System and method for fabricating diodes

• US 20040217457A1
• Filed: 05/03/2004
• Published: 11/04/2004
• Est. Priority Date: 05/01/2003
• Status: Active Grant

• Current Assignee: South Carolina University
• Original Assignee: South Carolina University
• Inventors: Sudarshan, T. S., Gao, Ying, Soloviev, Stanislav
• Granted Patent: US 7,061,021 B2
• Time in Patent Office: Days
• US Class Current: 257/678
• CPC Class Codes: H01L 21/0455 Making n or p doped regions or layers, e.g. using diffusion
• CPC Class Codes: H01L 29/1608 Silicon carbide
• CPC Class Codes: H01L 29/6606 the devices being controllable only by variation of the electric current supplied or the electric potential applied, to one or more of the electrodes carrying the current to be rectified, amplified, oscillated or switched, e.g. two-terminal devices
• CPC Class Codes: H01L 29/861 Diodes
• CPC Class Codes: Y10S 438/931 Silicon carbide semiconductor

Abstract

This invention is directed to a system and method of fabricating PN and PiN diodes by diffusing an acceptor impurity into a substrate. This invention is particularly advantageous for fabricating SiC diodes having linearly graded, deep pn junctions. One method that this invention uses to achieve its advantages is by diffusing an acceptor impurity into a substrate using a crucible, acceptor source, substrate, and furnace.

Claims

1. A method for fabricating a diode comprising the steps of:

providing a crucible;

providing a substrate comprised of wide band gap material having at least one epitaxial layer within said crucible;

providing an acceptor impurity in a gaseous state within said crucible; and

,

heating said crucible to at least 1400°

C. for at least 60 seconds so that said acceptor diffuses into said substrate to create a p layer thereby producing a PiN diode.

2. The method of claim 1 wherein said step of providing a substrate includes the step of providing a substrate doped by donor impurities with a density of at least 10¹⁷ cm⁻

3.

3. The method of claim 1 wherein said step of providing a substrate includes the step of providing a substrate doped with an n-type epitaxial layer with a density less than 10¹⁷ cm³.

4. The method of claim 1 wherein said step of providing a substrate includes the step of providing a SiC substrate.

5. The method of claim 1 including the steps of:

forming a photoresistant layer on said substrate; and

,

heating said photoresistant layer in a vacuum to at least 400°

C. at a rate of 2000°

C. per hour or less so that said heating forms a carbon film thereby providing a protection layer on said substrate.

6. The method of claim 5 wherein the step of heating said photoresist layer in a vacuum of pressure equal or below 1 Torr.

7. The method of claim 1 including the step of providing Aluminum in a gaseous state within said crucible.

8. The method of claim 1 wherein the step of providing an acceptor impurity in a gaseous state includes the step of providing Boron in a gaseous state.

9. The method of claim 8 wherein the step of providing an acceptor impurity in a gaseous state includes the steps of:

providing source material having Boron; and

,

evacuating said crucible to a pressure less than or equal to 1 Torr.

10. The method of claim 9 wherein the step of providing source material includes the step of providing SiC powder having at least 0.01% Boron by weight.

11. The method of claim 1 including the step of purifying said crucible prior to providing an acceptor impurity in a gaseous state by flushing said crucible with an inert gas.

12. The method of claim 1 including the step of removing a thickness of approximately 5-10 microns from the back side diffused layer of said diode.

13. The method of claim 1 including the step of forming mesa structures on said substrate so that a PiN diode is formed.

14. The method of claim 1 including the step of forming a local mask on said substrate so that local diffusion can be performed.

15. The method of claim 14 wherein the step of forming a local mask includes the steps of:

forming a local photoresist pattern having a thickness of more than 2 microns;

heating said photoresist pattern to at least 200°

C. at a rate of 400°

C. per hour or less to provide a hard-baked photoresist; and

,

heating said hard-baked photoresist in a vacuum at a pressure equal to or below 1 Torr to at least 600°

C. at a rate of 400°

C. per hour or less.

16. A diode produced by the method of claim 1.

17. The method of claim 1 including the steps of:

evacuating said crucible to a pressure of 760 Torr or less; and

,

inserting an inert gas into said crucible prior to heating said crucible to at least 1400°

C.

