Transfer Gate Fabrication Using Sacrificial Dielectric Pillar
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Solution Overview
Problem
The fabrication of pixels with vertical transfer gates for increased pixel density is complicated and results in image artifacts due to surface features on the trench sidewalls and bottom surface formed by reactive ion etching or reactive ion sputtering, leading to electron transport lag and dark current issues.
Innovation Solution
A method involving forming a dielectric pillar on a semiconductor substrate, growing an epitaxial layer surrounding the pillar, and removing the pillar to create a trench, which avoids the surface features that cause image artifacts, allowing for precise control of trench depth and reducing electron transport lag.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If reactive ion etching or reactive ion sputtering is used to form the trench, then the trench can be formed with vertical walls, but surface features on the sidewalls and bottom surface cause electron transport lag and dark current
Solution Approach 1:
A sacrificial mandrel is introduced as an intermediary structure to define the trench geometry. The mandrel is formed first, then epitaxial growth occurs around it, and finally the mandrel is removed. This intermediary approach avoids direct etching of the substrate, eliminating the harmful surface features that cause image artifacts while maintaining precise trench depth control through the mandrel's dimensions and the controlled epitaxial growth process.
Solution Approach 2:
The mandrel is formed in advance before the trench is created. By establishing the mandrel structure first, the trench geometry is predetermined, allowing for precise depth control during subsequent epitaxial growth. This preliminary action ensures that the trench will have the correct dimensions while avoiding the harmful effects of direct etching methods.
2Quantity of substance
If the trench depth is increased to 0.4 micrometers for vertical transfer gate, then pixel density increases, but electron transport lag and dark current increase due to surface features
Solution Approach 1:
The sacrificial mandrel serves as a mediator that enables the formation of deep trenches (0.4 micrometers) without creating harmful surface features. By using the mandrel to define the trench geometry and then removing it after epitaxial growth, the method achieves the required trench depth for vertical transfer gates while maintaining image quality and eliminating electron transport lag and dark current issues.
Solution Approach 2:
The patent replaces the mechanical/chemical etching process (reactive ion etching or sputtering) with an epitaxial growth process. Instead of removing material to create the trench, the method grows epitaxial layers around a mandrel and then removes the mandrel, leaving a clean trench. This substitution eliminates the surface features that cause reliability issues while achieving the necessary trench depth.
3Ease of manufacture
If traditional etching methods are used, then the fabrication process is simpler, but image artifacts result from surface features on trench walls
Solution Approach 1:
The sacrificial mandrel is used as an intermediary structure that simplifies the overall fabrication process by providing a straightforward template for trench formation. Rather than using complex etching processes that create harmful surface features, the mandrel-based approach with epitaxial growth and mandrel removal provides a cleaner, more reliable method that eliminates image artifacts while remaining manufacturable.
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 method enhances pixel density without the image artifacts associated with traditional etching techniques, providing a trench with controlled depth and improved image quality by eliminating electron transport lag and dark current issues.
Implementation Method 1
growing an epitaxial layer on the semiconductor substrate and surrounding the dielectric pillar
Data Source
AI summary
A method for forming a transfer gate includes (i) forming a dielectric pillar on a surface of a semiconductor substrate and (ii) growing an epitaxial layer on the semiconductor substrate and surrounding the dielectric pillar. The dielectric pillar has a pillar height that exceeds an epitaxial-layer height of the epitaxial layer relative to the surface. The method also includes removing the dielectric pillar to yield a trench in the epitaxial layer. A pixel includes a doped semiconductor substrate having a front surface opposite a back surface. The front surface forms a trench extending a depth zT with respect to the front surface within the doped semiconductor substrate along a direction z perpendicular to the front surface and the back surface. The pixel has a dopant concentration profile, a derivative thereof with respect to direction z being discontinuous at depth zT.