Abstract:
A method of depositing layers on a plurality of semiconductor substrates simultaneously, comprising the steps: 
       cleaning of at least one respective surface of the substrates in a first reactor at a first substrate temperature T red , transport of the substrates from the first reactor into a second reactor, and    subsequent deposition of at least one respective layer on the semiconductor substrates in the second reactor at a second substrate temperature T dep ,    wherein the semiconductor substrates are moved or stored during the cleaning step and during transport from the first reactor into the second reactor in an interruption-free manner in a reducing gas atmosphere as long as the substrate temperature is above a critical temperature T c  which is dependent on the substrate material and the material of the at least one layer to be deposited.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is for entry into the U.S. national phase under §371 for International Application No. PCT/EP2005/052123 having an international filing date of May 10, 2005, which claims priority under 35 USC §119 to German Patent Application No. 10 2004 024 207.0-33 filed on May 10, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     The present invention concerns a method of depositing layers on a plurality of semiconductor substrates simultaneously and a reactor arrangement for layer deposition on a plurality of substrates.  
         [0004]     2. Discussion of Related Art  
         [0005]     The development of silicon-germanium-carbon technology has made it possible to manufacture semiconductor devices which contain functional layers with a hitherto unattainable combination of a particularly small layer thickness and a high level of dopant concentration. In that way it was possible for the economically highly developed silicon technology also to be used for devices which are suitable for high frequency, and in part replace the hitherto dominant technology based on III-V semiconductors such as gallium arsenide.  
         [0006]     Si/SiGe—C technology profits on the one hand from the advantageous electronic properties of Si/SiGe heterostructures over pure Si structures and on the other hand from the dual effect of the incorporation of carbon. Carbon can compensate for lattice dislocation adaptation of SiGe in relation to Si and in addition represses the diffusion of dopants, which as a consequence leads to unwanted layer spread in transistor structures.  
         [0007]     The advantages of Si/SiGe—C technology however could come to fruition only on the basis of the accompanying development of new deposition methods which are adapted to the particular demands of the new semiconductor structures. Thus low-temperature deposition methods were developed, which as far as possible avoid diffusion-promoting process steps at high temperatures.  
         [0008]     WO 03/012840 A2 discloses such a low-temperature method of manufacturing thin epitaxial layers. In those methods a plurality of substrates are initially subjected to external preliminary cleaning and then subjected to a hydrogen treatment at a high temperature of between 750 and 1100° C. (“hydrogen prebake”). After that preliminary treatment the substrates are coated in a low-pressure hot or warm wall batch reactor at temperatures which are lower in comparison with the hydrogen treatment.  
         [0009]     It is further known from WO 03/012840 A2 for the hydrogen prebake operation to be carried out in a first reactor chamber and for the layer deposition operation to be carried out in a second reactor chamber. After the hydrogen prebake step the substrates are transferred from the first reactor chamber into the second. Transport is effected through a transport chamber in an inert atmosphere or vacuum.  
         [0010]     U.S. Pat. No. 6,488,788 B1 discloses a similar arrangement. A problem with the method described in those documents is transport between the reactor chambers after the hydrogen treatment. Due to the change in the gas atmosphere at elevated substrate temperatures, unwanted reactions can occur and fresh impurities can come about on the surface of the substrates which have just been cleaned, and they can adversely affect the homogeneity of the subsequent layer deposition operation. In addition handling of the wafers for transport from the first reactor chamber into the second chamber is complicated and time-consuming.  
         [0011]     Therefore the technical object of the invention is to provide a method of and an apparatus for layer deposition, in which the above-specified disadvantages do not occur.  
       DISCLOSURE OF INVENTION  
       [0012]     The method aspect of the above mentioned object is attained by a method of deposition of layers on a plurality of semiconductor substrates simultaneously, comprising the following steps: 
        simultaneous cleaning of at least one respective surface of the plurality of semiconductor substrates in a first reactor at a first substrate temperature T red ,     simultaneous transport of the plurality of semiconductor substrates from the first reactor into a second reactor, and     subsequent deposition of at least one respective layer on the semiconductor substrates in the second reactor at a second substrate temperature T dep .        
 
         [0016]     In the method according to the invention the semiconductor substrates which hereinafter are also referred to for brevity as substrates are further moved or stored during the cleaning step and during transport from the first reactor into the second reactor in an interruption-free manner in a reducing gas atmosphere as long as the substrate temperature is above a critical temperature T c  which is dependent on the substrate material and the material of the at least one layer to be deposited.  
