Abstract:
A method and device for depositing a semiconductor film. The method includes: a) carrying a semiconductor material by a carrier gas to a crucible installed in a vacuum deposition chamber via a passage; and b) heating the crucible to sublimate the semiconductor material to be vapor and depositing the vapor on a substrate. The device includes a semiconductor material feeding device, a passage, a vacuum deposition chamber, a crucible installed in the vacuum deposition chamber, and a substrate located above the crucible. The semiconductor material feeding device and the crucible are connected via the passage. The semiconductor material feeding device supplies semiconductor material and carrier gas. The semiconductor material is carried by the carrier gas and enters the crucible via the passage. The method and device can supply semiconductor materials continuously or periodically without opening a vacuum deposition chamber thereof and the uniformity of thin film can be controlled effectively.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Patent Application No. PCT/CN2009/073089, with an international filing date of Aug. 5, 2009, designating the United States, now pending, and further claims priority benefits to Chinese Patent Application No. 200910097899.2, filed Apr. 23, 2009. The contents of all of the aforementioned applications, including any intervening amendments thereto, are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a semiconductor film deposition technique, and more particularly to a method and device for depositing a semiconductor material to form a thin film on substrates using close-spaced sublimation techniques. 
         [0004]    2. Description of the Related Art 
         [0005]    In the manufacturing of the CdS/CdTe solar cell, a process called close-spaced sublimation which can produce high quality CdTe film is attracting attention recently. The process can produce a CdS/CdTe solar cell with the highest conversion coefficient (16.8%) in the world. The close-spaced sublimation process is a kind of vapor deposition process. Materials for forming the CdTe film (hereinafter referred to as a source) are placed in a crucible made of graphite. A glass sheet substrate on which CdTe film is deposited in the close-spaced sublimation process is located on the top of the crucible. A thermal insulating spacer is used to separate the glass sheet substrate and thermal conductive crucible. The distance between the glass sheet substrate and the top surface of the source is about between 0.5 and 5 cm. In this manner, the source sublimates and then deposits on the glass sheet substrate to form a semiconductor layer. In general, in the close-spaced sublimation process, as a source, CdTe is placed in the crucible prior to deposition of CdTe film. With the formation of CdTe film on the glass sheet substrate, the filling level of CdTe in the crucible decreases, leading to the increase of distance between the glass sheet substrate and the source. As a result, the microstructure and the resulting electrical properties of CdTe film changes with time. 
         [0006]    Conventional close-spaced sublimation processes load a starting material to an acceptable filling level in a crucible prior to deposition. To replenish the source consumed in the film deposition, it needs to be reloaded to the crucible. Since the heated vessel used in the method contains toxic vapors, which poses significant safety problem when it is opened for reloading during deposition, it is required to cool down prior to reloading the source. Thus, continuous production of CdTe film on the glass sheet substrate is interrupted to reload the source to the crucible. In practice, since only a small volume of CdTe is needed to form a thin CdTe film, a fully loaded crucible can be used for CdTe deposition for many days. However, with the deposition of CdTe film on the glass sheet substrate, the amount of CdTe source material in the crucible decreases with time, leading to the increase of the distance between the glass sheet substrate and the source and the change of morphology of CdTe polycrystalline. With the repetition of the CdTe film deposition, dispersions in thickness and in quality of the CdTe film increases gradually. Thereafter, it is not clear if the uniformity of deposition over time and across large substrates is achieved. Furthermore, the microstructure and morphology of CdTe particles left in the crucible change with deposition time, increasing the uncertainty of the film uniformity over time and large area substrate. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of the above-described problems, it is one objective of the invention to provide a method for depositing a semiconductor film, in which semiconductor materials are fed to a vacuum deposition device continuously or periodically without opening a vacuum deposition chamber thereof. 
         [0008]    It is another objective of the invention to provide a device for depositing a semiconductor film that supplies semiconductor materials continuously or periodically without opening a vacuum deposition chamber thereof. 
