Domain reversal control method for ferroelectric materials Number:7,115,513 from the United States Patent and Trademark Office (PTO) owispatent

Title: Domain reversal control method for ferroelectric materials

Abstract: A method for forming uniform, sharply defined periodic regions of reversed polarization within a unidirectionally polarized ferroelectric material proceeds as a two-step process. First, alignment keys are formed on upper and lower planar surfaces of a unidirectionally polarized ferroelectric material by producing a spaced pair of alignment key shaped domain reversed regions and etching alignment key shaped notches in the upper and lower surfaces where the domain reversed regions intersect the surface planes. These notches, being vertically aligned between the upper and lower surfaces, are then used to align photomasks over a surface coating of photoresist formed directly on the material surface or on SiO.sub.2 layers coating the material surface. The photoresist is then patterned and may also be thermally hardened to form a plurality of regularly spaced open strips, through which conducting metal contacts or ion-exchanged regions may be formed or contacts to the ferroelectric surface can be directly made by liquid conductors to produce the desired polarization reversals by application of an electric field at different temperatures of the material.

Patent Number: 7,115,513 Issued on 10/03/2006 to Chiang,   et al.

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Inventors:	Chiang; Tsung Yuan (Banciao, TW), Lin; Tze-Chia (Taipei, TW), Sher; Benny (Yongkang, TW), Chou; Ming-Hsien (Holmdel, NJ)
Assignee:	HC Photonics Corporation (Hsinchu, TW)
Appl. No.:	10/933,804
Filed:	September 3, 2004

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Current U.S. Class:	438/689 ; 216/23; 359/272; 438/725; 438/975
Current International Class:	H01L 21/302 (20060101)
Field of Search:	438/689,725,975 216/23 359/272

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References Cited [Referenced By]

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U.S. Patent Documents

	5009489		April 1991		Eguchi et al.
	5249250		September 1993		Yamada et al.
	5395495		March 1995		Nozaki
	5415743		May 1995		Harada
	5424867		June 1995		Nihei et al.
	5522973		June 1996		Harada
	5568308		October 1996		Harada
	5570225		October 1996		Harada
	5594746		January 1997		Harada
	6156255		December 2000		Byer et al.

Primary Examiner: Mills; Gregory
Assistant Examiner: Umez-Eronini; Lynette T.
Attorney, Agent or Firm: Saile Ackerman LLC Ackerman; Stephen B.

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Claims

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What is claimed is:

1. A domain reversal alignment method for forming photolithographic mask alignment keys on surfaces of a unidirectionally polarized ferroelectric material comprising: providing a substrate which is a unidirectionally polarized ferroelectric material having opposing first and second planar surfaces which are substantially parallel and wherein said polarization is directed substantially towards said first surface from said second surface; forming a layer of photosensitive material on the first surface of said substrate; patterning said photosensitive layer to produce openings having an alignment key shape, said openings exposing corresponding surface regions of said substrate surface; hardening said photosensitive layer by thermal processing and rendering said layer, thereby, an insulator; coating said surface regions with a conducting liquid; producing an electric field in a region within the substrate interior between said exposed substrate first surface regions and second substrate surface, said electric field reversing the unidirectional polarization within said interior region; and then removing said photosensitive layer; and etching substrate first and second surfaces to produce, thereby, surface indentations having said alignment key shape on both the first and second substrate surfaces, the indentation on the first surface being vertically aligned with a corresponding indentation on the second surface and said surface indentations forming alignment keys for the subsequent alignment of photolithographic masks over said surfaces.

2. The method of claim 1 wherein electric field is produced by placing an external source of potential difference between said first and second surfaces by electrically contacting said first and second surfaces, wherein said first surface is electrically contacted by said conducting liquid coated on said exposed surface regions.

3. The method of claim 1 wherein said conducting liquid is benzoic acid, phosphoric acid or solutions of the salts LiCl or KOH.

4. The method of claim 1 wherein said ferroelectric material is congruent LiTaO.sub.3 (CLT), congruent LiNbO.sub.3 (CLN), stoichiometric LiNbO.sub.3 (SLT), MgO doped CLT, or MgO doped CLN.

5. The method of claims 4 wherein the electric field is more than 20.75 kV/mm at room temperature for congruent LiNbO.sub.3 and for MgO doped CLN it is approximately 4 kV/mm at 180.degree. C.

6. The method of claim 1 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

7. A domain reversal alignment method for forming photolithographic mask alignment keys on surfaces of a unidirectionally polarized ferroelectric material comprising: providing a substrate which is a unidirectionally polarized ferroelectric material having opposing first and second planar surfaces which are substantially parallel and wherein said polarization is directed substantially towards said first surface from said second surface; forming a metallic or metal oxide conducting layer on the first surface of said substrate, said layer having an alignment key shape; producing an electric field within an interior region of the substrate between the first and second substrate surfaces and beneath said conducting layer, said electric field reversing the unidirectional polarization within said region; and then removing said conducting layers; and etching substrate first and second surfaces to produce, thereby, surface indentations having said alignment key shape on both the first and second substrate surfaces, the indentation on the first surface being vertically aligned with a corresponding indentation on the second surface and said surface indentations forming alignment keys for the subsequent alignment of photolithographic masks over said surfaces.

