Patent Publication Number: US-8539907-B2

Title: Apparatus for chemical bath deposition between two covers, wherein a cover is a substrate

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Taiwan Patent Application No. 099141425, filed on Nov. 30, 2010, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for film deposition, and in particular relates to an apparatus for chemical bath deposition. 
     2. Description of the Related Art 
     Development in the solar cell industry is driven by global environmental concerns and rising raw material prices. Among the various solar cells developed, dye-sensitized solar cell (DSSC) is advantageous as it can be fabricated with relatively lower costs due to its simpler fabrication process and ability for large area fabrication. 
     In a solar cell, a buffer layer plays an important role, because it is used as an n-type material and it protects the absorption layer of the solar cell. Therefore, it is important to fabricate a high quality buffer layer to improve the photoelectric conversion efficiency of solar cells. 
     Techniques for deposition of the buffer layer include: sputtering, vacuum evaporation, chemical bath deposition (CBD), spray pyrolysis, in which chemical bath deposition (CBD), is a widely used method due to its simple process and required low cost equipment. However, the conventional CBD process consumes a large amount of chemicals, and results in a large amount of waste. 
       FIG. 1  shows a conventional chemical bath deposition (CBD) apparatus which comprises a crucible  11 , top cover  12  and a plurality of substrates  13  for film deposition. Because the substrates  13  are vertically disposed in the crucible  11 , the upper portion and the lower portion of the substrate  13  have a non-uniform film thickness. Additionally, a large amount of solution for film deposition is needed, and after the deposition process, washing of the crucible  11  is needed. 
     Thus, there is a need to provide a chemical bath bat-deposition apparatus with a simple process, using fewer chemicals. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides an apparatus for chemical bath deposition, comprising: a first cover and a second cover, wherein the first cover and the second cover are disposed opposite to each other, and the first cover has at least two holes; and a spacer disposed between the first cover and the second cover, wherein the first cover, the spacer and the second cover form a film deposition space. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a cross-sectional representation of an apparatus for chemical bath deposition (CBD) in accordance with an conventional technique; and 
         FIGS. 2 and 3  show a series of cross-sectional schematic representations of an apparatus for chemical bath deposition (CBD) in accordance with an embodiment of the invention; 
         FIG. 4  shows a cross-sectional schematic representation of an apparatus for chemical bath deposition (CBD) in accordance with another embodiment of the invention; 
         FIGS. 5A-5B  show surface morphology representations of deposition film in accordance with an embodiment of the invention; 
         FIGS. 5C-5D  show a microscopic representation of deposition film in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     Referring to  FIG. 2 , the invention provides an apparatus for chemical bath deposition (CBD) which comprises a first cover  21 , a second cover  22  and a spacer  23 , wherein the first cover  21  and the second cover  22  are disposed opposite to each other, and the first cover  21 , the second cover  22  and the spacer  23  forms a film deposition space  25 . 
     In order to improve the seal-tightness between the first cover  21  and the second cover  22 , the edge of first cover  21  or the second cover  22  has a groove (not shown in  FIG. 2 ). The spacer  23  may be embedded in the groove of the first cover  21  or the second cover  22 . As a result, a solution (e.g. a solution for film deposition) is sealed in the film deposition space  25  formed by the first cover  21 , the second cover  22  and the spacer  23 . Thus, the solution leaking problem may be improved by the help of the groove. The groove comprises a circular, rectangular or irregular shape. The shape of the groove is not limited to the above-mentioned shape, and those skilled in the art may adjust the shape according to actual application needs. 
     The function of the first cover  21  is to reduce the evaporation loss of the solution. The first cover  21  has at least two holes  24 , and one hole  24   a  is used as a solution inlet, and the other hole  24   b  is used as a solution outlet. When the solution is prepared to be filled into the solution inlet  24   a , the solution outlet  24   b  is opened to balance the pressure in the film deposition space  25  to help the injection of solution. The holes have a diameter of about 3-5 mm. Note that the diameter may not be too large in order to avoid evaporation of the solution and degradation of film quality. 
