Abstract:
A high-speed wafer-processing apparatus and method that employs a vacuum chamber having at least two wafer transport robots and a process station. The vacuum chamber interfaces with a number of single-wafer load locks that are loaded and unloaded one wafer at a time by a robot in atmosphere. Four load locks are sized to allow for a gentle vacuum cycling of each wafer without significant pumpdown delays. The robots in the vacuum chamber move wafers sequentially from one of the load locks to a process station for processing and then to another one of the load locks for unloading by the atmospheric robot.

Description:
RELATED APPLICATIONS 
     The current application claims priority from Provisional Patent Application Ser. No. 60/334,251 entitled WAFER HANDLING APPARATUS AND METHOD filed on Nov. 29, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for moving workpieces in a vacuum chamber to maximize throughput. More specifically, it involves methods that minimize the amount of time between wafer processing by utilizing a parallel flow of workpieces for processing in the vacuum chamber. Each workpiece undergoes the same steps of introduction to the vacuum chamber, alignment to a suitable orientation, processing, and removal from the vacuum chamber. Four wafers are at various phases of the wafer handling and processing cycle simultaneously, thus maximizing utilization of the equipment while minimizing cycle time. 
     Up to twenty different types of tools are employed for affecting several hundred processing steps during the processing of wafers in the manufacture of microelectronic circuits. Most of these processing steps must be performed in a vacuum chamber at pressures less than 1×10 −3  torr, and each requires from about ten seconds to three minutes per wafer. Most of the processing tools operate on wafers one at a time in order to optimize control and reproducibility in a manufacturing environment. 
     In general, each operation on a wafer must be performed in a particular order, so that each operation must wait until completion of a preceding one, and, in turn, affects the time a wafer is available for a subsequent step. Tool productivity or throughput for vacuum processes that are relatively short, such as ion implantation, can be severely limited if the work flow to the processing location or platen is interrupted by sequential events, which may include, for example, the introduction of the wafer into the vacuum system, the orientation of a wafer in the vacuum chamber or the exchange of wafer carriers or cassettes. 
     It is desirable to shorten the duration of sequential events, i.e., those events that must be performed consecutively in order to increase throughput. However, the pump down times (to high vacuum) and the venting times (to atmospheric pressure) must be relatively long to reduce turbulence and ensure the wafer remains free of particles and foreign materials that could be redistributed from the load lock surfaces to the wafer. 
     The prior art has sought to address these concerns in a number of ways. For example, U.S. Pat. No. 5,486,080 of Sieradzki, which is incorporated herein by reference, employs two wafer transport robots to move wafers from two load locks past a process station. Both robots alternately transport each wafer from the cassette at one of the load locks along a path to an orientation position, through the process station, and back to the cassette until all the wafers in the cassette are processed. Pumpdown or venting of the other (second) load lock with another cassette holding multiple wafers occurs while the wafers in the cassette at the first load lock are processed. After processing the wafers from the first load lock, the first load lock is closed and vented while the second load lock is opened and the robot then transport the wafers from the second load lock through the process station. This procedure adequately achieves high throughputs for a cassette loaded batch of wafers (200 mm wafers), but does not address the requirements resulting from the use of 300 mm wafers. 
     With the continuing trend toward smaller and faster electronic devices, the use of cassettes to hold and transport wafers is now burdensome. For example, 300 mm wafers are transported in Front-Opening Unified Pods (FOUPs), which keep the wafers in an ultra-clean environment. The FOUPs interface to dedicated modules on process equipment, which automatically open their doors while an atmospheric robot removes and replaces wafers as required. These FOUPs are not intended to be loaded into vacuum, whereas cassettes used to transport 200 mm wafers may be directly placed in load locks and brought to vacuum. As such, a system optimized for 300 mm wafers has different requirements from earlier systems. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above-described limitations of handling wafers in a vacuum environment. The present invention provides an approach whereby at least four load locks interfacing with a vacuum chamber cycle in a sequence between vacuum and atmosphere to reduce the cycle time for processing a wafer. The steps of the cycle can be carried out simultaneously on different wafers to optimize the amount of time the process station is utilized. 
     The present invention provides a method for processing wafers. Each of the wafers undergoes the same cycle:
         it is placed in a load lock,   the load lock is roughed down to vacuum,   the wafer is removed from the load lock and placed on a holding station (where orientation is performed if necessary),   the wafer is transferred to the process station for processing (such as ion implantation),   the processed wafer is removed from the process station and placed in another load lock (different from the load lock in which it was roughed down), and   the load lock is vented to atmosphere where the wafer is removed and replaced with a new wafer.       

