Accurate positioning of a wafer

A load station for handling wafers, having the capability of bringing the center of a circular wafer that is maintained in a horizontal plane into coincidence with the axis of an overhead spindle carrier, which axis is perpendicular to the horizontal plane, where the initial position of the wafer in the horizontal plane is subject to two-dimensional position errors, and where the spindle carrier moves downward along its axis from an upper initial position to a lower final position, and where the spindle carrier has an underside that includes a vacuum pad for use in acquiring the wafer, the load station comprising:

• a base having a horizontal upper surface;
• a load ring assembly slidably resting on the upper surface of the base and including
• a load ring initially disposed in a first horizontal plane spaced above the upper surface of the base and having a center;
• at least three centering tabs spaced around the circumference of the load ring;
• hinge means connecting the centering tabs to the load ring for pivoting motion about horizontal axes that are tangent to the load ring, said hinge means located equidistant from the center of the load ring;
• biasing means operatively connected to the load ring and to the centering tabs, yieldably urging the centering tabs to an initial position in which portions of the centering tabs extend below the first horizontal plane and radially outward beyond the hinge means and bear on the horizontal surface of the base to yieldably support the load ring in its initial position above the upper surface of the base;
• the load ring having a top surface against which the spindle carrier pushes as it moves downward, pushing the load ring downward from its initial position and overcoming the biasing means to force the centering tabs to pivot from their initial positions and to arrive at final positions when the spindle carrier and load ring have reached their lower final positions;
• said centering tabs having first inwardly facing surfaces extending above the hinge means and through the horizontal plane in which the wafer is maintained so as to pivot inwardly as the load ring is forced downward by the descending spindle carrier, the first inwardly facing surfaces pushing against the edge of the wafer as the spindle carrier descends, to guide the wafer, bringing its center into coincidence with the center of the load ring;
• said centering tabs having second inwardly facing surfaces extending above the hinge means and above the first inwardly-facing surfaces so as to pivot inwardly as the load ring is forced downward by the descending spindle carrier, the second inwardly facing surfaces pushing against the unyielding periphery of the spindle carrier to slidably guide the load ring, bringing its center into coincidence with the axis of the spindle carrier;
• so that, when the spindle carrier has descended to its lower final position, the center of the wafer has been guided to the center of the load ring and the center of the load ring has been guided to the axis of the spindle;
• whereby, the center of the wafer has been brought into coincidence with the axis of the spindle carrier through the use of the load ring assembly.