Method and apparatus for transferring wafers between cassettes and a boat

A method of transferring a wafer from a source to a destination and compensating for any lateral offset of the actual location of the wafer center from the expected location of the wafer center in the source so that the wafer can be placed in the destination with the actual location of the wafer center very near a predetermined, desired, lateral location of the wafer center in the destination, said method comprising,

• removing the wafer from the source by a paddle having a center location which is known with respect to both said expected source location and said desired destination location,
• finding the amount of any lateral offset of the paddle center with respect to the wafer center,
• repositioning the paddle laterally by that amount of offset, and
• then delivering the wafer to the destination by the paddle.

A furnace for heating and/or processing wafers and comprising,

• a cassette having slots for holding wafers within the cassette,
• a boat having slots for holding wafers to be heated when the boat is loaded in a heating chamber,
• boat holder means for holding the boat at a wafer loading/unloading station while wafers are transferred between the slots of a cassette and the slots of a boat,
• wafer transfer means for transferring wafers between the slots of a cassette and the slots of a boat,
• vertical chamber means having an open lower end through which the boat is inserted at the beginning of a heating and/or processing cycle and withdrawn at the end of the cycle, and
• elevator means for raising the boat into the vertical chamber means from a chamber loading/unloading station and for lowering the boat out of the heating chamber means back to the chamber loading/unloading station,
• said elevator means and boat structure being separate and independent of each other so that a boat with its load of wafers can, as a unit, be placed onto and taken off of the elevator means at the chamber loading/unloading station and a boat can be loaded with wafers prior to the time the boat is placed on the elevator means and wafers can be unloaded from a boat after the boat is removed from the elevator means whereby the processing of wafers in the furnace is not delayed by the time required to load or to unload wafers from a boat.

A furnace for heating and/or processing wafers and comprising,

• a cassette having slots for holding wafers within the cassette,
• a boat having slots for holding wafers to be heated when the boat is loaded in a heating chamber,
• boat holder means for holding the boat at a wafer loading/unloading station while wafers are transferred between the slots of a cassette and the slots of a boat,
• wafer transfer means for transferring wafers between the slots of a cassette and the slots of a boat,
• vertical chamber means having an open lower end through which the boat is inserted at the beginning of a heating and/or processing cycle and withdrawn at the end of the cycle,
• elevator means for raising the boat into the vertical chamber means from a chamber loading/unloading station and for lowering the boat out of the heating chamber means back to the chamber loading/unloading station,
• wherein the boat slots are vertically spaced apart and are substantially vertically aligned with one another and wherein the cassette slots are vertically spaced apart and substantially vertically aligned with one another and wherein the wafer transfer means include a paddle which is movable vertically to pick up and to carry a wafer during a transfer of the wafer between the boat and cassette and which is movable radially into and out of the cassette and the boat and which is also movable in a circular arc in the course of making a transfer between a cassette and a boat,
• vertical, radial and rotational motor drive means for moving the paddle in respective vertical, radial and rotational directions and including controller means for controlling the actuation of the vertical, radial and rotational motor means,
• radial sensor means for sensing the amount of radial movement of the paddle which is involved in withdrawing a wafer from a cassette or boat,
• vertical sensor means for sensing the vertical position of a wafer in a slot in a boat or in a shelf of a cassette,
• theta sensor means for sensing and measuring the amount of offset of the center of the paddle with respect to the center of a wafer on the paddle, and
• wherein the sensor means send signals to the controller means for establishing in three coordinates by calibration the expected center of a wafer in each shelf and in each slot and for also compensating during the actual transfer for any lateral offset of the paddle center with respect to the wafer center by repositioning the paddle laterally by that amount of offset required so that any errors introduced during wafer pick-up are eliminated and the wafer transfer means conserve all of the available tolerances for the very critical wafer delivery.