     While each wafer must undergo these steps in sequence, allowing all the steps to be finished for one wafer, then proceeding with the next wafer is time consuming; the process station can be better utilized. The present invention is based, in part, on the recognition that parallel cycling of wafers better utilizes the system resources. Because the time required for placing the wafer onto the process station, processing the wafer and removing the wafer from the process station takes about one-quarter of the vacuum cycle and atmospheric wafer exchange time, the inventors recognized that four wafers could be at various phases of the cycle simultaneously to maximize efficiency. This is desirable because more rapid venting and roughing of wafers to and from the vacuum system adds particles and other foreign materials to the surfaces of the wafer resulting in reduced production yields. Thus, optimizing cycling could be achieved by simultaneously having a wafer in each state of a load lock cycle. Further, the inventors realized that grouping load locks in pairs and having a wafer brought to vacuum in one load lock and then exiting through a second load lock of the pair would result in the optimum efficiency of the load locks. In addition, the highest process station productivity would result by having two pairs of load locks operating in parallel, each pair accessed by an independent robot acting in vacuum, with each robot able to access the process station. To accomplish this, at least two robots acting in vacuum, at least four load locks, and at least two holding stations are necessary to achieve optimal usage of one process station. If the time for the in vacuum part of the cycle could be shortened further, additional load locks could provide additional cycle time benefits. 
     The steps of the method are performed using a first pair of load locks (LL 11  and LL 12 ), a second pair of load locks (LL 21  and LL 22 ), and a vacuum chamber that includes a first vacuum robot (VR 1 ) associated with the first pair of load locks (LL 11  and LL 12 ) and a second vacuum robot (VR 2 ) associated with the second pair of load locks (LL 21 , and LL 22 ). Each of the robots operates independently of each other for handling the wafers in vacuum. Each of the load locks includes a first portion opening to atmosphere and a second portion opening to the vacuum chamber. 
     Wafers are placed on and removed from the process station by first one vacuum robot and then the other. In steady-state operation the first wafer has completed processing. VR 1  removes the processed first wafer from the process station and the VR 2  places the second wafer on the process station. 
     A third wafer has previously been removed a LL 12  by VR 1  and placed on the first holding station. 
     VR 1  places the processed first wafer in LL 12  and LL 12  begins venting. Nearly simultaneously, a fourth wafer is introduced into vacuum through LL 22 . 
     The fourth wafer is removed from LL 22  by VR 2  and placed on the second holding station. VR 2  then removes the processed second wafer from the process station followed by VR 1 &#39;s placement of the third wafer onto the process station. 
     VR 2  then places the processed second wafer into LL 22  and this load lock begins venting. Nearly simultaneously, a fifth wafer is introduced into vacuum through LL 11  of the first pair of load locks. 
     The fifth wafer is removed from LL 11  by VR 1  and placed on the first holding station. VR 1  then removes the processed third wafer from the process station followed by VR 2 &#39;s placement of the fourth wafer onto the process station. 
     VR 1  then places the processed third wafer into LL 11  and this load lock is vented. 
     In this way wafers are continuously processed. Each of the steps: bringing the wafer to vacuum, handling the wafer in vacuum for orientation, processing the wafer, and venting to atmosphere so a new wafer can be cycled, are carried out sequentially on four wafers roughly ninety degrees out of phase with one another. More explicitly, if we define t p  as the time required to place a wafer on the process station, process the wafer, and remove the wafer from the process station, the time available for venting a load lock, exchanging the processed wafer with a new wafer, and pumping the load lock is approximately 3t p . If t p  is equal to 15 seconds, a full 45 seconds is available to vent a load lock, exchange the processed wafer with a new wafer, and pump the load lock. This results in high throughputs while ensuring slow vent and pump down time to maintain wafer cleanliness. 
     In another aspect of the present invention, the method processes wafers using four load locks, a vacuum chamber that includes a first and second vacuum robot for handling the wafers in vacuum, a process station, and a first and second holding station for preprocessing the wafers. The first and second vacuum robots operate independently of each other and each of the load locks has a first portion opening to atmosphere and a second portion opening to vacuum in a vacuum chamber. The first vacuum robot interfaces with the first and second load locks, and the second vacuum robot interfaces with the third and fourth load locks. 
     To start a processing cycle, the first load lock that contains a first wafer is roughed down to vacuum. After the first load lock reaches vacuum, the second portion of the load lock opens to vacuum for the removal of the first wafer by the first vacuum robot. The first vacuum robot places the first wafer onto the first holding station for preprocessing, such as orientation. In turn, a second wafer, which has been roughed down in the second lock and which just completed processing, is removed from the process station and is loaded at vacuum into the first load lock by the first vacuum robot. The first load lock vents to atmosphere to end a processing cycle for the second wafer. Once at atmosphere, the second wafer is removed from the first load lock and a third wafer is placed in the first load lock that is roughed down to vacuum to start a processing cycle for the third wafer. At the same time, but offset in the cycle by one quarter of the cycle, the second vacuum robot is carrying out the same steps, accessing the third and fourth load locks (that form a second pair), and using a second holding station and the same process station. Again, the processed wafers enter vacuum in one of the load locks and exit vacuum in the other of the pair. 
