CCD Imager

Discussion in 'Digital Photography' started by swnw, Sep 20, 2003.

  1. swnw

    swnw Guest

    Hi,

    Why CCD Imagers are usually smaller than 24 x 36 area?

    swnw
     
    swnw, Sep 20, 2003
    #1
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  2. "swnw" <> wrote in message news:bkhg24$13r7$...
    > Hi,
    >
    > Why CCD Imagers are usually smaller than 24 x 36 area?


    1. Cost. They can sell multiple CCDs for the same fixed setup cost.
    2. Compactness. They can be used in smaller camera housings, and with
    smaller lenses.

    Bart
     
    Bart van der Wolf, Sep 20, 2003
    #2
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  3. swnw

    Alfred Molon Guest

    Alfred Molon, Sep 20, 2003
    #3
  4. swnw

    swnw Guest

    Thanks, swnw.
     
    swnw, Sep 20, 2003
    #4
  5. swnw

    Eric Gisin Guest

    Because that is a huge amount of silicon, more than any microprocessor. You
    get very low yields fabricating such big chips, and very high prices.

    "swnw" <> wrote in message news:bkhg24$13r7$...
    |
    | Why CCD Imagers are usually smaller than 24 x 36 area?
     
    Eric Gisin, Sep 20, 2003
    #5
  6. "swnw" <> writes:

    >Why CCD Imagers are usually smaller than 24 x 36 area?


    Because of the way the cost of a chip scales with its size.

    Many good consumer cameras use a chip that is about 1/5 the dimensions
    of the 35 mm frame, about 5 x 7 mm. This is 1/25 the area of a
    full-frame chip.

    If you had a manufacturing process that turned out 100% usable chips,
    using a square sheet of silicon, you would get 25 times as many of the
    small chips as you would of the full-frame chips, given the same amount
    of raw materials. So the large chip has to cost at least 25 times as
    much as the cost of the small ones.

    In addition, the raw wafer is actually round, and you can pack small
    CCDs closer to the edges of the wafer than for the larger ones, so the
    factor is slightly larger than 25:1.

    But the think that really makes it more expensive is called "yield".
    Chip manufacturing isn't perfect, and every wafer has some flaws in it
    that are large enough to make the chip containing the flaw into junk.
    Now, suppose that the flaws are relatively few, and that when making
    the smaller CCDs 99% of them are good, and only 1% of them are bad.
    In other words, there is a 99% chance of no "fatal" flaws in any one
    CCD. When you make the larger full-frame CCDs, the odds of having no
    fatal flaws in an area 25 times as large is (roughly) 0.99^25 = 0.78.
    So 78% of the larger CCDs are good, and 22% of them are junk. The
    smaller proportion of good large chips means that the good ones have to
    cost more by a factor of 1/0.78 just to pay for the raw materials.

    As the rate of flaws goes up, it messes up the economics of the large
    chips much faster than the smaller ones. A 2% defect rate with the
    small-area chips means that 98% are good. With the larger CCDs, only
    0.98^25 = 0.60 or 60% work. If 97% of the small chips work (3%
    defects), only 47% of the large ones do. If 95% of the small chips
    work, only 28% of the large ones do. And if 90% of the small chips
    work, only 7.2% of the large ones work. If 80% of the small chips work,
    only 0.4% of the large ones work - essentially none at all.

    So, in the case where 90% of the small chips work, the large chips have
    to cost 25 times more because of area, and 90/7.2 = 12.5 times as much
    because of the difference in defect rate. So the *manufacturer's cost*
    for the large chip is 312 times the cost of the small one, not 25
    times.

    And then there are economies of production scale that make the
    difference larger yet.

    Apparently semiconductor fab houses will not tell you what their defect
    rate is - whether it's 1% or 5% or 10%. But the way the mathematics
    works, you can pretty much guarantee that the cost of a large chip goes
    up much faster than the relative area of the chip.

    Dave
     
    Dave Martindale, Sep 21, 2003
    #6
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