Image Sensor Photo Mask Gradient Etch Slope Control
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Solution Overview
Problem
The existing methods for fabricating CMOS image sensors result in a steep angle of approximately 70 degrees in the etching interfacial region between the pixel and logic circuit regions, leading to poor uniformity in subsequent color filtering processes and increased diffused reflection, which affects the photolithography process and device characteristics.
Innovation Solution
A method involving a photo mask with a gradually decreasing light intensity pattern across the interfacial region between the pixel and logic circuit regions, combined with an etch back process that maintains similar etch rates for the photoresist and insulation layers, to achieve a gentler slope angle of around 0.5 degrees, reducing the insulation layer thickness and improving process margins.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If selective etching of insulation layer is performed using conventional photoresist patterning, then the insulation layer thickness is reduced to decrease distance from photodiode to micro lens, but a steep slope of approximately 70 degrees is formed in the etching interfacial region, causing poor uniformity in subsequent color filtering and increased diffused reflection
Solution Approach 1:
The photoresist pattern is designed with spatially varying thickness: thicker in the pixel region and gradually thinner in the interfacial region toward the logic circuit region. This local variation in photoresist thickness creates a corresponding gradual slope in the etched insulation layer, transforming the abrupt 70-degree interface into a gentle transition. The local quality principle resolves the contradiction by making the photoresist and insulation layer properties position-dependent, achieving both thickness reduction and slope control in different regions.
Solution Approach 2:
The invention changes the physical parameter of photoresist thickness across the substrate surface. By controlling the photoresist deposition process or using a tilted substrate during spin coating, a gradient thickness profile is achieved. This parameter change in photoresist thickness directly translates to a gradient etch depth profile, creating the desired gentle slope in the interfacial region while maintaining adequate etching depth in the pixel region to reduce the photodiode-to-micro-lens distance.
2Shape
If dry etching conditions are used to generate polymer during etching to decrease slope angle, then the slope angle is reduced, but much polymer is generated as particles that are hard to remove in subsequent cleaning processes, deteriorating device characteristics and process yield
Solution Approach 1:
The photoresist pattern is prepared in advance with a predetermined thickness gradient before the etching process begins. This preliminary structuring of the photoresist layer acts as a template that guides the etching process, ensuring that the insulation layer is etched to the desired depth profile without requiring aggressive etching conditions. By pre-defining the slope geometry through photoresist thickness control, the need for polymer-generating slope control mechanisms is eliminated, avoiding particle contamination and maintaining high process yield.
3Reliability
If the area of photodiode is increased to improve light sensitivity, then light sensitivity is improved, but the number of pixels that can be integrated is reduced
Solution Approach 1:
The invention addresses the pixel density vs. sensitivity trade-off by manipulating the vertical dimension through selective insulation layer removal. By creating a three-dimensional structure where the insulation layer thickness varies spatially, the patent enables better light collection efficiency without increasing the horizontal photodiode area. The gradual slope structure improves light guidance and reduces reflection losses, effectively enhancing sensitivity through vertical structural optimization rather than horizontal area expansion, thereby allowing higher pixel integration density.
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 results in a more uniform and efficient fabrication process, reducing the distance between the photodiode and micro lens, enhancing light sensitivity, and improving the uniformity of subsequent color filtering layers, thereby increasing the efficiency of light reception and process yield.
Implementation Method 1
performing an etch back process over the insulation layer and the photoresist pattern in conditions that an etch rate of the photoresist pattern are substantially the same as that of the insulation layer
Implementation Method 2
A method involving a photo mask with a gradually decreasing light intensity pattern across the interfacial region between the pixel and logic circuit regions
Data Source
AI summary
A method for fabricating an image sensor includes forming an insulation layer over a substrate in a logic circuit region and a pixel region, forming a photoresist over the insulation layer, patterning the photoresist to form a photoresist pattern where the insulation layer in the pixel region is exposed and the insulation layer in the logic circuit region is not exposed, wherein a thickness of the photoresist pattern is gradually decreased in an interfacial region between the pixel region and the logic circuit region in a direction of the logic circuit region to the pixel region, and performing an etch back process over the insulation layer and the photoresist pattern in conditions that an etch rate of the photoresist pattern are substantially the same as that of the insulation layer.