Abstract:
The invention relates to an apparatus for evaporating solid materials, for instance, selenium for coating a substrate. The solid material is brought into a first crucible via a feeding source. In this crucible the material melts at a temperature which is preferably slightly higher than its melting point. The molten material flows via a transporting device, for instance, a pipe into a second crucible, where the material evaporates at a temperature higher than its boiling point and is transported to a substrate. In order to stop the evaporation within a very short time preferably within only one to two minutes, a cooling device for cooling the material beyond its melting point is arranged at the transporting device. With this cooling device the material in the transporting device can be cooled down beyond its melting point in a very short time.

Description:
PRIORITY 
       [0001]    The present application claims priority from to commonly owned and assigned application No. 60/950,778, Attorney Docket No. APPL-026/00US, entitled Vacuum Evaporation Apparatus for Solid Materials, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a vacuum evaporation apparatus for solid materials. 
       BACKGROUND OF THE INVENTION 
       [0003]    An important goal in modern research in energy is to find methods to convert sunlight into current, having only low costs. Thin film solar cells promise to have lower costs compared with common solar cells. 
         [0004]    These thin films can, for instance, be produced by a sputtering process or by evaporation of solid materials onto a substrate. A continuous vacuum evaporation device for metal has been described, where a substrate to be coated moves within a vacuum chamber (U.S. Pat. No. 4,880,960). A deposition material in particle form is stored in a reservoir closed on its lowermost part by a plate. A cable links the plate to a valve which closes the supply opening of a crucible heated to a temperature for evaporation of the deposition material. A control device using at least one electromagnet allows the plate to pivot and thus to open the valve. A quantity of powder falls into the crucible, then the plate and the valve are returned to closed position. A screen with a mesh having dimensions smaller than those of the deposition material particles in the crucible allows passage of the vapor only on subsequent deposition onto the substrate. 
         [0005]    Furthermore, there is described an apparatus for coating a substrate, wherein the apparatus comprises an evaporation source and a system for distributing the evaporated material onto the substrate (DE 102 24 908 A1). The system for distributing further comprises a line source, wherein the line source and the substrate can be moved relatively to each other. 
         [0006]    A method for depositing semiconductor layers on a substrate for evaporation in vacuum and by dosed supply of a base product in N evaporators has also been described (FR 2 456 144). The supply of the product is periodically. Each of the N evaporators is, one after another, heated up to a temperature lower than the boiling temperature of the base product and is loaded with a dose which is necessary for the continuous coating of the active portion of the evaporator and which is sufficient for a complete vaporisation during time T until the next loading. 
         [0007]    One problem, however, which exists, when a product is evaporated, is that the evaporation can hardly be stopped within a few seconds. This is because the material has to cool off and this needs some time. 
         [0008]    Therefore, it is an object of the present invention to solve the problem mentioned above. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention therefore relates to an apparatus for evaporating solid materials, for instance, selenium for coating a substrate. The solid material is brought into a first crucible via a feeding source. In this crucible the material melts at a temperature which is preferably slightly higher than its melting point. The molten material flows via a transporting device, for instance, a pipe into a second crucible, where the material evaporates at a temperature higher than its boiling point and is transported to a substrate. In order to stop the evaporation within a very short time preferably within one to two minutes, a cooling device for cooling the material beyond its melting point is arranged at the transporting device. With this cooling device the material in the transporting device can be cooled down beyond its melting point in a very short time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention is shown in the figures and explained by the following description. 
           [0011]      FIG. 1  shows a perspective general view of a vacuum evaporation apparatus for solid materials with a substrate to be coated. 
           [0012]      FIG. 2  shows a view in direction of A of the apparatus according to  FIG. 1  without the substrate. 
           [0013]      FIG. 3  shows a cut B-B through the apparatus according to  FIG. 1 . 
           [0014]      FIG. 4  shows a part of another embodiment of the apparatus according to  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  shows a perspective general view of a vacuum evaporation apparatus  1  for solid materials and a substrate  2 . This substrate  2 , e.g. a glass plate, is moved relatively to the apparatus  1  in the direction  7  or  8 , respectively. The apparatus  1  comprises a housing  3  having feet for a safe standing. In  FIG. 1  only the feet  4 ,  5 ,  6  can be seen. 
         [0016]    On top of the housing  3  a charging device  9  is arranged which has an upper part  10 , a middle part  11  and a lower part  12 . The middle part  11  is vacuum sealed and provides a charging valve  14 . On top of the upper part  10  a removable lid  13  is arranged. This lid  13  can be removed if the upper part  10  has to be provided with solid material, for instance selenium. 