         [0017]     The method according to the invention is based on the realization that an adverse effect on the homogeneity of layer deposition by impurities or reactions which have undesirably occurred after the hydrogen treatment of the substrates is a phenomenon which is dependent on temperature. It is possible to determine a critical temperature T c  which is dependent on the substrate material and above which layer deposition is adversely affected. In the case of silicon the critical temperature is at values somewhat above 200° C.  
         [0018]     In accordance with the invention the plurality of substrates, during the cleaning operation and during subsequent transport from the first reactor into the second reactor, is moved or stored without interruption in a reducing gas atmosphere as long as the substrate temperature is greater than or equal to the critical temperature T c . In that way it is possible to produce particularly homogenous layers in the subsequent layer deposition procedure.  
         [0019]     The method according to the invention also has the advantage of permitting rapid more robust substrate transport. Storage and transport of the substrates at temperatures above the critical temperature in a reducing gas atmosphere like a hydrogen atmosphere cause passivation of the substrate surfaces. Originally open valencies at the surface of the substrate are occupied by atoms of the gas of the reducing gas atmosphere. The passivation achieved in that way, for example hydrogen passivation, is maintained at temperatures below the critical temperature sufficiently long to ensure substrate transport in a nitrogen atmosphere without the risk of troublesome surface reactions. Unwanted reactions with oxygen or moisture during substrate transport even at low temperatures can be suppressed in that fashion.  
         [0020]     In a preferred embodiment of the method according to the invention the substrates during transport from the first reactor into the second reactor are transported out of the reducing gas atmosphere after the substrate temperature has fallen below the critical temperature. In that way on the one hand transport of the substrates can be begun while still in the reducing gas atmosphere, whereby further time is gained. On the other hand the entire operation of transporting the semiconductor substrates from the first reactor into the second reactor does not have to be performed in a reducing gas atmosphere. After the above-discussed passivation of the surfaces in a reducing gas atmosphere such as for example a hydrogen atmosphere above the critical temperature the substrates can also be moved in another gas atmosphere, for example in an inert gas atmosphere such as a nitrogen atmosphere or in a vacuum. Less complication and expenditure is required for that purpose than for providing reactor regions which have to be changed over to a reducing gas atmosphere.  
         [0021]     In a further embodiment of the method according to the invention transport of the substrates from the first reactor into the second reactor includes the following steps: 
        transporting the substrates from the first reactor into a first lock chamber of adjustable temperature and gas atmosphere at a substrate temperature above the critical temperature T c ,     cooling the substrates in the first lock chamber to a substrate temperature below the critical temperature T c , and     transporting the substrates from the first lock chamber into the second reactor.        
 
         [0025]     By virtue of using the first lock chamber the substrates can be transported after the hydrogen treatment in the first reactor in a reducing gas atmosphere. As a result the first reactor becomes free again for receiving fresh substrates. The substrates which have just been cleaned can then be cooled in the first lock chamber in the reducing gas atmosphere until they have fallen below the critical temperature T c . Transport of the substrates into the second reactor can then be effected in an inert gas atmosphere or in a vacuum. For that purpose for example the gas atmosphere in the first lock chamber is replaced or pumped out until the desired conditions for further transport have occurred. The lock chamber can then be opened and the substrates can be further transported to the second reactor.  
         [0026]     In a further configuration of the method according to the invention the first lock chamber is associated with the first reactor. Transport of the substrates from the first lock chamber into the second reactor includes in that respect the following steps: 
        if necessary changing over the gas atmosphere in the second lock chamber to a reducing gas atmosphere,     heating the substrates to a substrate temperature above the critical temperature T c , and     transporting the substrates into the second reactor in the reducing gas atmosphere.        
 
         [0030]     That method involves two lock chambers. The first lock chamber serves for transporting and storing the substrates prior to and after the hydrogen prebake operation. The second lock chamber serves for transporting and storing the substrates prior to and after the layer deposition operation. In this embodiment, the layers are heated with the introduction of the substrates into the second reactor in the reducing gas atmosphere in the second lock chamber to a substrate temperature above the critical temperature T c .  
         [0031]     In this case also it is conceivable that one or more further substrate carriers are already stored during deposition of a number of substrates in the second reactor in the second lock chamber and are brought to the temperature required for the deposition operation.  
         [0032]     A further embodiment of the method according to the invention provides for transport of the substrates from the first lock chamber into the second lock chamber through a transport chamber with a non-reducing gas atmosphere. It will be appreciated that in accordance with the invention that is effected only at temperatures below the critical temperature T c .  
