Waveguide formation using CMOS fabrication techniques
Conventional approaches to integrating waveguides within standard electronic processes typically involve using a dielectric layer, such as polysilicon, single-crystalline silicon, or silicon nitride, within the in-foundry process or depositing and patterning a dielectric layer in the backend as a post-foundry process. In the present approach, the back-end of the silicon handle is etched away after in-foundry processing to expose voids or trenches defined using standard in-foundry processing (e.g., complementary metal-oxide-semiconductor (CMOS) processing). Depositing dielectric material into a void or trench yields an optical waveguide integrated within the front-end of the wafer. For example, a shallow trench isolation (STI) layer formed in-foundry may serve as a high-resolution patterning waveguide template in a damascene process within the front end of a die or wafer. Filling the trench with a high-index dielectric material yields a waveguide that can guide visible and/or infrared light, depending on the waveguide's dimensions and refractive index contrast.
1. A method of making at least one optical waveguide in a silicon substrate having a front side, a back side, and at least one ridge extending from the front side of the silicon substrate and a dielectric layer of first dielectric material deposited on the front side of the silicon substrate over the at least one ridge, the first dielectric material having a first refractive index, the method comprising:
(A) etching a portion of the back side of the silicon substrate so as to form at least one trench in the dielectric layer via removal of the at least one ridge extending from the front side of the silicon substrate; and
(B) depositing a waveguide core material, having a second refractive index greater than the first refractive index, into the at least one trench so as to form a core of the at least one optical waveguide.
2. The method of claim 1 , wherein:
the silicon substrate comprises a first electronic device and a second electronic device formed in the silicon substrate, and
(A) comprises removing silicon disposed between the first electronic device and the second electronic device so as to form the at least one trench in the dielectric layer.
3. The method of claim 2 , wherein (A) further comprises defining a path for the optical waveguide between the first electronic device and the second electronic device.
4. The method of claim 3 , wherein:
the dielectric layer extends at least partially between the first electronic device and the second electronic device, and
(A) further comprises etching the at least one trench to extend at least partially into a plane containing the first electronic device and the second electronic device.
5. The method of claim 2 , wherein at least one of the first electronic device and the second electronic device comprises at least one of a transistor, a photodetector, a modulator, and a light source.
6. The method of claim 1 , wherein (A) comprises at least one of vapor phase etching, wet etching, deep reactive ion etching, and multi-step etching.
7. The method of claim 1 , wherein (A) comprises etching the at least one trench to a width of less than or equal to about 130 nm.
8. The method of claim 7 , wherein (A) comprises etching the width of the at least one trench to a precision of less than or equal to about 1 nm.
9. The method of claim 1 , wherein:
(A) comprises exposing a portion of the dielectric layer, through the silicon substrate, adjacent to the at least one trench; and
(B) comprises depositing a uniform layer of the waveguide core material over at least a portion of the dielectric layer adjacent to the at least one trench.
10. The method of claim 1 , wherein (B) comprises depositing at least one of polymer, amorphous silicon, silicon nitride, silicon-rich silicon nitride, aluminum nitride, aluminum oxide, titanium oxide, tantalum oxide, zinc oxide, and amorphous silicon germanium alloy in the at least one trench.
11. The method of claim 1 , further comprising, before (B):
depositing a second dielectric material, within the dielectric layer beneath the at least one trench or within the at least one trench, to form a cladding layer of the at least one waveguide.
12. The method of claim 1 , wherein the waveguide core material is at least partially transparent at at least one wavelength in a range from about 400 nm to about 2000 nm.
13. The method of claim 1 , further comprising:
(C) etching away at least a portion of the waveguide core material; and
(D) depositing a cladding layer on the waveguide core material etched in (C).
14. The method of claim 1 , further comprising:
forming a heater, in thermal communication with the core of the optical waveguide, to heat the optical waveguide so as to vary the second refractive index.