Patent Publication Number: US-2009217872-A1

Title: Backside coating prevention device, coating chamber device for coating plate-shaped substrates, and method of  coating

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a backside coating prevention device, a coating chamber device for coating plate-shaped substrates, and a method of coating. Particularly, the present invention relates to a backside coating prevention device for a coating chamber for coating plate-shaped substrates, a coating chamber device for coating plate-shaped substrates, 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 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 include one or more slit valves. A transport system including 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 and thereby coating continuously transported rectangular plate-shaped substrates, coating material may deposit not only on the front 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 
     One embodiment is directed to a backside coating prevention device adapted for a coating chamber for coating plate-shaped substrates, said coating chamber comprising a plurality of walls, a coating material source adapted for dispensing coating material into the coating chamber, a substrate support, a front side of the substrate support facing the coating material source, the substrate support being adapted for supporting on the front side one or more plate-shaped substrates each having a substrate front side and thereby defining a substrate front plane, wherein said backside coating prevention device comprises two or more screens, the screens being provided at least two of the walls of the coating chamber, each screen having a protruding member protruding from the respective wall. 
     According to another embodiment, a coating chamber device for coating plate-shaped substrates is a coating chamber comprising a plurality of walls, a coating material source adapted for dispensing coating material into the coating chamber, a substrate support, a front side of the substrate support facing the coating material source, the substrate support being adapted for supporting on the front side one or more plate-shaped substrates each having a substrate front side and thereby defining a substrate front plane, and a backside coating prevention device comprising two or more screens, the screens being provided at least two of the walls of the coating chamber, each screen having a protruding member protruding from the respective wall. 
     In a further embodiment it is provided a method of coating plate-shaped substrates in a coating chamber, comprising providing a plate-shaped substrate on a substrate support of the coating chamber, dispensing coating material from a coating material source provided in the coating chamber towards the plate-shaped substrate, and simultaneously preventing backside coating by two or more screens, the screens being provided at least two walls of the coating chamber, each screen having a protruding member protruding from the respective wall. 
     Further advantages, features, and details are evident from the dependent claims, the description and the drawings. 
     Embodiments are also directed to apparatuses for carrying out the disclosed methods and including apparatus parts for performing described method steps. Furthermore, embodiments are also directed to methods by which the described apparatus operates or by which the described apparatus is manufactured. It may include method steps for carrying out functions of the apparatus or manufacturing parts of the apparatus. The method steps may be performed by way of hardware components, firmware, software, a computer programmed by appropriate software, by any combination thereof or in any other manner. 
     It is contemplated that elements of one embodiment may be advantageously utilized in other embodiments without further recitation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of embodiments of the invention, briefly summarized above, may be had by reference to embodiments. The accompanying drawings relate to embodiments of the invention and are described in the following. Some of the above mentioned embodiments 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 an enlarged cross-sectional view of a screen of the backside coating prevention device shown in  FIG. 1 . 
         FIG. 3  shows a cross-sectional view of a screen of the backside coating prevention device according to embodiments described herein. 
         FIGS. 4   a  and  4   b , respectively, are cross-sectional views of screens of the backside coating prevention device according to embodiments described herein. 
         FIGS. 5   a  and  5   b , respectively, show cross-sectional views of screens of the backside coating prevention device according to embodiments described herein. 
         FIGS. 6   a  and  6   b , respectively, show cross-sectional views of screens of the backside coating prevention device according to embodiments described herein. 
         FIGS. 7   a  and  7   b , respectively, show cross-sectional views of screens of the backside coating prevention device according to embodiments described herein. 
         FIGS. 8   a  and  8   b , respectively, show cross-sectional views of screens of the backside coating prevention device according to embodiments described herein. 
         FIGS. 9   a  and  9   b , respectively, show cross-sectional views of screens of the backside coating prevention device according to embodiments described herein. 
         FIG. 10  shows a cross-sectional view of a screen of the backside coating prevention device according to embodiments described herein. 
         FIG. 11  shows a cross-sectional view of a screen of the 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 or 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. Throughout the description and in the claims, the coating chamber device may also be referred to as coating chamber. 
