Image Sensor Pad Metallization for Void-Free Cover Bonding
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Solution Overview
Problem
In image sensor devices, forming electrical connections on the side where epitaxial silicon has been formed is challenging due to the thickness of the epitaxial layer, and existing methods for filling pad openings with bonding glue often result in voids, which can hinder the bonding process with optically transmissive covers.
Innovation Solution
A method involving electrolessly plating a layer of metal, such as nickel, directly onto the pad base within the pad opening of the image sensor die, ensuring the metal layer is level with the top surface, thereby facilitating a void-free filling of the pad opening and enabling even spreading of bonding glue during the bonding process with an optically transmissive cover.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If bonding glue is used to fill pad openings in image sensor die, then electrical connections can be established, but voids form in the bonding process which hinders reliable connections
Solution Approach 1:
A metal layer is deposited onto the pad base before the bonding glue filling process. This preliminary metal layer creates a planarized surface that enables the bonding glue to spread evenly without forming voids, thus ensuring reliable electrical connections while achieving void-free bonding quality
Solution Approach 2:
The metal layer acts as an intermediary between the pad base and the bonding glue. It provides a uniform surface that mediates the bonding process, preventing void formation while maintaining electrical connectivity, thus resolving the contradiction between connection reliability and bonding quality
2Ease of manufacture
If the pad opening is filled directly with bonding glue, then the process is simple, but air gaps form which prevent reliable bonding with optically transmissive covers
Solution Approach 1:
The metal layer is deposited as a preliminary step before applying bonding glue. This creates a planarized surface that allows subsequent bonding glue application to be simple and effective, eliminating air gaps while maintaining process simplicity, thus resolving the contradiction between manufacturing ease and bonding reliability
3Reliability
If the epitaxial layer is thick to ensure proper image sensor formation, then device performance is maintained, but forming electrical connections on that side becomes challenging
Solution Approach 1:
Instead of attempting to form electrical connections through the thick epitaxial layer (vertical dimension), the solution creates pad openings and fills them with metal and bonding glue (horizontal dimension). This dimensional shift allows electrical connections to be formed without compromising the thick epitaxial layer needed for image sensor performance, thus resolving the contradiction between device performance and manufacturing ease
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 allows for reliable electrical connections and void-free bonding, enhancing the bonding process and preventing air gaps, thus ensuring the image sensor device's performance and efficiency.
Implementation Method 1
electrolessly plating a layer of metal directly onto the pad base to a top surface of the pad opening
Data Source
AI summary
Implementations of a pad for an image sensor may include a pad base coupled in a pad opening formed in an image sensor die and a layer of metal directly coupled to the pad base extending to a top surface of the pad opening. The pad base may directly couple with a first metallization layer of the image sensor die.