The hybrid pixel detectors used in the high energy physics experimentscurrently under construction use a three dimensional connection technique, theso-called bump bonding. As the pitch below 100um, required in theseapplications, cannot be fullfilled with standard industrial processes (e.g. theIBM C4 process), an in-house bump bond process using reflown indium bumps wasdeveloped at PSI as part of the R&D for the CMS-pixel detector. The bump deposition on the sensor is performed in two subsequent lift-offsteps. As the first photolithographic step a thin under bump metalization (UBM)is sputtered onto bump pads. It is wettable by indium and defines the diameterof the bump. The indium is evaporated via a second photolithographic step withlarger openings and is reflown afterwards. The height of the balls is definedby the volume of the indium. On the readout chip only one photolithographicstep is carried out to deposit the UBM and a thin indium layer for betteradhesion. After mating both parts a second reflow is performed for selfalignment and obtaining a high mechanical strength. For the placement of the chips a manual and an automatic machine wasconstructed. The former is very flexible in handling different chip and modulegeometries but has a limited throughput while the latter features a much highergrade of automatisation and is therefore much more suited for producinghundreds of modules with a well defined geometry. The reliability of this process was proven by the successful construction ofthe PILATUS detector. The construction of PILATUS 6M (60 modules) and the CMSpixel barrel (roughly 800 modules) will start in 2005.
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