Patent Publication Number: US-2009229519-A1

Title: Apparatus for manufacturing semiconductor thin film

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for manufacturing a semiconductor thin film that manufactures a silicon carbide semiconductor, more particularly a semiconductor thin film manufacturing apparatus that forms a semiconductor thin film on a substrate by epitaxial growth. 
     BACKGROUND ART 
     Silicon carbide (SiC) semiconductors, for example, having excellent heat resistance and mechanical strength, have been attracting attention for such reasons as their use as a material in blue light-emitting diodes and the like, and their application to electric elements having high power and low loss due to the demand for energy saving through high pressure resistance and low ion resistivity. An SiC semiconductor is formed by depositing an SiC thin film on an SiC substrate. As a means of depositing an SiC thin film on a substrate to form an SiC semiconductor, for example, epitaxial growth of SiC can be utilized. 
     The SiC thin film is deposited on the substrate by epitaxial growth in which raw material gases containing H 2  gas, SiH 4  gas and C 3 H 8  gas are reacted with each other on a heated SiC wafer surface. At this time, in order to grow the SiC thin film in a uniform manner, it is important that a flow of raw material gases over the SiC wafer is even, and also that the raw material gases are evenly mixed and that heat is evenly transferred to the substrate. 
     Based on this perspective, a CVD apparatus that can form a uniform thin film by forming a gas flow that is parallel to the substrate has been proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-252176). According to this CVD apparatus, it is possible to form a gas flow parallel to the surface of the substrate by regulating a gas flow that has passed a heating element on which the substrate is disposed. 
     However, even in the aforementioned CVD apparatus or the like, there are cases whereby uniformity in the film thickness and electric characteristics of the SiC thin film cannot be ensured due to reasons such as the existence of regions where raw material gases remain and the mixing of the raw material gases being uneven. In addition, temperature distribution on the substrate may become uneven for such reasons as the mixing of the raw material gases being uneven. Further, since raw material gases are decomposed at an upstream side (gas supply side), and a rate of growth at this side is lowered compared with a downstream side (gas exhaust side), it is necessary to make a supply rate of raw material gases uniform by reducing an opening diameter to increase a flow rate of raw material gases, toward the downstream portion. 
     In addition, impurities such as a reaction product of SiC or dirt may adhere to an inner wall of a heating element that heats an SiC wafer or an inner wall of the apparatus. Such impurities may easily come off from the inner wall due to such reasons as a high amount of gas flow at the time of SiC growth, which is as high as several liters/min to several tens of liters/min, or repetition of vacuuming and gas charging at the time of conveying a wafer. Consequently, these impurities may be scattered in a reaction tube and mixed in with raw material gases to adhere to or mix into an SiC wafer surface or an SiC layer, thereby impairing the functional capability of the resultant SiC semiconductor. 
     The heating element is usually disposed in the interior of the reaction tube via a heat insulating member made of a material having porosity such as glass wool which is a graphite material. However, impurities are often adsorbed even onto the heat insulating member, and a portion of the heat insulating member may come off and become an impurity. 
     As a means of growing a crystal while preventing adhesion of impurities such as those described above, a chemical vapor deposition apparatus that holds a substrate in such a manner that a main surface thereof on which a crystal is grown faces downward, has been proposed (see, for example, JP-A No. 9-82649). However, in this apparatus, as the edge of a substrate is held with a susceptor, regions where a thin film does not form occur. In addition, due to unevenness in the in-plane temperature of the substrate, in-plane uniformity may decrease. 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     The object of the present invention is to solve the aforementioned problems. That is, one object of the invention is to provide an apparatus for manufacturing a semiconductor thin film capable of forming an even thin film that is substantially free from adhesion of impurities, and capable of improving in-plane evenness of a grown thin film. 
     Means to Solve the Problems 
     An apparatus for manufacturing a semiconductor thin film of the present invention includes a reaction tube, a susceptor disposed in the inside of the reaction tube, and a negative pressure generator that applies a negative pressure to a substrate deposited on the susceptor to hold the substrate, wherein the substrate is placed so that an angle of a normal line to a crystal growth face of the substrate to a vertical downward direction is less than 180°. 