         [0017]    However, before the upper part  10  is filled with said solid material, the valve  14  has to be closed. Then the upper part  10  is flooded with a gas, e.g. air or nitrogen, via a pipe which is not shown in  FIG. 1 . After the upper part  10  has been flooded, the lid  13  is removed and the upper part  10  is provided with the solid material. 
         [0018]    Afterwards the lid  13  is replaced and fastened and the upper part  10  is evacuated by a pump not shown in  FIG. 1 . The upper part  10  can now serve as a reservoir for the solid material. 
         [0019]    With the charging valve  14  the quantity of solid material coming from the upper part  10  can be regulated, so that only a defined quantity of solid material passes the lower part  12 , before it reaches the interior of the housing  3 . 
         [0020]    The housing  3  is connected to a pump system via a tube or pipe  15 , so that a vacuum can be created in the housing  3 . The pump system is not shown in  FIG. 1 . The apparatus  1  comprises furthermore a first and a second mounting flange  16 ,  17 . On the opposite side of the wall of the housing where the pipe  15  is arranged, another pipe  18  can be seen. This pipe serves for the transport of a cooling media, e.g. water, thermal oil, gas etc. 
         [0021]    Onto the mounting flange  17  a third mounting flange  19  is arranged which holds an evaporation pipe  20  via a holding device  21  for holding the evaporation pipe  20 . The evaporation pipe  20  itself has a holding device  22  by its own, which is fixed to the holding device  21  of the mounting flange  19 . These two holding devices form a holder  23 . Although only one holder  23  is shown in  FIG. 1 , the evaporation pipe  20  can be fixed to the mounting flange  13  by more than one holder  23 . However, due to the weight of the evaporation pipe  20 , the holder  23  can be omitted. 
         [0022]    This evaporation pipe  20  is arranged on a device  24  in which a crucible for evaporating the material is arranged, which cannot be seen in  FIG. 1 . The evaporation pipe  20  furthermore has a lid  25  which is connected to the evaporation pipe  20  via a ring  26 , which is fastened by a fastener  27 , e.g. a screw. 
         [0023]    As illustrated in  FIG. 1 , the evaporation pipe  20  is fixed to the device  24  by a ring  28  which also comprises a fastener  29 . The evaporation pipe  20  is arranged to the device  24  in such a way that the vapor in the pipe  20  can only leave it by a distribution system of the evaporation pipe  20  which cannot be seen in  FIG. 1 . 
         [0024]    The device  24  is connected to a protrusion  30  of a device  31  for holding a crucible which is arranged inside the housing  3  via a pipe  32 . 
         [0025]      FIG. 2  shows a view in direction of A of the apparatus  1  according to  FIG. 1  without the substrate  2 . The evaporation pipe  20  being arranged to the mounting flange  19  is connected to the device  24  and fastened to it by the ring  28  and the fastener  29 . The evaporation pipe  20  comprises holes  33  to  38  forming a linear distribution system  39 , through which the vapor can leave the evaporation pipe  20  and can move towards the substrate. Then the vapor condenses on said substrate to form a film. 
         [0026]    A cut B-B through the apparatus  1  according to  FIG. 1  is shown in  FIG. 3 . There the charging device  9  is mounted on a wall  40  of the housing  3 . The upper part  10  serves as a storage room for a solid material, for instance selenium. Below the charging device  9  a storage container  41  is arranged in the housing  3  having a pipe  42  through which the material coming from the charging device  9  can get into a melting crucible  43 . 
         [0027]    In  FIG. 3  the storage container  41  has the body structure of a funnel. Furthermore, a pipe  42  and a feeder  48  are arranged in the interior of the housing  3 . This feeder  48 , for instance a screw conveyer, acts as a device for supplying the pipe  42  with solid material coming from the storage container  41 . 
         [0028]    The melting crucible  43  is arranged within the device  31  for holding the crucible  43 . The crucible  43  can be heated by thermal oil which is kept between two walls  44 ,  45  in an intermediate room  46 . Instead of heating the crucible  43  by thermal oil, it can also be heated by a resistance heater or an induction coil. On top of the crucible  43  a hood  47  is provided. But when the cooling process is started, this intermediate room  46  acts as quick shut down device for cooling, as it serves as a pathway for a cooling medium, for instance water. 