         [0009]    To achieve the above objectives, in accordance with one embodiment of the invention, there is provided a method for depositing a semiconductor film using close-spaced sublimation technique, the method comprising the steps of:
       a) carrying a semiconductor material by a carrier gas to a crucible installed in a vacuum deposition chamber via a passage; and   b) heating the crucible to sublimate the semiconductor material to be vapor and depositing the vapor on a substrate.       
 
         [0012]    In a class of this embodiment, the method further comprises providing a feeding distributor to uniformly distribute the semiconductor material carried by the carrier gas in the crucible. 
         [0013]    In a class of this embodiment, the feeding distributor is a perforated manifold. 
         [0014]    In a class of this embodiment, the perforated manifold is made from stainless steel, graphite, or silicon carbide. 
         [0015]    In a class of this embodiment, the carrier gas is nitrogen, argon, helium, or a mixture thereof. 
         [0016]    In a class of this embodiment, in the step b), a heated permeable membrane is provided in the crucible; the sublimated semiconductor material, together with the carrier gas, passes through the heated permeable membrane and deposits on the substrate with a surface temperature lower than that of the sublimated semiconductor material; non-vaporized solid semiconductor material is further sublimated to be vapor in the heated permeable membrane, and solid semiconductor material is blocked by the permeable membrane and cannot deposit on the substrate. 
         [0017]    In a class of this embodiment, the permeable membrane is heated to be 2-5° C. higher than the temperature of the crucible. 
         [0018]    In accordance with another embodiment of the invention, there provided is a device for depositing a semiconductor film using close-spaced sublimation technique, comprising a semiconductor material feeding device, a passage, a vacuum deposition chamber, a crucible installed in the vacuum deposition chamber, and a substrate located above the crucible, wherein the semiconductor material feeding device and the crucible are connected via the passage, the semiconductor material feeding device supplies a semiconductor material and a carrier gas, and the semiconductor material is carried by the carrier gas and enters the crucible via the passage. 
         [0019]    In a class of this embodiment, a feeding distributor is disposed in the crucible to uniformly distribute the semiconductor material. 
         [0020]    In a class of this embodiment, the feeding distributor is a perforated manifold, and the perforated manifold is connected with the passage. 
         [0021]    In a class of this embodiment, the crucible is equipped with a permeable membrane; the permeable membrane is heatable and allows vaporized semiconductor material and carrier gas to pass through; the substrate is located above the permeable membrane, and the semiconductor material is heated and sublimated in the crucible to be vapor; and the vapor passes through the permeable membrane and deposits on the substrate. 
         [0022]    In a class of this embodiment, the semiconductor material feeding device comprises a carrier gas tank connected with the passage, a hopper connected with the passage, and a feeding control device which controls the feeding rate of the semiconductor material in the hopper. 
