Retrograde well structure for a CMOS imager
First Claim
1. A pixel sensor cell for an imaging device, said pixel sensor cell comprising:
- a retrograde well of a first conductivity type in a substrate, wherein said retrograde well has a vertically graded dopant of said first conductivity type between a bottom of said retrograde well having a highest concentration of said dopant of said first conductivity type and a top of said retrograde well;
a photosentitive region formed in said retrograde well; and
a floating diffusion region of a second conductivity type formed in said retrograde well for receiving charges transferred from said photosensitive region.
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Abstract
A retrograde well structure for a CMOS imager that improves the quantum efficiency and signal-to-noise ratio of the imager. The retrograde well comprises a doped region with a vertically graded dopant concentration that is lowest at the substrate surface, and highest at the bottom of the well. A single retrograde well may have a single pixel sensor cell, multiple pixel sensor cells, or even an entire array of pixel sensor cells formed therein. The highly concentrated region at the bottom of the retrograde well repels signal carriers from the photosensor so that they are not lost to the substrate, and prevents noise carriers from the substrate from diffusing up into the photosensor. Also disclosed are methods for forming the retrograde well.
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Citations
34 Claims
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1. A pixel sensor cell for an imaging device, said pixel sensor cell comprising:
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a retrograde well of a first conductivity type in a substrate, wherein said retrograde well has a vertically graded dopant of said first conductivity type between a bottom of said retrograde well having a highest concentration of said dopant of said first conductivity type and a top of said retrograde well;
a photosentitive region formed in said retrograde well; and
a floating diffusion region of a second conductivity type formed in said retrograde well for receiving charges transferred from said photosensitive region. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 18)
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15. The pixel sensor cell of clam 12, wherein said photosensor is a photoconductor sensor.
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19. A pixel sensor cell for an imaging device, said pixel sensor cell comprising:
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a retrograde well of a first conductivity type formed in a substrate;
a photosensor formed in said retrograde well;
a reset transistor having a gate stack formed in said retrograde well;
a floating diffusion region of a second conductivity type formed in said retrograde well between said photosensor and reset transistor for receiving charges from said photosensor, said reset transistor operating to periodically reset a charge level of said floating diffusion region; and
an output transistor having a gate electrically connected to said floating diffusion region. - View Dependent Claims (20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34)
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Specification