Abstract:
A system for transporting substrates from an atmospheric pressure to high vacuum pressure and comprising: a rough vacuum chamber having an entry valve and an exit opening; a high vacuum chamber having an entry opening, the high vacuum chamber coupled to the rough vacuum chamber such that the exit opening and the entry opening are aligned; a valve situated between the exit opening and the entry opening; a first conveyor belt provided in the rough vacuum chamber; a second conveyor provided in the high vacuum chamber; a sensing element provided in the high vacuum chamber to enable detection of broken substrates on the second conveyor; and, a mechanism provided on the second conveyor belt enabling dumping of broken substrates onto the bottom of the high vacuum chamber.

Description:
RELATED CASES 
     This Application claims priority benefit from U.S. Provisional Application Ser. No. 61/568,129, filed on Dec. 7, 2011, the disclosure of which is incorporated here by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     This invention relates to transport of substrates and, specifically, to high throughput transport of substrates from atmospheric into vacuum environment for fabrication of solar cells. 
     2. Related Arts 
     Vacuum transport of substrates has been used in the manufacture of semiconductors for many years. A typical loadlock device has one valve as entry port to receive wafers from atmospheric environment, and valve as one exit port for delivering wafers into the vacuum processing system. In many systems the entry port also used to return wafers to the atmospheric environment. In such systems, a robot arm positioned on the atmospheric side places the substrate inside the loadlock and, after vacuum is established in the loadlock, a robot arm positioned on the vacuum side of the system (e.g., a mainframe) fetches the substrate from the loadlock and places it in a vacuum processing chamber, e.g., a plasma chamber. Once processing is completed, the vacuum-side robot places the processed substrate in the loadlock, and, after establishing proper pressure, the atmospheric-side robot fetches the processed substrate out of the loadlock. While such an architecture works well for semiconductor processing, solar cell fabrication requires much higher throughput. For example, while semiconductor processing proceeds at a rate of around 60-100 wafers per hour, solar cell fabrication proceeds at the rate of 1500-2500 wafers per hour. Thus, a new loadlock architecture is needed to facilitate such high throughput. 
     Additionally, due to the high speed processing of solar cells, and the relative low cost of each individual cell as compared to a semiconductor substrate, wafer breakage is a relatively frequent and an acceptable event in solar cell fabrication, while it is not acceptable in semiconductor processing. Still, the system needs to be able to recognize and handle events of wafer breakage. This is especially the case where wafer breaks inside the vacuum environment, where no manual identification and handling of the broken wafer can be performed without breaking vacuum and disassembling parts of the system. Accordingly, improvements in systems for manufacturing of solar cells are needed to identify and handle wafer breakage. 
     SUMMARY 
     The following summary is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
     Disclosed embodiments provide loadlock architectures that enable high throughput fabrication of solar cells. Disclosed embodiments enable identification of broken substrates and disposal of these substrates. Disclosed embodiments also enable replacement of broken substrates without halting processing. 
     Disclosed embodiments provide a two-stage loadlock wherein conveyor belts are used for wafer transport inside the loadlock. The first stage is a low vacuum loadlock that is configured for rapid pumping from atmospheric pressure to rough, partial vacuum environment, referred to also as low vacuum environment. A conveyor belt is situated inside the first stage and handles transport of wafers into the first stage and out to the second stage. Provisions are made to ensure that the wafers do not move during the rapid pump from atmospheric to partial vacuum pressure inside the first stage. The second stage is a high vacuum stage and has conveyor to transfer wafers into the high vacuum stage and into the vacuum processing chamber. The second stage includes means for identifying and disposing of broken wafers. Also, an optional loader can be provided in the second stage to replace broken wafers with good wafers, so that the system can continue processing at its regular cycle. 
     Various embodiments disclosed herein provide for a low volume roughing loadlock with perforated conveyor belts, each belt having at least one vacuum plate with vacuum channels coupled to vacuum conduits, and valves provided on the conduits to control the vacuum and the pumping and venting from the plate, such that the wafers are prevented from moving during pump or vent of the loadlock. 
     Also, a high vacuum chamber is provided, which serves a vacuum transition and buffer chamber from rough vacuum to high vacuum, and also serves as a broken wafer screening and replacement chamber prior to the process chambers. In disclosed embodiments, the HVLL chamber contains a wafer imaging source such as a CIS or camera. The CIS or camera could be mounted inside or outside of vacuum using a lens or window. 
     In disclosed embodiments, the HVLL contains a wafer storage device and a wafer dump. The wafer dump removes broken wafers from the conveyors and stores them in a safe location to be disposed of later. The wafer storage is used to replace the broken wafers and keep the process flow uninterrupted. 
