14'3 TopLevel Interconnection

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14.3 Top-Level Interconnection

• Autorouting automatic, but verifying it may
  require as much time as manual interconnection
  itself.
• Manual interconnection can consider wiring
  resistance, electromigration, noise coupling,
  heat distribution, etc.
• Maze Routing
• across and through circuit Blocks
• Saves 5-10 die area
• often requires 2-3 times as long to complete
• harder to modify
• Channel Routing
• simpler/easier to modify
• quicker to implement
• Most modern designs use this.

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(1) Channel Routing Principles

• minimum two-level interconnection. Ex) one
  Metal, one Poly or two Metals
• long Poly run is not good for signals
• 20-50 W/sq. for Poly, 5W/sq. for silicided Poly
• Short Poly jumpers are ok.
• Be careful which signals flow in Poly
• two metal design
• one for x-direction, the other for y-direction
• should run at right angles polyy, metal-1x,
  metal-2y, metal-3x, etc.

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• Metal Pitch, Pm
• Pm Wm Sm width spacing
• Ex) Wm 2 um, Sm 1.5 um ? Pm 2 1.5
  3.5 um
• Wiring Channel Width, Wc
• Wc N Pm Sm min.width leads Pitch
  spacing
• Ex) Pm 3.5 um, Sm 1.5 um, N 6 ? Wc 6
  3.5 1.5 22.5 um
• VIA w/o enlarged heads
• Wm gt Wv 2 Omv via width 2 min. M overlap
  VIA
• Ex) Wm2um, Wv 1.5um, Omv 0.5um ? LHS lt RHS
  1.5 20.5 2.5um ? see below for increased
  Metal head

Wider leads --Preferred.
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• primary routing channels 2 or 3 large channels
  that carry most wires
• each primary channel about 20 of all
  top-level signals
• ex) 100 signals ? 20 leads / primary channel
• Narrowest feeder channel 3-5 leads as difficult
  to know exactly difficult to increase channel
  width once the interconnection starts.
• IF using poly also, run Poly in only
  one-direction (// to metal-2) to avoid gridlock

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(2) Special Routing Techniques

• different leads have different considerations
  voltage drops, noise coupling,
  electromigration, etc.

• Kelvin Connections
• Leads have Common point to avoid voltage
  fluctuation due to large currents

Ground Return 100 uA
Ex) AB 10 sq.s of 30 mW/sq 0.3 W DR
C Star node or Kelvin Connection
DV 100uA0.3W 30 uV ? 10 fluctuation in
GND Current, 100 dB Av ? 10 30 uV 1E5 V/V
0.3 V fluctuation
Kelvin connection is marked by 45-deg angle
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• Applications of Kelvin Connections

• VR1 voltage regulator
• F1power lead F2GND lead
• K1, K2 Kelvin Connections
• also called, remote sensing

• F1, F2 Force leads S1, S2 Sense leads
• large current thru F1-F2 little current thru
  S1-S2

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(3) Noisy Signal, Sensitive Signal

• most noise problem is caused by
  capacitive-coupling of signals from one node to
  another node
• capacitively couple noise is a concern if
  significant energy at 1 MHz
• Analog signals are more noise sensitive than
  digital signal
• Ex) Amp input is noise sensitive and high R
  output is not sensitive and low R
• The following are Noise sensitive
• Inputs to high-gain amps and precision
  comparators
• Inputs to ADCs
• Outputs of precision voltage references
• Analog ground lines to high-precision circuitry
• Precision high-value R networks
• Very low-level signals, regardless of impedance
• Very low-current circuitry of any sort
• Circuit designers usually need to identify
  sensitive signals

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• If noisy and sensitive signal lines
• run adjacent to each other, then run
• another low-noise low-impedance signal
• as a shield.
• Shield line can be an extra GND line or
• Supply line
• Whenever possible, separate the
  sensitivecircuitry from noisy circuitry into
  separateportions of die

• Noisy signal and sensitive signal must not run
  on top of each other
• If crosses, then area of intersection should be
  minimum
• At each crossing, IF shielding isneeded, then
  Polysignal, Metal-1shield,Metal-2signal.
  Metal-1 shield to lowimpedance GND node.
• Shield extend 2-3 um beyond intersection

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• Sensitive signals should never run farther than
  necessary ? fewer opportunities for noise
  coupling
• sensitive signals running between circuit blocks
  should not pass other blocks ? keep number of
  long sensitive signals few to reduce chances of
  capacitive coupling.
• Noise coupling thru substrates ? minimized by
  proper grounding ? separate Grounds for
  sensitive circuits and noisy circuits ? three
  Grounds one for sensitive c, one for noisy c,
  and one for substrate contacts. Ex)
  substrate GND to a separate lead finger