8. The method of claim 7 wherein said ferroelectric material is congruent LiTaO.sub.3 (CLT), congruent LiNbO.sub.3 (CLN), stoichiometric LiNbO.sub.3 (SLT), MgO doped CLT, or MgO doped CLN.

9. The method of claim 8 wherein the electric field is more than 20.75 kV/mm at room temperature for congruent LiNbO.sub.3 and for MgO doped CLN it is approximately 4 kV/mm at 180.degree. C.

10. The method of claim 7 wherein the metallic conducting layer is Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo and the conducting metallic oxides are IrO.sub.2, RuO.sub.2 or SrTiO.sub.3.

11. The method of claim 7 wherein said electric field is produced by an external source of potential difference, said source electrically contacting said second surface and electrically contacting the conducting layer on said first surface.

12. A method for forming uniform domains of reversed polarization within a unidirectionally polarized ferroelectric material comprising: providing a substrate that is a unidirectionally polarized ferroelectric material having opposing first and second planar surfaces which are substantially parallel and wherein said polarization is directed substantially vertically towards said first surface and away from said second surface; forming alignment keys on said planar surfaces; using said alignment keys, forming, by a first process, a plurality of periodically spaced regions on said first surface and forming, by a second process, a plurality of periodically spaced regions on said second surface, all of said regions on said first surface being exposed or electrically contacted by a conducting layer and each of said regions being vertically aligned with a corresponding region on said second surface and all of said second surface regions being exposed or electrically contacted by a conducting layer; coating all exposed surface regions with a conducting liquid; electrically contacting the conducting liquid or conducting layer on said corresponding surface regions formed on said first and second surfaces; producing an electric field within said substrate between said corresponding electrically contacted regions and thereby reversing said unidirectional polarization within the substrate interior between said regions.

13. The method of claim 12 wherein said alignment keys are formed by a method comprising: forming a layer of photosensitive material on the first surface of said substrate; patterning said photosensitive layer to produce openings having an alignment key shape, said openings exposing corresponding surface regions of said substrate surface; hardening said photosensitive layer by thermal processing and rendering said layer, thereby, an insulator; producing an electric field in a region between said exposed substrate first surface regions and second substrate surface, said electric field reversing the unidirectional polarization within said region; and then removing said photosensitive layer; etching said first and second surfaces to produce, thereby, surface indentations having said alignment key shape on both the first and second substrate surfaces, the indentation on the first surface being vertically aligned with a corresponding indentation on the second surface and said surface indentations forming alignment keys.

14. The method of claim 13 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

15. The method of claim 12 wherein said alignment keys are formed by a method comprising: forming a conducting layer on the first surface of said substrate, said layer having an alignment key shape; producing an electric field within a region of the substrate between the first and second substrate surfaces and beneath said conducting layer, said electric field reversing the unidirectional polarization within said region; and then removing said conducting layer; etching said first and second surfaces to produce, thereby, surface indentations having said alignment key shape on both the first and second substrate surfaces, the indentation on the first surface being vertically aligned with a corresponding indentation on the second surface and said surface indentations forming alignment keys.

16. The method of claim 15 wherein the conducting layer is a layer of the metal Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo or the conducting metallic oxide IrO.sub.2, RuO.sub.2 or SrTiO.sub.3.

17. The method of claim 12 wherein said first process comprises: forming a first layer of photosensitive material on said first substrate surface; patterning said first layer using a first photolithographic mask aligned with the alignment keys on the first substrate surface, thereby forming periodically spaced openings in said first layer and exposing corresponding regions of said first surface; hardening said patterned first layer and rendering it an insulator by heating.

18. The method of claim 17 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

19. The method of claim 17 wherein said second process comprises: forming a second layer of photosensitive material on said second substrate surface; patterning said second layer using a second photolithographic mask, substantially identical to said first photolithographic mask, aligned with the alignment keys on the second substrate surface, thereby forming periodically spaced openings in said second layer that are vertically below said openings in the first layer and exposing corresponding regions of said second surface; hardening said patterned second layer and rendering it an insulator by heating.

20. The method of claim 19 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

21. The method of claim 12 wherein said first process comprises: forming a layer of photosensitive material on said first substrate surface; patterning said layer using a first photolithographic mask aligned with the alignment keys on the first substrate surface, thereby forming openings in said layer and exposing corresponding regions of said first surface; hardening said patterned layer and rendering it an insulator by heating.

