Abstract:
A substrate processing system includes a first load lock, a process chamber having a first opening to allow an exchange of a substrate between the first load lock and the first process chamber, first rollers in the process chamber; and second rollers in the first load lock, wherein the first rollers and the second rollers are configured to transport a substrate thereon through the first opening between the first load lock and the process chamber. At least some of the first rollers and the second rollers are idler rollers.

Description:
The present application is a Continuation-in-Part (CIP) Patent Application of and claims priority to commonly assigned U.S. patent application Ser. No. 11/847,956, entitled “Substrate processing system having improved substrate transfer system”, filed by the same inventors on Aug. 30, 2007, now U.S. Pat. No. 7,806,641, the disclosures of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This application relates to an apparatus for depositing material on a substrate, etching substrate, and thermal processing in a vacuum or reduced pressure environment. 
     BACKGROUND 
     Material deposition and other substrate processing are widely used in window glass coating, flat panel display manufacturing, coating on flexible films (such as webs), hard disk coating, industrial surface coating, semiconductor wafer processing, photovoltaic panels, and other applications. Materials are sputtered off a target or vaporized from a source and deposited on a substrate. Materials can also be deposited by chemical vapor deposition method. A deposition process may include the following steps: a substrate can be placed in the process chamber. The chamber is evacuated and back filled with a desirable gas. The deposition is carried out. The process chamber can be vented before the substrate is taken out. The evacuation of the process chamber can take much time and limit the throughput of the deposition system. In addition, residues of the atmosphere or reaction with atmosphere can also affect the quality of deposited material. 
     To address the issues of throughput and environment control, one can employ a load lock in connection with a processing chamber. A substrate can be loaded in the load lock in an atmospheric environment. The load lock is then evacuated to vacuum or to reach conditions similar to the process chamber. The substrate is then transported by a transport mechanism into the process chamber to conduct processing steps such as cleaning, etching, and deposition. The substrate is then removed from the process chamber to the load lock. Commercial vacuum processing equipment can include the following types: single load lock and single process chamber, in-line system with one load lock at each end of the system, cluster tool with one or more central substrate transfer modules. The single load lock and single process chamber system requires an active transportation mechanism to carry entire substrate over a long distance without mechanical support and can not carry heavy substrate. In addition, most single load lock and single process chamber system can only load one substrate at a time and have limited throughput. The in-line system requires two load locks and an active transport system either inside vacuum or through the vacuum envelope. The cluster tool requires one or more large central transfer modules to actively transport the work pieces. When several process chambers are present, the transport mechanism is required to move the work pieces between various process stations. The transport mechanism is required not to introduce gas or air to the vacuum or controlled environment. The transport mechanism can use low power and low out-gassing vacuum motors, or use bellows or seals to transfer motion from outside the vacuum. The typical moving mechanism carries the entire substrate and typically requires heavy and expensive mechanism. The seals or bellows have limited lifetime and are also expensive. The active motors require constant monitoring and are a major cause of failures. The complexity of the aforementioned active transport mechanisms increases the system cost and reduces reliability. 
     Therefore there is a need for a simple and reliable system for deposition or other vacuum processing operations having desired throughput and deposition quality. 
     SUMMARY 
     In one aspect, the present invention relates to a substrate processing system that includes a first load lock; a process chamber having a first opening to allow an exchange of a substrate between the first load lock and the first process chamber, wherein the first load lock is fixed to the process chamber throughout operation; first rollers in the process chamber; second rollers in the first load lock; and a first transport mechanism that can move the second rollers in translational movements relative to the first load lock, wherein the first rollers and the second rollers can transport a substrate thereon through the first opening between the first load lock and the process chamber, wherein at least some of the first rollers and the second rollers are idler rollers. At least one of the first load lock and the process chamber can tilt to a first tilt angle to allow the substrate to move under gravitation on the first rollers and the second rollers from the first load lock to the process chamber, and wherein at least one of the first load lock and the process chamber can tilt to a second tilt angle to allow the substrate to move under gravitation on the first rollers and the second rollers from the process chamber to the first load lock. 