     A typical application of the backside coating prevention device, the coating chamber and the coating method of the invention is 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. In addition, the coating chamber is not limited to a vacuum chamber. 
     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. 
       FIG. 1  illustrates a vacuum sputtering coating chamber  10  for thin-film coating of continuously transported rectangular plate-shaped glass substrates  100 , including a backside coating prevention device  200 , according to embodiments described herein. The coating chamber  10  comprises a bottom wall  12 , a top wall  14 , a front wall (not shown), a rear wall (not shown) and two opposite sidewalls  16 , the material of all walls being stainless steel or normal steel, the coating chamber  10  being vacuum-tight. The front wall includes a substrate feeding opening (not shown) and the rear wall includes a substrate discharge opening (not shown), the openings being designed as vacuum locks or airlocks, typically slit valves, for maintaining a vacuum in the coating chamber 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 up to about 10 −6  hPa (10 −6  Torr). Furthermore, at the top wall  14  a sputtering cathode  26  comprising a target of Ag is provided as a coating material source for dispensing coating material into the coating chamber. 
     On the bottom wall  12 , as a substrate support, a transport system  18  for continuously conveying of glass substrates  100  is mounted. The transport system  18  has a top side  24 , also herein referred to as front side  24 , facing the sputtering cathode  26  and is adapted for supporting on the front side  24  one or more plate-shaped substrates  100 . The transport system  18  comprises a plurality of rotatable rolls  20  arranged successively in parallel to each other throughout the coating chamber  10 , extending from one sidewall  16  to the opposite sidewall  16 . The rolls  20  are connected to a driving unit and a control unit (both not shown). Each roll  20  has a plurality of spaced apart rings  22  being each concentrically attached to the roll  20 . The rings  22  support the glass substrates  100  and, thereby, define a substrate support plane at the front side  24  of the substrate support. As such, the transport system  18  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 located on the substrate support plane below the sputtering cathode  26  and through the substrate feeding and discharge openings of the coating chamber  10 . 
     In an alternative design (not shown in the Figures) of the transport system, the diameter of the plurality of rolls may be much smaller than the diameter of the plurality of rings. Each ring may then be attached to a wheel which is attached to one of the rolls. Hence, each roll may have a plurality of spaced apart wheels being each concentrically attached to the roll. Each wheel may support one ring at the outermost circumference of the wheel. The rings support the glass substrates  100  and, thereby, define a substrate support plane at the front side  24  of the substrate support. 
     As shown in e.g.  FIG. 2 , each glass substrate  100  has a substrate front side  110  (also referred to herein as front side  110 ) to be coated and facing the sputtering cathodes  26  during transport of the glass substrate on the transport system  18 . Hence, the front side  110  of each glass substrate  100  defines a substrate front plane  120 . The substrate front plane  120  is shown in  FIG. 1  as a dashed line. Furthermore, each glass substrate  100  has a backside  105  opposite to the front side  110  and facing the transport system  18  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. 2  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. 2  is provided on the opposite lateral side of glass substrate  100  but in mirrored configuration. 
     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. 
     As can be seen from  FIG. 1 , the sputtering cathode  26  and, hence, the target thereof 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  110  of the substrate  100  over the whole area of the front side  110 , i.e. even on the lateral ends  112 . During transport of the glass substrates  100 , two gaps  50  will be formed between the lateral sides  114  of the rectangular plate-shaped glass substrates  100  arranged on the transport system  18  and the sidewalls  16  of the compartment. These gaps  50  extend along the sidewalls  16  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  50  and may undesirably be deposited on the backsides  105  of the glass substrates, typically due to scattering. 
     In view of the above, a backside coating prevention device according to embodiments described herein comprises two or more screens, the screens being provided at least two of the walls of the coating chamber  10 , each screen having a protruding member protruding from the respective wall. As is shown in  FIG. 1 , one example of such a backside coating prevention device includes two lateral screens  200 , each sidewall  16  of the coating chamber  10  being provided with one of the screens  200  above the substrate front plane  120 . 
