Abstract:
A gas inlet member of a CVD reactor includes a gas inlet housing having a gas distribution volume supplied with a process gas by a feed line and a multiplicity of gas lines, each formed as a tube and engaging openings of a gas outlet plate arranged in front of an inlet housing wall, and through which the process gas enters a process chamber. A coolant chamber adjoins the gas inlet housing wall and a coolant cools the gas inlet housing wall and outlet ends of the gas lines that are in heat-conductive contact with the gas inlet housing wall. The gas outlet plate is thereby thermally decoupled from the gas inlet housing wall such that the gas outlet plate, which is acted on by radiation heat coming from the process chamber, heats up more intensely than the outlet ends which extend into the openings of the gas outlet plate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a NATIONAL STAGE under 35 USC 371 of and claims priority to International Application PCT/EP2012/063072, filed 5 Jul. 2012, incorporated herein by reference, which claims priority to DE 10 2011 051 778.2 filed 12 Jul. 2011 and DE 10 2011 056 589.2 filed 19 Dec. 2011. 
     FIELD OF THE INVENTION 
     The invention relates to a gas inlet member of a CVD reactor, comprising a gas inlet housing which has at least one first gas distribution volume that can be supplied with a first process gas by a first feed line and has a multiplicity of gas lines which are each formed as a tube and engage in first openings of a gas outlet plate arranged in front of an inlet housing wall, and through which the first process gas enters a process chamber located beneath the gas outlet plate, and further comprising a clearance space provided between the gas inlet housing wall and the gas outlet plate. 
     BACKGROUND 
     A gas inlet member is known from US 2009/0169744A1 and US 2006/0263522A1, in which a gas distribution volume is connected via tubes to the ceiling of a process chamber. Through the tubes, the process gas fed into the gas distribution volume can enter the process chamber located beneath the gas outlet plate. The process chamber ceiling here forms a gas outlet plate that forms a housing wall of a gas distribution volume. 
     DE 103 20 597 A1 describes a method and a device for depositing semiconductor layers using two process gases, the process gases being introduced into the process chamber through a gas inlet member. 
     DE 697 06 248 T2 describes a gas inlet member formed as a shower head, the gas lines of which are positioned on the vertices of triangles. Cooling chambers for cooling the gas inlet wall are present inside the gas inlet member. 
     U.S. Pat. No. 6,544,341 B1 describes a gas inlet member comprising a plurality of chambers arranged one above the other and having a multiplicity of gas outlet openings that are positioned on the vertices of equilateral triangles. 
     U.S. Pat. No. 7,976,631 B2 describes a gas inlet member that has cooled structures protruding out of the gas inlet housing wall toward the process chamber. 
     US 2006/0021574 A1 describes a gas inlet member, the process gas flowing through a porous gas outlet plate. 
     FIGS. 3 and 4 of DE 10 2006 018 515 A1 describe a gas inlet member which is part of a CVD reactor. The gas inlet member is shaped like a shower head and is arranged above a process chamber, the base of which is formed by a susceptor, on which substrates to be coated, in particular semiconductor substrates, can be placed. A gas outlet plate which is in contact with a gas inlet housing wall during the deposition process is present directly beneath the gas inlet member. The gas outlet plate can be displaced towards the susceptor for cleaning. Since the gas inlet housing wall of the gas inlet member is cooled and only the susceptor is heated, it is possible due to this measure to bring the gas outlet plate to a higher temperature so that by introducing an etching gas, for example HCl, parasitic deposits can be cleaned from the gas outlet plate. 
     In the case of CVD reactors having gas inlet members, the process chamber ceiling of which is formed by the gas inlet housing wall itself, parasitic growth occurs on the cooled gas inlet housing wall. This can result in undesirable particle formation within the process chamber. Moreover, the outlets of the gas feed lines can clog. This has the consequence that the gas inlet members have to be frequently mechanically cleaned. For cleaning the gas outlet side of the gas inlet member, the entire reactor housing has to be opened. 
     It is therefore an object of the invention to provide measures by which the cleaning cycles can be extended. 
     The gas outlet plate known from the above-mentioned prior art has openings for the process gas to pass through. Although the gas outlet plate is in contact with the cooled gas inlet wall, the gas outlet plate is still heated by heat conduction via the process gas through the process chamber, or via heat radiation from the susceptor. The outlet ends of the openings in the gas outlet plate can thus reach a temperature at which the process gas passing through the openings decomposes. The decomposition products can react with one another in the region of the outlet. This can also result in parasitic reactions in the gas phase, where undesirable adducts can form. 
