CMOS Image Sensor Superlattice Infrared Absorption
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Solution Overview
Problem
Current semiconductor devices, such as CMOS image sensors, face limitations in charge carrier mobility and performance due to existing materials and processing techniques, despite advancements in strained silicon and superlattice technologies.
Innovation Solution
The development of a CMOS image sensor incorporating a superlattice structure with stacked groups of semiconductor and non-semiconductor monolayers, including a retrograde well and well configurations, which reduces effective mass and enhances charge carrier mobility, and includes a cap layer and shallow trench isolation to improve device performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional semiconductor materials and structures are used in CMOS image sensors, then manufacturing processes are simpler, but charge carrier mobility is limited and infrared light absorption is insufficient
Solution Approach 1:
The patent employs a superlattice structure composed of alternating layers of silicon and silicon-germanium materials. This composite material approach creates a periodic modulation of the conduction band that reduces effective mass and enhances charge carrier mobility, directly resolving the contradiction between maintaining manufacturing feasibility and improving electrical performance.
Solution Approach 2:
The invention modifies the crystal lattice parameters by introducing strained silicon layers within the superlattice structure. The strain engineering changes the band structure parameters, specifically reducing the effective mass of charge carriers and increasing mobility, while the layered structure allows control over the magnitude and direction of strain through layer thickness and composition ratios.
2Reliability
If standard silicon structures are used, then device fabrication is easier, but infrared light absorption capability is reduced
Solution Approach 1:
The superlattice structure of alternating silicon and silicon-germanium layers creates a periodic potential that enhances infrared light absorption through phonon-assisted transitions. The composite material system enables absorption of longer wavelength infrared photons that cannot be absorbed in conventional silicon, directly addressing the infrared absorption enhancement requirement.
Solution Approach 2:
The patent introduces a periodic modulation in the vertical dimension through the layered superlattice structure. This dimensional approach creates quantum well states and modifies the density of states, enabling enhanced infrared absorption through inter-subband transitions and phonon-assisted processes that are not available in bulk silicon.
3Reliability
If strained silicon layers are introduced to enhance mobility, then charge carrier mobility increases, but manufacturing precision requirements increase
Solution Approach 1:
The superlattice structure divides the strained silicon layer into multiple thin alternating layers with silicon-germanium. This segmentation approach allows the strain to be distributed across multiple interfaces, maintaining the mobility enhancement while reducing the cumulative stress and defect density that would occur in a single thick strained layer, thereby relaxing manufacturing precision requirements.
Solution Approach 2:
The invention applies strain locally at the interfaces between silicon and silicon-germanium layers rather than throughout a bulk strained layer. This localized strain at interfaces provides mobility enhancement while the overall structure can be grown with more relaxed thickness tolerances, as each individual layer can be thinner and the critical parameter becomes interface quality rather than bulk layer uniformity.
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 superlattice structure achieves higher charge carrier mobility, reduced crosstalk between pixels, and enhanced IR light absorption, leading to improved performance in CMOS image sensors.
Implementation Method 1
The superlattice structure achieves higher charge carrier mobility, reduced crosstalk between pixels, and enhanced IR light absorption
Implementation Method 2
which reduces effective mass and enhances charge carrier mobility
Implementation Method 3
enhanced IR light absorption
Implementation Method 4
reduced crosstalk between pixels
Data Source
AI summary
A method for making a CMOS image sensor may include forming a plurality of laterally adjacent infrared (IR) photodiode structures on a semiconductor substrate having a first conductivity type. Forming each IR photodiode structure may include forming a superlattice on the semiconductor substrate including a plurality of stacked groups of layers, with each group of layers including a plurality of stacked base semiconductor monolayers defining a base semiconductor portion, and a non-semiconductor monolayer(s) constrained within a crystal lattice of adjacent base semiconductor portions. The superlattice may have the first conductivity type. A semiconductor layer may be formed on the superlattice, along with a retrograde well extending downward into the semiconductor layer from a surface thereof and having a second conductivity type, a first well around a periphery of the retrograde well having the first conductivity type, and a second well above the retrograde well having the first conductivity type.