       FIG. 2  illustrates an enlarged cross-sectional view of one example of the screens  200  of the backside coating prevention device shown in  FIG. 1 . It is noted that the proportions of the screen  200  and the glass substrate  100  as shown in  FIG. 2  are not to scale as compared to the proportions of the screen  200  and the glass substrate  100  as shown in  FIG. 1 . Each lateral screen  200  is typically made of stainless steel and typically has an L-shaped cross section, i.e. it comprises two branches  202  and  204  arranged substantially perpendicularly to each other. Branch  202  is attached to the interior surface of the sidewall  16  of the coating chamber  10 . Branch  204  is provided at the bottom end of branch  202  and protrudes towards the centre of the coating chamber substantially in parallel to the substrate front plane  120 , i.e. substantially perpendicularly to the sidewall  16 . Therefore, branch  204  forms the protruding member and extends along sidewall  16  substantially in parallel to the transport path  60 . The installation height of branch  202  at the sidewall  16  in the coating chamber  10  is such that branch  204  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  120 . Both branches  202  and  204  of the lateral screens  200  further extend substantially in parallel to the substrate front plane  120  along the sidewall  16  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  200  is provided at least in the coating region  70 . 
     Typically, the material(s) of the screens 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  202  and  204  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  204  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  204  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  204  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  204  protrudes from the sidewall  16  such that the branch  204  is positioned between the sputtering cathode  26  and the substrate front plane  120 . More specifically, as mentioned above, each glass substrate  100  supported on the substrate support plane has a front side  110  and lateral ends  112  each comprising a lateral side  114 . As shown in  FIG. 2 , a gap  210  is formed between the front side  110  of the glass substrate  100  and the lower side of branch  204 , branch  204  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  110  of the glass substrate  100 , depending on the installation height of branch  202  as explained above. Therefore, gap  210  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  50  between the glass substrate  100  and the sidewalls  16  are prevented from passing to the backside  105  thereof, since they are deposited on the upper surface of branches  204 . The gap  210  between protruding branch  204  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  204  of the backside coating prevention device. 
     In one variation according to embodiments described herein, the protruding member, e.g. formed as branch  204 , 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  200  shown in  FIG. 2 , branch  204  has a lateral end formed as a front face  206 . The lateral end of branch  204  is positioned above the substrate front plane  120  and is spaced about 2 mm from the lateral side  114  of glass substrate  100  by gap  210 . Simultaneously, front face  206  of screen  200  is substantially aligned with the lateral side  114  of the glass substrate  100  supported on the transport system  18 . Therefore, contact or collisions of the glass substrate  100  with the protruding branches  204  of the backside coating prevention device are avoided. Furthermore, Ag particles, which are sputtered towards the gaps  50  between the glass substrate  100  and the sidewalls  16 , move along a straight trajectory and may also be deflected or scattered by collisions with other particles or with the sidewalls  16 . Because of the alignment of front faces  206  of the protruding branches  204  with the lateral sides  114  of the glass substrate  100 , most of the Ag particles, which are laterally sputtered towards the gaps  50  between the glass substrates  100  and the sidewalls  16 , are adsorbed or deposited on the upper surfaces and the front faces  206  of the branches  204 . 
     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  18  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  204  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 sides  110  of the glass substrates  100  having varying dimensions and, hence, from the substrate front planes  120  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  50  which are formed between the rectangular plate-shaped glass substrates  100  and the sidewalls  16  of the coating chamber  10 . Coating particles ejected laterally towards these gaps  50  are mainly deposited on the upper surfaces of the protruding branches  204  of the screens  200 . Thereby, passage of Ag particles through the gaps  50  between the glass substrates  100  and the sidewalls  16  towards the backsides  105  of the glass substrates  100  is reduced or substantially inhibited. 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. 3 . Like in  FIG. 2 , a cross-sectional view of only one screen  300  of such a backside coating prevention device is shown. However, typically two lateral screens  300  are included in the backside coating prevention device. Each lateral screen  300  includes two branches  302  and  304  which are arranged as described with respect to the embodiment shown in  FIG. 2 . 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  110  of at least one of the one or more plate-shaped substrates  100  on the substrate support. Branch  304  differs from branch  204  shown in  FIG. 2  in that the front face  306  of branch  304  is positioned above the glass substrate  100 . That means that branch  304  extends over the lateral part  112  of the front side  110  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  16  or other particles towards the gap  210  between the protruding branch  304  and the front side  110  of the glass substrate  100 . 