     After preprocessing of the first wafer completes, it is removed from the first holding station using the first vacuum robot and held by the first vacuum robot until the second vacuum robot removes a wafer it placed on the process station for processing. After placing the first wafer of the process station, the first vacuum robot removes the third wafer from the second load lock at vacuum and places it onto the first holding station for preprocessing. Following completion of the processing of the first wafer, the first vacuum robot removes the first wafer from the process station and loads the second load lock, which is open at vacuum, with the first wafer for venting to atmosphere to end the processing cycle for the first wafer. Once the first wafer is removed from the process station, the process station again becomes available to the second vacuum robot that can place a wafer for processing on the process station. When the process station again becomes available to the first vacuum robot, the first vacuum robot removes the second wafer from the first holding station and places the second wafer onto the process station for processing. The above steps are carried out in a concurrent manner using the third and fourth load locks, the second vacuum robot, the second holding station, and the process station to concurrently process wafers. The process station alternately receives and processes wafers provided by the first vacuum robot and the second vacuum robot. Optionally, an atmospheric robot operates in atmosphere to load and unload each load lock with wafers from a carrier to maintain a substantially constant flow of wafers. 
     The present invention also provides a system for processing a workpiece in vacuum. The system includes a vacuum chamber maintained at high vacuum that contains a process station, a first and second holding station, and a first and second vacuum robot for transferring workpieces from the holding stations to the process station in the vacuum chamber. The first vacuum robot and the first holding station are associated so that they work in conjunction with each other while the second vacuum robot and the second holding station are associated and work in conjunction with each other. Each association is mutually exclusive of the other. Four load locks interface with the vacuum chamber. Each of the load locks is capable of receiving a workpiece at atmospheric pressure and introducing it to the vacuum chamber for processing. In like fashion, each of the load locks is capable of receiving a workpiece at vacuum from the vacuum chamber at completion of workpiece processing and returning the workpiece to atmospheric pressure. Each of the load locks cycle between atmosphere and vacuum always with a wafer in it, under computer control in a sequence that results in a ninety degree phase offset in the cycle amongst the load locks. The load locks are grouped in pairs so that a first pair is associated with the first vacuum robot and first holding station, and the second pair is associated with the second vacuum robot and second holding station. Additionally, the system can include a docking station in atmosphere to receive and hold a FOUP. An atmospheric robot operating in atmospheric pressure can be used to transport wafers between the FOUPs and each of the load locks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the prior art system described in U.S. Pat. No. 5,486,080. 
         FIG. 2  illustrates an exemplary wafer handling system in accordance with an illustrative embodiment of the present invention. 
         FIG. 3  illustrates an exemplary timing diagram in an illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before beginning with the detailed description, it is helpful to first define a few terms as used throughout the specification and claims. 
     As used herein, the term “robot” refers to an articulated arm under independent control. 
     As used herein, the terms “parking station”, “transfer station”, “orientation station”, or “holding station” refers to a device that holds a wafer before processing, and can orient or align a wafer. 
       FIG. 1  shows a prior art wafer handling system. The prior art system employs two cassettes holding a plurality of wafers and two load locks. A cassette of wafers is loaded into one of the load locks. While one load lock is being vented, the cassette exchanged, and the load lock pumped down, the other load lock remains in a vacuum state and all the wafers from its cassette are processed in sequence. That is, a load lock is cycled to vacuum and back to atmosphere once for each cassette of wafers placed in the load lock. 
     In the prior art system illustrated in  FIG. 1 , processing of wafers occurs by transferring the wafers between robots via a transfer station. That is, robot # 1  removes a wafer from a cassette in the load lock # 1  and transfers the wafer to the transfer station  50 . Robot # 2  removes the wafer from the transfer station  50  and places the wafer on the platen  25  for processing at the process station. After the wafer is processed, robot # 1  removes the wafer from the platen  25  and returns this wafer to the cassette in load lock # 1 . This cycle is followed until all the wafers in the cassette are processed. 
     This system works well for 200 mm wafers that are transported in cassettes where the cassettes maybe introduced into the vacuum system. Since FOUP&#39;s cannot be placed into a vacuum system this system requires wafers to be transferred from a FOUP into a wafer holding device in the load lock. To compensate for the additional handling step, the prior art system must employ complex rapid atmospheric wafer handling that places wafers at increased risk of damage. 
       FIG. 2  illustrates a system  100  suitable for practicing the present invention. Nevertheless, those skilled in the art will understand that this is a diagrammatic representation of a three-dimensional form and that various forms and details can be implemented without departing from the scope of the present invention. The system  100  includes one or more docking stations  101   a  through  101   d  to receive one or more FOUPs  104   a  through  104   d.  Each of the docking stations  101   a  through  101   d  includes a door  102   a  through  102   d  that can be opened automatically by commercially available equipment, not shown. A robot  110  operating in atmosphere is capable of moving along a track  111  to load and unload wafers from each of the docking stations  101   a  through  101   d.  The robot  110  is capable of additional degrees of freedom including vertical, radial and azimuthal movements. Operation of the robot  110  in relation to the docking stations  101   a  through  101   d  is discussed below in more detail. Although the system  100  is discussed in connection with the use of FOUPs, one skilled in the art will recognize that in certain applications cassettes may be used in place of FOUPs as a transportation carrier for transport of the wafers or workpieces at atmosphere. 
