Abstract:
A wafer susceptor and a chemical vapor deposition apparatus. In one embodiment, the chemical vapor deposition apparatus includes a chamber, a susceptor, a heater and a gas supply system. The susceptor is disposed within the chamber and is rotatable around a rotation axis, wherein an upper surface of the susceptor is suitable for carrying a plurality of wafers, and a middle region of a lower surface of the susceptor is set with a first cavity. The heater is disposed under the susceptor and is used to heat the wafers on the susceptor. The gas supply system is used to introduce a reactive gas into the chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 100109747 filed in Taiwan, R.O.C. on Mar. 22, 2011, the entire contents of which are hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a deposition apparatus, and more particularly to a chemical vapor deposition (CVD) apparatus. 
       BACKGROUND OF THE INVENTION 
       [0003]    Presently, in processes for manufacturing compound semiconductor devices, a Chemical Vapor Deposition (CVD) apparatus, such as a metal-organic CVD (MOCVD), is usually used for growing chips required. The existing CVD apparatuses are classified into vertical type and horizontal type according to different design forms of a reaction chamber. The vertical type CVD apparatus is such designed that a precursor required for deposition reaction is introduced to a position above chips in the reaction chamber in a manner of being vertical to the chip surface. 
         [0004]      FIG. 1  is a schematic view of a conventional vertical type CVD apparatus. A CVD apparatus  100  includes a gas supply system  102 , a reaction chamber  104 , a susceptor  106 , and a heater  108 . The susceptor  106  is disposed in the reaction chamber  104 , and an upper surface of the susceptor  106  may support a plurality of wafers. Furthermore, in order to heat the chips on the susceptor  106  evenly, the susceptor  106  is generally designed to be capable of rotating around a rotation axis  110  in the reaction chamber  104 . 
         [0005]    The heater  108  is disposed below the susceptor  106  in the reaction chamber  104 , so as to heat the chips in the wafers on the susceptor  106 . The gas supply system  102  is disposed on the whole reaction chamber  104  and is located above the susceptor  106 . The gas supply system  102  may introduce a reaction gas  114  to a position above the wafers on the susceptor  106  in the reaction chamber  104 . 
         [0006]    In the deposition process, the susceptor  106  rotates around the rotation axis  110  together with the wafers thereon. Meanwhile, the heater  108  heats the wafers on the susceptor  106  through the susceptor  106 . When being heated by the heater  108 , the reaction gas  114  applied above the wafers on the susceptor  106  by the gas supply system  102  undergoes a reaction to grow a desired deposition layer on the wafer surface. Excessive reactant, undesired products, and waste gas are discharged out of the reaction chamber  104  through a exhaust port  112  at the bottom of the reaction chamber  104 . 
         [0007]      FIGS. 2A and 2B  are a top view and a cross-sectional view of a conventional susceptor. The susceptor  106  generally has a plurality of round recessed portions  116   a ,  116   b , and  116   c  disposed thereon, as shown in  FIG. 2A . As shown in  FIG. 2B , the wafers  118   a ,  118   b , and  118   c  are respectively disposed in the recessed portions  116   a ,  116   b , and  116   c , wherein the recessed portions have the same depth. 
         [0008]    However, in practice, it is found that when growing chips required by light emitting diode (LED) devices on the susceptor  106  in  FIG. 2B , the chips formed on the wafers on the central area of the susceptor  106 , for example, wafer  118   a  and a part of the wafers  118   b , especially chips on the wafer  118   a  at the central position, have an abnormally shorter wavelength, compared with that on the other wafers. Thus, the characteristics of the chips of the same production batch are not consistent, thereby resulting in the yield loss. 
         [0009]    Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies. 
       SUMMARY OF THE INVENTION 
       [0010]    Accordingly, in one aspect, the present invention is directed to a wafer susceptor and a CVD apparatus, in which a recessed portion is disposed at a central area of the susceptor, so that the susceptor has different thickness distribution. Thus, the problem of uneven temperature of the susceptor can be effectively solved. 
