Silica-on-silicon waveguides and related fabrication methods
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.
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 (XeF 2 ) 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 cm 2 .