Transfer Gate Insulating Film Layout for Faster Image Sensors
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Solution Overview
Problem
The settling time of a transistor with a transfer gate electrode increases due to high capacitance between the gate electrode and the semiconductor substrate, limiting the speed of the transfer transistor.
Innovation Solution
The imaging device incorporates a transfer transistor with a transfer gate electrode positioned on the first surface of the substrate via an insulating film, where the insulating film includes a first insulating film on the channel region and a thicker second insulating film on other regions, reducing the capacitance between the transfer gate electrode and the substrate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the transfer gate electrode is positioned close to the semiconductor substrate to improve control, then the control efficiency is improved, but the capacitance between the gate electrode and substrate increases
Solution Approach 1:
The insulating film is designed with non-uniform thickness, where the first region (under the gate electrode) has a smaller thickness to maintain strong control, while the second region (peripheral areas) has a larger thickness to reduce parasitic capacitance. This local differentiation resolves the contradiction by optimizing both control efficiency and capacitance reduction in their respective regions.
2Quantity of substance
If the insulating film thickness is increased to reduce capacitance, then the capacitance is reduced, but the control efficiency of the gate electrode deteriorates
Solution Approach 1:
By making the insulating film thickness non-uniform with a thinner first region under the gate electrode and a thicker second region in peripheral areas, the invention maintains strong gate control where needed while reducing parasitic capacitance in areas where control is less critical, thus resolving the contradiction between capacitance reduction and control efficiency.
3Speed
If the settling time is reduced to improve speed, then the transfer transistor speed increases, but the stability of charge transfer may be compromised
Solution Approach 1:
The invention changes the physical parameter of the insulating film thickness to optimize the balance between speed and stability. By reducing the thickness in the critical region under the gate electrode, the capacitance is reduced, allowing faster charging and discharging of the gate, thus increasing transfer speed while maintaining adequate stability through the optimized thickness profile.
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 reduces the capacitance of the transfer gate electrode, thereby decreasing the settling time of the transfer transistor and enhancing its speed.
Implementation Method 1
For a large capacitance between the gate electrode that controls the transfer of a group of signal charges (hereinafter also referred to as a transfer gate electrode) and the semiconductor substrate, the settling time of a transistor including the transfer gate electrode (hereinafter also referred to as a transfer transistor) increases
Data Source
AI summary
An imaging device capable of reducing the capacitance of a transfer gate electrode is provided. The imaging device includes a first substrate and a plurality of pixels provided in the first substrate. Each of the plurality of pixels includes a photoelectric conversion portion provided in the first substrate, a charge storage portion provided in the first substrate, and a transfer transistor provided on a first surface side of the first substrate to transfer charges from the photoelectric conversion portion to the charge storage portion. The transfer transistor includes a transfer gate electrode provided on the first surface of the first substrate via an insulating film interposed between the transfer gate electrode and the first surface. The insulating film includes a first insulating film located on a channel region formed on the first substrate, and a second insulating film located on a region other than the channel region on the first substrate. The second insulating film is thicker than the first insulating film.