     In the examples shown in  FIGS. 2 and 3 , respectively, during sputtering an amount of particles of the sputtered Ag material, which are laterally ejected towards the sidewalls  16 , is deposited on the surfaces of branches  202  and  302 , respectively. As a result, in addition to the backside coating prevention effect of the screens  200  and  300 , these embodiments reduce or inhibit contamination of the sidewalls  16  of the coating chamber  10  by Ag particles. Furthermore, maintenance of the coating chamber  10  may include a replacement of the lateral screens  200 ,  300  without the need for a sidewall cleaning procedure. In particular, the branches  202 ,  302  may cover sidewall  16  up to the top wall  14  of the chamber. 
       FIGS. 4   a  and  4   b , respectively, show further examples of backside coating prevention devices according to embodiments described herein. In these examples, L-shaped screens  400  and  450 , respectively, of the backside coating prevention device are attached to the sidewalls  16  in an inverted way as compared to the screens  200  and  300  shown in  FIGS. 2 and 3 . That means that a branch  402  of the screen  400  is mounted facing the lateral side  114  of the glass substrate  100  while branch  404  is attached to the top end of branch  402 . Branch  402  is mounted to sidewall  16  and extends across substrate front plane  120  into the upper portion, e.g. in the coating region, of the coating chamber. Branch  404  protrudes substantially in parallel to the substrate front plane  120  and 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  120 . 
     Correspondingly, branch  452  of screen  450  shown in  FIG. 4   b  is mounted facing the lateral side  114  of the glass substrate  100  while branch  454  is attached to the top end of branch  452 . Branch  452  is mounted to sidewall  16  and extends across substrate front plane  120  into the upper portion, e.g. in the coating region, of the coating chamber. Branch  454  protrudes partially over the front side  110  of the glass substrate  100  and substantially in parallel to the substrate front plane  120 . Branch  454  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  110  of the glass substrates  100  during transport. 
     According to a further variation of embodiments described herein, said lateral end of the protruding member of the screen is formed to taper away from the front side of a plate-shaped substrate  100  on the substrate support. In  FIGS. 5   a  and  5   b , respectively, examples of such a backside coating prevention device are shown, which correspond to the embodiment shown in  FIG. 2  except for the design of the protruding lateral ends of the protruding branches. For instance, L-shaped screen  500  of  FIG. 5   a  includes a branch  502  attached to the sidewall  16  and a branch  504  protruding above the substrate front plane  120 . The lateral end  506  of branch  504  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  550  shown in  FIG. 5   b  has two branches  552  and  554 . Branch  552  is attached to the sidewall  16  while branch  554  protrudes substantially in parallel to the substrate front plane  120 . In addition, the lateral end  556  of branch  554  has a tapered form such that the protruding central tip thereof is directed towards the opposing sidewall  16  of the coating chamber  10 . In the examples of  FIGS. 5   a  and  5   b , since the protruding laterals ends  506  and  556  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  500 ,  550  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  210  may be provided in the tapered portion of the protruding branches  504 ,  554  while the bottom surface of these branches  504 ,  554  is spaced from the substrate front plane  120  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. 
     In further examples according to embodiments described herein and shown in  FIGS. 6   a  and  6   b , respectively, the screens of  FIGS. 4   a  and  4   b  are modified to include tapered lateral ends. The example of  FIG. 6   a  includes a branch  602  attached to the sidewall  16  and a branch  604  protruding above the substrate front plane  120 . The lateral end  606  of branch  604  has a wedge-shaped slanted face such that the protruding tip thereof is directed towards the top of the coating chamber  10 . Screen  650  of the example shown in  FIG. 6   b  has two branches  652  and  654 , wherein branch  654  protrudes substantially in parallel to the substrate front plane  120 , and branch  652  is attached to the sidewall  16 . In addition, the lateral end  656  of branch  654  has a tapered form such that the protruding central tip thereof is directed towards the opposing sidewall  16  of the coating chamber  10 . The tapered lateral ends  606  and  656  of the backside coating prevention device of the examples as shown in  FIGS. 6   a  and  6   b  result in an improved clearance of the glass substrate during transport, such that contact or collisions of the glass substrate with the screens  600 ,  650  are prevented. 
