CMOS imager and method of formation
View Patent ↗A CMOS imager having multiple graded doped regions formed below respective pixel sensor cells is disclosed. A deep retrograde p-well is formed under a red pixel sensor cell of a semiconductor substrate to increase the red response. A shallow p-well is formed under the blue pixel sensor cell to decrease the red and green responses, while a shallow retrograde p-well is formed below the green pixel sensor cell to increase the green response and decrease the red response.
1. A method of forming photosensors for an imaging device, said method comprising the steps of:
providing a substrate having a top surface;
forming a first, second and third doped regions of a first conductivity type in said substrate, wherein said first, second and third doped regions extend from said top surface to substantially different depths in said substrate, said first, second and third doped regions being formed laterally relative to each other such that at least one doped region does not overlap with another doped region;
forming a first photosensor for sensing charges corresponding to a first color wavelength in association with said first doped region;
forming a second photosensor for sensing charges corresponding to a second color wavelength in association with said second doped region; and
forming a third photosensor for sensing charges corresponding to a third color wavelength in association with said third doped region.
2. The method of claim 1 , wherein said step of forming said first doped region further comprises forming a deep retrograde well located at a first depth in said substrate.
3. The method of claim 2 , wherein said step of forming said second doped region further comprises forming a shallow well located at a second depth in said substrate.
4. The method of claim 3 , wherein said step of forming said third doped region further comprises forming a shallow retrograde well located at a third depth in said substrate.
5. The method of claim 4 , wherein said first depth is substantially greater than said second and third depth.
6. The method of claim 5 , wherein said third depth is substantially greater than said second depth.
7. The method of claim 1 , wherein said first, second and third doped regions are formed sequentially.
8. The method of claim 1 , wherein said first, second and third doped regions are formed simultaneously.
9. The method of claim 1 , wherein said first conductivity type is p-type.
10. The method of claim 1 , wherein said first conductivity type is n-type.
11. A method of forming a color pixel cell for an imaging device, said method comprising the steps of:
providing a substrate having a top surface;
forming at least three multiple graded wells of a first conductivity type in said substrate, said at least three multiple graded wells extending from said top surface to substantially different depths in said substrate, said at least three multiple graded wells being formed laterally such that said graded wells do not overlap;
forming photosensitive elements respectively associated with said at least three multiple graded wells for producing charges in response to a respective color wavelength; and
forming a floating diffusion region of a second conductivity type in each one of said at least three multiple graded wells for receiving charges respectively transferred from a said photosensitive element associated with the same well.
12. The method of claim 11 , wherein one of said at least three multiple graded wells is a deep retrograde well formed at a first depth in said substrate.
13. The method of claim 12 , wherein one of said at least three multiple graded wells is a shallow well formed at a second depth in said substrate.
14. The method of claim 13 , wherein one of said at least three multiple graded wells is a shallow retrograde well formed at a third depth in said substrate.
15. The method of claim 11 , wherein said first conductivity type is p-type, and said second conductivity type is n-type.
16. The method of claim 1 , wherein said at least three multiple graded wells are formed sequentially.
17. The method of claim 11 , wherein said at least three multiple graded wells are formed simultaneously.
18. The method of claim 11 , wherein said first conductivity type is n-type, and said second conductivity type is p-type.
19. The method of claim 11 , wherein said photosensitive element is a photodiode sensor.
20. The method of claim 11 , wherein said photosensitive element is a photoconductor sensor.
21. The method of claim 11 , further comprising the step of forming a transfer gate on each of said at least three multiple graded wells, between a said photosensitive element and a said floating diffusion region.
22. The method of claim 21 , wherein said photosensitive element is a photogate sensor.
23. The method of claim 11 further comprising the step of forming a reset transistor in each of said at least three multiple graded wells for periodically resetting a charge level of said floating diffusion region, said floating diffusion region being the source of said reset transistor.
24. A method of forming a pixel array for an imaging device, said method comprising the steps of:
providing a substrate having a top surface;
forming at least two graded wells of a first conductivity type in said substrate, said at least two graded wells extending from said top surface to substantially different depths in said substrate, said at least two graded wells being spaced laterally such that said graded wells do not overlap; and
respectively forming at least two pixel sensor cells in said at least two graded wells, wherein each pixel sensor cell has an associated photosensitive element and a floating diffusion region of a second conductivity type, the photosensitive elements producing charges in accordance with a respective color wavelength which are respectively received by said floating diffusion regions.
25. The method of claim 24 , wherein said step of forming said at least two graded wells further comprises forming a deep retrograde well at a first depth in said substrate.
26. The method of claim 25 , wherein said step of forming said at least two graded wells further comprises forming a shallow well of a second depth in said substrate.
27. The method of claim 26 , wherein said step of forming said at least two graded wells further comprises forming a shallow retrograde well of a third depth in said substrate.
28. The method of claim 27 , wherein said first depth is substantially greater than said second and third depths.
29. The method of claim 28 , wherein said third depth is substantially greater than said second depth.
30. The method of claim 24 , wherein said first conductivity type is p-type, and said second conductivity type is n-type.
31. The method of claim 24 , wherein said at least two graded wells are formed sequentially.
32. The method of claim 24 , wherein said at least two graded wells are formed simultaneously.
33. The method of claim 24 , wherein said first conductivity type is n-type, and said second conductivity type is p-type.
34. A method of forming a color imaging sensor, said method comprising the steps of:
providing a substrate having a top surface;
forming a first defined region in said substrate for sensing a first color wavelength component;
forming a second defined region in said substrate for sensing a second color wavelength component; and
forming a third defined region in said substrate for sensing a third color wavelength component, wherein said first, second and third defined regions extend from said top surface to substantially different depths in said substrate, said first, second and third defined regions being formed laterally such that said defined regions do not overlap.
35. The method of claim 34 , wherein said step of forming said first defined region further comprises forming a deep retrograde well located at a first depth in said substrate.
36. The method of claim 35 , wherein said step of forming said second defined region further comprises forming a shallow well located at a second depth in said substrate.
37. The method of claim 36 , wherein said step of forming said third defined region further comprises forming a shallow retrograde well located at a third depth in said substrate.
38. The method of claim 37 , wherein said first depth is substantially greater than said second and third depth.
39. The method of claim 38 , wherein said third depth is substantially greater than said second depth.
40. The method of claim 34 , wherein each of said first, second and third defined regions senses charges of red, blue and green color wavelength, respectively.
41. The method of claim 34 , wherein said first, second and third defined regions are formed sequentially.
42. The method of claim 34 , wherein said first, second and third defined regions are formed simultaneously.
43. The method of claim 34 further comprising the step of forming an isolation region separating at least two of said first, second, and third defined regions.
44. The method of claim 34 further comprising the step of forming an isolation region separating said first, second, and third defined regions.
45. The method of claim 1 further comprising the step of forming an isolation region separating at least two of said first, second, and third doped regions.
46. The method of claim 1 further comprising the step of forming an isolation region separating said first, second, and third doped regions.
47. The method of claim 11 further comprising the step of forming an isolation region separating at least two of said three multiple graded wells.
48. The method of claim 11 further comprising the step of forming an isolation region separating said three multiple graded wells.
49. The method of claim 24 further comprising the step of forming an isolation region separating said graded wells.