     In disclosed embodiments, a system is provided, wherein wafers broken in the LVLL or moved into the LVLL are moved from the LVLL to the HVLL without blocking the wafer path or slot valve. This could be done by, e.g., a conveyor that extends through the slot valve between chambers after the valve is open and retracts before the valve closes. It is better if the belt extends only from the HVLL, so as to keep the LVLL volume as small as possible. Alternatively the perforated belt in the LVLL could extend thru the valve into the adjacent chamber or in the case of the out going loadlock the belt could extend into the atmospheric handling area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects and features of the invention would be apparent from the detailed description, which is made with reference to the following drawings. It should be appreciated that the detailed description and the drawings provides various non-limiting examples of various embodiments of the invention, which is defined by the appended claims. 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
         FIG. 1  is a schematic of a two-stage linear loadlock arrangement according to one embodiment. 
         FIG. 2  is a schematic of a replacement wafer station according to one embodiment. 
         FIGS. 3A and 3B  illustrate schematic of a retractable conveyor, according to one embodiment. 
         FIG. 4  is a schematic illustrating an extended movable conveyor belt according to one embodiment. 
         FIG. 5  is a schematic illustrating a conveyor arrangement according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments disclosed herein enable high throughput of substrates from atmospheric environment into vacuum environment. The high throughput enabled by these embodiments is particularly suitable for solar cell fabrication, although it may be used for fabrication of other items, especially when there&#39;s a need to transfer the processed item from atmospheric environment into vacuum environment. Examples where transfer of substrates from atmospheric environment into vacuum environment include: chemical vapor deposition (CVD), physical vapor deposition (PVD), ion implant, etc. The disclosed two-stage linear loadlock is particularly suitable to linear fabrication systems, wherein the substrates enter the loadlock from entry port on one side and exit through exit port on the opposite side, and are not returned to atmosphere through the entry port. 
     High speed processing is susceptible to substrate breakage. In standard semiconductor processing systems, such an event requires a shut down of the system, disassembly of the system, and manual cleaning of the substrate debris. However, in solar cell fabrication environment, shutting off the system and manual cleaning of debris are unacceptable, due to the long downtime required. As an example, if a semiconductor system is shut down for one hour, production of about 60 wafers may be lost. On the other hand, if a solar system is shut down for one hour, production of up to 2500 solar cells may be lost. Such a high lost of production is highly detrimental in the highly competitive, low margins market of solar cell fabrication. Accordingly, embodiments of the invention include features that enable continued processing regardless of wafer breakage. It should be appreciates, however, that the various features disclosed can be used independently of each other, or in any combination with each other. For clarity, each disclosed embodiment may include all of the features to illustrate the synergistic function of all of the features, but the use of all of the features in a single system is not mandatory. 
       FIG. 1  illustrates a schematic of a two-stage, linear loadlock arrangement according to one embodiment. The arrangement is referred to as a two-stage system, as the pressure is reduced from atmospheric to high vacuum level in two stages. The arrangement is referred to as linear as the wafers always travel in one linear direction and never reverse direction, as is done in standard, robot arm based. Semiconductor systems. 
     In this embodiment, a first loadlock chamber  100  is a low vacuum loadlock (LVLL) chamber that cycles from atmospheric pressure to rough, low (or intermediate) vacuum pressure. Low vacuum pressure means that it is below atmospheric pressure, but higher pressure than the high vacuum level required in the processing chamber. The volume of loadlock  100  is very small and is connected to vacuum pump  138  via vacuum conduits  139  and valve (A), to enable rapid pumping down from atmospheric pressure to rough vacuum pressure. Loadlock chamber  105  is a high vacuum loadlock (HVLL) and is maintained at high vacuum pressure by, e.g., a turbo pump  102 . The vacuum level maintained in the HVLL can be commensurable with the pressure required for processing. Regardless, the volume of the HVLL is much larger than that of the LVLL, such that when a gate is opened between these two chambers, the residual pressure in the LVLL is rapidly diluted to high vacuum level of the HVLL. Effectively, as wafers traverse these two loadlock chambers they are moved from atmospheric pressure to processing pressure in two stages. 
     Substrates  101  are loaded from atmospheric environment into loadlock  100  via valve  115 . From there they are transferred into loadlock  105  via valve  120 , and then to processing chamber  122  via valve  110 . Chamber  122  is not illustrated, as it is not relevant to the structure and/or operation of this embodiment. Chamber  122  is not necessarily a processing chamber, but rather may be, for example, a buffer station, a cooling station, a heating station, etc. The idea here is that chamber  122  is maintained in vacuum at a level similar to that in loadlock chamber  105 . 
