Flip chip assembly with sub-micron 3D re-alignment via solder surface tension

Abstract

We demonstrate experimentally a flip-chip assembly with submicron three-dimensional alignment accuracy. We employ solder surface tension to push the flipped chip into lithographically defined alignment stops. During reflow, surface tension forces of the melted solder can move a chip by more than a hundred microns. We use these motions to obtain self-alignment by constraining the motions to lithographically defined mechanical stops and chip edge butting. This approach is particularly useful in InP laser to Si photonic assemblies, where sub-micron alignment is required for low optical connection loss. In this report, our test vehicles comprise silicon photonic chips and laser placeholder chips made of silicon as well. To enable self-alignment of edge-emitting single-mode lasers, a significant re-alignment range is needed to overcome the laser cleaving tolerance of +/- 15 microns and the low +/- 10 microns placement accuracy of high-throughput pick-and-place tools. We employ in-situ infrared (IR) microscopy to look through the assembled chips during solder induced re-alignment. We show that the self-alignment of the chips starts at the moment the solders melt. Cross sectional analysis is used to confirm the alignment accuracy and contact on the lithographic stops. We discuss process window considerations related to standoff height and solder volume.