Abstract:
The invention relates to a CVD-reactor for depositing layers made of a reaction gas onto workpieces. Said reactor comprises an elongate, vertical reaction chamber that is defined by a reactor wall and a reactor base, an inlet line for guiding the reaction gas into the reaction chamber, entering into the region of the reactor base in the reaction chamber, a central outlet line that guides the used reaction gas out of the reaction chamber and that extends out of the reactor chamber in the region of the reactor base, a tier-like workpiece receiving element that is arranged in a central manner in the reaction chamber and can be rotated about the central axis thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase of PCT/EP2009/059360 filed Jul. 21, 2009, which claims priority of German Patent Application DE 10 2008 034 330.7 filed Jul. 23, 2008. 
     FIELD OF THE INVENTION 
     The present invention relates to a CVD (chemical vapor deposition) reactor for use in the vapor-phase deposition of coatings, in particular hard material coatings, onto workpieces and the surfaces thereof. 
     The expressions “vapor-phase deposition” and “CVD” (from the English expression “chemical vapor deposition”) are generally used as synonyms. They relate to a method for generating layers, in particular thin layers, on the surfaces of other materials (substrates) such as, for instance, workpieces for metal processing (for example cutting tips, saw blades, etc). Vapor-phase deposition is characterized in that chemical reactions of the chemical compounds contained in the reaction gas take place, and the desired main products of the chemical reactions are deposited on the surface of the substrate, forming a layer or coating thereon. Possible reaction by-products accumulate in gaseous form and must be removed from the gas mixture in order to ensure the layer properties. This occurs by way of an exhaust gas line that either conveys the gases into the atmosphere via a gas scrubber or supplies the gases to the neutralization process via a vacuum pump. Working under vacuum enables good process control even in rapid reactions and, thanks to low dwelling times, equalizes the reduction in layer thickness in the direction of flow that is due to the decrease in concentration (depletion). 
     The present invention relates in particular to a tubular reactor that is in particular a hot-wall reactor. In other words, the present invention relates to a CVD reactor in which the chemical (for example thermal) reactions can take place at a temperature (of the reaction gas) of more than 720° C. The deposition temperatures are preferably between 800 and 1200° C. The CVD reactor of the present invention therefore differs from reactors for semiconductor coating, in which the temperatures are mostly lower than 500° C. 
     BACKGROUND OF THE INVENTION 
     CVD reactors of the type mentioned above are well known in the prior art. The invention thereby takes as a starting point a CVD reactor such as is shown in  FIG. 3 . The shown CVD reactor comprises a cylindrical, vertical reactor chamber  10  that is delimited by a reactor wall  11  and a reactor base  12 . The reactor wall  11  is dome-shaped and comprises a hollow cylindrical section and a dome  14  facing away from the reactor base. A plurality of heaters  1  to  4  are provided along the reactor wall  11 , by means of which the reactor wall  11  can be heated. Suitable end parts  15  can be found in the upper and lower regions. The reactor is therefore basically disposed in a homogeneously heated space  16 . 
     The shown CVD reactor furthermore comprises a central inlet line  17  for the continuous inflow of reaction gas. The central inlet line is formed by a central pipe that can be rotated via a drive  18 . Downstream of the site where the inlet line  17  passes through the reactor base  12 , a preheating chamber  19  is integrated in the inlet pipe, which chamber is formed by a container that has flow diversions and/or baffles (not shown) provided therein. Downstream of the preheating chamber  19  a plurality of gas outlets  20  are provided in the central inlet pipe. 
     A centrally arranged, tiered workpiece receiving element  21  is furthermore disposed in the reactor chamber  10 . It comprises a plurality of tray-shaped receiving elements that are arranged one above the other. The tiers are respectively formed between two receiving elements. The gas outlets  20  of the central inlet pipe  17  are each arranged at the level of a tier and open out into the respective tier above the workpiece tray. At their radially outer end, the respective tiers are in fluid connection with the reactor chamber  10 . An end plate  22  is furthermore provided above the uppermost tier. 
     The central inlet pipe  17  that can be rotated by means of the drive  18  is rotatably mounted in the tiered workpiece receiving element  21 . 
     Furthermore, an outlet line  23  passes through the reactor base  12  and forms an outlet for used reaction gas out of the reactor chamber  10 . 
     There are several problems with respect to this solution. On the one hand, the preheating chamber  19  has limits with respect to its surface that comes into contact with the inflowing reaction gas, i.e. if the preheating chamber per se is not supposed to be enlarged, the surface can only be enlarged with difficulty and combined with the integration of further flow-impeding baffles. Efforts are being made to increase the amount of inflowing gas (reaction gas) and to thereby shorten the coating process and/or to keep the thickness of the layer within narrow limits over the reactor volume. However, if the amount of gas is increased, the heating capacity of the preheating chamber must also be increased. Furthermore, part of the reactive gas mixture is prematurely consumed on the inner surfaces. 
