Patent Publication Number: US-2009218214-A1

Title: Backside coating prevention device, coating chamber comprising a backside coating prevention device, and method of coating

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a backside coating prevention device, a coating chamber, and a method of coating. Particularly, the present invention relates to a backside coating prevention device adapted for a coating chamber for coating plate-shaped substrates, a coating chamber for coating plate-shaped substrates, the coating chamber comprising a backside coating prevention device, and a method of coating plate-shaped substrates. 
     BACKGROUND OF THE INVENTION 
     Thin-film coating of material on plate-shaped substrates may be accomplished in many ways, for example by evaporation or sputtering of the coating material. In some instances, for example in the manufacture of solar cells, it is desirable to coat exclusively one surface or maybe also the lateral faces of the plate-shaped substrates. 
     In known installations for coating continuously conveyed plate-shaped substrates, typically glass substrates, with thin layers by cathode sputtering, several compartments are located one after another. Each compartment comprises at least one sputtering cathode and process gas inlets, and is connected with a vacuum pump for evacuation. The compartments are connected to one another by means of openings, typically vacuum locks or airlocks, which may comprise one or more slit valves. A transport system comprising transport rolls for transporting the plate-shaped substrates along a path below the sputtering cathodes and passing the substrates through the openings between the compartments is provided. 
     When operating a sputtering cathode, a plasma is established and ions of the plasma are accelerated onto a target of coating material to be deposited onto the substrates. This bombardment of the target results in ejection of atoms of the coating material, which accumulate as a deposited film on the substrate below the sputtering cathode. 
     In known designs of a compartment for sputtering continuously transported rectangular plate-shaped substrates, coating material may be deposited not only on the front sides and, in some instances, on the lateral sides of the plate-shaped substrates as desired, but also on the backsides thereof, which is especially undesirable for glass substrates for solar cells. 
     SUMMARY OF THE INVENTION 
     In one aspect it is provided a backside coating prevention device adapted for a coating chamber for coating plate-shaped substrates, said coating chamber being adapted for coating continuously or discontinuously transported plate-shaped substrates, including a front wall having a substrate feeding opening and a rear wall having a substrate discharge opening, a coating material source adapted for dispensing coating material into the coating chamber, and a transport system, a front side of the transport system facing the coating material source, the transport system being adapted for continuously or discontinuously transporting a plurality of plate-shaped substrates along a transport path on the front side of the transport system, wherein said backside coating prevention device is adapted for providing a gas barrier at the front side of the transport system and adjacent to the backsides of the plurality of plate-shapes substrates for preventing backside coating of the plate-shaped substrates. 
     A further aspect is directed to a coating chamber for coating plate-shaped substrates, said coating chamber being adapted for coating continuously or discontinuously transported plate-shaped substrates, including a front wall having a substrate feeding opening and a rear wall having a substrate discharge opening, a coating material source adapted for dispensing coating material into the coating chamber, and a transport system, a front side of the transport system facing the coating material source, the transport system being adapted for continuously or discontinuously transporting a plurality of plate-shaped substrates along a transport path on the front side of the transport system, wherein said backside coating prevention device is adapted for providing a gas barrier at the front side of the transport system and adjacent to the backsides of the plurality of plate-shapes substrates for preventing backside coating of the plate-shaped substrates. 
     According to another aspect, a method of coating plate-shaped substrates in a coating chamber includes conveying a plurality of plate-shaped substrates through the coating chamber by a) feeding one of the plate-shaped substrates into the coating chamber through a substrate feeding opening in a front wall of the coating chamber and arranging the plate-shaped substrate on a transport system for continuously or discontinuously transporting the plurality of plate-shaped substrates along a transport path on the front side of the transport system, b) continuously or discontinuously transporting the plate-shaped substrate along the transport path, while providing a gas barrier at the front side of the transport system and adjacent to the backside of the plate-shaped substrate for preventing backside coating of the plate-shaped substrate and while dispensing coating material from a coating material source provided in the coating chamber towards a front side of the plate-shaped substrate, c) discharging the plate-shaped substrate through a substrate discharge opening in a rear wall of the coating chamber, wherein the plate-shaped substrate is discharged while another one of the plate-shaped substrates is being conveyed through the coating chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some of the above mentioned aspects will be described in more detail in the following description of typical embodiments with reference to the following drawings in which: 
         FIG. 1  shows a cross-sectional view of a coating chamber including a backside coating prevention device according to embodiments described herein. 
         FIG. 2  is a cross-sectional view of the coating chamber along line A-A shown in  FIG. 1 , including a schematic illustration of a part of the backside costing prevention device outside the coating chamber according to embodiments described herein. 
         FIG. 3  is a top view on a section of a transport system of the coating chamber shown in  FIGS. 1 and 2 . 
         FIG. 4  is a flow diagram of a coating method according to embodiments described herein. 
         FIG. 5  is a flow diagram of another coating method according to embodiments described herein. 
         FIG. 6  is a flow diagram of a further coating method according to embodiments described herein. 
         FIG. 7  shows a top view on a section of a transport system of another coating chamber according to embodiments described herein. 
         FIG. 8  is a top view on a section of a transport system of another coating chamber according to embodiments described herein. 
         FIG. 9  shows a top view on a section of a transport system of another coating chamber according to embodiments described herein. 
         FIG. 10  illustrates a part of another variation of a backside coating prevention device according to embodiments described herein. 
         FIG. 11  shows a part of a further variation of a backside coating prevention device according to embodiments described herein. 
         FIG. 12  shows a part of a yet further variation of a backside coating prevention device according to embodiments described herein. 
         FIG. 13  shows a part of another variation of a backside coating prevention device according to embodiments described herein. 
         FIG. 14  shows a part of a further variation of a backside coating prevention device according to embodiments described herein. 
         FIG. 15  shows a part of a yet further variation of a backside coating prevention device according to embodiments described herein. 
         FIG. 16  shows a part of a yet further variation of a backside coating prevention device according to embodiments described herein. 
         FIG. 17  shows a part of another variation of a backside coating prevention device according to embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the various embodiments, one ore more examples of which are illustrated in the figures. Each example is provided by way of explanation, and is not meant as a limitation of the invention. Within the following description of the drawings, the same reference numbers refer to the same components. Generally, only the differences with respect to the individual embodiments are described. 
     Typically, applications of the backside coating prevention device, of the coating chamber and of the coating method of the invention are in vacuum sputtering compartments of installations for coating continuously or discontinuously conveyed plate-shaped substrates with thin films. The invention is especially useful for coating plate-shaped glass substrates with thin metal films, for example with Ag films, in the manufacture of solar cells. 
     Without limiting the scope of the invention, the following is directed to a backside coating prevention device in a vacuum sputtering coating chamber for thin-film Ag coating of continuously transported rectangular plate-shaped glass substrates. Embodiments of the present invention can also be applied to other coating methods, such as thin-film vapour deposition, and other coating materials than Ag, e.g. other metals or alloys. Furthermore, other substrates, such as a web or plastic films, having modified shapes may be employed. Moreover, the substrate(s) may be delivered to the coating chamber continuously or may be provided in the coating chamber in a discontinuous mode. Additionally, the coating chamber may not be limited to a vacuum chamber. Typically, the glass substrate have a thickness in the range between 2 mm and 19 mm. For example, in typical solar cell applications the glass substrates have a thickness of 2 mm to 5 mm. Furthermore, the size of the glass substrates may be up to 3 meters by 6 meters in some applications. 
