Abstract:
A vapor deposition apparatus of the present invention has a substrate holder having a substrate holding surface for holding a substrate thereon, and a flow channel for supplying a source gas onto the substrate. The flow channel has an upper wall and a lower wall. An aperture portion is provided in the lower wall of the flow channel. The substrate holding surface of the substrate holder fits in the aperture portion while forming a space between the substrate holding surface and the aperture portion. A means for reducing leakage of gas through the space between the aperture portion and the substrate holder is provided. With this structure, since a means for reducing leakage of gas through the space between the aperture portion and the substrate holder is provided, the conductance with respect to outflow of gas increases, which in turn reduces variations in the amount of outflow gas. This results in high yield production of nitride semiconductor devices with a long life and high light-emission efficiency.

Description:
[0001]     This non-provisional application claims priority under 35 U.S.C. §119(a) on Japanese Patent Application No. 2004-279420 filed in Japan on Sep. 27, 2004, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention generally relates to vapor deposition apparatuses, and more particularly to vapor deposition apparatuses improved for high yield production of nitride semiconductor devices.  
         [0003]     Nitride-based group III-V compound semiconductor crystals represented by GaN, InGaN, AlGaN, AlInGaN, etc., have direct-transition-type band gaps and are expected to be applied to semiconductor laser devices. InGaN mixed crystals enable red-to-ultraviolet light emission and thus are attracting special attention as short-wavelength material. These crystals are already in practical use as light emitting diode devices with wavelengths ranging from ultraviolet to green and as bluish purple laser diode devices. Generally, these devices are produced by the metal organic chemical vapor deposition (MOCVD) method by using CVD apparatuses. Specifically, GaN-type, InGaN-type, AlGaN-type, InGaNP-type, InGaNAs-type, and InGaAlN-type nitride semiconductor films are grown over a substrate. CVD apparatuses that grow these semiconductor films with the use of organic metal material are referred to as MOCVD apparatuses.  
         [0004]      FIG. 9  shows a schematic cross-section of a conventional MOCVD apparatus (see, for example, Japanese Patent Publication No. 2001-19590). The conventional MOCVD apparatus has reaction chamber  31 . Reaction chamber  31  houses flow channel  32  that effectively supplies source gas onto substrate  33 , substrate holder  34  that holds substrate  33  on substrate holding surface  34   a , and heater  35  that heats substrate holder  34 . Flow channel  32  has upper wall  32   a  and lower wall  32   b  that has aperture portion  36 . Substrate holding surface  34   a  of substrate holder  34  fits in aperture portion  36  while forming a space between substrate holding surface  34   a  and aperture portion  36 . Substrate  33  and substrate holding surface  34   a  of substrate holder  34  come in contact with the gas flowing in flow channel  32 . As shown by the arrows, source gas is supplied from gas supply port  37  and flows through flow channel  32  onto substrate  33 , where the gas contributes to growth of nitride semiconductor films. Substrate  33  and substrate holder  34  are revolved by revolution of revolving axis  39 . Source gas that does not contribute to growth of semiconductor films is released from gas exhausting port  38 . Also provided in reaction chamber  31  is automatic carry-in/out equipment, not shown, that carries in and out substrate  33  and substrate holder  34  (see, for example, Japanese Patent Application Publication No. 2003-17544).  
         [0005]     Production of nitride-based semiconductor lasers made of GaN, AlN, InN, and mixed crystals thereof with the use of conventional MOCVD apparatuses is problematic in that the crystallinity and thickness of the grown film are not uniform throughout the substrate. Also, there are variations between substrates. As a result, the nitride-based semiconductor layers prepared over a substrate are found to suffer crystal distortions and multiple cracks.  
         [0006]     Crystal defects including cracks act as the center of non-emitting combination; the defects act as paths for current to cause leakage current, posing the problem of poor yields. Particularly with LD devices, the defects cause an increase in threshold current density, posing the problem of shortened device life. Thus, it is important to reduce crystal defects including cracks.  