     It is therefore a further object of the invention to provide measures by which undesirable pre-reactions can be reduced. 
     In the case of a CVD reactor in which the process chamber ceiling is cooled directly, the high heat dissipation requires that a high heat capacity must be provided in order to heat the susceptor to the desired process temperatures. 
     It is therefore a further object of the invention to increase the energy efficiency. 
     In a deposition process that uses hydrides, for example arsine or phosphine, which react in the process chamber together with organometallic compounds of main group III, moisture residues have negative effects on the layer growth. 
     It is therefore a further object of the invention to provide measures by which the moisture-inducing effects can be reduced. 
     At least one of these objects is achieved by the invention set forth in the claims. 
     SUMMARY OF THE INVENTION 
     First and foremost, it is proposed that the gas outlet plate is thermally decoupled with respect to the gas inlet housing wall. In contrast, the outlet ends of the gas lines are coupled to the cooled region of the gas inlet housing. If the lower side of the gas inlet member is heated by radiant heat from the heated susceptor of the process chamber, the process chamber ceiling in the region of the gas outlet plate can heat up more intensely than in the region of the outlets of the gas lines. For this purpose, the thermal conductivity of the material from which the gas outlet plate is made is lower than that of the material from which the gas lines are made. The gas lines through which a first process gas is introduced into the process chamber extend through the gas inlet housing wall. The gas lines are tubes that extend through the cooling chamber flushed by a coolant and connect the first gas distribution volume to the ceiling of the process chamber. According to the invention, these gas lines have extensions or are extended in such a manner that they extend as far as and into the openings of the gas outlet plate, which is spaced apart from the gas inlet housing wall. The gas lines can extend so far into the gas outlet plate that their outlets lie in the gas outlet plate surface facing the process chamber. The outlet areas do not have to be exactly flush in the surface of the gas outlet plate. Small steps, in particular of the size of a gap width around the gas lines, are possible, and may possibly be desirable. Furthermore, it is provided that the gas outlet plate is thermally decoupled with respect to the cooled gas inlet housing wall. For this purpose, the gas outlet plate is preferably spaced apart from the gas inlet housing wall by a clearance space in the form of a gap. This gap preferably extends over the entire surface of the gas outlet plate so that the gas outlet plate is connected only at its edge to the gas inlet member. 
     In a refinement of the invention, it is provided that with the gas inlet member formed like a shower head, at least one further process gas can be introduced into the process chamber. To this end, the gas inlet member forms a second gas distribution volume that likewise is connected to the gas inlet housing wall by means of tubes that form gas feed lines. The two gas distribution volumes can be arranged layered one above the other. The second gas feed lines through which the second process gas is introduced, which preferably is a hydride of the main group V, open into the gap between the gas inlet housing wall and the gas outlet plate. The outlets of the second gas lines lie flush in the gas inlet housing wall. The gas line extensions of the first gas lines, through which preferably an organometallic compound of an element of main group III is introduced into the process chamber, are preferably flush with the lower side of the gas outlet plate facing the process chamber. The openings through which the gas line extensions of the first gas lines protrude preferably have an opening width that is larger than the outer diameter of the gas line extensions, so that an annular gap is formed around the gas line extensions through which gas that is introduced into the gap between the gas inlet housing wall and the gas outlet plate can pass. 