         [0033]     In a further embodiment the gas atmosphere in the second reactor upon the introduction of the substrates is at a temperature required for heating the substrates to the second substrate temperature T dep .  
         [0034]     The method according to the invention can also be carried out without transport and storage of the substrates in lock chambers. In such an embodiment for example the first and the second reactors are arranged in mutually adjoining relationship and can be separated from each other by gas-tight and vacuum-tight closures. After the cleaning step in the first reactor and prior to the layer deposition step in the second reactor in this embodiment the following steps are carried out: 
        cooling the substrates in the first reactor to a substrate temperature which is equal to the deposition temperature T dep  in the second reactor,     changing over the gas atmosphere in the second reactor to a reducing gas atmosphere,     transporting the substrates into the second reactor, and     if necessary changing over the gas atmosphere in the second reactor to the gas atmosphere required for the layer deposition operation.        
 
         [0039]     The reducing gas atmosphere is preferably an ultra-pure reducing gas atmosphere. Ultra-pure in this context means that impurities are present only in a concentration of in total less than 10 ppb.  
         [0040]     Preferably the reducing gas atmosphere comprises hydrogen, wherein the residual content of oxidizing constituents such as for example oxygen and moisture such as water vapor is no higher than 1 ppb.  
         [0041]     As already explained in the opening part of this specification the critical temperature depends on the substrate material used and the material of the layer to be deposited. The critical temperature is typically lower than the deposition temperature T dep . In a preferred embodiment the following relationship applies between the first and second substrate temperatures T red  and T dep  and the critical temperature T c : 
 
200° C.&lt;T c &lt;T dep &lt;T red &lt;1000° C. 
 
         [0042]     In that case T red  denotes the temperature of the reducing gas atmosphere during the hydrogen prebake operation. For the situation of deposition on silicon the deposition temperature is typically between 500° C. and 700° C.  
         [0043]     Preferably the method according to the invention is performed simultaneously with at least 25 substrates.  
         [0044]     The method is particularly suitable for deposition on silicon substrates. It is however also possible to envisage other substrate materials, for example silicon-germanium, silicon carbide or germanium.  
         [0045]     Preferably at least one epitaxial layer is deposited in the second reactor. Additionally or alternatively the deposition step in another embodiment includes the deposition of at least one dielectric layer on the substrates.  
         [0046]     In a preferred embodiment the method according to the invention is carried out in parallel on a plurality of batches. While a first plurality of substrates is subjected to the deposition step at the same time at least one second plurality of substrates is subjected to the cleaning step. In that way it is possible to achieve a marked increase in the wafer throughput per hour in comparison with single-chamber systems and in comparison with implementation of the method according to the invention with only one plurality of substrates at the same time, that is to say only one batch.  
         [0047]     As the deposition step generally lasts markedly longer than the cleaning step a plurality of batches can also be cleaned during the deposition operation. After the prebake and cooling to below the critical temperature the second batch, that is to say the second plurality of substrates, is either stored in the lock chamber or in the transport chamber until the first batch, that is to say the first plurality, has been moved after the deposition step after out of the lock chamber of the deposition reactor. Storage in the transport chamber is advantageous in the situation where a third or still further batches are provided for cleaning in the prebake reactor while the first batch is being subjected to the deposition step in the deposition reactor.  
         [0048]     The foregoing consideration motivates an embodiment of the reactor arrangement according to the invention, comprising two or more second reactor chambers for carrying out the deposition operation. In this embodiment the cleaned batches do not have to be stored long but can be transported immediately to the respective next deposition chamber which comes free, or a lock chamber provided there. Parallel implementation of the deposition step in a plurality of deposition reactors permits a further increase in efficiency.  
         [0049]     A further embodiment of the method according to the invention provides that the first substrate temperature T red  is set by short-term irradiation of a respective substrate with a light source. In that way the thermal budget to which the substrates are exposed can be markedly reduced. In addition there is a saving on complicated and expensive heating devices at the prebake reactor chamber. Unlike heating the entire reactor chamber, this embodiment provides that each substrate of a batch is irradiated individually. Therefore it is only exposed for a short time to a defined heating output.  
         [0050]     Preferably in that case a respective substrate is irradiated either with a light flash or with a plurality of light flashes, in which the number, frequency spectrum, intensity and duration of the light flash or light flashes are so selected that the substrate is heated for a predetermined period of time to the first substrate temperature T red . To carry out a surface cleaning operation, a period of time of a maximum of 10 seconds in a hydrogen atmosphere at the first substrate temperature is sufficient.  