     While two substrates are illustrated inside loadlock  100  of  FIG. 1 , as can be appreciated, multiple rows of wafers can be loaded simultaneously into loadlock  100 . For instance, there could be a single-wafer per cycle, a single row of 3 wafers per cycle, two rows of three wafers for a total of six wafers per cycle, etc. The idea here is to balance transport time and pump time so as to provide the required throughput at the required cycle time. The cycle time should be calculated based on the processing time in the processing chambers, such that the chambers are always fed with fresh substrates and do not idle waiting for wafers to be transported, i.e., wafer transport should not form a bottleneck for the processing chambers. 
     To illustrate, conveyor  133  is shown with two wafers, conveyor  130  is shown with one wafer, and conveyor  131  is shown with no wafers. However, generally, the conveyors will carry the same number of wafers, so that at each cycle the exact number of wafers will be transferred from conveyor  133  to conveyor  130 , from conveyor  130  to conveyor  131 , and from conveyor  131  to processing chamber  122 . In this embodiment, each row has its own conveyor, and only one row can be seen in this perspective, and additional rows would be obscured by the illustrated conveyors. 
     Once the substrates have been transferred into loadlock  100 , the chamber is sealed and vacuum is rapidly drawn in order to obtain the required throughput. However, when high rate vacuum pumping is performed, the rapid air flow may cause the substrates to shift their position on top of the conveyor belt. This may lead to misalignment and/or breakage of wafers. Therefore, as illustrated more clearly in the callout of  FIG. 1 , according to one embodiment, a perforated flexible belt  134  having perforations  135  is used to hold the wafer(s) in place during turbulent pumping or venting of the loadlock. The perforated flexible belt  134  travels over a perforated base plate  136 , having perforations  137  on its upper surface upon which the perforated belt  134  travels. The base plate  136  has vacuum channels in its interior (not shown) leading to the perforations  137 . The perforated base plate  136  is coupled to the vacuum pump  138  via vacuum conduits  139  and valve (B), such that when valve (B) is set to vacuum, pump  138  draws vacuum through the perforated base plate  136  and through the perforated belt  134 , thus holding the wafer(s) in place. In this embodiment, the conveyor belt is stationary during pump down and during venting of the base plate. Note that venting of the base plate  136  may not be required in order to enable wafers transport onto belt  130 , since once the LVLL chamber is evacuated, the pressure equalizes such that there&#39;s no more vacuum force on the substrates. That is, after pumping the LVLL, the pressure in the LVLL will be the same as the pressure in the base plate, so that the gripping force will be gone. The gripping force only exists during the first part of the pump down. In operation, the base plate is pumped first to create a pressure delta between the rest of the LVLL volume and the base, which causes the wafer to be gripped. As the rest of the LVLL volume pumps down, the pressure delta gets smaller and smaller, as does the gripping force. Nevertheless, if venting is needed, it can be done by setting valve (B) to vent position, which may couple the base plate to atmosphere or to inactive gas, such as nitrogen, bringing the interior channels of base plate  136  to atmospheric pressure. 
     For example, when new wafers are introduced into the loadlock  100  and the valve  115  is closed, the conveyor belt valve (B) is set to vacuum for about 0.1 seconds before the chamber pump valve (A) is opened (set to vacuum position). This causes the wafers to be pressed against the belt and prevents the wafers from being moved around and possibly broken during pump down of the chamber  100 . Then, when gate  120  is opened to transfer the wafers into chamber  105 , the nitrogen vent valve (C) is opened while the conveyor belt vacuum valve (B) is still open (set to vacuum position). This holds the wafer down through the turbulent vent flow. Once the appropriate pressure is reached, the belt can be energized to move the wafer to the next stage. 
     There is a second function of the perforated belt, which is to prevent air from being trapped between the belt and the wafer. If air is trapped under the wafer, it will eventually get pumped out, e.g., during vacuum pumping, causing the wafer to slide/twist. This second function is independent of the vacuum channels in the base plate. Therefore, the feature of perforated belt can be employed even if the base plate  136  does not have vacuum channels. The vacuum pumping arrangement to base plate  136  is, therefore, optional. 
