Embedded optical interconnections on printed wiring boards

Optical interconnections with embedded photodetectors integrated onto printed wiring boards (PWB) have been demonstrated. High performance FR-4 was chosen as the substrate after several commercial products were examined in terms of their heat resistance and surface roughness. High performance FR-4 enables the use of higher curing temperatures for the waveguide materials than standard FR-4. One dominant issue in optical interconnections on PWB is the surface roughness of the PWB, which is not as smooth as semiconductor wafers, which have often been reported in the past as substrates for waveguide optical interconnections. The measured surface roughness of the high performance FR4 had both a short (+/0.5 pm on a IO pm period) and long period (+/1 pm on a 500 pm period) component. It is critical to planarize the shorter period roughness to minimize the optical scattering loss. To planarize the substrate surface, 16 pm of benzocyclobutene (BCB) was spin coated and cured at 250°C under Nz flow. The resulting surface roughness after planarizatiou was +/0.2 pm on a 500 p period. The waveguide cladding and core layers were next spin coated onto the planarized surface. The waveguide was composed of a 12 p n polysiloxane (PSB-KI) cladding layer and 4 pm of BCB as the core layer. The thin film photodetector (PD) was embedded into the core layer. The PD was a 1 pm thick InGaAs inverted metal-semiconductor-metal photodetector. The multimode channel waveguide was defmed using conventional photolithography and reactive ion etching. An optical input signal at a wavelength of 1.3 p n was launched into the waveguide through a prism coupler, propagated through the polymer waveguide, and was coupled intothe Fig. 1 One option for embedding a detector in a waveguide on SOP substrate. embedded thim film photodetector, resulting in an electrical output signal from the embedded photodetector.