     In a refinement of the invention, it is provided that the gas outlet plate has further openings through which the gas that is introduced into the gap can pass. This gas preferably is the second process gas, which can enter the process chamber in this manner. The gas outlet plate openings associated with the second process gas can be aligned with the outlets of the second gas lines. The tubes that form the first gas lines are preferably made of metal. The gas line extensions are integrally connected to the tubes. Thus, these are portions of the metal tubes which form the first gas lines and which project from the gas inlet housing wall. Due to this configuration, the outlet ends of the first gas lines are thermally coupled to the cooled portion of the gas inlet member. The outlets of the first gas lines are therefore kept at temperatures below the decomposition temperature of the first process gas that flows through the first gas lines into the process chamber. In contrast to these “cold” portions of the process chamber ceiling, the gas outlet plate can heat up. The gas outlet plate is thermally decoupled with respect to the cooled portion of the gas inlet member and is heated up by heat conduction or by heat radiation from the susceptor. While the first process gas enters the process chamber through “cold” discharge openings, the second process gas enters the process chamber through “hot” openings. 
     In a refinement of the invention which is of independent importance, it is provided that the first feed lines in the gas outlet plate are positioned on the vertices of equilateral triangles. This results overall in a hexagonal arrangement of the first feed lines in the gas discharge plane of the gas outlet plate. The second discharge openings in the gas outlet plate, through which the second process gas enters the process chamber, are preferably positioned on the centers of these triangles. Accordingly, the outlets of the second gas lines are also positioned on the vertices of equilateral triangles. The second process gas can enter the process chamber not only through these gas outlet plate openings associated with the second process gas, but also through the annular gap around the gas feed line extensions. However, a purge gas can also be introduced into the process chamber through these annular gaps so that contact between the two process gases is shifted into a region of hotter gas temperatures. For introducing a purge gas, it is also possible to structurally enhance the first gas lines. For example, the first gas lines can extend within a tube with a larger diameter, so that an annular gap is formed around the first gas lines through which a purge gas is introduced into the process chamber. The two tubes, which are nested into one another, open into the process chamber through the openings in the gas outlet plate. Here as well, a tube nested in this manner can be surrounded by an annular gap through which the second process gas is introduced into the process chamber. However, it is also possible for a further process gas to be introduced into the process chamber through the annular gap which is formed around the first gas line. 
     In a refinement of the invention, it is provided that the gas outlet plate is arranged in the reactor housing to be height-variable with respect to the gas inlet housing wall and the susceptor. If one or a plurality of substrates supported on the susceptor are to be coated with a layer in the process chamber, the gas outlet plate is in an operating position in which it is separated from the gas inlet housing wall by a small clearance space. The gas outlet plate is in a thermally decoupled position with respect to the gas inlet housing wall. The first gas lines protrude with their extensions into the openings of the gas outlet plate, the extensions being spaced at a lateral distance from the wall of the opening. A first process gas enters the process chamber through the extensions. The second process gas or a purge gas can enter the process chamber through the annular gap between the extensions and the opening wall. For cleaning the process chamber and in particular the gas outlet plate surface facing the process chamber, an etching gas can be introduced into the process chamber. Additionally or alternatively, however, for this purpose it is also possible to displace the gas outlet plate toward the heated susceptor and/or to increase the distance from the cooled gas inlet housing wall. By thus bringing the gas outlet plate closer to the hotter surface of the susceptor, the gas outlet plate can heat up to a higher temperature at which the parasitic deposits adhering to the lower side of the gas outlet plate can be removed. For vertical displacement, the device can have a lifting mechanism by means of which the distance between the gas inlet housing wall and the gas outlet plate can be varied. In a refinement of the invention, the gas outlet plate is secured on the lower side of the gas inlet member in such a manner that the gas outlet plate can be displaced in its direction of extent. By such horizontal displaceability of the gas outlet plate, the gas line extensions can be brought from an operating position, in which they freely extend into the openings, into another operating position in which in each case the wall of a cooled gas line extension rests against the wall of the opening. The gas outlet plate can thus be brought into heat-conductive contact with the gas line extensions. With these measures, it is possible to keep the gas outlet plate at temperatures below 450° C., preferably below 400° C., during the growth process. By modifying the position of the gas outlet plate with respect to the gas inlet housing wall, it is possible in a subsequent cleaning phase to bring the surface temperature of the