VLSI Design and Simulation - School of Engineeringchandy/courses/249s05/249ln05.pdf · ECE 249 VLSI...

VLSI Design and SimulationVLSI Design and Simulation

Lecture Lecture 55

Performance CharacterizationPerformance Characterization

ECE 249 VLSI Design and Simulation

Spring 2005

Lecture 5

Dept. of Electrical and Computer Engineering

University of Connecticut

TopicsTopics

•• PerformancePerformance Characterization Characterization

–– Resistance EstimationResistance Estimation

–– Capacitance EstimationCapacitance Estimation

–– Inductance EstimationInductance Estimation

Spring 2005

Lecture 5

•• Inverter Voltage Transfer CurveInverter Voltage Transfer Curve

Spring 2005

Lecture 5

•• Voltage versus Time curve (ideal)Voltage versus Time curve (ideal)

Spring 2005

Lecture 5

•• Gate delayGate delay

•• Voltage Voltage versus Time curveversus Time curve

Spring 2005

Lecture 5

•• Interconnect delayInterconnect delay

Spring 2005

Lecture 5

•• DelayDelay–– Primary determinant of the speed of a circuitPrimary determinant of the speed of a circuit

–– Due to resistances and capacitancesDue to resistances and capacitances•• Intrinsic resistance and capacitanceIntrinsic resistance and capacitance

•• Extrinsic resistance and capacitanceExtrinsic resistance and capacitance

Spring 2005

Lecture 5

Resistance EstimationResistance Estimation

•• Dependent on Dependent on resistivityresistivity of material of material

•• Directly proportional to lengthDirectly proportional to length

•• Inversely proportional to cross-sectionalInversely proportional to cross-sectionalareaarea

R = ρlA

Spring 2005

Lecture 5

R = ρLA

= RSLW

Spring 2005

Lecture 5

•• RRSS is the sheet resistance expressed in is the sheet resistance expressed interms of terms of ΩΩ// ��

Spring 2005

Lecture 5

50-15050-150DiffusionDiffusion

150-200150-200PolysiliconPolysilicon

00..005-0.15-0.1Top metalTop metal

Typical Resistance (Typical Resistance (ΩΩ// �� ))Interconnect MaterialInterconnect Material

Spring 2005

Lecture 5

Resistance EstimationResistance Estimation•• Intrinsic resistanceIntrinsic resistance

•• In linear regionIn linear region

IDS = k VGS −VT( )VDS −VDS2

Req =1

k VGS −VT( )=

µCoxWL

VGS −VT( )=

1µCox VGS −VT( )

µCox VGS −VT( )

Spring 2005

Lecture 5

•• Intrinsic resistanceIntrinsic resistance

–– Dependent on CDependent on Coxox and carrier mobility and carrier mobility

–– Temperature variantTemperature variant

–– Typically 1000-30000 Typically 1000-30000 ΩΩ// ��

Spring 2005

Lecture 5

Capacitance EstimationCapacitance Estimation

•• Capacitance in concert with interconnectCapacitance in concert with interconnectresistance is the primary determinant ofresistance is the primary determinant ofinterconnect delaysinterconnect delays

•• Intrinsic capacitanceIntrinsic capacitance

•• Interconnect capacitancesInterconnect capacitances

Spring 2005

Lecture 5

MOS device capacitancesMOS device capacitances

•• Overlap related capacitanceOverlap related capacitance

•• Channel related capacitancesChannel related capacitances

–– Dependent on region of operationDependent on region of operation

•• Diffusion to substrate capacitancesDiffusion to substrate capacitances

Spring 2005

Lecture 5

Source

CGCDCGCS

Spring 2005

Lecture 5

CGCBCSB

Spring 2005

Lecture 5

•• Overlap related capacitanceOverlap related capacitance

•• Usually can be ignored since Usually can be ignored since xxDD is very is verysmallsmall€

CGSO = CDSO =εoxtox

Aoverlap = CoxxDW

Spring 2005

Lecture 5

•• Channel related capacitancesChannel related capacitances–– CutoffCutoff

•• No channelNo channel

•• Therefore, no gate to source or drain capacitancesTherefore, no gate to source or drain capacitances

Spring 2005

Lecture 5

Source

CGCB =C0Cdep

C0 + Cdep

C0 = CoxWL

Cdep =εsilicond

Spring 2005

Lecture 5

•• Channel related capacitancesChannel related capacitances–– CutoffCutoff

•• No channelNo channel

•• Therefore, no gate to source or drain capacitancesTherefore, no gate to source or drain capacitances

–– As gate voltage increases, depletion region deepens,As gate voltage increases, depletion region deepens,causing causing CCdepdep to decrease, and thus decrease the to decrease, and thus decrease thegate to body capacitancegate to body capacitance

–– As gate voltage nears VAs gate voltage nears VTT, inversion channel forms, inversion channel formscausing a barrier for the gate to body capacitancecausing a barrier for the gate to body capacitance

Spring 2005

Lecture 5

MOS device capacitancesMOS device capacitancesCgb

C0Cdep

C0 + Cdep

Vt Vgs

Spring 2005

Lecture 5

•• Channel related capacitancesChannel related capacitances–– SaturationSaturation

