Image Sensor Pixel Isolation Using Shifted Lithography Patterns
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Solution Overview
Problem
The shrinking geometry size in image sensor device fabrication leads to challenges with photoresist masks, as high aspect ratio masks are prone to capillary forces, causing them to collapse, which hinders the formation of precise pixel structures necessary for high-resolution imaging.
Innovation Solution
A method involving multiple lithography processes and the use of optional shrinkage materials to form photoresist columns with low aspect ratios and ultrahigh aspect ratio trenches, preventing collapse and enabling the creation of deeper isolation regions without lateral diffusion, thus allowing for closer pixel spacing and higher resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If photoresist masks with high aspect ratio are used to produce smaller pixels, then manufacturing precision is improved, but the masks become more prone to collapse due to capillary forces
Solution Approach 1:
The patent applies segmentation by dividing the photoresist mask formation into multiple lithography processes. Instead of forming high aspect ratio masks in a single step, the process segments the mask formation into multiple lower aspect ratio steps, where each step creates a portion of the final mask structure. This segmentation reduces the aspect ratio of individual photoresist columns, making them more stable and less prone to collapse from capillary forces, while still achieving the desired small pixel dimensions through cumulative patterning.
Solution Approach 2:
The patent utilizes another dimension by forming trenches between adjacent photoresist columns in the vertical dimension. These trenches provide structural support to the photoresist columns, effectively reducing their aspect ratio and preventing collapse. The trenches create a three-dimensional support structure that enhances mask stability without compromising the horizontal resolution needed for small pixel production.
2Productivity
If pixel geometry size is reduced to increase pixel density, then productivity is improved, but photoresist masks collapse due to increased capillary forces
Solution Approach 1:
The patent segments the mask formation process into multiple lithography steps, allowing each photoresist column to have a lower aspect ratio that prevents collapse. This segmented approach enables the formation of closely spaced pixels (high density) while maintaining mask stability, as each segmentated mask structure is mechanically stronger than a single high aspect ratio mask would be.
Solution Approach 2:
The patent introduces trenches as an intermediary structural element between adjacent photoresist columns. These trenches act as mediators that provide mechanical support and reduce the effective aspect ratio of the photoresist columns. This intermediary structure enables higher pixel density to be achieved without the photoresist masks collapsing under capillary forces.
3Reliability
If multiple lithography processes are used to form low aspect ratio photoresist columns, then photoresist stability is improved, but device complexity increases
Solution Approach 1:
The patent segments both the mask formation and the trench formation into separate lithography processes. This segmentation of the fabrication steps, while increasing process complexity, provides a systematic and repeatable methodology that improves photoresist column stability. The segmented approach allows for optimized parameters in each step, ensuring reliable mask formation that would not be achievable in a single process.
Solution Approach 2:
The patent applies preliminary action by forming trenches between photoresist columns before subsequent processing steps. This preliminary structural preparation provides mechanical support that prevents collapse during later fabrication steps. The trenches are formed in advance to establish a stable framework that enables the low aspect ratio photoresist columns to maintain their integrity throughout the remaining device fabrication process.
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 allows for the formation of image sensor devices with increased pixel density and deeper isolation regions, reducing the risk of photoresist collapse and enhancing pixel-to-pixel isolation, thereby improving the resolution and efficiency of image sensor devices.
Implementation Method 1
photoresist masks with high aspect ratio are more prone to the effects of capillary forces. These effects are exacerbated as the aspect ratio of the mask increase and/or as the pitch decreases. As a result, photoresist masks may collapse, for example, due to the pulling effect of capillary forces between adjacent photoresist masks.
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.