     The general process that can be followed to transfer wafers through the two-stage system is illustrated in  FIG. 1  as well, providing the reader with a single view of the hardware and process flow. In this example, the process starts after loadlock  100  has been vented, e.g., by opening valve (C) to flow nitrogen into the loadlock  100  until it reaches atmospheric pressure or by flowing air into the chamber. In this embodiment, at this stage the loadlock  100  is empty, i.e., no wafers are positioned on conveyor belt  133  (the wafers have already been transferred to loadlock  105 ). Loadlock valve  115  is then opened, and conveyor belt  133  is activated to load wafers into loadlock  100 . The valve  115  is then closed and valve (B) is set to vacuum position, so that the vacuum suction in conveyor  133  from pump  138  holds down the wafers. Valve (A) is then set to vacuum to rapidly evacuate the loadlock  100 . When the appropriate vacuum level is reached in loadlock  100 , valves  120  is opened, and the conveyors are activated to transfer wafers from each convey  133  to conveyor  130 . Then valve  120  is closed and HVLL  105  is pumped to high vacuum level. Valve  110  is then opened and the wafer is transferred from conveyor  131  into the chamber  122 . In this way, the LVLL cycles between atmospheric pressure and low vacuum level, while the HVLL cycles between low vacuum and high vacuum levels. The process chamber  122  is never exposed to low vacuum level, but rather only to the high vacuum level of HVLL. When valve  120  is closed, valve (C) can be set to vent. The process then repeats itself for the next cycle. 
     In the embodiment illustrated in  FIG. 1 , provisions are made to detect and dispose of broken wafers. A camera  140  or a contact image sensor (CIS)  142  is used to create an image of the wafer in the HVLL  105 . The image is then used to determine whether each wafer is intact or broken. If the wafer is not intact, it is dumped and, optionally, is replaced by a wafer from a wafer stocker  150  positioned over each row of wafer conveyors. As shown in the example of  FIG. 1 , the feature of dumping the broken wafer is implemented by one or both of the conveyors  130  and/or  131  having swinging capability over one or both rotational axis. This is illustrated by the curved arrows and the conveyors shown in broken lines in their wafer dump position. The wafers can be dumped into collection tray  104 , which may be emptied at regular system servicing periods. 
     Also illustrated in  FIG. 1  is the optional storage station, also referred to as stocker,  150  for replacement wafers. In this embodiment, the storage station  150  can move vertically, as shown by the double-headed arrow, so as to deposit wafers of conveyor  131  as needed. Alternatively, conveyor  131  can be moved vertically to collect replacement wafers from the stocker  150 . The wafer storage or stocker  150  is loaded with wafers  101  at the start of each run and as needed during a run. For instance, if the storage  150  holds 4 wafers over 3 rows, then the first 12 wafers that are loaded into the system are not transferred to the processing chamber, but rather are loaded into the storage as replacement wafers. Then, broken wafers can be replaced with wafers from the storage  150  without interruption of the system and without skipping a processing cycle. Again, the idea is that at each cycle the processing chamber is fed with wafers so that it is not idling. The wafers remaining in the stocker  150  would then be the last wafers processed at the end of the run, less any that were used to replace broken wafers. 
       FIG. 2  illustrates an embodiment of wafer storage over one row of the wafer conveyor  131 . The view shown in  FIG. 2  is a cross-section along line Z-Z of  FIG. 1 . The wafer storage  150  includes a housing  252  that is transportable vertically, as illustrated by the double-headed arrow. Shelves  254  are positioned inside the housing  252  to support several replacement wafers  101 . Conveyor  131  is shown as having rotating shaft  260 , rollers  262 , and flexible belt  264  riding on rollers  262 . To fill the storage with wafers, the storage housing  252  is lowered around an empty conveyor  131 , until the top shelf  254  is below the conveyor  131 . Then, the conveyor  131  is energized and a wafer is brought into the loadlock chamber  105  and onto conveyor  131 . When the wafer on conveyor  131  is aligned with the top shelf  254 , the conveyor is stopped and the storage housing  252  shifts up one slot to pick up the wafer from the conveyor. This process is repeated until the storage shelves  254  are all full. Conversely, to replace a broken wafer, the storage housing  252  lowers until the bottom wafer remaining on the storage shelves  254  is transferred to the conveyor  131 . 
     In the embodiment of  FIG. 1 , the wafers traverse two loadlock chambers, i.e., from loadlock  100  to loadlock  105 . The wafers are transferred via slot  123 , which can be sealed by vacuum valve  120 . So long as the wafers are intact, full size wafers can easily traverse the gap of slot  123 , since the gap is much smaller than the size of the wafer. However, if the wafer is broken, small pieces can be dropped inside the slot  123 , and potentially damage the vacuum valve  120 .  FIGS. 3A and 3B  are close views illustrating a feature that enables handling broken wafers in loadlock chamber  100  and preventing pieces from being deposited in slot  123 .  FIG. 4  illustrates an embodiment of a system using the features of  FIGS. 3A and 3B . 