     A vacuum chamber  160  is provided with four load locks  120   a  through  120   d,  each of which is provided with an atmospheric valve  121   a  through  121   d,  which opens to allow transfer of a wafer in atmosphere from one of the docking stations  101   a  through  101   d  to a selected load lock. Each of the load locks  120   a  through  120   d  include a vacuum valve  122   a  through  122   d,  which opens to allow transfer of the wafer from the load lock into the vacuum chamber  160 . Load locks  120   a  through  120   d  are further equipped with venting and pumping means and with other valves and controls (not shown) that one skilled in the art will recognize. The load locks  120   a  through  120   d  are shown as being located side-by-side but can optionally be disposed as two pairs one above the other. 
     Within the vacuum chamber  160  is a process station  150  addressable by a robot  131  and a robot  132 . Robot  131  can address a holding station  141 , which can pre-orient a wafer for processing at process station  150 . Robot  132  can address a holding station  142  that is similar to the holding station  141 . A controller  170  is provided to control sequencing of wafers through the system  100  and to control activation, deactivation and overall coordination of mechanical and environmental operations during wafer handling and processing. The environmental operations include, for example, control of vent and pump down operations for each of the load locks  120   a  through  120   d  and control of the clean environment in the vacuum chamber  160 . The mechanical operations include, for example, instructing of each robot or wafer handler and control of certain valves and other mechanical devices. The controller  170  can optionally include an environmental controller  172  to control all or part of the environmental operations. 
     A variety of vacuum processes benefit from the present invention. One such vacuum process is ion implantation. With ion implantation, the implantation process time per wafer is often less than about 10 seconds. As such, with the use of an illustrative embodiment of the present invention, wafer throughputs of greater than 300 wafers per hour may be achievable for an ion implantation process. 
     In practice, one wafer is removed from a slot in a selected FOUP or cassette in one of the docking stations  101   a  through  101   d  by the robot  110 , and placed into one of the load locks  120   a  through  120   d,  which is pumped down to vacuum, and opened to the vacuum chamber  160 . The wafer is transported to one of the holding stations  141 ,  142  by one of the robots  131  and  132  depending upon which load lock the wafer is placed. Which robot and holding station is selected to handle the wafer depends on which load lock the wafer is loaded into at atmosphere. For example, if the wafer is loaded into load lock  120   a  or  120   b  at atmosphere, robot  131  and holding station  141  are employed to process the wafer through the vacuum chamber  160 . In similar fashion, if the wafer is loaded into load lock  120   c  or  120   d  at atmosphere, robot  132  and holding station  142  are employed to process the wafer through the vacuum chamber  160 . If the application is ion implantation, the holding stations may be alignment stations for alignment and pre-orientation of wafers. To place an unprocessed wafer onto the process station  150  with a minimum amount of delay, it is desirable to employ a sequence of robotic movement that instructs the robot  131  remove a processed wafer from the process station  150  and instructs the robot  132  place an unprocessed wafer from the holding station  142  onto the process station  150  as soon as the processed wafer is removed by the robot  131 . In this manner, a wafer enters the system  100  for processing about every ten to fifteen seconds. 
     Furthermore, all other wafer handling, atmospheric and vacuum, such as, alignment and orientation is performed as a background operation in parallel to the processing of a wafer on the process station  150 . After processing, the processed wafer is placed directly into one of the load locks  120   a  through  120   d,  which is vented and opened to atmosphere to allow removal of the processed wafer. 
     System  100  advantageously provides single-wafer vacuum entry and exit, and single-wafer processing. That is, one wafer rather than a batch of wafers is at risk at one time, thus providing greater flexibility in the selection and control of process variables such as beam incident angle, and cycle times at the various steps in the process. Further, system throughput does not depend on lot size as is the case when wafers are introduced into the vacuum system in batches of twenty-five. Furthermore, single wafer processing allows for the internal dimensions of each load lock to be minimized so that the internal volume of each load lock is significantly less than a load lock constructed to receive more than one wafer. The result of minimizing the internal volume of each load lock is shorter cycle times for pumping down and venting of each load lock due to the smaller volume to evacuate. This improvement in pump down and vent cycle times of the load locks by the system  100  is realized without increasing the risk of wafer contamination, because fast vacuum cycling of each load lock is not required. Moreover, single wafer processing is extremely suitable for process development work where small lot sizes and fast turnaround is more economical. Consequently, system  100  is able to provide significant advantages without compromising the throughput or particle contamination performance of the system. 
     The nature or configuration of the process station  150  is dependent in part upon the vacuum process being employed. For example, in an ion implantation implementation, the process station  150  can include an electrostatic chuck or platen that clamps the wafers using only backside contact. To aid in transferring wafers, the process station  150  optionally includes three lift pins that are actuated by a mechanism below the surface of a platen. In operation, a wafer is extended over the electrostatic chuck by one of the robots  131 ,  132  and the lift pins are raised. The selected robot is retracted and the pins are lowered. When the electrostatic chuck senses the presence of the wafer, the chuck applies a clamping voltage to ensure secure clamping. Having secured the wafer, the electrostatic chuck is tilted as appropriate and moved so as to traverse the wafer through a ribbon-shaped or scanned ion beam to accomplish uniform ion implantation. After implantation, the chuck returns to the start position, unclamps the wafer, the pins are raised, and the robot that loaded the platen with the wafer unloads the wafer from the process station. With the robot under the wafer, the pins are lowered to transfer the wafer onto an arm of one of the robot. 