22. The method of claim 21 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

23. The method of claim 21 wherein said second process comprises: forming an insulating layer on said second surface; forming a second layer of photosensitive material on said insulating layer; patterning said photosensitive material using a second photolithographic mask substantially identical to said first mask aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material.

24. The method of claim 23 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

25. The method of claim 23 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist.

26. The method of claim 12 wherein said first process comprises: forming a first layer of photosensitive material on said first substrate surface; patterning said first layer using a first photolithographic mask aligned with the alignment keys on the first substrate surface, thereby forming periodically spaced openings in said first layer and exposing corresponding regions of said first surface; hardening said patterned first layer by heating.

27. The method of claim 26 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

28. The method of claim 27 wherein said second process comprises: forming an insulating layer on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a second photolithographic mask substantially identical to said first photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material; forming ion-exchanged regions within the substrate below said exposed second surface regions.

29. The method of claim 28 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

30. The method of claim 28 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist.

31. The method of claim 12 wherein said first process comprises: forming an insulating layer on said first surface; forming a first layer of photosensitive material on said insulating layer; patterning said first layer of photosensitive material using a first photolithographic mask aligned by said alignment keys on said first surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned first layer of photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said first substrate surface; removing the patterned first layer of photosensitive material; forming ion-exchanged regions within the substrate below said exposed second surface regions; removing said insulating layer; forming a second layer of photosensitive material on said first surface; patterning said second layer using a second photolithographic mask aligned by said alignment keys on said first surface, said patterning leaving portions of said second layer positioned above said ion-exchanged regions and forming exposed first surface regions adjacent to each side of said ion-exchanged regions; heating said patterned second layer to harden it and render it an insulator.

32. The method of claim 31 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

33. The method of claim 31 wherein said second process comprises: forming an insulating layer on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a third photolithographic mask substantially identical to said second photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material.

34. The method of claim 33 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist.

35. The method of claim 31 wherein said second process comprises: forming an insulating layer on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a third photolithographic mask substantially identical to said second photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material; forming ion-exchanged regions within the substrate below said exposed second surface regions.

36. The method of claim 35 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist.

37. The method of claim 12 wherein said first process comprises: forming a first layer of photosensitive material on said first substrate surface; patterning said first layer using a first photolithographic mask aligned with the alignment keys on the first substrate surface, thereby forming periodically spaced openings in said first layer and exposing corresponding regions of said first surface; hardening said patterned first layer and rendering it an insulator by heating.

38. The method of claim 37 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

39. The method of claim 37 wherein said second process comprises: forming a second layer of photosensitive material on said second substrate surface; patterning said second layer using a second photolithographic mask substantially identical to said first photolithographic mask aligned with the alignment keys on the second substrate surface, thereby forming periodically spaced openings in said second layer and exposing corresponding regions of said second surface vertically below the periodically spaced openings on the first surface; forming a conducting layer over said patterned second layer, a portion of said conducting layer conformally filling each periodically spaced opening in said second layer and each said portion thereby electrically contacting the exposed surface regions of said second surface; removing said patterned second layer, leaving, thereby, only said portions of said conducting layer electrically contacting said exposed regions of said second surface.

40. The method of claim 39 wherein the conducting layer is a layer of the metallic conductors Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo or the conducting metallic oxides IrO.sub.2, RuO.sub.2 or SrTiO.sub.3 and the photosensitive material is photoresist.

41. The method of claim 37 wherein said second process comprises: forming an insulating layer on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a second photolithographic mask substantially identical to said first photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material; forming ion-exchanged regions within the substrate below said exposed second surface regions; removing said insulating layer; forming a second layer of photosensitive material on said second substrate surface; patterning said second layer using a third photolithographic mask substantially identical to said second photolithographic mask aligned with the alignment keys on the second substrate surface, thereby forming periodically spaced openings in said second layer and exposing corresponding regions of said second surface vertically below the periodically spaced openings on the first surface; forming a conducting layer over said patterned second layer, a portion of said conducting layer conformally filling each periodically spaced opening in said second layer and each said portion thereby electrically contacting the exposed surface regions of said second surface over said ion-exchanged regions; removing said patterned second layer, leaving, thereby, only said portions of said conducting layer electrically contacting said exposed regions of said second surface.

42. The method of claim 41 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist that is hardened by heating to 180.degree. C.

43. The method of claim 37 wherein said second process comprises: forming a layer of photosensitive material on said second substrate surface; patterning said second layer using a second photolithographic mask, substantially identical to said first photolithographic mask, aligned with the alignment keys on the second substrate surface, thereby forming periodically spaced openings in said second layer and exposing corresponding regions of said second surface vertically below the periodically spaced openings on the first surface; forming a conducting layer over said patterned second layer, a portion of said conducting layer conformally filling each periodically spaced opening in said second layer and each said portion thereby electrically contacting the exposed surface regions of said second surface over said ion-exchanged regions; removing said patterned second layer, leaving, thereby, only said portions of said metal layer electrically contacting said exposed regions of said second surface.

