Method of manufacturing a spectral filter for green and shorter wavelengths

• US 20040045932A1
• Filed: 06/04/2003
• Published: 03/11/2004
• Est. Priority Date: 06/04/2002
• Status: Active Grant

• Current Assignee: Lake Shore Cryotronics, Inc.
• Original Assignee: Lake Shore Cryotronics, Inc.
• Inventors: Kochergin, Vladimir, Swinehart, Philip
• Granted Patent: US 7,135,120 B2
• Time in Patent Office: Days
• US Class Current: 216/59
• CPC Class Codes: B82Y 20/00 Nanooptics, e.g. quantum optics or photonic crystals
• CPC Class Codes: G02B 5/201 in the form of arrays
• CPC Class Codes: G02B 5/204 in which spectral selection is performed by means of a conductive grid or array, e.g. frequency selective surfaces for use with wavelengths longer than the infrared light H01Q15/0006
• CPC Class Codes: G02B 5/207 comprising semiconducting materials
• CPC Class Codes: G02B 5/208 for use with infrared or ultraviolet radiation, e.g. for separating visible light from infrared and/or ultraviolet radiation
• CPC Class Codes: G02B 5/22 Absorbing filters G02B5/201 - G02B5/208 take precedence
• CPC Class Codes: G02B 6/1225 comprising photonic band-gap structures or photonic lattices
• CPC Class Codes: G03F 7/70575 Wavelength control, e.g. control of bandwidth, multiple wavelength, selection of wavelength or matching of optical components to wavelength
• CPC Class Codes: G03F 7/70941 Stray fields and charges, e.g. stray light, scattered light, flare, transmission loss
• CPC Class Codes: G03F 7/70958 Optical materials or coatings, e.g. with particular transmittance, reflectance or anti-reflection properties

Abstract

The UV, deep UV and/or far UV (ultraviolet) filter transmission spectrum of an MPSi spectral filter is optimized by introducing at least one layer of substantially transparent dielectric material on the pore walls. Such a layer will modify strongly the spectral dependences of the leaky waveguide loss coefficients through constructive and/or destructive interference of the leaky waveguide mode inside the layer. Increased blocking of unwanted wavelengths is obtained by applying a metal layer to one or both of the principal surfaces of the filter normal to the pore directions. The resulting filters are stable, do not degrade over time and exposure to UV irradiation, and offer superior transmittance for use as bandpass filters. Such filters are useful for a wide variety of applications including but not limited to spectroscopy and biomedical analysis systems.

Claims

1. A method of making a spectral filter comprising:

providing a substrate wafer of single-crystal semiconductor having a first surface and a second surface,

etching the substrate wafer to produce a structured layer having pores with controlled depths defined at least partially therethrough,

coating the pores with at least one layer of a material substantially transparent within the pass-band of said spectral filter, said material having a thickness of at least 10 nm, and

removing at least one un-etched portion of the substrate wafer.

2. The method of claim 1 further including prior to etching, providing the first surface of the substrate wafer with a surface topology which defines the cross-sectional shape, arrangement and location of the pores to be formed during etching.

3. The method of claim 2, wherein said surface topology is composed of regularly-arranged depressions on the first surface of substrate wafer.

4. The method of claim 2, wherein said surface topology is produced by producing a photoresist mask on the first surface of the substrate wafer and by a subsequent etching of the first surface through said photoresist mask.

5. The method of claim 4, wherein said etching is chosen from the group consisting of chemical etching, reactive ion etching, and ion milling.

6. The method of claim 5, wherein said chemical etching is chosen from the group consisting of alkaline etching and acidic etching.

7. The method of claim 2, wherein said surface topology is produced by disposing a layer of material with different chemical properties than those of wafer material on the first surface of substrate wafer, by producing a photoresist mask on the surface of said layer, by etching away the said material inside photoresist mask openings and by etching the wafer surface through formed openings in said disposed chemically different material.

8. The method of claim 7, wherein said chemically different layer is silicon dioxide and is disposed by a step chosen from the group consisting of:

thermal oxidation of the surface of wafer in the oxygen-contained atmosphere, chemical vapor deposition, and physical vapor deposition.

9. The method of claim 7, wherein the said layer is a silicon nitride layer disposed by a step chosen from the group consisting of chemical vapor deposition and physical vapor deposition.

10. The method of claim 7, wherein said chemically different layer is removed from the first surface of the wafer after forming said surface topology in the wafer.

11. The method of claim 1 wherein said semiconductor substrate wafer is a silicon wafer.

12. The method of claim 11, wherein said silicon wafer is a <

100>

-oriented wafer.

