cse477 lecture8

8/6/2019 cse477 lecture8

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CSE477 L08 Capacitance.1 Irwin&Vijay, PSU, 2002

CSE477VLSI Digital Circuits

Fall 2002

Lecture 08: MOS & WireCapacitances

Mary Jane Irwin ( www.cse.psu.edu/~mji )www.cse.psu.edu/~cg477

[Adapted from Rabaeys Digital Integrated Circuits, 2002, J. Rabaey et al.]
http://www.cse.psu.edu/~mjihttp://www.cse.psu.edu/~cg477http://www.cse.psu.edu/~cg477http://www.cse.psu.edu/~mji


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Review: Delay Definitions

t

Vout

Vin

input

waveform

output

waveform

tp = (tpHL + tpLH)/2

Propagation delay

t

50%

tpHL

50%

tpLH

tf

90%

10%

tr

signal slopes

Vin Vout


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CMOS Inverter: Dynamic

VDD

Rn

Vout = 0

Vin = VDD

CL tpHL = f(Rn, CL)

Todays focus

Next lectures focus

Transient, ordynamic, response determines themaximum speed at which a device can be operated.


4/34


Sources of Capacitance

Cw

CDB2

CDB1

CGD12

CG4

CG3

wiring (interconnect) capacitance

intrinsic MOS transistor capacitances

Vout2Vin

extrinsic MOS transistor (fanout) capacitances

Vout

VoutVin

M2

M1

M4

M3

Vout2

CL


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MOS Intrinsic Capacitances

Structure capacitances

Channel capacitances

Depletion regions of the reverse-biasedpn-junctions of the drain andsource


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MOS Structure Capacitances

xdSource

n+

Drain

n+W

Ldrawn

xd

Poly Gate

n+n+

tox

Leff

Top view

lateral diffusion

CGSO = CGDO = Cox xd W = Co W

Overlap capacitance (linear)


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MOS Channel Capacitances

SD

p substrate

B

GVGS +

-

n+n+

depletion

regionn channel

CGS = CGCS + CGSO CGD = CGCD + CGDO

CGB = CGCB

The gate-to-channel capacitance depends uponthe operating region and the terminal voltages



Review: Summary of MOS Operating Regions

Cutoff (really subthreshold) VGS VTq Exponential in V

GSwith linearV

DSdependence

ID = IS e(qV

GS/nkT) (1 - e -(qV DS /kT) ) (1 - VDS ) where n 1

Strong Inversion VGS >VT

q Linear (Resistive) VDS



Average Distribution of Channel Capacitance

Operation

Region

CGCB CGCS CGCD CGC CG

Cutoff Cox WL 0 0 Cox WL Cox WL + 2CoW

Resistive 0 Cox WL/2 Cox WL/2 Cox WL Cox WL + 2CoW

Saturation 0 (2/3)Cox WL 0 (2/3)Cox WL (2/3)Cox WL +2CoW

q

Channel capacitance components are nonlinear andvary with operating voltage

q Most important regions are cutoff and saturation

since that is where the device spends most of its time



MOS Diffusion Capacitances

S D

p substrate

B

GVGS +

-

n+n+

depletionregionn channel

CSB = CSdiff CDB = CDdiff

The junction (or diffusion) capacitance is from thereverse-biased source-body and drain-body pn-junctions.



Source Junction View

side walls

channel

W

xj

channel-stop

implant (NA+)

source

bottom plate

(ND)

LS

substrate (NA)

Cdiff = Cbp + Csw = CjAREA + Cjsw PERIMETER

= Cj LS W + Cjsw (2LS + W)

junctiondepth



Review: Reverse Bias Diode

All diodes in MOS digital circuits are reverse biased;the dynamic response of the diode is

determined by depletion-region charge orjunction capacitance

Cj = Cj0/((1 VD)/ 0)m

where Cj0 is the capacitance under zero-bias conditions (afunction of physical parameters), 0 is the built-in potential

(a function of physical parameters and temperature)

and m is the grading coefficient

q m = for an abruptjunction (transition from n to p-material isinstantaneous)

q m = 1/3 for a linear(or graded) junction (transition is gradual)

Nonlinear dependence (that decreases with increasingreverse bias)