     The semiconductor thin film manufacturing apparatus of the invention includes a negative pressure generator that applies a negative pressure to a substrate held onto a susceptor, in order to hold the substrate on the upper side. By holding (placing) a growth face of the substrate by means of the negative pressure generator so that an angle of a normal line to a crystal growth face of the substrate to a vertical downward direction is less than 180° (for example, in a vertical direction or in a horizontal direction), adhesion of impurities due to falling of impurities can be prevented. In addition, adhesion of impurities such as reaction products or dirt caused by a flow of a raw material gas or a carrying gas can also be prevented. Since the region to which a negative pressure is applied by the negative pressure generator is positioned at a surface onto which a thin film may not be formed, an even film can be formed on a surface for thin film formation. Further, as compared with the cases where a substrate is held onto a susceptor with a holder, the semiconductor thin film manufacturing apparatus of the invention, in which substrates are held in close contact with the susceptor, can provide even temperature within a substrate surface. As a result, in-plane evenness of a grown thin film can be improved. 
     In the susceptor, a through hole that penetrates the susceptor and a communicating part by which a part of the through hole is in communication with a position to which a substrate is placed. It is preferable to hold the substrate by distributing a carrier gas via the through hole to generate a negative pressure in the communicating part by a negative pressure generator. 
     Various types of means can be applied as the negative pressure generator. One preferable embodiment of the semiconductor thin film manufacturing apparatus of the invention is a means that generates a force to attract a substrate by distributing a carrier gas in a through hole to generate a negative pressure at a communicating part. In the aforementioned means that distributes a carrier gas, a circulating system in which a carrier gas that has passed a through hole is supplied to the through hole again can be applied. This system enables effective utilization of the carrier gas, and great advantages in terms of energy and environment can be achieved. 
     The through hole preferably has a venturi structure in which the diameter thereof decreases from an upstream side in a direction of a carrier gas flow toward the communicating part, and increases from the communicating part toward a downstream side in a direction of the carrier gas flow. 
     With a structure in which a pathway for a carrier gas flow is narrowed at the communicating part in the through hole, the flow rate of the carrier gas at the communicating part can be increased (venturi effect). As a result, the negative pressure at the communicating part can be even increased and the substrate can be held more stably. 
     Effect of the Invention 
     According to the present invention, an apparatus for manufacturing a semiconductor thin film that can form an even thin film with substantially no adhesion of impurities, and can improve in-plane evenness of a grown thin film, can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a partial cross-sectional view illustrating an outline of one embodiment of the semiconductor thin film manufacturing apparatus of the present invention. 
         FIG. 2  is a perspective view of a susceptor shown in  FIG. 1 . 
         FIG. 3  is a partial cross-sectional view illustrating an outline of another embodiment of the semiconductor thin film manufacturing apparatus of the invention. 
         FIG. 4  is a cross-sectional view illustrating an aspect of holding a substrate in a semiconductor thin film manufacturing apparatus of Example. 
         FIG. 5  is a cross-sectional view illustrating an aspect of holding a substrate in a semiconductor thin film manufacturing apparatus of Comparative Example. 
         FIG. 6  is a diagram illustrating an angle formed by a normal line to a crystal growth face of a substrate and a plumb line. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The semiconductor thin film manufacturing apparatus of the present invention will be illustrated by referring to  FIG. 1  and  FIG. 2 .  FIG. 1  is a partial cross-sectional view showing an embodiment of the semiconductor thin film manufacturing apparatus. In  FIG. 1 , the semiconductor thin film manufacturing apparatus  10  has a reaction tube  12 , an RF coil  14  disposed on the outer side thereof, a raw material supply tube  16  that distributes raw material gases to a reaction chamber  12 A in the reaction tube  12 , a carrier gas supply tube  18  that supplies a carrier gas, an exhaust tube  24 , and a vacuum pump  36 . A thermal insulating member  26  and a susceptor  20  are provided at the inside of the reaction tube  12  in this order. Holding parts  20 A that hold substrates  22 A and  22 B are provided at an upper part and a lower part in a vertical direction of the susceptor  20 . In the susceptor  20 , a through hole  30  that penetrates through the susceptor  20  is provided, and a communicating part  32  through which a part of the through hole  30  communicates with the holding part  20 A is provided. 
     When a carrier gas is distributed in the through hole  30  while the substrate  22 A is held, gas in the communicating part  32  is suctioned in a direction indicated by arrow A (see  FIG. 2 ), thereby the pressure is reduced to generate a negative pressure. By means of this negative pressure, the substrate  22 A is tightly fixed to the susceptor  20 . 
