Versatile method for optical performances characterization of off-axis CMOS pixels with microlens radial shift

In this paper, we present a versatile characterization method we developed at STMicroelectronics for off-axis pixels (i.e. over the image plane) on CMOS image sensor. The solution does not require optics, making it suitable for early design phases as for optimizations and investigations. It is based on a specific design of color filters and microlens masks, which consists in several blocks. Inside each block, the filters and the microlens are shifted by a given amount, relatively to the pixel. Each block is related to a given chief ray and then defines a point in the chief ray angle space. Then, the performances of these angular points can be measured by rotating the sensor, using conventional uniform illumination setup with controlled f-number. Then it is possible to map these data on the image plane, knowing the chief ray angle versus focal plane coordinate function. Finally, we present some characterizations and optimizations based on the fact that the shift is arbitrary defined during circuit layout step, so it is possible to test the sensor with higher chief ray angles than those present in the product, or to optimize the shift of the microlens versus the chief ray angle for a given pixel architecture.