Embedded Bonding Pad for Image Sensor Planarity
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Solution Overview
Problem
The profile and thickness of bonding pads on image sensors interfere with the formation of color filter layers and micro-lenses, degrading photosensitivity by increasing the distance light must travel to reach the pixels.
Innovation Solution
A method of forming a bonding pad within the passivation layer and inter-metal dielectric, ensuring it is co-planar with the passivation layer, reducing the step height and creating a smooth surface for the color filter layer and micro-lens formation, and using a conductive layer that is planarized to achieve adequate bonding properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If bonding pads are formed near the top surface of the image sensors, then bonding pads are available for wafer level testing and wire bonding, but the profile of the bonding pad adversely affects the process of forming the color filter layer and micro-lens
Solution Approach 1:
The bonding pad is embedded within the passivation layer, nesting the bonding pad structure inside the protective passivation layer. This embedding approach allows the bonding pad to be accessible for wire bonding while being contained within the planarized passivation layer structure, preventing profile interference with color filter and micro-lens formation.
Solution Approach 2:
The bonding pad is positioned at a different vertical dimension (embedded within the passivation layer) rather than protruding at the top surface. This dimensional repositioning allows the bonding pad to maintain its electrical connection function while eliminating the profile interference that would occur if it protruded at the same level as the color filter and micro-lens formation surfaces.
2Strength
If the thickness of the bonding pad is increased, then bonding properties for wire bonding are improved, but the distance that light must travel to reach the pixels increases, degrading photosensitivity
Solution Approach 1:
The bonding pad is nested within the passivation layer, which provides a planarized surface. This nesting allows the bonding pad to have sufficient thickness for adequate bonding properties while being contained within the overall planar structure of the passivation layer, preventing additional light travel distance through protrusion.
Solution Approach 2:
The passivation layer acts as an intermediary between the bonding pad and the external environment. It provides a planarized surface that maintains the bonding pad's thickness for adequate bonding while preventing the bonding pad profile from interfering with light path to the pixels, thus protecting photosensitivity.
3Manufacturing precision
If the bonding pad profile is reduced, then the formation of color filter layer and micro-lens is improved, but bonding properties may be compromised
Solution Approach 1:
The bonding pad is nested within the passivation layer, allowing it to have sufficient thickness for bonding properties while the passivation layer provides the planarized external surface. This nested structure simultaneously achieves both adequate bonding properties and improved color filter/micro-lens formation by eliminating profile interference.
Solution Approach 2:
The bonding pad thickness is optimized in the vertical dimension within the passivation layer, while the passivation layer provides the planarized surface in the horizontal dimension. This dimensional separation allows the bonding pad to have sufficient thickness for bonding without creating profile interference at the top surface.
Data Source
AI summary
A semiconductor device includes a semiconductor substrate having a front surface and a back surface, elements formed on the substrate, interconnect metal layers formed over the front surface of the substrate, including a topmost interconnect metal layer, an inter-metal dielectric for insulating each of the plurality of interconnect metal layers, and a bonding pad disposed within the inter-metal dielectric, the bonding pad in contact with one of the interconnect metal layers other than the topmost interconnect metal layer.