gas outlet plate side facing the process chamber to temperatures greater than 500° C. The combination is particularly preferred in which the gas outlet plate is displaceable not only vertically, but also horizontally. In a refinement of the invention, the openings in the gas outlet plate are modified in such a way that they have a sleeve that protrudes into the clearance space. The sleeve forms a tube portion into which the first gas line protrudes in a telescopic manner. Due to this configuration, the vertical position of the gas outlet plate can vary over a relatively wide range without the outlet of the gas line extensions extending out of their associated sleeve and freely protruding into the clearance space. Thus, process gases exiting the outlet of the first gas line also cannot enter the clearance space, even if the distance between the gas inlet housing wall and the gas outlet plate is increased. Mixing of process gases thus takes place only within the opening that is extended by the sleeve. The second process gas enters the opening through the annular gap between the inner wall of the sleeve and the outer wall of the gas line extension. In a refinement of the invention, it is provided that the walls of the opening are profiled. For example, the openings of the gas outlet plate can have chamfers or counterbores. These profiles can be situated on the gas outlet plate side facing away from the process chamber. Moreover, such chamfers also facilitate assembly since they cause a self-centering effect. If there are tolerance errors, the chamfers minimize any thermal bridges between the gas line extensions and the opening edge. In this case, it can be advantageous if the profiles, i.e., the chamfers or counterbores, are located on the gas outlet plate side facing the process chamber. The growth rate at the edge thus also decreases, so that the operating time between two cleaning steps is extended. Finally, it is advantageous when the cross-section of the opening has a noncircular contour, in particular, has projections that protrude into the opening and can establish thermal contact with the gas line extensions during a horizontal displacement of the gas outlet plate. 
     Exemplary embodiments of the invention are explained below with reference to accompanying drawings. In the figures: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows the structure of a reactor housing of a CVD reactor, 
         FIG. 2  shows the detail II-II in  FIG. 1 , 
         FIG. 3  shows the detail III-III in  FIG. 1 , 
         FIG. 4  shows the detail IV-IV in  FIG. 3 , 
         FIG. 5  schematically shows the arrangement of the gas discharge openings in the gas outlet plate  14 , 
         FIG. 6  shows a second exemplary embodiment of the invention in an illustration according to  FIG. 4 , 
         FIG. 7  shows a third exemplary embodiment of the invention in an illustration according to  FIG. 4 , 
         FIG. 8  shows a fourth exemplary embodiment of the invention in an illustration according to  FIG. 1  in a growth operating position, the gas outlet plate  14  being displaceable in the vertical direction, 
         FIG. 9  shows the enlarged detail IX-IX in  FIG. 8 , 
         FIG. 10  shows an illustration according to  FIG. 9 , but with the gas outlet plate  14  lowered in the vertical direction, 
         FIG. 11  shows a fifth exemplary embodiment of the invention in a growth process position, in which the opening  16  is formed by a sleeve  30  that protrudes into the clearance space  20 , 
         FIG. 12  shows an illustration according to  FIG. 11 , but with the gas outlet plate  14  lowered, 
         FIG. 13  shows as a sixth exemplary embodiment, an illustration according to  FIG. 2  with a profiled wall of the opening  16 , which is formed as a bevel facing toward the clearance space  20 , 
         FIG. 14  shows as a seventh exemplary embodiment, an illustration according to  FIG. 13 , the profile of the wall of the opening  16  being formed as a circumferential ring groove, the opening of which faces the clearance space  20 , 
         FIG. 15  shows as an eighth exemplary embodiment a further variant of a profile of the opening wall  16 , in this case a bevel  28  facing the process chamber, 
         FIG. 16  shows a ninth exemplary embodiment of the invention in an illustration according to  FIG. 1 , the gas outlet plate  14  being displaceable not only in the vertical direction but also in the horizontal direction, 
         FIG. 17  shows an enlarged illustration of the gas line extension  15  that protrudes into the opening  16  when the gas outlet plate  14  is lowered, the wall of the gas line extension being brought in heat-conductive contact with the opening wall due to a horizontal displacement of the gas outlet plate  14 , and 
         FIG. 18  shows the section according to the line XVIII-XVIII, the circumferential contour line of the opening  16  being configured here to be noncircular due to four projections  23  being provided that protrude radially into the opening, one projection  32  being in contact with the gas line extension  15 . 
     