       FIG. 1  illustrates a cross-sectional view of a coating chamber  10  designed as a vacuum sputtering chamber for thin-film coating of continuously transported rectangular plate-shaped glass substrates  100 . The coating chamber  10  includes a backside coating prevention device  200 ,  202 , according to embodiments described herein.  FIG. 2  shows a cross-sectional view of the coating chamber  10  according to  FIG. 1 , along line A-A. The coating chamber  10  comprises a bottom wall  12 , a top wall  14 , a front wall  16 , a rear wall  18  and two sidewalls  17 . The material of all walls is stainless steel and the coating chamber  10  is vacuum-tight. The front wall  16  includes a substrate feeding opening  20  and the rear wall  18  includes a substrate discharge opening  22 . The substrate feeding and discharge openings  20 ,  22  are designed as vacuum locks or airlocks, typically as slit valves, for maintaining a vacuum in the coating chamber  10  when feeding and discharging the glass substrates  100 . The coating chamber  10  further has process gas inlets (not shown) and is connected to vacuum pumps (not shown) for establishing a vacuum of about 10 −6  Torr. The pressure value of 10 −6  Torr should, of course, be understood as an example while other pressure values or ranges are also applicable. For example, a typical pressure range for sputtering is between 10 −3  hPa to 10 −2  hPa, a typical pressure range for evaporation is from lower than 10 −6  hPa to 10 −3  hPa, more typically in the range from 10 −5  hPa to 10 −4  hPa. Furthermore, at the top wall  14  one or more, typically two, sputtering cathodes  26  each comprising a target of Ag are provided as coating material source adapted for dispensing coating material into the coating chamber. 
     On the bottom wall  12 , as a substrate support, a transport system  30  for continuously conveying a plurality of glass substrates  100  is mounted, as is shown in  FIGS. 1 and 2 , especially in  FIG. 2 . The transport system  30  has a front side  31  facing the sputtering cathode  26  and is adapted for supporting on the front side  31  one or more plate-shaped glass substrates  100 . The transport system  30  comprises a plurality of, typically two, rotatable rolls  32  arranged in parallel to each other throughout the coating chamber  10  successively from the front wall  16  to the rear wall  18 . The rolls  32  extend from one sidewall  17  to the opposite sidewall  17 . Furthermore, each roll  32  is positioned below a cover panel  36  of the transport system  30  and comprises a plurality of spaced apart rings  33  being each concentrically attached to the roll  32 . The rings  33  extend through openings  34  in the cover panel  36  of the transport system  30  and support the glass substrates  100  and, thereby, define a substrate support plane  120  above the cover panel  36 . The substrate support plane  120  is shown in  FIGS. 1 and 2  as a dashed line. Front sides  105  of the glass substrates  100  supported on the rings  33  face the sputtering cathodes  26 . The cover panel  36  is disposed at a front side  31  of the transport system and has an installation height such that it is positioned typically about 2 mm to about 10 mm, more typically about 2 mm to about 5 mm, most typically at least about 2 mm beneath the substrate support plane  120 . The resulting distance of at least about 2 mm between the cover panel  36  and the substrate support plane  120  or transported glass substrates  100 , respectively, allows for vibrations or sagging of the glass substrates  100  during transport. At the same time contact or collisions of the glass substrates  100  with the cover panel  36  are avoided. Furthermore, the distance between cover panel  36  and substrate support plane  120  defines a space below the substrate support plane. 
     The rolls  32  are connected to a driving unit  40 , which is connected to a control unit  50 , the latter being herein also referred to as control means. Both units  40  and  50  are provided outside the coating chamber as shown schematically in  FIG. 2 . The transport system  30  is made for conveying the plate-shaped glass substrates  100  in a transport direction along a transport path  60 . The transport path  60  is defined by the transported glass substrates  100  and is positioned on the substrate support plane  120  below the sputtering cathodes  26  and through the substrate feeding and discharge openings  20 ,  22  of the coating chamber  10 . During coating operation, the transport direction extends from the substrate feeding opening  20  to the substrate discharge opening  22 . 
     As shown in e.g.  FIG. 2 , each glass substrate  100  has one front side  105  to be coated and facing the sputtering cathodes  26  during transport of the glass substrate on the transport system  30 . Each glass substrate  100  further includes a backside  110  opposite to the front side  105  and facing the transport system  30  during transport of the glass substrate thereon, and two lateral ends  112  each comprising a lateral side  114 . During transport of the glass substrates  100 , as illustrated in  FIG. 1 , gaps  210  are formed between successively transported rectangular plate-shaped glass substrates  100  on the transport system  30 . The gaps  210  extend across the full width of the transport path  60 . Typically, when coating a plurality of successively transported substrates, an efficient way of operating the sputtering cathodes is a continuous mode. Ag particles, which are sputtered in the present coating chamber  10  by the sputtering cathodes  26  towards the glass substrates  100 , move along a straight trajectory and may also be deflected by collisions with other particles or with the walls of the coating chamber  10 . A number of the Ag coating material particles which travel to the front sides  105  of the glass substrates  100  may pass the gaps  210  between successively transported rectangular plate-shaped glass substrates  100  and may undesirably be deposited on the backsides  110  of the glass substrates  100 . Moreover, additional lateral gaps  500  are formed between the lateral sides  114  of successively transported rectangular plate-shaped glass substrates  100  and the sidewalls  17  of the coating chamber  10 , as shown in  FIG. 2 . Therefore, the sputtered Ag particles may also pass the lateral gaps  500  and may be deposited on the backsides  110  of the glass substrates  100 . 
     Therefore, as is illustrated in  FIG. 1 to 3 , in the coating chamber  10  a backside coating prevention device is provided, the backside coating prevention device according to embodiments described herein comprising a barrier gas supply unit. In a typical example shown in  FIG. 1 to 3 , the barrier gas supply unit includes a plurality of barrier gas conduits  200 , a plurality of barrier gas outlets  202  and a barrier gas source  204 . The barrier gas may be an inert gas, for example Ar gas of a suitable purity grade which is typically chosen to be the same purity grade as that of the sputter gases used in the process. However, the barrier gas is not necessarily inert as also gas mixtures containing reactive gases may be used. For example, a mixture of Ar, N 2 , and O 2  may be used in a reactive process. Such mixture is not inert due to the O 2 . In general, any gas not having a negative influence on the process may qualify as a barrier gas to be used in the disclosed apparatus and process. Typical but non-limiting examples of barrier gases include inert gases and/or gases having high molecular weight. The barrier gas conduits  200  are attached to the backside of the cover panel  36  and extend in parallel to the rolls  32  of the transport system  30 . Each roll  32  in the example shown in  FIG. 1 to 3  is positioned between two barrier gas conduits  200 . Each barrier gas conduit  200  is connected via a valve  206 , typically a control valve, more typically a pressure control valve, to the barrier gas source  204 . It will be understood by those skilled in the art that a mass flow controller (MFC) may be used as an alternative to valve  206  or in combination with valve  206 . For example, a main MFC may be provided together with additional valves or MFCs at the individual inlets. The valve  206 , also referred to as a barrier gas valve, and the barrier gas source  204  are provided outside the coating chamber  10 . In typical designs of the backside coating prevention device according to embodiments described herein, one common valve  206  may be provided for all barrier gas conduits  200 . In another typical design, each barrier gas conduit  200  may have one separate valve  206 . The valve(s)  206  are typically operated electromagnetically and are controlled by control unit  50 . Furthermore, each barrier gas conduit  200  is connected to one of the barrier gas outlets  202 . The barrier gas outlets  202  are provided in the cover panel  36  in parallel to the rolls  32  and transverse, typically perpendicular, to the transport path  60 , i.e. transverse, typically perpendicular, to the transport direction during coating operation. 
     As is illustrated in the top view on the cover panel  36  according to  FIG. 3 , the barrier gas outlets  202  are longitudinal slits formed in the cover panel  36 . The barrier gas outlets  202  are arranged in parallel to each other and extend across the full width of the cover panel  36 . Furthermore, as is shown in  FIG. 1 , because of the installation height of the cover panel  36 , the barrier gas outlets  202  are typically about 2 to about 10 mm, more typically about 2 to about 5 mm, most typically at least about 2 mm spaced apart from the substrate support plane  120 , i.e. from the backsides  110  of the transported glass substrates  100 . 
