Why short wires scale, but long wires don

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Why short wires scale, but long wires dont
Shubu Mukherjee VSSAD, Alpha Technology Compaq
Computer Corporation Shrewsbury, Massachusetts
Stephen Felix Alpha Technology Compaq Computer
Corporation Shrewsbury, Massachusetts
Disclaimer We dont discuss repeaters in this
talk
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Resistance of Wires (R)
R ? L / (t W), ? resistivity
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Capacitance of Wires (C)
W
h
t
Metal Layer 3
v
wire X
Metal Layer 2
Metal Layer 1
Clateral ? L t / h, Cvertical ? L W / v
Ctotal for wire X 2 Clateral 2 Cvertical
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Wire Delay
L
R ? L / (t W)
current flow
t
C 2 (? L t / h ? L W / v)
W

• Delay of Short Wires ? C, assuming constant drive
  
• Delay of Long Wires ? RC
• Whats a long wire?
• intrinsic RC delay of wire significant fraction
  of gate delay
• gt 0.3 - 0.7 mm in 0.125 technology, depends on
  metal layer
• note die sizes 20 x 20 mm2

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Scaling Equation example

• R ? L / (t W)
• Let f scaling factor
• typically, f 0.7 every generation
• Let fR scaling factor of R
• fR f / f, constant, if t constant (past)
• fR f / (f f) 1 / f, if t scales as f
  (future)
• resistance increases!

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Wire Delay Scaling in the Past

• Assume t v constant
• Delay of short wires ? C ? Clateral Cvertical
• Clateral constant, Cvertical scales as f2
• if Clateral 0, delay reduces by f2 50
• if Clateral Cvertical, delay reduces by 1 -
  (1 f2) / (1 1) 25
• Delay of long wires ? RC
• RC scales as C, because R is constant, 25 - 50

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Wire Scaling in the Future
Key Assumption Violated t v NOT constants New
Assumption all dimensions scale the same way
W
L
PAST
W
FUTURE
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Wire Delay Scaling in the Future
current flow
L
R ? L / (t W)
C 2 (? L t / h ? L W / v)
t
W
Delay of short wires ? C delay reduces by (1 -
f) ? 30 every generation Delay of long wires ?
RC delay constant in every generation, because
L, W, t, h scale in the same way
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Conclusions

• Past
• delay on a short wire reduces by 25 - 50 per
  generation
• delay on a long wire reduces by 25 - 50 per
  generation
• Future
• delay on a short wire reduces by 30 per
  generation
• delay on a long wire is constant
• Many short wires become long wires in next
  generation
• intrinsic constant RC appears relatively higher
  compared to faster gates