         [0051]     In accordance with a further aspect of the invention there is provided a reactor arrangement for layer deposition on a plurality of substrates. The reactor arrangement according to the invention comprises: 
        a first reactor chamber which is adapted to carry out simultaneous cleaning of the plurality of substrates,     at least one second reactor chamber which is adapted for the deposition of at least one respective layer on each of the plurality of substrates,     a first heating device for setting the substrate temperature of the substrates in the first reactor chamber,     a second heating device for setting the substrate temperature of the substrates in the second reactor chamber,     a device for producing a gas atmosphere of predetermined composition and predetermined pressure selectively in the first reactor chamber or in the second reactor chamber or in both reactor chambers, and     a transport device which is adapted to transport the plurality of substrates simultaneously from the first reactor chamber into the second reactor chamber.        
 
         [0058]     In addition provided in the reactor arrangement according to the invention is a control device which is connected to the heating devices and the device for producing the gas atmosphere and which is adapted to control the heating devices and the device for producing the gas atmosphere in such a way that the substrates during the cleaning operation and during a subsequent transport step of the cleaned substrates from the first reactor into the second reactor are moved or stored in an interruption-free manner in a reducing gas atmosphere as long as the substrate temperature is above a critical temperature T c  which is dependent on the substrate material and the material of the at least one layer to be deposited.  
         [0059]     The reactor arrangement according to the invention is adapted to carry out the layer deposition method in accordance with the first aspect of the invention. The control device of the reactor arrangement according to the invention co-ordinates the interplay of the heating devices and the device for producing the gas atmosphere of predetermined composition and predetermined pressure. Control is effected with the proviso that the substrates, during the cleaning operation and during subsequent transport of the cleaned substrates from the first reactor into the second reactor are moved or stored in an interruption-free manner in a reducing gas atmosphere as long as the substrate temperature is above the critical temperature T c . Advantages of the reactor arrangement according to the invention arise on the one hand directly out of the description of the advantages of the method according to the invention.  
         [0060]     In the reactor arrangement according to the invention the first and second reactor chambers are separate from each other and each serve for carrying out respective clearly defined method steps. Thus the second reactor chamber is used for the layer deposition procedure and not for carrying out a cleaning operation on the substrates. That has advantages in regard to the purity of the atmosphere in the substrate cleaning operation (prebake) and in the layer deposition operation, as well as the frequency of necessary cleaning procedures for the reactor chambers themselves. Substrate cleaning is typically effected at a temperature which is increased in relation to the layer deposition operation. If substrate cleaning and substrate deposition are carried out in one chamber, the temperature of the chamber must be increased for the cleaning operation and later reduced again for the layer deposition operation. In the layer deposition operation, a deposit also occurs at the walls of the reactor chamber. Because of the thermal cycles in successive cleaning and deposition steps, that wall deposit in a reactor arrangement in accordance with the state of the art can peel off, in particular during the high-temperature phases, by virtue of the layer stresses which occur in the thermal cycles in the deposit on the walls. Thus the gas atmosphere can be contaminated during the substrate cleaning operation, whereby the purity of the substrate surfaces is detrimentally affected. That risk is at least reduced in the reactor arrangement according to the invention. The deposited layers are very substantially free of foreign particles and thus enjoy particularly good layer quality.  
         [0061]     In the reactor arrangement according to the invention the second reactor chamber can also be cleaned more easily and less frequently because the chamber can always kept at the same temperature, the deposit on the walls of the reactor is less and, because of the constant temperature, no thermally produced stresses occur in the wall deposit layer, which result in it peeling off. That permits the reactor according to the invention to have a longer service time between necessary reactor cleaning procedures, irrespective of whether that is effected in the form of in-situ cleaning or in the form of external cleaning with a reactor change. The reactor arrangement according to the invention therefore enjoys increased efficiency in comparison with known arrangements.  
         [0062]     In an embodiment of the reactor arrangement according to the invention the first reactor chamber and the second reactor chamber each have at least one respective opening for the through-transport of the plurality of substrates. Both openings are closable by means of at least one gas-tight and vacuum-tight closure. In this embodiment the reactor chambers are in mutually adjoining relationship at least in a region which includes at least one opening. This arrangement without lock chambers is particularly compact in terms of the space it occupies. It will be noted however that it suffers from the disadvantage that the through-put of substrates is reduced in comparison with an arrangement with lock chambers.  
