Fine-pitch Bumping for Pixel Detectors

Hybrid pixel detector modules are now state of the art in high energy particle detection. These modules consist of a particle sensing element, the sensor, and one or more electronic readout chips which are flip chip bonded to the sensor tile. One advantage of hybrid pixel detectors is the separate development, optimization and processing on wafer level of the sensor and the electronic readout chip. Because of the direct interconnection between pixel and readout cell, the high spatial resolution demands small solder joints in a tight pitch for an accurate and fast detection of the particle track location.

Several high energy physic experiments all over the worls are running with pixel detector modules fabricated at Fraunhofer IZM. This type of modules are the innermost part of two of the largest particle detectors world-wide - the ATLAS and CMS detector at the Large Hadron Collider (LHC) at CERN, Switzerland. About 1500 Pixel Detector Modules has been assembled at Fraunhofer IZM for the ATLAS detector. Furthermore, pixel detector modules with a pixel size of only 55 x 55 µm² are used for x-ray imaging applications. In this case the electronic read-out chip contains a matrix of 256 x 256 micro bumps with a size of 25 µm each for interconnection of sensor pixel and electronic read-out cell.

These are in total 65.536 bumps per chip and more than 6 million bumps on a 200 mm read-out chip CMOS wafer. ECD bumping (electrochemical deposition) is therefore the best suited technology because it can be used for pitches even below 40 µm. 25 µm pixel interconnection bumps are produced by SnAg-, AuSn- or In- electroplating on 200 mm readout chip wafers with an excellent uniformity. Solderable metallization has to be deposited on the sensor pixel pads. Sensor chips of different materials can be processed ranging from silicon and GaAs wafer to single chip diamond substrates. Finally, this fine pitch application requires also a high accuracy flip chip bonding process for read-out chip to senor asembly.

Fraunhofer IZM has a long term experience in manufacturing of hybrid pixel detector modules from research and development stage up to the production of more than 1000 modules for large particle detector projects.