Interposer with planar sidewall surface for optical coupling and methods of forming the same

An assembly comprising:

• a composite semiconductor package including an interposer and a semiconductor die that is attached to the interposer, wherein the interposer comprises metal wiring structures and optical waveguides that are embedded in dielectric layers and further comprises a stepped outer sidewall, wherein the stepped outer sidewall comprises a first outermost vertical surface segment located within a vertical plane, a second outermost vertical surface segment located within the vertical plane, a laterally-recessed sidewall segment that is laterally recessed inward relative to the vertical plane, a first connecting horizontal surface segment that connects an edge of the first outermost vertical surface segment to an edge of the laterally-recessed sidewall segment, and a second connecting horizontal surface segment that connects an edge of the second outermost vertical surface segment to another edge of the laterally-recessed sidewall segment;
• a molding compound;
• and
• a fiber access unit having a first end that is optically coupled to a subset of the optical waveguides through an optical glue portion that is interposed between the fiber access unit and a laterally-recessed sidewall segment of the stepped outer sidewall, wherein:
• a sidewall of the optical glue portion is aligned with a sidewall of the molding compound;
• and
• the first connecting horizontal surface segment comprises a horizontal surface segment of the molding compound.

An assembly comprising:

• a composite semiconductor package including an interposer and a semiconductor die that is attached to the interposer, wherein the interposer comprises metal wiring structures and optical waveguides that are embedded in dielectric layers and further comprises a stepped outer sidewall, wherein the stepped outer sidewall comprises a first outermost vertical surface segment located within a vertical plane, a second outermost vertical surface segment located within the vertical plane, a laterally-recessed sidewall segment that is laterally recessed inward relative to the vertical plane, a first connecting horizontal surface segment that connects an edge of the first outermost vertical surface segment to an edge of the laterally-recessed sidewall segment, and a second connecting horizontal surface segment that connects an edge of the second outermost vertical surface segment to another edge of the laterally-recessed sidewall segment;
• a molding compound;
• a fiber access unit having a first end that is optically coupled to a subset of the optical waveguides through an optical glue portion that is interposed between the fiber access unit and a laterally-recessed sidewall segment of the stepped outer sidewall, wherein a sidewall of the optical glue portion is aligned with a sidewall of the molding compound;
• and
• a packaging substrate bonded to the interposer, wherein the semiconductor die is more distal from the packaging substrate than the interposer is from the packaging substrate.