Selective Epitaxy for Image Sensor LDD Regions
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Solution Overview
Problem
Conventional CMOS image sensors face limitations such as low sensitivity and reduced full well capacity due to smaller photosensitive element areas, leading to lower dynamic range and signal-to-noise ratio, and are constrained by design rules for lightly-doped drain (LDD) implants which limit the minimization of isolation distances between pixel transistors and photosensitive elements.
Innovation Solution
The solution involves epitaxially growing doped silicon in LDD regions instead of conventional implantation, eliminating the need for an LDD implant mask, allowing for reduced isolation distances and enabling larger photosensitive elements without violating design rules, and using a doped sacrificial blanket film as an alternative to form LDD regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional LDD implantation with mask is used, then manufacturing precision is maintained, but device complexity increases and isolation distance cannot be minimized
Solution Approach 1:
The patent removes the LDD implant mask from the fabrication process entirely. Instead of using a mask to define LDD regions, the invention uses selective epitaxial growth that occurs only in predetermined regions without requiring a mask, thereby eliminating the mask-related design rules and minimizing isolation distances.
Solution Approach 2:
The patent replaces the mechanical/chemical implantation process with a growth-based process. Instead of implanting dopants through a mask, the invention uses selective epitaxial growth of doped silicon regions, substituting the implantation mechanism with a growth mechanism that inherently provides precision without mask constraints.
2Productivity
If photosensitive element area is reduced to increase pixel density, then productivity increases, but sensitivity and full well capacity deteriorate
Solution Approach 1:
The patent addresses the photosensitive element size limitation by optimizing the spatial arrangement in the planar dimension. By minimizing isolation distances through the removal of mask constraints, the invention effectively increases the usable photosensitive area within the same pixel pitch, allowing higher pixel density without sacrificing sensitivity.
3Area of stationary object
If isolation distance between pixel transistors and photosensitive elements is minimized, then pixel area increases, but manufacturing precision requirements increase due to LDD implant mask constraints
Solution Approach 1:
The patent removes the LDD implant mask and its associated design rules, allowing isolation distances to be minimized without the precision constraints that previously governed mask-based LDD definition. This extraction of the mask constraint enables greater design freedom in minimizing isolation distances.
Solution Approach 2:
The selective epitaxial growth process inherently defines the LDD regions through self-alignment mechanisms, eliminating the need for external mask guidance. The growth occurs automatically in the predetermined regions based on the underlying structure, providing self-service precision without mask constraints.
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 smaller isolation regions between photosensitive elements and pixel transistors, enabling larger pixel sizes and improved sensitivity while maintaining or exceeding standard CMOS technology pitches, thus enhancing image sensor performance by increasing dynamic range and signal-to-noise ratio.
Implementation Method 1
Instead of implanting silicon dopants in the LDD regions, doped silicon may be epitaxially grown in the LDD regions
Implementation Method 2
a doped sacrificial blanket film may be used to form LDD regions by diffusing the dopants from the film into the LDD regions
Data Source
AI summary
Embodiments of the present invention are directed to an image sensor having pixel transistors and peripheral transistors disposed in a silicon substrate. For some embodiments, a protective coating is disposed on the peripheral transistors and doped silicon is epitaxially grown on the substrate to form lightly-doped drain (LDD) areas for the pixel transistors. The protective oxide may be used to prevent epitaxial growth of silicon on the peripheral transistors during formation of the LDD areas of the pixel transistors.