CMOS Image Sensor Trench Isolation via Plasma Doping
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Solution Overview
Problem
CMOS image sensors face challenges with crystalline defects in trench isolation regions, leading to increased dark current and noise due to incomplete impurity regions surrounding the trenches, which are difficult to form using traditional ion implantation methods, especially as trenches deepen and narrow.
Innovation Solution
A method involving plasma doping to form a channel stop impurity region that surrounds the bottom and sidewalls of trenches, using dopant gases like BF3 with dilution gases, ensuring the impurity region is thinly formed and continuous, preventing dark current and noise issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If ion implantation is used to form impurity region in trench, then impurity doping is achieved, but the impurity region cannot completely surround the trench sides and bottom
Solution Approach 1:
The patent replaces the mechanical ion implantation process with a chemical vapor deposition (CVD) process. Instead of physically implanting ions that cannot reach all surfaces, the CVD process uses gaseous precursors that diffuse and deposit conformally on all trench surfaces including sides and bottom, achieving complete coverage through chemical reactions rather than mechanical bombardment.
Solution Approach 2:
The patent changes the fundamental doping parameter from high-energy ion implantation to low-energy chemical vapor deposition. This parameter change allows the doping mechanism to transition from directional particle injection to isotropic chemical deposition, enabling uniform impurity distribution throughout the trench structure.
2Manufacturing precision
If ion implantation is performed at high energy to form impurity region, then doping is achieved, but the impurity region becomes thick which shrinks the photodiode depletion region
Solution Approach 1:
The patent changes the energy parameter from high-energy ion implantation to low-energy chemical vapor deposition. This allows precise control of impurity region thickness through deposition time and temperature, preventing excessive thickness that would shrink the depletion region and reduce saturation current.
Solution Approach 2:
The patent substitutes the mechanical high-energy ion implantation with chemical vapor deposition, where impurity incorporation occurs during film formation. This chemical approach naturally limits impurity region thickness to the film thickness, providing better control and preventing depletion region shrinkage.
3Productivity
If trenches are deepened and narrowed for higher integration, then device integration is improved, but ion implantation becomes less effective at forming surrounding impurity regions
Solution Approach 1:
The patent replaces mechanical ion implantation with chemical vapor deposition, which is particularly advantageous for deep and narrow trenches. The gaseous precursors can diffuse into deep trenches and deposit conformally on all surfaces, whereas ion implantation becomes increasingly ineffective as trench depth increases due to scattering and shadowing effects.
Solution Approach 2:
The patent transitions from a directional doping approach (ion implantation from above) to a three-dimensional conformal deposition approach (CVD coating all surfaces). This dimensional change allows effective doping of deep and narrow trenches by utilizing the vapor phase to access all surfaces equally, regardless of trench geometry.
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 reduces dark current and noise by ensuring complete coverage of the trench surfaces with impurity regions, maintaining the photodiode's depletion region and saturation current, thereby enhancing imaging characteristics.
Implementation Method 1
doping an impurity into a bottom and a sidewall of the trench to form a channel stop impurity region, wherein the channel stop impurity region surrounds the bottom and the sidewall of the trench; The doping step may be performed by plasma doping.
Data Source
AI summary
A semiconductor device includes: a trench device isolating region formed in a substrate to define a photodiode active region; a channel stop impurity region formed in the substrate contacting the device isolating region, wherein the channel stop impurity region surrounds a bottom and a sidewall of the device isolating region; and a photodiode formed within the photodiode active region.