     In accordance with a goal of achieving high wafer throughput, for example greater than 250 wafers per hour, without excessively rapid pumping, venting, or fast robotic motions in vacuum, the following exemplary process flow illustrates the hardware and process steps to achieve this end. Consider the case of a system capable of processing 240 wafers per hour. The total cycle time per wafer through the system  100  is about sixty seconds. Those skilled in the art will recognize that this cycle time is merely illustrative and may be shortened resulting in yet higher throughput, but the use of sixty seconds facilitates the discussion below. 
     It is undesirable for any other steps to intrude on this timing, or the process time is curtailed possibly resulting in a limited throughput of the system. Other time periods that need to be as long as possible are the periods for venting and pump down of each of the load locks  120   a  through  120   d.  A time between ten and twenty seconds is desirable for each of these operations, to minimize particulate contamination of a wafer. Furthermore, when each of the load locks  120   a  through  120   d  is opened to atmosphere, the robot  110  in atmosphere collects a wafer, returns it to its slot and collects another wafer from a different location, sometimes in a different cassette or FOUP, and places the just collected wafer in the recently vacated load lock, requiring a time of between six and fourteen seconds. Accordingly, to maximize efficiency, it is desirable to use four or more load locks. 
     Indeed, clean pumping and venting is facilitated by minimizing the load lock volume, so the use of single-wafer load locks is desirable. Using an estimated time budget of about forty seconds for each of the load locks  120   a  through  120   d  to complete the pumping and venting cycle, the vent time and the pumpdown time for each of the load locks  120   a  through  120   d  is about sixteen seconds for each operation, and the atmospheric load and unload time by the robot  110  is about ten seconds. 
     The time periods of the pump down and vent operations can be reduced to increase throughput, but at the expense of less clean processing. Clean processing requires minimum disturbance of any particulate contamination in each of the load locks  120   a  through  120   d  and avoidance of the creation of condensation by too rapid quasi-adiabatic expansion of moist air in each of the load locks  120   a  through  120   d  during pump down. As such, it becomes clear that each of the load locks  120   a  through  120   d  should be as small as reasonably possible, and for a 300 mm wafer, with a 1 cm clearance on all sides, the volume of each of the load locks  120   a  through  120   d  is estimated to be about two liters. 
     It is a further advantage of the system  100  that each of the load locks  120   a  through  120   d  can be opened after the end of every process step, several seconds before the next processing step begins. This tends to make the pressure in the system identical during every process step, removing a source of systematic process variation that occurs in other process flows. 
     In one exemplary embodiment of the system  100 , the load locks  120   a  through  120   d  are physically arranged in two stacks of two load locks. The robot  110  moving on the track  111  in atmosphere can address both stacks and all FOUPs  104   a  through  104   d,  or cassettes. The robot  110  can include a vacuum chuck, and is capable of relatively high speed movement. Each of the robots  131  and  132  operate independently of each other in vacuum. The robot  131  accesses a first pair of load locks, one of the handling stations  141 ,  142 , and the process station  150 . The robot  132  accesses a second pair of load locks, the other of the handling stations  141 ,  142 , and the process station  150 . Those skilled in the art will recognize that is possible to arrange and stack more than two pair of load locks. Moreover, because the robot  110  in atmosphere can use a vacuum chuck to securely hold the wafers by the backside, it is permitted to use high accelerations and fast motions. Thus a single robot may be sufficient to meet throughput requirements, but more than one robot  110  can be used if desired. 
       FIG. 2  is referred to below to discuss an exemplary set of wafer transport paths. To facilitate the discussion below the exemplary set of wafer transport paths focuses on the FOUP  104   a  holding one or more wafers located at the docking station  101   a,  the load locks  120   a  and  120   b,  the first vacuum robot  131 , and the holding station  141 . In practice, a FOUP may hold up to twenty-five wafers in a vertical array and as such, the exemplary set of wafer transport paths are repeated twenty-five times until all wafers in the FOUP have completed processing. Nonetheless, those skilled in the art will recognize that the details discussed below apply equally to an exemplary set of wafer transport paths for the FOUPs  104   b  through  104   d  loaded into the docking stations  101   b  through  101   d.    FIG. 3  discussed below further illustrates the exemplary set of wafer transport paths as applied to the load locks  120   a  through  120   d.  For the ease of the discussion below, the exemplary wafer transport paths employ the operation of load locks  120   a,    120   b,  first vacuum robot  131 , and first handling station  141 . Nevertheless, those skilled in the art will recognize that the discussion of the exemplary set of wafer transport paths are equally applicable to the operation of load locks  120   c  and  120   d,  the second vacuum robot  132 , and the handling station  142 , because each grouping of wafer handling devices operate in a concurrent manner. 