     In  FIGS. 7   a  and  7   b , embodiments of the backside coating prevention device are shown, comprising screens having protruding members being inclined with respect to the sidewalls  16 . The inclination of the protruding members is in these embodiments different from 90 degrees. In other words, the protruding members are not substantially perpendicular with respect to the sidewall but are slanted. According to the example of  FIG. 7   a , a branch  702  of a screen  700  is attached to the sidewall  16 , while a slanted branch  704  is mounted facing downwards at the bottom end of branch  702 . Branch  702  is mounted to the sidewall  16  at such an installation height that a protruding lateral end  706  of branch  704  is spaced in the range from 1.5 mm to 5 mm from the front side of the glass substrate  100 , while it is directed towards the front side  110  of the glass substrate  100 . In other words, gap  210  has a width in the range from 1.5 mm to 5 mm. Accordingly, in the example of  FIG. 7   b , a branch  754  of a screen  750  is attached in an inclined way at a bottom end of a branch  752  mounted at the sidewall  16 , slanted branch  754  facing upwards. The installation height of branch  752  is such that a protruding lateral end  756  of branch  754  is spaced in the range from 1.5 mm to 5 mm from and above the front side of the glass substrate  100 . In other words, gap  210  has a width in the range from 1.5 mm to 5 mm. Again, backside coating of the glass substrate  100  is reduced or prevented, since the resulting small width of the gaps  210  between screens  700  and  750 , respectively, and the glass substrate  100  during transport reduces or inhibits passage of sputtered Ag particles to the backside of the glass substrate  100 . 
       FIGS. 8   a  and  8   b , respectively, show other examples of backside coating prevention devices according to embodiments described herein. In the example of  FIG. 8   a , a branch  802  of a screen  800  is mounted at the sidewall  16  while a branch  804  is attached to the top end of branch  802  in an inclined way with respect to branch  802 . The inclination of the protruding member, i.e. of branch  804  in this example, is different from 90 degrees. Branch  802  is mounted to sidewall  16  below the substrate front plane  120 . Branch  804  is further directed upwards such that its lateral end  806  is positioned above the lateral side  114  of glass substrate  100 . In the example shown in  FIG. 8   b , branch  852  is attached to the sidewall at such an installation height that it is facing the lateral side  114  of glass substrate  100 . Branch  852  is mounted to sidewall  16  and extends across substrate front plane  120  into the upper portion, e.g. in the coating region, of the coating chamber. An inclined branch  854  having a lateral end  856  is mounted at the top end of branch  852  and is directed downwards to the front side of the glass substrate  100 . Branches  804  and  854  are each 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  110  of the glass substrate  100 . Because of the resulting small width of the gaps  210  between the screens  800  and  850 , respectively, and the glass substrate  100  on the transport system, both embodiments allow for a reduced or prevented backside coating of the glass substrate  100 . 
     In other embodiments of the backside coating prevention device, the screens may have protruding members formed by a variation of the thickness of the screens. For instance, examples of the backside coating prevention device shown in  FIGS. 9   a  and  9   b , respectively, have stepped cross-sectional shapes. That means that, as is shown in  FIG. 9   a , a screen  900  is a panel  902  comprising a protrusion  904  at its bottom end. Thereby, a stepped screen  900  is formed, which is mounted at an installation height at sidewall  16  such that protrusion  904  is positioned spaced from and above glass substrate  100  during transport. Typically, the gap width of gap  210  between the bottom surface of protrusion  904  and substrate front plane  120  is in the range of 1.5 mm to 5 mm, more typically about 2 mm. Screen  950  shown in  FIG. 9   b  is similar to screen  900  but mounted in an inverted way as compared to the example of  FIG. 9   a . Screen  950  includes a panel  952  having a protrusion  954  protruding from the top end of panel  952 . Panel  952  is mounted at the sidewall  16  at such installation height that protrusion  954  has an bottom surface extending substantially in parallel to and being spaced apart from the front side of glass substrate  100 . Typically, the gap width of gap  210  between the bottom surface of protrusion  954  and substrate front plane  120  is in the range of 1.5 mm to 5 mm, more typically about 2 mm. Both examples result in a reliable reduction or prevention of a backside coating of glass substrate  100  because of the small width of the gaps  210  between screens  900  and  950 , respectively, and the front side of glass substrate  100  in the range from 1.5 to 5 mm, typically about 2 mm. Moreover, screens  900  and  950  additionally shield a part of the sidewall  16  facing the lateral side  114  of the glass substrate, thus preventing contamination of the sidewall  16  by coating material in this area of the sidewall  16 . 
