Characterization of Ni/SnPb-TiW/Pt Flip Chip Interconnections in Silicon Pixel Detector Modules

In contemporary high energy physics experiments, silicon detectors are essential for recording the trajectory of new particles generated by multiple simultaneous collisions. Modern particle tracking systems may feature 100 million channels, or pixels, which need to be individually connected to read-out chains. Silicon pixel detectors are typically connected to readout chips by flip-chip bonding using solder bumps. High-quality electro-mechanical flip-chip interconnects minimizes the number of dead read-out channels in the particle tracking system. Furthermore, the detector modules must endure handling during installation and withstand heat generation and cooling during operation. Silicon pixel detector modules were constructed by flip-chip bonding 16 readout chips to a single sensor. Eutectic SnPb solder bumps were deposited on the readout chips and the sensor chips were coated with TiW/Pt thin film UBM (under bump metallization). The modules were assembled at Advacam Ltd, Finland. We studied the uniformity of the solder bumps using Scanning White Light Interferometry (SWLI). According to our results, the Ni/SnPb-TiW/Pt interconnections fulfill the requirements in [1] for flip-chip bonding pixel detector modules. This study proposes a way to decrease the number of dead channels of the silicon pixel detector modules by precisely measuring the soldered bump diameter to ensure that they fulfil the specifications.