A transfer system for use with a tool for treating a work-piece at sub-atmospheric pressure such as an ion implanter for implanting silicon wafers. An enclosure defines a low pressure region for treatment of work-pieces placed at a work-piece treatment station within the low pressure region. Multiple work-piece isolation load locks transfer work-pieces, one or two at a time, from a higher pressure region to the lower pressure for treatment and back to said higher pressure subsequent to said treatment. A first robot transfers work-pieces within the low pressure region from the load locks to a treatment station within the low pressure region. Multiple other robots positioned outside the low pressure region transfers work-pieces to and from the multiple work-piece isolation load locks from a source of said work-pieces prior to treatment and to a destination of said work-pieces after said treatment.