44. The method of claim 43 wherein the conducting layer is a layer of the metallic conductors Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo or the conducting metallic oxides IrO.sub.2, RuO.sub.2 or SrTiO.sub.3.

45. The method of claim 43 wherein the photosensitive material is photoresist.

46. The method of claim 37 wherein said second process comprises: forming an insulating layer on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a third photolithographic mask substantially identical to said first photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material; forming ion-exchanged regions within the substrate below said exposed second surface regions; removing said insulating layer; forming a second layer of photosensitive material on said second substrate surface; patterning said second layer using a fourth photolithographic mask substantially identical to said second photolithographic mask aligned with the alignment keys on the second substrate surface, thereby forming periodically spaced openings in said second layer and exposing corresponding regions of said second surface vertically below the periodically spaced openings on the first surface; forming a conducting layer over said patterned second layer, a portion of said conducting layer conformally filling each periodically spaced opening in said second layer and each said portion thereby electrically contacting the exposed surface regions of said second surface over said ion-exchanged regions; removing said patterned second layer, leaving, thereby, only said portions of said metal layer electrically contacting said exposed regions of said second surface.

47. The method of claim 46 wherein the conducting layer is a layer of the metallic conductors Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo or the conducting metallic oxides IrO.sub.2, RuO.sub.2 or SrTiO.sub.3.

48. The method of claim 12 wherein said first process comprises: forming a layer of photosensitive material on said first substrate surface; patterning said layer using a first photolithographic mask aligned with the alignment keys on the second substrate surface, thereby forming periodically spaced openings in said second layer and exposing corresponding regions of said second surface vertically below the periodically spaced openings on the first surface; forming a conducting layer over said patterned layer, a portion of said conducting layer conformally filling each periodically spaced opening in said second layer and each said portion thereby electrically contacting the exposed surface regions of said second surface; removing said patterned layer, leaving, thereby, only said portions of said metal layer electrically contacting said exposed regions of said second surface.

49. The method of claim 48 wherein the conducting layer is a layer of the metallic conductors Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo or the conducting metallic oxides IrO.sub.2, RuO.sub.2 or SrTiO.sub.3.

50. The method of claim 48 wherein said second process comprises: forming a layer of photosensitive material on said second substrate surface; patterning said first layer using a second photolithographic mask aligned with the alignment keys on the first substrate surface, thereby forming periodically spaced openings in said first layer and exposing corresponding regions of said first surface; hardening said patterned first layer by heating.

51. The method of claim 50 wherein said photosensitive material is photoresist and it is hardened by being raised to a temperature of approximately 180.degree. C.

52. The method of claim 48 wherein said second process comprises: forming an insulating layer on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a second photolithographic mask substantially identical to said first photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material.

53. The method of claim 52 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist and it is hardened by heating to approximately 180.degree. C.

54. The method of claim 48 wherein said second process comprises: forming an insulating layer on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a second photolithographic mask substantially identical to said second photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material; forming ion-exchanged regions within the substrate below said exposed second surface regions.

55. The method of claim 54 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist and it is hardened by heating to approximately 180.degree. C.

56. The method of claim 12 wherein said first process comprises: forming an insulating layer on said first surface; forming a first layer of photosensitive material on said insulating layer; patterning said first layer of photosensitive material using a first photolithographic mask aligned by said alignment keys on said first surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned first layer of photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said first substrate surface; removing the patterned first layer of photosensitive material; forming ion-exchanged regions within the substrate below said exposed second surface regions; removing said insulating layer; forming a second layer of photosensitive material on said first surface; patterning said second layer using a second photolithographic mask aligned by said alignment keys on said first surface, said patterning leaving portions of said second layer positioned above said ion-exchanged regions and forming exposed first surface regions adjacent to each side of said ion-exchanged regions; forming a conducting layer over said patterned second layer, a portion of said conducting layer conformally filling each periodically spaced opening in said second layer and each said portion thereby electrically contacting the exposed surface regions of said second surface; removing said patterned layer, leaving, thereby, only said portions of said conducting layer electrically contacting said exposed regions of said second surface.

57. The method of claim 56 wherein the conducting layer is a layer of the metallic conductors Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo or the conducting metallic oxides IrO.sub.2, RuO.sub.2 or SrTiO.sub.3.

58. The method of claim 56 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist.

59. The method of claim 56 wherein said second process comprises: forming an insulating layer on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a second photolithographic mask substantially identical to said first photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material.

60. The method of claim 59 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist and it is hardened by heating to approximately 180.degree. C.

