CMOS Image Sensor Capacitive Coupled Photodiode Dark Noise Reduction
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Solution Overview
Problem
CMOS image sensors face challenges with high dark noise and junction leakage due to the formation of direct ohmic contacts, which lead to increased dark current and poor signal-to-noise ratios, particularly in the 3T type CMOS image sensors.
Innovation Solution
The implementation of a capacitive coupled photodiode structure that eliminates the need for direct ohmic contacts by using a capacitive element with an insulation layer between conductive plates, reducing junction leakage and dark current, and integrating a source follower transistor via the capacitive element, while maintaining compatibility with standard CMOS processing technology.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If direct ohmic contacts are formed in the photodiode structure, then electrical connection is achieved, but junction leakage and dark current increase
Solution Approach 1:
The patent introduces a capacitive coupling structure as an intermediary between the photodiode and the readout circuit. Instead of direct ohmic contact, a capacitor is inserted to transfer the signal, which blocks the direct path for dark current while still allowing AC signal transmission. This mediator structure eliminates the harmful direct contact path while preserving the useful signal transfer function.
Solution Approach 2:
The patent replaces the traditional direct electrical contact (ohmic contact) with a capacitive coupling mechanism. The capacitor transfers the photodiode signal through electric field coupling rather than direct charge carrier flow, substituting a field-based mechanism for a contact-based mechanism, thereby eliminating junction leakage at the contact interface.
2Ease of operation
If direct ohmic contacts are formed, then signal readout is enabled, but dark noise increases due to contact formation and plasma etching
Solution Approach 1:
The capacitive coupling structure serves as an intermediary that enables signal readout without requiring direct ohmic contact. The capacitor allows the AC signal from the photodiode to be transferred to the readout circuit while blocking DC components including dark current, thus enabling signal readout while eliminating the source of dark noise associated with contact formation.
Solution Approach 2:
The patent extracts and removes the direct ohmic contact element from the photodiode structure. By taking out the problematic contact formation step and replacing it with capacitive coupling, the source of dark noise generation (contact formation and associated plasma etching) is completely eliminated while signal readout functionality is preserved.
3Reliability
If capacitive coupling structure is implemented, then dark noise is reduced, but device complexity increases
Solution Approach 1:
The patent merges the capacitor structure with existing pixel cell components. The capacitive coupling elements are integrated into the photodiode structure itself, sharing common regions and processes with other pixel components, thereby reducing the overall complexity increase that would result from adding separate discrete components.
Solution Approach 2:
The capacitive coupling structure serves multiple functions simultaneously: it transfers the photodiode signal to the readout circuit, blocks dark current, and can be integrated with existing pixel cell components. This multi-functionality reduces the need for additional separate structures, thereby limiting the increase in device complexity.
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 significantly reduces dark noise and improves the signal-to-noise ratio without increasing pixel cell size, eliminating sources of junction leakage associated with direct contact formation and plasma etching processes.
Implementation Method 1
a capacitive element having a first conductive plate overlying a portion of the n- doped region of the photodiode and a second conductive plate
Implementation Method 2
The photodiode receives an incident light and generates a corresponding electric charge
Data Source
AI summary
A complementary metal oxide semiconductor (CMOS) image sensor device includes a capacitive coupled photodiode that is formed within a region of a semiconductor substrate. The photodiode receives an incident light and generates a corresponding electric charge. The CMOS image sensor device includes a reset transistor coupled to the photodiode for reverse biasing the photodiode with a predetermined voltage. The CMOS image sensor device further includes a buffer circuit and a capacitor, which is interposed between the photodiode and the buffer circuit. The capacitor is configured to transfer the electric charge to the buffer circuit. The buffer circuit may include an emitter follower or a source follower transistor.