     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  120 . An example of these embodiments is shown in  FIG. 10 . 
     According to  FIG. 10 , a screen  1000  comprises a branch  1002  attached to the sidewall  16  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  120 , as shown in  FIG. 10 . 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. 10 , substantially all Ag coating particles travelling towards the gaps  50  between the glass substrate  100  and the sidewalls  16  are deposited on the upper surfaces of the protruding branches  1004 . Hence, by providing screens  1000  on both sidewalls  16  of the coating chamber, passage of Ag particles through the gaps  50  between the glass substrates  100  and the sidewalls  16  towards the backside  105  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. 10 , 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  120 , resulting in a first position of the protruding member. In other examples, the installation height of branch  1004  shown in  FIG. 10  may be adjusted so that the lower side of branch  1004  is substantially aligned with the substrate front plane  120 , 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. 10  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. 10  is positioned on the substrate front plane  120 . Alternatively, other parts of the lateral end  1006  may be positioned on the substrate front plane  120 . 
     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. 10 , 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. 10 , 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  16 . Furthermore, for each screen the branch which is attached to the sidewall  16  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. 10  will also result in a backside prevention effect as mentioned above for the example of  FIG. 10 . 
     In an alternative embodiment of the example shown in  FIG. 10 , the screens of the backside coating prevention device do not include a protruding branch  1004 . Instead, only a single branch  1002  is provided and extends across substrate front plane  120 . Thus, a gap is formed between a lateral surface of branch  1002  and lateral side  114  of the glass substrate. In this embodiment, the thickness of branch  1002  is adjusted so that the width of the gap formed between branch  1002  and lateral side  114  is about 1.5 to about 10 mm, typically about 1.5 to about 5 mm, and most typically about 2 mm. Thus, it is not necessary to provide a screen with a specific profile but, alternatively, a plain panel having a specific thickness may be provided as the backside coating prevention device. 
     Furthermore, it will be understood by those skilled in the art that in the above embodiments described with reference to  FIG. 10 , i.e. the embodiments having a lateral gap  1010  instead of a vertical gap  210 , the dimensions of the branches, especially of the protruding branches, will be adjusted with respect to the width and the thickness of the glass substrates to be coated. Moreover, it will be understood by those skilled in the art that in all embodiments described herein the coating chamber  10  will be designed for glass substrates of specific dimensions so that the dimensions, especially the lengths, of the backside coating prevention devices can be specifically adjusted to those dimensions of the substrates. Thus, by knowing the dimensions of the glass substrates for which the chamber is designed, the skilled person can determine the correct dimensions of the backside coating prevention devices so that a specific gap width is achieved in the chamber during operation. 
     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. 11  showing a cross-sectional view of one example of embodiments described herein, a screen  2000  may be an elongated panel  2004  mounted at its lower side at fixture  2010  of an elongated holder  2008  protruding from a side wall  16 . According to another example (not shown), the lower side of panel  2004  may be directly attached to the holder  2008 , with no fixture or spacing between the lower side of the panel  2004  and the upper side of holder  2008 . The panel  2004  may extend substantially in parallel to the transport path  60  and, hence, to the substrate front plane  120 . As illustrated in  FIG. 11 , the holder  2008  may be an elongated protrusion of the side wall  16 . Hence, it may be an integral part of the side wall  16 , as shown in  FIG. 11 . Alternatively, the holder  2008  may be an elongated member attached at the sidewall  16 . According to the example shown in  FIG. 11 , the holder  2008  and the panel  2004  extend along the sidewall  16  in parallel to each other and in parallel to the transport path  60 . Furthermore, in the present example, the dimensions of panel  2004  are such that a front side  2006  of the panel  2004  is substantially aligned with the lateral side  114  of the substrate  100 . Typically, the holder  2008  may be provided at an installation height at sidewall  16  such that the lower side of panel  2004  is positioned spaced from and above glass substrate  100  during transport, such that the gap  210  is formed between the panel  2004  and the substrate  100 . The gap width of the gap  210  between the bottom surface of the panel  2004  and the substrate front plane  120  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  2000  as shown in  FIG. 11 , the holder  2008  may be provided at the sidewall  16  in an installation height such that the upper side of the holder  2008  is substantially aligned with the front side  110  of the substrate and, at the same time, the gap  210  is formed between the panel  2004  and the substrate  100 . Each above mentioned design of the example shown in  FIG. 11  results in a reliable reduction or prevention of a backside coating of glass substrate  100 , because of the small width of the gaps  210  between screen  2000  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, respectively, the edges of the protruding members  204 ,  304 ,  404 ,  454 ,  504 ,  554 ,  604 ,  654 ,  704 ,  754 ,  804 ,  854 , and  1004  as well as of the panels  900 ,  950 , and  2004  and of the holder  2008  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. 
