Retrograde well structure for a CMOS imager

• US 6,858,460 B2
• Filed: 11/12/2002
• Issued: 02/22/2005
• 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): Smith, Matthew
• Assistant Examiner(s): MALSAWMA, LALRINFAMKIM HMAR
• Application Number:
  US10/291,670
• Publication Number:
  US 20030180982A1
• Time in Patent Office: 833 Days
• Field of Search: 438/57, 438/60, 438/75, 438/84, 438/144, 438/145
• US Class Current: 438/60
• 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 method of forming a photosensor for an imaging device, said method comprising the steps of:

forming 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 top of said retrograde well; and

forming a photosensor at an upper surface of the retrograde well.

2. The method of claim 1, wherein said step of forming a retrograde well is an ion implantation step.

3. The method of claim 1, wherein the first conductivity type is p-type.

4. The method of claim 3, wherein the retrograde well is doped with boron.

5. The method of claim 1, wherein the first conductivity type is n-type.

6. The method of claim 5, wherein the retrograde well is doped with a dopant selected from the group consisting or arsenic, antimony, and phosphorous.

7. The method 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, and within the range of about 5×

10¹⁸ to about 1×

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

8. The method of claim 7, wherein the photosensor forming step is a photogate sensor forming step.

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

10¹⁶ to abut 1×

10¹⁸ atoms per cm³ at the bottom of the retrograde well, and within the range of about 1×

10¹⁵ to 5×

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

10. The method 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, and about 5×

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

11. The method of claim 1, wherein the photosensor forming step is a photodiode sensor forming step.

12. The method of claim 1, wherein the photosensor forming step is a photoconductor forming step.

13. The method of claim 1, wherein the photosensor further comprises a transfer gate.

14. A method of forming a pixel sensor cell for an imaging device, said method comprising the steps of:

forming 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;

forming a photosensitive region in the retrograde well;

forming a photosensor on an upper surface of the photosensitive region for controlling the collection of charge therein; and

forming a floating diffusion region of a second conductivity type in the retrograde well for receiving charges transferred from said photosensitive region.

15. The method of claim 14, wherein said step of forming a retrograde well is an ion implantation step.

16. The method of claim 14, wherein the first conductivity type is p-type, and the second conductivity type is n-type.

17. The method of claim 16, wherein the retrograde well is doped with boron.

18. The method of claim 14, wherein the first conductivity type is n-type, and the second conductivity type is p-type.

19. The method of claim 18, wherein the retrograde well is doped with a dopant selected from the group consisting of arsenic, antimony, and phosphorous.

20. The method of claim 14, 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, and within the range of about 5×

10¹⁴ to about 1×

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

21. The method of claim 14, 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, and within the range of about 1×

10¹⁵ to about 5×

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

22. The method of claim 14, wherein the retrograde well has a dopant concentration of about 3×

10¹⁷ atoms per cm³ at the bottom of the retrograde well, and about 5×

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

23. The method of claim 14, wherein the photosensor is a photodiode sensor.

24. The method of claim 14, wherein the photosensor is a photoconductor sensor.

25. The method of claim 14, further comprising a step of forming a transfer gate on the retrograde well between the photosensor and the floating diffusion region.

26. The method of claim 25, wherein the photosensor is a photogate sensor.

27. The method of claim 14, further comprising a step of forming a reset transistor in the retrograde well for periodically resetting a charge level of the floating diffusion region, said floating diffusion region being the source of the reset transistor.

28. The method of claim 14, wherein the photosensitive region comprises a doped region of a second conductivity type.

29. A method of forming a pixel array for an imaging device, said method comprising the steps of:

forming 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; and

forming a plurality of pixel sensor cells in the retrograde well, wherein each pixel sensor cell has a photosensitive region, a photosensor formed on the photosensitive region, and a floating diffusion region of a second conductivity type.

30. The method of claim 29, wherein said step of forming a retrograde well is an ion implantation step.

31. The method of claim 29, wherein the first conductivity type is p-type, and the second conductivity type is n-type.

32. The method of claim 29, wherein the first conductivity type is n-type, and the second conductivity type is p-type.

33. The method of claim 29, 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, and within the range of about 5×

10¹⁴ to about 1×

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

34. The method of claim 29, 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, and within the range of about 1×

10¹⁵ to about 5×

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

35. The method of claim 29, wherein the retrograde well has a dopant concentration of about 3×

10¹⁷ atoms per cm³ at the bottom of the retrograde well, and about 5×

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

36. The method of claim 29, wherein the photosensitive region comprises a doped region of a second conductivity type.

37. The method of claim 29, wherein the photosensor of each pixel sensor cell is a photodiode sensor.

38. The method of claim 29, wherein the photosensor of each pixel sensor cell is a photoconductor sensor.

39. The method of claim 29, further comprising a step of forming a transfer gate for each pixel sensor cell on the retrograde well between the photosensor and the floating diffusion region.

40. The method of claim 29, wherein the photosensor of each pixel sensor cell is a photogate sensor.