         [0011]    In another aspect, the present invention is directed to a wafer susceptor and a CVD apparatus, which can effectively solve the problem that the characteristics and the wavelength of chips placed at a central area of the susceptor are abnormal. 
         [0012]    In a further aspect, the present invention is also directed to a wafer susceptor and a CVD apparatus, which can improve the uniformity of temperature distribution of the susceptor, thereby improving the consistency of the characteristics of chips of the same production batch, and achieving the purpose of improving the production yield. 
         [0013]    In yet another aspect, the present invention provides a CVD apparatus. The CVD apparatus includes a reaction chamber, a susceptor, a heater, and a gas supply system. The susceptor is disposed in the reaction chamber, and can rotate around a rotation axis. An upper surface of the susceptor is suitable for supporting a plurality of wafers, and a first recessed portion is disposed in a central area of a lower surface of the susceptor. The heater is located below the susceptor, and is used for heating wafers on the susceptor. The gas supply system is used for introducing a reaction gas into the reaction chamber. 
         [0014]    According to an embodiment of the present invention, a center of the first recessed portion coincides with a center of the susceptor. 
         [0015]    According to another embodiment of the present invention, the center of the first recessed portion deviates from the center of the susceptor. 
         [0016]    According to another embodiment of the present invention, the diameter of the first recessed portion is in the range of  1 / 4  to 4 times of the diameter of each of the wafers. 
         [0017]    According to another embodiment of the present invention, the depth of the first recessed portion is in the range of 0.1 mm to the thickness of the susceptor minus 0.5 mm. 
         [0018]    According to another embodiment of the present invention, the first recessed portion is an annular recessed portion. In an example, the width of the annular recessed portion is in the range of ⅛ to 2 times of the diameter of each of the wafers. In another example, an average diameter of the annular recessed portion is in the range of ¼ to 2 times of the diameter of each of the wafers, and the average diameter of the annular recessed portion is an average of the inner diameter and the outer diameter of the annular recessed portion. 
         [0019]    According to another embodiment of the present invention, the susceptor further includes a plurality of recessed portions and a second recessed portion disposed in an upper surface of the susceptor. The wafers are correspondingly accommodated in the recessed portions, and the second recessed portion is disposed in a bottom of a recessed portion at the central position of the recessed portions. In an example, the diameter of the second recessed portion is smaller than the diameter of each of the wafers, and the depth of the second recessed portion is in the range of 1 μm to 500 μm. In another example, a center of the second recessed portion coincides with a center of a recessed portion at the central position of the recessed portions. 
         [0020]    According to another embodiment of the present invention, the first recessed portion has an inclined side, so that the diameter of the first recessed portion gradually increases from a bottom of the first recessed portion towards the lower surface of the susceptor. In an example, the diameter of the first recessed portion at the lower surface of the susceptor is in the range of ¼ to 2 times of the diameter of each of the wafers. 
         [0021]    According to another embodiment of the present invention, the CVD apparatus is an MOCVD apparatus. 
         [0022]    In another aspect, the present invention further provides a wafer susceptor, which is applicable in a CVD apparatus. The wafer susceptor includes a plurality of first recessed portions and at least one second recessed portion. The first recessed portions are disposed at an upper surface of the wafer susceptor, and are used for supporting a plurality of wafers. The second recessed portion is disposed at an opposite lower surface of the susceptor, and forms a gap space. 
         [0023]    By application of the wafer susceptor and the CVD apparatus of the present invention, among other things, the problem of uneven temperature of a susceptor can be effectively solved, and the problem that the characteristics and wavelength of chips placed at a central area of the susceptor are abnormal can be solved, so as to improve the consistency of the characteristics of chips of the same production batch. 