     Implementations of the system may include one or more of the following. The second rollers can include a first stack of second rollers and a second stack of second rollers at a different height from the first stack of rollers. The first stack of second rollers and the second stack of second rollers can carry two different substrates. The first transport can move the first stack of second rollers in translational movements relative to the first load lock, wherein the first rollers and the first stack of second rollers can transport the substrate substantially through the first opening between the first load lock and the process chamber. The first transport mechanism can move the second stack of rollers to substantially the same height as the first rollers, and wherein the first rollers and the second stack of rollers are configured transport the substrate substantially through the first opening between the first load lock and the process chamber. The substrate processing system can further include a second transport mechanism can move the substrate on the first rollers and the second rollers. At least one of the first load lock and the process chamber can tilt to allow the substrate to move under gravitation on the first rollers and the second rollers, wherein the tilt angle is below 10 degrees relative to the horizontal direction. The substrate processing system can further include a third transport mechanism, wherein at least one of the first rollers and the second rollers is an active roller that can be rotated by the third transport mechanism. The process chamber can include at least one deposition source that can provide a material to be deposited on the substrate. The first load lock can include at least one deposition source that can provide a material to be deposited on the substrate. The substrate processing system can further include a gate that can close the first opening to isolate the interior of the first process chamber from the interior of the first load lock, wherein the first load lock comprises an opening through which the substrate is loaded into the first load lock therein or unloaded from the first load lock therein. The substrate processing system can further include a second load lock that can connect with the process chamber through a second opening in the chamber wall; and third rollers in the second load lock, wherein the second rollers and the third rollers can transport a substrate thereon through the second opening between the second load lock and the process chamber. The first load lock and the second load lock can be disposed on opposite sides of the process chamber. The third rollers can include a first stack of rollers and a second stack of rollers at a different height from the first stack of rollers. The first load lock, the process chamber, and the second load lock can tilt to allow the substrate to move under gravitation on at least one of the first rollers, the second rollers, and the third rollers. 
     In another aspect, the present invention relates to a substrate processing system that includes a first load lock; a process chamber having a first opening to allow an exchange of a substrate between the first load lock and the first process chamber, wherein the first load lock is fixed to the process chamber throughout operation; first rollers in the process chamber; a first stack of second rollers in the first load lock; a second stack of second rollers in the first load lock, wherein the second stack of second rollers are at a different height from the first stack of rollers, wherein the first rollers and the first stack of the second rollers can transport a first substrate thereon through the first opening between the first load lock and the process chamber, and wherein the first rollers and the second stack of the second rollers can transport a second substrate thereon through the first opening between the first load lock and the process chamber; a first transport mechanism that can move the first stack of second rollers and the second stack of second rollers relative to the first load lock, wherein the first transport mechanism can move the first stack of second rollers and the second stack of second rollers in translational movements relative to the first load lock, wherein at least one of the first load lock and the process chamber can tilt to allow the substrate to move under gravitation on at least one of the first rollers and the second rollers; and a third transport mechanism, wherein at least one of the first rollers, the first stack of second rollers, and the second stack of the second rollers is an active roller that can be rotated by the third transport mechanism. 
     In another aspect, the present invention relates to a substrate processing system that includes a first load lock; a first process chamber having a first opening to allow an exchange of a substrate between the first load lock and the first process chamber, wherein the first load lock is fixed to the process chamber throughout operation; a second process chamber having a second opening to allow an exchange of the substrate between the first process chamber and the second process chamber; a second load lock having a second opening to allow an exchange of the substrate between the second load lock and the second process chamber; first rollers in the first load lock, wherein the first rollers can be moved in translational movements relative to the first load lock; second rollers in the first process chamber, wherein the first rollers and the second rollers can transport the substrate thereon through the first opening between the first load lock and the first process chamber; third rollers in the second process chamber, wherein the second rollers and the third rollers can transport the substrate thereon through the second opening between the first process chamber and the second process chamber; and fourth rollers in the second load lock, wherein the third rollers and the fourth rollers can transport the substrate thereon through the third opening between the second process chamber and the second load lock, wherein at least some of the first rollers, the second rollers, the third rollers, and the fourth rollers are idler rollers, wherein the first load lock, the first process chamber, the second process chamber, and the second load lock can tilt to allow the substrate to move under gravitation on at least one of the first rollers, the second rollers, the third rollers, and the fourth rollers. 