         [0007]     The present inventors studied the cause of crystal defects including cracks. As a result, it has been found that the concentration distribution and amount of supply of source gas supplied on the substrate vary, which is because the amount of outflow of gas through the space between aperture portion  36  of flow channel  32  and substrate holder  34  is not constant. In MOCVD growth of nitride-based semiconductors, it was found that this effect was important and the uniformity of crystallinity in a crystal film and the uniformity of the thickness in a crystal film plane were not secured.  
         [0008]     The present inventors have found the causes of variations in concentration distribution and amount of supply of source gas, which will be described below.  
         [0009]      FIG. 10  is a view describing the cause of variations in the amount of outflow of gas through the space between the aperture portion of the flow channel and the substrate holder. In MOCVD apparatuses, there is space  21  between aperture portion  36  of flow channel  32  and substrate holder  34 . Provision of space  21  is because substrate holder  34  needs to be revolved in the direction R for the purpose of uniform crystal growth throughout the substrate plane. Space  21  is provided also because substrate holder  34  needs to be movable for substrate  33  on substrate holder  34  to be carried in from the outside of reaction chamber  31 .  
         [0010]     Substrate  33  is carried in as follows. First, heater  35  is kept apart from flow channel  32 . Substrate holder  34  with substrate  33  thereon is then mounted on heater  35  (referred to as catching). For positioning, engagement is provided in the part that heater  35  and substrate holder  34  touch. Heater  35  and substrate holder  34  are then moved to flow channel  32 , and set such that substrate  33  is placed appropriately relative to flow channel  32 .  
         [0011]     Because substrate holder  34  expands when heated, or in view of the catching accuracy of substrate holder  34  and heater  35 , the engagement need some tolerance. Also some tolerance is necessary to space  21 , which is between aperture portion  36  of flow channel  32  and substrate holder  34 . At the time of automatic carry-in/out, if revolving axis  39  is off the center of substrate holder  34 , this location is referred to as a catching error. In this case, substrate holder  34  is revolved with a varying space relative to aperture portion  36  of flow channel  32 . Because of the accuracy of axis processing, revolving axis  39  is generally eccentric to some extent. The axis eccentricity causes variations in space  21 , which is between aperture portion  36  of flow channel  32  and substrate holder  34  (the axis eccentricity causing wobbling  22 ). Under these circumstances the amount of outflow gas  23  varies, which in turn causes biased-flow of source gas, i.e., bias of gas concentration distribution on substrate  33 . Also, the gas concentration distribution on substrate  33  is not steady.  
         [0012]     The problem of variations in space  21 , which is between aperture portion  36  of flow channel  32  and substrate holder  34 , also occurs at the time of automatically remounting flow channel  32  and substrate holder  34  after removal thereof for washing. Further, because of the processing accuracy of flow channel  32  and substrate holder  34 , it is difficult to repeat the initial positioning, which means space  21  of different size after renewal of flow channel  32  and substrate holder  34 .  
         [0013]     Since the extent of the above problem varies between apparatuses, growth conditions need to be optimized for each individual apparatus. In nitride-based semiconductor, re-evaporation of the crystal happens because of its high saturated vapor pressure. In accordance with variations in the concentration distribution and amount of supply of source gas, the ratio of III source gas and V source gas also varies. Consequently, crystallinity does not become uniform in a crystal film plane and the thickness does not become uniform in a crystal film plane. Thus, variations in the amount of outflow gas seriously affect crystal growth.  
       SUMMARY OF THE INVENTION  
       [0014]     In view of the foregoing and other problems, it is an object of the present invention to provide a vapor deposition apparatus improved for stable gas distribution throughout the substrate.  
         [0015]     It is another object of the present invention to provide a vapor deposition apparatus improved for a stable amount of outflow gas.  
         [0016]     It is another object of the present invention to provide a vapor deposition apparatus improved to prevent crystallinity in a crystal film plane and the thickness in a crystal film plane from varying.  
         [0017]     It is another object of the present invention to provide a vapor deposition apparatus that eliminates the need for optimization of growth conditions for each individual apparatus.  
         [0018]     It is another object of the present invention to provide a vapor deposition apparatus that realizes high-yield production of light emitting devices of nitride semiconductor with a long life and high light-emission efficiency.  