    
    
     DETAILED DESCRIPTION 
     The CVD reactor according to the invention has a gas storage and metering system, not illustrated, in which the process gases are mixed with carrier gases and fed through lines to the CVD reactor. In  FIG. 1 , in which the cross-section of a reactor housing  1  is illustrated, only two gas feed lines  21 ,  23  for a first process gas and a second process gas, respectively, are illustrated. The feed lines  21 ,  23  each open into gas distribution volumes  5  and  6 , respectively. The gas distribution volumes  5 ,  6  lie one above the other and are separated from one another by a separator plate  12 . Each gas distribution volume  5 ,  6  is connected via separate tubes, which form the gas lines  8 ,  9 , to a gas inlet housing wall  10 . The process gas exiting the outlets of the gas lines  8 ,  9  reaches a process chamber  22  situated below the gas inlet member  2 , the base of the process chamber being formed by a susceptor  3 . One or a plurality of substrates to be coated are present on the susceptor  3 . The susceptor  3  is heated to a process temperature from below by means of a heater  4 . The temperature can range between 500° and 1000° Celsius. The reactor housing  1  is connected to a vacuum pump, not illustrated, so that total pressures of typically 50 to 800 mbar can be set inside the process chamber. The diameter of the susceptor  3  corresponds to approximately 300 mm to 600 mm. The diameter of the gas inlet member  2 , which is shaped like a shower head, corresponds approximately to the diameter of the susceptor  3 . 
     Directly above the gas inlet housing wall  10 , a coolant chamber  7  is present through which a coolant flows so that the gas inlet housing wall  10  is temperature-controlled to a temperature between 60° and 200° Celsius. 
     A gas-flushed gap  20  is present below the gas inlet housing wall  10 . The second gas lines  9  associated with the second process gas open into the gap  20 . The gas lines are formed by tubes that pass through the coolant chamber  7 . 
     The first gas lines  8 , which likewise are formed from tubes and through which the first process gas flows, traverse not only the coolant chamber  7  but also the second gas distribution volume  6 . The tubes forming the first gas lines  8  continue through the gas inlet housing wall  10  and form gas line extensions  15  which initially traverse the gap  20  and then enter an opening  16  in a gas outlet plate  14 . 
     The gas outlet plate  14  is held only at its edge, which is circular in the exemplary embodiment. As can be seen from  FIG. 2 , the gas outlet plate is held at the edge, supported from below in the exemplary embodiment, by a wall which forms the edge of the gas inlet member  2 . 
     The gas outlet plate  14  can be made of a heat-conductive or a nonheat-conductive material. The gas outlet plate  14  can be made of quartz, graphite or a metal. 
     The openings  16  into which the gas line extensions  15  extend have a larger diameter than the gas lines  8 , so that an annular gap  17  is formed around each of the outlet portions of the gas lines  8 . The outlets  8 ′ of the gas lines  8  are flush with the surface of the gas outlet plate  14  facing the process chamber  22 . 
     The gas outlet plate  14  has further openings  18  with a smaller diameter than the openings  16 . The openings  18  are in an aligned position with the outlets  9 ′ of the second gas lines  9 . 
     The intermediate wall  11  which delimits the cooling chamber  7  from the second gas distribution volume  6 , the intermediate wall  12  which separates the two gas distribution volumes  5 ,  6  from one another, the gas inlet housing wall  13  through which the feed lines  21 ,  23  pass, and the gas inlet housing wall  10  run parallel to one another. The gas inlet housing wall  10  runs in the horizontal direction. The gas outlet plate  14  likewise runs in the horizontal direction. In this respect, the gas outlet plate  14  runs substantially parallel to the gas inlet housing wall  10 . Both plates (the gas outlet plate  14  and also the gas inlet housing wall  10 ) do not have to have smooth surfaces. Although the preferred exemplary embodiment illustrated in the drawings shows flat surfaces, the surfaces of the gas inlet housing wall  10  and the surfaces of the gas outlet plate  14  can also be structured or profiled. 
     In the exemplary embodiment illustrated in  FIGS. 1 to 4 , the gas inlet housing wall  10  is actively cooled. The susceptor  3  is actively heated. A first process gas in the form of an organometallic compound of an element of main group III is introduced through the feed line  23 . This takes place together with a carrier gas. A hydride of an element of main group V is likewise introduced together with a carrier gas through the feed line  21 . These gases exit the outlets  8 ′ and  9 ′, respectively, and enter the process chamber  22  and heat up therein. Gas phase reactions take place in which the process gases are pre-decomposed or intermediate products are formed. As a result of pyrolytic decomposition of the process gases III-V, layers are deposited on the surface of the substrate. 
