Joe wrote:
>>>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.
<<
Foveon at the IEEE talk I attended was hesistant to reveal exact yields but
claimed that the SD9
sensor had very high yields with very few chips with actual dead pixels. This
is in contrast to
their earlier 3 sensor beam splitter cameras which often had a significant
number of dead pixels.
Of course the yield Joe discusses above assumes sensors have to be perfect.
This is not really
true except for chips for scientific applications. In cameras dead pixels are
interpolated in
software after a calibration test done before shipping the camera. How many
dead pixels is
acceptable depends on the vendor. But we might expect the number of dead pixels
to be deeemed
acceptable to ship, to increase with chip size! Foveon said they only have
problems with some
weak pixels (in there present chip size) which have to have their output scaled
rather than
completely interpolated from adjacent pixels if the pixels were dead.
Joe is right about how dense these chips are. The Foveon chips is something
like 6 transistors per
sensor. Approx 60 million transistors total for the SD9 sensor. This is about
the size of a
Pentium III.
Regards,
Tim Hughes
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