         [0063]     An alternative embodiment provides a first lock chamber which is connected to the first reactor chamber by way of a common opening closable with a sealingly closing closure.  
         [0064]     In addition in a further embodiment there is a second lock chamber which is connected to the second reactor chamber by way of a common opening closable with a sealingly closing closure.  
         [0065]     In this embodiment, there can advantageously be provided between the first lock chamber and the second lock chamber a transport chamber which is connected to them by a respective opening closable with a sealingly closing closure, wherein provided in the transport chamber is a transport device which is adapted to transport the plurality of substrates from the first lock chamber into the second lock chamber. That permits interruption-free transport of the plurality of wafers in unison. The transport device used for that purpose can be for example a transport robot. Preferably a multi-finger robot is used, which in an embodiment is adapted to simultaneously transport  25  wafers.  
         [0066]     In a further embodiment the first heating device which is used for setting the temperature during the hydrogen prebake operation is adapted to heat the gas atmosphere in the first reactor chamber to a maximum of about 1000° C.  
         [0067]     The first and second heating devices are preferably of such a design and are each controlled in a suitable fashion such that the temperature of the substrates which is required for the hydrogen prebake operation and the deposition operation respectively is already achieved during the introduction of the substrates into the first and second reactor chambers respectively. The efficiency of the procedure can be further enhanced in that way because, immediately after the introduction of the substrates into the respective reactor chamber, it is possible to begin with the cleaning operation and the deposition sequence respectively.  
         [0068]     The step of heating the substrates can be effected as already described hereinbefore in connection with an embodiment of a method according to the invention, alone or with the assistance of short-term irradiation of the substrates. For that purpose in an embodiment of the reactor arrangement the first heating device includes an irradiating device which is adapted to deliver radiation pulses of a predetermined number, a predetermined frequency spectrum, a predetermined intensity and a predetermined duration, in such a way that a substrate irradiated with the radiation pulse or the radiation pulses at a predetermined spacing from the irradiating device is heated for a predetermined period of time to the first substrate temperature T red .  
         [0069]     Preferably moreover there is provided a first pressure control device which is associated with the first reactor chamber and which is adapted to set the pressure of the gas atmosphere in the first reactor chamber to atmospheric pressure or a slightly reduced pressure and to maintain that pressure.  
         [0070]     Furthermore the reactor arrangement according to the invention in another embodiment has a cooling device which is adapted to cause in the first reactor chamber a temperature drop in the gas atmosphere by a maximum of 40 K/min. The through-put of the first reactor can be increased by rapid temperature changes of that kind. A cooling device of that kind is not required if heating of the substrates is effected solely with the irradiating device. It is however appropriate if the heating device includes on the one hand an irradiating device and on the other hand a heating means which involves the entire reactor atmosphere, in which respect a “base temperature” in the prebake reactor is set with the heating means and the irradiating device heats the substrates, starting from that base temperature.  
         [0071]     In a particularly preferred embodiment the device for producing the gas atmosphere is additionally connected to the first lock chamber and is adapted to produce in the first lock chamber a gas atmosphere of predetermined composition and predetermined pressure. In addition the first heating device, the device for producing the gas atmosphere and the control device are additionally adapted to set a reducing gas atmosphere in the first lock chamber to a temperature above the critical temperature T c  and to maintain that temperature. The device for producing the gas atmosphere and the control device are also preferably adapted to set and maintain an ultra-pure hydrogen atmosphere with a residual content of oxidizing constituents of a maximum of 1 ppb.  
         [0072]     Preferably moreover the device for producing the gas atmosphere is additionally connected to the second lock chamber and adapted to produce in the second lock chamber a gas atmosphere of predetermined composition and predetermined pressure. Furthermore the second heating device, the device for producing the gas atmosphere and the control device are additionally adapted to set a reducing gas atmosphere in the second lock chamber to a temperature above the critical temperature T c  and to maintain said temperature.  
         [0073]     Further preferred embodiments of the method according to the invention and the reactor arrangement according to the invention are set forth in the appendant claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0074]     Embodiments by way of example of the invention are described hereinafter with reference to the Figures in which:  
         [0075]      FIG. 1  is a diagrammatic simplified plan view of a reactor arrangement for low-temperature epitaxy,  
         [0076]      FIG. 2  shows sectional views of the two reactors of the reactor arrangement of  FIG. 1  along lines A-A, A-B and B-B,  
         [0077]      FIG. 3  shows a flowchart of a first embodiment of the method according to the invention, and  
         [0078]      FIG. 4  shows a table view of the process implementation of a second embodiment in which at least two batches are processed in parallel. 