•• Channel is pinched offChannel is pinched off

•• Gate to source capacitance existsGate to source capacitance exists

•• Gate to drain capacitance is zeroGate to drain capacitance is zero

CGCB =23CoxWL

CGCD = 0

Spring 2005

Lecture 5

•• Channel related capacitancesChannel related capacitances–– LinearLinear

•• Channel is formedChannel is formed

•• Therefore, no gate to body capacitanceTherefore, no gate to body capacitance

CGCS = CGCD =CoxWL2

Spring 2005

Lecture 5

C0Cdep

C0 + Cdep

Vt Vgs

VDS + Vt

Spring 2005

Lecture 5

C0Cdep

C0 + Cdep

Vt Vgs

VDS + Vt

Spring 2005

Lecture 5

MOS device capacitanceMOS device capacitance

•• Channel related capacitanceChannel related capacitance

•• Worst caseWorst case

•• CCoxox ranges from 1.7 ranges from 1.7-6-6 fFfF//µµmm22

•• For a 1.5For a 1.5µµ by by 11.5.5µµ channel channel€

Cg = CoxWL

Cg = (6)(1.5)(1.5)=13.5 fF

Spring 2005

Lecture 5

•• Diffusion to substrate capacitanceDiffusion to substrate capacitance

•• Junction capacitanceJunction capacitance

•• CCjj is the is the bottom-platebottom-plate capacitance per area capacitance per area

Cdiff = C jLSW

Spring 2005

Lecture 5

•• Side wall or peripherySide wall or periphery capacitance (drain and source capacitance (drain and sourcesidewalls)sidewalls)

•• CCjswjsw is the is the side wallside wall capacitance per linear distance capacitance per linear distance

Cdiff = C jsw(2LS +W )

Spring 2005

Lecture 5

•• CCjj is typically is typically .5-2.5-2 fFfF//µµmm22

•• CCjswjsw is typically is typically .28-.4.28-.4 fFfF//µµmm

•• For a 1.5For a 1.5µµ by 1.5 by 1.5µµ diffusion region diffusion region

Cdiff = C jLSW + C jsw(2LS +W )= 2(1.5)(1.5) + .28(3.0 +1.5)= 5.8 fF

Spring 2005

Lecture 5

Interconnect capacitancesInterconnect capacitances

Cplate =εditdiWL

Spring 2005

Lecture 5

•• When h is comparable in magnitude to t,When h is comparable in magnitude to t,fringing electric fields can increase the totalfringing electric fields can increase the totaleffective parasitic capacitanceeffective parasitic capacitance

•• The effect is magnified as the ratio of w to hThe effect is magnified as the ratio of w to hdecreasesdecreases

•• If w=h, the effective capacitance can be up toIf w=h, the effective capacitance can be up to10 times 10 times CCplateplate

Spring 2005

Lecture 5

Cross-InterconnectCross-Interconnectcapacitancescapacitances

•• Can be very difficult to computeCan be very difficult to compute

•• Requires three dimensional field simulationsRequires three dimensional field simulations

•• Usually provided by process measurementsUsually provided by process measurements

Spring 2005

Lecture 5

Cross InterconnectCross InterconnectCapacitancesCapacitances

Perimeter (Perimeter (fFfF//µµmm))Area (Area (fFfF//µµmm2)2)..2525µµm processm process

..002929..001717Metal2 over polyMetal2 over poly

..050544..050577Metal1 over polyMetal1 over poly

..002525..010133Metal2 over oxideMetal2 over oxide

..004545..030366Metal2 over Metal1Metal2 over Metal1

..040400.030.030Metal1 over oxideMetal1 over oxide

..005454..008888Poly over oxidePoly over oxide

Spring 2005

Lecture 5

•• Can dominate the effect of the gateCan dominate the effect of the gatecapacitancecapacitance

•• Example: 100Example: 100µµm metal1 line over oxidem metal1 line over oxide–– AreaArea capacitance: 100 capacitance: 100µµm x 1m x 1µµm x .030fF/m x .030fF/µµmm22 = 3 fF = 3 fF

–– Fringing capacitance: 100Fringing capacitance: 100µµm x 2 x .m x 2 x .040fF040fF//µµm = 8m = 8.0.0 fFfF

–– Total capacitance: Total capacitance: 1111 fF fF

Spring 2005

Lecture 5

InductanceInductance

•• For the most part is not an issueFor the most part is not an issue

•• Wire inductance is on the order of 10s of pH perWire inductance is on the order of 10s of pH permmmm

•• Small enough to ignore except for very highSmall enough to ignore except for very highperformance chipsperformance chips

•• Inductance is usually higher for I/O interfacesInductance is usually higher for I/O interfaces

Spring 2005

Lecture 5

Delay DefinitionsDelay Definitions

(VOH+ VOL)/2

Spring 2005

Lecture 5

Interconnect delayInterconnect delay

•Lumped RC modelVin Vout

•Charge Vin to VDD

•The transient output voltage is

Vout (t) =VDD 1− e−

2=VDD 1− e

−tdlhRC

tdlhRC

= −ln 12

tdlh ≈ .69RC

Spring 2005

Lecture 5

Interconnect delayInterconnect delay

•Distributed RC ladder model

Vin Vout

C/N C/N C/N C/N

R/N R/N R/N R/N

•More accurate than lumped RC model

•More difficult to solve for large N

•Need full-scale SPICE simulation

Spring 2005

Lecture 5

Elmore DelayElmore Delay

•• Single line model not useful forSingle line model not useful forgeneralized RC tree networksgeneralized RC tree networks

C1 C2 C3

R1 R2 R3

Spring 2005

Lecture 5

Next classNext class

•• More Performance CharacterizationMore Performance Characterization

VLSI Design and Simulation - School of Engineeringchandy/courses/249s05/249ln05.pdf · ECE 249 VLSI...

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