         [0023]    Advantages of the invention are summarized below. The semiconductor material is carried into the crucible installed in the vacuum deposition chamber by the carrier gas via the passage, leading to the semiconductor material to enter continuously or periodically the vacuum deposition device without opening the vacuum deposition chamber thereof. Distributing the semiconductor material carried by the carrier gas in the crucible uniformly through the feeding distributor overcomes technical difficulties in the art in which the filling level of CdTe in the crucible decreases with the formation of CdTe film on the glass substrate, leading to the increase of distance between the glass sheet substrate and the source. As a result, the uniformity of thin film on the substrate can be controlled effectively in the invention. The introduction of the permeable membrane is to withhold the non-vaporized semiconductor material in the crucible and allow the carrier gas and vaporized semiconductor material to pass through and then deposit on the substrate to form a thin film. The permeable membrane is thermal conductive to avoid condensation of the vaporized semiconductor material in the permeable membrane. The condensation will jam the pores in the permeable membrane and block the flow of vapor. Moreover, the non-vaporized semiconductor powder can also be further vaporized to be vapor in the permeable membrane. The feeding rate of semiconductor material is controlled to meet the requirement of thin film deposition exactly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a sectional view of a device for depositing a semiconductor film according to one embodiment of the invention; 
           [0025]      FIG. 2  is a planar view of a perforated manifold in a crucible according to one embodiment of the invention; 
           [0026]      FIG. 3  is a sectional view of a vapor deposition device according to one embodiment of the invention; 
           [0027]      FIG. 4  is a sectional view of a vapor deposition device according to another embodiment of the invention; 
           [0028]      FIG. 5  is a longitudinal sectional view illustrating a deposition process on a substrate in which a semiconductor material is conveyed by a metal conveyer according to one embodiment of the invention; and 
           [0029]      FIG. 6  is a schematic diagram of a semiconductor material feeding device according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0030]    As shown in  FIG. 1 , a device for depositing a semiconductor film  10  comprises a semiconductor material feeding device  20  and a vacuum deposition chamber  14 . The structure of the semiconductor material feeding device  20  and the vacuum deposition chamber  14  will be fully described thereafter. Two different practices of depositing a semiconductor material on a glass sheet substrate  60  to form a semiconductor layer are performed. One is to deposit the semiconductor material on the substrate  60  that is placed on the top of the crucible  32  until the thickness of the semiconductor layer meets requirement, while the other practice is to deposit the semiconductor material on a moving substrate that is conveyed by a metal conveyer  36 . The substrate is moved forward with the metal conveyer  36 . 
         [0031]    The device for depositing a semiconductor film  10  is used to deposit a semiconductor material to form a thin film on the glass sheet substrate  60 , for example, a CdS and CdTe thin film in CdS/CdTe solar cells. However, it should be noted that other substrates and deposition materials can also be utilized in accordance with the invention. For example, other materials comprise those that can be sublimated to be vapor at moderate temperatures and other substrates comprise metal substrates such as foils. 
         [0032]    The device for depositing a semiconductor film  10  comprises an insulated housing  12  defining the vacuum deposition chamber  14 . The semiconductor material is deposited on the glass sheet substrate  60  in the vacuum deposition chamber  14 . The housing  12  is heated in any suitable manner, for example, by a halogen lamp  34 , to keep the temperature therein at between 400 and 650° C. The vacuum deposition chamber comprises a vapor deposition device  30 . The vapor deposition device  30  comprises a crucible  32  where the semiconductor material is heated and sublimated to be vapor. The crucible  32  comprises a feeding distributor used to distribute the semiconductor material uniformly. The feeding distributor can be a perforated manifold  37  or any distributor capable of distributing the semiconductor material uniformly. The distributor employed in the embodiment is the perforated manifold  37 . The vapor deposition device  30  further comprises a heatable permeable membrane  40  fixed on the crucible  32 . The substrate  60  is placed above the permeable membrane  40 . The permeable membrane  40  and substrate  60  are separated with a spacer  35  made of insulating materials. 
         [0033]    In the embodiment of  FIG. 1 , the spacer  35 , made of insulating material ceramic, separates the permeable membrane  40  from the substrate  60 , ensuring that the temperature of the permeable membrane is higher than that of the substrate. The distance between the permeable membrane  40  and the substrate glass is between 2 and 30 mm, preferably, 10 mm. The perforated manifold  37  is made of stainless steel, graphite, or silicon carbide. The semiconductor material is heated in the heated crucible and sublimated to be vapor and deposits on the glass sheet substrate  60  after passing through the permeable membrane. The semiconductor material in the crucible is heated during the vapor deposition to a slightly higher in temperature than the glass substrate, which is about 500 to 750° C. 
