Microlens Array Alignment for CMOS Image Sensor Brightness Uniformity
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Solution Overview
Problem
Conventional CMOS image sensors face issues with non-symmetric brightness distribution and improper color balance due to their non-symmetric pixel layout, which is exacerbated at corners and edges, and existing methods struggle to correct these issues without lengthy and costly modifications.
Innovation Solution
A method involving iterative alignment of the microlens array, starting with pre-simulation to determine a shrink-factor, producing sample silicon dies, evaluating image quality, calculating alignment errors, and adjusting the shrink-factor and de-centering values through post-simulation until satisfactory brightness distribution is achieved, using iterative processes to refine the microlens array alignment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a microlens array is shifted to match incident rays at large angles, then the sensor can accept rays at large angles, but non-symmetric brightness distribution appears over the image output
Solution Approach 1:
The patent applies asymmetry by intentionally shifting the microlens array relative to the pixel array in a non-symmetric manner. The shift amount is calculated based on the principal ray angles at different sensor locations, allowing each region to optimally receive oblique rays while maintaining overall image quality through controlled asymmetric alignment
2Manufacturing precision
If symmetric layouts are imposed on each pixel to resolve non-symmetric brightness distribution, then brightness uniformity may be improved, but many limitations and restrictions are imposed on layout designers and additional silicon real estate is required
Solution Approach 1:
The patent changes the alignment parameter (shift amount) of the microlens array rather than modifying the fundamental symmetric layout structure. By adjusting the shift parameter based on principal ray angle calculations, the patent achieves brightness uniformity improvement without imposing layout restrictions or requiring additional silicon area
3Manufacturing precision
If the entire CMOS pixel layout is modified to correct non-symmetric brightness, then image quality may be improved, but lengthy and expensive turn-around processes are required
Solution Approach 1:
The patent performs preliminary calculation of the optimal microlens shift amount using principal ray angle analysis before the actual sensor fabrication. This pre-determined shift value is then applied during manufacturing, avoiding the need for lengthy post-fabrication layout modifications and reducing turn-around time
4Adaptability or versatility
If conventional shifting techniques are used to match incident rays, then large angle ray acceptance is achieved, but improper color balance occurs in the image output
Solution Approach 1:
The patent optimizes the shift parameter by calculating it based on principal ray angles and evaluating its effect on both brightness distribution and color balance. The optimal shift value is determined to simultaneously achieve large angle ray acceptance while maintaining accurate color balance, rather than using fixed or arbitrary shift amounts
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 corrects non-symmetric brightness distribution and color balance issues, improving image quality by iteratively tuning the microlens array alignment, ensuring uniform brightness and accurate color representation across the entire image.
Implementation Method 1
a microlens to direct incoming light to the photodiode through the color filter
Data Source
AI summary
A method for aligning a microlens array in a sensor die to resolve non-symmetric brightness distribution and color balance of the image captured by the sensor die. The method includes performing a pre-simulation to simulate a microlens array alignment in a silicon die and to determine a shrink-factor and de-centering values, calculating the error in a real product's alignment in process and image offset, performing a post simulation based on offset calculation on the real product and re-design of the microlens alignment, and repeating the steps of calculating the error and performing the post-simulation until a satisfactory brightness distribution is obtained. The sensor die has sensor pixels, each pixel comprising a photodiode and a microlens for directing incoming light rays to the photodiode, wherein optical axis of the microlens is shifted with respect to optical axis of the photodiode by a preset amount determined by at least one iteration of alignment process.