Image Sensor Ion Implantation Isolation and Contact Layers
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Solution Overview
Problem
Existing image sensors face challenges in achieving proper physical contact between photodiodes and metal interconnections, electrical isolation of color contacts in stacked RGB configurations, and reducing charge sharing and dark current, which affect image quality and sensitivity.
Innovation Solution
The solution involves forming ion implantation isolation layers and contact layers in a CMOS image sensor, with stacked color image sensing units on a substrate, and bonding substrates with metal interconnections to enhance contact and electrical isolation, while creating a potential difference between the source and drain of transfer transistors to prevent charge sharing and facilitate swift photocharge transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If wafer-to-wafer bonding is performed to contact photodiode with metal interconnection, then physical contact is achieved, but contact reliability is insufficient
Solution Approach 1:
An N-type contact layer is introduced as an intermediary between the photodiode and metal interconnection. This contact layer serves as a mediator that facilitates reliable electrical contact while compensating for alignment inaccuracies during wafer bonding, thereby improving contact reliability without requiring extreme manufacturing precision.
Solution Approach 2:
The electrical parameters (doping concentration, thickness) of the contact layer are optimized to achieve ohmic contact. By adjusting these parameters, the contact resistance is minimized and contact reliability is improved, while the contact layer's physical properties provide tolerance for alignment variations during bonding.
2Productivity
If stacked RGB photodiode configuration is used to increase pixel density, then productivity increases, but electrical isolation between color contacts becomes difficult
Solution Approach 1:
The stacked photodiode structure is segmented with dedicated isolation regions between different color contacts. These isolation regions electrically separate the contacts for different colors (e.g., blue, green, red) while maintaining their vertical stacking arrangement, thus preserving high pixel density while achieving reliable electrical isolation.
Solution Approach 2:
Isolation layers are introduced as intermediary structures between contacts of different colors in the stacked configuration. These layers act as electrical barriers that prevent cross-talk between color channels while allowing the compact stacked architecture to maintain high pixel density.
3Reliability
If heavily doped source and drain are used in transfer transistor, then electrical conductivity increases, but charge sharing phenomenon occurs reducing image quality
Solution Approach 1:
The doping concentration is made non-uniform in the source and drain regions. By applying local quality variations (heavily doped contact regions vs. lightly doped channel regions), the source and drain achieve good electrical conductivity for charge transfer while the reduced doping in critical areas prevents charge sharing and maintains image quality.
4Productivity
If photodiode area is reduced to increase pixel count, then productivity increases, but light receiving capability deteriorates
Solution Approach 1:
The photodiode structure transitions from a planar two-dimensional layout to a three-dimensional stacked configuration. By stacking multiple color photodiodes vertically, the light receiving area is effectively increased in the vertical dimension while maintaining a compact footprint, thus increasing pixel count without sacrificing light receiving capability.
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 improves contact reliability, reduces charge sharing and dark current, and enhances image sensitivity and quality by ensuring effective electrical isolation and efficient photocharge transfer.
Implementation Method 1
forming an ion implantation isolation layer in the image sensing device
Implementation Method 2
forming ion implantation contact layers electrically connected to the first, second and third color image sensing units
Implementation Method 3
bonding the first and second substrates to each other to allow the image sensing device to contact the metal interconnection
Data Source
AI summary
An image sensor includes a metal interconnection and readout circuitry over a first substrate, an image sensing device, and an ion implantation isolation layer. The image sensing device is over the metal interconnection, and an ion implantation isolation layer is in the image sensing device. The image sensing device includes first, second and third color image sensing units, and ion implantation contact layers. The first, second and third color image sensing units are stacked in or on a second substrate. The ion implantation contact layers are electrically connected to the first, second and third color image sensing units, respectively.