Method of manufacturing a spectral filter for green and longer wavelengths

• US 20050199510A1
• Filed: 04/25/2005
• Published: 09/15/2005
• Est. Priority Date: 10/16/2002
• Status: Active Grant

• Current Assignee: Lake Shore Cryotronics, Inc.
• Original Assignee: Lake Shore Cryotronics, Inc.
• Inventors: Kochergin, Vladimir, Swinehart, Philip
• Granted Patent: US 7,628,906 B2
• Time in Patent Office: Days
• US Class Current: 205/640
• CPC Class Codes: B82Y 20/00 Nanooptics, e.g. quantum optics or photonic crystals
• CPC Class Codes: G02B 27/1073 characterized by manufacturing or alignment methods
• CPC Class Codes: G02B 5/20 Filters polarising elements G02B5/30
• CPC Class Codes: G02B 5/201 in the form of arrays
• CPC Class Codes: G02B 6/1225 comprising photonic band-gap structures or photonic lattices

Abstract

A method of manufacture for optical spectral filters with omnidirectional properties in the visible, near IR, mid IR and/or far IR (infrared) spectral ranges is based on the formation of large arrays of coherently modulated waveguides by electrochemical etching of a semiconductor wafer to form a pore array. Further processing of said porous semiconductor wafer optimizes the filtering properties of such a material. The method of filter manufacturing is large scale compatible and economically favorable. The resulting exemplary non-limiting illustrative filters are stable, do not degrade over time, do not exhibit material delamination problems and offer superior transmittance for use as bandpass, band blocking and narrow-bandpass filters. Such filters are useful for a wide variety of applications including but not limited to spectroscopy, optical communications, astronomy and sensing.

Claims

1. A method of making a spectral filter comprising:

providing a substrate wafer of single-crystal (100)-oriented n-doped silicon having a first surface and a second surface,

providing etching starting points at a first surface of said semiconductor wafer,

electrochemically etching the substrate wafer beginning at said start points to produce a structured layer having pores with controlled depths defined at least partially therethrough, said pores having coherently modulated cross-sections at least over the part of said depth, and

depositing onto the pore walls at least one layer of substantially transparent material.

2. The method of claim 1, wherein said etching start points compose a regularly-arranged array of depressions on the first surface of substrate wafer.

3. The method of claim 1, wherein said etching start points are located 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.

4. The method of claim 3, wherein said etching is chosen from the group consisting of chemical etching, reactive ion etching, and ion milling.

5. The method of claim 4, wherein said chemical etching is chosen from the group consisting of alkaline etching and acidic etching.

6. The method of claim 1, wherein said etching starting points are 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 chemically different material inside the photoresist mask openings and by etching the wafer surface through the thus-formed openings in said disposed chemically different material.

7. The method of claim 6, 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 the wafer, chemical vapor deposition, wet chemical oxidation and physical vapor deposition.

8. The method of claim 6, 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.

9. The method of claim 6, wherein said chemically different layer is removed from the first surface of the wafer after forming said surface topology in the wafer.

10. The method of claim 1, wherein said electrochemical etching includes connecting the substrate as an electrode, contacting the first surface of the substrate with an electrolyte, setting a voltage between said electrodes, illuminating the second surface of a substrate wafer and continuing etching to form said pores extending to a desired depth substantially perpendicular to said first surface.

11. The method of claim 10, wherein said electrolyte is a fluoride-containing, acidic electrolyte.

12. The method of claim 11, wherein said electrolyte contains hydrofluoric acid in a range of 1% to 50% by volume.

13. The method of claim 11, wherein said electrolyte additionally contains an oxidizing agent.

14. The method of claim 11, wherein said electrolyte additionally contains a hydrogen reducing agent selected from the group of chemicals consisting of mono-functional alkyl alcohols, di-functional alkyl alcohols, and tri-functional alkyl alcohols.

15. The method of claim 11, wherein said electrolyte additionally contains a viscosity-modifying agent.

16. The method of claim 11, wherein said electrolyte additionally contains an electrical conductivity-modifying agent.

17. The method of claim 11, wherein said electrolyte additionally contains a wetting agent.

18. The method of claim 11, wherein at least one electrochemical etching parameter selected from the group consisting of electrical current density, illumination intensity, electrolyte temperature, electrolyte composition and/or applied voltage is changed in a predetermined fashion with time during the electrochemical etching process.

19. The method of claim 1, wherein each of said at least one layer of substantially transparent material is deposited onto the pore walls by a technique selected from the group consisting of chemical vapor deposition, atomic layer deposition, photochemical decomposition and thermal oxidation.

20. The method of claim 1, wherein at least one layer of absorptive and/or reflective material is deposited on the pore walls over at least part of the pore depths.

21. The method of claim 20, wherein each of said at least one layer of reflective and/or absorptive material is deposited by a technique selected from the group consisting of chemical vapor deposition, atomic layer deposition, photochemical decomposition, electroplating, electroless plating, die casting and molding.

22. The method of claim 1, further including the removal of the nonporous remainder of the wafer at the distal ends of the pores.

23. The method of claim 22, wherein said removal of the unwanted remainder of the wafer comprises a step selected from the group consisting of reactive ion etching, chemical etching, grinding, mechanical and/or chemical-mechanical polishing.

24. The method of claim 23, wherein the chemically resistant layer is deposited on the pore walls prior to said removal of the unwanted remainder of the wafer.

25. The method of claim 24, 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, atomic layer deposition or thermal oxidation.

26. The method of claim 25, further including removing the chemically resistant layer from the pore walls after the removal of the said unwanted remainder of the wafer.

27. The method of claim 26, further including coating the first, second or both surfaces of said spectral filter with at least one layer of material intended to suppress the reflection from said surfaces of said spectral filter in at least some wavelength ranges within the transparency wavelength range of said spectral filter.

28. The method of claim 27 wherein said antireflective structure is disposed by a technique chosen from the group consisting of thermal oxidation, chemical vapor deposition, atomic layer deposition, physical vapor deposition and/or thermal evaporation.

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

30. The method of claim 29 wherein said sealing step comprises at least one of the group consisting of anodic bonding, thermal bonding, glass frit bonding, brazing or adhesive bonding.

31. The method of claim 1 wherein a roughness suppression procedure is performed subsequently to said etching of the pores into the substrate wafer.

32. The method of claim 31 wherein said roughness suppression procedure includes chemical etching of said pore walls.

33. The method of claim 32 wherein said chemical etching takes place in a heated alkaline solution.

34. The method of claim 31 wherein said roughness suppression procedure includes thermal oxidation of the pore walls followed by chemical etching of the formed oxide layer on the pore walls