Joe explained:
The scaling law is the Poisson distribution: Flaws are randomly
distributed. Even one flaw kills the chip. Double the area of the
chip, double the average number of flaws in the chip. If the average
number of flaws per chip is low enough, then most chips will by luck be
flawless. As the average rises, then more and more chips are unlucky.
What saves us is that many chips have more than one flaw, allowing
others to have no flaws. I suspect that for image sensors, which are
quite large, the average yield (fraction of the chips on a wafer that
are good) is low, perhaps 25 0f memory serves. The cost per wafer is
more or less fixed, so the cost per chip basically depends on the yield,
and one of the most closely guarded business secrets is the actual
yield.
In this regime (~25% yield), changes in chip size have large effects on
yield, but the real law isn't so simple as the square or the cube.
Joe - you probably know that memory chips don't follow these yield laws
because they have built-in redundancy (spare rows or columns in the array
that can be configured during testing to replace faulty bits). It's the
same with image sensors but it's done in the software - so called pixel
mapping. If there is a faulty pixel it can be interpolated as the average
of the adjacent pixels. This means that those defects no longer cause the
whole chip to be failed. The software can even fix pixels that fail during
normal lifetime. See excellent Oly description for E-10:
http://www.imaging-resource.com/NEWS/998087928.html
br
jez
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