61. The method of claim 56 wherein said second process comprises: forming an insulating layer of on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a second photolithographic mask substantially identical to said second photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said insulating layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material; forming ion-exchanged regions within the substrate below said exposed second surface regions.

62. The method of claim 61 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist and it is hardened by heating to approximately 180.degree. C.

63. The method of claim 56 wherein said second process comprises: forming a layer of photosensitive material on said second substrate surface; patterning said second layer using a second photolithographic mask, substantially identical to said first photolithographic mask, aligned with the alignment keys on the second substrate surface, thereby forming periodically spaced openings in said second layer and exposing corresponding regions of said second surface vertically below the periodically spaced openings on the first surface; forming a conducting layer over said patterned second layer, a portion of said conducting layer conformally filling each periodically spaced opening in said second layer and each said portion thereby electrically contacting the exposed surface regions of said second surface over said ion-exchanged regions; removing said patterned second layer, leaving, thereby, only said portions of said metal layer electrically contacting said exposed regions of said second surface.

64. The method of claim 63 wherein the conducting layer is a layer of the metallic conductors Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo or the conducting metallic oxides IrO.sub.2, RuO.sub.2 or SrTiO.sub.3.

65. The method of claim 63 wherein said photosensitive material is photoresist.

66. The method of claim 56 wherein said second process comprises: forming an insulating layer on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a second photolithographic mask substantially identical to said first photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said SiO.sub.2 layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material; forming ion-exchanged regions within the substrate below said exposed second surface regions; removing said insulating layer; forming a second layer of photosensitive material on said second substrate surface; patterning said second layer using a third photolithographic mask substantially identical to said second photolithographic mask aligned with the alignment keys on the second substrate surface, thereby forming periodically spaced openings in said second layer and exposing corresponding regions of said second surface vertically below the periodically spaced openings on the first surface; forming a conducting layer over said patterned second layer, a portion of said conducting layer conformally filling each periodically spaced opening in said second layer and each said portion thereby electrically contacting the exposed surface regions of said second surface over said ion-exchanged regions; removing said patterned second layer, leaving, thereby, only said portions of said metal layer electrically contacting said exposed regions of said second surface.

67. The method of claim 66 wherein the conducting layer is a layer of the metallic conductors Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo or the conducting metallic oxides IrO.sub.2, RuO.sub.2 or SrTiO.sub.3.

68. The method of claim 66 wherein said insulating layer is a layer of SiO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4 or hardened photoresist formed to a thickness between approximately 500 and 5000 angstroms by sputtering or e-beam deposition and wherein the photosensitive material is photoresist.

69. The method of claim 56 wherein said second process comprises: forming a layer of photosensitive material on said second substrate surface; patterning said second layer using a second photolithographic mask, substantially identical to said first photolithographic mask, aligned with the alignment keys on the second substrate surface, thereby forming periodically spaced openings in said second layer and exposing corresponding regions of said second surface vertically below the periodically spaced openings on the first surface; forming a conducting layer over said patterned second layer, a portion of said conducting layer conformally filling each periodically spaced opening in said second layer and each said portion thereby electrically contacting the exposed surface regions of said second surface over said ion-exchanged regions; removing said patterned second layer, leaving, thereby, only said portions of said conducting layer electrically contacting said exposed regions of said second surface.

70. The method of claim 69 wherein the conducting layer is a layer of the metallic conductors Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo or the conducting metallic oxides IrO.sub.2, RuO.sub.2 or SrTiO.sub.3.

71. The method of claim 69 wherein the photosensitive material is photoresist.

72. The method of claim 56 wherein said second process comprises: forming an insulating layer on said second surface; forming a layer of photosensitive material on said insulating layer; patterning said photosensitive material using a second photolithographic mask substantially identical to said first photolithographic mask and aligned by said alignment keys on said second surface, said patterning exposing periodically spaced surface regions of said SiO.sub.2 layer vertically below said exposed first surface regions; hardening said patterned photosensitive material by heating; etching through said insulating layer by applying an etching solution to said exposed surface regions, thereby exposing corresponding regions of said second substrate surface; removing the patterned photosensitive material; forming ion-exchanged regions within the substrate below said exposed second surface regions; removing said insulating layer; forming a second layer of photosensitive material on said second substrate surface; patterning said second layer using a third photolithographic mask substantially identical to said second photolithographic mask aligned with the alignment keys on the second substrate surface, thereby forming periodically spaced openings in said second layer and exposing corresponding regions of said second surface vertically below the periodically spaced openings on the first surface; forming a conducting layer over said patterned second layer, a portion of said conducting layer conformally filling each periodically spaced opening in said second layer and each said portion thereby electrically contacting the exposed surface regions of said second surface over said ion-exchanged regions; removing said patterned second layer, leaving, thereby, only said portions of said metal layer electrically contacting said exposed regions of said second surface.

