Backside Illuminated Sensor Metal Extension for Wafer Testing
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Solution Overview
Problem
Backside illuminated image sensors with thin substrates require a thick carrier wafer for support, which obstructs access to bond pads, preventing wafer level testing such as probe testing and design performance testing.
Innovation Solution
The solution involves creating extended bond pads that can be accessed from the substrate's backside without penetrating the thick carrier wafer, allowing for wafer level testing by forming metal extensions or openings in the substrate to reach the bond pads, enabling probe testing while the sensor is still on the wafer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a thick carrier wafer is used to support the thin substrate, then mechanical strength is improved, but access to bond pads is blocked preventing wafer level testing
Solution Approach 1:
The patent extends bond pads from the front surface into the substrate thickness dimension, creating via holes that provide access to bond pads from the backside of the substrate. This dimensional extension allows testing access without removing the carrier wafer, resolving the contradiction between mechanical support and testing access.
Solution Approach 2:
The patent divides the bond pad structure into multiple segments: the original bond pad on the front surface, conductive vias extending through the substrate, and extended bond pad regions on the backside. This segmentation allows different portions to serve different functions - front side for normal operation, backside for testing - while maintaining structural integrity.
2Illumination intensity
If the substrate is thinned to allow light transmission, then optical performance is improved, but mechanical strength deteriorates requiring a carrier wafer
Solution Approach 1:
The substrate is segmented functionally into a thin optical region for light transmission and a structured region with extended bond pads and vias for mechanical support and electrical access. This segmentation allows the substrate to be thin enough for optimal light transmission while maintaining sufficient mechanical strength through the added structural elements.
Solution Approach 2:
The patent creates a composite structure combining the thin substrate material with the carrier wafer material, and further integrates conductive materials in the via regions. This composite approach allows the thin substrate to maintain mechanical strength through the combined structure while preserving optical transmission properties.
3Ease of operation
If extended bond pads are created via via holes, then access for testing is improved, but manufacturing complexity increases
Solution Approach 1:
The extended bond pads and via holes are formed during the substrate processing stage, before the substrate is mounted to the carrier wafer. This preliminary action integrates the testing access structure into the main fabrication flow, avoiding the need for complex post-assembly modifications and reducing overall manufacturing complexity.
Data Source
AI summary
A method of providing metal extension in a backside illuminated image sensor is provided in the present disclosure. In one embodiment, a first set of pads and a second set of pads, and a metal layer are provided in a backside illuminated image sensor. The first set of pads are electrically coupled to the second set of pads through the metal layer, and a pad in the second set of pads is exposed to the surface of the backside illuminated image sensor for testing. In an alternative embodiment, a first set of pads, at least one second pad directly positioned over the first set of pads are provided in a backside illuminated image sensor. The first set of pads are electrically coupled to the at least one second pad and the at least one second pad is exposed to the surface of the backside illuminated image sensor for testing.