         [0024]    These and other aspects of the present invention will become apparent from the following description of the preferred embodiment taken in conjunction with the following drawings, although variations and modifications therein may be effected without departing from the spirit and scope of the novel concepts of the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The accompanying drawings illustrate one or more embodiments of the invention and together with the written description, serve to explain the principles of the invention. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment, and wherein: 
           [0026]      FIG. 1  is a schematic view of a conventional vertical type CVD apparatus; 
           [0027]      FIG. 2A  is a top view of a conventional susceptor; 
           [0028]      FIG. 2B  is a cross-sectional view of the conventional susceptor; 
           [0029]      FIG. 3  is a schematic view of a vertical type CVD apparatus according to an embodiment of the present invention; 
           [0030]      FIG. 4  is a cross-sectional view of a susceptor according to a first embodiment of the present invention; 
           [0031]      FIG. 5  is a cross-sectional view of a susceptor according to a second embodiment of the present invention; 
           [0032]      FIG. 6A  is a cross-sectional view of a susceptor according to a third embodiment of the present invention; 
           [0033]      FIG. 6B  is a top view of the susceptor according to the third embodiment of the present invention; 
           [0034]      FIG. 7A  is a cross-sectional view of a susceptor according to a fourth embodiment of the present invention; 
           [0035]      FIG. 7B  is a top view of the susceptor according to the fourth embodiment of the present invention; and 
           [0036]      FIG. 8  is a cross-sectional view of a susceptor according to a fifth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. 
         [0038]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” or “has” and/or “having” when used herein, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. 
         [0039]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0040]    As used herein, “around”, “about” or “approximately” shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term “around”, “about” or “approximately” can be inferred if not expressly stated. 
         [0041]    When growing LED chips in a CVD apparatus, chips on wafers at a central area of a susceptor, especially chips on wafers at the central position of the susceptor, have a shorter wavelength than others. Inventors find the problem that the chips on the central area of the susceptor is relatively short, which is because that the reaction temperature of wafers on the central area is higher than that of wafers in other areas in the deposition process. That is to say, a heater of a conventional CVD apparatus cannot heat the wafers on the susceptor evenly. Accordingly, in one aspect, the present invention provides several CVD apparatuses, and susceptors of the CVD apparatuses have different thickness distribution designs, so as to improve the consistency of the characteristics of chips of the same production batch. 
         [0042]      FIG. 3  is a schematic view of a vertical type CVD apparatus according to an embodiment of the present invention. In this embodiment, a CVD apparatus  200  includes a reaction chamber  204 , a susceptor  206 , a heater  208 , and a gas supply system  202 . In an exemplary embodiment, the CVD apparatus  200  may be, for example, an MOCVD apparatus. 
         [0043]    The susceptor  206  is disposed in the reaction chamber  204 . The susceptor  206  has an upper surface  216  and a lower surface  218  opposite to the upper surface  216 , and the upper surface  216  of the susceptor  206  is used for supporting a plurality of wafers. The heater  208  is disposed in the reaction chamber  204 , and is located below the susceptor  206 , so as to heat the wafers on the susceptor  206 . For example, the heater  208  utilizes the heat provided by a resistance wire, and conducts the heat to the wafers on the susceptor  206  through heat convection, heat radiation, and heat conduction. Moreover, in order to evenly heat the wafers on the susceptor  206 , the susceptor  206  may rotate around a rotation axis  210  in the reaction chamber  204 , for example, clockwise or counter clockwise, as shown in  FIG. 3 . The gas supply system  202  is disposed at an upper side of the reaction chamber  204 , and is located above the susceptor  206 . The gas supply system  202  is used for introducing the reaction gas  212  into the reaction chamber  204 , and releasing the reaction gas  212  to surfaces of the wafers on the upper surface  216  of the susceptor  206  from top to bottom. 
         [0044]    When a deposition process is carried out in the CVD apparatus  200 , the susceptor  206  is driven by a rotating base  209  located below the susceptor  206  to rotate around the rotation axis  210 , so that the wafers supported on the susceptor  206  rotate around the rotation axis  210  together. Meanwhile, the heater  208  heats the susceptor  206 , so as to further heat the wafers on the upper surface  216  of the susceptor  206  through the conduction of the susceptor  206 . At this time, the reaction gas  212  above the susceptor  206  and released by the gas supply system  202  undergoes a reaction, so as to grow a desired deposition layer on surfaces of the wafers on the susceptor  206 . Excessive reactants, byproducts, and waste gas are exhausted out of the reaction chamber  204  through a exhaust port  214  at a bottom of the reaction chamber  204 . 