         [0034]    The semiconductor material feeding device  20  is connected with the crucible  32  via a passage  38 . The passage  38  connects with the perforated manifold  37 . The semiconductor material feeding device  20  supplies the semiconductor material and carrier gas. The semiconductor material, carried by the carrier gas and passing through the passage  38  and perforated manifold  37 , is uniformly distributed in the crucible. The crucible  32  is heated to vaporize the semiconductor material to be vapor. The semiconductor vapor, together with the carrier gas, passes through the heated permeable membrane and then deposits on the substrate. The surface temperature of the substrate is slightly lower than that of the semiconductor vapor. Non-vaporized semiconductor material inside the heated permeable membrane is further sublimates to be vapor, while the solid semiconductor material is blocked by the permeable membrane and cannot deposit on the glass sheet substrate. 
         [0035]    The carrier gas is nitrogen, argon, helium, or a mixture thereof. The preferred semiconductor material is powder. Two different practices of introducing the semiconductor material into the crucible  32  in the vacuum deposition chamber have been performed. In one such practice, the semiconductor material is continuously introduced into the crucible  32  by the carrier gas without opening the vacuum deposition chamber and without interrupting the continuous deposition process, the feeding rate of the semiconductor material meets the deposition requirement exactly. Another such practice of introducing the semiconductor material into the crucible  32  by the carrier gas is performed periodically without opening the vacuum deposition chamber. 
         [0036]    The permeable membrane  40  is supported with an L-shaped holder  31  and fixed at the top of the crucible  32 . The L-shaped holder  31  is preferably made of an insulating ceramic material, for example, alumina ceramic. A preferred structure for the permeable membrane is that only the carrier gas and semiconductor vapor are allowed to pass through, while all non-vaporized semiconductor material is blocked inside the crucible. A heater can be disposed inside the permeable membrane  40  to adjust the temperature thereof. The permeable membrane  40  itself can functions as a heater and thus a voltage is applied at both ends thereof to adjust the temperature. The permeable membrane  40  is heated to be 2-5° C. higher than the temperature of the crucible. As a result, the semiconductor vapor cannot condense at the surface of the permeable membrane and inside the pores. Otherwise, the passage of the semiconductor vapor in the permeable membrane is jammed by the condensation of the vapor, leading to the blockage of vapor flowing in the pores. The preferable material for the crucible  32  is graphite. 
         [0037]    The permeable membrane  40  is selected from the group consisting of graphite, silicon carbide, silicon nitride, and boron nitride. The preferable material of the permeable membrane  40  is heatable graphite. The permeable material made of heatable graphite has good thermal conductivity. The pores in the permeable membrane  40  are distributed regularly. The preferred size of the pores in the membrane is in micro-range and its porosity is above 25% such that only the carrier gas and the semiconductor vapor can pass through. The non-sublimated semiconductor material is blocked in the crucible until it is sublimated to be vapor in the permeable membrane  40 . The thickness of the permeable membrane  40  is 1-10 mm. In the present embodiment, the permeable membrane is 2 mm in thickness. 
         [0038]    As shown in  FIG. 1 , the semiconductor material feeding device  20  comprises a carrier gas cylinder  22  connected with the passage  38 , a hopper  28  connected the passage  38 , and a feeding control device controlling the feeding rate of the semiconductor material in the hopper. 
         [0039]    The carrier gas cylinder  22  generates the carrier gas. The semiconductor material feeding device  20  further comprises a rotary screw  26  equipped with an actuator  27 , a vibrating feeder, or a combination thereof. The feeding rate of the semiconductor material is precisely controlled by the rotating speed of the rotary screw  26  in the hopper  28 . Another method of controlling the feeding rate is achieved by varying vibrating frequency of the vibrating feeder. 
