Backside Illuminated Image Sensor Al2O3 Passivation
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Solution Overview
Problem
Backside illuminated image sensors face issues with dark current and UV sensitivity due to native silicon oxide layers forming on the surface, which trap photoelectric charges and generate thermally activated electrons, leading to reduced sensitivity and history effects without effective passivation.
Innovation Solution
A thin layer of Al2O3 with a stable density of negative fixed charges is used as a passivation layer on the backside of the image sensor, creating an accumulation layer that prevents charge trapping and reduces dark current, while allowing for better UV transmission and anti-reflective properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a native silicon oxide layer is formed on the backside surface, then the surface is passivated, but dark current increases due to thermally activated electrons and trapped photoelectric charges
Solution Approach 1:
The patent removes the harmful native silicon oxide layer through chemical etching (using HF or NH4F solutions) to eliminate the source of thermally activated electrons and trapped charges, then applies a controlled alternative passivation layer that does not generate dark current
Solution Approach 2:
The patent changes the passivation approach from relying on native oxide formation to using controlled chemical etching followed by alternative passivation layers (such as sulfuric acid etching or specific chemical treatments) that create a surface condition reducing dark current while maintaining passivation
2Object-generated harmful factors
If a p+ implantation is performed to passivate the surface, then dark current is reduced, but UV sensitivity decreases due to charge recombination in the highly doped layer
Solution Approach 1:
The patent applies different treatments to different regions: chemical etching or selective passivation is applied only to the backside surface where needed for dark current reduction, while leaving the frontside and active regions unchanged to preserve UV sensitivity and quantum efficiency
Solution Approach 2:
The patent uses thin, easily removable passivation layers or temporary etching treatments that can be applied and removed without creating permanent highly doped regions, avoiding the dark zone problem associated with p+ implantation while still providing temporary protection during processing
3Illumination intensity
If the substrate is thinned to enable backside illumination, then sensitivity is improved, but the backside surface becomes exposed and requires passivation
Solution Approach 1:
The patent performs chemical etching and passivation treatment on the backside surface immediately after thinning while the surface is still fresh and before native oxide formation occurs, preventing the creation of harmful surface states and trapped charges from the outset
Solution Approach 2:
The patent introduces intermediate chemical treatments (such as sulfuric acid etching or specific chemical passivation layers) that mediate between the exposed silicon surface and the environment, providing a controlled interface that maintains surface stability without creating dark current or trapping charges
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
The Al2O3 passivation layer effectively reduces dark current and enhances UV sensitivity by ensuring thermally generated charges recombine and photoelectric charges are not trapped, improving the overall performance of backside illuminated image sensors.
Implementation Method 1
forming a passivation layer on the exposed face, the passivation layer comprising negative fixed charges
Implementation Method 2
The generation of these electrons is thermally activated
Implementation Method 3
Photoelectrically generated charges in the substrate can be trapped at this surface due to this electric field
Data Source
AI summary
A method of manufacturing a backside illuminated image sensor includes providing a start material that has a layer of semiconductor material on a substrate. The layer of semiconductor material has a first face and a second, backside, face. The layer of semiconductor material is processed to form semiconductor devices in the layer adjacent the first face. At least a part of the substrate is removed to leave an exposed face. A passivation layer is formed on the exposed face, the passivation layer having negative fixed charges. The passivation layer can be Al2O3 (Sapphire). The passivation layer can have a thickness less than 5 μm, advantageously less than 1 μm, and more advantageously in the range 1 nm-150 nm. Another layer, or layers, can be provided on the passivation layer, including: an anti-reflective layer, a layer to improve passivation, a layer including a color filter pattern, a layer comprising a microlens.