| On the left is shown the pixel for green light detection. The structure of this single junction pixel can be the same as that of current CMOS imagers, with a buried photodiode, transfer gate, and floating diffusion region. On the right side is shown the stacked junction pixel for blue and red detection. The blue and red absorbing regions are separated by a boron implant and by shallow trench isolation (STI). Complete charge transfer from the red diode to the floating diffusion can be realized through the careful design of the ion implant and the device layout. Since buried photodiode structures are used for all three detectors, dark current, reset noise and charge transfer are the same as standard CMOS sensors. Only 2-3 extra masks are needed for ion implantation, and one color filter is eliminated, so fabrication costs should be the same or less than standard CMOS image sensors. Due to the thinner red and blue diodes, the full well capacity will be higher than the usual single diode. Simulations indicate that 2-3 times higher capacities can be obtained. Combined with the expectation that the noise should be similar to the standard structure, both dynamic range and signal-to-noise ratio should have a corresponding 2-3x improvement. |