     The first cover  21  has good corrosion resistance, and acid/base resistance, and is made of material such as aluminum alloy, glass, quartz, aluminum oxide or polymer, wherein the polymer comprises polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE) or polypropylene (PP). In one embodiment, the material of the first cover  21  is preferred to be polytetrafluoroethylene (PTFE). Because PTFE has low surface energy, a film would not likely form on the PTFE following the deposition process. Thus, after the deposition process, the first cover  21  made of PTFE, would be easy to wash. 
     Additionally, the first cover  21  also provides a pressure to the second cover  22 , and the pressure prevents the solution from leaking out therefrom, to improve the seal-tightness of the first cover  21  and the second cover  22 . The spacer  23  is used to seal the first cover  21  and the second cover  22 , and thus the seal-tightness of the spacer  23  my also be improved by the pressure which is provided by the first cover  21 . 
     The second cover  22  may be a substrate for film deposition, and the material of the second cover comprises glass, stainless steel or polyimide (PI). Moreover, a substrate for film deposition may be disposed on the inner surface of the first cover  21 , while the second cover  22  is another substrate for film deposition, wherein the two substrates for film deposition may be disposed simultaneously. Note that the substrate for film deposition may be the second cover or further be disposed on the inner surface of the first cover, and those skilled in the art may adjust the arrangement of the substrate according to actual application needs. 
     The function of the spacer  23  is to seal the first cover  21  and the second cover  22 , and the spacer  23  has good elastic property, good acid/base resistance, and low surface energy. The material of the spacer  23  comprises rubber, silicone or polytetrafluoroethylene (PTFE). In one embodiment, an O-ring with a diameter of about 60 mm-200 mm and a thickness of about 2 mm-15 mm is used as the spacer  23 . 
     Note that the height of the solution for film deposition filled in the film deposition space  25  is determined by the height of the spacer  23 . In general, the height of the spacer  23  is about 2 mm-10 mm. Thus, waste is reduced due to the reduced heights. 
     Referring to  FIG. 3 , the invention provides an apparatus for chemical bath deposition (CBD) which further comprises a heater  26  disposed below the second cover  22 . In another embodiment, while a substrate for film deposition is disposed on the inner surface of the first cover  21 , the heater  26  may be disposed on the first cover  21 . The function of the heater  26  is to provide the high temperature environment needed for the film deposition. The heater  26  may be a conventional heater or a heat storage material. For example, a material (such as stainless steel or copper block) with high thermal conductivity is soaked into hot water, and then the material is taken out from the heated water to be used as a heat storage material. 
     Note that because the second cover  22  itself is a substrate for film deposition and the heater  26  is directly disposed below the second cover  22 , the substrate for film deposition is heated directly. In the conventional techniques, the solution rather than the substrate is heated. Thus, the advantages of the apparatus of the invention are to save energy and reduce film deposition time. 
     The apparatus of the invention further comprises a magnetic material in the first cover  21 . When the second cover  22  is disposed on the heater  26 , the magnetic material of the first cover  21  may be attracted by the heater  26  (made of magnetic material) to provide an additional pressure. The additional pressure may improve the seal-tightness between the first cover  21  and the second cover  22  and prevent the solution from leaking. 
     Furthermore, referring to  FIG. 3 , the invention provides an apparatus for chemical bath deposition (CBD) which further comprises a shaking device  27  disposed below the heater  26 . In another embodiment, the shaking device  27  may be directly disposed below the second cover  22 . In yet another embodiment, a shaking device incorporating a heating mechanism is disposed below the second cover  22 . The shaking device  27  may be shaken by forward spinning, reverse spinning, shaking, rotation or revolution to obtain a uniform film. 
     Referring to  FIG. 4 , the invention also provides a second embodiment. The difference between the  FIG. 4  and  FIG. 2  is that the first cover  21  has an outer edge, and the outer edge has an extension portion  21   a , and the extension portion  21   a  contacts the spacer  23  in  FIG. 4 . The extension portion  21   a  has a length L and a width D. The length L may be adjusted to a desired height according to the solution needed. The width D may be adjusted according to the size of the spacer  23  needed. If a small sized spacer  23  is provided, a small contact area between the spacer  23  and the first cover  21 /second cover  22  will be obtained. Thus, the seal-tightness between the first cover  21  and the second cover  22  may further be improved by reducing the contact area. 
