CMOS Image Sensor Under Layer for Pad Protection and Adhesion
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Solution Overview
Problem
The fabrication of CMOS image sensors faces issues such as uneven photoresist coating and residual material, leading to reduced image quality, as well as peeling of the color filter array due to inadequate adhesion between the silicon nitride passivation layer and the photoresist material, and damage to the pad during the developing and rework processes.
Innovation Solution
A method involving the formation of a semiconductor substrate with a passivation layer, an opening with tapered sidewalls to expose the pad, followed by the deposition of an under layer to cover the pad and passivation layer, and subsequent formation of a color filter array and U-lenses, where the under layer is carefully removed to prevent residuals and enhance adhesion, using materials like oxide, nitride, or non-photosensitive organic materials for improved uniformity and protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an opening exposing the pad is formed before CFA and U-lenses formation, then wafer acceptable test can be performed, but uneven coating or residuals of photoresist material occur reducing image quality
Solution Approach 1:
An under layer is formed in advance before the color filter array formation, covering the pad area. This preliminary action prevents photoresist material from directly contacting the pad, thereby avoiding uneven coating and residuals while still allowing the opening to be present for testing purposes.
Solution Approach 2:
The under layer acts as an intermediary between the pad and the photoresist material. It provides a protective interface that prevents direct interaction between the photoresist and pad, eliminating the harmful effects of direct exposure while maintaining the structural integrity needed for testing.
2Device complexity
If color filter array is formed on silicon nitride passivation layer, then fabrication process is simplified, but adhesion is not good enough causing peeling of color filter units
Solution Approach 1:
The under layer serves as an intermediary adhesion promoter between the silicon nitride passivation layer and the color filter array. It provides a surface that enhances bonding, preventing peeling of color filter units while maintaining the simplified fabrication process of forming CFA directly on the passivation layer structure.
Solution Approach 2:
The structure becomes a composite system with the under layer having properties that bridge the silicon nitride passivation layer and the photoresist-based color filter array. This composite structure combines the advantages of both interfaces, providing both mechanical support and adhesion enhancement.
3Ease of operation
If opening exposing pad is formed, then pad access is enabled, but developing process and rework process bring damage to the pad
Solution Approach 1:
The under layer is formed in advance to cover and protect the pad area before any developing or rework processes occur. This preliminary protective layer prevents harmful chemicals or mechanical actions from directly contacting and damaging the pad, while still allowing pad access through the opening for testing purposes.
Solution Approach 2:
The under layer provides beforehand cushioning protection to the pad against potential damage from developing and rework processes. It acts as a sacrificial or protective barrier that absorbs or deflects harmful effects before they can reach the vulnerable pad structure.
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 method ensures uniform under layer thickness, prevents residual material on the pad, protects the pad from damage, and enhances the adhesion of the color filter array, thereby improving image quality by reducing peeling and maintaining the integrity of the sensor components.
Implementation Method 1
a method for forming the opening is, for example, to perform an etching process
Implementation Method 2
an under layer is formed on the semiconductor substrate to cover the pad and the passivation layer
Data Source
AI summary
A method for fabricating an image sensor, which includes the following steps, is provided. A semiconductor substrate including a sensor array, a pad and a passivation layer is provided, and the passivation layer covers the sensor array and the pad. An opening, which comprises tapered sidewalls not perpendicular to a bared surface of the pad, is formed in the semiconductor substrate to expose the pad. An under layer is formed on the semiconductor substrate, and covers the pad and the passivation layer. A color filter array is formed on the under layer and over the corresponding sensor array. A planar layer is formed on the color filter array. A portion of the under layer is removed to expose the pad. A plurality of U-lenses is formed on the planar layer.