     Embodiments may include one or more of the following advantages. The disclosed systems and methods can reduce system complexity and manufacturing cost comparing to some convention systems. For example, a single load lock chamber can continuously feed the process chamber with substrates without delay. A substrate in the process chamber in the disclosed substrate processing system can be transported on rollers by a small force or by gravitation applied to the substrate. The rollers are rotated by the movement of the substrate instead of being directly driven by a heavy and complex motorized transport mechanism (e.g. active transport mechanism) as in some conventional systems. The removal of such active transport systems for the substrates can also reduce the size and footprint of the substrate processing systems. 
     Another advantage of the disclosed system is that the substrate mechanism is more reliable compared to some conventional systems that include bellows or sealing components which are prune to failures. The substrate carried by rollers can reliably be moved by a small force or by gravitation in the disclosed substrate processing system. 
     Another advantage of the disclosed system is that the contamination related to motorized mechanisms in some conventional systems can be reduced or eliminated by using a cleaner gravity driven transport mechanism for the substrates. 
     The disclosed systems and methods can also improve throughput by using a continuous workflow between an input load lock and an output load lock. 
     The details of one or more embodiments are set forth in the accompanying drawings and in the description below. Other features, objects, and advantages of the invention will become apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a substrate processing system having an improved substrate transport system and a process chamber. 
         FIG. 1B  is a cross-sectional perspective view of the improved substrate transport system of  FIG. 1A . 
         FIG. 1C  is a perspective view of the substrate processing system of  FIG. 1A  with deposition sources or other processing sources installed in load lock and process chamber. 
         FIG. 1D  is a side cross-sectional view of the substrate processing system having the improved substrate transport system of  FIG. 1A . 
         FIG. 1E  is a side cross-sectional view of the substrate processing system of  FIG. 1A  illustrating the loading of a first substrate in the load lock. 
         FIGS. 1F and 1G  are side cross-sectional views of the substrate processing system of  FIG. 1A  illustrating the loading of the first substrate in the process chamber. 
         FIG. 1H  is a side cross-sectional view of the substrate processing system of  FIG. 1A  showing that a second substrate is loaded in the load lock while the first substrate is in the process chamber. 
         FIG. 1I  is a side cross-sectional view of the substrate processing system of  FIG. 1A  showing that the removal of the first substrate from the process chamber. 
         FIGS. 1J and 1K  are side cross-sectional views of the substrate processing system of  FIG. 1A  illustrating the loading of a second substrate from the load lock into the process chamber. 
         FIG. 2  is a cross-sectional view of another substrate processing system having an improved substrate transport system and a process chamber. 
         FIG. 3A  is a cross-sectional view of another substrate processing system having an improved substrate transport system and a process chamber while the substrate processing system is tilted to one direction. 
         FIG. 3B  is a cross-sectional view of the substrate processing system in  FIG. 3A  while the substrate processing system is tilted to another direction. 
         FIG. 4  is a cross-sectional view of another substrate processing system having an improved substrate transport system and a multiple of process chambers. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1A-1K , a substrate processing system  100  includes a load lock  110  and a process chamber  120 . The process chamber  120  can be evacuated during material deposition and other processing steps such as cleaning, etching etc. The substrate processing system  100  can also include sources  150  for providing materials to be deposited or processed on a substrate  140 ,  141 . The process chamber  120  can include proper electrodes for establishing DC or AC voltages between the substrate and the other components in the process chamber  120  (e.g. the chamber wall or a backing plate on the target). A vacuum pump can be connected to the process chamber  120  to produce a vacuum environment around the substrate  140 ,  141  during processing operations. The sources  150  can include one or more targets and one or more magnetron sources. The sources  150  can also be shower head for distributing chemicals, ion source for substrate cleaning, heating or cooling source. The interiors of the load lock  110  can be connected to the interior of the process chamber  120  by openings  121 . The opening  121  can be closed to isolate the interiors of the load lock  110  from the interior of the process chamber  120  by an isolation valve  122 . The load lock  110  can optionally include a source  151  for deposition or other processing operations. 
