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 two-dimensional Gaussian and advanced Monte Carlo simulations, the doping concentration in this trunk is about 20% of the maximum concentration in the case of a 85° mask angle. The simulations predict the experimental results fairly well. The asymmetry effect of high energy ion implantations can also be visualized in photoresist by means of a damaged regio