     Moreover, while the wafers from the FOUP  104   a  in the docking station  101   a  are processed, other FOUPs can be inserted or removed from each of the docking stations  101   b  through  101   d,  to maintain a continuous or near continuous supply of unprocessed wafers. Such operations can occur with flexible scheduling in background to the sequence of operations described below. Each new load of wafers will in due course be processed by a similar sequence of steps to that described below. After the last wafer from one location has been removed, the next load of wafers can be removed from a selected one of the docking stations  101   b,    101   c,  or  101   d,  in a similar sequence. For convenience in tracking wafers, each wafer removed can be returned to the slot in the cassette or FOUP from which it was removed. 
       FIG. 3  illustrates an exemplary timing diagram for the system  100  that illustrates the parallel processing of wafers so that each subsequent wafer lags an immediately preceding wafer by about ninety degrees in cycle phase through the process. As such, the load locks  120   a  through  120   d  operate in a sequential manner so that at a given point in time one load lock is open to vacuum, one load lock is open to atmosphere, one load lock is venting, and one load lock is pumping down. That is, about every twelve to fifteen seconds, the next load lock in the sequence carries out the same step as its predecessor. The time for each of these steps is not required to be equal, but each load lock goes through this same sequence repetitively, recommencing the full cycle at equally phased intervals. Thus, it follows that every operation in the sequence is repeated about every twelve to fifteen seconds later. 
     The actions of the robots  131 ,  132 , the holding stations  141 ,  142  and the load locks  120   a  through  120   d  alternate between right and left, where load locks  120   a  and  120   b,  robot  131 , and holding station  141  are classified as left in the system  100 , and load locks  120   c  and  120   d,  robot  132 , and holding station  142  are classified as right in the system  100 . Furthermore, a wafer introduced into a first load lock on the left side of the system  100  is removed after processing from a second load lock on the left side of the system  100 . That is, a wafer loaded into the load lock  120   a  at atmosphere for processing is unloaded from the load lock  120   b  at atmosphere when the processing is complete. 
     The same holds true for the right side of the system  100 . For example, an unprocessed wafer loaded into load lock  120   c  at atmosphere and subsequently processed, is unloaded from load lock  120   d  at atmosphere at the completion of the processing. In this manner, it is possible to have a load lock on each respective side of the system  100  open to the vacuum chamber awaiting exchange of a wafer. This ensures that the load locks are always pumped down or vented with a wafer. Moreover, this allows each load lock to realize a time of about thirty seconds per wafer to vent, exchange a processed wafer for an unprocessed wafer and pump down to high vacuum. This allotted cycle time of about fifty to sixty seconds helps to maintain the requisite level of cleanliness in each load lock to prevent particulate contamination of a wafer during pump down and venting. The fifty second cycle time allots about fifteen seconds for venting, ten seconds for wafer exchange at atmosphere, and about fifteen for pump down with the remaining time used for system pressure equalization. 
     As such, those skilled in the art should recognize and understand the process flow and timing for multiple wafers based on the description of the process flow and timing discussed in connection with  FIGS. 2 and 3 . To illustrate the concurrent and parallel operations of the system  100 , the discussion begins at a point in the process after a number of wafers from the FOUP  104   a  or cassette holding a lot of wafers have completed processing (i.e., steady state). It is understood that at initiation and termination of a lot of wafers by the system  100  certain operations or components may be idle or become idle. The path of each sequential wafer in the lot can be determined by adding an appropriate multiple of about fifteen seconds, and applying the below rules, because each preceding wafer in the lot follows a similar sequence. In the following discussion the origin of the timing is taken as the moment the load lock valve  121   a  for the load lock  120   a  begins to close. 
     The robot  110  collects a wafer from a slot in the FOUP  104   a  in the docking station  101   a  and introduces it into the load lock  120   a.  At about time zero (0) seconds, the atmospheric valve  121   a  closes, and the load lock  120   a  is roughed down to a pressure of around 1 Pa. At about time twelve (12) seconds, the vacuum valve  122   a  opens to a high vacuum (for example 1×10 −3  Pa), causing a brief rise in pressure in the vacuum chamber  160  lasting about one to four seconds. It will be seen that the timing of this event is often considered important. At about time sixteen (16) seconds the robot  131  begins to fetch the wafer from the load lock  120   a,  and at about time twenty-two (22) seconds the robot  131  places the wafer on the holding station  141 . The holding station  141  determines the correct orientation of the wafer. 
     While the holding station  141  determines the correct orientation of the wafer, at about time twenty-three (23) seconds, the robot  131  begins to remove an already processed wafer from the process station  150 , where its processing just completed. The processed wafer is placed in the load lock  120   a  by the robot  131  since it was loaded as an unprocessed wafer from load lock  120   b.  At about time twenty-four (24) seconds, the vacuum valve  122   d  opens. Those skilled in the art will recognize that because of the above timing any pressure burst in the vacuum chamber  160  occurs after the processing of a wafer has completed, and before the processing of the next wafer commences. Further, at about time twenty-four (24) seconds, the robot  132  commences placing another wafer onto the process station  150  from the holding station  142 . 