         [0029]      FIG. 3  shows furthermore the pipe  32  with an inner pipe  49  in that preferably a resistance heater is arranged. The resistance heater is not shown in  FIG. 3 , however. This pipe  32  connects the crucible  43  with a crucible  50 . This crucible  50  contains molten material  64 . In this crucible  50  a heater is arranged, preferably a resistance heater. The inner pipe  49 , the pipe  32  as well as the crucibles  50  and  43  can consist of an appropriate metal, ceramics or graphite. The inner pipe  49  is surrounded by a quick shut down device  51  in form of pipes for cooling the molten material in the pipe  49 . However, when the heating process takes place, these quick shut down devices act as a pathway for a heating medium, e.g. thermal oil. The shut down device  51  for cooling the molten material in the pipe  49  is arranged on both sides of the protrusion  30 . 
         [0030]    Another shut down device  52  for cooling is arranged also in form of pipes around the outer parts of the crucible  50 . On top of the device  24  holding the crucible  50  the evaporation pipe  20  is arranged. 
         [0031]    This evaporation pipe  20  comprises a first inner pipe  53  having the distribution system  39 , showing to the side where the substrate  2  passes the apparatus  1 . The inner pipe  53  is surrounded by at least one half pipe. In  FIG. 3  the inner pipe  53  is surrounded by two half pipes  54 ,  55 . Such an arrangement is shown for instance in FIG. 7 of DE 102 24 908 A1. The inner pipe  53  typically has a high temperature heating device, for example a resistance heater, so that the vapor in the interior  56  of the inner pipe  53  cannot condense. 
         [0032]    In  FIG. 3  there is shown a dividing wall  68  which is connected to the wall  44  of the crucible  43  via a bar  69 . This dividing wall  68  prevents material falling into the liquid material  65  inside the crucible  43  to have an effect on the uncontrolled cooling of the liquid material in the region of the inner pipe  49 . The bar  69  and the dividing wall  68  can consist of the same material as the crucible  43 , for instance an appropriate metal, graphite or ceramics. 
         [0033]    To control the temperature in the apparatus  1 , temperature measuring devices can be applied, so that the temperature of the material in the crucibles  43 ,  50 , the pipe  49  and the interior of the pipe  20  can be controlled. The temperature can therefore be adapted to the temperature needed at every stage of the process. 
         [0034]    When a coating process is started, the upper part  10  of the charging device  9  is filled with the material used for coating the substrate  2 . If the material is selenium, it preferably has the form of granules. 
         [0035]    By opening the charging valve  14 , the material gets into the storage container  41  without breaking the vacuum. The charging valve  14  will be closed and the material will be transported by the feeder  48  to the pipe  42 , to then eventually fall into the crucible  43 . In this crucible  43  the material is molten, the crucible  43  having a temperature which is higher than the melting point of the solid material. In the case of selenium the temperature only has to be slightly higher than its melting point of 221° C. 
         [0036]    The quantity of material falling into the crucible  43  can be controlled by the feeder  48 , which can be run automatically, so that a variation of the filling amplitude in the crucibles  43 ,  50  is kept at a minimum. In the housing  3  as well as in the charging device  9  having the lid  13  on top of it, there is a stable vacuum throughout the whole process. 
         [0037]    When the heating process is started thermal oil runs through the shut down devices  46 ,  51  and  52 . The thermal oil thus acts as a heating medium. The material in the crucibles  43 ,  50  and in the pipe  49  hence is heated by that thermal oil. 
         [0038]    Once the material got into the crucible  43  it is molten, so that it can pass the pipe  49  and can get into the crucible  50 . In this crucible  50  the temperature is much higher in order to convert the liquid material  64  into its gaseous form, i.e. vapor. This vapor rises into the interior  56  of the evaporation pipe  20  and leaves said pipe  20  via the linear distribution system  39 . The vapor then moves in the direction of the substrate  2  in order to generate a layer of that material as the substrate  2  passes the apparatus  1 . The heater arranged at the inner pipe  53  hinders the vapor to condense at the surface of the inner pipe  53  or in the holes of the distribution system  39 . 
         [0039]    When the process is stopped the heaters are shut off and the quick shut down devices  51 ,  52  for cooling are switched on, so that the cooling media, e.g. water, thermal oil, gas, runs through these shut down devices  46 ,  51 ,  52  for cooling in order to cool the material down. For a quick cooling of the molten material  64  in the crucible  50 , it is important that the crucible  50  does not comprise too much molten material  64 . Thus the volume of the crucible  50  is kept quite small compared to the volume of the crucible  43 . 
         [0040]    It is important to cool the material in the crucibles  43 ,  50  down to a temperature which lays beyond the melting point. After the material is cooled down, the cooling media is removed from the shut down devices  51 ,  52  so that the shut down devices  51 ,  52  can be filled with air again, having a heat capacity which is much lower compared to that of the cooling media. 