         [0019]     It is another object of the present invention to provide an MOCVD apparatus that realizes high light-emission efficiency and high-yield production of long-lived light emitting devices of nitride semiconductor.  
         [0020]     In order to accomplish the above and other objects, the vapor deposition apparatus according to the present invention is a vapor deposition apparatus comprising: a substrate holder comprising a substrate holding surface for holding a substrate thereon; a flow channel for supplying a source gas onto the substrate, the flow channel comprising an upper wall and a lower wall; and an aperture portion provided in the lower wall of the flow channel. The substrate holding surface of the substrate holder fits in the aperture portion while forming a space between the substrate holding surface and the aperture portion. The apparatus also comprises a means for reducing leakage of gas through the space between the aperture portion and the substrate holder.  
         [0021]     According to the invention, since a means for reducing leakage of gas through the space is provided between the aperture portion and the substrate holder, variations in the amount of outflow gas can be decreased.  
         [0022]     The means for reducing leakage of gas is preferably formed by bending the space from the inside of the flow channel to the outside thereof.  
         [0023]     The means for reducing leakage of gas preferably comprises: an upward dent portion dented in an upward direction, the upward dent portion being provided along a periphery of the aperture portion and in a thickness portion of the lower wall of the flow channel; and a brim projecting from a side wall of the substrate holder in a lateral direction, the brim fitting in the upward dent portion while forming a space between the brim and the upward dent portion, when the substrate holding surface of the substrate holder is in a state of fitting in the aperture portion.  
         [0024]     With this structure, the space formed between the aperture portion of the flow channel and the substrate holder has a bent passage. The bent passage is composed of a first passage extending downward from the inside of the flow channel, a second passage extending in a lateral direction from the end of the first passage, and a third passage extending from the end of the second passage down to the outside of the flow channel.  
         [0025]     Since the space is formed of a bent passage, the conductance with respect to outflow gas decreases, thus significantly reducing the amount of outflow gas through the space.  
         [0026]     Another embodiment of the means for reducing leakage of gas comprises a brim projecting from a side wall of the substrate holder in a lateral direction, while forming a space between the brim and the lower wall of the flow channel, when the substrate holding surface of the substrate holder is in a state of fitting in the aperture portion.  
         [0027]     With this structure, the space formed between the aperture portion of the flow channel and the substrate holder has a bent passage composed of a first passage extending downward from the inside of the flow channel and a second passage extending in a lateral direction from the end of the first passage to the outside of the flow channel.  
         [0028]     When the space has a passage bent in this manner, the conductance with respect to outflow gas also decreases, thus reducing the amount of outflow gas through the space.  
         [0029]     The vapor deposition apparatus may have a mechanism for revolving the substrate holder. With this structure, even if the substrate holder is rotated, variations in the amount of outflow gas can be decreased.  
         [0030]     The vapor deposition apparatus preferably comprises: a heater for heating the substrate, the substrate holder holding the substrate being mounted on the heater, the heater being vertically movable and provided below the aperture portion of the flow channel; a mounting mechanism for mounting the substrate holder on the heater; and a moving mechanism for moving the heater with the substrate holder mounted thereon while fitting the substrate holding surface of the substrate holder in the aperture portion of the flow channel. According to this structure, variations in the amount of outflow gas are reduced at the time of automatically carrying in/out the substrate.  
         [0031]     The substrate holder preferably comprises a disk comprising a brim provided along its side wall.  
         [0032]     The vapor deposition apparatus is preferably used as an MOCVD apparatus for vapor deposition of a nitride semiconductor.  
         [0033]     In the vapor deposition apparatus of the present invention, a means for reducing leakage of source gas through the space between the aperture portion of the flow channel and the substrate holder is provided. Source gas flowing from upstream in the flow channel is therefore not leaked through the space between the aperture portion of the flow channel and the substrate holder, or the amount of the gas leakage is significantly reduced. This reduces variations in outflow gas and thus assures uniformity of crystallinity and layer thickness of thin films throughout the substrate plane. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]      FIG. 1  is a schematic cross-section of a vapor deposition apparatus according to embodiment 1 of the present invention.  
