STMicro patent application US20120086091 proposes to place metal shields 23 between color filters in BSI pixel array:
"...a disadvantage of [BSI] sensors is that they are particularly prone to color mixing phenomena between neighboring color filters. In particular, a light ray may cross two adjacent filters 15 of different colors before reaching a photosensitive area 9. A light ray may also be essentially filtered by a filter 15 of a first color, and reach the photosensitive area 9 of a neighboring pixel, associated with another color. Such phenomena are currently called “optical crosstalk” and adversely affect the quality of the images acquired by the sensor. They especially occur when light rays reach filters 15 with poorly adapted angles of incidence, for example, in case of a poor alignment of microlenses 17, or in case of parasitic reflections in the sensor. Such phenomena are particularly strong in the peripheral regions of the sensor, in which the pixels are illuminated by light rays having relatively high angles of incidence, which may exceed 30°."
"Walls 23 enable to prevent any direct passing of light between adjacent filtering elements. Walls 23 extend approximately along the entire height, for example, along at least 90% of the height of the filtering layer, and are preferably made of aluminum, of tungsten."