     The substrate  22 A before distributing a carrier gas in the through hole  30  is preferably temporarily fixed with a holding means. The diameter of the through hole  30  is preferably from 5 mm to 20 mm, and is more preferably from 5 mm to 10 mm. The diameter of the communicating part  32  is preferably from 5 mm to 20 mm, and is more preferably from 5 mm to 10 mm. 
     The holding part  20 A is positioned at an upper part in a vertical direction (an upper side of the reaction chamber  12 A), and two or more of those may be provided in the rest of the region. Herein, the “upper part in a vertical direction” refers to a part at a position that is higher than the bottom of the reaction chamber  12 A. When the holding part  20 A is also provided on a lateral side of the reaction chamber  12 A, it is preferable that a negative pressure generator is provided for each of the holding parts  20 A so that the substrate will not detach from the susceptor. The substrate  22 A is placed so that an angle of a normal line to a crystal growth face of the substrate  22 A to a vertical downward direction is less than 180°. 
     As shown in  FIG. 6 , the angle θ formed by a normal line to a crystal growth face of the substrate  22 A (Y) and a line in a vertical downward direction (X) is preferably 90° or less (90° is more preferable). Herein, the “angle of a normal line to a crystal growth face to a vertical downward direction” refers to an angle which is smaller. 
     As shown in  FIG. 1 , in the reaction chamber  12 A, raw material gases that have been introduced react at the surfaces of substrates  22 A and  22 B, and therefore thin films are deposited on the surfaces of the substrates  22 A and  22 B. 
     Next, a structure of the susceptor will be illustrated by referring to  FIG. 2 .  FIG. 2  is a perspective view extracting only the susceptor  20 . As shown in  FIG. 2 , for example, the susceptor  20  has a hexagonal cross-sectional shape and has a hollow part having a rectangular shape. This hollow part in the susceptor  20  corresponds to the reaction chamber  12 A to which raw material gases are distributed. The wall thickness of the susceptor  20  is, for example, preferably from about 10 mm to about 30 nm. The shape of the susceptor is not limited to the embodiment shown in  FIG. 2 , and may be designed to be in a plate shape or the like, as appropriate. 
     The susceptor  20  is preferably formed from a graphite member coated with silicon carbide. At an upper part in a vertical direction of the susceptor  20  is provided the holding part  20 A, where the substrate  22 A is held by contacting the holding part  20 A, and the substrate  22 A is heated. 
     The susceptor  20  is designed to generate heat by means of induction heating of an RF coil  14  disposed outside the reaction tube  12 , as shown in  FIG. 1 , and to heat the substrate indirectly. The RF coil  14  generates a high frequency magnetic flux and induces an eddy current in the susceptor  20 , thereby heating the susceptor  20  by means of Joule heat generated by the eddy current. The temperature of the substrate heated by the susceptor  20  is preferably 1300° C. or higher. Particularly, in the case of growing an SiC thin film, the substrate  20 A (and  20 B) may be heated by the susceptor  20  to a temperature of preferably 1300° C. or higher, more preferably from about 1400° C. to about 2000° C. The heating temperature of the susceptor  20  is controlled by a controlling means (not shown) based on the surface temperatures of the susceptor  20  and the substrate. 
     When two kinds of raw material gases are used, these gases may be mixed and supplied from a raw material supplying tube  16 , or may be supplied from separate raw material supplying tubes to the reaction chamber  12 A. The carrier gas supplying tube  18  has a branched structure in order to supply a carrier gas to the reaction chamber  12 A and the through hole  30 , respectively. The raw material supplying tube  16  and the carrier gas supplying tube  18  is provided with MFC  16 A, and  18 A and  18 B, respectively, so that supply amounts of the gases can be regulated. 
     In the case of forming an SiC thin film, C 3 H 8  (propane) and SiH 4  (silane) are used as raw material gases. In this case, a H 2  gas can be used as a carrier gas to be supplied with the raw material gases. An SiC wafer (SiC substrate) can be preferably used as a substrate. 
     If necessary, a mixing chamber may be provided between the supplying tubes (i.e., the raw material supplying tube  16  and the carrier gas supplying tube  18 ) and the reaction chamber  12 A. In the mixing chamber, a mixing shower plate having pores and a diffusion shower plate having pores are disposed. The raw material gas and the carrier gas supplied into the mixing chamber are mixed in order to have an even distribution of concentration, by passing through the pores in the mixing shower plate. The diameter and the number of the pores provided in the mixing shower plate can be determined in view of the type of raw material of the raw material gas, the degree of mixing, and the like. 
     The thermal insulating member  26  insulates the reaction tube  12  from the heat of the susceptor  20 , and is preferably formed from glass wool which is a graphite material. The thermal insulating member  26  is disposed so as to be in tightly contact with the inner wall of the reaction tube  12 , and the susceptor  22  is fixed on the inner wall of the thermal insulating member  26 . 