     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  being transported in a direction parallel to the longitudinal axis of the tube-shaped vessel, the circular front and rear lids being the front and rear walls as defined above, the sidewalls as defined above being the areas of the tube-shaped wall facing the lateral ends of the glass substrates during transport. 
     Furthermore, in each of the above embodiments and examples 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  16  above and/or below the substrate front plane  120 , i.e. above and/or below the screens  200 ,  300 ,  400 ,  450 ,  500 ,  550 ,  600 ,  650 ,  700 ,  750 ,  800 ,  850 ,  900 ,  950 ,  1000  and  2000 , 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. 
     According to one embodiment, a backside coating prevention device adapted for a coating chamber for coating plate-shaped substrates is provided, said coating chamber comprising a plurality of walls, a coating material source adapted for dispensing coating material into the coating chamber, a substrate support, a front side of the substrate support facing the coating material source, the substrate support being adapted for supporting on the front side one or more plate-shaped substrates each having a substrate front side and thereby defining a substrate front plane, wherein said backside coating prevention device comprises two or more screens, the screens being provided at least two of the walls of the coating chamber, each screen having a protruding member protruding from the respective wall. 
     In a modification of the above embodiment, said protruding member is positioned to be spaced in the range from 1.5 mm to 5 mm from the one or more plate-shaped substrates during coating. 
     In a modification of any of the above embodiment and the above modification thereof, said protruding member protrudes from the respective wall such that the protruding member is positioned between the coating material source and the substrate front plane. 
     In a modification of any of the above embodiment and the above modifications thereof, said protruding member is substantially aligned with the substrate front plane. 
     According to a further embodiment, a coating chamber for coating plate-shaped substrates comprises a plurality of walls, a coating material source adapted for dispensing coating material into the coating chamber, a substrate support, a front side of the substrate support facing the coating material source, the substrate support being adapted for supporting on the front side one or more plate-shaped substrates each having a substrate front side and thereby defining a substrate front plane, and a backside coating prevention device comprising two or more screens, the screens being provided at least two of the walls of the coating chamber, each screen having a protruding member protruding from the respective wall. 
     In a modification of the above further embodiment, said protruding member is positioned to be spaced in the range from 1.5 mm to 5 mm from the one or more plate-shaped substrates during coating. 
     In a modification of any of the above further embodiment and the above modification thereof, said protruding member protrudes from the respective wall such that the protruding member is positioned between the coating material source and the substrate front plane. 
     In a modification of any of the above embodiments and the above modifications thereof, the substrate front side of each of said one or more plate-shaped substrates is to be coated and faces the coating material source during coating, said one or more plate-shaped substrates each having a backside, which is opposite to the substrate front side and faces the substrate support during being supported thereon, and two lateral sides, said protruding member has a lateral end protruding into the coating chamber, said lateral end being positioned to be spaced apart from but substantially aligned with one of the lateral sides of at least one of the one or more plate-shaped substrates on the substrate support, or said lateral end being positioned to extend over a lateral part of the substrate front side of at least one of the one or more plate-shaped substrates on the substrate support. 
