IMD-Level Micro-Lenses for Image Sensor Light Transmission
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Solution Overview
Problem
Solid state imaging devices face reduced sensitivity due to thick inter-metal dielectric layers and metal interconnects, which obstruct light alignment and reflection, particularly in system-on-chip applications with increasing metallization levels and shrinking pixel sizes.
Innovation Solution
The implementation of IMD-level micro-lenses with refractive indices greater than the IMD layers, and barrier layers with limited net thickness of 100 angstroms or less, along with a cap layer on copper metal lines, to enhance light transmission and reduce reflection, allowing more light to reach the photosensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If thick inter-metal dielectric layers are used to accommodate multiple levels of metallization, then device complexity and integration capability are improved, but light transmission to photosensors deteriorates due to increased distance and reflection
Solution Approach 1:
The patent introduces an intermediary optical structure (lens system) between the interconnection layer and photosensor to redirect and focus light through the thick IMD layer. This mediator enables light to navigate the complex interconnection structure and reach the photosensor effectively, resolving the contradiction between thick IMD for high integration and light transmission requirements.
Solution Approach 2:
The patent modifies optical parameters by introducing lenses with specific focal lengths and positions within the IMD layer. By changing the optical path parameters (focal point, lens curvature, layer thicknesses), the system achieves effective light transmission despite the increased physical thickness of the interconnection structure.
2Adaptability or versatility
If more levels of metallization are added to increase integration, then device functionality is improved, but light reflection increases reducing photosensor sensitivity
Solution Approach 1:
The lens structure acts as an optical intermediary that collects and focuses light passing through multiple metal interconnect layers. This mediator compensates for the increasing light reflection and scattering caused by additional metallization levels, maintaining photosensor sensitivity while enabling higher system-on-chip integration.
Solution Approach 2:
The patent converts the harmful effect of thick IMD and multiple metal layers (which normally block and reflect light) into a benefit by using the lens to focus the transmitted light. The very structures that cause light loss are transformed into an opportunity for optical focusing that enhances light delivery to the photosensor.
3Productivity
If pixel area is reduced to increase pixel density, then productivity is improved, but alignment precision between lens and photosensor becomes more critical and difficult to achieve
Solution Approach 1:
The patent addresses alignment precision by introducing the vertical dimension (depth) as an additional degree of freedom. By positioning lenses at specific depths within the IMD layer and using focal length control, the system achieves precise optical coupling between lens and photosensor even when lateral dimensions are reduced for higher pixel density.
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 solution improves light sensitivity and compatibility with thicker interconnection structures and shrinking pixel areas, maintaining compatibility with existing manufacturing methods and tooling.
Implementation Method 1
The micro-lens array is formed over the color filter with the micro-lenses having a refractive index greater than that of the IMD layers
Implementation Method 2
barrier layers with limited net thickness of 100 angstroms or less, along with a cap layer on copper metal lines, to enhance light transmission and reduce reflection
Data Source
AI summary
An image sensor device is provided. A substrate has a photosensor region formed therein and/or thereon. An interconnection structure is formed over the substrate, and includes metal lines formed in inter-metal dielectric (IMD) layers. At least one IMD-level micro-lens is/are formed in at least one of the IMD layers over the photosensor region. Preferably, barrier layers are located between the IMD layers. Preferably, each of the barrier layers at each level has a net thickness limited to 100 angstroms or less at locations over the photosensor region, except at locations where the IMD-level micro-lenses are located. The IMD-level micro-lenses and the etch stop layers preferably have a refractive index greater than that of the IMD layers. A cap layer is preferably formed on the metal lines, especially when the metal lines include copper. An upper-level micro-lens may be located on a level that is above the interconnection structure.