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Why short wires scale, but long wires don’t

Explore the effects of wire scaling in computer devices on short and long wires. Discover the implications for delay reduction and intrinsic RC ratio in past and future technology generations.

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Why short wires scale, but long wires don’t

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  1. Why short wires scale, but long wires don’t Shubu Mukherjee VSSAD, Alpha Technology Compaq Computer Corporation Shrewsbury, Massachusetts Stephen Felix Alpha Technology Compaq Computer Corporation Shrewsbury, Massachusetts Disclaimer: We don’t discuss repeaters in this talk

  2. L current flow t W Resistance of Wires (R) R =  L / (t W),  = resistivity

  3. 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

  4. 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 • What’s a long wire? • intrinsic RC delay of wire significant fraction of gate delay • > 0.3 - 0.7 mm in 0.125 technology, depends on metal layer • note: die sizes ~ 20 x 20 mm2

  5. 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!

  6. L R =  L / (t W) current flow t C = 2 * ( L t / h +  L W / v) W 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%

  7. t t 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

  8. 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

  9. 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

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