     The thickness of substrates  22 A and  22 B may be appropriately determined depending on purposes and, in the present embodiment, is preferably around 400 μm. The conveyance tray  28  on which the substrate  22 B is placed is preferably formed from a polycrystalline SiC member. 
     The exhaust tube  24  is provided with a vacuum pump  36 , and is constructed so that growth under reduced pressure can be carried out, and the raw material gas in the reaction tube  12  can be discharged from the apparatus. 
     In the following, a process of forming a semiconductor thin film using the semiconductor thin film manufacturing apparatus of the invention will be illustrated by taking the case of forming an SiC semiconductor as an example. First, H 2  gas, SiH 4  gas and C 3 H 8  gas are supplied to the reaction chamber  12 A from the supplying tubes . The ratio of the supplied H 2  gas, SiH 4  gas and C 3 H 8  gas here is approximately 12000/2/3 (=H 2 /SiH 4 /C 3 H 8 ) by volume. 
     When the mixing chamber is provided between the supplying tubes and the reaction chamber  12 A, respective gases (raw material gases) are mixed with each other upon passing through the pores in the mixing shower plate, and are then diffused upon passing through the pores in the diffusion shower plate and supplied to the reaction chamber  12 A. The raw material gases are sufficiently mixed with the mixing shower plate and the diffusion shower plate so that the mixture has an even distribution of concentration. 
     The raw material gas that has been supplied to the reaction chamber  12 A and reached the vicinity of the susceptor  20  is also heated by the susceptor  20 . The raw material gas which has entered the reaction chamber  12 A are heated to about 1500° C. when passing through a pathway formed on the side of a surface of the substrate, and are reacted on the substrate  24 . As a result, SiC is deposited on the substrate to form an SiC thin film. Thereafter, the raw material gases which have passed over the substrates  22 A and  22 B are discharged from the apparatus via the exhaust tube  24  and the vacuum pump  26 . 
     MFC 16 A,  18 A and  18 B provided in the supplying tubes are controlled by a controlling means such as CPU and the like (not shown), respectively, and the flow rate and pressure of the raw material gas in the reaction chamber  12 A are regulated by the controlling means. 
     The process of forming an SiC semiconductor usually includes a step of introducing a carrier gas and an etching gas prior to the introduction of raw material gas, and carrying out etching of a substrate surface. The SiC substrate at this time is preferably heated so that a temperature at the surface thereof is from around 1300° C. to around 1600° C. Examples of the carrier gas include H 2  gas, and examples of the etching gas include hydrogen chloride and H 2  gas. 
     According to the semiconductor thin film manufacturing apparatus of the present invention, the pathway for the raw material gas is formed below the substrate  22 A. Therefore, the substrate surface to which a thin film is formed is constantly oriented downward in a gravity force direction. Consequently, adhesion of reaction products or impurities such as broken pieces of thermal insulating materials to a surface for forming a thin film of the substrate  22 A or the thin film itself can be prevented. In addition, since the surface to be formed with an SiC thin film of the substrate  22 A is oriented downward in a gravity force direction, the surface is exposed to an ascending heat flow, and both of heating efficacy at high temperature and uniformity in the temperature gradient are excellent. Moreover, the temperature gradient of the substrate  22 A can be uniformized. 
     In addition, since there is very little region in the substrate for being held with a holder, the yield of thin film growth can be improved. Further, since no gap is formed between the substrate and the susceptor, the thin film is not deposited on the backside of the substrate and the backside of the substrate does not have to be re-polished. The construction in which a carrier gas is supplied to a through hole to hold a substrate by means of a negative pressure is also cost effective, since new installation of instruments such as a vacuum pump to suction the substrate is not necessary. 
     The semiconductor thin film manufacturing apparatus of the invention can be modified in various ways, based on the above-described construction. 
     For example, in the reaction chamber  12 A shown in  FIG. 1 , the height of a raw material gas exhaust port L 1  is preferably smaller than the height of a raw material gas supply port L 0 . 
     When the height of the raw material gas exhaust port L 1  is smaller than the height of the raw material gas supply port L 0 , the flow rate of the raw material gas at the exhaust side can be increased. Accordingly, the supply amount of the raw material gas at the exhaust side of the reaction chamber  12 A can be reduced, thereby counterbalancing the difference in the growth rates of SiC thin films at the raw material gas supply side and at the exhaust side, and improving uniformity in the growth rate of SiC thin films. 
