In the manufacture of semiconductors, a common tool, referred to as a cluster tool, is one of the key units used in the manufacture of wafers.  A typical commercial device has a generally circular central area with chambers attached along the circumference.  The chambers extend outward around the central area.  When wafers are processed they are moved first from an input output station on the circumference of the central chamber into the central chamber and then from the central chamber into an attached or circumferential chamber where processing is performed.  In this tool as in substantially all manufacturing systems used today, the practice is to process wafers one at a time.  A wafer may be moved into a chamber for processing and then back to the central chamber.  This can be followed by a further move to another circumferential chamber and then following processing, back to the central chamber.  Eventually the wafer when fully processed is moved out of the tool altogether.  The movement out goes again through an input/output station or chamber which in connection with vacuum systems is generally called a load lock where the wafer moves from vacuum to atmosphere.  A unit of this sort is described for example in U.S. Pat.  No.  4,951,601. 
Another tool indexes wafers along a central axis and feeds wafers through surrounding processing chambers.  In this tool, all wafers are fed simultaneously to the next processing stop.  Wafers cannot move independently although they can be processed independently.  They all remain at a process station for the same amount of time but the processes at each station can be independently controlled subject of course to the maximum time permitted by the time permitted for the stop.  Although the first described tool could be made to operate in this way, in fact however, it may move wafers so that they do not progress to the adjacent processing chamber in sequence and all are not required to have the same dwell time at a processing chamber. 
When either of these systems is operating the central area is generally at vacuum but it may also be at some other preselected or predetermined and controlled environment.  This central section for example can have presented a gas that is useful to the processes being preformed in the process chambers.  The chambers or compartments along the outer surface of the central zone are generally at a vacuum too but may also have a pre-selected and controlled gaseous environment.  Processing is also generally performed in a vacuum by moving wafers while in vacuum from the central chamber to an attached chamber or compartment.  Generally once the wafer reaches a chamber or compartment for processing, the chamber or compartment is sealed off from the central chamber.  This prevents materials and/or gases used in the processing chamber or compartment from reaching the central zone preventing contamination of the atmosphere in the central zone as well as attached processing chambers and/or preventing contamination of wafers located in the central zone waiting to be processed or further processed.  This also permits the processing chamber to be set at a vacuum level different than that used in the central chamber for the particular processing to be carried out in the chamber.  For example, if the processing technology of a chamber requires more of a vacuum, then with a seal in place between the central zone and the chamber, the chamber itself can be further pumped down to match the process requirements for the particular processes to be performed within that chamber.  Alternatively, if less of a vacuum is required, the pressure may be increased without affecting the pressure of the central chamber.  After processing of the wafer is completed, the wafer is moved back to the central chamber and then out of the system.  In this way the wafer may progress through this tool sequentially through the chambers and all of the available processes.  Alternatively the wafer may proceed through only selected chambers and be exposed to only selected processes. 
Variations on these processes are also in use in equipment offered to the field.  However, they all tend to rely on a central area or zone that is integral to the various processes.  Also since the predominant usage of such equipment is to make wafers, the discussion will primarily be in terms of wafers.  However, it should be understood that most of the processes under discussion are also applicable to substrates in general and that the discussions should be taken to also apply to such substrates and such manufacturing equipment. 
Recently there has been described a system that is distinct from these, in that it is linear in shape rather than circular and wafers move for processing from one chamber to the next chamber.  Since the wafer moves in sequence from one chamber to an adjacent chamber, there is no need for the central zone as part of the equipment.  In this tool a wafer enters the unit and is generally attached to a chuck that travels with the wafer as it moves through the system.  In this unit, processing is performed for equal amounts of time in each chamber. 
This system has a smaller footprint than is typical in this field since the footprint approximates the footprint of the processing chambers only and does not include a large central zone.  This is an advantage of this type equipment.  This system is described in a pending published patent application, Publication No.  2006/0102078 A1.  This particular system has a uniform dwell time at each process station.  This allows for some differences in processing limited of course by the length of the longest dwell period.  If one requires independently controlled dwell times at the various stations, another approach may be preferred.  Also this type of equipment has the disadvantage that if one station is down for repair or maintenance, then the whole system is itself unavailable for processing.