Backside Metal Pad for Image Sensor Light Loss Reduction
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Solution Overview
Problem
CMOS image sensors suffer from light loss and restricted design due to metal interconnection routing, which limits the area available for light reception and degrades image sensitivity.
Innovation Solution
The image sensor is designed with a metal pad formed on the backside of a semiconductor substrate, electrically connected to a via metal through a super via or damascene process, allowing light to be incident directly onto the photodiode from the backside, thereby reducing optical path length and improving sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal interconnection layers are formed over the photodiode and transistor on the frontside, then electrical connection is achieved, but the area of light receiving part is restricted and light loss occurs
Solution Approach 1:
The patent inverts the conventional structure by forming the metal pad on the backside of the semiconductor substrate rather than on the frontside. This inversion allows light to incident directly onto the photodiode without passing through metal interconnection layers, eliminating light loss while maintaining electrical connection through the backside-mounted metal pad that connects to via metal extending through the substrate
2Reliability
If metal interconnection layers are formed on the frontside, then electrical connection is achieved, but the optical path length increases and design flexibility is restricted
Solution Approach 1:
The patent moves the metal pad from the frontside to the backside of the semiconductor substrate, inverting the conventional arrangement. This allows the optical path to be shortened as light incident on the backside travels a shorter distance to reach the photodiode, while electrical connection is maintained through via metals that extend from the frontside through the substrate to connect with the backside metal pad
3Length of stationary object
If the number of metal interconnection layers is restricted to shorten optical path, then optical path length is reduced, but design flexibility is restricted
Solution Approach 1:
The patent resolves the design flexibility limitation by utilizing the third dimension (vertical depth) through via metals that extend through the substrate thickness. This allows metal interconnection to occur in the depth dimension rather than only in planar layers, enabling flexible routing and connection designs while maintaining a short optical path in the light-receiving plane
Data Source
AI summary
Disclosed are an image sensor and a method for manufacturing the same. The image sensor includes a semiconductor substrate formed on a first surface thereof with a readout circuitry and a photodiode area; a metal interconnection layer formed on the first surface; a connection via metal extending from the first surface to a second surface of the semiconductor substrate, the connection via metal having a projection part projecting from the second surface; an insulating layer formed on the first surface of the semiconductor substrate to expose the projection part while surrounding a portion of a lateral side of the projection part; and a metal pad formed on the insulating layer such that the metal pad covers the projection part, thereby shortening an optical path to reduce light loss and improve image sensitivity.