Measurement of High Sensitivity and Low Crosstalk of Zero-Space Microlens for 2.8- $\mu\hbox {m}$ -Pitch Active Pixel Sensor

This paper describes the difference between normal microlenses and zero-space microlenses in image sensors. The investigation of both sensitivity and crosstalk has been implemented for normal microlenses and zero-space microlenses that are fabricated on a 2.8-μm pinned photodiode array of 640 (row) by 480 (column) based on an 0.18-μm -CMOS process. The simulation results show that the zero-space microlens technology increases the pixel sensitivity by about 75.61% and decreases the crosstalk by about 9.4% compared to the normal microlens technology. This paper presents the sensitivity analysis of both normal microlenses and zero-space microlenses based on the wafer test data from the Teradyne IP750, which is a special wafer test platform for CMOS sensors. The results of the statistical data of the wafer test show that the sensitivity of the zero-space microlenses has been improved by about 73.6% on the red pixel (69.6% on the green pixel and 76.3% on the blue pixel) compared with that of the normal microlenses. Using a similar statistical method, the crosstalk characteristics of both the zero-space microlenses and the normal microlenses have been measured as well. Compared with the normal microlenses, the testing results show that the crosstalk of green light in red light is reduced by about 7.244% and the crosstalk of blue light in red light is reduced by about 41.447% due to the use of the zero-space microlens technology. More importantly, this paper also provides the comparison and analysis of the real images taken by CMOS image sensors with the normal microlenses and the zero-space microlenses, respectively. From the comparison of image results, the image luminance of the zero-space microlenses is higher than that of the normal microlenses, and the image definition of the zero-space microlenses is clearer than that of the normal microlenses as well. So far, the silicon results show that the zero-space microlens technology is a better choice than- - the normal microlens technology for CMOS image sensors of a small-size pixel.