Light Trapping Materials for Optical Crosstalk Reduction

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Solution Overview

Problem

Optical crosstalk in solid state imagers, such as CCD and CMOS imagers, leads to image degradation and contrast reduction due to stray light entering neighboring photosensors, which is exacerbated by decreasing pixel sizes and increasing pixel density, and existing light shields are imperfect and often reflect light, causing further errors.

Innovation Solution

Incorporating light trapping materials like nanotextured or porous silicon between layers in the imager structure to absorb stray light through total internal reflection, reducing optical crosstalk by creating a material with a higher index of refraction than the surrounding layers, thereby trapping light rays that would otherwise cause aberrations.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If light shields are formed in upper device layers, then light shielding is provided, but light piping and shadowing increase, and optical crosstalk is not fully prevented

Engineering Contradiction:

Improveoptical crosstalkVSAvoidimage accuracy

Core Design Contradiction:

Object-affected harmful factorsVSReliability

Solution Approach 1:

The light trapping structure extends vertically into the substrate below the photosensor surface, utilizing the third dimension (depth) to intercept stray light before it reaches neighboring photosensors. This vertical light trapping approach complements traditional lateral light shielding by operating in a different spatial dimension.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Solution Approach 2:

A dielectric layer is introduced as an intermediary material between the photosensor and the light trapping structure. This dielectric layer serves as a medium that allows light to pass through to the photosensor while also enabling the formation of the light trapping structure above it, mediating between light transmission and light trapping functions.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Productivity

If pixel size is reduced to increase pixel density, then more pixels fit in the imager array, but optical crosstalk between neighboring photosensors increases

Engineering Contradiction:

Improvepixel densityVSAvoidoptical crosstalk

Core Design Contradiction:

ProductivityVSObject-affected harmful factors

Solution Approach 1:

The light trapping structure is implemented as discrete elements associated with individual photosensors or groups of photosensors, segmenting the light management function at the pixel level. This allows each pixel region to independently trap its own stray light without interfering with adjacent pixels, enabling high pixel density while maintaining isolation.

Inventive Principle:

Principle #1Segmentation

3Object-affected harmful factors

If light shields are formed in metal interconnect layers, then light shielding is achieved, but conductive connections are limited and light still passes through imperfect shields

Engineering Contradiction:

Improvestray lightVSAvoidmetallization layer constraints

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The light trapping structure utilizes a porous or textured dielectric material that provides effective light trapping through its physical structure rather than relying on opaque metal layers. This porous approach allows light to be scattered and trapped multiple times within the structure, achieving effective shielding without blocking electrical conductivity paths.

Inventive Principle:

Principle #31Porous materials

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

The use of light trapping materials significantly reduces optical crosstalk, enhancing image quality by ensuring that incident light is directed to the correct photosensor, minimizing blurring and contrast reduction, and reducing light piping and shadowing issues.

Implementation Method 1

Incorporating light trapping materials like nanotextured or porous silicon between layers in the imager structure to absorb stray light through total internal reflection

Methodology Applied

Scientific EffectTotal internal reflection: Total Internal Reflection