73. The method of claim 72 wherein the conducting layer is a layer of the metallic conductors Al, Cu, Ni, Cr, NiCr, Au, Ag, Ta, Ti, Pt, Mo or the conducting metallic oxides IrO.sub.2, RuO.sub.2 or SrTiO.sub.3.

74. The method of claim 72 wherein said photosensitive material is photoresist.

75. The method of claim 12 wherein the ferroelectric material is congruent LiTaO.sub.3 (CLT), congruent LiNbO.sub.3 (CLN), stoichiometric LiNbO.sub.3 (SLT), MgO doped CLT, or MgO doped CLN.

76. The method of claim 75 wherein the electric field is more than 20.75 kV/mm at room temperature for congruent LiNbO.sub.3 and for MgO doped CLN it is approximately 4 kV/mm at 180.degree. C.

77. The method of claim 12 wherein said conducting liquid is benzoic acid, phosphoric acid or a solution of the salts KOH or LiCl.

78. The method of claim 12 wherein said substrate is heated to an optimal temperature that is less than approximately 250.degree. C. during application of the electric field.

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Description

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a fabrication method for creating patterned domain reversals in ferroelectric materials. The method can be applied to any ferroelectric material.

2. Description of the Related Art

Ferroelectric materials have internal electric dipole moments which can be made to manifest themselves on a macroscopic domain scale without the presence of external electric fields (hence the term "ferroelectric" by analogy with "ferromagnetic" for materials with domain scale magnetic dipole moments). These macroscopic polarizations are responsible for the optical properties of the materials through the important effects they have on the propagation of electromagnetic radiation. When the polarization of such materials is linearly dependent (or only very weakly non-linearly dependent) on the electric field strength of an electromagnetic wave propagating through the material, the effect of the linear polarization is to produce a constant refractive index, which is responsible for modifying the speed of the wave through the material. In such a linear case, an incident oscillating electromagnetic field at frequency .omega. produces an oscillation of the polarization at the same frequency co which, in turn, produces a re-radiated electromagnetic field also of the same frequency but out of phase with the incident wave. The original incident wave, combined with the phase-varying re-radiated waves along the forward propagation direction of the incident wave, creates a net transmitted wave that moves through the material at an apparently slower speed but same frequency. The speed, v(.omega.), of the transmitted wave in the crystal, is defined as c/n(.omega.), where c is the speed of the wave in vacuum (ie., the speed of light) and n(.omega.) is the index of refraction of the medium which, as indicated, depends on the frequency .omega. of the wave (ie., the medium is generally dispersive). Another important parameter of the medium is k(.omega.), the propagation constant of the radiation, which is defined as: k(.omega.)=2.pi.n(.omega.)/.lamda., where .lamda. is the wavelength of the wave in vacuum.

When the polarization of the crystal is made to change by the imposition of an external electric field that is not the oscillating field of the incident electromagnetic wave, then interactions between the wave and the material can occur which are not simply describable by a constant index of refraction that simply changes the wave speed. For example, the so-called electrooptic effect results when the application of a constant electric field is used to rotate the dipole moment directions of a crystal and to thereby change both the speed and the polarization direction of an incident wave.

If the polarization at a position x within the crystal is a non-linear function of the field at that position, the propagation of an electromagnetic wave can be affected in additional ways. For example, the propagation of a wave with frequency .omega..sub.1 will lead to the propagation of a secondary wave with frequency 2.omega..sub.1, which is the second harmonic of the wave. If two waves, of different frequencies .omega..sub.1 and .omega..sub.2 simultaneously propagate through the non-linear crystal, there will be generated additional waves of frequencies .omega..sub.1+.omega..sub.2, .omega..sub.1-.omega..sub.2, 2.omega..sub.1 and 2.omega..sub.2. Each wave periodically modulates the polarization which the other wave sees and which it sees itself and, as a result, new waves are generated.