         [0045]    In one aspect of the present invention, the susceptor has different thickness designs to solve the problem of uneven temperature of the susceptor, so as to improve the consistency of the characteristics of the chips.  FIG. 4  is a cross-sectional view of a susceptor according to a first embodiment of the present invention. Generally, as shown in  FIG. 4 , a susceptor  206   a  includes a plurality of recessed portions  220   a ,  220   b , and  220   c , which are recessed in the upper surface  216   a  of the susceptor  206   a , for supporting corresponding wafers. The recessed portions are, for example, round, so as to match the shape of the wafers, and the recessed portions have the same depth. Preferably, the recessed portions have the depth equal to or slightly greater than the thickness of the wafers. The recessed portion  220   a  is located at the central position of the upper surface  216   a  of the susceptor  206   a , the recessed portions  220   b  surround a periphery of the recessed portion  220   a , and the recessed portions  220   c  surround a periphery of the recessed portions  220   b . As shown in  FIG. 4 , a wafer  222   a  may be disposed in the recessed portion  220   a  at the central position of the susceptor  206   a , a plurality of wafers  222   b  may be disposed in the recessed portions  220   b  at the periphery of the recessed portion  220   a  respectively, and a plurality of wafers  222   c  may be disposed in the recessed portions  220   c  at the periphery of the recessed portions  220   b . The wafers  222   a ,  222   b , and  222   c  have the same diameter  228 . 
         [0046]    In this embodiment, a recessed portion  230   a  is recessed in a central area  232  of a lower surface  218   a  of the susceptor  206   a , so as to form a gap space. The recessed portion  230   a  is located just below the recessed portion  220   a  at the central position of the upper surface  216   a  of the susceptor  206   a . In an example, the recessed portion  230   a  is also round, and has a center coinciding with the center of the susceptor  206   a . The diameter  226   a  of the recessed portion  230   a  is in the range of ¼ to 4 times of the diameter  228  of the wafer  222   a . Furthermore, the depth  224   a  of the recessed portion  230   a  is in the range of 0.1 mm to the thickness  242  of the susceptor  206   a  minus 0.5 mm. 
         [0047]    In the susceptor  206   a , the recessed portion  230   a  is disposed on the central area  232  of the lower surface  218   a , such that the thickness of the susceptor  206   a  at the recessed portion  220   a  at the central position is smaller than the thickness at the other recessed portions  220   b  and  220   c . Therefore, referring to  FIGS. 3 and 4 , due to the arrangement of the recessed portion  230   a , the heat of the heater  208  transferred to the part of the susceptor  206   a  at the recessed portion  230   a  and adjacent areas through radiation and convection is reduced, such that the heat received by the part of the susceptor  206   a  is close to or equal to the heat received by the other parts of the susceptor  206   a . In this way, the processing temperature of the wafer disposed on the central area of the upper surface of the susceptor  206   a  is consistent with the processing temperature of the wafers disposed on the other areas. Thus, the consistency of the characteristics of the chips of the same production batch is improved. 
         [0048]      FIG. 5  is a cross-sectional view of a susceptor according to a second embodiment of the present invention. A susceptor  206   b  of this embodiment has a structure substantially the same as that of the susceptor  206   a  of the first embodiment, and the difference therebetween lies in that the center of a recessed portion  230   b  recessed in the central area  232  of a lower surface  218   b  of a susceptor  206   b  deviates from the center of the susceptor  206   b , which is different from the center of the recessed portion  230   a  of the susceptor  206   a  coinciding with the center of the susceptor  206   a . In other words, in the susceptor  206   b , the recessed portion  230   b  is not located just below the recessed portion  220   a  at the central position of an upper surface  216   b  of the susceptor  206   b , but is disposed at a side of the recessed portion  220   a  at the central position. 
