Image Sensor Pixel Isolation Using Shifted Lithography Masks
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Solution Overview
Problem
The fabrication of image sensor devices with micron or sub-micron pixels is challenged by the collapse of photoresist masks due to high aspect ratios and capillary forces, which are exacerbated by decreasing pitch sizes.
Innovation Solution
A method involving multiple lithography processes forms photoresist columns with low aspect ratios and ultrahigh aspect ratio trenches, using shrinkage materials to enhance adhesion and prevent collapse, allowing for deeper ion implantation and increased pixel density without compromising structural integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If photoresist masks with high aspect ratio are used to produce micron or sub-micron pixels, then pixel density and resolution are improved, but the masks become more prone to collapse due to capillary forces
Solution Approach 1:
The photoresist mask is segmented into multiple thinner layers (first photoresist layer, second photoresist layer, third photoresist layer) stacked on top of each other. This segmentation reduces the aspect ratio of individual layers while maintaining the overall height needed for deep ion implantation masking, thereby preventing collapse due to capillary forces during development.
Solution Approach 2:
The patent uses composite photoresist structures combining multiple photoresist materials with different properties. Each layer can be optimized for specific functions (adhesion, pattern definition, mechanical strength), creating a composite mask system that is both stable and capable of defining high-density pixel patterns.
2Manufacturing precision
If the pitch of photoresist masks is decreased to increase pixel density, then resolution is improved, but capillary forces are exacerbated causing mask collapse
Solution Approach 1:
By segmenting the mask into multiple thin layers spaced apart vertically, the horizontal pitch can be reduced for higher pixel density while each individual layer maintains sufficient thickness to resist capillary forces during development, preventing the collapse that would occur with a single high-aspect-ratio layer at reduced pitch.
Solution Approach 2:
The patent moves the solution from the horizontal plane to the vertical dimension by stacking photoresist layers. This allows pitch reduction in the horizontal direction for higher density while maintaining adequate layer thickness in the vertical direction to withstand capillary forces, effectively decoupling the two conflicting requirements.
3Length of stationary object
If deeper ion implantation is performed to enhance pixel isolation, then isolation depth is improved, but photoresist mask collapse occurs due to high aspect ratio
Solution Approach 1:
The stacked photoresist layer structure enables deep ion implantation by providing cumulative thickness across multiple layers while each individual layer remains thin enough to be developed without collapse. The layers are positioned to mask regions requiring deep isolation while maintaining structural integrity throughout the implantation process.
4Reliability
If multiple lithography processes are used to form low aspect ratio photoresist columns, then manufacturing complexity increases, but photoresist collapse is prevented
Solution Approach 1:
The patent divides the single lithography step into multiple sequential lithography processes, each forming a thinner photoresist layer. This segmentation reduces the aspect ratio of each layer, preventing collapse, while the cumulative effect of multiple layers achieves the required masking depth for the application.
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 formation of image sensor devices with enhanced pixel isolation and resolution by reducing the risk of photoresist collapse, enabling deeper ion implantation and higher pixel density without lateral diffusion, thus improving image sensor performance.
Implementation Method 1
using shrinkage materials to enhance adhesion and prevent collapse
Implementation Method 2
A first implantation process is performed using the first photoresist layer as a mask to form first isolation regions in the device substrate
Data Source
AI summary
A method includes performing a first lithography process using a first pattern of a first photomask to form a first photoresist pattern on a front side of a device substrate; performing a first implantation process using the first pattern as a mask to form first isolation regions in the device substrate; after performing the first implantation process, performing a second lithography process using a second pattern of a second photomask to form a second photoresist pattern on the front side of the device substrate, the second pattern being shifted from the first pattern by a distance less than the first pitch and in the first direction; performing a second implantation process using the second photoresist pattern as a mask to form second isolation regions in the device substrate and spaced apart from the first isolation regions; and forming pixels between the first and second isolation regions.