     During a typical coating operation, a plurality of rectangular plate shaped glass substrates  100  are conveyed one after another through the coating chamber  10 , while the sputtering cathodes  26  are operated continuously. Each glass substrate  100  is fed into the coating chamber  10  through the substrate feeding opening  20  and arranged on the rings  33  of the transport system  30 . After that, each glass substrate  100  is continuously transported by the transport system  30  along the transport path  60  below the operating sputtering cathodes  26 . Finally, each glass substrate  100  is discharged through the substrate discharge opening  22 . Since a plurality of glass substrates  100  is to be coated and in order to improve the effectiveness of the coating processing, two or more glass substrates  100  may simultaneously be transported on the transport system  30  in the coating chamber  10  one after another. Therefore, each time two rectangular plate shaped glass substrates  100  are successively transported through the coating chamber  10 , one of the gaps  210  is formed between the two glass substrates  100 . This gap  210  is moving along the transport path  60  during transport of the glass substrates  100 . 
     According to typical embodiments of the coating method described herein, a gas barrier is provided at the front side of the transport system and adjacent to the backsides of the plate-shaped substrates for preventing backside coating of the plate-shaped substrate. More typically, a gas barrier may be provided between the front side of the transport system and the plate-shaped substrates for preventing backside coating of the plate-shaped substrates. In one example, an Ar gas barrier is established in the coating chamber  10  beneath the glass substrates  100 , i.e. in the space defined between the cover panel  36  and the backsides  110  of the glass substrates  100 , and also in the gaps  210 , which are formed between successively transported rectangular plate-shaped glass substrates  100 . Particles of Ag coating material are ejected from the sputtering cathodes  26  towards the glass substrates  100  and also towards the gaps  210  between successive glass substrates  100 . Because of the Ar gas barrier, passage of Ag particles through the gaps  210  between successively transported glass substrates  100  towards the backsides  110  of the glass substrates  100  is reduced or substantially inhibited. Furthermore, the Ar gas barrier also extends through the lateral gaps  500  formed between the lateral ends  114  of the glass substrates  100  and the sidewalls  17  of the coating chamber  10 . Therefore, also Ag particles traveling to the lateral gaps  500  between the glass substrates  100  and the sidewalls  17  are prevented to pass these lateral gaps  500  and to deposit on the backsides  110  of the glass substrates  100 . In summary, the Ar gas barrier established by the backside coating prevention device according to embodiments described herein at least reduces or even prevents that Ag target material sputtered towards the glass substrates  100  enters the region below the glass substrates  100  via the gaps formed around the glass substrates  100 . 
     The following is an example of a coating method according to embodiments described herein, the beginning of which is shown schematically in  FIG. 4 . As soon as a front end of a first glass substrate  100  ((n−1)th glass substrate; n being an integer ≧2) has entered the coating chamber  10 , control unit  50  switches on a constant Ar barrier gas flow. This barrier gas flow is established from the barrier gas source  204  through the barrier gas conduits  200  to the barrier gas outlets  202  by opening the corresponding barrier gas valves  206 . Typically, the gas flow rate depends on the type of substrate to be processed, the type, size, and/or geometry of coating chamber  10 , as well as on the size of gap  210 . Exemplary flow rates are from about 20 sccm up to about 500 sccm. Typically, the barrier gas flow is adjusted so that the maximum amount of barrier gas in the chamber will be of the same order of magnitude as the process gas. The sputtering cathodes  26  are then switched on or, alternatively, are already working. The first glass substrate  100  is continuously transported below the operating sputtering cathodes  26  and through the coating chamber  10  while being coated on its front side  105  with Ag particles and supplied on its backside  110  with Ar barrier gas. After the rear end of the first glass substrate  100  has entered the coating chamber  10 , the front end of a second (nth) glass substrate  100  is fed into the coating chamber  10  through the substrate feeding opening  20 . The second glass substrate  100  is arranged on the rings  33  of the transport system  30  and transported thereon. The barrier gas flow through the barrier gas outlets  202  is kept constant as before. Again, the second glass substrate  100  is continuously transported below the operating sputtering cathodes  26  and through the coating chamber  10  while being coated on its front side  105  with Ag particles and supplied on its backside  110  with Ar barrier gas. During conveying of the second glass substrate  100 , after a first and a second period of time, the front end and the rear end of the continuously transported first glass substrate  100  consecutively arrive at and are discharged through the discharge opening  22 . The first and the second periods of time depend on the length, i.e. the distance between the front and the rear ends, of the first glass substrate  100 , as the skilled person is aware. Thereafter, the front end of the second glass substrate  100  arrives at the substrate discharge opening  22  and is discharged from the coating chamber  10 . Finally, after a period of time depending on the length of the second glass substrate  100 , its rear end is discharged through the substrate discharge opening  22 , thus completing the process of coating the second glass substrate  100 . 
     In the course of above coating operation, because of the constant Ar barrier gas flow, a gas barrier is established beneath the first and second glass substrates  100 , in the gap  210  which is formed between the first and second glass substrates  100  during transport, and in the lateral gaps  500  between the glass substrates  100  and the sidewalls  17  of the coating chamber. Therefore, passage of Ag particles through the gaps  210  and the lateral gaps  500  is reduced or substantially inhibited. Thereby, backside coating of the first and second glass substrates  100  is avoided. The method steps illustrated above for the second glass substrate  100  may be repeated with a third (n+1) and other successively transported glass substrate(s)  100 , while the backsides  110  thereof are supplied with an Ar barrier gas flow, in order to coat a plurality of glass substrates  100  with thin Ag films and simultaneously reduce or prevent coating on the backsides  110  thereof. 
     According to the above example of a coating method, a constant Ar barrier gas flow is continuously supplied through the barrier gas outlets  202  after the first glass substrate  100  has entered the coating chamber  10  until the last glass substrate  100  has been discharged from the coating chamber  10 . The control unit  50  controls the Ar barrier gas flow by opening and closing the barrier gas valves  206 . In this example the control unit  50  switches the Ar barrier gas flow on at the time the front end of the first glass substrate  100  is fed into the coating chamber and switches it off at the time the rear end of the last glass substrate  100  is discharged from the coating chamber. Both switching times can be calculated by the control unit  50  based on predetermined information about the length of the glass substrates  100 , about the width of the gaps  210  between successively transported glass substrates  100  and based on the transport speed which is controlled by the control unit  50 . Alternatively, the switching times may be determined based on information derived from sensors connected to the control unit  50 , typically movement sensors. Such sensors may, for instance, be positioned outside the coating chamber  10  at the front and rear walls  16 ,  18  near the substrate feeding opening  20  and substrate discharge opening  22 . In this example of the coating method, instead of a plurality of barrier gas valves  206 , one common barrier gas valve  206  connected to all barrier gas conduits  200  may be used. 
     In another variation of a coating method according to embodiments described herein, the beginning of which is schematically shown in  FIG. 5 , the Ar barrier gas flow may be controlled during transport. This controlling of the Ar gas flow results in a variable or even discontinuous supply of the Ar barrier gas to one or more barrier gas outlets  202 . In the present example, for each barrier gas conduit  200  a separate barrier gas valve  206  is provided. This means each barrier gas outlet  202  is connected via one of the barrier gas conduits  200  to the corresponding barrier gas valve  206 , which is connected to the barrier gas source  204 . Each time one of the gaps  210  between successively transported glass substrates  100  comes near any of the barrier gas outlets  202 , the control unit  50  transmits a switching command to the barrier gas valve  206  connected to the respective barrier gas outlet  202 , in order to reduce or stop the barrier gas flow. After the gap  210  has passed the barrier gas outlet  202 , barrier gas flow through this barrier gas outlet is resumed by a command of the control unit  50  to the corresponding barrier gas valve  206 . 