     The organometallic components enter the process chamber  22  through the first gas lines  8 . Since the first gas lines are thermally coupled to the coolant chamber  7 , the outlets  8 ′ of the first gas lines have a temperature that is lower than the decomposition temperature of the first process gas. Since the gas outlet plate  14  is thermally decoupled with respect to the coolant chamber  7  and the gas inlet housing wall  10 , its surface facing the process chamber  22  has a significantly higher temperature. During a growth process, this temperature can range between 250° and 600° C. The heat transfer between the susceptor  3  and the cooling chamber  7  is thus reduced, thereby increasing the energy efficiency of the CVD reactor. At the same time, a certain thermal activation of the second process gas takes place, the second process gas being less reactive than the first process gas. 
     With the configuration according to the invention of the gas inlet member  2 , the process chamber ceiling can be etched in situ. Moreover, due to the hot process chamber ceiling, deposition of parasitic reaction products in the process chamber is reduced, and the nature of the deposit changes. The parasitic deposit deposited in the process chamber according to the invention has a higher stability. During etching, the process chamber ceiling, i.e., the lower side of the gas outlet plate  14 , is heated to temperatures above 500° Celsius and, if necessary, even up to 1,000° C. This takes place substantially due to heat radiation or heat conduction from the susceptor via the gas in the process chamber. HCl, optionally together with a carrier gas, for example N 2  or H 2 , can be introduced through the gas lines  8  and/or the gas lines  9 . 
     Since during the operation of the device, the gas inlet housing wall  10  and the gas outlet plate  14  have temperatures that differ significantly from one another, and a uniform gas supply has to be ensured, the dimensions of the annular gaps  17  and of the clearance space must be selected accordingly. In addition to the thermal expansion of the gas outlet plate  14 , the influences of gravity must also be taken into account since the gas outlet plate  14  is held only at the edge. The diameters of the openings  16  are in any case selected in such a way that no stresses are transmitted to the gas line extensions  15  during thermal expansion of the gas outlet plate  14 . 
     In the exemplary embodiment illustrated in  FIG. 6 , the gas outlet plate  14  has temperature-regulating channels  19 . A hot or cold liquid can flow through these temperature-regulating channels  19 , so that not only can the gas outlet plate  14  be hotter than the gas inlet member  2 , but it may also be arranged to heat up the gas inlet member  2  and to cool the gas outlet plate  14 , for example if a layer is to be deposited by condensation onto a substrate supported on the susceptor  3 . 
     In a preferred configuration, the gas lines that are illustrated only in a simplified cross-section in  FIGS. 1 to 4  do not lie in a common cross-sectional plane. 
       FIG. 5  shows that the first gas lines  8  or the outlets  8 ′ thereof are positioned on the vertices of an equilateral triangle. Thus, each outlet  8 ′ is spaced at the same distance a from the outlet  8 ′ nearest to it, resulting in a hexagonal arrangement. 
     The openings  18  of the gas outlet plate  14  through which the second process gas flows into the process chamber are likewise positioned in a hexagonal association with one another. The openings lie approximately in the centers of the equilateral triangles having the edge length a. 
     While in the exemplary embodiments illustrated in  FIGS. 1 to 4 and 6 , the second process gas can also enter the process chamber through the annular gaps  17  and therefore can come into contact with the first process gas directly in front of the outlet  8 ′, in the exemplary embodiment illustrated in  FIG. 7 , an additional annular flow channel  26  is provided. For this purpose, the tube forming the first gas line  8  is surrounded by a tube  27  that has a larger diameter. A purge gas which is fed into a further gas distribution volume  25  can be introduced into the process chamber  2  purge gas through the gap  26  between the tubes  8  and  27 . The gas distribution volume  25  is separated from the gas distribution volume  6  by an intermediate wall  24 . The intermediate wall  12  separates the gas distribution volume  25  from the gas distribution volume  5 . 
     Due to the purge gas exiting the annular gap  17 , the first process gas exiting the outlet  8 ′ comes into contact with the second process gas exiting the gap  17  only in a hotter region of the process chamber  2 . A further measure is thus provided so as to reduce adduct formation or other parasitic reactions in the process chamber. The gas outlet plate  14  can be formed in one or multiple parts. Moreover, the gas outlet plate  14  forms a temperature shield of the cooled gas inlet member  2 . The gas outlet plate  14  can be heated up by the heat radiation emanating from the susceptor  3 . Only cold islands remain in the region of the outlets  8 ′. 
     The exemplary embodiment illustrated in  FIG. 7  can also be operated in a different manner; for example, a further process gas can be introduced into the process chamber through the annular channel  26 . A purge gas or also a process gas can be introduced into the process chamber  22  through the gas lines  8 . 
