Vertical CMOS Image Sensor Plug Fabrication
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Solution Overview
Problem
The existing methods for fabricating vertical CMOS image sensors suffer from severe lateral diffusion of implant plugs, which deteriorates isolation characteristics between adjacent photodiodes, limiting the miniaturization of unit pixels and integration of the image sensor.
Innovation Solution
The method involves controlling the dose and energy of ion implantation by using phosphorous and arsenic ions, specifically implanting phosphorous at a low dose and arsenic at a high dose, with precise energy levels to form plugs that minimize lateral diffusion and improve vertical connectivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If phosphorous ions are implanted at high energy and high dose to form plugs at depth of 2 μm, then the plug depth is achieved, but severe lateral diffusion occurs which deteriorates isolation characteristics
Solution Approach 1:
The ion implantation process is divided into multiple sequential steps with different energies and doses. First, phosphorous is implanted at high energy (1.2 MeV) to reach the target depth, then additional phosphorous is implanted at lower energy (500 keV) through a photoresist mask to complete the plug formation. This segmentation allows control over the vertical depth while limiting lateral diffusion by using lower energy for the final doping stage.
Solution Approach 2:
The implantation parameters (energy and dose) are changed between different implantation steps. The first implantation uses high energy (1.2 MeV) and high dose (5×10^15 atoms/cm²) to achieve the required plug depth. The second implantation uses lower energy (500 keV) and lower dose (5×10^14 atoms/cm²) to complete the doping while minimizing lateral diffusion. This parameter optimization resolves the contradiction between achieving depth and maintaining isolation.
2Reliability
If high dose phosphorous implantation is used to form plugs, then the electrical connection is improved, but lateral diffusion increases which limits unit pixel size reduction
Solution Approach 1:
The doping process is segmented into two distinct implantation steps. The first step uses high dose (5×10^15 atoms/cm²) at high energy to establish the primary electrical connection. The second step uses lower dose (5×10^14 atoms/cm²) at lower energy through a photoresist mask to complete the plug formation with minimal lateral spread. This segmentation ensures adequate electrical connection while limiting the area expansion that would constrain pixel miniaturization.
3Measurement precision
If phosphorous implantation is performed at high energy to reach 2 μm depth, then the red photodiode signal is sensed, but the isolation characteristics between adjacent photodiodes deteriorate
Solution Approach 1:
The implantation process is divided into sequential steps where the first high-energy implantation (1.2 MeV) establishes the depth required for red photodiode signal sensing. The second implantation step uses lower energy (500 keV) through a photoresist mask to complete the plug formation with controlled lateral diffusion. This segmentation maintains the signal sensing capability while improving isolation characteristics by limiting the lateral spread in the final doping stage.
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 reduces lateral diffusion, enhances isolation characteristics, and allows for smaller unit pixel sizes, thereby improving the integration and production efficiency of vertical CMOS image sensors.
Implementation Method 1
phosphorous and arsenic ions are implanted at a controlled dose and energy
Data Source
AI summary
A method of fabricating a vertical CMOS image sensor is disclosed, to improve the integration with the decrease in size of pixel by minimizing the lateral diffusion, in which phosphorous and arsenic ions are implanted while controlling the dose and energy, the method including forming a first photodiode in a semiconductor substrate; forming a first epitaxial layer on the semiconductor substrate; forming a first plug by sequentially implanting first and second ions in the first epitaxial layer; forming a second photodiode in the first epitaxial layer; forming a second epitaxial layer in the first epitaxial layer; forming an isolation area in the second epitaxial layer; and forming a third photodiode and a second plug in the second epitaxial layer.