41. The semiconductor of claim 39 formed by the process of claim 1.

18. A method for fabricating a diode comprising the steps of:

providing a crucible;

providing a substrate comprised of wide band gap material having a n⁺ layer, a n⁻

layer and a p⁺ layer within said crucible;

providing an acceptor impurity in a gaseous state within said crucible; and

,

heating said crucible to at least 1400°

C. for at least 60 seconds so that said Boron diffuses into said substrate to create a p-layer thereby fabricating a PiN diode.

19. The method of claim 18 wherein said step of providing a substrate includes the step of providing a substrate doped by donor impurities with a density of at least 10¹⁷ cm⁻

3.

20. The method of claim 18 wherein said step of providing a substrate includes the step of providing a substrate with an n-type epitaxial layer with a density less than 10¹⁷ cm⁻

3.

21. The method of claim 18 wherein said step of providing a substrate includes the step of providing a substrate with a p-type layer.

22. The method of claim 18 wherein said step of providing a substrate includes the step of providing a SiC substrate.

23. The method of claim 18 including the steps of:

forming a photoresist layer on said substrate; and

,

heating said photoresist layer in a vacuum to at least 400°

C. at a rate of 2000°

C. per hour or less so that said heating forms a carbon film thereby providing a protection layer on said substrate.

24. The method of claim 23 wherein the step of heating said photoresist layer in a vacuum of pressure equal or below 1 Torr.

25. The method of claim 18 wherein the step of providing an acceptor impurity in a gaseous state includes the steps of:

providing source material having Boron; and

,

evacuating said crucible to a pressure below or equal to 1 Torr.

26. The method of claim 25 wherein the step of providing source material includes the step of providing SiC powder having at least 0.01% Boron by weight.

27. The method of claim 18 including the step of forming mesa structures on said substrate so that a PiN diode is formed.

28. The method of claim 18 including the steps of:

evacuating said crucible to a pressure of 760 Torr or less; and

,

inserting an inert gas into said crucible prior to heating said crucible to at least 1400°

C.

29. A diode produced by the method of claim 18.

42. The semiconductor of claim 39 formed by the process of claim 18.

30. A system for fabricating diodes comprising:

a housing;

a crucible contained within said housing;

a heating member for heating said crucible;

a substrate contained within said crucible having donor impurities with a density of at least 10¹⁷ cm<sup>−

3</sup> and a n-type epitaxial layer with a density of less than or equal to 10¹⁷ cm<sup>−

3</sup>; and

,

a gaseous acceptor impurity source contained within said crucible so that when said crucible is heated, said acceptor diffuses into said substrate to form a PiN diode.

31. The system of claim 30 including a gaseous Aluminum source contained within said crucible so that said Aluminum diffuses into said substrate when heated.

32. The system of claim 30 including an inert gas contained within said crucible.

33. The system of claim 30 wherein said substrate is a SiC substrate.

34. The system of claim 30 including a graphite mask for local diffusion having a thickness of at least one micron.

35. The system of claim 30 including insulation material generally surrounding said crucible.

36. The system of claim 30 wherein said acceptor impurity includes Boron.

37. The system of claim 36 wherein said gaseous acceptor impurity source includes at least 0.01% Boron by weight.

38. The system of claim 37 wherein said gaseous acceptor impurity source includes SiC powder.

39. A silicon carbide semiconductor comprising:

a n⁺ layer;

a n⁻

layer carried by said n⁺ layer;

a p-layer carried by said n⁻

layer having a graded pn junction between said n⁻

layer and said p-layer where said gradient is at least 10²³/cm⁴.

40. The semiconductor of claim 39 wherein said pn junction is located at least 0.5 micron from the top surface of said p layer.

43. The semiconductor of claim 39 formed by the process of ion implantation.

44. The semiconductor of claim 39 formed by the process of epitaxial growth.