         [0041]    Cleaning the apparatus is also effortless, because the evaporation pipe  20  can be easily removed by removing the flange  19 . After this, the flanges  16 ,  17  can be removed so that the housing  3  is open to two sides. 
         [0042]    Therefore, maintenance of the apparatus  1  is also easy because all devices comprise clutches for quick coupling. Therefore, the device  9  can also be exchanged easily. After having cleaned the apparatus  1  a new process can be started using a new and different material for the coating process. 
         [0043]    The charging device  9  can comprise a star feeder with for instance four segments. One of those segments is arranged just above the valve  14 , the segment comprising no material. 
         [0044]    The charging procedure is started by closing the valve  14  and venting the upper part  10 . The lid  13  is then removed from the device  9  and the upper part  10  is charged with the solid material. Thereafter the lid  13  is replaced and the upper part  10  is evacuated by a pump not shown in  FIG. 3 . After the pressure has been compensated between the upper part  10  of the device  9  and the housing  3  again, the valve  14  can be opened. 
         [0045]    By turning the star feeder the material falls through the opening of the valve  14  so that it can get into the crucible  43  via the storage container  41 . Afterwards the valve  14  is closed again. 
         [0046]    The charging device  9  can also comprise a cup charger having at least one cup. This at least one cup can be canted like a crucible. 
         [0047]    After the device  9  is closed by the lid  13 , the valve  14  is closed and the upper part  10  is vented. The lid  13  is removed again and the at least one cup is filled with a material. The lid is put on top of the device  9  again and the valve  14  is opened after that the device  9  has been evacuated. Then the at least one cup is canted and the material falls through the opening of the valve  14  into the storage container  41  of the housing  3 . From there it runs through the pipe  42  to finally reach the crucible  43 . 
         [0048]    The device  9  can furthermore comprise a container charger having a rotary hoisting-gear and a hoisting-gear with a charger basket. 
         [0049]    When the charging process is started, the valve  14  is closed and the device  9  is vented. The device  9  is then lifted and pivoted to a side. The charging basket is lowered, so that the basket can be changed by a basket filled with material. The basket is then pulled into the device  9  by the hoisting-gear. The device  9  is pivoted over the valve  14  and then put upon the valve  14 . The device is then evacuated and the valve opened, so that the material can get into the crucible  43 . 
         [0050]    In this embodiment the material can be selenium having the form of granules. 
         [0051]      FIG. 4  shows a part of the embodiment of the apparatus  1  according to  FIG. 3  drawn to a larger scale. It shows the crucible  50  surrounded by pipes  57 ,  58 ,  59 ,  60  forming the shut down device  52  for cooling arranged in the device  24 . On top of the device  24  with the crucible  50  the evaporation pipe  20  is arranged. This pipe  20  has the linear distribution system  39  having openings  61 ,  62 ,  63  through which the vapor can leave the evaporation pipe  20 . The crucible  50  is connected to the crucible  43  via the inner pipe  49  surrounded by the outer pipe  32 . Again, as can be seen in  FIG. 4 , the pipe  49  is surrounded by the shut down cooling device  51 . The crucible  43  which is separated from the crucible  50  by the pipe  49  is formed out of the same material as the pipe  49  and the crucible  50 . However, the pipe  49  and the crucibles  50 ,  43  can be formed out of different materials. The pipe  49  and the crucibles  50 ,  43  preferably consist of graphite, ceramics or an appropriate metal. 
         [0052]    As shown in  FIG. 3  there can be seen the dividing wall  68  connected to the crucible  43  by the bar  69 . The dividing wall  68  and the bar  69  can consist of the same material as the crucible  43 . 
         [0053]    Again the liquid material  70 ,  65 , for example selenium, in the two crucibles  50 ,  43  has approximately the same altitude due to the circumstance that the quantity of the solid material can be controlled automatically via a computer by the valve  14 , not shown in  FIG. 4 , and the feeder  48 , not shown in  FIG. 4  either. However, the volume of the crucible  50  is much smaller compared to that of the crucible  43 . 
         [0054]    Instead of the quick shut down devices  46 ,  51 ,  52  it is possible to arrange a screen above the crucible  50  to prevent the vapor from leaving the evaporation pipe  20 . However, especially in the case when selenium is used as a material, it is hard to prevent selenium from passing the screen because of its high vapor pressure. Such an arrangement would have to be very complex in its construction. Therefore, the shut down devices  46 ,  51 ,  52  are preferred.