CMOS image sensors are now widely used in digital imaging systems as pixel size has steadily decreased to allow higher-resolution imaging. When the pixel size scales below 2 &mgr;m, however, microlens performance is significantly affected by diffraction from the edges of the image sensor pixel. This results not only in quantitative performance degradation, but also in a qualitative shift in functionality. We perform a systematic analysis of microlens design during lateral scaling of CMOS image sensor pixels. The optical efficiency and optical crosstalk are calculated with a first-principles finite-difference time-domain (FDTD) method. We find that there are two regimes of operation for three-metal-layer pixels depending on pixel size and wavelength: a refraction-dominated regime for pixel sizes larger than 1.45 &mgr;m and a diffraction-dominated regime for pixel sizes smaller than 1.45 &mgr;m. In the refraction-dominated regime, the microlens can be designed and optimized to perform its concentrating function. In the diffraction-dominated regime, the optimal radii of curvature for microlenses are very large and a flat microlens layer, in fact, becomes the best choice and performance is severely degraded. Under these circumstances, the microlens no longer fulfills its optical function as a focusing element. To extend the functionality of the microlens beyond the 1.45 &mgr;m node, we predict that a one-metal-layer dielectric stack or shorter will be required.
Recent efforts in CMOS image sensor design have focused on reducing pixel size to increase resolution given a fixed package size. This scaling comes at a cost, as less light is incident on each pixel, potentially leading to poor image quality caused by photon shot noise. One solution to this problem is to allow the imaging or objective lens to capture more light by decreasing its f-number. The larger cone of accepted light resulting from a lower f-number, however, can lead to decreased optical efficiency and increased spatial optical crosstalk at the pixel level when the microlens is not able to properly focus the incident light. In this work, we investigate the effects of imaging lens f-number on sub-2µm CMOS image sensor pixel performance. The pixel is considered as an optical system with an f-number, defined as the ratio of the pixel height to width, and we predict the performance of a realistic pixel structure subject to illumination from an objective lens. For our predictions, we use finite-difference time-domain (FDTD) simulation with continuous-wave, diffraction-limited illumination characterized by the f-number of the imaging lens. The imaging lens f-numbers are chosen to maintain resolution and incident optical power as pixel size scales, while the pixel f-number is varied by modifying the height of the pixel structure. As long as pixel f-number is scaled to match the imaging f-number when pixel size is scaled, optical efficiency and crosstalk for on-axis illumination will not be significantly affected down to the 1.2 &mgr;m pixel node. We find the same trend for system MTF, which does not seem to suffer from diffraction effects.
Over the last decade, the pixels that make up CMOS image sensors have steadily decreased in size. This scaling has two effects: first, the amount of light incident on each pixel decreases, reducing the photodiode signal and making optical efficiency, i.e., the collection of each photon, more important. Second, spatial optical crosstalk increases because diffraction comes into play when pixel size approaches the wavelength of visible light. To counter these two effects, we have investigated and compared three methods for guiding incident light from the microlens down to the photodiode. Two of these techniques rely on total internal reflection (TIR) at the boundary between dielectric media of different refractive indices. The first involves filling the central pixel area with a high-index dielectric material, while in the second approach, material between the pixels is removed and air is used as a low-index cladding. The third method uses reflection at a metal-dielectric interface to confine the light. Simulations were performed using commercial finite-difference time-domain (FDTD) software on a realistic 1.75 μm pixel model for on-axis as well as angled incidence. We evaluate the optical efficiency and spatial crosstalk performance of these methods compared to a reference pixel and examine the influence of several design parameters.