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Method for self-writing a waveguide in the underfill to couple the substrate waveguide to the VCSEL and photodetector

IP.com Disclosure Number: IPCOM000124272D
Publication Date: 2005-Apr-14
Document File: 4 page(s) / 115K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for self-writing a waveguide in the underfill to couple the substrate waveguide to the vertical cavity surface emitting laser (VCSEL) and photodetector (PD). Benefits include improved performance, improved ease of manufacturing, and product simplification.

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Method for self-writing a waveguide in the underfill to couple the substrate waveguide to the VCSEL and photodetector

Disclosed is a method for self-writing a waveguide in the underfill to couple the substrate waveguide to the vertical cavity surface emitting laser (VCSEL) and photodetector (PD). Benefits include improved performance, improved ease of manufacturing, and product simplification.

Background

              Flip-chip packages without underfill have reliability problems. The coupling of light between a flip-chip attached source/photodetector and a substrate waveguide is poor due to light beam divergence.

              However, conventional organic underfill does not meet the requirements for optical flip-chip packages because it cannot guide the light transmission between the flip-chip die and the substrate. The underfill cannot reduce light divergence and causes significant optical signal loss

              No conventional solution exists to the reliability problems. Microlens systems have been proposed to collimate and steer optical beams between optical components in a package. Proposed solutions may or may not include the use of underfill of a specific refractive index between the die and the substrate.

General description

      The disclosed method uses a photoactive underfill to form a self-written waveguide core to guide the light between a flip-chip mounted source and a substrate waveguide or a substrate and a flip-chip mounted photodetector. After assembly and underfill dispense, the light source is powered on to expose the light path between the source and the substrate waveguide and between the substrate waveguide and the PD. The light exposes the photoactive underfill to create a light pipe or short waveguide core between the components. The underfill is subsequently cured to form the cladding in the unexposed areas. The formed waveguide prevents light from diverging, guides optical signal transmission between the optical flip-chip die and the planar waveguide on the substrate, and significantly reduces the optical signal loss. Because the disclosed underfill can eliminate light divergence, it eliminates the complicated lens system that is required to collimate/refocus the diverged light.

              The resin mixture consists of a photosensitive acrylate, an epoxy which is not photosensitive, and a curing agent.

              A light with a correct wavelength from the laser of the optical flip-chip die passes through the uncured underfill. The light selectively cures the resin along the light trace and forms a waveguide core with a higher refractive index. After formation of the waveguide core, the unexposed part of the underfill is cured thermally. The thermally cured underfill has a lower refractive index and acts as the cladding of the waveguide. The formed waveguide in the underfill effectively guides optical signal transmission between the optical flip-chip die and the substrate, reducing light divergence and minimizing th...