         [0040]    In the embodiment, the feeding controlling device comprises the rotary screw  26  and the actuator  27  which drives the rotary screw  26  to rotate at a certain speed. The rotary screw  26  is controlled by the actuator  27 , introducing the semiconductor powder  21  into the passage  38  at a certain rotating speed and further into the perforated manifold  37  in the vapor deposition device  30 . The flow rate of the carrier gas is controlled by an adjustable valve  24  to maintain the carrier gas at a certain flow rate, ensuring that the semiconductor material  21  flows into the crucible  32  at an expected rate. A preferred feeding approach is: the feeding rate of the semiconductor material carried by the carrier gas to the crucible  32  is controlled to meet the requirement of semiconductor deposition, so that no accumulation or scarcity of the semiconductor material occurs in the crucible  32 . As shown in  FIG. 1 , the material feeding device  20  further comprises an observation window  23  disposed on the passage  38 , which is used to observe the flow of the semiconductor material. 
         [0041]    As shown in  FIG. 2 , the perforated manifold  37  comprises a plurality of parallel channels  1 ,  1 ′,  2 ,  2 ′,  3 ,  3 ′,  4 ,  4 ′, and  5  and two entry passages  34  and  34 ′ at both ends thereof. The preferred width of these parallel channels is 5-10 mm. The entry passage  34  is connected with the channels  1 ,  2 , and  3 , and the entry passage  34 ′ with the channels  1 ′,  2 ′, and  3 ′. The entry passage  34  has an entry  33 , and the entry passage  34 ′ has an entry  33 ′. Through the entry  33  and  33 ′ and the passage  38  and  38 ′ at both ends of the vapor deposition device  30 , the perforated manifold  37  in the crucible  32  is connected with the semiconductor material feeding device  20 . 
         [0042]    The carrier gas from the carrier gas cylinder carries the semiconductor material powder to the perforated manifold  37  via the entry  33 . As a result, the semiconductor material powder is distributed uniformly in the crucible  32 . Another semiconductor material feeding device  20 ′ introduces the carrier gas and semiconductor material powder into the crucible  32  via the entry  33 ′ in the same manner as above. As such, there is a good distribution of the carrier gas and entrained semiconductor powder along the entire space of the crucible  32 , ensuring that the vapor in the space between the permeable membrane  40  and the glass sheet substrate  60  is distributed uniformly on the whole surface of the glass sheet substrate  60 , and ensuring that the semiconductor film deposited on the surface of the glass sheet substrate  60  is uniform. 
         [0043]      FIGS. 1 ,  3 , and  4 , respectively, discloses different embodiments of the vapor deposition devices  30 ,  30 ′, and  30 ″. More specifically, the vapor deposition device  30  as shown in  FIG. 1  has an L-shaped block  31  made of ceramic to hold the permeable membrane  40  on the top of the crucible  32 . The vapor deposition device  30 ′ as shown in  FIG. 3  has a pair of pins  39  made of graphite inside the crucible  32  to support the permeable membrane  40 . Both the vapor deposition devices  30  and  30 ′ are suitable for the deposition of the semiconductor material on the glass sheet substrate periodically or continuously. In these two vapor deposition devices, the size of the crucible must match the size of the substrate. 
         [0044]    The vapor deposition device  30 ″ as shown in  FIG. 4  comprises the metal conveyer  36  used to convey and support the glass sheet substrate  60  for continuous production of a thin film on the glass sheet substrate. The glass sheet substrate  60  is directly placed on the metal conveyer  36 . The metal conveyer  36  and the crucible  32  are separated with the spacer  35 . The spacer  35  is made of a ceramic material with low friction coefficient. There is no gap between the metal conveyer  36  and the crucible  32  and thus no semiconductor vapor leakage occurs on both sides of the crucible  32 . The carrier gas, passing through the permeable membrane  40 , discharges along the other two sides of the crucible  32 . The loss of the semiconductor vapor is minimized by reducing the gap between the glass sheet substrate  60  and the spacer  35  and opening space between two adjacent glass sheet substrates  60 . The preferred gap between the glass sheet substrate  60  and the spacer  35  is between 0.2-0.5 mm. With the help of metal conveyer  36 , the glass sheet substrate can be moved in and out of the vacuum deposition chamber  14 . The size of the crucible in the vapor deposition device  30 ″ may be equal to or smaller than that of the glass sheet substrate. The metal conveyer  36  can also be located on the spacer  35  as shown in  FIG. 1 . 
