Method and apparatus for controlling thermal transfer in a cyclic vacuum processing system

Apparatus for providing thermal transfer with a semiconductor wafer in an ion implantation system including a vacuum chamber which is vented during one portion of an operating cycle and is vacuum pumped to a low pressure during another portion of said cycle, said apparatus comprising:

• a disc adapted for rotation about a central axis and having locations for mounting a plurality of semiconductor wafers, each of said locations including a platen against which a wafer is sealed to define a thermal transfer region, each of said platens including a passage providing gas flow between said thermal transfer region and said chamber, each of said locations further including valve means for controllably opening and closing said passage in said platen;
• and
• control means for closing each of said valve means when the pressure in said chamber reaches a predetermined intermediate pressure,
• whereby gas at said intermediate pressure is trapped in said thermal transfer region during ion implantation and conducts thermal energy between said wafer and said platen.