CMOS photodiodes for narrow linewidth applications

In recent years CMOS image sensors have gained a major market share for general imaging applications. However, when standard CMOS image sensors are employed in applications that require the detection of light with a very small spectral width, like 3D-time-of-flight imaging or other applications with laser light illumination, problems arise, that are negligible in standard imaging applications with broadband illumination. For a given wavelength a strong variation of the sensitivity upon small process related variations of the dielectric stack on top of the photodiodes leads to large die to die variations. In this paper a method is presented that decreases these sensitivity variations by introducing multiple optical path lengths of the dielectric stack within each photodiode. Using this method the maximum quantum efficiency variation for process induced thickness variations could be reduced significantly for a broad range of wavelengths without any additional processing steps.