     In addition, as shown in  FIG. 3 , a venturi structure may be adopted in which the diameter of the thorough hole  30  decreases from an upstream side in a direction of the carrier gas flow toward the communicating part  32 A, and increases from the communicating part  32  toward a downstream side in a direction of the carrier gas flow. In  FIG. 3 , the same symbols as those of  FIG. 1  exert the same functions as those in the case of  FIG. 1 , and illustration thereof will be omitted (the same will also apply in  FIG. 4  and  FIG. 5  described later). 
     By adopting such a form that pathway for the carrier gas is narrowed at the communicating part of the through hole, the flow rate of the carrier gas at the communicating part can be increased. As a result, a negative pressure at the communicating part is increased, and a substrate can be held more stably. 
     Inclination angles θ 1  to θ 4  indicating amounts of inclination of the venturi structure in  FIG. 3  are preferably from 1° to 30°, more preferably from 5° to 10°, respectively. 
     In addition, as shown in  FIG. 3 , the carrier gas supply tube  18  and the raw material supply tube  16  may be combined into one supply tube in midstream to supply both of the carrier gas and the raw material gas to the reaction chamber  12 A and the through hole  30 . In this case, since the raw material gas is distributed also to the through hole, a thin film may be formed on a portion of the backside of the substrate  22 A through the communicating part  32 . However, since the pressure at the communicating part  32  is reduced, the amount of the thin film formed is small and the film is not formed on the entire surface of the substrate, as is the case with previous apparatus. Therefore productivity is not affected. 
     Effective utilization of the carrier gas and the raw material gas can be achieved by adopting the aforementioned structure in which the carrier gas and the raw material gas are distributed in combination, and also adopting a system in which gases discharged from the apparatus by a vacuum pump are used again. 
     The disclosure of Japanese Patent Application No. 2005-368173 is incorporated herein by reference in its entirety. In addition, all publications, patent applications and technical specifications described in the present description are incorporated herein by reference to the same extent as that of the case where incorporation of individual publications, patent applications, and technical specifications by reference is specifically and individually described. 
     EXAMPLE 1 
     Formation of an SiC epitaxial thin film on a substrate was conducted using a semiconductor thin film manufacturing apparatus shown in  FIG. 1 . The substrate was held by distributing a carrier gas (hydrogen gas: 100 sccm) in a through hole  30  (diameter: 8 mm) while temporarily holding the substrate by a holding member  50 , as shown in  FIG. 4  illustrating a cross-sectional view of the reaction tube  12 . The diameter of a communicating part was 8 mm. 
     The substrate used was a 4H-SiC wafer with a 8° off (0001) Si face. Epitaxial growth was performed by chemical vapor deposition (CVD method). The apparatus used was a horizontal hot wall-type CVD apparatus. Other growth conditions and results are shown in the following Table 1. As seen from Table 1, falling of foreign matters onto the substrate placed on the upper side and deposition of a thin film to the back surface of the substrate were not observed, and in-plane evenness was favorable. The number of defects and the presence or absence of thin film growth on the backside were determined with an optical microscope and with naked eye. 
     EXAMPLE 2 
     Formation of an SiC epitaxial thin film on a substrate was conducted using a semiconductor thin film manufacturing apparatus equipped with a similar susceptor to that used in Example 1, except that a through hole had a venturi structure as shown in  FIG. 3 . Angles θ 1 , θ 2 , θ 3  and θ 4  were 8°, respectively. The diameters of both ends of the through hole were 8 mm, respectively. Other growth conditions and results are shown in the following Table 1. As seen from Table 1, falling of foreign matters onto the substrate placed on the upper side and deposition of a thin film to the back surface of the substrate were not observed, and in-plane evenness was favorable. 
     COMPARATIVE EXAMPLE 
     Formation of an SiC epitaxial thin film on a substrate was conducted using a semiconductor thin film manufacturing apparatus shown in  FIG. 1 , except that the substrate was fixed with a holding member and no through hole was formed, as shown in  FIG. 5  illustrating a cross-sectional view of the reaction tube. Conditions of the substrate used and the like were the same as those in Example 1. Other main growth conditions and results are shown in the following Table 1. As seen from Table 1, no falling of foreign matters onto the upper side of the substrate was observed, but a thin film was deposited on the back surface of the substrate. In addition, in-plane evenness was on a low level as compared with those observed in Examples. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Example 1 
                 Example 2 
                 Comparative example 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Upper 
                 Lower 
                 Upper 
                 Lower 
                 Upper 
                 Lower 
               
