Image Sensor Gate Oxide Nitridation Noise Reduction
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Solution Overview
Problem
In integrated circuits, particularly in optical sensors, the nitridation process for gate oxides increases Random Telegraph Signal (RTS) noise due to the formation of interface traps, which degrades imaging quality, especially at low light levels, as it leads to undesirable flickering pixels and reduced signal-to-noise ratio.
Innovation Solution
Protecting thick gate oxide layers from nitridation using a photoresist layer during the nitridation process, ensuring only thin gate oxide layers are exposed to nitrogen, thereby reducing the incorporation of nitrogen at the oxide/Si interface and minimizing RTS noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If gate oxide nitridation is performed to block boron penetration, then boron penetration is reduced, but interface states and traps increase causing higher RTS noise
Solution Approach 1:
The patent applies selective nitridation where only specific gate oxide regions are nitrided based on their thickness. Thin gate oxides receive nitridation treatment to block boron penetration, while thick gate oxides are excluded from nitridation to avoid creating harmful interface traps. This local differentiation resolves the contradiction by applying the beneficial effect only where needed without causing the harmful side effect.
Solution Approach 2:
The patent segments the gate oxide layers into two categories: thin gate oxides that require nitridation for boron protection and thick gate oxides that should remain non-nitrided to maintain low noise characteristics. By dividing the treatment approach based on oxide thickness, the patent simultaneously achieves boron penetration blocking where necessary and RTS noise reduction where necessary.
2Reliability
If uniform nitridation is applied to all gate oxides, then boron penetration is blocked across all devices, but RTS noise increases in all devices including those with thick gate oxide
Solution Approach 1:
The patent implements non-uniform nitridation by using thickness-based selection criteria. Gate oxides thinner than a specified threshold receive nitridation treatment, while thicker gate oxides are protected from nitridation. This local quality approach ensures boron penetration blocking is applied only to vulnerable thin gate oxides, preventing RTS noise generation in thick gate oxide devices that would otherwise be unnecessarily degraded.
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 suppresses the formation of traps at the interface between the thick gate oxide and silicon, resulting in lower RTS noise and improved imaging quality by preventing unnecessary nitridation of thick gate oxides, which are less susceptible to boron penetration.
Implementation Method 1
the photoresist acts as a barrier layer to suppress the incorporation of nitrogen into the thick gate oxide layer
Implementation Method 2
nitridation is performed on the thin gate oxide layer to form a nitrided gate oxide layer that has an improved ability to prevent penetration of boron or other dopants
Data Source
AI summary
An image sensor is described in which the imaging pixels have reduced noise by blocking nitridation in selected areas. In one example, a method includes forming a first and second gate oxide layer over a substrate, forming a layer of photoresist over the first gate oxide layer, applying nitridation to the photoresist and the second gate oxide layer such that the first gate oxide layer is protected from the nitridation by the photoresist, and forming a polysilicon gate over the first and second gate oxide layers.