Dark Current Reduction in Back-Illuminated Imaging Sensors and Method of Fabricating Same

• US 20070235829A1
• Filed: 05/23/2007
• Published: 10/11/2007
• Est. Priority Date: 02/11/2005
• Status: Active Grant

• Current Assignee: SRI International, Inc.
• Original Assignee: Sarnoff Corporation (SRI International, Inc.)
• Inventors: Bhaskaran, Mahalingam, Levine, Peter, Swain, Pradyumna
• Granted Patent: US 7,723,215 B2
• Time in Patent Office: Days
• US Class Current: 257/437
• CPC Class Codes: H01L 27/1464 Back illuminated imager structures
• CPC Class Codes: H01L 27/14643 Photodiode arrays; MOS imagers
• CPC Class Codes: H01L 27/148 Charge coupled imagers individual charge coupled devices H01L29/765
• CPC Class Codes: H01L 31/18 Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
• CPC Class Codes: H01L 31/1892 methods involving the use of temporary, removable substrates
• CPC Class Codes: Y02E 10/50 Photovoltaic [PV] energy

Abstract

A method for fabricating a back-illuminated semiconductor imaging device on a semiconductor-on-insulator substrate, and resulting imaging device is disclosed. The device includes an insulator layer; a semiconductor substrate, having an interface with the insulator layer; an epitaxial layer grown on the semiconductor substrate by epitaxial growth; and one or more imaging components in the epitaxial layer in proximity to a face of the epitaxial layer, the face being opposite the interface of the semiconductor substrate and the insulator layer, the imaging components comprising junctions within the epitaxial layer; wherein the semiconductor substrate and the epitaxial layer exhibit a net doping concentration having a maximum value at a predetermined distance from the interface of the insulating layer and the semiconductor substrate and which decreases monotonically on both sides of the profile from the maximum value within a portion of the semiconductor substrate and the epitaxial layer. The doping profile between the interface with the insulation layer and the peak of the doping profile functions as a “dead band” to prevent dark current carriers from penetrating to the front side of the device.

Claims

1. A method for fabricating a back-illuminated semiconductor imaging device which reduces dark current, comprising the steps of:

providing a substrate comprising;

a mechanical substrate,

an insulator layer, and

a semiconductor substrate;

growing an epitaxial layer on the semiconductor substrate while simultaneously causing diffusion of one or more dopants into the epitaxial layer such that, at completion of the growing of the epitaxial layer, there exists a net dopant concentration profile in the semiconductor substrate and the epitaxial layer which has a maximum value at a predetermined distance from the interface of the insulating layer and the semiconductor substrate and which decreases monotonically on both sides of the profile from the maximum value within the semiconductor substrate and the epitaxial layer; and

fabricating one or more imaging components in the epitaxial layer.

2. The method of claim 1, further comprising the steps of:

growing a second insulation layer on the semiconductor substrate before said step of fabricating;

implanting doping ions through the second insulation layer; and

removing the second insulation layer.

3. The method of claim 1, wherein the shape of the net dopant concentration profile is approximately Gaussian.

4. The method of claim 1, further comprising the step of removing at least a portion of the mechanical substrate.

5. The method of claim 4, further comprising the step of:

removing at least a portion of the insulator layer following complete removal of the mechanical substrate.

6. The method of claim 5, wherein the step of removing at least a portion of the insulator layer results in a remaining insulator layer having a thickness such that the remaining insulator layer functions as an anti-reflection coating for electromagnetic radiation over a predetermined range of wavelengths.

7. The method of claim 4, further comprising the step of depositing at least one layer of a material which functions as an anti-reflection coating for electromagnetic radiation over a predetermined range of wavelengths.

8. The method of claim 4, further comprising the steps of removing the entire insulating layer and depositing at least one layer of a material which functions as an anti-reflection coating for electromagnetic radiation over a predetermined range of wavelengths.

9. The method of claim 1, further comprising the step of setting the maximum value of the net dopant concentration profile such that a potential barrier corresponding to the doping maximum is at least about 10 times greater than kT, where k is the Botzmann constant and T is absolute temperature in Kelvins.

10. The method of claim 2, wherein the step of fabricating one or more imaging components includes the step of fabricating CMOS imaging components, charge-coupled device (CCD) components, photodiodes, avalanche photodiodes, or phototransistors, in any combination.

11. A back-illuminated semiconductor imaging device configured to operate over a predetermined range of wavelengths, comprising:

an insulator layer;

a semiconductor substrate, having an interface with the insulator layer;

an epitaxial layer grown on the semiconductor substrate by epitaxial growth; and

one or more imaging components in the epitaxial layer in proximity to a face of the epitaxial layer, the face being opposite the interface of the semiconductor substrate and the insulator layer, the imaging components comprising a plurality of junctions within the epitaxial layer;

wherein the semiconductor substrate and the epitaxial layer exhibit a net doping concentration having a maximum value at the interface of the semiconductor substrate and the insulator layer and decreasing monotonically at a rate greater than or equal to a predetermined average rate of change of slope with distance within the substrate and epitaxial layer with increasing distance from the interface within a portion of the semiconductor substrate and the epitaxial layer between the interface and the junctions.