         [0035]      FIG. 2 ( a ) is a cross-section of a vapor deposition apparatus that has a mechanism for revolving the substrate holder and a mechanism for automatically carrying in/out the substrate holder according to embodiment 1 of the present invention.  
         [0036]      FIG. 2 ( b ) is a plan view of a mechanism for carrying in/out the substrate holder.  
         [0037]      FIG. 3  is an enlarged schematic cross-section of the part of fitting of the flow channel and substrate holder shown in  FIG. 1 .  
         [0038]      FIG. 4  is a view showing a three-dimensional shape of the flow channel shown in  FIG. 1  according to embodiment 1 of the present invention.  
         [0039]      FIG. 5  is a graph of a comparison between the thickness distributions of GaN layers grown by using a conventional MOCVD apparatus and an MOCVD apparatus according to embodiment 1 of the present invention.  
         [0040]      FIG. 6  is an enlarged schematic cross-section of the part of fitting of the flow channel and substrate holder according to embodiment 2 of the present invention.  
         [0041]      FIG. 7  is a view showing a comparison between the thickness distributions of GaN layers grown by using a conventional MOCVD apparatus and an MOCVD apparatus according to embodiment 2 of the present invention.  
         [0042]      FIG. 8  is a view schematically showing another specific example of the orifice structure.  
         [0043]      FIG. 9  is a schematic cross-section of a conventional MOCVD apparatus.  
         [0044]      FIG. 10 ( a ) is a view describing the cause of variations in the amount of outflow gas through the space between the aperture portion of the flow channel and the substrate holder.  
         [0045]      FIG. 10 ( b ) is a plan view of the part of fitting of the flow channel and substrate holder. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]     Preferred embodiments of the present invention will be described referring to drawings. It will be appreciated that the present invention is not limited to these embodiments.  
         [0047]     A feature of the present invention is provision of a means for reducing leakage of gas through the space between the aperture portion and the substrate holder in order to reduce variations in the amount of outflow gas. This will be described in detail in embodiments 1 and 2 below.  
       Embodiment 1  
       [0048]      FIG. 1  is a cross-section of a vapor deposition apparatus according to embodiment 1. As in a conventional apparatus, this vapor deposition apparatus has, in reaction chamber  11 , flow channel  12  that effectively supplies source gas onto substrate  13 , substrate holder  14  that holds substrate  13 , and heater  15  that heats substrate holder  14 . Flow channel  12  has upper wall  12   a  and lower wall  12   b . Through flow channel  12 , source gas flows in parallel with substrate  13  from gas supply port  17  towards gas exhausting port  18 . Lower wall  12   b  of flow channel  12  has aperture portion  16 . Substrate holding surface  14   a  of substrate holder  14  fits in aperture portion  16  while forming space  21  between aperture portion  16  and substrate holder  14 . Substrate  13  and substrate holding surface  14   a  of substrate holder  14  come in contact with the gas flowing in flow channel  12 . Source gas is supplied from gas supply port  17  and flows through flow channel  12  onto substrate  13 , where the gas contributes to growth of nitride semiconductor films. Source gas that does not contribute to growth of semiconductor films is released from gas exhausting port  18 .  
         [0049]     This vapor deposition apparatus is designed to reduce leakage of gas through space  21 , which is between aperture portion  16  of flow channel  12  and substrate holder  14 . A means for the reduction of gas leakage is composed of a combination of upward dent portion  12   c  and brim  14   b . Upward dent portion  12   c  is dented in an upward direction, and is provided along the periphery of aperture portion  16  and in a thickness portion of lower wall  12   b  of flow channel  12 . Brim  14   b  projects from the side wall of substrate holder  14  in a lateral direction. This will be described in greater detail later.  
         [0050]     As shown in  FIG. 2 , the vapor deposition apparatus of the present invention may have a revolving mechanism for revolving substrate holder  14  and a mechanism for automatically carrying in/out substrate holder  14 . As shown in the figure, substrate holder  14  is revolved by a revolving mechanism connected to revolving axis  19  that is mounted to heater  15 . In the embodiment shown, substrate holder  14  is revolved by gear  83  mounted to revolving axis  19 . Gear  83  is in turn revolved by a revolving means such as motor  84 . Revolution may be transmitted by a belt or the like instead of by gear  83 . Revolving axis  19 , mounted to heater  15 , protrudes from reaction chamber  11  to the outside thereof, where the axis is connected to gear  83 .  