               
                   
                 substrate 
                 substrate 
                 substrate 
                 substrate 
                 substrate 
                 substrate 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Hydrogen carrier gas 
                 25 
                 25 
                 25 
                 25 
                 25 
                 15 
               
               
                 supply amount (slm) 
               
               
                 Silane raw material gas 
                 10 
                 10 
                 10 
                 10 
                 10 
                 10 
               
               
                 supply amount (sccm) 
               
               
                 Propane raw material gas 
                 5 
                 5 
                 5 
                 5 
                 5 
                 5 
               
               
                 supply amount (sccm) 
               
               
                 Growth rate (μm/Hr) 
                 8 
                 8 
                 8 
                 8 
                 8 
                 8 
               
               
                 In-plane evenness of 
                 2.0 
                 1.1 
                 1.9 
                 1.1 
                 4.3 
                 1.2 
               
               
                 grown thin film 
               
               
                 (deviation/average) (%) 
               
               
                 Substrate size 
                 1 inch 
                 1 inch 
                 1 inch 
                 1 inch 
                 1 inch 
                 1 inch 
               
               
                 Defects due to foreign 
                 0 
                 2 
                 0 
                 2 
                 0 
                 3 
               
               
                 matters in thin film 
               
               
                 (number/wafer) 
               
               
                 Thin film growth on 
                 Absent 
                 Absent 
                 Absent 
                 Absent 
                 Present 
                 Absent 
               
               
                 backside of substrate 
               
               
                   
               
            
           
         
       
     
     As seen from the above Table 1, in-plane evenness in Comparative Example was on a low level. Unevenness in a temperature within the substrate surface is considered to be a cause for this result. In addition, since a substrate edge was held with a holding member in Comparative Example, thin film was not formed at the holding part, and growth of thin film on the backside of the substrate was observed. On the other hand, in each of Examples 1 and 2, no falling of foreign matters on the substrate placed on the upper side was observed, no thin film was deposited on the backside of the substrate, and in-plane evenness was favorable. 
     EXPLANATION OF SYMBOLS 
     
         
           10 —Semiconductor thin film manufacturing apparatus 
           12 —Reaction tube 
           12 A—Reaction chamber 
           14 —RF coil 
           16 —Raw material supplying tube 
           18 —Gas supplying tube 
           16 A,  18 A,  18 B—MFC 
           20 —Susceptor 
           20 A—holding part 
           22 A and  22 B—Substrates 
           24 —Exhaust tube 
           26 —Heat insulating member 
           28 —Conveyance tray 
           30 —Through hole 
           32 —Communicating part