Vertical-Cavity Surface-Emitting Laser (VCSEL)is a useful light source that can be applied to chip-to-chip and board-to-board optical data link. To package the developed optical interconnect, a VCSEL array was flip-chip bonded on a transparent glass substrate that is a propagation path of laser beam and also support waveguide. The size of (1x4)VCSEL array used is 1000$\mu$m×70$\mu$m , and 200 $\mu$m thick. In dealing with such a small VCSEL array, Under Bump Metallurgy (UBM) process is difficult.
When the VCSEL is flip chip bonded without UBM, the VCSEL`s window can be damaged because solders spread to window along the metal pad and cover it. If subsequent process temperature is higher than 250$^\circ$C, the performance of VCSEL will be degraded and waveguide can be damaged during the flip chip bonding.
In the present study, we tried a flip chip bonding by using indium solder bumps to overcome difficulties. In the part of glass substrate, indium solders were deposited on top of Au/Ni/Ti UBM pad by thermal evaporation method. Solder bumps were formed by using lift-off method. Adopting indium can prevent a degradation of VCSEL by decreasing the processing temperature below 150$^\circ$C and decrease the bonding pressure because indium is malleable. To optimize the flip chip bonding condition of VCSEL array, the die shear test and the spreading test for indium solder bump were carried out. The die shear strength was increased as the bonding temperature and pressure were increased. Except some cases, indium solder bump didn`t spread much along the metal line or chip pad during the flip-chip bonding and reflow process. During the die shear test, facture occured mostly between indium solder bump and VCSEL chip pad. In the soldering operation, indium easily forms oxides that adhere onto the indium solder bump surface and prevent the solder solution from having contact with the VCSEL chip pad to be joined. To prevent oxidation of indium bump and increase the adhesion between indium bump and VCSEL chip pad, thin silver layer was coated onto the indium surface.
In this study it was found that the optimum condition of flip chip bonding for VCSEL array using indium bump is 150$^\circ$C, 500 gf. Also thin silver layer coating is very effective method to prevent oxidation of indium bump and enhance the adhesion between indium bump and VCSEL chip pad. By preventing indium solder from oxidation in bonding process, flux is no longer needed. In addition, surface morphology of indium solder could be improved by coating thin silver layer. Finally, VCSEL array was flip chip bonded on the fused silica substrate by using the optimum bonding condition. To drive VCSEL array, the substrate was bonded on the PCB test board by wire bonding method. As a result, uniform laser beam propagation profile and power could be obtained in this bonding condition.
