Ultra low-loss CMOS compatible silicon waveguides

Abstract

A low loss optical waveguiding structure for silicon-on-insulator (SOI)-based arrangements utilizes a tri-material configuration including a rib / strip waveguide formed of a material with a refractive index less than silicon, but greater than the refractive index of the underlying insulating material. In one arrangement, silicon nitirde may be used. The index mismatch between the silicon surface layer (the SOI layer) and the rib / strip waveguide results in a majority of the optical energy remaining within the SOI layer, thus reducing scattering losses from the rib / strip structure (while the rib / strip allows for guiding along a desired signal path to be followed). Further, since silicon nitirde is an amorphous material without a grain structure, this will also reduce scattering losses. Advantageously, the use of silicon nitride allows for conventional CMOS fabrication processes to be used in forming both passive and active devices.

Description

CROSS-REFERENCE TO RELATED APPLICATION [0001] The present application claims the benefit of U.S. Provisional Application No. 60 / 638,216, filed Dec. 21, 2004.TECHNICAL FIELD [0002] The present invention relates to the formation of low loss silicon optical waveguides in a silicon-on-insulator (SOI) structure and, more particularly, to the inclusion of a rib / slab of a CMOS-compatible material having a refractive index intermediate that of silicon and silicon dioxide to achieve the desired lower loss. BACKGROUND OF THE INVENTION [0003] The optical waveguides typically formed in SOI-based structures comprise silicon (with a refractive index of approximately 3.47), the silicon formed in a “slab”, “rib” or “strip” geometry on the surface of the SOI substrate. A strip waveguide typically comprises crystalline silicon, where crystalline silicon is known to exhibit less loss that a polysilicon form of the same material. However, strip waveguides tend to exhibit relatively high optical loss th...

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Benefits of technology

[0007] The need remaining in the prior art is addressed by the present invention, which relates to the formation of low loss silicon optical waveguides in a silicon-on-insulator (SOI) structure and, more particularly, to the inclusion of a rib / slab of a CMOS-compatible material having a refractive index intermediate that of silicon and silicon dioxide to achieve the desired lower loss.

[0008] In accordance with the present invention, a rib / slab of a material having a refractive index less than that of silicon, but greater than that of silicon dioxide is formed over a portion of the surface SOI layer of a SOI-based structure. In a preferred embodiment, silicon nitride may be used as this material (inasmuch as it is particularly well-suited for CMOS-based fabrication processes). The thickness of the rib / slab is controlled as a trade-off between optical coupling into the rib / slab and optical loss. By virtue of using a material with a refractive index less than that of silicon, the contrast in refractive index helps in confining the majority of the optical signal within the SOI layer, thus significantly reducing optical loss through the rib / slab portion of the waveguide.

Problems solved by technology

However, strip waveguides tend to exhibit relatively high optical loss through their (relatively rough) sidewalls, which serve as scattering surfaces for a propagating optical signal.

That is, since an etching process is used to form the sidewalls of the strip, the etchant tends to roughen the exposed sidewall surfaces.

The bulk losses of polysilicon are greater than that of crystalline silicon, resulting in higher optical losses along the rib waveguide.

While this method is somewhat successful in reducing scattering losses, it cannot be used in applications involving “active” waveguide structures, since such a high temperature process will result in unwanted dopant migration within the SOI layer.

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