+

-

VD



Junction Capacitance


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Reverse-Bias Diode Junction Capacitance

0

0 . 5

1

1 . 5

2

- 5 - 4 - 3 - 2 - 1 0 1

VD (V)

Cj(

fF)

linear (m=1/3)

abrupt (m=1/2)

Cj0


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MOS Capacitance Model

CGS

CSB CDB

CGD

CGB

S

G

B

D

CGS = CGCS + CGSO CGD = CGCD + CGDO

CGB = CGCB

CSB = CSdiff CDB = CDdiff


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Transistor Capacitance Values for 0.25

Cox

(fF/ m2)

Co

(fF/ m)

Cj

(fF/ m2)

mj b

(V)

Cjsw

(fF/ m)

mjsw bsw

(V)

NMOS 6 0.31 2 0.5 0.9 0.28 0.44 0.9

PMOS 6 0.27 1.9 0.48 0.9 0.22 0.32 0.9

Example: For an NMOS with L = 0.24 m, W = 0.36 m,LD = LS = 0.625 m

CGC = Cox WL =

CGSO = CGDO = Cox xd W = Co W =

so Cgate_cap = CoxWL + 2CoW =

Cbp = Cj LS W =

Csw = Cjsw (2LS + W) =

so Cdiffusion_cap =


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Transistor Capacitance Values for 0.25

Cox

(fF/ m2)

Co

(fF/ m)

Cj

(fF/ m2)

mj b

(V)

Cjsw

(fF/ m)

mjsw bsw

(V)

NMOS 6 0.31 2 0.5 0.9 0.28 0.44 0.9

PMOS 6 0.27 1.9 0.48 0.9 0.22 0.32 0.9

Example: For an NMOS with L = 0.24 m, W = 0.36 m,LD = LS = 0.625 m

CGC = Cox WL = 0.52 fF

CGSO = CGDO = Cox xd W = Co W = 0.11 fF

so Cgate_cap = CoxWL + 2CoW = 0.74 fF

Cbp = Cj LS W = 0.45 fF

Csw = Cjsw (2LS + W) = 0.45 fF

so Cdiffusion_cap = 0.90 fF


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Review: Sources of Capacitance

Vout

Cw

VinCDB2

CDB1

CGD12

M2

M1

M4

M3

Vout2

CG4

CG3

wiring (interconnect) capacitance

intrinsic MOS transistor capacitances

Vout2Vin

extrinsic MOS transistor (fanout) capacitances

Vout

CL

ndrain

pdrain


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Gate-Drain Capacitance: The Miller Effect

A capacitor experiencing identical but opposite voltageswings at both its terminals can be replaced by acapacitor to ground whose value is two times the originalvalue

Vin

CGD1

M1

Vout V

V

VinM1

Vout

V

V

2CGB1

M1 and M2 are either in cut-off or in saturation.

The floating gate-drain capacitor is replaced by acapacitance-to-ground (gate-bulk capacitor).


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Drain-Bulk Capacitance: Keq s (for 2.5 m)

high-to-low low-to-high

Keqbp Keqsw Keqbp Keqsw

NMOS 0.57 0.61 0.79 0.81PMOS 0.79 0.86 0.59 0.7

We can simplify the diffusion capacitance calculations

even further by using a Keq to relate the linearizedcapacitor to the value of the junction capacitance underzero-bias

Ceq = Keq Cj0


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Extrinsic (Fan-Out) Capacitance

The extrinsic, or fan-out, capacitance is the total gate capacitanceof the loading gates M3 and M4.

Cfan-out = Cgate (NMOS) + Cgate (PMOS)

= (CGSOn + CGDOn + WnLnCox ) + (CGSOp + CGDOp + WpLpCox )

Simplification of the actual situation

q Assumes all the components of Cgate are between Vout and GND (orVDD )

q

Assumes the channel capacitances of the loading gates are constant


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Layout of Two Chained Inverters

InOut

Metal1

VDD

GND

1.2 m=2

1.125/0.25

0.375/0.25

PMOS

NMOS

Polysilicon

W/L AD ( m2) PD ( m) AS ( m2) PS ( m)NMOS 0.375/0.25 0.3 1.875 0.3 1.875

PMOS 1.125/0.25 0.7 2.375 0.7 2.375

0.125 0.5


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Components of CL (0.25 m)C Term Expression Value (fF)