         [0045]    A preferred operation mode of feeding the semiconductor material and a preferred vacuum deposition device are described below. When the crucible is equal to the glass sheet substrate in size, the semiconductor material powder is introduced to the perforated manifold  37  in the heated vapor deposition device  30  and then flows into the crucible  32  where the semiconductor material is sublimated to be vapor. The vaporized semiconductor passes through the heated permeable membrane  40  located on the top of the crucible  32  and then deposits to form a semiconductor film on the glass sheet substrate that is placed on the vapor deposition device  30  and conveyed by a pair of the metal conveyer  36 . After the deposition is over, the semiconductor material-coated substrate  60  on the metal conveyer  36  is moved away from the vapor deposition device  30  by the metal conveyer  36 . After that, another glass sheet substrate on the metal conveyer  36  is moved to the top of the crucible  32  rapidly where the semiconductor vapor passes through the permeable membrane  40  and then deposits on the surface of the glass sheet substrate  60 . 
         [0046]    During this deposition process, the loss of the semiconductor material depends on the movement speed of the metal conveyer  36  and the open distance between two adjacent substrates. Another operation mode of deposition of the semiconductor film is as shown in  FIG. 4 , in which the glass sheet substrate  60  are different from the crucible  32  in size, e.g., the width of the crucible  32  is less than the length of the glass sheet substrate  60  along the conveying direction of the metal conveyer  36 , as illustrated in  FIG. 5 . In this embodiment of operation, the metal conveyer  36  conveys the glass sheet substrate  60  at a certain speed along the direction of arrow as illustrated in  FIG. 5 . In the meantime, the semiconductor vapor, which is formed by sublimating in the crucible  32  and carried by the carrier gas, passes through the permeable membrane  40  and then deposits on the surface of the glass sheet substrate to form a semiconductor film. In the same manner, the semiconductor material consumed during the deposition is complemented to the crucible  32  by the carrier gas at a certain rate from the hopper  28  located outside the vacuum deposition chamber. The opening distance between two adjacent glass substrates can be adjusted as required in practice. To reduce the loss of the semiconductor vapor, the opening distance between two adjacent substrates in the vacuum deposition chamber is controlled to be less than 1 cm. 
         [0047]    As shown in  FIG. 6 , another embodiment of the semiconductor material feeding device  20  for deposition of semiconductor material on the glass sheet substrate, i.e., the semiconductor material powder  21 , passing through the passage  38  from the hopper  28  by rotating the rotary screw  26 , enters the vapor deposition device  30 . The feeding control device comprises a container  29  equipped with a vibratory feeder and a shutter  52 . The passage  38  is provided with a plurality of holes  42  receiving the semiconductor material to flow into the container  29 . The shutter  52  blocks all or some of the holes  42 . The container  29  is connected with the hopper  28  via another passage which is not shown in  FIG. 6 . 
         [0048]    When the amount of the semiconductor material required by the device for depositing a semiconductor film  10  cannot be achieved by varying the rotating speed of the rotary screw  26 , the feeding rate of the semiconductor material can be controlled as follows. The shutter  52  blocks part of the holes underneath, and some of the semiconductor material from the hopper  28  enters the container  29  by passing through the holes  42  which are not blocked by the shutter  52  and the rest is carried to the perforated manifold  37  by the carrier gas. When the semiconductor material  21  has been accumulated to a certain of amount in the container  29 , it can be fed back to the hopper  28  using a specific passage (not shown in  FIG. 6 ) or ramped up to the passage  38  by a vibratory machine  25 . Thus, the feeding rate of the semiconductor material to the vapor deposition device  30  is controlled precisely. The amount of the semiconductor material accumulated in the container  29  is monitored with the observation window  23 . 
         [0049]    While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.