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EE 587 SoC Design & Test. Partha Pande School of EECS Washington State University [email protected] SoC Physical Design Issues Wire Inductance. Wire Inductance. Wide wires in clock distribution & upper level metal layers These wires have low resistance

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Ee 587 soc design test
EE 587SoC Design & Test

Partha Pande

School of EECS

Washington State University

[email protected]

Wire inductance
Wire Inductance

  • Wide wires in clock distribution & upper level metal layers

  • These wires have low resistance

  • Exhibit significant inductive effects

  • New materials with low-resistance interconnect


  • Complete interconnect model should include inductance

  • With increasing frequency and a decrease in resistance due to wide wires and the use of copper, inductance will begin to influence clocks/busses:

    Z = R + jL

  • Inductance, by definition, is for a loop not a wire

    • inductance of a wire in an IC requires knowledge of return path(s)

    • inductance extraction for a whole chip is virtually impossible...










Transmission line model
Transmission Line Model

  • Follow Board Notes

Inductance effects
Inductance Effects

  • Lumped RLC line



VO = Zo Vin





Treat RC problem as a resistive divider:



wn = 1/sqrt(LC)

z=RC/2sqrt(LC) = damping factor






+ (R + sL)





s2 + s2zwn + wn2

s2LC + sRC + 1

z > 1 we have two real poles (RC effects)

z < 1 we have two complex poles (RLC effects)

Poles are P1,2 = wn [- z sqrt(z2-1)]



Inductance effects1
Inductance Effects

  • Follow board notes

Other inductance effects
Other Inductance Effects

  • For most gates Ron is in the order of K so typically R >> jwL

    • response is dominant by RC delay for most signals

  • Only the large drivers have a small enough Ron to allow the inductance to control the dynamic response

    • clocks

    • busses

  • For clocks, self-inductance term can dominate the response (especially if shielding is used)

  • For busses, mutual inductance term dominates and creates noise events that could cause malfunction

  • For power supplies, inductance can also be a problem due to the Ldi/dt drop (in addition to the IR drop) as supplies scale down

Capacitive and inductive noise
Capacitive and Inductive Noise

For most wires, jL < (Rwire+Rdrive) for the frequency and R of interest. So, for delay, L is not a big issue currently.

But L can be  20 - 30% of R so noise may be seen on adjacent line (mutual coupling)

Dangerous scenario is a combination

of localized capacitive coupling noise and

long range mutual inductive coupling noise

+ -

- +

Return path current

Double noise events

Propagation delay
Propagation Delay

Rt=Rl, Lt=Ll, Ct=Cl, CT=CL/Ct

Propagation delay cont d
Propagation Delay (Cont’d)

  • 50% propagation delay

  • where ζ and wnare the damping factor and natural frequency of the circuit

  • Function of both the interconnect and gate impedance

Propagation delay cont d1
Propagation Delay (Cont’d)

  • If the ratio of the total resistance of the line to the lossless characteristic impedance increases, inductive effects can be neglected

  • If the ratio of the driver resistance to the lossless characteristic impedance increases, inductive effects can be neglected

  • If the ratio between the time required to charge the load capacitance through the gate and wire resistance to the time of fight increase then inductive effects can be neglected

Dependence of delay on interconnection length
Dependence of Delay on Interconnection Length

  • If the gate parasitic impedances (CL and Rtr ) are neglected then the propagation delay can be expressed as

  • For the limiting case where L →0, the above equation reduces to

  • For the limiting case R ->0, the delay is given by

Repeater insertion revisited
Repeater Insertion revisited

  • Lower repeater size and less number of repeaters

  • The amount of inductance effects present in an RLC line depends on the ratio between the RC and the LC time constants of the line

  • As Inductance effect increases the LC time constant dominates the RC time constant and the delay of the line changes from a quadratic to a linear dependence on the line length.

  • Optimum number of repeaters for the minimum propagation delay decreases

Inductance power dissipation
Inductance & Power Dissipation

  • The dynamic power is given as

  • Increasing inductance effects results in fewer number of repeaters as well as smaller repeater size

  • Significantly reduces total capacitance

  • Faster rise time results in lower short-circuit power

Inductance extraction
Inductance Extraction

  • Inductance can only be defined for a closed current loop

  • The inductance of the loop is proportional to the area of the loop

  • At low frequency resistive impedance dominates

  • Current uses as many returns as possible to have parallel resistances

  • Situation is different at higher frequencies

Mutual inductance
Mutual Inductance

  • Causes extra noise and delay effects

Inductive noise in a bus
Inductive Noise in a bus

  • Physically, a wide bus with all the lines switching in the same direction behaves as one wide line

  • Hence, the effective inductance of a line that is part of a bus is far larger than the self-inductance of that line

  • LC time constant of the line becomes much larger

Antenna effects
Antenna Effects

  • As each metal layer is placed on the chip during fabrication, charge builds up on the metal layers due to CMP1, etc.

  • If too much charge accumulates on gate of MOS transistor, it could damage the oxide and short the gate to the bulk terminal

  • Higher levels of metal accumulate more charge so they are more troublesome (i.e., metal 5 is worse than metal 1)

  • Need to discharge metal lines during processing sequence to avoid transistor damage (becomes a design/layout issue)

+ + + + + + + + + + +

Metal 2

Metal 1



+ + + + + + + + + + + + + + + + + + + + + + + + + + +





This transistor could be damaged

1. CMP is chemical mechanical polishing which is used to planarize each layer before the next layer is placed on the wafer.

Preventing antenna effects
Preventing Antenna Effects

  • A number of different approaches for antenna repairs:

  • Diode Insertion - Make sure all metal lines are connected to diffusion somewhere to discharge the metal lines during fabrication

-diodes costs area

- need to optimize number

and location

- causes problems for

design verification tool

+ + + + + + + + + + + + + + + + + + + + + + + + + + +



Antenna diode

Preventing antenna effects1
Preventing Antenna Effects

  • Note that there are always diodes connecting to source/drain regions of all transistors and charge on each layer is drained before next layer is added…so why are we worried?

  • Gate input of next device may not be connected to a diode until it’s too late…charge accumulation on metal exceeds threshold

Should put antenna

diode here.

Keep area of upper layer metals

small near next transistor

Preventing antenna effects2
Preventing Antenna Effects

  • Second approach is to add buffers to interconnect to break up long wire routes and provide more gate area for antenna ratio

  • Third approach is to use metal jumpers to from one layer of metal to another

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + ++++++++ + + + + + + + +

Metal 1/polish

vias (charge removed)


+ + + +

Class presentation
Class Presentation

  • The first class presentation assignment will be posted soon.

  • One of you has to present the basic concepts discussed in the paper to the class

  • Presentation time ~20 minutes

  • After the presentation everybody has to participate in the discussion