     Information about the length of the glass substrates  100  and the width of the gaps  210  between successively transported glass substrates  100  may be fed into the control unit  50  before starting the coating process shown in  FIG. 5 . Alternatively, such information may be determined during transport by corresponding sensors connected to the control unit  50 . The latter also controls the driving unit  40  of the transport system  30  and, hence, the transport speed of the glass substrates  100 . Therefore, the control unit  50  is able to determine the moving position of each gap  210  between successive glass substrates  100 , and the barrier gas valves  206  are accordingly controlled. Thereby, in the present example shown in  FIG. 5 , the amount of Ar barrier gas required for backside coating prevention during coating may be reduced, since the barrier gas flow from a barrier gas outlet  202  directly through the gaps  210  between successively transported glass substrates  100  is reduced or avoided. At the same time, the barrier gas flow through the other barrier gas outlets  202  is maintained as long as they are positioned below the transported glass substrates  100 . Therefore, an Ar gas barrier below the glass substrates  100  is established and maintained, the gas barrier having a gas pressure and a gas flow into the gaps  210 , which are sufficient to safely avoid backside coating. 
     One example of typical embodiments described herein is directed to a coating method for glass substrates  100  which are shorter than the distance of the front wall  16  and the rear wall  18  of the coating chamber  10 . This means that one or more gaps  210  between successively transported glass substrates  100  are arranged inside the coating chamber  10  during transport. For this case, the coating method explained above and illustrated in  FIG. 5  is utilized. Again, backside coating of the glass substrates  100  is avoided while the amount of barrier gas required for backside prevention is reduced. 
     In another example of a coating method according to embodiments described herein, the beginning of which is shown schematically in  FIG. 6 , the glass substrates  100  are equally long or longer than the distance between the front wall  16  and the rear wall  18 . Therefore, during a certain time period of transport through the coating chamber  10 , each glass substrate  100  spans the distance between the front wall  16  and the rear wall  18  of the coating chamber  10 , i.e. between the substrate feeding and discharge openings  20  and  22 . As a result, during this time period, no gap  210  between successive glass substrates  100  is inside the coating chamber  10 . Therefore, the barrier gas flow to all barrier gas outlets  202  is reduced or switched off by control unit  50  controlling the barrier gas valves  206 . The time period during which no gap  210  is arranged inside the coating chamber may be determined as explained above, i.e. based on the information of the length of the glass substrates  100 , of the width of the gaps  210  between successive glass substrates  100  and of the transport speed, or based on data from corresponding sensors. As soon as one of the gaps  210  between two successive glass substrates  100  enters the coating chamber  10 , barrier gas flow through all barrier gas outlets  202  is started and an Ar gas barrier is established beneath the glass substrates  100 . Alternatively, barrier gas may be supplied consecutively only to such barrier gas outlets  202  which are next to and/or below the moving gap  210 . In another modification, only such barrier gas outlets  202  are provided with barrier gas, which are positioned before and after one of the gaps  210  moving through the coating chamber  10  and which at the same time are positioned below the successive glass substrates  100  forming this gap  210 . 
     In case of coating glass substrates  100  having varying lengths and including glass substrates  100  which are shorter as well as glass substrates  100  which are equally long or longer than the distance between the front and the rear walls  16  and  18  of the coating chamber  10 , a combination of at least parts of the methods shown in  FIG. 4  or  5  and  FIG. 6  may be utilized. That means, as long as no gap  210  between successively transported glass substrates  100  is arranged in the coating chamber, the Ar barrier gas flow may be reduced or switched off. As soon as one of the gaps  210  enters the coating chamber during transport of the glass substrates  100 , one of the barrier gas step sequences shown in  FIGS. 4 and 5  is started. In each of these cases, backside coating of the glass substrates is avoided while the required amount of barrier gas is kept as small as possible. 
     In  FIG. 7  a further variation of embodiments of the backside coating prevention device described herein is shown, comprising a specific design of the barrier gas outlets provided in the cover panel  36  of the transport system  30 . According to  FIG. 7 , instead of each of the barrier gas outlets  202  shown in  FIG. 3  and extending across the full width of the cover panel  36 , a plurality of barrier gas outlets  302  formed as longitudinal apertures are provided in line. In the present example, each ring  33  extending through an opening  34  of the cover panel  36  is positioned between two parallel longitudinal apertures  302 . As will be understood by the person skilled in the art, in this variation of embodiments the barrier gas flow will be adjusted to the specific design of the barrier gas outlets  302 . Other suitable modifications of the embodiments of the backside coating prevention device as described herein are possible, e.g. a combination of the designs of the barrier gas outlets shown in  FIGS. 3 and 7 , as the skilled person will be aware of. 
       FIGS. 8 and 9  illustrate other examples of typical embodiments of the backside coating prevention device.  FIG. 8  shows barrier gas outlets  402  being shorter than the barrier gas outlets  202  indicated in  FIG. 3 , such that they do not extend across the full width of the cover panel  36 . Furthermore, an additional barrier gas outlet  404  is provided in the lateral end portion of the cover panel  36 , as shown in  FIG. 8 , in parallel to the sidewall  17 , i.e. perpendicular to the barrier gas outlets  402 . The opposite lateral end portion (not shown) of cover panel  36  is also provided with such an additional barrier gas outlet  404  in a mirror-inverted design. The barrier gas outlets  404  are connected to additional barrier gas conduits (not shown). In case of an embodiment having only one common barrier gas valve  206  for all barrier gas conduits as mentioned above and suitable for the method illustrated in  FIG. 4 , the barrier gas conduits of the barrier gas outlets  404  may also be connected to the common valve  206 . Supplying barrier gas through the resulting system of barrier gas outlets  404  allows for establishing a strong Ar gas barrier in lateral gaps  500  between the transported glass substrates  100  and the sidewalls  17 , thus promoting the backside coating preventing effect. Alternatively, the additional barrier gas outlets  404  may be connected to the barrier gas source  204  by additional barrier gas conduits connected to one or more additional barrier gas valves (not shown) which are separately controlled by the control unit  50 . The latter example is especially suitable for a variation of the coating method including at least a part of the step sequence as illustrated in  FIG. 6 , i.e. in case that some or all of the glass substrates  100  are equally long or longer than the distance of the front wall  16  and the rear wall  18 . This example allows for a separate supply of barrier gas to the barrier gas outlets  404  during the time period when one of the glass substrates spans the distance between the front and the rear walls  16  and  18  of the coating chamber  10  and the barrier gas flow through the barrier gas outlets  402  is reduced or stopped. Thereby, a backside coating prevention mainly with respect to the lateral gaps  500  between the glass substrates  100  and the sidewalls  17  is effected. 
       FIG. 9  shows a modified design of the example shown in  FIG. 8 , the modification being due to the fact that the barrier gas outlets  402  and  404  illustrated in  FIG. 8  are split into a plurality of shorter longitudinal apertures. The backside prevention effect of the design according to  FIG. 9  corresponds to the one of the example shown in  FIG. 8 . 
     Furthermore, it will be understood by those skilled in the art that in the above embodiments the coating chamber  10  will be designed for glass substrates of specific dimensions. Therefore, the dimensions of the backside coating prevention devices and the features of the corresponding coating method, e.g. the amount of the barrier gas flow, can be specifically adjusted to those dimensions of the glass substrates. Thus, by knowing the dimensions of the glass substrates for which the coating chamber and the coating method is designed, the skilled person can determine the correct dimensions of the backside coating devices and the correct features of the corresponding coating method such that a suitable gas barrier for prevention of backside coating is achieved. 
     In a modification of the above examples of embodiments as described herein, the barrier gas outlets may be provided in alignment with the sputtering cathodes  26 . That means that some or all of the barrier gas outlets described above, typically the barrier gas outlets  202 ,  302 ,  402 ,  502 ,  504 , are provided only in one or more coating regions  70  below the sputtering cathodes  26 , resulting in a reduced amount of barrier gas required for backside coating prevention. 