     
    
     DETAILED DESCRIPTION  
       [0079]     An embodiment by way of example of the reactor arrangement is described hereinafter with simultaneous reference to  FIGS. 1 and 2 .  FIG. 1  shows a diagrammatic simplified plan view of a reactor arrangement  10  for low temperature epitaxy. The reactor arrangement  10  includes the following components: a loading and unloading chamber  12  for the entry and exit of cassettes  14  with substrates  16 , a transfer chamber  18  with integrated robot handler  20 , and two process systems  22  and  24 , also referred to herein as reactors. The reactor  22  serves for cleaning the substrates in a hydrogen atmosphere. The reactor  24  serves for coating the substrates. The robot handler is a multi-finger robot which simultaneously transports  25  wafers.  
         [0080]     The two reactors  32 ,  42  involve on the one hand a low-pressure CVD reactor  42  for epitaxial deposition and on the other hand a system  32  which, under atmospheric conditions or a slightly reduced pressure, permits the implementation of a hydrogen treatment (prebake) at elevated temperatures (up to about 1000° C.). That reactor has forced cooling and is thus suitable for rapid temperature changes (up to about 40 K/min).  
         [0081]     The structure of the reactors  22  and  24  can be seen from the diagrammatic and simplified sectional view in  FIG. 2 . The sectional view is along the lines A-A, A-B and B-B in  FIG. 1 , in which respect the distance A-B is not shown true to scale in relation to  FIG. 1 . The reactors  22  and  24  each have a respective reactor chamber  220 ,  240  and a respective lock chamber  222 ,  242 . The lock chambers are also referred to as loadlock systems. The lock chambers are each disposed below the respectively associated reactor. All the above-mentioned chambers are separated among each other and from each other outwardly by sealingly closing doors which are symbolically indicated by broken lines in  FIGS. 1 and 2 .  
         [0082]     Provided in each of the lock chambers is a respective lift  224 ,  244 , by means of which quartz boats  226 ,  246  with a plurality of substrates  16  can be transported to and fro between the reactor chamber and the lock chamber associated therewith. A plurality of lifts can also be provided in each chamber, which can be operated independently of each other. Loading of the quartz boats  226  and  246  is effected in the respective lock chamber  222  and  242 .  
         [0083]     The loading and unloading chamber  12  and the transport chamber  18  can be flushed with inert gas. For the sake of enhanced simplicity that is shown in  FIG. 1  only in relation to the transport chamber, more specifically by means of a feed line  26  and a valve  28  for controlling the gas flow by way of the line  26 . The feed of inert gas is shown by a broken-line arrow  102  in  FIG. 1  at the upper end of the illustrated portion of the feed line. An illustration of the carbon dioxide gas containers and a detailed discussion of the feed line system which is usual in the art are not included here. The transport chamber and the loading and unloading chamber can also be evacuated by means of pumps  30  and  32  respectively.  
         [0084]     The loadlock systems  222  and  242  are also to be flushed with inert gas by way of suitable devices which are shown solely in  FIG. 1 . The feed flow of inert gas into the first lock chamber  222  beneath the first reactor chamber  220  by way of a feed line  34  is controlled by means of a valve  36 . The feed flow of inert gas into the second lock chamber  242  beneath the second reactor chamber  240  by way of a feed line  38  is controlled by means of a valve  40 .  
         [0085]     The loadlock systems  222  and  242  additionally include devices for flushing with hydrogen. The hydrogen feed flow is identified by arrows with sold lines. The feed flow of hydrogen into the first lock chamber  222  by way of feed lines  42  is controlled by means of a valve  44 . The feed flow of hydrogen into the second lock chamber  222  by way of feed lines  46  is controlled by means of a valve  48 . The loadlock systems are adapted to leak-tightly enclose a hydrogen atmosphere.  
         [0086]     In addition the controlled feed flow of process gases during the deposition operation into the second reactor chamber  240  by way of suitable feed lines is symbolized by way of example by an arrow with a dash-dotted line as well as a feed line  50  and a valve  52 . Depending on the respective number of process gases to be used during the deposition operation, it will be appreciated that the required number of valves and feed lines is to be provided.  
