Vertical capacitors for image sensors

Discussion in 'Digital Photography' started by Alfred Molon, Jun 5, 2006.

  1. Alfred Molon

    Alfred Molon Guest

    I wonder if it would be possible to use a technology used by DRAM makers
    to increase cell capacity, when the area is very small - vertical
    capacitors instead of horizontal ones. Would solve the problem of ever
    shrinking sensor cell sizes (at least the dynamic range problem).
     
    Alfred Molon, Jun 5, 2006
    #1
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  2. But the problem is also photons/second. If you shrink the
    cell size by 2x (area by 4x) the photons/second/sensor
    drops by 4x. So if the well can hold more, you then
    must expose longer, essentially making a lower ISO device.
    Of course in some situations, that would be just fine.
    In others, your ISO 100 just turned to ISO 25 and you
    might get a lot of blurred shots. But it would be nice
    for landscapes ;-).

    Roger
     
    Roger N. Clark (change username to rnclark), Jun 6, 2006
    #2
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  3. Alfred Molon

    ASAAR Guest

    I think that you're considering a complication that wouldn't
    occur, as resolution is already high enough for most purposes.
    Assuming most sensors are already at the 6mp or 8mp level, why not
    simply make the cells taller (extend vertically), keeping the sensor
    area constant? Why would the cell dimensions have to be shrunk by
    2x, allowing from 24mp to 32mp on the same sized sensor? As you
    say, that would reduce the usable ISO significantly, which is why I
    don't see that being something that any camera designer would
    consider doing, unless there would be some other overwhelming,
    unstated benefit. Lower chip(sensor) cost probably isn't one of
    those benefits.
     
    ASAAR, Jun 6, 2006
    #3
  4. Alfred Molon

    Alfred Molon Guest

    More dynamic range, useful when taking outdoor shots.
     
    Alfred Molon, Jun 6, 2006
    #4
  5. Alfred Molon

    ASAAR Guest

    ??? I understand how dynamic range could be increased if taller,
    "vertical" capacitors could be successfully used in sensors. Did I
    write anything implying the opposite, or did you misinterpret my
    reply? My point was that if "vertical capacitors" could work, why
    would a manufacturer want to shrink their other two dimensions,
    greatly increasing the number of megapixels that would fit on a
    sensor having the same area, but with the disadvantage of reducing
    the highest usable ISO, as well as the number of photons that could
    be captured, in effect foregoing the greater dynamic range that
    might be possible. Roger's reply seemed to indicate that your
    "vertical capacitor" idea would be used primarily to increase
    resolution, whereas I thought your idea was to keep the resolution
    about the same, but using the changed sensor to increase the
    sensor's dynamic range.
     
    ASAAR, Jun 6, 2006
    #5
  6. SNIP
    Or somewhere in the middle, increased resolution *and* dynamic range
    ....

    It'll be interesting to see Canon's Photokina introductions, press
    release 2-3 weeks before Photokina, opening on September 26th.

    I wouldn't be surprized if they indeed come up with a 22 MP full
    24x36mm frame model (to stay ahead of competition and make inroads to
    medium format body and Digital Back users). It would most likely be a
    more studio oriented camera with a true sub ISO 100 or perhaps a
    binning capability. It is still possible with current technology,
    although they might also increase the ADC to 14 or 16 bits (which
    would require a Digic3 generation).

    Bart
     
    Bart van der Wolf, Jun 6, 2006
    #6
  7. Alfred Molon

    Don Stauffer Guest

    But keeping the area of a capacitor the same and increasing the spacing
    of the plates DECREASES the capacitance, and hence the dynamic range.
     
    Don Stauffer, Jun 6, 2006
    #7
  8. Alfred Molon

    ASAAR Guest

    Excellent point. When sensors have been discussed before, have
    the ones able to collect more photons been described as having a
    greater well height? I seem to recall that, but I may be mistaken.
    If so, I guess that the "height" wouldn't describe any real
    measurement, but would be used just to make it easier to visualize
    how sensors operate. Capacitance could be increased by reducing the
    distance between the plates, but it may be that reducing the height
    would introduce other problems, such as greater rates of charge
    leakage, something similar to electron tunneling, etc. It may be
    that the only way to effectively increase the capacitance would be
    by using the traditional method - increase the sensor's size. :)
     
    ASAAR, Jun 6, 2006
    #8
  9. Alfred Molon

    Scott W Guest

    clearly more bits would be a bit win. For a system that is really
    working well you would have just one ISO, the lowest. If the A/D had
    enough range to capture the full signal there would be no point in
    raising the gain for higher ISOs.
    of electrons of noise, is to run at pretty high ISO and then we are
    lossing most of the top end range.

    BTW I though the current cameras were at 12 bits not 14, but am not
    sure of this.

    Scott
     
    Scott W, Jun 6, 2006
    #9
  10. Alfred Molon

    Alfred Molon Guest

    I was suggesting to increase the area of the capacitor by making it
    vertical, i.e. grow towards the inside of the sensor. The light
    sensitive area on top would channel electrons into this vertical
    capacitor below.
     
