Range Modulated Implants for CMOS Image Sensor Isolation
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Solution Overview
Problem
The existing methods for forming implanted regions in CMOS image sensors are costly and prone to alignment errors due to repetitive photolithography steps required for deep isolation regions, which complicates the reduction of pixel pitch and increases fabrication complexity.
Innovation Solution
A method involving a stack of alternating layers over an etch stop layer, with a thin screen oxide layer and a patterned photoresist, allows for ion implantation at multiple depths with reduced photolithographic steps, forming range modulated implants to achieve deep and narrow isolation regions efficiently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If multiple repetitive photolithography steps are used to form deep isolation regions, then the desired implant depth is achieved, but fabrication cost increases and alignment errors occur
Solution Approach 1:
The patent segments the single deep implantation process into multiple shallower implantation steps, each protected by a sacrificial oxide layer of different thickness. This allows achieving the equivalent of a deep implant while using simpler, less costly photolithography steps at each stage, reducing alignment complexity while maintaining precision.
Solution Approach 2:
The patent introduces sacrificial oxide layers as intermediary protective structures between the photoresist mask and the substrate during each implantation step. These oxide layers enable precise depth control without requiring direct alignment to the substrate surface, thereby reducing alignment errors and simplifying the photolithography requirements.
2Quantity of substance
If pixel pitch is decreased to increase pixel density, then more pixels fit on the sensor, but photodiodes require deeper formation to avoid sensitivity loss
Solution Approach 1:
The patent applies segmentation to the isolation region formation process, creating multiple discrete implantation zones at different depths through sequential steps with sacrificial oxide layers. This enables the formation of deep isolation regions necessary for deep photodiodes in high-density pixel arrays without requiring a single complex deep implantation step.
3Manufacturing precision
If repetitive photoresist deposition and patterning is performed, then multiple implant depths are achieved, but a significant portion of fabrication line capacity is consumed
Solution Approach 1:
The patent performs preliminary formation of sacrificial oxide layers with precisely controlled thicknesses before each implantation step. This preliminary action establishes predetermined depth markers that guide the implantation process, eliminating the need for complex real-time adjustments and reducing the number of iterative photolithography cycles required, thereby improving fabrication line capacity.
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 reduces the number of photolithographic steps, minimizes alignment errors, and enables the formation of deep and narrow isolation regions, enhancing pixel density and image quality while lowering fabrication costs.
Implementation Method 1
Ion implantation may be performed to form implants at a first depth by implanting ions through the layers of the stack that remain on the substrate
Data Source
AI summary
Image sensors may include a plurality of photodiodes. The photodiodes may be isolated from each other using isolations regions formed from p-well or n-well implants. Deep and narrow isolation regions may be formed using a multi-step process that selectively places implants at desired depths in a substrate. If desired, the multi-step process may include only one photolithographic patterning step, which in turn can help reduce costs, fabrication time, and alignment errors. The process may include passing ions through a stack of alternating layers of material such as alternating layers of oxide and nitride. After each implant, a layer in the stack may be removed and ions may be passed through the layers remaining in the stack to form an implant at a different depth in the substrate.