CMOS Image Sensor Gate Silicidation for Dark Signal Reduction
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Solution Overview
Problem
CMOS image sensors experience increased dark signal issues due to metallic contamination from wet cleaning processes, leading to erroneous or distorted image signals, as the tungsten silicide gate electrodes are exposed and prone to contamination.
Innovation Solution
A method for forming a CMOS image sensor involving the sequential patterning of a gate insulating layer and doped polysilicon layers to create transfer and reset gates, with a floating diffusion layer in between, followed by the formation of metal gate silicide and source/drain metal silicide using different metal layers, and the use of protective insulation and spacer layers to prevent contamination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If wet cleaning processes are performed to clean the substrate, then cleaning effectiveness is improved, but metallic contamination of the photodiode increases due to exposed tungsten silicide gate electrodes
Solution Approach 1:
An insulating layer is introduced as an intermediary between the tungsten silicide gate electrode and the photodiode. This insulating layer prevents direct contact and contamination between the metal gate and the photodiode during wet cleaning processes, while still allowing the cleaning to effectively remove contaminants from the substrate surface.
Solution Approach 2:
The patent converts the potentially harmful exposure of the tungsten silicide gate electrode during wet cleaning into a beneficial structure by adding the insulating layer. This layer not only prevents contamination but also becomes part of the device structure, isolating the metal gate from the photodiode region and preventing dark signal generation.
2Speed
If tungsten silicide is used in the gate electrode to reduce resistance, then operational speed is improved, but dark signal increases due to photodiode contamination from the metal gate
Solution Approach 1:
The insulating layer serves as a mediator that allows the tungsten silicide gate to maintain its low-resistance, high-speed performance while preventing it from generating dark signals in the photodiode. The layer physically separates the metal gate from the photodiode, blocking the generation of leakage currents.
Solution Approach 2:
The gate structure is segmented into distinct functional layers: the tungsten silicide layer for low resistance and the insulating layer for isolation. This segmentation allows each layer to perform its specific function without interfering with or contaminating other components.
3Ease of operation
If the tungsten silicide gate electrode is exposed during wet cleaning, then access for cleaning is improved, but contamination of the photodiode occurs at the exposed portions
Solution Approach 1:
The insulating layer acts as a protective intermediary that allows wet cleaning processes to access and clean the substrate surface effectively, while simultaneously preventing metal contaminants from reaching the photodiode region during the cleaning operation.
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 minimizes dark signal effects by preventing metal contamination and optimizing the operational speed of the CMOS image sensor, ensuring accurate image signal transmission.
Implementation Method 1
carrying out a selective silicidation process using a metal gate layer to form metal gate silicide on the exposed gate
Implementation Method 2
sequentially forming a protective insulation layer and a mold layer on an entire surface of the substrate
Implementation Method 3
planarizing the mold layer until the protective insulation layer is exposed
Data Source
AI summary
A CMOS image sensor and a method for forming the same are provided. According to the method, a gate insulating layer and a doped polysilicon layer which are sequentially stacked on a substrate are patterned to form a transfer gate and a reset gate set apart from each other. A floating diffusion layer between the transfer gate and the reset gate, a light receiving element at a side of the transfer gate away from and opposite to the floating diffusion layer and a source/drain region at a side of the reset gate away from and opposite to the floating diffusion layer are formed. An insulation layer and a mold layer are sequentially formed on an entire surface of the substrate, and the mold layer is planarized until the insulation layer is exposed. The exposed insulation layer is removed to further expose an upper surface of the gates. A selective silicidation process is carried out using a metal gate layer to form a metal gate silicide on the exposed gate. The sequential steps in the selective silicidation process alleviate the metal contamination prevalent in various wet cleaning processes that may increase the malfunction of CMOS image sensors.