     In a modification of any of the above embodiments and the above modifications thereof, said lateral end is formed to taper away from the substrate front side of a plate-shaped substrate on the substrate support. 
     In a modification of any of the above embodiments and the above modifications thereof, said protruding member is substantially aligned with the substrate front plane. 
     In a modification of any of the above embodiments and the above modifications thereof, each screen comprises a first branch and a second branch, said first branch being attached to one of the walls and said second branch forming the protruding member. 
     In a modification of any of the above embodiments and the above modifications thereof, said first and second branches are arranged substantially perpendicularly to each other. 
     In a modification of any of the above embodiments and the above modifications thereof, said coating material source is adapted to dispense coating material at least into a coating region of the coating chamber and each screen is provided at least in the coating region. 
     In a modification of any of the above embodiments and the above modifications thereof, said coating chamber is adapted for coating continuously or discontinuously transported plate-shaped substrates, the substrate support being a transport system adapted for continuously or discontinuously transporting the plate-shaped substrates along a transport path, said backside coating prevention device being provided at two opposite sidewalls of the coating chamber and comprising at least two of the screens, each of the two sidewalls being provided with at least one screen thereof, each screen having the protruding member protruding from the respective sidewall. 
     In a modification of any of the above embodiments and the above modifications thereof, said protruding member extends along the respective sidewall substantially in parallel to the transport path. 
     In a modification of any of the above embodiments and the above modifications thereof, said coating chamber is adapted for coating by sputtering, the coating material source being a sputtering cathode. 
     According to another embodiment, a method of coating plate-shaped substrates in a coating chamber is provided, comprising providing a plate-shaped substrate on a substrate support of the coating chamber, dispensing coating material from a coating material source provided in the coating chamber towards the plate-shaped substrate, and simultaneously preventing backside coating by two or more screens, the screens being provided at least two walls of the coating chamber, each screen having a protruding member protruding from the respective wall. 
     In a modification of the above another embodiment, said preventing of backside coating is effected by said screens, each screen having the protruding member being positioned so that each protruding member is spaced in the range from 1.5 mm to 5 mm from the plate-shaped substrate during coating. 
     In a modification of any of the above another embodiment and the above modification thereof, said preventing of backside coating is effected by said screens, each screen having the protruding member protruding from the respective wall such that the protruding member is positioned between the coating material source and the plate-shaped substrate. 
     In a modification of any of the above another embodiment and the above modifications thereof, said plate-shaped substrate has a substrate front side to be coated and facing the coating material source during coating, a backside opposite to the front side and facing the substrate support during being supported thereon, and two lateral sides, said protruding member has a lateral end protruding into the coating chamber, said lateral end being spaced apart from but substantially aligned with one of the lateral sides of the plate-shaped substrate on the substrate support, or said lateral end extending over a lateral part of the substrate front side of the plate-shaped substrate on the substrate support. 
     In a modification of any of the above another embodiment and the above modifications thereof, said lateral end is tapering away from the front side of the plate-shaped substrate. 
     In a modification of any of the above another embodiment and the above modifications thereof, said plate-shaped substrate has a substrate front side to be coated and thereby defines a substrate front plane, and wherein said protruding member is substantially aligned with the substrate front plane. 
     In a modification of any of the above another embodiment and the above modifications thereof, each screen comprises a first branch and a second branch, said first branch being attached to one of the walls and said second branch forming the protruding member. 
     In a modification of any of the above another embodiment and the above modifications thereof, said first and second branches are arranged substantially perpendicularly to each other. 
     A modification of any of the above another embodiment and the above modifications thereof comprises providing the plate-shaped substrate by feeding the plate-shaped substrate into the coating chamber and arranging the plate-shaped substrate on the substrate support, the substrate support being a transport system for continuously or discontinuously transporting the plate-shaped substrate along a transport path, continuously or discontinuously transporting the plate-shaped substrate by the transport system along the transport path, while dispensing coating material from the coating material source and preventing backside coating of the plate-shaped substrate by at least two of the screens provided at two opposite sidewalls of the coating chamber, each of the two sidewalls being provided with at least one screen thereof, said protruding member of each screen extending along the respective sidewall substantially in parallel to the transport path, and discharging the plate-shaped substrate from the coating chamber. 
     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.