         [0087]     It should be added that the information set forth in the context of this description and shown in the Figures regarding the number and arrangement of the valves does not have to correspond to the actual conditions. The present description is limited to establishing that feed lines are present and that the feed flow can be controlled by means of a respective one or more valves. The configuration, arrangement and number of the feed lines and valves is established in accordance with the technical necessities by means of measures which are usual in the art.  
         [0088]     All chambers can be evacuated by means of pumps  30 ,  32 ,  54  and  56 .  
         [0089]     In addition a respective cooling system which is integrated into each of the reactors  22 ,  24 , for example by means of a water-cooled casing, permits the substrates to be moved out of the reactor chambers  220 ,  240  at high temperatures and subsequently permits rapid cooling thereof in the associated lock chambers  222 ,  242 . The cooling system of the first reactor  22  is illustrated diagrammatically by means of a block  58 . That reactor has a forced cooling arrangement and is thus suitable for fast temperature changes (up to about 40 K/min). The cooling system of the second reactor  24  is diagrammatically illustrated by means of a block  60 .  
         [0090]     The first and second reactor chambers as well as the first and second lock chambers are each individually heatable with corresponding heating devices. In  FIG. 1 , by virtue of the diagrammatic illustration therein, heating systems  62  and  64  at the first and second reactors  22 ,  24  are each only shown as a whole. Irrespective thereof however each of the four specified chambers is equipped with its own heating device. That is shown in  FIG. 2 . The heating system  62  at the first reactor  22  has a heating device  620  for the first reactor chamber  220  and a heating device  622  for the first lock chamber  222 . The heating system  64  at the second reactor  24  has a heating device  640  for the second reactor chamber  240  and a heating device  642  for the second lock chamber  242 .  
         [0091]     Provided in all four chambers, namely the lock chambers  222 ,  242  and the reactor chambers  220  and  240 , are temperature sensors, representatively identified in  FIG. 1  by blocks  66  and  68  at the first and second reactors, and also illustrated in  FIG. 2  by blocks  660  in the first reactor chamber,  662  in the first lock chamber,  680  in the second reactor chamber and  682  in the second lock chamber. Furthermore, provided in all chambers of the reactor arrangement are pressure sensors which are not shown here for the sake of simplicity of the illustration.  
         [0092]     A control circuit  70  is connected at the input side to the temperature and pressure sensors of all chambers. It is further connected at the output side to the above-mentioned valves, pumps, heating devices and cooling devices and controls the operating condition thereof.  
         [0093]     The mode of operation of the control circuit will be apparent from the description hereinafter of an embodiment of the method according to the invention with reference to the flowchart in  FIG. 3 .  
         [0094]     Unless expressly specified otherwise hereinafter, all chambers are under permanent inert gas flushing. The residual content of moisture and oxygen is monitored by way of suitable measuring devices and should have a fall from the loading and unloading chamber  12  by way of the transport chamber  18 , the first lock chamber  222  towards the first reactor chamber  220 , and the second lock chamber  242  towards the second reactor chamber  240 , to lower values.  
         [0095]     If active transport operations do not prevent that, doors basically remain closed.  
         [0096]     The method is started at a step S 10 . After external pre-cleaning of the substrates (for example Piranha/SC1/HF-dip/SC2/DI) in a step S 12  they are introduced into the loading and unloading chamber  12  in a step S 14  and remain here under inert gas flushing until a sufficiently low residual content of moisture and oxygen is reached.  
         [0097]     Then in a step S 16  transfer of the substrates is effected by means of robots  20  into a process boat  226  of the prebake system  22 , the boat being disposed in the first loadlock chamber  222 . If necessary a rapid further reduction in the residual content of moisture and oxygen can be achieved by an alternate succession of pumping out and inert gas or hydrogen flushing.  
         [0098]     The subsequent introduction of the process boat  226  into the first reactor chamber  220  also takes place in a step S 18  under inert gas or hydrogen, at a reactor temperature of between 500 and 750° C. The subsequent prebake treatment takes place in a step S 20  under a reducing atmosphere (hydrogen) at temperatures ≧800° C.  
         [0099]     After the boat comes out in a hydrogen atmosphere and again at between 500 and 750° C. in a step S 22  the substrates remain in the lock chamber  222  and here cool down in hydrogen in a step S 24  to about 200° C.  
         [0100]     It is only now that in a step S 26  in nitrogen the wafers are transferred into the boat  246  in the loadlock chamber  242  beneath the CVD reactor chamber  240 . Then in a step S 28  the system is switched over to a hydrogen atmosphere which is also maintained during the introduction operation in a step S 32 . In that case the reactor chamber  240  is already set to the region of the temperature of the later deposition procedure (between about 550 and 650° C.).  
