Image Sensor Capping Structure for Low Dark Current
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Solution Overview
Problem
CMOS image sensors face challenges with dark current, which is electrical current passing through photodetectors even in the absence of photons, primarily due to band discontinuity between germanium epitaxial structures and silicon capping structures.
Innovation Solution
The introduction of a second capping structure comprising both the group IV chemical element (germanium or silicon) and a second chemical element with a different energy band gap, positioned between the first capping structure and the epitaxial structure, reduces conduction and valence band discontinuities, thereby minimizing dark current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a silicon capping structure is used to cover the germanium epitaxial structure, then the manufacturing process is simplified and the sensor can be produced using standard CMOS processes, but band discontinuity occurs between the silicon capping structure and germanium epitaxial structure, generating dark current
Solution Approach 1:
A graded buffer layer comprising silicon-germanium alloy with gradually varying germanium concentration is introduced between the silicon capping structure and germanium epitaxial structure. This intermediate layer acts as a transition zone that progressively matches the lattice constant and energy band structure between silicon and germanium, reducing dislocation density and minimizing band discontinuity. The graded composition profile (increasing germanium content from bottom to top) provides a smooth transition that eliminates the abrupt interface, thereby reducing dark current generation while maintaining compatibility with standard CMOS manufacturing processes.
2Object-generated harmful factors
If the energy band gap difference between capping structure and epitaxial structure is reduced, then dark current is minimized, but the device structure becomes more complex requiring additional layers and materials
Solution Approach 1:
The germanium concentration in the buffer layer is continuously varied as a gradient from approximately 0% at the silicon interface to 100% at the germanium epitaxial structure interface. This parameter change approach creates a graded buffer layer where the composition transitions smoothly, progressively matching the lattice constant and energy band structure between silicon and germanium. The continuous variation of compositional parameter eliminates abrupt interfaces and reduces dislocation density, achieving low dark current while using a single continuous layer rather than multiple discrete layers, thus limiting the increase in device complexity.
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 configuration effectively reduces dark current in CMOS image sensors to below 0.5 nanoamps, enhancing the sensor's performance and precision, particularly in applications like time-of-flight sensors.
Implementation Method 1
dark current, which is electrical current passing through photodetectors even in the absence of photons, primarily due to band discontinuity between germanium epitaxial structures and silicon capping structures
Data Source
AI summary
In some embodiments, a method for forming a semiconductor device is provided. The method includes etching a substrate to form a recess within a surface of the substrate. An epitaxial material is formed within the recess, a capping structure is formed on the epitaxial material, and a capping layer is formed onto the capping structure. The capping layer laterally extends past an outermost sidewall of the capping structure. Dopants are implanted into the epitaxial material. Implanting the dopants into the epitaxial material forms a first doped region having a first doping type and a second doped region having a second doping type.