CMOS Photodiode Lateral Extension Reduces Dark Current
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Solution Overview
Problem
Image sensors based on CMOS technology suffer from high dark current issues due to parasitic currents generated at the contact region of the photodiode and space charge region beneath the spacer, which degrades image resolution at low luminance.
Innovation Solution
The solution involves eliminating the contact region of the space charge region of the photodiode and surface dielectric at the spacer by creating a lateral surface extension of the upper pn junction beneath the spacer, using a plasma doping process to produce a thin surface layer that does not significantly alter the photodiode's characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the photodiode structure is extended beneath the spacer to eliminate contact region, then dark current is reduced, but manufacturing complexity increases
Solution Approach 1:
The photodiode structure is segmented into distinct regions: the upper pn junction with lateral surface extension beneath the spacer, and the bulk photodiode region. This segmentation allows the surface extension to be optimized for eliminating contact region issues while the bulk region maintains standard photodiode functionality, thus reducing dark current without overly complicating the overall device.
Solution Approach 2:
The invention extends the pn junction laterally beneath the spacer in the horizontal dimension, rather than only vertically. This lateral extension creates a surface layer that eliminates the contact region between the photodiode and spacer, reducing dark current pathways while maintaining the vertical stacking efficiency needed for compact device architecture.
2Manufacturing precision
If plasma doping process is used to create thin surface layer, then manufacturing precision is improved, but process complexity increases
Solution Approach 1:
The plasma doping process parameters (ion energy, doping concentration, plasma composition) are precisely controlled to achieve the desired thin surface layer thickness and dopant profile. By optimizing these parameters, the process achieves high manufacturing precision for the surface extension while managing process complexity through established plasma technology.
Solution Approach 2:
The invention replaces traditional mechanical or thermal diffusion methods with plasma doping to create the thin surface layer. Plasma doping offers superior precision and control over dopant distribution at shallow depths, achieving the required manufacturing precision while using a well-established semiconductor fabrication technique rather than developing entirely new processes.
3Object-generated harmful factors
If lateral surface extension is created beneath spacer, then parasitic current is reduced, but photodiode characteristics may be altered
Solution Approach 1:
The lateral surface extension is created with specific local properties: a thin surface layer with controlled thickness and dopant concentration. This local modification beneath the spacer targets only the problematic contact region, reducing parasitic currents without significantly altering the electrical characteristics of the main photodiode bulk region. The local quality change is confined to where it is needed.
Solution Approach 2:
The surface extension is designed to be thin and localized, providing just enough modification to eliminate the contact region issue without excessive action that would alter photodiode characteristics. The partial extension beneath the spacer is sufficient to reduce parasitic currents while maintaining the overall photodiode performance.
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 without impairing the photodiode's electrical characteristics, improving image resolution by minimizing parasitic currents at low luminance.
Implementation Method 1
a surface layer having a second type of conductivity, the opposite of the first, is produced within this initial semiconductor region before the lateral spacers of the insulated gate are produced
Implementation Method 2
the surface layer is produced by implantation using a plasma doping process
Implementation Method 3
Image sensors based on semiconductor components rely on the principle of converting photons into electron/hole pairs in silicon
Data Source
AI summary
An integrated circuit includes at least one photodiode associated with a transfer transistor. The photodiode is formed with an upper pn junction. The transfer transistor includes a lateral spacer located on a side facing the photodiode. An upper layer of the upper pn junction includes a lateral surface extension lying beneath the spacer. A lower layer of the upper pn junction forms a source/drain region for the transfer transistor. An edge of the lateral surface extension lying beneath the spacer and adjacent a gate of the transfer transistor contacts a substrate of the integrated circuit. An oxide layer insulating the gate from the underlying substrate does not overlie the lateral surface extension of the upper layer underneath of the lateral spacer.