13. The method of claim 11 wherein said etching is electrochemical etching and includes connecting the substrate as an electrode, contacting the first surface of the substrate with an electrolyte, setting a current density which will influence etching erosion, and continuing etching to form said pores extending to a desired depth substantially perpendicular to said first surface.

14. The method of claim 13, wherein said electrochemical etching occurs in a fluoride-containing, acidic electrolyte.

15. The method of claim 14, wherein said electrolyte contains hydrofluoric acid in a range of 1% to 50% by volume.

16. The method of claim 14, wherein said electrolyte additionally contains an oxidizing agent.

17. The method of claim 14, wherein said electrolyte additionally contains a hydrogen-contributing reducing agent.

18. The method of claim 17, wherein said hydrogen reducing agent is selected from the group of chemicals consisting of mono functional alkyl alcohols, tri functional alkyl alcohols, and tri functional alkyl alcohols.

19. The method of claim 14, wherein said electrolyte additionally contains a viscosity-increasing agent.

20. The method of claim 14, wherein said electrolyte additionally contains a conductivity-modifying agent.

21. The method of claim 14, wherein said electrolyte additionally contains a wetting agent

22. The method of claim 12, wherein said silicon wafer is an n-type doped wafer.

23. The method of claim 22 further including illuminating a second surface of the substrate wafer that lies opposite the first surface during electrochemical etching.

24. The method of claim 13, wherein electrochemical etching parameters are selected from the group consisting of electrical current density, illumination intensity and/or applied voltage are set approximately constant during the electrochemical etching process.

25. The method of claim 13, wherein at least one electrochemical etching parameter selected from the group consisting of electrical current density, illumination intensity and/or applied voltage is changing in a predetermined fashion with time during the electrochemical etching process.

26. The method of claim 12, wherein said silicon wafer is a p-type doped wafer.

27. The method of claim 26, wherein the electrolyte additionally contains at least one organic additive.

28. The method of claim 27, wherein the said at least one organic additive is selected from the group consisted of acetonitrile, dimethylformamide, dimethylsulfoxide, diethylenglycol, formamide, hexamethylphosphoric triamide, isopropanol, triethanolamine, 2-methoxyethyl ether, triethylphosphite, and triethyleneglycol dimethyl ether.

29. The method of claim 27 further including illuminating a second surface of the substrate wafer that lies opposite the first surface during electrochemical etching.

30. The method of claim 27, wherein electrical current density is set approximately constant during electrochemical etching process.

31. The method of claim 29, wherein at least one electrochemical etching parameter selected from the group consisting of electrical current density, illumination intensity and/or applied voltage is changing in a predetermined fashion with time during the electrochemical etching process.

32. The method of claim 1 wherein said semiconductor substrate wafer is of material chosen from the full possible range of alloys and compounds of zinc, cadmium, mercury, silicon, germanium, tin, lead, aluminum, gallium, indium, bismuth, nitrogen, oxygen, phosphorus, arsenic, antimony, sulfur, selenium and tellurium.

33. The method of claim 32, wherein said semiconductor wafer is a <

100>

-oriented wafer.

34. The method of claim 32 wherein said etching is electrochemical etching and includes connecting the substrate as an electrode, contacting the first surface of the substrate with an electrolyte, setting a current density which will influence etching erosion, and continuing etching to form said pores extending to a desired depth substantially perpendicular to said first surface.

35. The method of claim 13, wherein said electrochemical etching occurs in an acidic electrolyte.

36. The method of claim 1, wherein each of said at least one layer of substantially transparent material is deposited by chemical vapor deposition.

37. The method of claim 1, wherein the at least one layer of substantially transparent material on the pore walls includes a thermally grown silicon dioxide layer, and the method further includes deposition of at least one additional layer using one of many variations of chemical vapor deposition.

38. The method of claim 1, further including filling the pores completely with a second transparent material after coating the pores with a said at least one layer of transparent material.

39. The method of claim 1, wherein removal of the unwanted remainder of the wafer comprises a step selected from the group consisting of Reactive Ion Etching, chemical etching, mechanical or chemical-mechanical polishing.

40. The method of claim 39, wherein the chemically resistant layer is deposited on the pore walls prior to said removal of the unwanted remainder of the wafer.

41. The method of claim 40, wherein said chemically-resistant layer comprises Si₃N₄ or silicon dioxide having a thickness from about 5 nm to about 500 nm and is applied by one of the many variants of chemical vapor deposition or thermal oxidation.

