Development of an ultrahigh vacuum, i n v a c u o wafer transfer facility integrated processing

Abstract

An ultrahigh vacuum, in vacuo wafer transfer system has been completed for development of integrated wafer processing. Design decisions which determined the structural layout and the interior design of the vacuum transfer vessel are elucidated in this paper. Among the criteria considered were (i) the areal floor space occupied by the transfer device, (ii) the average travel distance per transfer, and (iii) an awareness that mechanical function must be incorporated without leading to wafer particulate contamination. Of these criteria, the first two influenced the structural arrangement of the transfer system while the last criterion influenced the interior design. Elementary calculations are presented for a simple rotary and linear transfer tube design. These calculations show that, for integration of more than four nodes, the linear transfer design occupies less floor space than a rotary design would occupy. Calculations show that the rotary design does offer a shorter average transfer distance. However, trafficking of the process steps will produce comparable average travel distances per transfer between the rotary and linear designs. To reduce wafer particulate contamination, the interior design included face-down susceptors in all deposition chambers, vertical wafer storage, isolation valves off line from the linear transfer, and concealed gear trains. The vacuum transfer vessel currently interconnects a chemical vapor deposition unit, a surface analytical unit, a substrate modification chamber, a molecular-beam epitaxy chamber, and a vertical load lock. Vacuum performance curves for samples entering the transfer vessel from air show that water desorption from the sample carrier and wafer must be enhanced during sample entry into the transfer system; else, water desorption can inadvertently occur during subsequent processing steps.