# Edge Couplers in Silicon Photonic Integrated Circuits: A Review
## 4 Structural Transformations of Edge Couplers in Vertical Direction
### Edge Couplers with Index-Matching Cladding  
Apart from edge couplers based on cascaded multi-stage tapers, there are also some research works about edge couplers with a Si inverse taper buried in index-matching claddings [88; 89; 90; 91; 92; 93; 94; 95]. The typical structure of an edge coupler assisted with index-matching cladding is presented in Figure 13, and we can observe that unlike the Si layer underneath the SiO\({}_{2}\) cladding, other kinds of materials including SiN, SiON, and polymers are utilized to engineer the refractive index between the value of the fiber core and silicon waveguides. In some cases, there may be an additional intermediate layer between the silicon waveguides and the top index-matching cladding. [PERSON] et al. applied the combination of two types of polymers--that is, WIR30-490 with a higher index as the top cladding and ZPU450 with a lower index as the secondary waveguide connected with a silicon taper [88]. [PERSON] et al. decreased the thickness of the SiO\({}_{2}\) cladding and designed a thick polymer waveguide as its top cladding [89]. [PERSON] et al. had an adjustment on the basis of typical silicon dioxide and substituted it with silicon-rich oxide with a higher refractive index of around 1.6 [90]. [PERSON] et al. applied initiated chemical vapor deposition to form the polymer cladding on top of the amorphous silicon core [91]. [PERSON] et al. designed a structure composed of both the SU-8 three-dimensional taper and the silicon oxynitride intermediate. SiON was deposited surrounding silicon waveguide and could transmit light gradually from the SU-8 3D taper to the bottom silicon layer [92]. [PERSON] et al. used an intermedium layer made of SiON that surrounded the Si core and was surrounded by a SiO\({}_{2}\) top cladding. In this structure, the SiON intermediate layer can be regarded as a secondary core encircling the Si waveguide [93]. [PERSON] et al. had a similar design with [PERSON]'s work in [93] with SiON as the transition material as well as the secondary core in the coupler [94]. [PERSON] et al. designed a SiO\({}_{2}\) spacer to isolate the Si taper and SiON layer, and the Si taper is knife-shaped with slanted sidewalls [95]. In [89], due to the narrow tip width of the Si inverse taper (approximately 40 nm), EBL is applied to form the Si taper on the SOI wafer, and thermal oxidation is applied to grow a thin layer with 30 nm thickness of SiO\({}_{2}\) on the Si taper. A top cladding of SU8 polymer is spin-coated on the thin  
Figure 13: Sketch of silicon taper in index-matching cladding. SiO\({}_{2}\) layer and then patterned with EBL. The thickness composition of the Si, SiO\({}_{2}\), and SU-8 layer is delicately optimized to make the effective index in the mode converter close to that of the coupling fiber. The extraordinary coupling performance of the TM mode is achieved at an ultra-low loss about 0.36 dB within a 300 um-long footprint. An additional 3-dB loss for 1.5 um misalignment in both the vertical and horizontal directions is measured. It is straightforward to increase the modal overlap between the fiber core and the photonic circuits regarding aspects of both size and effective index with the aid of the refractive index matching a cladding of proper thickness. The fabrication process of the index matching layer is simpler than that of the edge couplers with multiple assisting waveguides and multi-stage tapers mentioned in previous sections. One prerequisite is to determine appropriate kinds of index matching material or to synthesize a specific type of material according to the requirements of the refractive index value. Theoretically, the refractive index of the matching cladding should be close to that of the fiber core to decrease the index mismatch. The selection of optical lithography or electron beam lithography depends on the feature size in the entire configuration, which is commonly the taper tip width.