Interconnection techniques of GaAs pixel detector on silicon ASIC electronics

To realize innovative instrumentation for mammography, a digital mammographic head based on several gallium arsenide detectors and on the Medipix chip (developed by the Medipix collaboration at CERN) has been designed. This device is able to perform single photon counting readout and allows a full detection efficiency at the mammographic energies. To make such detection units, industrial processes for the production of GaAs pixel detectors and for their bump-bonding to the read-out VLSI electronics have been developed by Alenia Marconi Systems (AMS S.p.A.). The detection unit is a 200 μm thick GaAs matrix of 64×64 square pixels (170 μm pitch), previously optimized and fully characterized, connected via bump bonding to the electronic chip. Specific studies were performed to develop a process that meets the requirements of low cost, high yield and high reliability for the bump bonding process of the GaAs detector to the silicon electronics. To ensure a high-quality electrical contact between the matrix detector and the silicon read out electronics a bump-bonding process has been set-up. It is based on a In evaporation process. In this work we present the technical characteristics of the bump bonding process and the results obtained in terms of bump-bonding yield and detection performance of the GaAs based detection units which have been realized and fully tested and are now ready to be implemented in the mammographic head.