Backside Illuminated Image Sensor Black Level Calibration
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Backside illuminated CMOS image sensors face challenges in accurately calibrating the black level due to differences in dark current characteristics between dark and imaging pixels, caused by stress differences from metal light blocking layers, making it difficult and expensive to fabricate light blocking pixels and resulting in inaccurate image data.
Innovation Solution
A system and method for a backside illuminated imaging system that includes a black reference pixel array with a light shielding layer and a buffer layer to prevent light exposure, integrated with readout and control circuitry for accurate black level calibration, which reduces stress differences and faithfully represents the black level of the imaging array.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If metal light blocking layers are formed on backside illuminated CMOS image sensors to create dark pixels for black level calibration, then light blocking capability is improved, but stress differences between dark and imaging pixels cause inaccurate black level calibration
Solution Approach 1:
An intermediate buffer layer is introduced between the metal light blocking layer and the imaging pixels. This buffer layer acts as a stress compensation mechanism that equalizes the stress state between dark pixels and imaging pixels, thereby eliminating the stress-induced dark current differences while maintaining the light blocking function of the metal layer.
Solution Approach 2:
The stress state parameter of the pixel structure is modified by adding the buffer layer. This changes the physical condition of the dark pixels to match the imaging pixels, ensuring that both types of pixels operate under equivalent stress conditions and produce comparable dark current characteristics.
2Measurement precision
If dark pixels with metal light blocking layers are used for black level calibration, then dark current estimation is possible, but fabrication cost and complexity increase due to etch damage and defects
Solution Approach 1:
The buffer layer serves as a protective intermediary that prevents etch damage from propagating to the underlying pixel structures during metal layer fabrication. This intermediate layer absorbs the mechanical stress and chemical damage from the etching process, simplifying the fabrication workflow and reducing defect rates.
3Illumination intensity
If black reference pixels are shielded with metal layers to block light, then light blocking is achieved, but stress differences cause the black reference pixels to display different dark current characteristics from imaging pixels
Solution Approach 1:
The buffer layer acts as a stress-mediating structure that compensates for the stress introduced by the metal light blocking layer. This ensures that the black reference pixels experience the same stress conditions as the imaging pixels, making their dark current characteristics reliable representatives of the imaging array's black level.
Data Source
AI summary
An imaging system capable of black level calibration includes an imaging pixel array, at least one black reference pixel, and peripheral circuitry. The imaging pixel array includes a plurality of active pixels each coupled to capture image data. The black reference pixel is coupled to generate a black reference signal for calibrating the image data. Light transmitting layers are disposed on a first side of a pixel array die including the imaging system and cover at least the imaging pixel array and the black reference pixel. A light shielding layer is disposed on the first side of the pixel array die and covers a portion of the light transmitting layers and the black reference pixel without covering the imaging pixel array.