"we were able to make a BSI imager with a record efficiency over a broad spectrum. In the full visible light spectrum (400-800nm), the efficiency is fairly constant between 80 and 90%. And even in the infrared, at 850nm, the efficiency is still 80%."
"For our record BSI sensor, we grew high-quality EPI layers on standard silicon. By tuning the growth conditions, it is possible to engineer specific doping profiles in the detection layer. This way, a built-in electric field can be created to pull the generated carriers from the substrate to the diode at the surface, where they are collected, largely avoiding crosstalk.
Another technique that we experimented with is to separate the pixels with deep and narrow trenches filled with highly doped polysilicon. This creates a built-in electric field at the sides of the trench, effectively creating a wall between the pixels and eliminating crosstalk. One obvious disadvantage of this method is that you lose part of the detection surface, i.e. the fill factor will slightly decrease."
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| . Crosstalk measurement for BSI imagers with graded EPI layer, showing the percentage of charge picked up by neighboring pixels (left) first generation graded EPI layer of 35µm (right) second generation graded EPI layer of 12µm |
"At imec’s fab, we’ve been able to thin wafers down to 12µm. The best uniformity that can be reached is <2µm thickness variability on 200mm wafers."
