Fabrication of high-aspect-ratio 5 μm ultra-small pitch indium bump arrays by vacuum thermal deposition under variable rate

As the pixel pitch of infrared focal plane detectors is scaled down to 5 μm, the vacuum thermal evaporation process for fabricating indium bumps for device flip-chip interconnects faces significant challenges. Simple control of the substrate temperature and deposition rate is insufficient to achieve the high aspect ratio and uniformity indium bumps because of the serious clogging issues. To address this problem, this work investigates the evolution of the volume and profile of the indium bumps fabricated under different deposition conditions, and proposes a dynamic deposition rate approach to maximize the aspect ratio growth rate of the indium bumps. This research enables the successful fabrication of 5 μm pitch indium bump arrays with aspect ratio of 0.92 for 5 μm-pitch indium bumps and 0.78 for 5 μm-pitch indium balls after reflowing. Furthermore, the influence of the under-bump metallization (UBM) size on the height of the reflowed indium balls produced by the high-aspect-ratio indium bumps fabricated using the dynamic deposition rate approach is investigated and analyzed using a spherical cap model. The deformation of large-array indium balls after flip-chip interconnection process also have been investigated and finally achieved an interconnection ratio greater than 99 %. This work lays the groundwork for the further development of ultra-small pixel infrared detectors.