Assembly of very fine pitches Infrared focal plane array with indium micro balls

Flip chip is a high-density and highly reliable interconnection technology, which is mandatory for the fabrication of high end heterogeneous imaging arrays. In the field of cooled infrared detectors, indium balls interconnections technology is well mastered and widely used down to $10\mu m$ pitch. Control of ultra-fine pitch $(\lt 10\mu m)$ and high interconnections count flip-chip bonding technology represents a challenge on the roadmap of next generation devices. In the context of the common laboratory DEFIR with LYNRED (ex SOFRADIR), CEA-LETI has addressed the issue of assembling very small pitches focal-plane-arrays (FPA). First, we developed new wafer level processes and managed to produce Under Bump Metallization (UBM) and indium micro balls at pitches as in e as 7,5 $\mu m$ and 5 $\mu m$. These balls arrays were used to develop bonding processes on mechanical and electrical test vehicles up to a resolution of 1280 x1024 pixels. We investigated in details the influence of many aspects of the process such as the flatness difference that can be caught up, the alignment accuracy, the formation of Inter Metallic Compounds (IMC) at UBM/In interface. These new processes were successfully used to solder CMT (Cadmium Mercury Tellurium) TV range (640x512 pixels) detectors at 7,5 $\mu m$ pitch onto ROICs specially designed to fulfil the project’s needs. First electro-optical characterizations revealed a connection rate higher than 99% andit has been possible to acquire IR pictures. In addition, some ageing tests have been performed on these functional detectors and an electrical (with daisy chains) 2k
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demonstrator at 5 $\mu m$ pitch is to be achieved.