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

Area of Science:

• Optoelectronics
• Semiconductor device physics

Background:

• Smaller pixels in CMOS image sensors are crucial for achieving higher digital imaging resolution.
• Diffraction effects become significant for pixel sizes below 2 micrometers, impacting microlens optical performance.

Purpose of the Study:

• To perform a first-principles electromagnetic analysis of microlens behavior during lateral scaling of CMOS image sensor pixels.
• To determine the critical pixel size at which diffraction limits microlens functionality.

Main Methods:

• Electromagnetic analysis based on first principles.
• Modeling of microlens behavior in three-metal-layer CMOS image sensor pixels.

Main Results:

• Diffraction prevents microlenses from focusing light effectively in pixels smaller than 1.4 micrometers.
• This limitation severely degrades performance across red, green, and blue color channels for both on and off-axis pixels.
• The analysis was conducted for a standard three-metal-layer pixel architecture.

Conclusions:

• Microlens functionality is significantly compromised below the 1.4 micrometer pixel node due to diffraction.
• One-metal-layer or backside-illuminated pixel designs are necessary to overcome these limitations.
• These advanced designs are predicted to enable continued microlens functionality at smaller pixel scales.