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Politics : Formerly About Advanced Micro Devices
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To: Steve Porter who wrote (54141)4/5/1999 11:43:00 AM
From: Shane Geary  Read Replies (1) of 1570747
 
Steve,

I won't be going back researching the Cyrix thread (!!) but..

As you know, copper can offer 4 benefits:

(i) Lower R at a given linewidth (ie lower RC delay)
(ii) Same R at a smaller linewidth (ie possible lower die size)
(iii) Possibly lower processing costs (fewer steps that traditional schemes - ie W plugged vias and Al wiring)
(iv) Possibly lower defect density at very tight metal pitches

A lot of the discussion on the benefits of Cu have revolved around (i). There is no correct answer at 0.18um geometries IMHO - it is too dependant on part-specifics and can be designed around (a point that Process Boy has been making). For example, global routing is done at the top metal layers with wide Al lines, whereas local routing (where RC delay is less important) is done at the earlier metal layers with tight pitches.

To say whether "to Cu or not Cu" requires trading off the costs and benefits of all 4 of the above. Again, no easy/correct answer.

Look at the large size of the K7 die at 0.25um (actually drawn transistor gate length is 0.22um I believe). Could a key benefit of going to Cu at 0.18um be the reduction in die size greater than that achieved by a linear shrink?

People have mentioned the Alpha - from what I recall, the Alpha die size is very large as a result of relatively wide metal pitches, even at the first few metal levels (the pitch of the first 2 metal levels has a large impact on die size (and/or # of metal levels required)). The wide pitches are required to minimize RC delay (the Alpha always being much more sensitive to RC delay because of the high MHz). I always thought that the Alpha would be the chip that would get the biggest benefit from Cu.

I guess what I am trying to say is that I think there are certain benefits to using Cu at 0.18um, and that parts can be designed to take advantage of Cu at 0.18um, but that parts can be designed to reduce the disadvantage of not using Cu also.

Let me pose a question: Why has IBM introduced Copper on a 0.22um process? Does the PowerPC750 get no benefit? Was it just for the PR?
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