         [0051]     The substrate is carried in as follows. As shown in  FIG. 2 , revolving axis  19  is vertically movable by a driving device, not shown. Before substrate  13  is carried in, heater  15  is kept apart from flow channel  12  (for example, below flow channel  12 ). Under this condition, substrate holder  14  with substrate  13  thereon is carried on fork  86  into reaction chamber  11 . Fork  86  is stopped at a position where substrate holder  14  is situated over heater  15 . Next, revolving axis  19  is moved upward to mount substrate holder  14  on heater  15  (catching), after which revolving axis  19  is stopped temporarily. For positioning, engagement is provided in the part that heater  15  and substrate holder  14  touch. Revolving axis  19  is moved for above to engage heater  15  with substrate holder  14 . Next, fork  86  is pulled out of reaction chamber  11 . Revolving axis  19  is then vertically moved to set substrate holder  14  so that substrate holding surface  14   a  of substrate holder  14  fits in aperture portion  16 . For carrying-out of substrate  13 , the above procedure is performed in reverse order.  
         [0052]     Generally, the amount of source gas that outflows through the space between aperture portion  16  of flow channel  12  and substrate holder  14  is proportionate to the difference between the cross-sectional area of flow channel  12  and the area of the space. Practice shows that the amount of leakage of gas through the space is especially larger at the upstream side of the substrate. Further, as described above, variations in the space cause variations in the amount of supply of source gas onto the substrate.  
         [0053]     The operation of reducing leakage of gas through the space between the aperture portion of the flow channel and the substrate holder, realized in this embodiment, will be described below.  
         [0054]      FIG. 3  is an enlarged schematic cross-section of the part of fitting of aperture portion  16  of flow channel  12  and substrate holder  14  shown in  FIG. 1 . As shown in the figure, in this embodiment, a means for reducing leakage of gas is of orifice structure  25  defined by aperture portion  16  of flow channel  12  and substrate holder  14 .  
         [0055]     That is, space  21  formed between aperture portion  16  of flow channel  12  and substrate holder  14  is a bent passage composed of first passage  101  extending downward from the inside of flow channel  12 , second passage  102  extending in a lateral direction from the end of first passage  101 , and third passage  103  extending from the end of second passage  102  down to the outside of flow channel  12 . More specifically, the means for reducing leakage of gas is composed of upward dent portion  12   c  and brim  14   b . Upward dent portion  12   c  is dented in an upward direction, and is provided along the periphery of aperture portion  16  and in a thickness portion of lower wall  12   b  of flow channel  12 . Brim  14   b  projects from the side wall of substrate holder  14  in a lateral direction. When substrate holding surface  14   a  of substrate holder  14  is in a state of fitting in aperture portion  16 , brim  14   b  fits in upward dent portion  12   c  while forming a space between brim  14   b  and upward dent portion  12   c.    
         [0056]     This structure decreases the conductance with respect to outflow of source gas even when the area of space  21  between aperture portion  16  of flow channel  12  and substrate holder  14  is the same as ever. This significantly reduces outflow of source gas through space  21 , which is between aperture portion  16  of flow channel  12  and substrate holder  14 , or stops the outflow. Since variations in the amount of outflow of gas are reduced, the flow of source gas becomes stable and the uniformity of crystallinity and layer thickness of thin films is secured throughout the substrate plane.  
         [0057]      FIG. 4  is a view showing a three-dimensional shape of flow channel  12  shown in  FIG. 1  according to embodiment 1. FIGS.  4 ( a ),  4 ( b ),  4 ( c ), and  4 ( d ) respectively show the upper surface, cross-section, side surface, and lower surface of flow channel  12 . As shown in  FIG. 4 , lower wall  12   b  of flow channel  12  of embodiment 1 has aperture portion  16 . Upward dent portion  12   c  that is dented in an upward direction is provided along the periphery of aperture portion  16  and in a thickness portion of lower wall  12   b  of flow channel  12 . Upward dent portion  12   c  and substrate holder  14  together constitute an orifice structure. Upper wall  12   a  and lower wall  12   b  of flow channel  12  are connected together by two side walls  12   d  and  12   d.    