42. The method of claim 40, further including removing the chemically resistant layer from the pore walls after the removal of the said unwanted remainder of the wafer.

43. The method of claim 1, further including coating the first, second or both surfaces of said spectral filter with at least one layer of material that absorbs in at least some wavelength ranges outside the transparency wavelength range of said spectral filter.

44. The method of claim 43 wherein said absorptive material is disposed by the technique chosen from the group consisting of chemical vapor deposition, physical vapor deposition, thermal evaporation and/or electroplating.

45. The method of claim 1, further including coating first, second or both surfaces of said spectral filter with a multilayer that is highly reflective in at least some wavelength ranges outside the transparency wavelength range of said spectral filter.

46. The method of claim 45 wherein said multilayer is applied by the technique chosen from the group consisting of chemical vapor deposition, physical vapor deposition or thermal evaporation.

47. The method of claim 1 further including sealing said spectral filter with two flat plates of material that is transparent within the transparency range of said spectral filter.

48. The method of claim 47 wherein said sealing step comprises at least one of the group of anodic bonding, thermal bonding, adhesive bonding.

49. A method of making a spectral filter, said method comprising:

providing a substrate wafer of single-crystal semiconductor having a first surface and a second surface,

etching the substrate wafer to produce a structured layer having pores with controlled depths defined at least partially therethrough,

removing at least one un-etched part of the substrate wafer, and

coating the pores with at least one layer of a material substantially transparent within the pass-band of said spectral filter material and having a thickness of at least 10 nm.

50. The method of claim 49, further including, prior to etching, providing the first surface of the substrate wafer with a surface topology that defines the cross sectional shape, arrangement and location of the pores to be formed during etching.

51. The method of claim 50, wherein said surface topology is composed of regularly arranged depressions on the first surface of substrate wafer.

52. The method of claim 50, wherein said surface topology is produced by producing a photoresist mask on the first surface of the substrate wafer and by a subsequent etching of the first surface.

53. The method of claim 52, wherein said etching is chosen from the group consisting of chemical etching, reactive ion etching, and ion milling.

54. The method of claim 53, wherein said chemical etching is the process chosen from the group consisting of alkaline etching and acidic etching.

55. The method of claim 50, wherein said surface topology is produced by disposing a layer of material with different chemical properties than those of wafer material on the first surface of substrate wafer, by producing a photoresist mask on the surface of said layer, by etching away the said material inside the photoresist mask openings and by etching the wafer surface through the so-formed openings in said chemically different material.

56. The method of claim 55, wherein said chemically dissimilar layer is silicon dioxide and is disposed by the method chosen from the group consisting of thermal oxidation of the surface of wafer in an oxygen-containing atmosphere, chemical vapor deposition, and physical vapor deposition.

57. The method of claim 55, wherein the said chemically dissimilar layer is silicon nitride layer applied by the method chosen from the group consisting of chemical vapor deposition and physical vapor deposition.

58. The method of claim 55, wherein said chemically dissimilar layer is removed from the first surface of the wafer after forming said surface topology.

59. The method of claim 49 wherein said semiconductor substrate wafer is a silicon wafer.

60. The method of claim 59, wherein said silicon wafer is of <

100>

-orientation.

61. The method of claim 59 wherein said etching is electrochemical etching and includes connecting the substrate as an electrode, contacting the first surface of the substrate with an electrolyte, setting a current density which will influence etching erosion, and continuing etching to form said pores extending to a desired depth substantially perpendicular to said first surface.

62. The method of claim 61, wherein said electrochemical etching occurs in a fluoride-containing acidic electrolyte.

63. The method of claim 62, wherein said electrolyte contains hydrofluoric acid in a range of 1% to 50%.

64. The method of claim 62, wherein said electrolyte additionally contains an oxidizing agent.

65. The method of claim 64, wherein said electrolyte additionally contains a hydrogen-contributing reducing agent.

66. The method of claim 65, wherein said hydrogen-contributing reducing agent is selected from the group of chemicals consisting of mono functional alkyl alcohols, tri functional alkyl alcohols and tri functional alkyl alcohols.

67. The method of claim 62, wherein said electrolyte additionally contains at least one viscosity increasing agent.

68. The method of claim 62, wherein said electrolyte additionally contains at least one conductivity modifying agent.

69. The method of claim 62, wherein said electrolyte additionally contains at least one wetting agent

70. The method of claim 60, wherein said silicon wafer is an n-type doped wafer.

71. The method of claim 70 further including illuminating a second surface of the substrate wafer that lies opposite the first surface during electrochemical etching.