     In the film deposition process, the solution is injected from the hole  24 , and a desired thickness of film is obtained by controlling the time and temperature of the film deposition process. After the deposition process, a washing process is conducted by injecting air, argon, nitrogen or de-ionized water from the inlet  24   a  into the space  25  to wash the first cover  21  and the second cover  22 . The waste solution is then exhausted from the outlet  24   b . The simple washing process is provided to reduce cost. 
     Additionally, before conducting the above-mentioned film deposition process, the surface composition or the surface morphology of the substrate may be changed by a specific compound. For example, before fabricating the buffer layer of CIGS cell, the substrate is etched by the bromine water to change the surface morphology thereof, or the substrate is dipped into a potassium cyanate (KCN) solution to change the surface composition thereof. Thus, the toxic bromine water and the toxic potassium cyanate (KCN) solution are limited in the small sized film deposition space  25  by the apparatus of the invention. Thus, the use of the toxic material is reduced. 
     In prior art, a crucible is often used as a solution container, and the film is not only grown on a substrate but also on the crucible. Thus, the crucible is needed to be washed after every cycle of the film deposition process. Note that the invention provides an apparatus formed by the first cover, the second cover and the spacer without the crucible, and the film is only grown on the substrate for film deposition (such as the second cover). Thus, the utilization rate of the solution is improved, and the use of the solution is reduced and no additional washing crucible process is needed. 
     Therefore, the apparatus for CBD of the invention has several advantages as follows. 
     (1) By the design of the first cover, the second cover and the spacer, the utilization degree of the solution is improved, the use of the solution is reduced and waste is reduced. 
     (2) The substrate for film deposition is heated directly to save energy and reduce film deposition time. 
     (3) The first cover or the second cover is used as a container rather than the crucible to simplify the washing process and reduce time and cost. 
     EXAMPLE 
     Example 1 
     0.00185 M of cadmium sulfate (CdSO 4 ), 1.5 M of ammonia (NH 4 OH) and 0.075 M of thiourea ((NH 2 ) 2 CS) were mixed together to prepare a solution for film deposition. 
     The apparatus  20  is shown in  FIG. 3 , and the second cover  22  had an area of about 50 cm 2 , the spacer  23  had a diameter of about 80 mm, the spacer  23  had a thickness of about 6 mm and the height of the solution was about 3 mm. 
     The film deposition process is described as follows: 
     (1) a second cover (glass substrate)  22  was disposed on a heater  26 ; 
     (2) a spacer  23  and the first cover  21  (made of PTFE) were disposed on the second cover  22 ; 
     (3) a solution for film deposition was injected from a hole  24  into a film deposition space  25 ; 
     (4) the solution was heated at 70° C. for 20 minutes to obtain a cadmium sulfide (CdS) film with a thickness of 80 mm, or at 70° C. for 40 minutes to obtain a cadmium sulfide (CdS) film with a thickness of 100 mm; and 
     (5) After the film deposition process, a washing process was conducted, and de-ionized water was injected into the space  25  through an inlet  24   a  and was exhausted through outlet  24   b  to wash the apparatus  20 . 
     Example 2 
     Example 2 is similar with Example 1, with the difference being that a shaking device (YSC. Company)  27  was added in Example 2. 
       FIGS. 5A-5B  show the surface morphology representations of the deposition film under the condition at 70° C. for 30 minutes. Compared with  FIG. 5A  (Example 1),  FIG. 5B  (Example 2) shows a more uniform film.  FIGS. 5C-5D  respectively show the microscopic representations of the deposition film of  FIG. 5A-5B  (×100 times). As shown in  FIG. 5C , some holes appeared in the film of Example 1. Compared with  FIG. 5C ,  FIG. 5D  (Example 2) shows a smooth film. Thus, the quality of the film was improved by incorporating the shaking device with the apparatus of the invention. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.