Pixel Well Contact Impurity Control for Image Sensor White Spot Reduction
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Solution Overview
Problem
In solid state image pickup devices, high-concentration ion implantation for forming well contacts leads to implantation damage and mixing of impurities, causing dark current and white spots in images due to intense electric fields between doped regions and photodiodes.
Innovation Solution
A semiconductor substrate with a pixel well contact system where the impurity concentration of the doped region is set to 1×1019 cm−3 or less, using boron as the high-concentration impurity for the contact portion, and isolating it from the photoelectric converter to reduce crystal defects and mixing of heavy metals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high-concentration ion implantation is performed to form a well contact, then the well potential can be stabilized, but implantation damage and mixing of impurities occur causing dark current and white spots
Solution Approach 1:
The patent applies local quality by creating a multi-layered contact structure with different impurity concentrations at different locations. The well contact includes a first contact region with high impurity concentration (1×10^19 cm^-3 or more) for stable electrical contact, and a second contact region with lower impurity concentration (1×10^19 cm^-3 or less) adjacent to the photodiode. This spatial differentiation of impurity concentration allows the contact to provide stable well potential while minimizing dark current and white spot defects caused by high-concentration implantation damage near the photodiode.
2Reliability
If a well contact is provided in each pixel, then shading can be prevented, but the complexity of the manufacturing process increases
Solution Approach 1:
The patent merges the well contact formation process with the existing source/drain formation process of peripheral circuit transistors. By using the same ion implantation steps that create source/drain regions for MIS transistors in peripheral circuits to also form the well contact regions in pixel regions, the manufacturing process complexity is reduced. The contact holes are formed through the interlayer insulating film to reach the pixel wells, and the same high-concentration ion implantation used for source/drain formation is utilized to create the contact regions, thereby preventing shading without significantly increasing process complexity.
3Manufacturing precision
If BF2 is used for ion implantation to satisfy shallow junction requirement, then junction depth is reduced, but mixing of heavy metal impurities occurs causing white spots
Solution Approach 1:
The patent applies parameter changes by carefully controlling the ion implantation conditions, particularly the dose and energy parameters. For the first contact region, high-concentration ion implantation (1×10^19 cm^-3 or more) is performed to create a low-resistance contact. For the second contact region adjacent to the photodiode, the impurity concentration is limited to 1×10^19 cm^-3 or less. This parameter differentiation in impurity concentration allows the use of BF2 for shallow junction formation while preventing excessive impurity mixing that would cause white spots, by ensuring that regions close to the photodiode maintain lower impurity levels.
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 significantly reduces the number of white spots by minimizing dark current and electric field influences, maintaining image quality without adding new components, and achieving a stable well potential.
Implementation Method 1
a pixel well contact supplying a reference voltage to the pixel well
Implementation Method 2
An impurity concentration of the first impurity doped region is lower than that of a source/drain of the MIS transistor
Implementation Method 3
a photodiode 121 performing photoelectric conversion
Data Source
AI summary
To reduce white spots by optimizing an impurity concentration of a p-type impurity doped region of a well contact, a size of a contact portion, a position of an n-type region serving as a photoelectric converter, and so on.In a solid state image pickup device in which a semiconductor substrate 11 includes a pixel region where a plurality of pixels are arranged, each pixel including a photoelectric converter 21, and a pixel well 12 shared by the respective pixels, a well contact 14 supplying a reference voltage to the pixel well 12 includes: an electrode 15 supplying a reference voltage; a p-type impurity doped region 16 placed in a surface of the pixel well 12; and a contact portion 17 placed in the p-type impurity doped region 16 so as to be connected to the electrode 15 and having a higher concentration than the p-type impurity doped region 16. The p-type impurity doped region 16 is doped with at least a p-type impurity, with an impurity concentration of 1×1019 cm−3 or less.