UNIVERSIT . . AT BONN Physikalisches Institut - Prof. Dr. Norbert ...

40 3. CIX 0.2 detector
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Fig. 3.17: CIX 0.2 sensor layouts. a) Pixel metallization of the CdTe and CdZnTe sensors.
b) Pixel-side layout of the Si sensors. The red circles are the openings in the passivation for
the gold stud bumps. c): Backside layout of the Si sensors with multi-guard ring structure.
Red squares indicate the passivation openings for the HV contacts.
3.4.2 Module assembly
Working with CdTe and CdZnTe sensors places several constraints on the applicable chipsensor
interconnect technologies. First, both materials are temperature sensitive to the
extent that the exposure to temperatures above 150 ◦ C for longer periods of time (>
several minutes) can already significantly decrease the material quality [52]. Therefore
conventional solder-based bump bonding techniques with process temperatures in excess
of 250 ◦ C are ruled out. Secondly, the ASICs and the sensors were shipped in the form of
single dies. This constraint makes the use of low temperature Indium bump bonding very
difficult as In bump bonding is a wafer-scale process.
A suited interconnect method is the so-called gold stud bumping technique. Fig. 3.18
shows the general steps that are involved in this procedure. It starts with the deposit
of small gold studs onto the ASIC surface. Typically these studs have diameters on the
order of several dozen micrometers with a small section of the original gold wire (typ. 10-
20 µm wide) protruding from the top. In some cases the top of the bumps is smoothed by
compressing the bump with a flat object (called coining). The placement of the individual
bumps can be performed on either single die or whole wafers but in the case of wafer scale
fabrication throughput becomes an issue. After the bumps are in place, the sensor is
positioned above the ASIC and pressed down with a certain force (flip-chip), depending
on the number of bumps. The forces, which are typically utilized, range on the order of
0.2 N/bump. The tip of the gold studs then forms a permanent bond with the surface of
the sensor metallization, which is further facilitated by heating the whole module up to
well below the melting point of gold but still high enough to slightly soften the material.
However, CdTe and CdZnTe complicate matters significantly not only due to their thermal
constraints, but also because of their mechanical tolerances. If too high forces are applied
to the metallization of the sensors, the electrodes can break or loose contact with the
sensor surface. This is the reason why the bumps have to be deposited on the chips
and why the additional application of ultrasonic vibrations to improve the adhesion of
the bumps is prohibited. These low force and low temperature requirements make it