HLValue (fF)

LHCGD1 2 Con Wn 0.23 0.23

CGD2 2 Cop Wp 0.61 0.61

CDB1 Keqbpn ADnCj + Keqswn PDnCjsw 0.66 0.90

CDB2 Keqbpp ADpCj + Keqswp PDpCjsw 1.5 1.15

CG3 (2 Con )Wn + Cox WnLn 0.76 0.76

CG4 (2 Cop )Wp + Cox WpLp 2.28 2.28

Cw from extraction 0.12 0.12CL 6.1 6.0


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Wiring Capacitance

The wiring capacitance depends upon the length andwidth of the connecting wires and is a function of thefan-out from the driving gate and the number of fan-outgates.

Wiring capacitance is growing in importance with thescaling of technology.


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Parallel Plate Wiring Capacitance

electrical field lines

W

H

tdi dielectric (SiO2)

substrate

Cpp = ( di /tdi )

WL

current flow

permittivity

constant

(SiO2= 3.9)

L


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Permittivity Values of Some Dielectrics

Material di

Free space 1Teflon AF 2.1

Aromatic thermosets (SiLK) 2.6 2.8

Polyimides (organic) 3.1 3.4

Fluorosilicate glass (FSG) 3.2 4.0

Silicon dioxide 3.9 4.5

Glass epoxy (PCBs) 5

Silicon nitride 7.5

Alumina (package) 9.5

Silicon 11.7


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Sources of Interwire Capacitance

interwire

fringe

pp

Cwire = Cpp + Cfringe + Cinterwire

= ( di /tdi )WL

+ (2 di )/log(tdi /H)

+ ( di /tdi )HL


28/34


Impact of Fringe Capacitance

(from [Bakoglu89])

H/tdi = 1

H/tdi = 0.5

Cpp

W/tdi


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Impact of Interwire Capacitance

(from [Bakoglu89])


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Insights

For W/H < 1.5, the fringe component dominates theparallel-plate component. Fringing capacitance can

increase the overall capacitance by a factor of 10 or more.

When W < 1.75H interwire capacitance starts to dominate

Interwire capacitance is more pronounced for wires in the

higher interconnect layers (further from the substrate) Rules of thumb

q Never run wires in diffusion

q Use poly only for short runs

q Shorter wires lower R and C

q Thinner wires lower C but higher R

Wire delay nearly proportional to L2


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Wiring Capacitances

Field Active Poly Al1 Al2 Al3 Al4

Poly 88

54

Al1 30 41 57

40 47 54

Al2 13 15 17 36

25 27 29 45

Al3 8.9 9.4 10 15 41

18 19 20 27 49

Al4 6.5 6.8 7 8.9 15 35

14 15 15 18 27 45

Al5 5.2 5.4 5.4 6.6 9.1 14 3812 12 12 14 19 27 52

fringe in aF/ m

pp in aF/ m2

Poly Al1 Al2 Al3 Al4 Al5

Interwire Cap 40 95 85 85 85 115

per unit wire length in aF/ m for minimally-spaced wires


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Dealing with Capacitance

Low capacitance (low-k) dielectrics (insulators) such as

polymide or even air instead of SiO2q family of materials that are low-k dielectrics

q must also be suitable thermally and mechanically and

q compatible with (copper) interconnect

Copper interconnect allows wires to be thinner withoutincreasing their resistance, thereby decreasing interwirecapacitance

SOI (silicon on insulator) to reduce junction capacitance


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Next Time: Dealing with Resistance

MOS structure resistance - Ron

Wiring resistance

Contact resistance


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Next Lecture and Reminders Next lecture

q MOS resistance

- Reading assignment Rabaey, et al, 4.3.2, 4.4.1-4.4.4

Reminders

q Lecture lectures 9+10 will be combined on the 26th

q HW2 due today

q Project specifications dues October 3rd

q HW3 due Oct 10th

q Evening midterm exam scheduled

- Wednesday, October 16th from 8:15 to 10:15pm in 260 Willard

- Only one midterm conflict filed for so far

cse477 lecture8

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