         [0049]    In an embodiment, for example, the diameter  226   b  of the recessed portion  230   b  is in the range of ¼ to 4 times of the diameter of the wafer  222   a . Moreover, for example, the depth  224   b  of the recessed portion  230   b  is in the range of 0.1 mm to the thickness  242  of the susceptor  206   b  minus 0.5 mm. 
         [0050]    In the susceptor  206   b , the recessed portion  230   b  in the central area  232  of the lower surface  218   b  and deviating from the center of the susceptor  206   b  can not only form a gap space but also make the thickness of the susceptor  206   b  at the recessed portion  220   a  at the central position and the adjacent areas smaller than the thickness at the other recessed portions, for example, the thickness at recessed portions  220   b  and/or  220   c . Thus, referring to  FIGS. 3 and 5 , due to the arrangement of the recessed portion  230   b , the heat of the heater  208  transferred to the part of the susceptor  206   b  at the recessed portion  230   b  and adjacent areas through radiation and convection is reduced, such that the heat received by the part of the susceptor  206   b  is close to or equal to the heat received by the other parts of the susceptor  206   b . It should be noted that, as the susceptor  206   b  will rotate with respect to the heater, there will be no problem of uneven temperature distribution caused by the deviating design of the recessed portion  230   b  below the susceptor  206   b . Moreover, through the design of the off-center distance of the recessed portion  230   b  and the range of the diameter of the recessed portion  230   b , the distribution of the temperature gradient of the heated susceptor  206   b  can be further adjusted. Therefore, the processing temperature of the wafers disposed in the upper surface  216   b  of the central area and adjacent areas of the susceptor  206   b  is consistent with the processing temperature of the wafers disposed on the other areas. Thus, the consistency of the characteristics of the chips of the same production batch is improved. 
         [0051]      FIGS. 6A and 6B  are a cross-sectional view and a top view of a susceptor according to a third embodiment of the present invention respectively. A susceptor  206   c  of this embodiment has a structure substantially the same as that of the susceptor  206   a  of the first embodiment, and the difference therebetween lies in that a recessed portion  230   c  recessed in the central area  232  of a lower surface  218   c  of the susceptor  206   c  is an annular recessed portion, which is different from the recessed portion  230   a  of the susceptor  206   a  being a round recessed portion, as shown in  FIG. 6B . In an embodiment, the center of the annular recessed portion  230   c  may coincide with the center of the susceptor  206   c , as shown in  FIG. 6B . In another embodiment, the center of the annular recessed portion  230   c  may deviate from the center of the susceptor  206   c.    
         [0052]    In an exemplary embodiment, as shown in  FIG. 6A , the width  226   c  of the annular recessed portion  230   c  is in the range of ⅛ to 2 times of the diameter  228  of the wafer  222   a . Furthermore, referring to  FIG. 6B , the annular recessed portion  230   c  has an average diameter, which is an average of the inner diameter R 1  and the outer diameter R 2  of the annular recessed portion  230   c . In an embodiment, the average diameter of the annular recessed portion  230   c  is in the range of ¼ to 2 times of the diameter of the wafer  222   a . For example, in the embodiment as show in  FIG. 6B , the inner diameter R 1  and the outer diameter R 2  of the recessed portion  230   c  are larger than the diameter  228  of the wafer  222   a ; therefore, the recessed portion  230   c  is disposed below the periphery of the recessed portion  220   a  of the upper surface  216   c , and passes through the part below the recessed portions  220   b  at the outer side of the recessed portion  220   a . In another embodiment, the depth  224   c  of the recessed portion  230   c  is in the range of 0.1 mm to the thickness  242  of the susceptor  206   c  minus 0.5 mm. 
