Self-aligned Contact Pad for Image Sensor Backside Grounding
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Solution Overview
Problem
Current processes for forming metal contact pads in image sensors, particularly for backside illuminated products, are cumbersome and costly due to the need for many photo lithography and etching steps, resulting in high costs and long cycle times, and often fail to ground the backside of the sensor as there is no connection from the metal shield to the contact pad.
Innovation Solution
The implementation of self-aligned etching and chemical mechanical polishing (CMP) steps reduces the number of masking steps required to form a grounded backside, metal grid, and metal shield, achieving these device architectures with just three masking steps, utilizing step-height differences and aluminum deposition to create a self-aligned contact pad.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If traditional photo lithography and etching steps are used to form metal contact pads, then the contact pads can be formed, but the process becomes cumbersome and costly with many additional fabrication steps
Solution Approach 1:
The sidewall of the trench serves as a self-aligned mask for aluminum deposition, eliminating the need for separate photo lithography and etching steps. The structure itself provides the alignment reference, making the process self-service and reducing fabrication complexity
Solution Approach 2:
The trench is formed first with precise dimensions, and then aluminum is deposited on its sidewall. This preliminary formation of the trench structure enables subsequent self-aligned contact pad formation without requiring additional masking steps
2Reliability
If traditional processes are used for backside illuminated products, then metal contact pads can be formed, but the backside of the sensor cannot be grounded due to no connection from metal shield to contact pad
Solution Approach 1:
The metal shield, contact pad, and grounding path are merged into a single continuous aluminum structure formed by depositing aluminum on the trench sidewall. This eliminates the need for separate grounding connections and ensures electrical continuity from the backside to the contact pad
Solution Approach 2:
The aluminum is deposited on the vertical sidewall of the trench rather than on a horizontal surface, creating a three-dimensional conductive path that simultaneously forms the contact pad and provides backside grounding through the metal shield
3Manufacturing precision
If multiple photo lithography and etching steps are used, then metal contact pads can be formed, but the cycle time increases resulting in long production times
Solution Approach 1:
The trench sidewall automatically serves as the alignment mask for aluminum deposition, eliminating the need for multiple photo lithography and etching cycles. This self-aligned approach dramatically reduces fabrication cycle time while maintaining precision
Solution Approach 2:
The complex photo lithography and etching steps are extracted and replaced by a simpler aluminum deposition process on the trench sidewall, reducing the number of fabrication steps and accelerating production
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 simplifies the process flow, reduces costs, and enables a grounded backside, improving subsequent micro lens and color filter processes by using a flat surface, while maintaining device performance and functionality.
Implementation Method 1
aluminum deposition to create a self-aligned contact pad
Implementation Method 2
chemical mechanical polishing (CMP) steps reduces the number of masking steps required
Data Source
AI summary
An image sensor includes a plurality of photodiodes disposed in a semiconductor material to convert image light into image charge, and a metal grid, including a metal shield that is coplanar with the metal grid, disposed proximate to a backside of the semiconductor material. The metal grid is optically aligned with the plurality of photodiodes to direct the image light into the plurality of photodiodes, and a contact pad is disposed in a trench in the semiconductor material. The contact pad is coupled to the metal shield to ground the metal shield.