WAFER CHARGING CONTROL IN HIGH-CURRENT IMPLANTERS

Wafer charging in high-current ion implanters is reviewed, and the operation of the electron flood gun in the Varian 160-10 implanter is examined. It is shown that flood gun electrons with energies up to 350 eV do reach the wafers and can cause damage when wafers are excessively overflooded. An in situ flood gun monitor using a capacitive pickup sensor is described. Experiments with the capacitive charge sensor have further shown that (i) wafers can self-charge during pump-down or venting of the target chamber, (ii) a slight overflooding is preferable to under flooding, and (iii) for perfect neutralization the flood gun emission current should vary with the magnetic scanning of the ion beam across the wafers. Using metal oxide semiconductor capacitors as test vehicles, it is shown that other factors also can affect charging damage to devices during implantation, such as the thickness of the field oxide or photoresist relative to the ion penetration depth, the proper grounding of the back sides of wafers during implantation, and the polarity of the silicon underneath the gate oxide. Finally, the benefits of proper electron flood control are demonstrated, and operating procedures are recommended.