Retrograde well structure for a CMOS imager

• US 7,619,672 B2
• Filed: 01/22/2004
• Issued: 11/17/2009
• Est. Priority Date: 06/16/1999
• Status: Active Grant

• Current Assignee: Aptina Imaging Corporation (ON Semiconductor Corporation)
• Original Assignee: Aptina Imaging Corporation (ON Semiconductor Corporation)
• Inventors: Durcan, Mark, Rhodes, Howard E.
• Primary Examiner(s): Ho; Tuan V
• Assistant Examiner(s): CHEN, CHIA WEI A
• Application Number:
  US10/761,319
• Publication Number:
  US 20040150736A1
• Time in Patent Office: 2,126 Days
• Field of Search: 348/308, 257/231, 257/233, 257/291, 257/292, 257/185, 257/431
• US Class Current: 348/308
• CPC Class Codes: H01L 27/14601 Structural or functional details thereof
• CPC Class Codes: H01L 27/14603 Special geometry or disposition of pixel-elements, address-lines or gate-electrodes
• CPC Class Codes: H01L 27/14609 Pixel-elements with integrated switching, control, storage or amplification elements scanning details of imagers (circuitry of solid-state image sensors H04N25/00); circuitry of imagers H04N25/70
• CPC Class Codes: H01L 27/14643 Photodiode arrays; MOS imagers
• CPC Class Codes: H01L 27/14683 Processes or apparatus peculiar to the manufacture or treatment of these devices or parts thereof not peculiar thereto H01L21/00
• CPC Class Codes: H01L 27/14689 MOS based technologies

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.

Claims

1. An imager comprising:

an array of pixel sensor cells formed in a retrograde well on a substrate, the retrograde well being doped with a vertically graded dopant profile, wherein each pixel sensor cell has a photosensitive region and a photosensor formed at the photosensitive region;

a circuit formed in the substrate and electrically connected to the array for receiving and processing signals representing an image output by the array and for providing output data representing the image; and

a processor for receiving and processing data representing the image.

2. The imager of claim 1, wherein said array of pixel sensor cells and said processor are formed on a single substrate.

3. The imager of claim 1, wherein said array of pixel sensor cells is formed on a first substrate, and said processor is formed on a second substrate.

4. The imager of claim 1, wherein the retrograde well has a dopant concentration within the range of about 1×

10¹⁶ to about 2×

10¹⁸ atoms per cm³ at the bottom of the retrograde well.

5. The imager of claim 4, wherein the retrograde well has a dopant concentration within the range of about 5×

10¹⁴ to about 1×

10¹⁷ atoms per cm³ at the top of the retrograde well.

6. The imager of claim 1, wherein the retrograde well has a dopant concentration within the range of about 5×

10¹⁶ to about 1×

10¹⁸ atoms per cm³ at the bottom of the retrograde well.

7. The imager of claim 6, wherein the retrograde well has a dopant concentration within the range of about 1×

10¹⁵ to 5×

10¹⁶ atoms per cm³ at the top of the retrograde well.

8. The imager of claim 1, wherein the retrograde well has a dopant concentration of about 3×

10¹⁷ atoms per cm³ at the bottom of the retrograde well.

9. The imager of claim 8, wherein the retrograde well has a dopant concentration of about 5×

10¹⁵ atoms per cm³ at the top of the retrograde well.

10. The imager of claim 1, wherein the retrograde well is a first retrograde well, and said circuit is formed in a second retrograde well.

11. A CMOS imager comprising:

an array of pixel sensor cells formed in a retrograde well in a substrate, the retrograde well being doped with a vertically graded dopant concentration, wherein each of said pixel sensor cells is separated by an isolation region that electrically isolates said pixel cells from each other, and each said pixel sensor cell comprises;

a photoconversion device;

a reset transistor;

a source follower transistor;

a row select transistor; and

a floating diffusion region in electrical communication with said photoconversion device and said source follower transistor.

12. The CMOS imager of claim 11 wherein the photoconversion device further comprises a transfer transistor positioned to gate charges between said photoconversion device to said floating diffusion region.

13. The CMOS imager of claim 11 wherein the photoconversion device is a photogate.

14. The CMOS imager of claim 11 wherein the photoconversion device is a photodiode.

15. The CMOS imager of claim 11 wherein the photoconversion device is a photoconductor.

16. The CMOS imager of claim 11 wherein said retrograde well is provided to reflect signal carriers back to the photoconversion device.

17. The CMOS imager of claim 11 wherein said vertically graded dopant concentration of the retrograde well is highest at a top of the well and lowest at a bottom of the well.

18. The CMOS imager of claim 11, wherein said vertically graded dopant concentration of the retrograde well is lowest at a top of the well and highest at a bottom of the well.

19. The CMOS imager of claim 18, wherein said vertically graded dopant concentration at the top of the retrograde well is within the range of about 5×

10¹⁴ to 1×

10¹⁷ atoms per cm³ and the concentration at the bottom of the retrograde well is within the range of about 1×

10¹⁶ to 2×

10¹⁸ atoms per cm³.

20. The CMOS imager of claim 18, wherein said vertically graded dopant concentration at the top of the retrograde well is within the range of about 1×

10¹⁵ to 5×

10¹⁶ atoms per cm³ and the concentration at the bottom of the retrograde well is within the range of about 5×

10¹⁶ to 1×

10¹⁸ atoms per cm³.

21. The CMOS imager of claim 18, wherein said vertically graded dopant concentration at the top of the retrograde well is about 5×

10¹⁵ atoms per cm³ and the concentration at the bottom of the retrograde well is about 3×

10¹⁷ atoms per cm³.