     In a further variation of the embodiments of the backside coating prevention device described herein, the barrier gas conduits and barrier gas outlets may be incorporated in a cooling arrangement for cooling the glass substrates  100 , thus saving space inside the coating chamber. 
     In another modification of the embodiments described herein, the backside coating prevention device further comprises at least two screens provided at two opposite sidewalls of the coating chamber, the sidewalls extending from the front wall to the rear wall of the coating chamber, each of the two sidewalls being provided with at least one of the screens, each screen having a protruding member protruding from the respective sidewall, each screen having the protruding member positioned so that each protruding member extends along the respective sidewall in parallel to the transport path and is spaced in the range from 1.5 mm to 5 mm from the plate-shaped substrates during coating. 
     As mentioned above and shown in e.g.  FIG. 10 , each glass substrate  100  has the front side  105  to be coated and facing the sputtering cathodes  26  during transport of the glass substrate on the transport system  30 . Moreover, each glass substrate  100  has the backside  110  opposite to the front side  105  and facing the transport system  30  during transport of the glass substrate thereon, and two lateral ends  112  each comprising one lateral side  114 . It is noted that  FIG. 10  only shows one of the two lateral ends  112  of glass substrate  100 . It will be understood by those skilled in the art, that the arrangement shown in  FIG. 10  is also provided on the opposite lateral side  114  of the glass substrate  100  but in a mirrored configuration. During transport of the glass substrates  100 , as mentioned above and as is visible particularly in  FIG. 10  showing only one lateral end  112  of one of the glass substrates  100 , two lateral gaps  500  will be formed between the lateral sides  114  of the rectangular plate-shaped glass substrates  100  arranged on the transport system  30  and the sidewalls  17  of the compartment. The lateral gaps  500  extend along the sidewalls  17  of the coating chamber  10  in parallel to the transport direction. A number of ejected atoms of the target material may pass these lateral gaps  500  and may undesirably be deposited on the backsides  110  of the glass substrates  100 . 
     In view of the above, in addition to the barrier gas supply unit as described above, the backside coating prevention device according to embodiments described herein may optionally include two or more screens, the screens being provided at at least two of the walls of the coating chamber  10 , the screens being optionally provided below the substrate support plane. Each screen has a protruding member protruding from the respective wall. In the example of the backside coating prevention device as shown in  FIG. 2 , two lateral screens  2000  may optionally be included, each sidewall  17  of the coating chamber  10  being provided with one of the screens  2000  below the substrate support plane  120 . 
       FIG. 10  illustrates an enlarged cross-sectional view of one of the screens  2000  shown in  FIG. 2 . Each lateral screen  2000  is made of stainless steel and typically has an L-shaped cross section, i.e. it comprises two branches  2002  and  2004  arranged perpendicularly to each other. Branch  2002  is attached to the interior side of the sidewall  17  of the coating chamber  10 . Branch  2004  is provided at the top end of branch  2002  and protrudes towards the centre of the coating chamber in parallel to the substrate support plane  120 , i.e. perpendicularly to the sidewall  17 . Therefore, branch  2004  forms the protruding member and extends along sidewall  17  in parallel to the transport path  60 . The installation height of branch  2002  at the sidewall  17  in the coating chamber  10  is such that branch  2004  is positioned about 2 to about 10 mm, typically about 2 to about 5 mm, most typically at least about 2 mm beneath the substrate support plane  120 . Both branches  2002  and  2004  of the lateral screens  2000  further extend in parallel to the substrate support plane  120  along the sidewall  17  throughout the coating chamber  10 , typically at least throughout a sputtering region of the coating chamber  10 . That means that the sputtering cathode  26  forming the coating material source is adapted to dispense coating material at least into the coating region  70  of the coating chamber  10  and each screen  2000  is provided at least in the coating region  70 . 
     In a typical embodiment of this variation of the backside coating prevention device, the protruding branch  2004  is positioned to be at least 2 mm spaced apart from the one or more plate-shaped substrates  100  during coating. Furthermore, branch  2004  protrudes from the sidewall  17  such that the substrate support plane  120  is positioned between the sputtering cathode  26  and branch  2004 . More specifically, as mentioned above, each glass substrate  100  supported on the substrate support plane  120  has the backside  110  and two lateral ends  112  each comprising one lateral side  114 . As shown in  FIG. 10 , a gap  2010  is formed between the backside  110  of the glass substrate  100  and the branch  2004 , branch  2004  being about 2 to about 10 mm, typically about 2 to about 5 mm, most typically at least about 2 mm spaced apart from the backside  110  of the glass substrate  100 , depending on the installation height of branch  2002  as explained above. Therefore, gap  2010  has a width of about 1.5 mm to about 10 mm, typically about 1.5 mm to about 5 mm, most typically at least about 2 mm. As a result of this small width, most of the Ag particles sputtered towards the lateral ends  112  of the glass substrate  100  are prevented from passing to the backside  110  thereof, since they are deposited on the upper surfaces of branches  2004  and the lateral sides  114  of the glass substrate. The gap  2010  between protruding branch  2004  and the glass substrate  100  also allows for vibrations or sagging of the glass substrate  100  during transport, preventing contact or collisions of the glass substrate  100  with the protruding branches  2004  of the backside coating prevention device. 
     During coating operation, glass substrates  100  are successively fed into the coating chamber  10  through the substrate feeding opening  20 , continuously conveyed by the transport system  30  along the transport path  60  on the substrate support plane  120  below the operating sputtering cathode  26 , and discharged through the substrate discharge opening  22 . Particles of Ag coating material are ejected from the sputtering cathode  26  towards the glass substrates  100  and also laterally towards the lateral gaps  500  which are formed between the rectangular plate-shaped glass substrates  100  and the sidewalls  17  of the coating chamber  10 . Coating particles ejected laterally towards these lateral gaps  500  are mainly deposited on the upper surfaces of the protruding branches  2004  of the screens  2000 . Thereby, passage of Ag particles through the lateral gaps  500  between the glass substrates  100  and the sidewalls  17  towards the backside  110  of the glass substrates  100  is reduced or substantially inhibited. Moreover, in this example, coating of the lateral sides  114  of the glass substrates  100  is not prevented, as is desired for some applications of the glass substrates. 
     Furthermore, according to embodiments described herein, the backside coating prevention device may include screens each comprising a protruding member having a lateral end which protrudes into the coating chamber and is positioned on the substrate support plane  120 . An example of this variation of embodiments is shown in  FIG. 11 . Like in  FIG. 10 , a cross-sectional view of only one screen  3000  of such a backside coating prevention device is illustrated in  FIG. 11 . However, typically two lateral screens  3000  are included in this example of the backside coating prevention device. 
     According to  FIG. 11 , a screen  3000  comprises a branch  3002  attached to the sidewall  17  of the coating chamber  10  and a branch  3004  formed as protruding member. Branch  3004  is mounted at the upper end of branch  3002  and protrudes into the coating chamber  10 . Branch  3004  has a lateral end  3006  facing the lateral side  114  of the glass substrate  100 . The installation height of branch  3004  is such that the lateral end  3006  is positioned on the substrate support plane  120 . Since in this example, screen  3000  is L-shaped, i.e. branches  3002  and  3004  are arranged perpendicularly to each other, the whole branch  3004  is positioned on the substrate support plane  120 . The length of branch  3004  is such that its lateral end  3006  is at least about 2 mm spaced apart, more specifically about 2 to about 10 mm, typically about 2 to about 5 mm, most typically about 2 mm spaced apart from the lateral side  114  of the glass substrate  100 . As a result, only a small gap  3010  of a width of at least about 2 mm is provided between branch  3004  and the lateral side  114  of the glass substrate  100 . Through this gap  3010 , only a negligible amount of Ag particles ejected from the sputtering cathodes  26  towards the lateral ends  112  of the glass substrate  100  will pass. 