     Depending on the height H of the clearance space  20  and the output of the heater  4 , there is an upper limit of the maximum temperature to which the lower side of the gas outlet plate  14  can be heated. In a growth process, the temperature of the lower side of the gas outlet plate is at a temperature lower than 450° C. or even lower than 400° C. However, in a cleaning step, the temperature should be at least 500° C. To be able to achieve this with simple means, the capability for changing the position of the gas outlet plate  14  is provided in the exemplary embodiments illustrated in  FIGS. 8 to 18 . For this purpose, the reactor has a displacement means  31  by means of which the gas outlet plate  14  can be displaced in the vertical direction. Hooks  31 ′ engage from below with the edge of the gas outlet plate  14 .  FIG. 8  shows the gas outlet plate  14  in a production process position in which the gas outlet plate  14  is brought to a height at which the outlet  8 ′ of the first gas line  8  is flush with the lower side of the gas outlet plate  14 . The tubular gas line extensions  15  formed by the first gas lines  8  protrude into the openings  16  of the gas outlet plate  14  while leaving an annular gap therebetween. 
     The dimension of the height H of the clearance space  20  can be increased. This takes place by lowering the gas outlet plate  14  by means of the lifting elements  31 . In doing so, the gas outlet plate  14  moves away from the cooled gas inlet housing wall  10  and approaches the heated top side of the susceptor  3 . As can be seen in  FIG. 10 , the gas line extensions  15  then no longer protrude into the openings  16 . In this operating position, the gas outlet plate  14  can heat up to over 500° C. during the cleaning process. In the exemplary embodiment, lifting elements  31  are provided so as to displace the gas outlet plate  14  between the stationarily-held gas inlet member  2  and the susceptor  3  that is held stationary relative thereto. However, alternative configurations are also possible; for example, the distance H can also be increased by lifting the gas inlet member  2 . It is also conceivable to lower the entire assembly comprising the gas inlet member  2  and the gas outlet plate  4  toward the susceptor  3 , or to lift the susceptor  3  toward the assembly. It is important in each case that the ratio of the distances between the gas outlet plate  14  and the gas inlet member  2  on the one hand, and between the gas outlet plate  14  and the susceptor  3  on the other, changes. 
     In the fifth exemplary embodiment illustrated in  FIGS. 11 and 12 , the openings  16  are formed by sleeves  30  which are inserted into corresponding openings of the gas outlet plate  14 . The sleeves  30  are flush with the surface of the gas outlet plate  14  that faces the process chamber, but protrude into the clearance space  20 . In this configuration, the gas outlet plate  14  can be lowered without the outlet  8 ′ of the first gas line  8  leaving the opening  16 . Also, in the lowered position of the gas outlet plate  14 , the annular gap  17  between the outer wall of the gas line extension  15  and the inner wall of the opening  16 , i.e., the inner wall of the sleeve  30 , is maintained. The sleeve  30  can be made of a suitable material, for example a metal, graphite, coated graphite or quartz. The sleeve is inserted into an opening of the gas outlet plate  14 . The sleeve can also be held there in a positive-locking manner. 
     The sixth exemplary embodiment illustrated in  FIG. 13  shows a profile of the wall of the opening  16 . The profile  28  is formed there by a chamfer that is provided on the side of the gas outlet plate  14  that faces the gas inlet housing wall  10 . 
     The seventh exemplary embodiment illustrated in  FIG. 14  shows a differently shaped profile  29  of the wall of the opening  16 . The profile  29  is formed here by a step, the step forming a groove that opens toward the side facing away from the process chamber. 
     In the eighth exemplary embodiment illustrated in  FIG. 15 , the opening  16  has a wall profile  28  that is formed as a chamfer. The entire opening wall extends on the inner surface of a truncated cone, the truncated cone-shaped space opening toward the process chamber. 
     The ninth exemplary embodiment illustrated in  FIG. 16  shows a gas outlet plate  14  that is displaceable in the horizontal direction. As the result of a horizontal displacement, the openings  16  can be brought into heat-conductive contact with the cooled gas line extensions  15 . 
       FIG. 17  shows an enlarged illustration in which the thermal contact is established by horizontally displacing the gas outlet plate  14 . Here, the gas outlet plate  14  has been in addition lowered in the vertical direction. 
       FIG. 18  shows a specially shaped cross-sectional contour of an opening  16 . Four semicircular projections  32  protrude into the clearance space of the opening  16  from a circular baseline in uniform angular distribution. By means of the vertical displacement, one of the projections  32  can be brought into heat-conductive contact with the gas line extension  15 . 
     All features disclosed are (in themselves) pertinent to the invention. The disclosure content of the associated/accompanying priority documents (copy of the prior application) is also hereby included in full in the disclosure, including for the purpose of incorporating features of these documents in the claims in the present application. The subsidiary claims in their optional subordinated formulation characterize independent inventive refinements of the prior art, in particular to undertake divisional applications based on these claims. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 List of reference numerals 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  1 
                 Reactor housing 
               
