Image Sensor Fabrication Without LTO Passivation
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Solution Overview
Problem
The use of a low temperature oxide (LTO) layer as a passivation layer in CMOS image sensors leads to poor step coverage and cracking at boundary regions, causing damage to micro lenses and increasing light channel, which is particularly problematic in image sensors with line widths of 0.13 μm or below.
Innovation Solution
A method for fabricating an image sensor without an LTO-based passivation layer, involving the formation of a metal line, etching a passivation layer to create an opening, forming over coating layers and micro lenses, and performing an etch-back process to remove the over coating plugs, thereby avoiding the use of the LTO layer and reducing damage to micro lenses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an LTO layer is formed as a passivation layer over micro lenses, then the micro lenses are protected from external damage factors such as moisture and particles, but the LTO layer causes poor step coverage and cracking at boundary regions, leading to micro lens damage and increased light channel
Solution Approach 1:
The patent removes the LTO passivation layer from the micro lens region entirely, extracting the harmful element while preserving the protective function through alternative means (process control during pad contact formation). This resolves the contradiction by eliminating the source of cracking while maintaining micro lens integrity through modified fabrication procedures.
Solution Approach 2:
The patent performs preliminary protective actions by carefully controlling the pad contact formation process before damage can occur. The etch stop layer is strategically positioned and the etching process is precisely controlled to prevent exposure of the metal line while avoiding damage to the micro lens, eliminating the need for LTO protection in the first place.
2Reliability
If an LTO layer is used as passivation layer, then micro lenses are protected during fabrication, but the LTO layer increases light channel which reduces light receiving efficiency in image sensors with line widths of 0.13 μm or below
Solution Approach 1:
The LTO layer is completely removed from the structure, extracting both the harmful light channel effect and the protective function. The protective function is then achieved through alternative process control methods, while the light channel is eliminated, improving light receiving efficiency in sub-0.13 μm image sensors.
3Ease of manufacture
If a pad contact opening is formed by etching through the passivation layer, then the metal line is exposed for pad contact formation, but the etching process damages the micro lens when an LTO layer is present
Solution Approach 1:
The etch stop layer acts as an intermediary element between the passivation layer and the micro lens. It provides a controlled etching interface that allows pad contact opening formation while preventing etch damage to the micro lens, eliminating the need for LTO layer protection during this critical process step.
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 reduces defects related to LTO layer peeling and light channel increase, enhancing light receiving efficiency and improving yield and performance by protecting micro lenses during the pad contact process.
Implementation Method 1
The etch stop layer 102 is formed to stop etching during an etching process for forming a pad contact
Implementation Method 2
An ashing process using oxygen (O2) plasma is performed to remove the photoresist pattern 105
Implementation Method 3
The LTO layer 111 is formed to protect the color filter arrays and the micro lenses from external damage factors such as moisture, particles, and slight thermal stress
Data Source
AI summary
A method for fabricating an image sensor including a first region, which is a light receiving region, and a second region, which is a pad region, includes forming a metal line in the second region over a substrate structure comprising a photodiode, forming a passivation layer over the substrate structure, selectively etching the passivation layer to form an opening exposing the metal line where a pad contact is to be formed, forming a first over coating layer (OCL1) in the first region while forming an over coating layer (OCL) plug over the opening in the second region, forming color filters, a second over coating layer (OCL2), and micro lenses in sequential order over the OCL1 in the first region, forming a photoresist pattern exposing the OCL plug, performing an etch-back process to remove the OCL plug, and removing the photoresist pattern.