     Therefore, when using the backside coating prevention device including two screens  3000  according to the example shown in  FIG. 11  during coating operation, substantially all Ag coating particles traveling towards the lateral gaps  500  between the glass substrate  100  and the sidewalls  17  are deposited on the upper surfaces of the protruding branches  3004 , the lateral ends  3006  thereof and at the lateral sides  114  of the glass substrate  100 . Hence, by providing the screens  3000  on both sidewalls  17  of the coating chamber, passage of Ag particles through the lateral gaps  500  between the glass substrates  100  and the sidewalls  17  towards the backside  110  of the glass substrate  100  is reduced or substantially inhibited. 
     Furthermore, it will be understood by those skilled in the art that in the above embodiments described with reference to  FIG. 11 , i.e. the embodiments showing a lateral gap  3010 , the dimensions of the branches, especially of the protruding branches, will be adjusted with respect to the width of the glass substrates to be coated. In particular, it will be understood by those skilled in the art that the coating chamber  10  will be designed for glass substrates of specific dimensions so that the dimensions, especially the lengths, of the screens of the backside coating prevention devices shown in  FIGS. 10 and 11  can be specifically adjusted to those dimensions of the glass substrates. Thus, by knowing the dimensions of the glass substrates for which the coating chamber is designed, the skilled person can determine the correct dimension of the screens of the embodiments of  FIGS. 10 and 11 , so that a specific gap width between the screens and the glass substrates is achieved in the coating chamber during operation. 
     As mentioned above, and as shown in  FIGS. 12 to 17  illustrating further examples of embodiments of a backside coating prevention device described herein, each glass substrate  100  has a substrate front side  105  (also referred to herein as front side  105 ) to be coated and facing the sputtering cathodes  26  during transport of the glass substrate on the transport system  30 . Hence, the front side  105  of each glass substrate  100  defines a substrate front plane  1200 . The substrate front plane  1200  is shown in  FIG. 12  as a dashed line. Furthermore, each glass substrate  100  has a backside  110  opposite to the front side  105  and facing the transport system  30  during transport of the glass substrate thereon. In addition, each glass substrate has two lateral ends  112 , each comprising a lateral side  114 . It is noted that  FIG. 12  only shows one of the two lateral ends  112  of glass substrate  100 . It will be understood by those skilled in the art that the arrangement shown in  FIG. 12  is provided on the opposite lateral side of glass substrate  100  but in mirrored configuration. 
     In each of the examples and embodiments disclosed herein, the sputtering cathode  26  and, hence, the target thereof may extend over the lateral ends  112  of the glass substrate, in order to apply a coating of a uniform thickness, e.g. of a substantially constant thickness, onto the front side  105  of the substrate  100  over the whole area of the front side  105 , i.e. even on the lateral ends  112 . During transport of the glass substrates  100 , two gaps  500  will be formed between the lateral sides  114  of the rectangular plate-shaped glass substrates  100  arranged on the transport system  30  and the sidewalls  17  of the compartment. These gaps  500  extend along the sidewalls  17  of the coating chamber  10  substantially in parallel to the transport direction. A number of ejected atoms of the target material may pass these gaps  500  and may undesirably be deposited on the backsides  110  of the glass substrates, typically due to scattering. 
     In view of the above, a backside coating prevention device according to embodiments described herein may optionally comprise two or more screens, the screens being provided at at least two of the walls of the coating chamber  10 , each screen having a protruding member protruding from the respective wall, the screens being optionally provided above the substrate front plane. As is shown in  FIG. 12 , one example of such a backside coating prevention device may include two lateral screens  600 , each sidewall  17  of the coating chamber  10  being provided with one of the screens  600  above the substrate front plane  1200 . 
       FIG. 12  illustrates an enlarged cross-sectional view of one example of the screens  600  of the backside coating prevention device according to embodiments disclosed herein. It is noted that the proportions of the screen  600  and the glass substrate  100  as shown in  FIG. 12  are not to scale. Each lateral screen  600  is typically made of stainless steel and typically has an L-shaped cross section, i.e. it comprises two branches  602  and  604  arranged substantially perpendicularly to each other. Branch  602  is attached to the interior surface of the sidewall  17  of the coating chamber  10 . Branch  604  is provided at the bottom end of branch  602  and protrudes towards the centre of the coating chamber substantially in parallel to the substrate front plane  1200 , i.e. substantially perpendicularly to the sidewall  17 . Therefore, branch  604  forms the protruding member and extends along sidewall  17  substantially in parallel to the transport path  60 . The installation height of branch  602  at the sidewall  17  in the coating chamber  10  is such that branch  604  is positioned about 1.5 to about 10 mm, typically about 1.5 to about 5 mm, most typically about 2 mm above the substrate front plane  1200 . Both branches  602  and  604  of the lateral screens  600  further extend substantially in parallel to the substrate front plane  1200  along the sidewall  17  throughout the coating chamber  10 , typically at least throughout a sputtering region of the coating chamber  10 . That means that the sputtering cathode  26  forming the coating material source is adapted to dispense coating material at least into a coating region  70  of the coating chamber  10  and each screen  600  is provided at least in the coating region  70 . 
     Typically, the material(s) of the screens of any example of embodiments disclosed herein is (are) vacuum-compatible and may be at least one element selected from the group consisting of Aluminum, an Aluminum alloy, or stainless steel in any of the embodiments described herein. However, other materials which are vacuum-compatible may be contemplated. The thickness of the screens or of the protruding member in any embodiment described herein, e.g. the thickness of any of the branches  602  and  604  in the present embodiment, may for example be a few mm, typically in the range from about 1 mm to about 10 mm, more typically from about 2 mm to about 5 mm. Moreover, in the embodiments described herein, typical dimensions of the protruding member, e.g. the dimensions of branch  604  in the present embodiment substantially in parallel to the transport direction, may be in the range from about 20 cm to about 100 cm. Furthermore, typical dimensions of the protruding member of any embodiment described herein, e.g. the dimensions of branch  604  of the present embodiment substantially perpendicular to the transport direction, may be in the range from about 10 cm to about 50 cm. That means that according to embodiments described herein, the dimensions of the protruding member of the screens may be L×W (Length×Width)=(10-50 cm)×(20-100 cm), wherein according to particular embodiments the Width W extends substantially in parallel to the transport direction. 
     In a typical embodiment, the protruding branch  604  is positioned to be about 1.5 mm to about 10 mm spaced apart from the one or more plate-shaped substrates  100  during coating. Furthermore, branch  604  protrudes from the sidewall  17  such that the branch  604  is positioned between the sputtering cathode  26  and the substrate front plane  1200 . More specifically, as mentioned above, each glass substrate  100  supported on the substrate support plane has a front side  105  and lateral ends  112  each comprising a lateral side  114 . As shown in  FIG. 12 , a gap  2100  is formed between the front side  105  of the glass substrate  100  and the lower side of branch  604 , branch  604  being about 1.5 to about 10 mm, typically about 1.5 to about 5 mm, most typically about 2 mm spaced apart from the front side  105  of the glass substrate  100 , depending on the installation height of branch  602  as explained above. Therefore, gap  2100  has a width of about 1.5 mm to about 10 mm, typically about 1.5 mm to about 5 mm, most typically about 2 mm. As a result of this small width, most of the Ag particles sputtered towards the gaps  500  between the glass substrate  100  and the sidewalls  17  are prevented from passing to the backside  110  thereof, since they are deposited on the upper surface of branches  604 . The gap  2100  between protruding branch  604  and glass substrate  100  also allows for vibrations or sagging of the glass substrate  100  during transport, preventing contact or collisions of the glass substrate  100  with the protruding branches  604  of the backside coating prevention device. 