               
                   
                  2 
                 Gas inlet member 
               
               
                   
                  3 
                 Susceptor 
               
               
                   
                  4 
                 Heater 
               
               
                   
                  5 
                 First gas distribution volume 
               
               
                   
                  6 
                 Second gas distribution volume 
               
               
                   
                  7 
                 Coolant chamber 
               
               
                   
                  8 
                 First gas line 
               
               
                   
                  8′ 
                 Outlet 
               
               
                   
                  9 
                 Second gas line 
               
               
                   
                  9′ 
                 Outlet 
               
               
                   
                 10 
                 Gas inlet housing wall 
               
               
                   
                 11 
                 Intermediate wall 
               
               
                   
                 12 
                 Intermediate wall 
               
               
                   
                 13 
                 Gas inlet housing wall 
               
               
                   
                 14 
                 Gas outlet plate 
               
               
                   
                 15 
                 Gas line extension 
               
               
                   
                 16 
                 Opening 
               
               
                   
                 17 
                 Annular gap 
               
               
                   
                 18 
                 Opening 
               
               
                   
                 19 
                 Temperature-regulating channel 
               
               
                   
                 20 
                 Clearance space 
               
               
                   
                 21 
                 Feed line 
               
               
                   
                 22 
                 Process chamber 
               
               
                   
                 23 
                 Feed line 
               
               
                   
                 24 
                 Intermediate wall 
               
               
                   
                 25 
                 Gas distribution volume 
               
               
                   
                 26 
                 Flushing gas line 
               
               
                   
                 27 
                 Tube 
               
               
                   
                 28 
                 Profile 
               
               
                   
                 29 
                 Profile 
               
               
                   
                 30 
                 Sleeve 
               
               
                   
                 31 
                 Displacement means 
               
               
                   
                 31′ 
                 Hook 
               
               
                   
                 32 
                 Projection 
               
               
                   
                 a 
                 Edge length 
               
               
                   
                 H 
                 Height