Robot arm (16) end effectors (10, 110, 210) of this invention rapidly and
cleanly transfer semiconductor wafers (12) between a wafer cassette (14)
and a processing station. The end effectors include proximal and distal
rest pads (24, 26, 124, 126) having pad and backstop portions (32, 34,
132, 134) that support and grip the wafer either by wafer peripheral edge
contact or within an annular exclusion zone (30) that extends inward from
a peripheral edge of the wafer. An active contact point (50, 150, 222) is
movable by a vacuum actuated piston (52, 152) between a retracted
wafer-loading position and an extended position that urges the wafer
against the distal rest pads to grip the wafer at its edge or within the
exclusion zone. The end effector further includes fiber optic light
transmission sensors (90, 102, 202, 214) for determining various wafer
surface, edge, thickness, tilt, and location parameters. The sensors
provide robot arm extension and elevation positioning data supporting
methods of rapidly and accurately placing and retrieving a wafer from
among a stack of closely spaced wafers stored in the wafer cassette. The
methods effectively prevent accidental contact between the end effector
and the wafers while effecting clean, secure gripping of the wafer at its
edge or within its exclusion zone.