Strongly Asymmetric Doping Profiles at Mask Edges in High-Energy Ion Implantation

The application of high energy ion implantation is restricted by an asymmetric doping profile at the mask edges. As a result, buried interconnect cannot easily be formed. Moreover, the holding voltage and threshold voltage of CMOS-processes with retrograde wells may be strongly affected by this asymmetry. It arises from the 7" wafer tilt, which is frequently used to avoid channeling, even in the case of nearly perpendicular (82-85") mask edges. On the mask side, which is incoming to the ion beam, a trunk to the surface has experimentally been observed. According to 2-D-Gaussian and advanced Monte Carlo simulations, the doping concentration in this trunk is about 20 percent of the maximum concentration in the case of a 85" mask angle. The simulations are fairly well able to predict the experimental results. The asymmetry effect of high energy ion implantations can also be visualized in photoresist by means of a damaged region.