Wafer handling apparatus and method

DETAILED DESCRIPTION A method and apparatus of heating a silicon wafer to bake off a solvent used in applying a photoresist to the wafer or to bake off a liquid used in washing or cleaning the wafer include a hot plate and a controlled positioning of the wafer with respect to the hot plate.
The method and apparatus provide flexibility in the rate and the way in which the wafer is heated to the equilibrium or bake out temperature while minimizing the total amount of time required for the baking operation.
The hot plate has a large mass in comparison to the mass of the wafer and is maintained at a closely controlled, substantially constant temperature level which is higher than the equilibrium or bake out temperature level to which the wafer is to be heated.
The wafer is placed in close proximity to the hot plate at the beginning of the heating cycle, and the wafer is then repositioned relative to the hot plate to vary the thickness of the air gap between the wafer and the hot plate.
The heating of the wafer is controlled as a function of the distance of the wafer from the hot plate.
The wafer handling method and apparatus of the present invention insure centering of the wafer at the work station by means of two wafer sensors located at the work station in locations effective to detect the presence of the wafer at the work station.
The sensors detect the presence of the leading part of the wafer as the wafer enters the work station area and signal a controller to slow down the speed of conveyor drive belts and the speed at which the wafer enters the work station area.
The sensors signal the controller to stop the drive belts when the trailing part of the wafer uncovers the optical paths of both of the sensors and the wafer is centered with the work station.
If the wafer is off-center with the work station, this off-center condition is sensed by the wafer sensors; and the wafer is forced back to center.
The drive belts are driven at the slow speed while an edge of the wafer is engaged with a curved surface that permits the wafer to be rotated and shifted laterally until the wafer is centered and the optical paths of both sensors are unblocked by the recentered wafer