     The robot  131  removes the first wafer from the holding station  141  at about time twenty-five (25) seconds, and is ready to place that wafer onto the process station  150 . At about time thirty-one (31) seconds, the robot  132  begins to remove the processed wafer from the process station  150 . At about time thirty-two (32) seconds, the vacuum valve  122   b  opens to introduce a new wafer to the vacuum chamber  160 , causing a pressure burst in the vacuum chamber  160 . With a minimum of delay, the robot  131  commences to move the first wafer from holding station  141  onto the process station  150 , following the removal of the processed wafer from the process station  150  by the robot  132 . Robot  131 , after placing the first wafer onto the process station  150 , removes the new wafer from the load lock  120   b  and places the new wafer onto the holding station  141  for orientation. At about time thirty-three (33) seconds, processing of the first wafer commences, and stops at about time thirty-eight (38) seconds. At about time forty (40) seconds, the robot  131  commences removal of the now processed first wafer from the process station  150 . At about time forty-one (41) seconds the robot  131  places the first wafer into load lock  120   b  and the vacuum valve  122   b  closes at about time forty-two (42) seconds. Venting of the load lock  120   b  occurs from about time forty-two (42) seconds through about time fifty-four (54) seconds. The atmospheric valve  121   b  opens as soon as the pressure equalizes in the load lock  120   b.  The robot  110  in atmosphere picks the initially processed wafer out of the load lock  120   b  and returns that wafer to the slot in the FOUP  104   a  from which it came, completing the operation at about time fifty-seven (57) seconds. 
       FIG. 3  illustrates an exemplary timing diagram for the present invention, beginning at a point in time T 0  after a number of wafers from a lot of wafers have cycled through system  100  for wafer processing (i.e., steady state). As such, at time T 0 , first load lock  120   a  is at atmosphere for the loading of a wafer  10 , second load lock  120   b  is at vacuum loading a wafer  6  (which entered the system from load lock  120   a ) from process station  150 , third load lock  120   c  is roughing down with wafer  9  for processing, fourth load lock  120   d  is venting with a wafer  5  (which entered the system from load lock  120   c ) that has completed processing. As such, robot  131  picks wafer  6  from process station  150  and loads it into load lock  120   b  for return to atmosphere. Robot  132  picks a wafer  7  off of holding station  142  and places wafer  7  onto process station  150  for processing once robot  131  removes wafer  6  therefrom. Wafer  7  was brought to vacuum in load lock  120   d.  While the load locks and robots perform their respective operations, holding station  141  orients a wafer  8  previously loaded by robot  131  from load lock  120   b  and holding station  142  is idle waiting for wafer  9  to reach vacuum. The process station  150  is idle for the unloading of wafer  6  and the loading of wafer  7 . All even numbered wafers come from load locks  120   a  and  120   b , while odd numbered wafers come from load locks  120   c  and  120   d.    
     Subsequently, at time T 1 , the following operations occur in system  100 . Load lock  120   a  begins roughing down with the wafer  10 , load lock  120   b  vents with wafer  6 , load lock  120   c  opens at vacuum for the removal of wafer  9 , and load lock  120   d  opens at atmosphere for unloading of wafer  5 . In parallel to the operations of the load locks, robot  131  removes wafer  8  from transfer station  141  and idles waiting to place it onto process station  150 , and robot  132  picks wafer  9  from load lock  120   c , and loads it onto holding station  142 . Holding station  141  idles while wafer  8  is removed, and holding station  142  idles to receive wafer  9 . Process station  150  processes wafer  7 . 
     Thereafter, at time T 2 , the following operations are carried out in the system  100 . Load lock  120   a  opens at vacuum for the removal of wafer  10 , load lock  120   b  opens at atmosphere for removal of wafer  6 , load lock  120   c  is open at vacuum awaiting the loading of wafer  7  from process station  150 , and load lock  120   d  is open at atmosphere for the loading of wafer  11 . At T 2 , robot  131  places wafer  8  onto process station  150 . Before robot  131  places wafer  8  onto process station  150 , robot  132  removes wafer  7  from process station  150  and loads wafer  7  into load lock  120   c  for return to atmosphere. Holding station  141  remains idle awaiting the next wafer and holding station  142  orients wafer  9 . Process station  150  idles to remove wafer  7  and receive wafer  8 . 
     Further, at time T 3 , the following operations are carried out in system  100 . First load lock  120   a  is open to vacuum and robot  131  is unloading wafer  10 , load lock  120   b  is open to atmosphere for the loading of a wafer  12 . Load lock  120   c  is venting with wafer  7 , and load lock  120   d  is roughing down with wafer  11 . As the load locks are operating, robot  131  picks wafer  10  from load lock  120   a  and places it onto holding station  141 , while robot  132  removes wafer  9  from holding station  142 , and idles waiting for the processing of wafer  8  to complete. Holding station  141  idles to receive wafer  10  from robot  131  and holding station  142  idles for the removal just oriented wafer  9 . Process station  150  processes wafer  8 . 