     In one variation according to embodiments described herein, the protruding member, e.g. formed as branch  604 , has a lateral end protruding into the coating chamber, wherein the lateral end is positioned to be spaced apart from but substantially aligned with one of the lateral sides  114  of at least one of the one or more plate-shaped substrates  100  on the substrate support. In the screen  600  shown in  FIG. 12 , branch  604  has a lateral end formed as a front face  606 . The lateral end of branch  604  is positioned above the substrate front plane  1200  and is spaced about 2 mm from the lateral side  114  of glass substrate  100  by gap  2100 . Simultaneously, front face  606  of screen  600  is substantially aligned with the lateral side  114  of the glass substrate  100  supported on the transport system  30 . Therefore, contact or collisions of the glass substrate  100  with the protruding branches  604  of the backside coating prevention device are avoided. Furthermore, Ag particles, which are sputtered towards the gaps  500  between the glass substrate  100  and the sidewalls  17 , move along a straight trajectory and may also be deflected or scattered by collisions with other particles or with the sidewalls  17 . Because of the alignment of front faces  606  of the protruding branches  604  with the lateral sides  114  of the glass substrate  100 , most of the Ag particles, which are laterally sputtered towards the gaps  500  between the glass substrates  100  and the sidewalls  17 , are adsorbed or deposited on the upper surfaces and the front faces  606  of the branches  604 . 
     During coating operation, glass substrates  100 , typically of substantially identical dimensions, are successively fed into the coating chamber  10  through the substrate feeding opening, continuously conveyed by the transport system  30  along the transport path  60  on the substrate support plane below the operating sputtering cathode  26 , and discharged through the substrate discharge opening. Consequently, since the plate-shaped glass substrates have typically the same thicknesses, the front sides of the glass substrates define a common substrate front plane. Alternatively, in embodiments described herein, glass substrates of such varying dimensions or thicknesses may be successively fed into the coating chamber  10  that the protruding member of the backside coating prevention device is positioned to be spaced at least 1.5 mm from the one or more plate-shaped substrates during coating. That means in the present embodiment, that branch  604  is about 1.5 to about 10 mm, typically about 1.5 to about 5 mm, most typically about 2 mm spaced apart from the front side  105  of the glass substrates  100  having varying dimensions and, hence, from the substrate front planes  1200  defined thereby. Particles of Ag coating material are ejected from the sputtering cathode  26  towards the glass substrates  100  and also laterally towards the gaps  500  which are formed between the rectangular plate-shaped glass substrates  100  and the sidewalls  17  of the coating chamber  10 . Coating particles ejected laterally towards these gaps  500  are mainly deposited on the upper surfaces of the protruding branches  604  of the screens  600 . Thereby, passage of Ag particles through the gaps  500  between the glass substrates  100  and the sidewalls  17  towards the backsides  110  of the glass substrates  100  is reduced or substantially inhibited. Furthermore, coating of the lateral sides  114  of the substrates  100  is reduced or avoided. Moreover, the coating on the front sides of the glass substrates  100  is uniform even at the lateral ends  112  thereof, as is especially desired when glass substrates for solar cells are processed. 
     A further variation of embodiments is now described with reference to  FIG. 13 . Like in  FIG. 12 , a cross-sectional view of only one screen  700  of such a backside coating prevention device is shown. However, typically two lateral screens  700  are included in the backside coating prevention device. Each lateral screen  700  includes two branches  702  and  704  which are arranged as described with respect to the embodiment shown in  FIG. 12 . However, in the present embodiment the backside coating prevention device includes a protruding member having a lateral end being positioned to extend over a lateral part of the front side  105  of at least one of the one or more plate-shaped substrates  100  on the substrate support. Branch  704  differs from branch  604  shown in  FIG. 12  in that the front face  706  of branch  704  is positioned above the glass substrate  100 . That means that branch  704  extends over the lateral part  112  of the front side  105  of the glass substrate  100  during transport, i.e. it extends partially over the front side of the transport path  60 . Thereby, backside coating of glass substrate  100  is prevented, even if an amount of the Ag particles laterally ejected from the sputtering cathode  26  is deflected or scattered by the sidewalls  17  or other particles towards the gap  2100  between the protruding branch  704  and the front side  105  of the glass substrate  100 . 
     In the examples shown in  FIGS. 12 and 13 , respectively, during sputtering an amount of particles of the sputtered Ag material, which are laterally ejected towards the sidewalls  17 , is deposited on the surfaces of branches  602  and  702 , respectively. As a result, in addition to the backside coating prevention effect of the screens  600  and  700 , these embodiments reduce or inhibit contamination of the sidewalls  17  of the coating chamber  10  by Ag particles. Furthermore, maintenance of the coating chamber  10  may include a replacement of the lateral screens  600 ,  700  without the need for a sidewall cleaning procedure. In particular, the branches  602 ,  702  may cover sidewall  17  up to the top wall  14  of the chamber. 
     According to a further variation of embodiments described herein, said lateral end of the protruding member of the screen may be formed to taper away from the front side of a plate-shaped substrate  100  on the substrate support. In  FIGS. 14 and 15 , respectively, examples of such a backside coating prevention device are shown, which correspond to the embodiment shown in  FIG. 12  except for the design of the protruding lateral ends of the protruding branches. For instance, L-shaped screen  800  of  FIG. 14  includes a branch  802  attached to the sidewall  17  and a branch  804  protruding above the substrate front plane  1200 . The lateral end  806  of branch  804  has a wedge-shaped slanted face such that the protruding tip thereof is directed towards the top wall  14  of the coating chamber  10 . Screen  850  shown in  FIG. 15  has two branches  852  and  854 . Branch  852  is attached to the sidewall  17  while branch  854  protrudes substantially in parallel to the substrate front plane  1200 . In addition, the lateral end  856  of branch  854  has a tapered form such that the protruding central tip thereof is directed towards the opposing sidewall  17  of the coating chamber  10 . In the examples of  FIGS. 14 and 15 , since the protruding laterals ends  806  and  856  of the screens  600  and  650  are formed tapering away from the front side of the glass substrate  100 , the clearance of the glass substrate  100  during transport is improved. Therefore, contact or collisions of the glass substrate  100  with the screens  800 ,  850  because of vibrations or sagging of the glass substrate  100  during transport can be avoided more safely while backside coating of the glass substrate is reduced or prevented. Simultaneously, a sufficient width of gap  2100  may be provided in the tapered portion of the protruding branches  804 ,  854  while the bottom surface of these branches  804 ,  854  is spaced from the substrate front plane  1200  less than the sufficient minimum gap width. Thus, the unwanted deposition of coating material on the backside of the glass substrate can be further reduced due to the very small gap width. 
     According to embodiments disclosed herein, the backside coating prevention device may have screens each comprising a protruding member being substantially aligned with the substrate. Furthermore, according to embodiments described herein, the backside coating prevention device may have screens each comprising a protruding member being substantially aligned with the substrate front plane  1200 . An example of these embodiments is shown in  FIG. 16 . 
     According to  FIG. 16 , a screen  1000  comprises a branch  1002  attached to the sidewall  17  of the coating chamber  10  and a branch  1004  formed as a protruding member. Branch  1004  is mounted at the bottom end of branch  1002  and protrudes into the coating chamber  10 . Branch  1004  has a lateral end  1006  facing the lateral side  114  of the glass substrate  100 . The installation height of branch  1004  may be adjusted so that the upper side of branch  1004  is substantially aligned with the substrate front plane  1200 , as shown in  FIG. 16 . Further in this example, screen  1000  is L-shaped, i.e. branches  1002  and  1004  are arranged substantially perpendicularly to each other. The length of branch  1004  is such that its lateral end  1006  is laterally spaced from the lateral side  114  of the glass substrate  100  by about 1.5 to about 10 mm, typically about 1.5 to about 5 mm, and most typically about 2 mm. As a result, only a small lateral gap  1010  having a width of about 1.5 to about 10 mm, typically about 1.5 to about 5 mm, and most typically about 2 mm is provided between branch  1004  and the lateral side  114  of the glass substrate  100 . Through this gap  1010 , only a negligible amount of Ag particles ejected from the sputtering cathodes  26  will pass. 
