  The hybrid pixel detectors used in the high energy physics experiments
currently under construction use a three dimensional connection technique, the
so-called bump bonding. As the pitch below 100um, required in these
applications, cannot be fullfilled with standard industrial processes (e.g. the
IBM C4 process), an in-house bump bond process using reflown indium bumps was
developed 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-off
steps. As the first photolithographic step a thin under bump metalization (UBM)
is sputtered onto bump pads. It is wettable by indium and defines the diameter
of the bump. The indium is evaporated via a second photolithographic step with
larger openings and is reflown afterwards. The height of the balls is defined
by the volume of the indium. On the readout chip only one photolithographic
step is carried out to deposit the UBM and a thin indium layer for better
adhesion. After mating both parts a second reflow is performed for self
alignment and obtaining a high mechanical strength.
  For the placement of the chips a manual and an automatic machine was
constructed. The former is very flexible in handling different chip and module
geometries but has a limited throughput while the latter features a much higher
grade of automatisation and is therefore much more suited for producing
hundreds of modules with a well defined geometry.
  The reliability of this process was proven by the successful construction of
the PILATUS detector. The construction of PILATUS 6M (60 modules) and the CMS
pixel barrel (roughly 800 modules) will start in 2005.
