ST and Commissariat A L'energie Atomique Et Aux Energies Alternatives patent application "Back side illumination image sensor with low dark current" by Jean-Pierre Carrere, Patrick Gros D'aillon, Stephane Allegret-Maret, and Jean-Pierre Oddou proposes an additional conductive layer on the BSI image sensor backside to screen the back of the imager from plasma processing-introduced defects:
"The degradation comes from plasma-based treatments used during the treatments of the back side of the sensor, particularly for forming the back side metallic contact pads, of aluminum for example, and for forming the layer protecting the colored filters, which aims to protect these filters during the slicing of the sensors along the slicing lines of the semiconductor wafer.
More precisely, the plasma charges the silicon oxide-silicon nitride stack triggering a trapping of positive charges (holes) in the anti-reflective silicon nitride layer, holes which will then migrate to the interface with the substrate to create a dark current by recombination with electrons."
In order to reduce the dark current, the application proposes: "an additional layer situated above the anti-reflective layer and including at least a lower part containing hydrogenated amorphous silicon (a-Si:H) or hydrogenated amorphous silicon nitride (a-SiNx:H), in which the Si/N ratio of the number of silicon atoms per cubic centimeter to the number of nitrogen atoms per cubic centimeter is greater than 0.7, and preferably greater than or equal to 1.2.
The function of this additional layer is to absorb, or even to stop, ultraviolet radiation, and/or to evacuate the trapped charges in the anti-reflective layer during subsequent plasma treatments, which will therefore reduce the residual positive charges possibly trapped in the anti-reflective layer and consequently reduce the dark current.
The thickness of this lower part of the additional layer is preferably less than a few tens of nanometers, for example a hundred nanometers, or even less than ten nanometers."