         [0101]     In a step S 34  a CVD-typical sequence of process steps takes place for setting the process parameters for epitaxial deposition, with the control of pressure, temperature and the flow of the process gases in the reactor chamber  240  by the control device  70 .  
         [0102]     After deposition has occurred the reactor  240  is flushed clear in a step S 36  and the pressure matched to that of the loadlock chamber  242 .  
         [0103]     The exit of the boat is followed in a step S 40  by transfer back into the loading and unloading chamber  12  by the robot  20 .  
         [0104]     After removal of the cassettes in a step S 42  the process is concluded in a step S 44 .  
         [0105]      FIG. 4  shows a second embodiment of the method according to the invention in a tabular view. In this embodiment, at least two batches are processed in parallel relationship. That makes it possible to speed up the method and thus achieve an increase in the wafer through-put, in comparison both with a dual-chamber system in which there is only ever one batch that is processed, and also in relation to a conventional single-chamber system.  
         [0106]     All steps S 50 -S 80  through which a batch passes are shown in the left-hand column in the tabular view of  FIG. 4 . Those steps substantially correspond to the operating procedure already described with reference to  FIG. 3 . The column which is next to it at the right (“Location”) states where the method step of a respective line is performed. The next column (“Medium”) specifies the gas atmosphere which is set during the method step in question. The right-hand column specifies the temperature at which the method step is performed. In that respect the abbreviation RT denotes room temperature. An arrow (=&gt;) between two temperature values indicates that the temperature is altered from the temperature value to the left of the arrow to the temperature value to the right of the arrow, during implementation of the method step.  
         [0107]     The method of this embodiment is described in greater detail hereinafter. In a step S 50  the substrates are arranged in the transfer chamber. The transfer chamber is flushed. Then, in a step S 52 , transfer of a first batch into the first lock chamber  222  beneath the prebake reactor chamber  220  takes place. After conclusion of the transfer the lock chamber  222  is flushed with nitrogen and pumped out in a step S 54  in order to obtain an atmosphere which is as pure as possible. The steps S 50 -S 54  are carried out at room temperature. Then the process boat is introduced into the prebake reactor chamber  220  in a step S 56 . That step takes place in a nitrogen atmosphere. The reactor chamber  220  is heated to a temperature of 650° C. In a subsequent step S 58  the gas atmosphere of the prebake reactor chamber is replaced by hydrogen and the temperature increased to 900° C. In a step S 60  the prebake operation then takes place at 900° C. in a hydrogen atmosphere. Thereafter in a step S 62  cooling to 650° C. is effected and in a step S 64  the boat is moved out into the lock chamber  222  and here cooled to a temperature below 200° C. The lock chamber  222  is then flushed with nitrogen and cooled down to room temperature in a step S 66 .  
         [0108]     The above-described process implementation of a prebake operation is subsequently repeated for a further batch while at the same time the first batch is subjected to further processing as described hereinafter.  
         [0109]     The first batch is transported at room temperature in a nitrogen atmosphere out of the lock chamber  222  of the prebake reactor into the lock chamber  242  of the epitaxy reactor. The lock chamber  242  is then flushed with nitrogen and pumped out in a step S 70 . In a subsequent step S 72  the lock chamber  242  is flushed with hydrogen at room temperature. In a subsequent step S 74  the temperature of the lock chamber  242  is increased from room temperature to 600° C. and the process boat with the first batch is introduced into the reactor chamber  240 . Then, the epitaxy process takes place at a temperature of about 600° C. in the reactor chamber  240 , with process gases being introduced into the reactor chamber  240  in that operation. After the conclusion of the epitaxy process, in a step S 78  the boat is moved back in a nitrogen atmosphere into the lock chamber  242  and the temperature in the lock chamber is reduced from 600° C. to room temperature again. Finally the first batch is transported back into the transfer chamber.  
         [0110]     As the epitaxy process in the step S 76  takes up the longest period of time (for example 2 hours), a plurality of batches can successively pass through the prebake process of steps S 50 -S 66  during the epitaxy process. Batches whose treatment has been finished and which are waiting for epitaxy can be stored in the transfer chamber.  
         [0111]     The above-described method, in comparison with a single-chamber system, achieved an increase in the wafer through-put from 29.3 wafers per hour to 35.3 wafers per hour, with each bath containing 100 product wafers.