Microlens performance limits in sub-2mum pixel CMOS image sensors.

CMOS image sensors with smaller pixels are expected to enable digital imaging systems with better resolution. When pixel size scales below 2 mum, however, diffraction affects the optical performance of the pixel and its microlens, in particular. We present a first-principles electromagnetic analysis of microlens behavior during the lateral scaling of CMOS image sensor pixels. We establish for a three-metal-layer pixel that diffraction prevents the microlens from acting as a focusing element when pixels become smaller than 1.4 microm. This severely degrades performance for on and off-axis pixels in red, green and blue color channels. We predict that one-metal-layer or backside-illuminated pixels are required to extend the functionality of microlenses beyond the 1.4 microm pixel node.

Optical confinement methods for continued scaling of CMOS image sensor pixels.

The pixels that make up CMOS image sensors have steadily decreased in size over the last decade. This scaling has two effects: first, the amount of light incident on each pixel decreases, making optical efficiency, i.e., the collection of each photon, more important. Second, diffraction comes into play when pixel size approaches the wavelength of visible light, resulting in increased spatial optical crosstalk. To address these two effects, we investigate and compare 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, while the third uses reflection at a metal-dielectric interface to confine the light. Simulations are performed using a finite-difference time-domain (FDTD) method on a realistic 1.75-mum 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 find significant (10%) improvement for the TIR designs with properly chosen parameters and nearly full spatial crosstalk elimination using metal to confine the light. We also show that these improvements are comparable to those achieved by thinning the image sensor stack.