When waves at two different frequencies .omega..sub.1 and .omega..sub.2 and two different propagation constants k.sub.1 and k.sub.2 propagate a distance L through a non-linear crystal, one wave (k.sub.1) can transfer power to the other wave (k.sub.2) through the non-linearity of the polarization. The amount of power transferred after the wave has traveled a distance L in the crystal can be shown to be proportional to: L.sup.2(sin x/x).sup.2, where x=(k.sub.3-k.sub.1-k.sub.2)L/2=L.DELTA.k/2 and k.sub.3 is the propagation constant of the polarization wave. If .DELTA.k is not zero, the transfer of power reaches a maximum value when the wave has propagated a distance called the coherence length, L.sub.c, where L.sub.c.DELTA.k/2=.pi./2, (ie. L.sub.c=.pi./.DELTA.k). If .DELTA.k=0, the incident waves and the polarization wave are said to be phase matched and the power transfer increases along the entire length of the crystal and is proportional to L.sup.2, where L is the length of the entire crystal. If .DELTA.k is not zero, the maximum power transfer occurs within the coherence length, then goes to zero, then rises again in the next coherence length. In general, the power transfer within the coherence length is the maximum transfer possible, no matter through how many coherence lengths the waves propagate. Since obtaining a phase matched condition is very difficult in practice (it has been obtained using birefringent crystals) and will only occur at particular k values, an alternative approach to maximizing power transfer is through "quasi-phase matching" or QPM. QPM is obtained by changing the phase between the propagating wave and the polarization wave by .pi./2 every time the propagating wave moves through an additional coherence length. This can be accomplished by rotating the polarization direction within the crystal by 180.degree. in successive coherence lengths. Unlike true phase matching, QPM can be obtained for a wave of arbitrary k value, providing the polarizations in the crystal can be rendered appropriately periodic in successive coherence lengths. Thus, by satisfying the QPM condition, the maximum power transfer is incremented in each successive passage of the wave through a coherence length, rather than falling to zero. Such a periodic rotation of polarization domains (domain reversals) of width L.sub.c is usually accomplished by the application of electric fields, electron beam patterning or proton exchange/heat treatment (of which more will be said in the following) and the process is called "poling." Although quasi-phase matching does not produce the amount of power transfer produced by genuine phase matching (because the factor (sin x/x).sup.2 is 1 for .DELTA.k=0, but is 4/.pi..sup.2 for L.sub.c .DELTA.k=.pi./2), it is much better than the non-QPM case. Much inventive effort has been expended in finding ways of rotating crystal polarizations in a periodic manner with the coherence length being the period.

A case of particular interest in modern technology occurs when .omega..sub.2=2.omega..sub.1, which is called frequency doubling or second harmonic generation (SHG). Obtaining a meaningful power transfer between an incident wave and its frequency doubled second harmonic allows the production, for example, of coherent green or blue light by the passage of near infra-red radiation from a solid state laser through a non-linear ferroelectric crystal. Since coherent infra-red radiation is easier to produce by laser action than coherent blue or green radiation, being able to obtain the green or blue by SHG is quite important. Such green or blue light is important for reading and writing optical storage disks. The coherence length needed to obtain efficient frequency doubling is given by: L.sub.c=.pi./(2k.sub.1-k.sub.2). Note that 2k.sub.1-k.sub.2 is not zero because of the dispersion of the material, so true phase matching is generally not possible. As noted above, use of QPL by poling non-linear ferroelectric crystals such as congruent lithium niobate (LiNbO.sub.3, or CLN) or stoichiometric lithium tantalate (LiTaO.sub.3 or SLT) allows frequency doubling of radiation within the entire range of frequencies for which these crystals are transparent: (0.32 microns 5.5 microns) for CLN and (0.27 microns 5.5 microns) for SLT. The following prior art teaches a variety of methods for patterning and reversing polarization domains to achieve QLM in non-linear ferroelectric materials.

Nihei et al. (U.S. Pat. No. 5,424,867), teaches a method for fabricating an optical wavelength converter with a high threshold for optical damage. It is pointed out that a crystal may be damaged by a second harmonic wave having a relatively low power of 2 mW at a wavelength of 477 nm. Within the method, periodic domain reversals are formed on a LiNbO.sub.3 substrate which is covered by a thin, transparent conducting film of indium-tin-oxide (ITO). The film then dissipates surface charges formed by the polarization process without, at the same time, producing reflection or scattering which would degrade the signal. Within the method domain reversals within the crystal substrate are accomplished by an electron beam having an energy between 20 30 kV. The period of the domain reversals is set to 4.7 microns which allows a first order period of 946 nm to produce a second order harmonic of period 477 nm.

Yamada et al. (U.S. Pat. No. 5,249,250) notes that the depth of a domain reversal and the width of a domain reversal (pitch) are related so that a narrow pitch (high frequency) results in a shallow depth. A shallow depth reduces the region of the crystal through which the SHG will occur. To solve this problem, Yamada teaches the formation of domain reversed regions by first polarizing the crystal uni-directionally (a single domain), then irradiating the crystal surface with a 15 kV electron beam in a pattern of regularly spaced parallel strips. An important aspect of this method is that the inverted domain structure can be formed without adversely affecting the index of refraction of the crystal.

Nozaki et al. (U.S. Pat. No. 5,395,495) teaches a method of forming domain reversals within a ferroelectric crystal wherein a high resistance layer is first formed on a uni-directionally polarized crystal surface and a charged particle beam is then directed into the crystal through the layer. This method alleviates the problem of forming effective domain reversals in the vicinity of the crystal surface.

Harada et al. (U.S. Pat. No. 5,415,743) teaches a method of forming sharply defined domain reversals that extend through the entire thickness of the ferroelectric crystal. The method teaches the formation of proton-exchanged regions on a unipolarized crystal and then heating the regions by the application of external electric fields. Also taught in the method is the formation of a Ti-diffused region which is also heated by an electric field, forming ion-implanted regions which are heated by an electric field and irradiating selected regions with light, followed by subsequent field treatment. The electric field in each of these methods is provided by a corona discharge.

