CHARGE CONTROL FOR HIGH-CURRENT ION IMPLANT

The electrostatic potential of high-current positive ion beams may rea ch hundreds of volts unless the space charges are compensated with ele ctrons. potential in excess of 10 V induced over sub-100-Angstrom gate oxides will deteriorate oxide integrity. Thus, all high-current impla nters incorporate either secondary electron flood (SEF) or plasma elec tron flood (PEF) hardware to generate and inject electrons into the io n beam to reduce beam potential. This paper discusses optimum conditio ns for wafer-charging control during ion implantation, including inher ent limitations of beam neutralization through residual gas ionization . We compare PEF and SEF, and suggest a simple test that determines wh ether electrons with energies higher than 10-20 eV are generated and t ransported to wafers. Experimental data show that device structures in fluence implant-induced oxide wearout. To minimize wearout of thin-gat e oxides, the optimization of charge neutralization conditions should take into account the conductivity of the Si substrate and the well st ructure.