RE: Future of Photography (was filmscanners: real value?)

["Austin Franklin" <austin@darkroom.com>]

>  They make them smaller for cost reasons, you can put more dies
> > (chips) on a
> > single wafer, which makes them cheaper.  That's not quite
> the same as
> > yield...
> >
> > > VERY LARGE ICs have been made in the past, but they are very
> > > expensive to
> > > make because the yields are so poor.
> >
> > Well, not necessarily true.  If you mean large, as in a lot of
> > transistors,
> > that is true.  If you mean physically large, simply because of
> > process, that
> > is not true.
>
> There is a certain probability that any spot on a wafer will be bad.
> Therefore, the larger the chip (physically) the greater the
> probability that
> it will be bad. One caveat is that wafer quality is getting
> better all the
> time and is far superior than what it used to be.

I believe you could possibly be confusing physical size because of process,
(that uses the same number of transistors), with physical size because of
more transistors, but using the same process.  They are different.

Processes that use physically larger transistors typically give a higher
yield than those that use smaller ones.  Unless it is an old process.  A
.25u process is far more reliable, ie, gives a higher yield, than a .12u
process.  The only thing you typically have to do to change processes is
scale the masks...the circuit is exactly the same.

It is true, that the more transistors you have, the higher the rate of
failure, that's simple statistics.  It isn't true that because the die is
larger simply because of process size, that the yield goes down...in fact it
typically goes up.