         [0058]      FIG. 5  is a graph of a comparison between the thickness distributions of GaN layers grown by using a conventional MOCVD apparatus and an MOCVD apparatus according to embodiment 1 of the present invention. In each example shown, a GaN layer was grown on a substrate of 2 inches. As seen from the graph, the MOCVD apparatus according to this embodiment improves the thickness distributions throughout the substrate.  
         [0059]     Also, an AlGaN layer was grown on a substrate by using the MOCVD apparatus according to this embodiment. The Al composition and layer thickness were uniform throughout the substrate plane. Thus, crystal distortions were inhibited which would otherwise have been caused by non-uniform composition and layer thickness of the thin film on the substrate, and accordingly no cracks were found.  
         [0060]     In this embodiment, as shown in  FIG. 3 , the lower surface of substrate holder  14  is lid-shaped covering the entire upper surface of heater  15  and the upper portion of the side wall of heater  15 . This is for ease of positioning when substrate holder  14  is set on heater  15 . The mechanism for positioning is not limited to the lid structure; the surfaces of contact may constitute a concave/convex combination.  
       Embodiment 2  
       [0061]      FIG. 6  is an enlarged schematic cross-section of the part of fitting of the flow channel and substrate holder according to embodiment 2 of the present invention. As shown in the figure, in this embodiment, the means for reducing leakage of gas is composed of brim  14   b  projecting from the side wall of substrate holder  14  in a lateral direction, while forming space  21  between brim  14   b  and lower wall  12   b  of flow channel  12 , when substrate holding surface  14   a  of substrate holder  14  is in a state of fitting in aperture portion  16 . Lower wall  12   b  of flow channel  12  is as conventionally designed. That is, only providing disk shaped substrate holder  14  having brim  14   b  provided on its periphery results in orifice structure  25  defined by aperture portion  16  of flow channel  12  and substrate holder  14 . In this case, space  21 , which is formed between aperture portion  16  of flow channel  12  and substrate holder  14 , is a bent passage composed of first passage  101  extending downward from the inside of flow channel  12 , and second passage  102  extending in a lateral direction from the end of first passage  101  to the outside of flow channel  12 .  
         [0062]     In this embodiment, as in embodiment 1, a revolving mechanism and a substrate automatic carry-in/out equipment may be provided, though not shown. While in this embodiment the three-dimensional shape of the flow channel is basically the same as that in embodiment 1, the aperture portion may be shaped similarly to the aperture portions of flow channels of conventional vapor deposition apparatuses.  
         [0063]     This structure, as in embodiment 1, decreases the conductance with respect to outflow of source gas even when the area of space  21  between aperture portion  16  of flow channel  12  and substrate holder  14  is the same as ever.  
         [0064]      FIG. 7  is a graph of a comparison between the thickness distributions of GaN layers grown by using a conventional MOCVD apparatus and an MOCVD apparatus according to embodiment 2 of the present invention. In each example shown, a GaN layer was grown on a substrate of 2 inches. As seen from the graph, the MOCVD apparatus according to this embodiment improves the thickness distributions throughout the substrate.  
         [0065]     With this structure, as in embodiment 1, since variations in the amount of outflow of gas were reduced, the flow of source gas became stable and the uniformity of crystallinity and layer thickness of thin films throughout the substrate plane were secured.  
         [0066]     While in embodiments 1 and 2 specific examples of the orifice structure have been shown, the present invention is not limited to the examples; any orifice structure that does not allow gas to flow therethrough can be applied to the present invention. For example, as shown in  FIG. 8 , such an orifice structure can be conveniently used that the space between aperture portion  16  of flow channel  12  and substrate holder  14  is bent from the inside of the flow channel to the outside thereof.  
         [0067]     The Embodiments herein described are to be considered in all respects as illustrative and not restrictive. The scope of the invention should be determined not by the Embodiments illustrated, but by the appended claims, and all changes which come within the meaning and range of equivalency of the appended claims are therefore intended to be embraced therein.