/m2

Poly 860 57 16 9 aF/m2

Poly2 52 aF/m2

M1 31 13 aF/m2

M2 32 aF/m2

Sub(fr) 310 250 76 59 39 aF/m

Poly(fr) 61 38 28 aF/m

M1(fr) 51 33 aF/m

M2(fr) 52 aF/m

© Digital Integrated Circuits2nd Inverter

INTERCONNECTINTERCONNECTResistanceResistance

© Digital Integrated Circuits2nd Inverter

Wire Resistance Wire Resistance

W

L

H

R = H W

L

Sheet ResistanceRo

R1 R2

© Digital Integrated Circuits2nd Inverter

Interconnect Resistance Interconnect Resistance

© Digital Integrated Circuits2nd Inverter

Polycide Gate MOSFETPolycide Gate MOSFET

n+n+

SiO2

PolySilicon

Silicide

p

Silicides: WSi 2, TiSi2, PtSi2 and TaSi

Conductivity: 8-10 times better than Poly

© Digital Integrated Circuits2nd Inverter

Sheet ResistanceSheet Resistance

© Digital Integrated Circuits2nd Inverter

Example: Intel 0.25 micron ProcessExample: Intel 0.25 micron Process

5 metal layersTi/Al - Cu/Ti/TiNPolysilicon dielectric

© Digital Integrated Circuits2nd Inverter

InterconnectInterconnectModelingModeling

© Digital Integrated Circuits2nd Inverter

The Lumped ModelThe Lumped Model

Vout

Driver

cwire

VinClumpe d

RdriverVout

© Digital Integrated Circuits2nd Inverter

The Lumped RC-ModelThe Lumped RC-ModelThe Elmore DelayThe Elmore Delay

© Digital Integrated Circuits2nd Inverter

The Ellmore DelayThe Ellmore DelayRC ChainRC Chain

© Digital Integrated Circuits2nd Inverter

Wire ModelWire Model

Assume: Wire modeled by N equal-length segments

For large values of N:

© Digital Integrated Circuits2nd Inverter

The Distributed RC-lineThe Distributed RC-line

© Digital Integrated Circuits2nd Inverter

Step-response of RC wire as a Step-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

© Digital Integrated Circuits2nd Inverter

RC-ModelsRC-Models

© Digital Integrated Circuits2nd Inverter

Driving an RC-lineDriving an RC-line

Vi n

Rs Vo ut(rw,cw,L)

© Digital Integrated Circuits2nd Inverter

Design Rules of ThumbDesign Rules of Thumb

rc delays should only be considered when tpRC >> tpgate of the driving gate

Lcrit >> tpgate/0.38rc rc delays should only be considered when the

rise (fall) time at the line input is smaller than RC, the rise (fall) time of the line

trise < RC when not met, the change in the signal is slower

than the propagation delay of the wire

© MJIrwin, PSU, 2000

© Digital Integrated Circuits2nd Inverter

Homework 4Homework 41. For the AMIS 0.5um technology, create an equivalent RC/Elmore SUE model

using the Mosis parametric test results (amis05.txt from web site). This model should include the effective gate capacitance, source and drain parasitic junction capacitances and equivalent resistance for NMOS and PMOS, L=0.5um as a function of W in um. Use this model to estimate the sizes of the transistors in the ring oscillator test for the standard and wide case from the given performance data. (Do this carefully, this is a useful model!!)

2. Rabaey Chap. 4 on-line problems: 1, 4, 7, 12

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