Harada et al. (U.S. Pat. No. 5,568,308) teaches the formation of domain reversals in a MgO-LN non-linear unipolarized ferroelectric crystal by first proton-exchanging an appropriate region and then applying patterned electrodes to a surface of the crystal and creating an electric field between the electrodes. The electrodes consist of a separated pair formed on an upper surface of the crystal wherein one of the pair is comb-shaped and the other of the pair is rectangular. In an alternative embodiment, the field is provided by a corona wire.

Harada et al. (U.S. Pat. No. 5,570,225) teaches the formation of domain reversals in a MgO--LiNbO.sub.3 or a MgO--LiTaO.sub.3 non-linear unipolarized ferroelectric crystal by first proton-exchanging an appropriate region, diffusing Ti through the region or diffusing Li through the region and then applying electrodes to a surface of the crystal and creating an electric field by applying a direct or pulsed current to the electrodes.

Harada et al. (U.S. Pat. No. 5,522,973) teaches the formation of domain reversals in a MgO--LiNbO.sub.3 or a MgO--LiTaO.sub.3 non-linear unipolarized ferroelectric crystal by first proton-exchanging an appropriate region and then applying patterned electrodes to a surface of the crystal and creating an electric field between the electrodes. The electrodes consist of a separated pair formed on an upper surface of the crystal wherein one of the pair is comb-shaped and the other of the pair is rectangular. In an alternative embodiment, the field is provided by a corona wire.

Harada et al. (U.S. Pat. No. 5,594,746) teach a method for forming domain reversals in a ferroelectric crystal using a corona wire and a pattern of electrodes formed on the crystal. There is also taught an apparatus for creating the domain reversals comprising the electrode, the corona wire, a power source and an evacuated chamber.

Byer et al. (U.S. Pat. No. 6,156,255) teach a method for forming patterned domain reversals in a non-linear ferroelectric material using an electric field applied by means of spaced conductors. There is also taught a method of first characterizing the material to determine the most efficient way to achieve the desired result. The characterization seeks to establish the proper application of surface treatments as well as the best geometry of the crystal.

The methods disclosed above produce some disadvantageous effects. In particular, Ti indiffusion, Li outdiffusion and proton exchange followed by heat treatment, produces undesirable changes in the index of refraction of the material. The use of corona discharge methods and the use of high temperature thermal cycles and vacuum processing increase fabrication complexity and expense. In addition, such processes can damage crystal surfaces and, thereby, adversely affect the refractive properties of the crystal. The disavantages of the various methods can be summarized as follows:

(1) Many of the methods cited above produce high temperatures which cause low throughput and process control difficulties. A ferroelectric material should be heated to the Curie temperature to reduce the magnitude of the external field required to create domain reversals in all methods except the E-field method (electric field between applied electrodes) and the corona discharge method. Thus, the average operating temperature is approximately 500.degree. C., which can cause wafer breakdowns, lead to stresses in the heating and cooling cycles and also result in the diffusion of metals into the ferroelectric material.

(2) The necessity of high vacuum processing slows down the process cycle and reduces throughput. Domain reversal techniques based on electron discharge require high vacuum to prevent interference from the environment and arcing between discrete electrodes. A vacuum of the order of 10.sup.-7 Torr takes a long time to produce.

(3) The depth of the domain reversal is often too shallow to use a bulk device like a solid state laser chip. Most domain reversal processes use surface chemical reactions, such as Ti in-diffusion, Li.sub.2O out-diffusion or proton exchange to lower the Curie temperature. In these cases, the depth of domain inversion is on the order of several microns. It is, therefore, only suitable to use such domain-reversed crystals in conjunction with surface devices such as wave-guides or surface acoustic wave (SAW) devices. To use bulk devices as radiation sources would require a ferroelectric device with domain reversals to a depth of millimeters.

(4) The scanning rate of domain reversals is very slow in most methods. To obtain a high quality (sharply defined) domain reversal, requires the use of slow methods, such as electron-beam writing or corona discharge.

Based on the limitations described above, we conclude that the E-field method (electric field produced between electrodes deposited on the material) has more advantages than the other methods. The chief disadvantages of this method are seen when the domain reversals must have a short period (narrow width) of about 10 microns or less and the wafer thickness is about 0.5 mm in CLN. In this case, the domain walls do not retain a planarity between the upper and lower surface planes of the crystal (between the +z and the -z surfaces). Moreover, in thicker wafers, the domain wall planarity is even worse. To alleviate these problems, the present invention teaches a double-sided poling process for ferroelectric domain reversals which also includes a new alignment technique. The method facilitates well defined poling in large