Low-temperature Junction Formation for Thinned High Energy Physics Sensors

In order to support the high luminosity upgrades at ATLAS and CMS, thinned silicon sensors for hybrid pixel detectors are critical to improve radiation hardness, reduce detector mass, and address high occupancy rates. Because silicon wafer processing tools are not equipped to handle thin wafers, the thinning step cannot be performed until the after the majority of the processing steps are complete, requiring post-processing to create the doped region that constitutes the diode contact at the backside of the wafer. This is challenging, because the high temperature anneal required to activate the dopant after ion implantation would be damaging to the existing front side structures. Current techniques for solving this problem, such as using Silicon-on-insulators wafers, are very expensive. A new microwave annealing technology has been developed which can activate the backside implant at low temperature, without damaging the frontside structures. We have verified this approach on a prototype sensor that was bump-bonded to a readout ASIC and used successfully to measure an Fe-55 x-ray spectrum.