Statistical Modelling of Channel Occupancy in Trunked PAMR Systems

Semiconductor wafers are transferred from and to holder elements or "boats", in which the wafers are processed, by transfer apparatus including a movable head having rotatable holder elements which hold the wafers. Wafers are moved out of and into the boats by vertically moving lifting elements. The lifting elements move the wafers from a boat to the rotatable holder elements, and vertical lifting elements move the wafers from the rotatable holder elements into boats after the head has moved itself from one position to another. Typically, there are three positions or stations for the head, and vertical lifting elements are positioned at each station for moving the wafers vertically to and from the boats which are located at each station.