Toshiba 2.2um Pixel Reverse Engineering

Chipworks blog published an article Toshiba 3.33um and 2.2um pixels reverse engineering. 3.33um pixel is quite usual 2-way shared 4T pixel layout. The poly level picture of 2.2um pixel reveals some interesting things - see the picture below. Chipworks says:

"The more recent Toshiba 3.2 Megapixel CIS, featured a variation on this 5T layout, but with a surprising twist. From a circuit perspective, the structure is still the same, with pairs of pixels still sharing the T3, T4 and T5 transistors. However structurally there is a difference - additional polysilicon slabs have been placed in between adjacent pixels in a column. We believe they help electrically separate pixels that lie on a common diffusion – at a cost. The performance trade-off must have been considered because additional polysilicon structures generally reduce the fill factor (the amount of light reaching the photo-cathode versus that hitting the pixel) for the CIS pixel.

Additionally, there is an extra slab of polysilicon extended over the centre of the active area of the red pixels. This may reduce sensitivity to red light since it must pass through the poly slab (note that wavelengths shorter than red light have a higher absorption coefficient and will be less able to penetrate the silicon) but it would increase the RGB color separation of the pixels."