     A transport system for transporting a substrate  140 ,  141  ( FIG. 1E  to  FIG. 3B ) can include a plurality of rollers  125 ,  203  mounted in the process chamber  120 , and a plurality of rollers  115 ,  201 ,  116 , and  202  mounted in the load lock  110 . The rollers  125 ,  115 ,  116  are idler rollers that are not rotated by an active transport mechanism such as motors or motor driven conveyance belt as used in some conventional systems. The idler rollers can be rotated around an axis or a point. The rollers  125 ,  115 ,  116  rotate passively following the movement of a substrate placed on the rollers  125 ,  115 ,  116 . The rollers  201 - 203  are active rollers (or motorized rollers) that are rotated by a drive mechanism  205  (not shown I  FIGS. 1E-3B  for clarity of the drawings). The positions of the rollers  201 - 203  are shown to be next to the opening  121  for illustration propose. The rollers  201 - 203  can also be located elsewhere that can move the substrates  140 ,  141  through the opening. The substrate  140 ,  141  can be placed on the rollers  115 ,  201 ,  116 ,  202  or the rollers  125 ,  203  and held down the rollers by gravity, magnetic forces, or mechanical stopper near edge of the substrate. The substrate  140 ,  141  placed on the rollers  115 ,  201 ,  116 ,  202 , and  125 ,  203  can be moved by a tilt in the process chamber  120  or the load lock  110  (as described in relation to  FIG. 2-3B  below). The substrate  140 ,  141  placed on the rollers  115 ,  201 ,  116 ,  202 , and  125 ,  203  can also be moved by a push or a pull by a mechanism  127  (as described below). The mechanism  127  can be either motorized or manually operated by an operator. The force required to move substrate  140  and  141  is small and the mechanism  127  can also be supported by rollers  125 ,  203 . The mechanism  127  is therefore simpler and more reliable than conventional substrate transfer mechanism. The substrate  140 ,  141  can be guided by a guard rail during the movement. In the present specification, the term “roller” is used to refer to a mechanism that can rotate and allow a substrate to move thereon. A “roller” can include a roller, a ball that can rotate freely, a wheel, and other mechanisms. 
     In some embodiments, the substrates  140 ,  141  can be moved by the active rollers  201 - 203  while the substrates are also supported by the rollers  115 ,  201 ,  116 ,  202 , and  125 ,  203 . The surfaces of the active rollers  201 - 203  can provide proper friction to drive the substrates  140 ,  141 . The active rollers  201 - 203  can serve several functions: When the substrate processing system  100  is substantially level, do not have enough tilt, or for some reason are stuck in the transport path, the active rollers  201 - 203  can provide the driving force to “take up”, push, or pull the substrates  140 ,  141 . When needed, the active rollers  210 - 203  can also be deactivated (or disengaged) from the drive mechanism  205  (shown in  FIG. 1D ) so that they become follower rollers to allow the substrates  140 ,  141  to be driven by gravity. 
     The load lock  110  can include an opening  131  for loading and unloading substrates. A door  135  can seal the opening  131  to keep the load lock  110  in a vacuum environment. The load lock  110  can include multiple stacks of rollers  115 ,  201 ,  116 ,  202  mounted on a rack  145 , which each can be used to carry and transport a different substrate  140  or  141 . A multiple of substrates  140  and  141  can therefore be simultaneously held in the load lock  110 , loaded or unloaded while another substrate is being processed in the process chamber  120 . The load lock  110  can include a transport mechanism  117  that can move the stacks of rollers  115 ,  201 ,  116 ,  202  in a direction  118  such that the top surface the stack of rollers  115 ,  201  or the stack of rollers  116 ,  202  can be registered to the top surface of the rollers  125 ,  203 . A substrate  140  or  141  can therefore move substantially in planar fashion from the rollers  125 ,  203  to the stack of rollers  115 ,  201  or vice versa. The mechanism  117  can also adjust the height of the rollers  116 ,  202  to be in registration to the rollers  125 ,  203  such that a substrate  140  or  141  move substantially in a plane from the rollers  125 ,  203  to the stack of rollers  116 ,  202  or vice versa. 