     At time T 4 , system  100  carries out the following operations. First load lock  120   a  receives wafer  8  at vacuum from process station  150  via robot  131 , second load lock  120   b  roughs down with wafer  12 , load lock  120   c  opens at atmosphere for the unloading of wafer  7 , and fourth load lock  120   d  opens at vacuum for the unloading of wafer  11 . Robot  131  picks wafer  8  from process station  150 , and loads first load lock  120   a  with wafer  8 . Robot  132  places wafer  9  onto process station  150  for processing. While robots  131  and  132  handle wafers, holding station  141  orients wafer  10  and handling station  142  idles for the removal of wafer  9 . Process station  150  idles for the unloading of wafer  8  by robot  131  and the loading of wafer  9  by robot  132 . 
     At time T 5 , the following operations are carried out in system  100 . Load lock  120   a  vents with wafer  8 , load lock  120   b  opens at vacuum for the removal of wafer  12 , load lock  120   c  is open at atmosphere for the loading of wafer  13 , and load lock  120   d  is open at vacuum unloading wafer  11 . At time T 5 , robot  131  unloads the wafer  10  from holding station  141  and waits for process station  150  to process wafer  9 . Robot  132  picks wafer  11  from load lock  120   d  and places it onto holding station  142  for orientation. Holding station  141  idles for the removal of wafer  10  and holding station  142  idles to receive wafer  11 . Process station  150  processes wafer  9 . 
     Further, at time T 6 , first load lock  120   a  opens at atmosphere for unloading for wafer  8 , load lock  120   b  remains open at vacuum awaiting the unloading of wafer  12 , load lock  120   c  is roughing down with wafer  13 , and load lock  120   d  is open at vacuum for the loading of wafer  9  from process station  150 . Correspondingly, second robot  132  picks wafer  9  from process station  150 , and loads it into load lock  120   d . Robot  131  loads wafer  10  onto process station  150  after robot  132  removes wafer  9  therefrom. Holding station  141  idles awaiting the loading of wafer  12  and holding station  142  orients wafer  11 . Process station  150  idles for the removal of wafer  9  and the placement of wafer  10 . 
     Subsequently, at time T 7 , system  100  carries out the following steps. Load lock  120   a  is open at atmosphere for the loading of wafer  14 , load lock  120   b  is at vacuum unloading wafer  12 , load lock  120   c  opens at vacuum with wafer  13 , and load lock  120   d  vents with wafer  9 . Robot  131  picks wafer  12  from load lock  120   b  and loads it onto holding station  141 . Robot  132  unloads wafer  11  from holding station  142 , and idles awaiting the completion of processing of wafer  10  on process station  150 . Holding station  141  idles to receive wafer  12  and holding station  142  idles for the removal of wafer  11 . Process station  150  processes wafer  10 . 
     System  100 , at time T 8 , carries out the following steps. Load lock  120   a , containing wafer  14 , roughs down. Load lock  120   b  is open at vacuum for the loading of wafer  10  from process station  150 , load lock  120   c  is open at vacuum for the removal of wafer  13 , and load lock  120   d  opens at atmosphere for the unloading of wafer  9 . Robot  131  picks wafer  10  from process station  150  and loads it into load lock  120   b  for venting to atmosphere. Robot  132  loads wafer  11  onto process station  150  once wafer  10  is removed therefrom. Holding station  141  orients wafer  12  while holding station  142  idles awaiting the loading of wafer  13  from load lock  120   c . Process station  150  idles for the removal of wafer  10  and the placement of wafer  11 . 
     System  100  continues in the above-detailed sequence of steps or operations until each wafer in the lot of wafers loaded into a selected docking station has been processed by and through the system. Nevertheless, those skilled in the art will recognize that system  100  can perform the above-described steps and operations in a continuous or near-continuous manner so long as an adequate supply of wafers for processing is provided to the system. That is, a number of docking stations, such as four, can be associated with the system to hold subsequent lots of unprocessed wafers for processing by the system. Moreover, the above discussion details that advantageous pairing of load locks to provide a load lock that is empty and open at vacuum awaiting the receipt of a processed wafer. The ability to provide a load lock open at vacuum waiting to receive a processed wafer as soon as processing is complete allows system  100  to realize a beneficial reduction in wafer processing cycle time. 
     While the present invention has been described with reference to an illustrative embodiment thereof, one skilled in the art will appreciate that various changes in form and detail may be made without parting from the intended scope of the present invention as defined in the pending claims. For example, robots  131  and  132  might be placed one above the other, or even integrated onto a common center to act as a robot with two concentric independent articulating arms. Further, load locks  120   a  through  120   d  need not be disposed in a straight line, but could be stacked two upon two. Furthermore, holding stations  141  and  142  need not be as shown, but could be located in any suitable position, including integrating them within each of robots  131  and  132 .