     Therefore, when using in a coating process a backside coating prevention device including two screens  1000  according to the example shown in  FIG. 16 , substantially all Ag coating particles travelling towards the gaps  500  between the glass substrate  100  and the sidewalls  17  are deposited on the upper surfaces of the protruding branches  1004 . Hence, by providing screens  1000  on both sidewalls  17  of the coating chamber, passage of Ag particles through the gaps  500  between the glass substrates  100  and the sidewalls  17  towards the backside  110  of the glass substrate  100  is reduced or substantially inhibited. 
     Hence, according to embodiments disclosed herein, the protruding member may be substantially aligned with the substrate front plane. As shown in the example of  FIG. 16 , the installation height of branch  1004  may be adjusted so that the upper side of branch  1004  is substantially aligned with the substrate front plane  1200 , resulting in a first position of the protruding member. In other examples, the installation height of branch  1004  shown in  FIG. 16  may be adjusted so that the lower side of branch  1004  is substantially aligned with the substrate front plane  1200 , resulting in a second position of the protruding member. Furthermore, the protruding member may be installed at any other position substantially aligning the protruding member with the substrate front plane. For example, the installation height of branch  1004  shown in  FIG. 16  may be adjusted so that the protruding member is positioned between the first and the second position of the protruding member. Typically, the lateral end of the protruding member may be positioned on the substrate front plane. For instance, the upper part of the lateral end  1006  of the branch  1004  shown in  FIG. 16  is positioned on the substrate front plane  1200 . Alternatively, other parts of the lateral end  1006  may be positioned on the substrate front plane  1200 . 
     Moreover, in embodiments disclosed herein, the protruding member may be positioned such that it is aligned with the substrate. Hence, the protruding member may have any position in which it is positioned opposite to, e.g. facing, a lateral side of a substrate supported on the substrate support. For instance, in a variation of the example shown in  FIG. 16 , branch  1004  may have any position in which the lateral end  1006  of the branch  1004  may face, at least partially, the lateral side  114  of the substrate  100 . 
     In variations of the example shown in  FIG. 16  and of other examples of embodiments described herein, the screens of the backside coating prevention device may each have a protruding branch arranged in an inclined way with an inclination different from 90 degrees with respect to the sidewall  17 . Furthermore, for each screen the branch which is attached to the sidewall  17  may be mounted thereon at such an installation height, and the protruding branch may have such a length, that the lateral end of the protruding branch is positioned at least about 1.5 mm spaced apart from the lateral side  114  of the glass substrate  100 . These modifications of the example illustrated in  FIG. 16  will also result in a backside prevention effect as mentioned above for the example of  FIG. 16 . 
     Furthermore, according to embodiments described herein, the backside coating prevention device may have screens each comprising a protruding member comprising a panel and a holder, the holder being provided at a respective wall. The panel may be provided at the holder. The panel may be an elongated panel. The holder may be an elongated holder or may include a plurality of holder elements. The holder may be a protruding integral part of the respective wall. Alternatively, the holder may be a member provided at the respective wall. 
     Hence, as illustrated in  FIG. 17  showing a cross-sectional view of one example of embodiments described herein, a screen  4000  may be an elongated panel  4004  mounted at its lower side at fixture  4010  of an elongated holder  4008  protruding from a sidewall  17 . According to another example (not shown), the lower side of panel  4004  may be directly attached to the holder  4008 , with no fixture or spacing between the lower side of the panel  4004  and the upper side of holder  4008 . The panel  4004  may extend substantially in parallel to the transport path  60  and, hence, to the substrate front plane  1200 . As illustrated in  FIG. 17 , the holder  4008  may be an elongated protrusion of the sidewall  17 . Hence, it may be an integral part of the sidewall  17 , as shown in  FIG. 17 . Alternatively, the holder  4008  may be an elongated member attached at the sidewall  17 . According to the example shown in  FIG. 17 , the holder  4008  and the panel  4004  extend along the sidewall  17  in parallel to each other and in parallel to the transport path  60 . Furthermore, in the present example, the dimensions of panel  4004  are such that a front side  4006  of the panel  4004  is substantially aligned with the lateral side  114  of the substrate  100 . Typically, the holder  4008  may be provided at an installation height at sidewall  17  such that the lower side of panel  4004  is positioned spaced from and above glass substrate  100  during transport, such that the gap  2100  is formed between the panel  4004  and the substrate  100 . The gap width of the gap  2100  between the bottom surface of the panel  4004  and the substrate front plane  1200  is in the range of about 1.5 mm to 5 mm, more typically about 2 mm. According to a specific design of the screen  4000  as shown in  FIG. 17 , the holder  4008  may be provided at the sidewall  17  in an installation height such that the upper side of the holder  4008  is substantially aligned with the front side  105  of the substrate and, at the same time, the gap  2100  is formed between the panel  4004  and the substrate  100 . Each above mentioned design of the example shown in  FIG. 17  results in a reliable reduction or prevention of a backside coating of glass substrate  100 , because of the small width of the gaps  2100  between screen  4000  and the front side of glass substrate  100  in the range from about 1.5 to 5 mm, typically about 2 mm. 
     In a modification of the backside coating prevention device, in the above embodiments and examples described herein, respectively, the edges of the protruding members may have a rounded from, in order to avoid sharp edges which might damage the glass substrates  100  in case of vibrations or sagging during transport. 
     Furthermore, in each of the above embodiments and examples including screens, also one or more additional screens having a shape as described above in any of the embodiments and examples, respectively, may be provided at each sidewall  17  above and/or below the substrate front plane  1200 , i. e. above and/or below the screens typically substantially in parallel thereto. Thereby, prevention of backside coating of the glass substrates  100  is promoted. 
     In addition, in further variations of the above embodiments and examples, respectively, the substrates  100  may be conveyed vertically instead of horizontally through the coating chamber. In such a case, as will be understood by the skilled person, the screens may be installed at other positions in the coating chamber, e.g. at the top and the bottom wall of the coating chamber, or may have correspondingly adapted modified profiles, in order to allow an installation at the sidewalls. 
     Moreover, in another modification of the above embodiments and examples, respectively, the coating chamber  10  may be a tube-shaped vessel having a tube-shaped wall closed by circular front and rear lids. The glass substrates  100  are transported in a direction parallel to the longitudinal axis of the tube-shaped vessel. Furthermore, the circular front and rear lids of this modification correspond to the front and rear walls as defined above. The sidewalls as defined above correspond to the areas of the tube-shaped wall facing the lateral ends  112  of the glass substrates  100  during transport. 
     A typical example of a material of the glass substrate  100 , which may also be referred to as baseline substrate, is soda lime float glass and may have a standard or reduced iron content. In addition, in the embodiments described herein, a pre-coated glass substrate may be used. For example, the glass substrate  100  may be coated with a transparent conductive oxide. Further, the glass substrate  100  may have an amorphous and/or microcrystalline silicon p-i-n structure or an amorphous and/or microcrystalline silicon p-i-n-p-i-n tandem cell structure. Moreover, in case of coating a substrate for solar cells, substrates having a solar cell layer stack may be used in embodiments described herein. Furthermore, typical dimensions of glass plates used as glass substrate  100  according to embodiments described herein are in the range of about 1×1 sqm to about 3×6 sqm, typically about 2.2×2.6 sqm or about 1.1×1.3 sqm. Typically, the thickness of the glass substrate  100  according to embodiments described herein is in the range of about 2 mm to about 5 mm. 
     The written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the claims. Especially, mutually non-exclusive features of the embodiments described above may be combined with each other. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.