       FIG. 1E  shows that a substrate  140  has been loaded on the rollers  116 ,  202  inside the load lock  110 . The substrate  140  can be loaded through the opening  131  when the door  135  is open. The racking  145  on which the rollers  115 ,  201 ,  116 ,  202  are mounted can be moved vertically by the transport mechanism  117  such that the top surfaces of the stack of rollers  116 ,  202  are aligned to the inside of the opening  121  and to the rollers  125 ,  203  in the process chamber  120 . The isolation valve  122  is then opened to make the load lock  110  to be in the same vacuum environment as that of the process chamber  120 . The substrate  140  is being pulled by the mechanism  127  from the load lock  110  into the process chamber  120 , as shown in  FIGS. 1F and 1G . The mechanism  127  can engage with the substrate  140  by a magnetic attractive force between a first magnet mounted at the tip of the mechanism  127  and another magnet of opposite polarity, or paramagnetic material such as 410 Stainless Steel mounted on a holder for the substrate  140 . The low friction rolling motions of the rollers  125 ,  203 ,  116 ,  202  allow smooth and low-resistance movement of the substrate  140  into the process chamber  120 . The mechanism  127  can be driven by a low-power motor or manually by an operator. Many inexpensive commercial transportation mechanisms for vacuum systems can be used for the mechanism  127  from, for example, Transfer Engineering of Fremont, Calif. The substrate  140  is placed on the rollers  116 ,  202 . 
     After the mechanism  127  has placed the substrate  140  on the rollers  125 ,  203  in the process chamber  120 , as shown in  FIG. 1H , the isolation valve  122  is closed. The substrate  140  is ready for processing such as cleaning and material depositions. A second substrate  141  can be loaded into the load lock  110  through the opening  131  while the substrate  140  can be processed in the isolated vacuum environment in the process chamber  120 . After the second substrate  141  is placed on the rollers  116 ,  202 , the load lock  110  can be sealed and evacuated to a similar vacuum environment as that in the process chamber  120 . 
     After the processing of the substrate  140 , the isolation valve  122  can be lifted. The substrate  140  can be pushed by the mechanism  127  on the rollers  125 ,  203  and rollers  115 ,  201 . The substrate  140  is moved out of the process chamber  120  into the load lock  110  by the mechanism  127 , as shown in  FIG. 1I . 
     After the substrate  141  is placed on the rollers  115 ,  201 ,  116 ,  202 , the substrates  140 ,  141  on the rollers  115 ,  201 ,  116 ,  202  are moved upward by the mechanism  117  to align the second substrate  141  to the opening  121 . The second substrate  141  can be pulled by the mechanism  127  from the load lock  110  into the process chamber  120 , as shown in  FIG. 1J . The pulling can be implemented by an attractive magnetic force between magnets that are mounted at the end of the mechanism  127  and on the edge of a holder for the substrate  140  (or  141 ). The pulling can also be implemented by mechanically engaging the mechanism  127  to the edge of a holder for the substrate  140  (or  141 ). The mechanism  127  can be disengaged from the substrate  140  (or  141 ) by pulling the mechanism  127  while holding the substrate or holder of the substrate against a mechanical stopper. The mechanism  127  from the substrate  140  (or  141 ) can be disengaged from the substrate  140  or  141  by a stronger magnetic force mounted either inside load lock  110  or mounted on the rack  145 . 
     After the substrate  141  is placed on the rollers  125 ,  203  in the process chamber  120 , as shown in  FIG. 1K , the isolation valve  122  can shut the opening  121 . The substrate  141  is ready for processing such as material depositions in a vacuum environment. The substrate  140  can be unloaded from the load lock  110  and new substrates can be loaded into the load lock  110  through the opening  131 . The steps illustrated in the  FIGS. 1E to 1K  can be repeated to process a large number of substrates. It should be noted that the substrate  140  or  141  can be formed by a single piece, or include multiple substrates placed or held on a carrier. It should also be noted that there can be more than two stacks of rollers in the load lock to achieve the effect. 
     It should be noted that the disclosed substrate processing system is compatible with a single load lock or a multiple of load locks. For example, the substrate processing system  100  can include a single load lock  110  that handles the loading and unloading of the substrates. The substrates in the load lock can be preheated in preparation for the processing in the process chamber  120 . Furthermore, the load lock  110  can optionally include a source  151  that can allow certain process steps (e.g. deposition, cooling, etching) to be conducted in the load lock  110 . 