72. The method of claim 71, wherein electrochemical etching parameters are selected from the group consisting of electrical current density, illumination intensity, temperature and/or applied voltage are set approximately constant during the electrochemical etching process.

73. The method of claim 61, wherein at least one electrochemical etching parameter selected from the group consisting of electrical current density, illumination intensity and/or applied voltage is varied in a predetermined fashion with time during the electrochemical etching process.

74. The method of claim 60, wherein said silicon wafer is a p-type doped wafer.

75. The method of claim 74, wherein the electrolyte additionally contains at least one organic additive.

76. The method of claim 75, wherein the said at least one organic additive is selected from the group consisted of acetonitrile, dimethylformamide, dimethylsulfoxide, diethylenglycol, formamide, hexamethylphosphoric triamide, isopropanol, triethanolamine, 2-methoxyethyl ether, triethylphosphite, triethyleneglycol dimethyl ether.

77. The method of claim 75 further including illuminating a second surface of the substrate wafer that lies opposite the first surface during electrochemical etching.

78. The method of claim 75, wherein electrical current density is set approximately constant during electrochemical etching process.

79. The method of claim 77, wherein at least one electrochemical etching parameter selected from the group consisting of electrical current density, illumination intensity and/or applied voltage is changing at a predetermined fashion during electrochemical etching process.

80. The method of claim 49 wherein said semiconductor substrate wafer is of material chosen from the full possible range of alloys and compounds of zinc, cadmium, mercury, silicon, germanium, tin, lead, aluminum, gallium, indium, bismuth, nitrogen, oxygen, phosphorus, arsenic, antimony, sulfur, selenium and tellurium.

81. The method of claim 80, wherein said semiconductor wafer is of a <

100>

-orientation.

82. The method of claim 80 wherein said etching is electrochemical etching and includes connecting the substrate as an electrode, contacting the first surface of the substrate with an electrolyte, setting a current density which will influence etching erosion, and continuing etching to form said pores extending to a desired depth substantially perpendicular to said first surface.

83. The method of claim 82, wherein said electrochemical etching occurs in an acidic electrolyte.

84. The method of claim 49, wherein said removal of the undesired remainder of the wafer includes a method selected from the group consisting of reactive ion etching, chemical etching, mechanical or chemical-mechanical polishing.

85. The method of claim 84, wherein the chemically resistant layer is deposited on the pore walls prior to said removal of the undesired remainder of the wafer.

86. The method of claim 85, wherein said chemically-resistant layer comprises Si₃N₄ or silicon dioxide having a thickness from about 5 nm to about 500 nm and is applied by one of the many variations of chemical vapor deposition or by thermal oxidation.

87. The method of claim 85, further including removing the chemically resistant layer from the pore walls after the removal of the said undesired remainder of the wafer.

88. The method of claim 49, wherein each of said at least one layer of substantially transparent materials is deposited by chemical vapor deposition.

89. The method of claim 49, wherein the at least one layer of substantially transparent material includes a thermally grown silicon dioxide layer, and the method further includes deposition of at least one additional layer using at least one of the many variations of chemical vapor deposition.

90. The method of claim 49, further including filling the pores with a different transparent material after coating the pores with a said at least one layer of transparent material.

91. The method of claim 49, further including coating the first, second or both surfaces of said spectral filter with at least one layer of material that absorbs in at least some wavelength ranges outside the transparency wavelength range of said spectral filter.

92. The method of claim 91 wherein said absorptive material is disposed by the technique chosen from the group consisting of chemical vapor deposition, physical vapor deposition, thermal evaporation or electroplating.

93. The method of claim 49, further including coating first, second or both surfaces of said spectral filter with a multilayered material that is highly reflective in at least some wavelength ranges outside the transparency wavelength range of said spectral filter.

94. The method of claim 93 wherein said multilayer is applied by the technique chosen from the group consisting of one of the many variations of chemical vapor deposition, by physical vapor deposition or by thermal evaporation.

95. The method of claim 49 further including sealing said spectral filter with two flat plates of a material that is transparent within the transparency range of said spectral filter.

96. The method of claim 95 wherein said sealing comprises a step selected from at least one of the group of anodic bonding and thermal bonding.

97. A method of making a spectral filter for green and shorter wavelengths comprising:

providing a substrate having a first surface and a second surface,

etching the substrate to produce a structured layer having pores with controlled depths defined at least partially therethrough, and

coating the pores with at least one layer of a material substantially transparent within the pass-band of said spectral filter.