Buried Self-Aligned Focusing Element for CMOS Image Sensor Crosstalk Reduction
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Solution Overview
Problem
In CMOS image sensors, focusing light into small photodiodes below 2.2 micron technology is challenging due to the narrow optical pass-throughs in the metal stack, leading to reduced sensitivity and increased crosstalk between pixels, as conventional techniques to enhance sensitivity, such as reducing the metal stack height or using embedded microlenses, are complex and inefficient.
Innovation Solution
The implementation of a buried self-aligned focusing element (SAFE) within the metal stack to extend the focal length of the microlens, allowing for higher metal stack heights and improved light focusing without diverging light, thereby increasing sensitivity and reducing crosstalk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the metal stack height is reduced to increase sensitivity, then light focusing improves, but circuit timing and gain are adversely affected
Solution Approach 1:
The patent divides the focusing function into two segments: the top microlens handles initial light collection, while the buried SAFE embedded in the metal stack provides secondary focusing. This segmentation allows the metal stack to maintain its full height for circuit performance while still achieving effective light focusing through the combined action of both elements.
Solution Approach 2:
The buried SAFE acts as an intermediary element within the metal stack that mediates between the top microlens and the photodiode. It provides the additional focusing power needed for sub-2.2 micron pixels without requiring reduction of the metal stack height, thus preserving circuit timing and gain while improving light focusing.
2Measurement precision
If an embedded microlens is used to increase sensitivity, then light focusing improves, but process complexity increases significantly
Solution Approach 1:
The patent merges the formation of the buried SAFE with the existing metal stack fabrication process. The SAFE is created using the same deposition and etching steps that form the metal interconnect layers, eliminating the need for separate microlens fabrication equipment and processes. This integration significantly reduces process complexity compared to conventional embedded microlens approaches.
Solution Approach 2:
The metal stack layers serve dual functions: they provide electrical interconnects for the circuit and simultaneously host the buried SAFE for optical focusing. This multi-functionality eliminates the need for dedicated microlens fabrication processes and materials, simplifying the overall manufacturing process while achieving both electrical and optical objectives.
3Measurement precision
If the photodiode aperture is reduced for higher resolution, then image detail improves, but alignment of embedded microlens becomes increasingly difficult
Solution Approach 1:
The buried SAFE is formed using a self-aligned process where the high-index material is deposited conformally on the metal stack and then etched back. The alignment is automatically determined by the metal stack geometry and etch profiles, eliminating the need for separate alignment steps. This self-alignment mechanism maintains precise positioning even as photodiode apertures shrink to sub-2.2 micron dimensions for higher resolution.
Solution Approach 2:
The SAFE is formed during the metal stack fabrication process itself, before final photolithography and patterning steps. By establishing the optical focusing element early in the process using the metal stack as a template, the system pre-establishes the alignment reference that will guide subsequent processing steps, ensuring consistent alignment as feature sizes decrease.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the sensitivity of CMOS image sensors by maintaining high metal stack heights while reducing crosstalk, enabling higher resolution and transistor count without the complexity of conventional methods.
Implementation Method 1
a buried self-aligned focusing element (SAFE) within the metal stack to extend the focal length of the microlens
Data Source
AI summary
An image sensor includes an optical sensor region, a stack of dielectric and metal layers, and an embedded layer. The optical sensor is disposed within a semiconductor substrate. The stack of dielectric and metal layers are disposed on the front side of the semiconductor substrate above the optical sensor region. The embedded focusing layer is disposed on the backside of the semiconductor substrate in a Backside Illuminated (BSI) image sensor, supported by a support grid, or a support grid composed of the semiconductor substrate.