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.

1 . A low loss silicon-on-insulator (SOI)-based optical waveguiding structure comprising 
 a silicon substrate;    an insulating layer disposed over the silicon substrate, the insulating layer having a first refractive index value;    a relatively thin silicon surface layer disposed over at least a portion of the insulating layer, the silicon exhibiting a refractive index value greater than the first refractive index value of the insulating layer; and    an optically transparent semiconductor waveguiding structure disposed over a portion of the relatively thin silicon surface layer, the optically transparent waveguiding structure having a refractive index value less than the refractive index value of silicon but greater than the first refractive index value of the insulating layer.    
     
     
         2 . A low loss SOI-based optical waveguiding structure as defined in  claim 1  wherein the optically transparent waveguiding structure is formed as a rib waveguide.  
     
     
         3 . A low loss SOI-based optical waveguiding structure as defined in  claim 1  wherein the optically transparent semiconductor waveguiding structure comprises a material selected from the group consisting of SiN, SiON and SiC.  
     
     
         4 . A low loss SOI-based optical waveguiding structure as defined in  claim 1  wherein the structure further comprises a relatively thin oxide layer disposed between the relatively thin silicon surface layer and the optically transparent semiconductor waveguiding structure.  
     
     
         5 . A low loss SOI-based optical waveguiding structure as defined in  claim 4  wherein the structure is used to form an active optical device, with the relatively thin silicon surface layer comprising regions of oppositely doped conductivity, and further including electrical contact areas formed within the silicon surface layer.  
     
     
         6 . A low loss SOI-based optical waveguiding structure as defined in  claim 5  wherein the regions are disposed to form a p-n junction within the relatively thin silicon surface layer.  
     
     
         7 . A low loss SOI-based optical waveguiding structure as defined in  claim 6  wherein the doped regions are disposed in a horizontal configuration.  
     
     
         8 . A low loss SOI-based optical waveguiding structure as defined in  claim 6  wherein the doped regions are disposed in a vertical configuration.  
     
     
         9 . A low loss SOI-based optical waveguiding structure as defined in  claim 1  wherein the optically transparent waveguiding structure includes a tapered region to reduce optical reflections therealong.  
     
     
         10 . A low loss SOI-based optical waveguiding structure as defined in  claim 9  wherein the silicon surface layer includes a tapered region to further reduce optical reflections.  
     
     
         11 . A low loss SOI-based optical waveguiding structure as defined in  claim 1  wherein the silicon surface layer exhibits a tapered doping profile between a first, passive device region and a second, active device region.  
     
     
         12 . A low loss SOI-based optical waveguiding structure as defined in  claim 1  wherein the insulating layer comprises silicon dioxide with a refractive index of approximately 1.47 and the optically transparent semiconductor waveguiding structure comprises silicon nitride with a refractive index of approximately 2.0.  
     
     
         13 . A low loss SOI-based optical waveguiding structure as defined in  claim 1  wherein the structure further comprises a cladding layer disposed to surround the exposed portions of the optically transparent waveguiding structure.  
     
     
         14 . A low loss SOI-based optical waveguiding structure as defined in  claim 13  wherein the cladding layer comprises a CMOS-compatible oxide layer.