Dual Size Microlens Formation for Image Sensors
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in fabricating dual-sized microlenses for both large-area and small-area pixels, leading to issues with dynamic range and signal-to-noise ratio due to the traditional methods of forming microlenses, which do not effectively translate when combining different pixel sizes.
Innovation Solution
A method of forming dual-size microlenses by creating blocks of microlens material with varying thicknesses and selectively adjusting the volume of the microlens material prior to reflow, allowing for the formation of microlenses with similar effective focal lengths over both small-area and large-area pixels, using techniques such as spin-coating, photoresist exposure, and etch-stop processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional microlens formation methods are used, then manufacturing simplicity is maintained, but manufacturing precision deteriorates when fabricating dual-sized microlenses for mixed pixel arrays
Solution Approach 1:
The microlens array is segmented into two distinct types: first microlenses corresponding to small-area pixels and second microlenses corresponding to large-area pixels. This segmentation allows each microlens type to be optimized independently for its specific pixel size, achieving precise focal length matching without requiring a completely new fabrication approach for the entire array.
Solution Approach 2:
Different microlens regions are assigned different optical properties (focal lengths) according to their underlying pixel sizes. Small-area pixels receive first microlenses with first focal lengths, while large-area pixels receive second microlenses with second focal lengths. This local quality differentiation ensures that each microlens-pixel combination operates at optimal performance without compromising the overall array.
2Manufacturing precision
If microlenses are formed without selective volume adjustment, then manufacturing simplicity is maintained, but manufacturing precision deteriorates due to inability to achieve equivalent focal lengths for dual-sized pixels
Solution Approach 1:
The method performs preliminary volume adjustment of microlens material blocks before the final reflow process. By selectively removing material to create voids in smaller blocks (corresponding to small-area pixels) while leaving larger blocks (corresponding to large-area pixels) substantially intact, the process pre-configures the material volumes needed to achieve equivalent focal lengths after reflow, simplifying the overall manufacturing precision achievement.
Solution Approach 2:
The invention changes the volume parameter of microlens material blocks selectively based on their intended pixel size. Smaller blocks have material removed to reduce their volume, while larger blocks maintain their volume. This parameter change ensures that after reflow, both microlens types achieve equivalent effective focal lengths despite their different underlying pixel areas.
3Adaptability or versatility
If uniform microlens material blocks are used for all pixels, then manufacturing simplicity is maintained, but manufacturing precision deteriorates due to inability to accommodate varying pixel sizes
Solution Approach 1:
The microlens array is segmented into two distinct types: first microlenses corresponding to small-area pixels and second microlenses corresponding to large-area pixels. This segmentation allows each microlens type to be optimized independently for its specific pixel size, achieving precise focal length matching without requiring a completely new fabrication approach for the entire array.
Solution Approach 2:
The fabrication process maintains universality by using a single reflow step for all microlens blocks, regardless of their final size. The multi-functionality is achieved by combining selective material removal with the universal reflow process, allowing both small and large microlenses to be formed in one cycle while achieving precise focal length matching for both pixel types.
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 enables the production of microlenses with equivalent effective focal lengths for both pixel sizes, improving the dynamic range and signal-to-noise ratio of image sensors by optimizing the formation process to accommodate varying pixel sizes.
Implementation Method 1
heating the blocks of microlens material to reflow them into microlenses
Implementation Method 2
exposing the photoresist layer to a pattern of light
Data Source
AI summary
A method of forming microlenses for an image sensor having at least one large-area pixel and at least one small-area pixel is disclosed. The method includes forming a uniform layer of microlens material on a light incident side of the image sensor over the large-area pixel and over the small-area pixel. The method also includes forming the layer of microlens material into a first block disposed over the large-area pixel and into a second block disposed over the small-area pixel. A void is also formed in the second block to reduce a volume of microlens material included in the second block. The first and second blocks are then reflowed to form a respective first microlens and second microlens. The first microlens has substantially the same effective focal length as the second microlens.