Front-Side Image Sensor Substrate Layout for Low Dark Current
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Solution Overview
Problem
The implementation of a P+ doped substrate in industrial manufacturing lines for SOI substrates is challenging due to boron diffusion during cleaning or heat treatment steps, leading to contamination and inaccurate doping control.
Innovation Solution
A front-side type image sensor substrate is designed with a P− type doped semiconducting support substrate, a P+ type doped semiconducting epitaxial layer, an electrically insulating layer, and a semiconducting active layer, where the epitaxial layer is formed between the support substrate and the insulating layer to minimize dark current and contamination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a P+ doped support substrate is used to minimize dark current, then dark current is reduced, but boron diffusion occurs during manufacturing causing contamination and inaccurate doping control
Solution Approach 1:
The substrate is divided into two distinct parts: a P- doped support substrate and a separate P+ doped epitaxial layer. This segmentation allows the P+ layer to provide dark current minimization while the P- support substrate avoids boron diffusion contamination during manufacturing.
Solution Approach 2:
An electrically insulating layer is introduced as an intermediary between the P- support substrate and the P+ epitaxial layer. This intermediary structure enables the P+ layer to function for dark current reduction while isolating it from the support substrate, preventing boron diffusion into the environment during manufacturing.
2Reliability
If a P+ doped support substrate is implemented, then electron migration from substrate to active layer is minimized, but manufacturing precision is compromised due to uncontrolled boron diffusion
Solution Approach 1:
The substrate structure is segmented into a P- doped support substrate and a separately formed P+ doped epitaxial layer. This allows precise control of doping levels in the epitaxial layer through controlled epitaxial growth and doping processes, while the support substrate maintains its P- doping without boron diffusion issues.
Solution Approach 2:
The doping concentration parameter is changed from P+ in the support substrate to P- in the support substrate, with the P+ doping applied only to the epitaxial layer. This parameter change enables precise control of electron migration prevention while avoiding boron diffusion contamination during manufacturing.
3Reliability
If the support substrate is biased at lower voltage to build up majority carriers, then carrier build-up occurs at the interface, but boron diffusion increases during cleaning and heat treatment steps
Solution Approach 1:
The P+ doped layer is extracted from the support substrate and placed as a separate epitaxial layer on top of the P- doped support substrate. This extraction allows the P+ layer to provide majority carrier build-up for interface optimization while removing the source of boron diffusion contamination from the support substrate.
Solution Approach 2:
An electrically insulating layer is introduced as an intermediary between the P- support substrate and the P+ epitaxial layer. This intermediary structure enables the P+ layer to provide majority carrier build-up at the interface while preventing boron diffusion into the environment during cleaning and heat treatment steps.
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 significantly reduces the exposed area of P+ doped material, minimizing contamination and allowing for precise control of doping levels, thereby effectively minimizing dark current in the image sensor.
Implementation Method 1
The choice of a P+ type doped support substrate aims at minimizing the migration of electrons from the support substrate to the active layer, which is likely to originate a dark current
Implementation Method 2
The electrically insulating layer 2 of silicon oxide is intended to electrically insulate the active layer 3 from the support substrate 1, for the purpose of preventing electrons from passing from the support substrate to the active layer
Implementation Method 3
boron diffusion out of the support substrate and scattering into the environment of the manufacturing line are observed
Data Source
AI summary
A front-side type image sensor may include a substrate successively including: a P− type doped semiconducting support substrate, an electrically insulating layer and a semiconducting active layer, and a matrix array of photodiodes in the active layer of the substrate. The substrate may include, between the support substrate and the electrically insulating layer, a P+ type doped semiconducting epitaxial layer.