    Alfred Molon, Jun 6, 2006
    #10
  11. Alfred Molon

    ASAAR Guest

    While that would increase the surface area of the capacitors, they
    probably work by having photons strike only one of the capacitor's
    "plates", creating a potential difference between the top and bottom
    surfaces. What you're now describing would have the light (photons)
    impinge equally on what, if rotated 90 degrees, would have been one
    edge of the top and bottom plates (as well as some of the dielectric
    between them). The surface area of these plates may be larger, but
    they're no longer presenting one of the surfaces, just their edges
    to the impinging photons. This probably wouldn't cause much of a
    signal (ie, voltage), if any, to register.
     
    ASAAR, Jun 6, 2006
    #11
  12. Alfred Molon

    Alfred Molon Guest

    I was thinking of a "greek pi" structure with a light sensitive area
    above (the horizontal line of the pi) and a vertical capacitor below
    (the two vertical lines of pi).
     
    Alfred Molon, Jun 6, 2006
    #12
  13. Alfred Molon

    ASAAR Guest

    And I was thinking that your geometry probably doesn't work for
    sensors. My guess (since I haven't studied sensors) is that the
    light sensitive area would have to be one of the capacitor's two
    plates. That is, one (but not both) of your two vertical lines,
    which in your design would be hidden from the incoming photons,
    which would strike not the broad side of one plate, but the thin
    edge of both plates as well as the thin dielectric separating them.

    To continue with the guesses, the way the sensor might work is
    that when photons strike one of the two plates, a voltage potential
    between the two plates is created, and the capacitance is used to
    preserve the voltage long enough for the camera to read it. To
    increase the dynamic range you'd have to increase the number of
    photons that could be collected, and that would be proportional to
    the capacitance. There are several ways the capacitance could be
    increased. One is to increase the plate's surface area, and one
    way that is done is to replace a tiny P&S sensor with a larger
    sensor, such as a 24mm x 36mm FF sensor. The other way would be to
    place the two plates closer together, but current sensor technology
    may already be too close to the mechanical or quantum limits to make
    any further capacitance increases practical. There are probably a
    couple of people here that have a better understanding of how
    sensors operate and are fabricated, so whether they concur or
    disagree with me and thee, we'll have to wait and see. :)
     
    ASAAR, Jun 6, 2006
    #13
  14. SNIP
    Correct, that's why I said "increase the ADC *to* 14 or 16 bits"
    (emphasis added to original quote). There may be one or two 'backs'
    with 12+ bit ADCs, I haven't tested them. Roger Clark's tests indicate
    that sensors (at least the ones in his cameras) are Photon Noise
    limited, which indicates we rather need deeper wells and higher
    quantum efficiency before requiring higher resolution ADCs..

    Bart
     
    Bart van der Wolf, Jun 7, 2006
    #14
  15. Alfred Molon

    Scott W Guest

    Ok missed that, I read increase from 14 to 16 not to 14 or 16, my bad.

    Scott
     
    Scott W, Jun 7, 2006
    #15
  16. Alfred Molon

    Don Stauffer Guest

    That is not a PHYSICAL height. That is a plot of voltage vs electron
    number. It is a graph, not a drawing of the capacitor.
     
    Don Stauffer, Jun 7, 2006
    #16
  17. Alfred Molon

    Don Stauffer Guest

    That is what I thought you meant- thickening the dielectric layer. That
    reduces the capacitance. Or, do you mean somehow depositing the plates
    on the SIDE of a vertical channel? I am not sure normal semiconductor
    processing can do that very well. Ordinarily the thickness of dielectric
    layers are thin compared to the dimensions of features.

    Remember, current IC processing gives us primarily an area design.
    While some folks have talked about some way to actually build 3D
    circuits with some form of polycrystalline material, that has not
    succeeded well as far as I know.
     
    Don Stauffer, Jun 7, 2006
    #17
  18. Alfred Molon

    Alfred Molon Guest

    No, thickening the dielectric layer would reduce the capacity. I was
    talking about adding a 3D component. Like building a skyscraper instead
    of a one floor house to increase the total living area.
     
    Alfred Molon, Jun 7, 2006
    #18
  19. Alfred Molon

    ASAAR Guest

    Umm, do you think that describing the well height with

    kind of says that it's not a PHYSICAL height? Or were you saying
    that the "well height", however defined, is related in no way to any
    physical property of the sensor's elements, such as capacitance?
     
    ASAAR, Jun 7, 2006
    #19
  20. Alfred Molon

    stauffer Guest

    It is related to physical properties, but not directly. I mean "well
    height" is not a physical dimension. One can use the analogy that a
    capacitor is a well for electrons, or a bucket. But that is an analogy
    only. We can say that the capacity of a well or bucket includes the
    depth of the well or bucket. So the capacity of a capactance is only
    by ANALOGY proportional to a depth. It physically is actually
    reciprocally related to depth.
     
    stauffer, Jun 8, 2006
    #20
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