Biasing Light Shielding Structures in Stacked Image Sensors
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Solution Overview
Problem
Conventional imaging systems face challenges in properly grounding tungsten light shields and biasing in-pixel light grids, leading to increased costs and undesirable topography, as well as undesired coupling effects due to inadequate doping and complex processing steps.
Innovation Solution
The implementation of through-oxide vias and integral metal routing paths allows for direct biasing of light shielding structures, while in-pixel grids are coupled to peripheral contacts or conductive straps to achieve effective voltage biasing, minimizing copper contamination and simplifying the manufacturing process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a via is formed from the tungsten light shield to a deep contact in the semiconductor substrate, then the light shield can be grounded, but extra processing steps are required and cost increases
Solution Approach 1:
The light shield structure is merged with the deep trench isolation structure, allowing both functions to be achieved through a single integrated structure formed by the same doping process, eliminating the need for separate via formation steps
Solution Approach 2:
The deep trench isolation structure serves multiple functions: it provides electrical isolation between pixels and simultaneously provides a grounded path for the light shield, eliminating the need for dedicated grounding vias
2Reliability
If deep p-wells or n-wells are used to form Ohmic contact, then grounding can be achieved, but the doping at the bottom may not be adequate to form a robust Ohmic contact
Solution Approach 1:
The doping concentration is locally optimized at the bottom of the deep trench isolation structure where contact is needed, with higher doping levels specifically at the contact region to ensure robust Ohmic contact while maintaining lower doping in other regions
3Reliability
If an aluminum strap is used to couple the tungsten light shield to the aluminum wire bond pad, then grounding can be achieved, but an undesirable topography is produced
Solution Approach 1:
The grounding function is merged into the deep trench isolation structure itself, which is formed as an integral part of the pixel structure, avoiding the need for separate aluminum straps that would create topography issues
4Reliability
If in-pixel light grids are used to improve isolation between active photodiodes, then isolation can be improved, but the grids are not shorted to ground causing undesired coupling effects
Solution Approach 1:
The in-pixel light grids are extracted from the active pixel region and relocated to the deep trench isolation structures between pixels, where they can be properly grounded and cannot cause coupling effects between active photodiodes
Data Source
AI summary
An imaging system may include an image sensor die stacked on top of a digital signal processor (DSP) die. Through-oxide vias (TOVs) may be formed in the image sensor die and may extend at least partially into in the DSP die to facilitate communications between the image sensor die and the DSP die. The image sensor die may include light shielding structures for preventing reference photodiodes in the image sensor die from receiving light and in-pixel grid structures for preventing cross-talk between adjacent pixels. The light shielding structure may receive a desired biasing voltage through a corresponding TOV, an integral plug structure, and/or a connection that makes contact directly with a polysilicon gate. The in-pixel grid may have a peripheral contact that receives the desired biasing voltage through a light shield, a conductive strap, a TOV, and/or an aluminum pad.