Abstract: A semiconductor substrate centering mechanism includes a plurality of substrate support pins, each pin having a top surface. The top surfaces of the pins define a plane in which the substrate is supported. Each pin has a tab mounted eccentrically at the top surface of the pin. The tabs extend upwardly relative to the top surfaces of the pins. The centering mechanism further includes a pin rotation mechanism adapted to rotate each pin. The pin rotation mechanism rotates the pins between a first position in which the tabs define an envelope that is larger than a circumference of the substrate and a second position in which the tabs define a centered position for the substrate. A telescoping arrangement of nesting shield segments may also be provided for each pin to prevent processing fluid from reaching a shaft of the pin.