Integration of LEDs and VCSELs using fluidic self-assembly

In this paper the integration of LEDs and VCSELs with silicon electronics using fluidic self-assembly (FSA) is reviewed. FSA is a micromechanical technique in which devices fabricated in one substrate can be integrated with systems on a target substrate. The devices to be integrated are etched into shapes or "blocks" that match receptor sites or "holes" in the target substrate. After etching, the blocks are suspended in a slurry. This slurry is placed in a recirculation pump along with the target substrate. The pump moves the slurry over the target substrate where the blocks fall into the holes or are swept off the substrate to be circulated again. FSA has been demonstrated with yields > 99% for devices of 30μm and larger. Smaller devices have yet to be tested. A five mask process has been developed that successfully integrates GaAs/AlGaAs LEDs and VCSELs on silicon. The critical steps are: 1) etching the receptor sites in the silicon, 2) isolating the sites from the remainder of the substrate, and 3) bonding/contacting the blocks after assembly. The latter bonding step involves thin film metal multilayers on the block and in the receptor that form a hard solder after a low temperature bake step. The remaining steps of planarizing and interconnecting the devices are similar to those used for VLSI.