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A METHODOLOGY TO SIZE POWER BUS WITHIN THE ELECTROMIGRATION AND VOLTAGE DROP CONSTRAINTS

IP.com Disclosure Number: IPCOM000008117D
Original Publication Date: 1997-Jun-01
Included in the Prior Art Database: 2002-May-20
Document File: 2 page(s) / 87K

Publishing Venue

Motorola

Related People

Larry Kan: AUTHOR [+3]

Abstract

In IC design, for a given metal line width of a power supply bus, e.g. VCC, the number of circuits allowed to be hooked up or, more specifically its bus length, has its limit. This is because of the lii- tations of both the metal electromigration rules and the tolerable IR drop along the power bus. There often exists the case where one rule dominates the other within any given metal width range. The tradi- tional way to size the power bus is through trial and error and experience. This method provides a straight forward way to optimize the metal width and metal length.

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M-LA Technical Developments

A METHODOLOGY TO SIZE POWER BUS WITHIN THE ELECTROMIGRATION AND VOLTAGE DROP CONSTRAINTS

by Larry Kan, Ruey Yu and Willie Anderson

INTRODUCTION

  In IC design, for a given metal line width of a power supply bus, e.g. VCC, the number of circuits allowed to be hooked up or, more specifically its bus length, has its limit. This is because of the lii- tations of both the metal electromigration rules and the tolerable IR drop along the power bus. There often exists the case where one rule dominates the other within any given metal width range. The tradi- tional way to size the power bus is through trial and error and experience. This method provides a

straight forward way to optimize the metal width and metal length.

DISCUSSION

  As shown in Figure 1, a simple model of a power bus driving a chain of n identical inverters (this can be any cell in a circuit). The optimal point of width(W) and length(L) can be modeled math- ematically based upon metal electromigration rule and IR drop tolerance.

v -------

; ; I I

I i I I

I

I

3 -c90

I I

I I

vss -----___ 4

n, (n-l),(n-2) ,...... 2, 1

Fig. 1 IR Drop Circuit Model

QMolnmk 1°C. 1997 4 June I997

[This page contains 15 pictures or other non-text objects]

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8 M-LA Technical Developments

  For n number of cells, each cell consumes average DC current of i, at E activity rate, or percentage of cells activated along the power bus. The total current is,

I = hi

  From the metal electromigration rule (of Motorola UDRII process, see attachments), for finished widths greater 3 microns,

Imax = kW'.=

Where k is constant, for bus width W, t...