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From: (MitchAlsup)
Subject: Re: Merced troubles
Date: 11 Aug 1998
Newsgroups: comp.arch

>> Both copper and SOI (Silicon on Insulator) technologies have
>> associated cost penalties, but the (impure) raw materials are
>> of negligible expense.

The raw material cost of Al vs. Cu is negligable, even Au and
Ag are acceptable in cost.

Aluminum is easy and clean to process [deposit, coat, bake,
print, develop, etch]; But even Al is soluable in Si, and barrier
metals [Ti, W] are used as an interface layer between Al and
SI to prevent degradation over time. Even Al to Al junctions
usually get a barrier metal.

Copper is a dirty process when processed like aluminum. This
has something to do about the kinds of acids which etch Cu
and don't etch the Photorisist. Copper is comming arround in
processing due to a new method of processing. Instead of
patterning a layer over Al and etching the undesired metal
away, the next layer of dielectric oxide is deposited, and then
the tracks for the copper are etched into theis oxide, and then
the Cu is deposited, Finally, the excess Cu is polished off,
leaving only the nice clean little tracks of Cu where the oxide
was removed. [Look Ma, no messy acids].

Copper is worth the additional expense in deep submicron
designs because wire has become the dominant delay
mechanism (over gates). This is expecially true on longer
(bus) wires. In a typical 0.18ยต process, and a large 500 MHz
microprocessor, Cu interconnect could boost frequency to
666 MHz without redesigning

Mitch Alsup

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