Silica-on-silicon waveguides and related fabrication methods

• US 8,818,146 B2
• Filed: 06/12/2012
• Issued: 08/26/2014
• Est. Priority Date: 06/13/2011
• Status: Active Grant

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: Vahala, Kerry, Lee, Hansuek, Chen, Tong, Li, Jiang
• Primary Examiner(s): Peng, Charlie
• Assistant Examiner(s): TAVLYKAEV, ROBERT FUATOVICH
• Application Number:
  US13/494,707
• Publication Number:
  US 20120321245A1
• Time in Patent Office: 805 Days
• Field of Search: 385/14, 385129-131, 438/39, 438/40, 438/42, 438/43, 438/713
• US Class Current: 385/14
• CPC Class Codes: G02B 6/136 by etching

Abstract

A method of manufacturing a waveguide eliminates a prior art reflow step and introduces certain new steps that permit fabricating of an ultra-low loss waveguide element on a silicon chip. The ultra-low loss waveguide element may be adapted to fabricate a number of devices, including a wedge resonator and a ultra-low loss optical delay line having an extended waveguide length.

Claims

1. A method for fabricating a waveguide, comprising:

placing a silicon substrate in a furnace;

introducing steam into the furnace;

raising a temperature inside the furnace to a first temperature level wherein a silicon dioxide layer is formed on a major surface of the silicon substrate;

eliminating a moisture content in the silicon substrate by heating the silicon substrate at a second temperature level in an oxygen-rich environment;

forming a first assembly by applying a photo-resist layer upon a portion of the major surface of the silicon dioxide layer;

immersing the first assembly into a bath containing an etching solution selected for etching silicon dioxide;

forming a second assembly by allowing the etching solution to act upon the silicon dioxide layer of the first assembly for a first period of time that is selected in order to;

a) expose a portion of the silicon substrate, and b) form a wedge structure in the silicon dioxide layer;

forming a third assembly by extending the first period of time by a second period of time in order to eliminate a foot region formed upon a sloping surface of the wedge structure;

after eliminating the foot region, forming a fourth assembly by removing the photo-resist layer from the third assembly; and

forming a waveguide component from the fourth assembly by exposing the fourth assembly to a xenon difluoride (XeF₂) environment that eliminates a portion of the silicon substrate and forms a support pillar below the wedge structure.

2. The method of claim 1, wherein the first period of time is further selected to allow the etching solution to act upon the silicon dioxide layer to form a slope angle ranging from about 7 degrees to about 90 degrees in the sloping surface of the wedge structure.

3. The method of claim 2, wherein an adhesion promoter is incorporated into the photo-resist layer, the adhesion promoter providing an adhesion factor between the photo-resist layer and the silicon dioxide layer, the adhesion factor selected in accordance with the first period of time and the slope angle.

4. The method of claim 3, wherein eliminating the moisture content in the silicon substrate comprises heating the silicon substrate at about 1000 degrees Celsius for about 24 hours in an oxygen-rich environment.

5. The method of claim 4, wherein the etching solution is a buffered hydrofluoric solution.

6. The method of claim 4, wherein the wedge structure after elimination of the moisture content has a trapezoidal cross-section with the sloping surface forming one leg of the trapezoidal cross-section.

7. The method of claim 4, wherein the waveguide component is formed as a wedge microcavity having a substantially circular shape with a diameter exceeding 500 microns.

8. The method of claim 7, wherein the wedge microcavity is formed as a whispering-gallery-mode waveguide.

9. The method of claim 8, wherein the whispering-gallery-mode waveguide is formed as a spiral structure.

10. The method of claim 9, wherein a diameter-to-propagation-length ratio of the spiral structure is determined, at least in part, by the slope angle, with a greater slope angle providing a smaller diameter-to-propagation-length ratio.

11. The method of claim 9, wherein the spiral structure comprises:

a first spiral waveguide;

a second spiral waveguide interspersed with the first spiral waveguide; and

a loopback coupler portion configured to couple the first spiral waveguide to the second spiral waveguide in a configuration that accommodates propagation of light in the first spiral waveguide in a first direction, a directional reversal in the loopback coupler portion, and propagation of the light in the second spiral waveguide in an opposite direction.

12. The method of claim 11, further comprising:

fabricating the spiral structure to provide an overall propagation length of about 7 meters on a silicon wafer that is approximately 5 cm×

5 cm is area.

13. A waveguide comprising:

a wedge structure formed from a silicon dioxide layer, in part by eliminating a foot region, the wedge structure having a linear sloping surface with a linear slope angle ranging from >

7 degrees to 90 degrees, the linear sloping surface further characterized by an absence of the foot region;

wherein the waveguide is configured as a whispering-gallery-mode microcavity.

14. The waveguide of claim 13, wherein the waveguide is configured as a wedge microcavity having a substantially circular shape with a diameter exceeding 500 microns.

15. The waveguide of claim 14, wherein the wedge microcavity further includes a support pillar formed on a silicon substrate, the wedge structure supported upon the silicon substrate by the support pillar.

16. The waveguide of claim 13, wherein the wedge structure is incorporated into a first waveguide sub-assembly, the first waveguide sub-assembly comprising:

a first spiral waveguide;

a second spiral waveguide interspersed with the first spiral waveguide; and

a loopback coupler portion configured to couple the first spiral waveguide to the second spiral waveguide in a configuration that accommodates propagation of light in the first spiral waveguide in a first direction, a directional reversal in the loopback coupler portion, and propagation of the light in the second spiral waveguide in an opposite direction.

17. The waveguide of claim 16, wherein a waveguide length of the first waveguide sub-assembly is about 7 meters, and a mounting area of a silicon substrate upon which the first waveguide sub-assembly is located is about 5 cm×

5 cm.

18. The waveguide of claim 16, wherein the wedge structure is incorporated into a second waveguide sub-assembly constructed substantially similar to the first waveguide sub-assembly, the first and second waveguide sub-assemblies coupled in a series arrangement for obtaining a delay line having an extended waveguide length.

19. The waveguide of claim 18, further including a third waveguide sub-assembly and a fourth waveguide sub-assembly each of which is constructed substantially similar to the first waveguide sub-assembly, the first, second, third and fourth waveguide sub-assemblies coupled in a series arrangement for further extending the waveguide length of the delay line.

20. The waveguide of claim 19, wherein each of the first, second, third and fourth waveguide sub-assemblies is located in a respective quadrant of a silicon substrate.

21. The waveguide of claim 20, wherein the waveguide length of the serially-coupled first, second, third and fourth waveguide sub-assemblies is about 27 meters, and a cumulative footprint area of the first, second, third and fourth waveguide sub-assemblies is about 100 cm².