     In some embodiments, the substrate processing system  100  can also include a second load lock in addition to the load lock  110 . The second load lock can be positioned on the opposite side of the process chamber from the first load lock  110 . The interior of the second load lock  130  connects to the interior of the process chamber  120  another opening in the process chamber. A separate isolation valve can open or shut the second opening. A transport mechanism can move substrates between the process chamber  120  and the two load locks. In some embodiments, the load lock  110 , the process chamber  120 , and the second load lock can be arranged as a continuous workflow. The load lock  110  can be assigned to load substrates into the process chamber  120 . The second load lock can be responsible for unloading substrates from the process chamber  120 . The loading and unloading of the substrates as well as the processing of the substrates in the process chamber can all be conducted in parallel. The throughput of the substrate processing system can be significantly improved. Similar to the load lock  110 , the second load lock can include stacks of rollers that can hold and transport a multiple of substrates. 
     In some embodiments, the disclosed substrate processing system can be tilted to one direction to create a bias. For example, as shown in  FIG. 2 , the substrate processing system  200  can be tilted toward the process chamber  120 . Because the gravitational force constantly pulls the substrate  140  toward the process chamber  120 , the transport mechanism can push the substrate  140  left upward into the load lock  110 . One or more stoppers can be included in the load lock  110  to hold the substrate  140  in place once the substrate is in the load lock  110 . The mechanism  127  can be withdrawn from the process chamber  120  to allow the isolation valve  122  to shut and the substrate to receive processing steps. After processing of the substrate, the isolation valve  122  can open. The mechanism  127  can push the substrate out of the process chamber  120  and into the load lock  110 . 
     In some embodiments, the disclosed substrate processing system can be tilted to in a plurality of directions to move the substrates by gravitational force. For example, as shown in  FIGS. 3A and 3B , the substrate processing system  300  can be tilted toward the load lock  110  ( FIG. 3A ), or away from the load lock  110  ( FIG. 3B ). The different tilt configurations of the substrate processing system  300  can move the substrate  140  between the process chamber  120  and the load lock  110 . The substrate processing system  300  does not require a transport mechanism that directly pushes or pulls a substrate between the load lock  110  and the process chamber  120 . 
     The tilt angles of the substrate processing systems  200  and  300  can be controlled within a range, for example, less than 33 degrees, or 30 degrees, or 25 degrees, or 20 degrees relative to the horizontal direction. In the present specification, the term “horizontal” is defined as a direction perpendicular to the gravity direction. Some of the rollers in the disclosed substrate processing systems are idler rollers that are not driven by active transport mechanism such as motors. They can be passively rotated by gravity or by the movement of the substrate sitting on top. The substrate can be moved by a mechanism  127  using either a motorized or a manual mechanism. 
     It is understood that the tilting of the disclosed substrate processing system is not limited to tilt movement around a single rotation axis. For example the disclosed substrate processing system can tilt around two rotation axes to move a substrate in two dimensions in the process chamber or a load lock. For example, the substrate can be reliably positioned in two, three, four, or more locations in the process chamber or the load locks. The multiple holding locations for the substrate in the process chamber can allow multiple substrates to be processed at different process station, including in parallel processing operations. The multiple holding locations for the substrate in the load locks allow multiple substrates to be loaded and unloaded from the substrate processing system. 
     In some embodiments, the disclosed substrate processing system can include multiple process chambers. For example, as shown in  FIG. 4 , the substrate processing system  400  can have two or more interconnected a process chambers  410  and  420 , and load locks  430  and  440  for loading and unloading substrates to and from the process chambers  410  and  420 . The substrate processing system  400  can include a tilt as shown in  FIG. 4 , or operate substantially level as described in  FIGS. 1A to 1K . The substrate processing system  400  can also tilt back and forth around one or more rotation axis to move the substrates among the process chambers  410 ,  420  and load locks  430 ,  440 . Moreover, a continuous workflow can be set up from the load lock  430  to the load lock  440  through the multiple process stations provided by the process chambers  410 ,  420 . 
     It is understood that the disclosed process chamber is compatible with many different types of processing operations such as physical vapor deposition (PVD), thermal evaporation, thermal sublimation, sputtering, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), ion etching, or sputter etching. It should also be noted that the load lock in the disclosed substrate processing system can include one, two, or more than two stacks of rollers, each of which can carry and transport one or more substrates.