Retrograde well structure for a CMOS imager

• US 6,483,129 B2
• Filed: 08/01/2001
• Issued: 11/19/2002
• Est. Priority Date: 06/16/1999
• Status: Expired due to Term

• Current Assignee: Aptina Imaging Corporation (ON Semiconductor Corporation)
• Original Assignee: Micron Technology, Inc.
• Inventors: Rhodes, Howard E., Durcan, Mark
• Primary Examiner(s): Wilson, Allan R.
• Application Number:
  US09/918,450
• Publication Number:
  US 20010050382A1
• Time in Patent Office: 475 Days
• Field of Search: 257/185, 257/431, 257/655, 257/917
• US Class Current: 257/185
• 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. 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.

2. The pixel sensor cell of claim 1, wherein the first conductivity type is p-type, and the second conductivity type is n-type.

3. The pixel sensor cell of claim 2, wherein said retrograde well is doped with boron.

4. The pixel sensor cell of claim 1, wherein the first conductivity type is n-type, and the second conductivity type is p-type.

5. The pixel sensor cell of claim 4, wherein said retrograde well is doped with a dopant selected from the group consisting of arsenic, antimony, and phosphorous.

6. The pixel sensor cell of claim 1, wherein said retrograde well has a dopant concentration within the range of about 1×

10¹⁶ to about 2×

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

7. The pixel sensor cell of claim 6, wherein said retrograde well has a dopant concentration within the range of about 5×

10¹⁴ to about 1×

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

8. The pixel sensor cell of claim 1, wherein said retrograde well has a dopant concentration within the range of about 5×

10¹⁶ to about 1×

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

9. The pixel sensor cell of claim 8, wherein said retrograde well has a dopant concentration within the range of about 1×

10¹⁵ to about 5×

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

10. The pixel sensor cell of claim 1, wherein said retrograde well has a dopant concentration of about 3×

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

11. The pixel sensor cell of claim 10, wherein said retrograde well has a dopant concentration of about 5×

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

12. The pixel sensor cell of claim 1, further comprising a photosensor formed on said photosensitive region for controlling the collection of charges in said photosensitive region.

13. The pixel sensor cell of claim 12, wherein said photosensor is a photodiode sensor.

14. The pixel sensor cell of claim 12, wherein said photosensor is a photogate sensor.

16. The pixel sensor cell of claim 1, further comprising a transfer gate formed on said retrograde well between said photosensor and said floating diffusion region.

17. The pixel sensor cell of claim 1, further comprising a reset transistor formed in said retrograde well for periodically resetting a charge level of said floating diffusion region, said floating diffusion region being the source of said reset transistor.

18. The pixel sensor cell of claim 1, wherein said photosensitive region comprises a doped region of a second conductivity type.

15. The pixel sensor cell of clam 12, wherein said photosensor is a photoconductor sensor.

19. A pixel sensor cell for an imaging device, said pixel sensor cell comprising:

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.

20. The pixel sensor cell of claim 19, wherein said photosensor further comprises a doped region of a second conductivity type located in said retrograde well.

21. The pixel sensor cell of claim 19, wherein said photosensor is a photodiode sensor.

22. The pixel sensor cell of claim 19, wherein said photosensor is a photoconductor sensor.

23. The pixel sensor cell of claim 19, further comprising a transfer gate located between said photosensor and said floating diffusion region.

24. The pixel sensor cell of claim 23, wherein said photosensor is a photogate sensor.

25. The pixel sensor cell of claim 19, wherein the first conductivity type is p-type, and the second conductivity type is n-type.

26. The pixel sensor cell of claim 25, wherein said retrograde well is doped with boron.

27. The pixel sensor cell of claim 19, wherein the first conductivity type is n-type, and the second conductivity type is p-type.

28. The pixel sensor cell of claim 27, wherein said retrograde well is doped with a dopant selected from the group consisting of arsenic, antimony, and phosphorous.

29. The pixel sensor cell of claim 19, wherein said retrograde well has a dopant concentration within the range of about 1×

10¹⁶ to about 2×

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

30. The pixel sensor cell of claim 29, wherein said retrograde well has a dopant concentration within the range of about 5×

10¹⁴ to about 1×

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

31. The pixel sensor cell of claim 19, wherein said retrograde well has a dopant concentration within the range of about 5×

10¹⁶ to about 1×

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

32. The pixel sensor cell of claim 31, wherein said retrograde well has a dopant concentration within the range of about 1×

10¹⁵ to about 5×

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

33. The pixel sensor cell of claim 19, wherein said retrograde well has a dopant concentration of about 3×

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

34. The pixel sensor cell of claim 31, wherein said retrograde well has a dopant concentration of about 5×

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