CMOS Image Sensor Alignment Mark Design
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Solution Overview
Problem
CMOS image sensor devices face alignment failures due to the low transmittance of blue photoresist in color filters, particularly at wavelengths around 633 nm, which affects exposure processes and leads to precision issues during the formation of alignment marks.
Innovation Solution
The method involves forming alignment marks within a distance of less than 3 mm from the substrate edge, using a photoresist with low transmittance at 550-650 nm wavelengths, and employing a solvent edge rinse or wafer edge exposure process to remove the color filter photoresist, allowing precise alignment and exposure of the marks using an appropriate beam wavelength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If blue photoresist is used in color filter with wavelength around 633 nm, then color filter performance is improved, but alignment precision deteriorates due to low transmittance
Solution Approach 1:
An alignment mark structure is introduced as an intermediary element that mediates between the low-transmittance blue photoresist and the alignment process. The alignment mark comprises a first alignment mark portion extending in a first direction and a second alignment mark portion extending in a second direction perpendicular to the first direction, creating a cross-shaped structure that enhances visibility and alignment accuracy despite the photoresist's low transmittance at 633 nm wavelength
Solution Approach 2:
The alignment mark structure transitions from traditional single-direction or point markers to a two-dimensional cross-shaped structure with portions extending in perpendicular directions. This dimensional expansion provides multiple reference axes for alignment, improving precision by allowing alignment verification along both x and y axes simultaneously, thereby overcoming the limitations imposed by low photoresist transmittance
2Device complexity
If alignment mark is placed far from substrate edge, then alignment process is simplified, but manufacturing complexity increases due to additional process steps
Solution Approach 1:
The alignment mark structure is designed with non-uniform local characteristics where the first and second alignment mark portions extend in perpendicular directions with potentially different lengths or configurations. This local quality variation optimizes alignment visibility and precision in different directions while maintaining overall process simplicity, allowing the structure to adapt to specific manufacturing requirements without adding complex process steps
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 effectively prevents alignment failures by exposing the alignment marks and ensuring accurate alignment during the formation of image sensor devices, even with low transmittance photoresists, thereby improving the precision and reliability of the image sensor manufacturing process.
Implementation Method 1
An exposure apparatus comprising a beam with wavelength of 550 ̃650 nm aligned through a mask to the alignment mark exposes the photoresist
Data Source
AI summary
A method for forming an image sensor device. An alignment mark is formed on or in a substrate with distance from the alignment mark to the substrate edge less than about 3 mm. An array of active photosensing pixels is formed on the substrate. At least one dielectric layer is formed covering the substrate and the array. A color filter photoresist is formed on the least one dielectric layer. Subsequent to removal of the color filter photoresist from the alignment mark, the color filter photoresist is exposed with alignment to the alignment mark.