12. The imaging device of claim 11, wherein the net doping concentration decreases monotonically from a value greater than 1.0E19/cm³ to the intrinsic value of the carrier concentration of the epitaxial layer within a predetermined distance from the interface.

13. The imaging device of claim 12, wherein the predetermined distance from the interface is at the face of the epitaxial layer.

14. The imaging device of claim 11, wherein the net doping concentration decreases monotonically from a value greater than 1.0E19/cm³ to less than 5.0E12/cm³ within a distance of approximately 3 microns or less.

15. The imaging device of claim 11, wherein a doping concentration at the interface of the semiconductor substrate and the insulator layer is high enough to stabilize a quantum efficiency of the device with respect to time and incident flux.

16. The imaging device of claim 11, wherein the epitaxial layer and semiconductor substrate both comprise silicon and the insulator layer comprises an oxide of silicon.

17. The imaging device of claim 11, wherein the insulator layer has a thickness selected to function as an anti-reflection coating over the predetermined range of wavelengths.

18. The imaging device of claim 11, further comprising at least one layer of a material deposited on the insulating layer opposite the semiconductor substrate that functions as an anti-reflection coating for electromagnetic radiation over a predetermined range of wavelengths.

19. The device of claim 11, wherein light detected by the imaging components is incident on a face of the insulator layer opposite the interface between the semiconductor substrate and the insulator layer.

20. The device of claim 11, wherein the predetermined range of wavelengths includes an ultraviolet range.

21. The imaging device of claim 11, wherein the one or more imaging components includes CMOS imaging components, charge-coupled device (CCD) components, photodiodes, avalanche photodiodes, or phototransistors, in any combination.

22. The imaging device of claim 11, wherein the semiconductor substrate has a thickness in the range from about 5 nanometers to about 100 nanometers.

23. The imaging device of claim 11, wherein the epitaxial layer has a thickness in the range from about 1 micrometer to about 400 micrometers.

24. A back-illuminated semiconductor imaging device configured to operate over a predetermined range of wavelengths and reduce dark current, comprising:

an insulator layer;

a semiconductor substrate, having an interface with the insulator layer;

an epitaxial layer grown on the semiconductor substrate by epitaxial growth; and

one or more imaging components in the epitaxial layer in proximity to a face of the epitaxial layer, the face being opposite the interface of the semiconductor substrate and the insulator layer, the imaging components comprising a plurality of junctions within the epitaxial layer;

wherein the semiconductor substrate and the epitaxial layer exhibit a net doping concentration having a maximum value at a predetermined distance from the interface of the insulating layer and the semiconductor substrate and which decreases monotonically on both sides of the profile from the maximum value within a portion of the semiconductor substrate and the epitaxial layer.

25. The imaging device of claim 24, wherein the shape of the net dopant concentration profile is approximately Gaussian.

26. The imaging device of claim 24, wherein the maximum value of the net dopant concentration profile is high enough such that a potential barrier corresponding to the doping maximum is at least about 10 times greater than kT, where k is the Botzmann constant and T is absolute temperature in Kelvins.

27. The imaging device of claim 24, wherein the maximum value of the net dopant concentration profile is high enough to stabilize a quantum efficiency of the device with respect to time and incident flux.

28. The imaging device of claim 24, wherein the epitaxial layer and semiconductor substrate both comprise silicon and the insulator layer comprises an oxide of silicon.

29. The imaging device of claim 24, wherein the insulator layer has a thickness selected to function as an anti-reflection coating over the predetermined range of wavelengths.

30. The imaging device of claim 24, further comprising at least one layer of a material deposited on a side of the insulating layer opposite the semiconductor substrate which functions as an anti-reflection coating for electromagnetic radiation over a predetermined range of wavelengths.

31. The imaging device of claim 24, wherein light detected by the imaging components is incident on a face of the insulator layer opposite the interface between the semiconductor substrate and the insulator layer.

32. The imaging device of claim 24, wherein the one or more imaging components includes CMOS imaging components, charge-coupled device (CCD) components, photodiodes, avalanche photodiodes, or phototransistors, in any combination.

33. The imaging device of claim 24, wherein the semiconductor substrate has a thickness in the range from about 5 nanometers to about 100 nanometers.

34. The imaging device of claim 24, wherein the epitaxial layer has a thickness in the range from about 1 micrometer to about 400 micrometers.

35. A back-illuminated semiconductor imaging device configured to operate over a predetermined range of wavelengths and reduce dark current, comprising:

at least one layer of a material that functions as an anti-reflection coating for electromagnetic radiation over a predetermined range of wavelengths;

a semiconductor substrate deposited on the at least one layer that functions as an anti-reflection coating;

an epitaxial layer grown on the semiconductor substrate by epitaxial growth; and

at least one imaging component in the epitaxial layer in proximity to a face of the epitaxial layer, the face being opposite an interface of the semiconductor substrate and the at least one layer that functions as an anti-reflection coating, the imaging components comprising a plurality of junctions within the epitaxial layer;

wherein the semiconductor substrate and the epitaxial layer exhibit a net doping concentration having a maximum value at a predetermined distance from the interface of the at least one layer that functions as an anti-reflection coating and the semiconductor substrate and that decreases monotonically on both sides of the profile from the maximum value within a portion of the semiconductor substrate and the epitaxial layer.