Focal Plane Array Pixel Definition by Fermi Energy Modulation
Solution Overview
Problem
Focal plane arrays require complex fabrication processes and are prone to surface leakage current due to deep etching of trenches to define individual pixels, which complicates the manufacturing and introduces potential signal crosstalk.
Innovation Solution
Pixels are defined by a controlled variation of the surface Fermi energy, eliminating the need for deep etching and minimizing surface leakage through shallow etching of a single cap layer, thereby simplifying the fabrication process and reducing inter-pixel crosstalk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If deep etching of trenches is used to define individual pixels, then pixel isolation is achieved, but fabrication complexity increases and surface leakage current is introduced
Solution Approach 1:
The patent extracts the pixel definition function from the deep trench etching process and relocates it to the cap layer. By removing only the cap layer material between pixels rather than etching deep trenches through the entire detector structure, the patent achieves pixel isolation while avoiding the complexity and reliability issues of deep etching.
Solution Approach 2:
The patent applies preliminary action by growing a cap layer over the entire detector array before pixel definition. This cap layer serves as a sacrificial layer that is subsequently removed in a shallow etching process to define pixels, eliminating the need for complex deep trench etching while maintaining pixel isolation.
2Reliability
If deep etching of trenches is used to define individual pixels, then pixel isolation is achieved, but surface leakage current increases
Solution Approach 1:
The patent extracts the pixel definition function from the deep trench etching process and relocates it to the cap layer. By removing only the cap layer material between pixels rather than etching deep trenches through the entire detector structure, the patent achieves pixel isolation while avoiding the complexity and reliability issues of deep etching.
Solution Approach 2:
The patent applies preliminary action by growing a cap layer over the entire detector array before pixel definition. This cap layer serves as a sacrificial layer that is subsequently removed in a shallow etching process to define pixels, eliminating the need for complex deep trench etching while maintaining pixel isolation.
3Ease of manufacture
If shallow etching of cap layer is used to define pixels, then fabrication is simplified and surface leakage is reduced, but pixel definition must be achieved differently
Solution Approach 1:
The patent introduces a cap layer as an intermediary element that facilitates simplified pixel definition. This cap layer acts as a mediator between the detector structure and the pixel definition process, allowing shallow etching to define pixels without exposing the underlying detector layers and their p-n junctions.
Solution Approach 2:
The patent extracts the pixel definition function from the deep trench etching process and relocates it to the cap layer. By removing only the cap layer material between pixels rather than etching deep trenches through the entire detector structure, the patent achieves pixel isolation while avoiding the complexity and reliability issues of deep etching.
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 approach results in a simpler fabrication process and high-quality images with minimal surface leakage and zero inter-pixel crosstalk, maintaining high performance across a wide temperature range with an optical fill factor of 100% and spectrally averaged quantum efficiency of 85%.
Implementation Method 1
Each pixel comprises a pixel pad having a perimeter; a gap surrounding the perimeter of the pixel pad. The pixels are defined by having a first Fermi energy at a surface of each the pixel pad and a second Fermi energy at a surface at the gap between the pixels, whereby variation between the first Fermi energy and the second Fermi energy produces a corresponding lateral variation in the electric field and the electrostatic potential in the detector layer below the surface of the pixel pad and the gap.
Implementation Method 2
The detector layer comprises a bulk semiconductor whose energy bandgap, and therefore cutoff wavelength, is determined by material composition
Data Source
AI summary
Pixels in a focal plane array are defined by controlled variation of the Fermi energy at the surface of the detector array. Varying the chemical composition of the semiconductor at the detector surface produces a corresponding variation in the surface Fermi energy which produces a corresponding variation in the electric field and electrostatic potential in the bulk semiconductor below the surface. This defines pixels by having one Fermi energy at the surface of each pixel and a different Fermi energy at the surface between pixels. Fermi energy modulation can also be controlled by applying an electrostatic potential voltage V1 to the metal pad defining each pixel, and applying a different electrostatic potential voltage V2 to an interconnected metal grid covering the gaps between all the pixel metal pads. Methods obviate the need to etch deep trenches between pixels, resulting in a more manufacturable quasi-planar process without sacrificing performance.