Backside Illuminated Image Sensor Alignment Mark Detection
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Solution Overview
Problem
Back-side illuminated image sensors face challenges in accurately and precisely aligning alignment marks on the front side of the wafer for processing, as these marks are difficult to detect and align with conventional tools when patterning the backside of the wafer.
Innovation Solution
The solution involves forming alignment marks on the front side of the wafer using reticles of different scales (e.g., 1× and 4×) and selectively etching the substrate to make these marks detectable from the backside, allowing for precise registration and alignment using optical detection systems with photo detectors and visible or infrared light.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If alignment marks are formed on the front side of the wafer, then alignment reference is provided for backside patterning, but the marks are difficult to detect and align with conventional tools when patterning the backside
Solution Approach 1:
The patent introduces an intermediary etched region that exposes the alignment mark formed on the front side, making it detectable from the backside. This intermediary structure acts as a mediator between the front-side alignment mark and the backside patterning process, allowing conventional optical tools to detect the alignment mark through the transparent substrate and the exposed region.
Solution Approach 2:
The patent utilizes the third dimension (depth/thickness of the substrate) to resolve the detection problem. By etching through the substrate thickness to expose the alignment mark, the solution transforms a two-dimensional surface detection problem into a three-dimensional solution where the alignment mark becomes accessible from the backside through the transparent substrate.
2Productivity
If the substrate is thinned to enable backside processing, then backside illuminated sensor performance is improved, but alignment mark detection from the backside becomes more challenging
Solution Approach 1:
The etched region serves as an intermediary that facilitates alignment mark detection while working within the thinned substrate structure. This intermediary element enables the alignment process without requiring additional substrate thickness, thus maintaining the benefits of substrate thinning for backside illumination while solving the detection challenge.
3Ease of manufacture
If conventional alignment tools are used for backside patterning, then existing equipment can be utilized, but accurate alignment with front side patterns cannot be achieved
Solution Approach 1:
The alignment mark structure is designed to be self-service compatible with conventional optical alignment tools. By creating an etched region that exposes the alignment mark and utilizing the substrate's transparency, the system enables conventional tools to perform their intended function of optical detection and alignment without requiring specialized equipment, thus achieving both ease of manufacture and alignment precision.
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 method enables accurate and precise patterning of back-side illuminated sensors by making alignment marks detectable from the backside, facilitating improved alignment and processing of critical layers, thereby enhancing the manufacturing efficiency of back-side illuminated image sensors.
Implementation Method 1
optical detection systems with photo detectors and visible or infrared light
Data Source
AI summary
A method for fabricating a backside illuminated image sensor is provided. An exemplary method can include providing a substrate having a front surface and a back surface; forming an alignment mark at the front surface of the substrate, wherein the alignment mark is detectable for alignment from the back surface; and processing the substrate from the back surface by performing registration from the back surface and using the alignment mark as a reference.