         [0053]    In the susceptor  206   c , the annular recessed portion  230   c  disposed on the central area  232  of the lower surface  218   c  can not only form a gap space, but also make the thickness of the susceptor  206   c  at the recessed portion  220   a  at the central position and/or the thickness of the adjacent recessed portions  220   b  smaller than that at the other recessed portions, for example, the thickness at the recessed portions  220   c . Thus, referring to  FIGS. 3 and 6A  together, due to the arrangement of the annular recessed portion  230   c , the heat of the heater  208  transferred to the part of the susceptor  206   c  at the recessed portions  230   c  and adjacent areas through radiation and convection is reduced, such that the heat received by the part of the susceptor  206   c  is close to or equal to the heat received by the other parts of the susceptor  206   c . Moreover, through the design of the off-center distance, the average diameter, and the range of the depth of the annular recessed portion  230   c , the distribution of the temperature gradient of the heated susceptor  206   c  can be further adjusted. Therefore, the processing temperature of the wafers disposed on the central area and the adjacent areas of the susceptor  206   c  is consistent with the processing temperature of the wafers disposed on the other areas. Thus, the consistency of the characteristics of the chips of the same production batch is improved. 
         [0054]      FIGS. 7A and 7B  are a cross-sectional view and a top view of a susceptor according to a fourth embodiment of the present invention respectively. A susceptor  206   d  of this embodiment has a structure substantially the same as that of the susceptor  206   c  of the third embodiment, and the difference therebetween lies in that, in addition to an annular recessed portion  230   d  disposed in a lower surface  218   d , an upper surface  216   d  of the susceptor  206   d  further has a recessed portion  234  disposed therein. The recessed portion  234  is recessed in the bottom of the recessed portion  220   a  at the central position, as shown in  FIG. 7A . In an embodiment, the center of the recessed portion  234  may coincide with the center of the recessed portion  220   a , as shown in  FIGS. 7A and 7B . In detail, this embodiment may be a multiple-depth recessed portion, which has a first depth for bearing a wafer and a second depth as a gap part below the wafer. In another embodiment, the center of the recessed portion  234  may deviate from the center of the recessed portion  220   a  at the central position. 
         [0055]    In an exemplary embodiment, as shown in  FIG. 7A , the depth  226   d  of the annular recessed portion  230   d  may be in the range of ⅛ to 2 times of the diameter  228  of the wafer  222   a . Similarly, referring to  FIG. 7B , the annular recessed portion  230   d  has an average diameter, in which the average diameter is an average of the inner diameter R 1  and the outer diameter R 2  of the annular recessed portion  230   d . In an embodiment, the average diameter of the annular recessed portion  230   d  may be in the range of ¼ to 2 times of the diameter of the wafer  222   a . Similarly, as shown in  FIG. 7B , the inner diameter R 1  and the outer diameter R 2  of the recessed portion  230   d  are greater than the diameter  228  of the wafer  222   a . Furthermore, the depth  224   d  of the recessed portion  230   d  may be in the range of 0.1 mm to the thickness  242  of the susceptor  206   d  minus 0.5 mm. 
         [0056]    Referring to  FIG. 7A , as the diameter  244  of the recessed portion  234  is smaller than the diameter  228  of the wafer  222   a , the range of the recessed portion  220   a  for accommodating the wafer  222   a  can cover the whole recessed portion  234 . In an embodiment, the depth  236  of the recessed portion  234  may be in the range of 1 μm to 500 μm. Moreover, a distance  238  between a side of the recessed portion  234  and an adjacent side of the recessed portion  220   a  may be, for example, 2 mm. 
         [0057]    In the susceptor  206   d , the annular recessed portion  230   d  disposed on the central area  232  of the lower surface  218   d  and the recessed portion  234  disposed in the recessed portion  220   a  at the central position of the upper surface  216   d  can not only form two gap spaces, but also make the thickness of the recessed portion  220   a  at the central position of the susceptor  206   d  and the thickness of the adjacent recessed portions  220   b  smaller than the thickness of the other recessed portions, for example, the recessed portions  220   c . Therefore, referring to  FIGS. 3 and 7A , due to the arrangement of the annular recessed portion  230   d  and the recessed portion  234 , the heat of the heater  208  transferred to the part of the recessed portion  230   d  and the adjacent areas of the susceptor  206   d  through radiation and convection is reduced, such that the heat received by the part of the susceptor  206   d  is close to or equal to the heat received by the other parts of the susceptor  206   d . Moreover, through the design of the off-center distance, the average diameter, and the range of the depth of the annular recessed portion  230   d , in combination with the arrangement of the recessed portion  234  at the upper surface  216   d  of the susceptor  206   d , the distribution of the temperature gradient of the heated susceptor  206   d  can be further adjusted. Therefore, the processing temperature of the wafers disposed on the central area and the adjacent areas of the susceptor  206   d  is consistent with the processing temperature disposed on the other areas. Thus, the consistency of the characteristics of the chips of the same production batch is improved. 
