Vertically tapered spot size converter and method for fabricating the same

A method for fabricating a vertically tapered spot-size converter on a substrate, comprising:

• growing a waveguide core over the substrate;
• coating the waveguide core with a photoresist layer of positive photoresist;
• transferring patterns of a photomask to the photoresist, the patterns being defined by openings in the photomask, each opening having a cross-section comprising a region of constant width and at least one region of non-constant width, the non-constant width reducing in a direction extending away from the region of constant width, comprising:
• aligning the photomask with a negative focus offset with respect to an external surface of the photoresist layer;
• exposing the photoresist layer to electromagnetic radiation through the openings of the photomask while maintaining the negative focus offset, thereby forming exposed areas and unexposed areas in the photoresist layer;
• developing the photoresist layer in a solvent to obtain uncovered regions of the waveguide core, the uncovered regions of the waveguide core being aligned with the exposed areas;
• thermally treating the unexposed areas of the photoresist layer;
• etching the uncovered regions of the waveguide core to provide the waveguide core with a vertically tapered profile, the vertically tapered profile being provided by the patterns of the photomask;
• and
• stripping off the exposed areas of the photoresist layer;
• growing a cladding layer extending over the waveguide core;
• and
• patterning and etching the cladding layer and the waveguide core to define the vertically tapered spot-size converter.