     According to an embodiment illustrated in  FIGS. 3A and 3B , to mitigate wafer broken in loadlock  100 , after the slot valve  320  between chambers  100  and  105  is opened ( FIG. 3B ), the conveyor assembly  330  from the HVLL chamber  105  is extended through the valve  320  and slot  323  to near the LVLL  100  conveyor  333  (alternatively, the conveyor from the LVLL can be extended through the slot into the HVLL chamber). At this position, the two belts are close enough to each other such that broken pieces can easily be transported from belt  333  to belt  330 . Once the wafer and broken pieces have been transferred to belt  333 , the conveyor is retracted ( FIG. 3A ) and the valve  320  is closed. In this way, as shown in  FIG. 4 , any broken wafers are transported to the HVLL chamber  105  where the CIS  142  or camera  140  will be used to determine whether they are broken. If broken wafers are found, belt  330  is energized to transfer the broken pieces to belt  131 . Belt  131  can then be tilted about its tilting axis to its inclined orientation so as to dump the broken pieces into tray  104 . The belt  131  can then be returned to its horizontal position and load a replacement wafer from the stocker  150 . Incidentally, the callout in  FIG. 4  illustrates that several rows of wafers can be handled simultaneously, as explained but not shown in  FIG. 1 , so as to avoid clutter in  FIG. 1 . Also, the embodiment of  FIG. 4  omits the vacuum pumping of the base plate, as this feature is optional. A perforated belt is still used, so that air may not be trapped between the wafers and the belt. 
       FIG. 4  illustrates another optional feature, which is equalization valve D. As can be seen, an opening is provided at the bottom of the LVLL chamber, which leads to piping to equalization valve D. The other side of equalization valve D leads to the HVLL. The purpose of this valve is as follows. After pumping the LVLL, the pressure will be around 1 torr or so. This is not a very good vacuum level, and consequently there are still a lot of air molecules in the LVLL. The HVLL is typically at a pressure around 1×10 −5  torr. If we simply open valve  120 , the air molecules in the LVLL will rush past the wafer and through valve  120  in order to equalize pressure between the low vacuum level and high vacuum level. That rush can cause the wafer to slide/twist. The equalization valve D provides an airflow path for equalization of pressure that causes air to flow down toward the bottom of the chamber. So, while there is still a rush of air, the direction of flow is such that it pushes the wafer down against the conveyor instead of pushing it forward toward the valve. As a result, the wafer does not move. 
       FIG. 5  illustrates another method for dealing with broken wafers in the LVLL  100 . According to this embodiment, a thin perforated conveyor belt  537 , such as a perforated Mylar belt, is stretched between the LVLL chamber  100  to the HVLL chamber  105 . Belt  537  is configured to move wafers from entry valve  115  all the way through slot  527 , and into HVLL chamber  105 . In this way, there&#39;s no transfer of wafers from one belt onto another over the slot  527 , thereby avoiding the potential for breakage pieces falling into slot  527 . From belt  537  the wafers may pass directly to belt  131 . 
     The special arrangement of the thin belt is illustrated in the close view in the callout of  FIG. 5 . Specifically, the thin belt  537  is designed such that valve  520  can be closed directly on the belt, when the belt is not in motion. Since both chambers  100  and  105  are under vacuum environment, the valve need not press too hard on the thin belt  537 , since the small flow through the belt is tolerable and will not adversely affect the performance of the system. 
     As can be understood, in the embodiment of  FIG. 5 , the thin belt, such as Mylar perforated belt, is threaded through a narrow slit between the low vacuum load lock  100  and high vacuum load lock  105 . The conveyor belt  537  is energized intermittently rather than continuously, wherein during each energized state it transports one, or one column, of wafers, referred to as “one pitch” or “one cycle.” When the conveyor belt  537  stops its motion, the valve  520  closes and presses on the conveyor belt  537 , to thereby separate the environment inside the high vacuum load lock  105  from that of low vacuum load lock  100 . Such an arrangement minimizes the gaps between conveyor belts that the wafers have to traverse so as to minimize breakage. 
     While this invention has been discussed in terms of exemplary embodiments of specific materials, and specific steps, it should be understood by those skilled in the art that variations of these specific examples may be made and/or used and that such structures and methods will follow from the understanding imparted by the practices described and illustrated as well as the discussions of operations as to facilitate modifications that may be made without departing from the scope of the invention defined by the appended claims.