         [0058]      FIG. 8  is a cross-sectional view of a susceptor according to a fifth embodiment of the present invention. A susceptor  206   e  of this embodiment has a structure substantially the same as that of the susceptor  206   a  of the first embodiment, and the difference therebetween lies in that a recessed portion  230   e  recessed in the central area  232  of a lower surface  218   e  of the susceptor  206   e  has an inclined side  240 . That is to say, unlike the side of the recessed portion  230   a  of the susceptor  206   a  lessentially perpendicular to the bottom, the inclined side  240  of the susceptor  206   e  is not perpendicular to a bottom  246 , but inclines outwards, such that an angle θ included between the inclined side  240  and the bottom  246  of the susceptor  206   e  is greater than 90 degrees. Therefore, in the susceptor  206   e , the diameter of the recessed portion  230   e  is increased gradually from the bottom  246  of the recessed portion  230   e  towards the lower surface  218   e  of the susceptor  206   e , so that the thickness of the part of the susceptor  206   e  at the recessed portion  230   e  is increased from the bottom  246  of the recessed portion  230   e  towards the lower surface  218   e  of the susceptor  206   e.    
         [0059]    In an embodiment, the center of the recessed portion  230   e  may coincide with the center of the susceptor  206   e , as shown in  FIG. 8 . In another embodiment, the center of the recessed portion  230   e  may deviate from the center of the susceptor  206   e.    
         [0060]    In an embodiment, the diameter  226   e  of the recessed portion  230   e  at the lower surface  218   e  of the susceptor  206   e  may be in the range of ¼ to 2 times of the diameter  228  of the wafer  222   a . Moreover, the depth  224   e  of the recessed portion  230   e  may be in the range of 0.1 mm to the thickness  242  of the susceptor  206   e  minus 0.5 mm. 
         [0061]    In the susceptor  206   e , the recessed portion  230   e  having the inclined side  240  disposed on the central area  232  of the lower surface  218   e  can not only form a gap space, but also make the thickness of the susceptor  206   e  at the recessed portion  220   a  at the central position and the adjacent areas smaller than the thickness at other recessed portions, for example, the thickness at the recessed portions  220   b  and/or  220   c . Therefore, referring to  FIGS. 3 and 8  together, due to the arrangement of the recessed portion  230   e , the heat of the heater  208  transferred to the part of the susceptor  206   e  at the recessed portion  230   e  and the adjacent areas through radiation and convection is reduced, such that the heat received by the part of the susceptor  206   e  is close to or equal to the heat received by the other parts of the susceptor  206   e . Therefore, the processing temperature of the wafers disposed in the upper surface  216   e  on the central area and the adjacent areas of the susceptor  206   e  is consistent with the processing temperature disposed on the other areas. Thus, the consistency of the characteristics of the chips of the same production batch is improved. 
         [0062]    It can be known from the embodiments, among other things, the present invention has an advantage that the susceptor has different thickness distribution since the susceptor of the CVD apparatus of the present invention has recessed portions disposed on the central area. Thus, the problem of uneven temperature of the susceptor is effectively solved. 
         [0063]    It can be known from the embodiments, among other things, the present invention has another advantage that the present invention can effectively solve the problem that the characteristics and the wavelength of chips placed at the central area of the susceptor are abnormal. 
         [0064]    It can be known from the embodiments, among other things, the present invention has another advantage that the present invention can improve the uniformity of the temperature distribution of the susceptor, thus improving the consistency of the characteristics of the chips of the same production batch, so as to improve the production yield. 
         [0065]    The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching. 
         [0066]    The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to activate others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.