     Furthermore, owing to the flow diversions in the preheating chamber the speed of the reaction gas is reduced. As a result, there is the added problem that in the preheating chamber a certain deposition process already occurs on the surfaces of the preheating chamber. This is a problem in particular in the case of reaction gases that contain Lewis acids and Lewis bases. Deposition problems are often encountered therewith. One example is the system TiCl4/CH3CN. The preheating chamber that is disposed in the rotatable central inlet pipe  17  and that rotates therewith is complex to dismantle and take apart since the premature coating welds the parts of the chamber together, and thus cleaning of the preheating chamber, i.e. the removal of the deposits in the preheating chamber, is problematic and time-consuming. In the case of very reactive gas mixtures, a thick layer formation can occur locally which, in extreme cases, leads to blockages. 
     Finally, it is also difficult with the preheating chamber to precisely regulate and control the temperature of the gases to be preheated. Another major disadvantage is that all reactive gases must be mixed prior to the rotating preheating chamber. 
     As is apparent from  FIG. 3 , the reaction gas is fed into the central inlet pipe  17  via a stationary gas inlet that is connected to the reactor base  12 . The central inlet pipe  17  is rotated by means of the drive  18 . This results in an interface between the stationary gas inlet and the central inlet pipe  17 . This interface is formed in the region of the reactor base  12  and causes considerable problems in particular as regards sealing so that the escape of reaction gas at said interface and thus out of the reactor cannot be completely avoided in particular owing to the support of the seal at the specified high temperatures. This is a serious problem especially in the case of highly reactive, in particular corrosive starting substances. 
     When using starting products with low vapor pressures, this point must be heated to temperatures of greater than 200° C., which requires an expensive and complex technique for sealing the rotating duct. 
     Known CVD reactors do not provide any solutions to the aforementioned problems. DD 111 935 discloses, for example, the downward introduction of a reaction gas via a central inlet pipe that is provided with gas outlets and that enters the reactor chamber in the region of the reactor lid. It is furthermore proposed herein to cool the central inlet line. The latter is in stark contrast to the specification to preheat the reaction gas before introduction into the reactor chamber. Furthermore, DD 111 935 explicitly teaches a flow outwards from the central inlet pipe towards the reactor wall so as to prevent an increased deposition rate on the reactor wall. 
     DE 197 43 922 proposes the continuous switching of the flow of reaction gas into the reactor chamber. Therein, the reaction gas flows outside a central pipe, over the workpieces to be coated, into the central pipe via an opening in the central pipe that is facing away from the reactor base and then out of the reactor, or—after switching—the reaction gas flows in the reverse direction upwards through the central pipe, out of the opening facing away from the reactor base and from there downwards over the workpieces to be coated and out of the reactor chamber. A relatively complex two-way valve arrangement that has to withstand high temperatures is provided herefor, and furthermore, the integration of gas preheating and mixing before coating of the workpieces is difficult, in particular when reactants with low vapor pressures are present, since the preheating would have to be carried out at different outlets depending on the different valve positions. 
     EP 0 164 928 A2 relates to a vertical hot-wall CVD reactor having two parallel inlet and outlet lines disposed in the edge regions, said lines passing through the base of the reactor into the reactor chamber and extending from there up to the uppermost tier of a tiered workpiece receiving element. The workpiece receiving element is rotatably mounted. The substrates to be coated are thereby supposed to be placed in the centre of the rotational axis of the workpiece receiving element and thus precisely centrally between the inlet and outlet lines. 
     US 2006/0159847 A1 comprises two inlet pipes that presumably extend through the base and from there up to the uppermost tier of a workpiece receiving element. The workpiece receiving element itself is not rotatable. The inlet lines can rather be rotated about 360°, and the workpiece receiving element is herein also supposed to be arranged centrally between the inlet and the outlet. 
     US 2007/0246355 A1 also discloses an inlet line that extends from the base up to the uppermost tier as well as an outlet line in the region of the base. A rotatable workpiece receiving element is furthermore provided. 
     With respect to EP 0 270 991 B1, a stationary inlet line is arranged centrally, i.e. in the middle relative to the reactor chamber and is surrounded by a rotatable outlet line. 
     Furthermore, two workpiece receiving elements are disposed to the left and right of the inlet and outlet line and are accommodated in an inner reactor chamber that is in turn surrounded by an outer reactor chamber. The central outlet line is rotatable. If this outlet line is rotated, the base plate rotates therewith and takes with it the workpiece receiving elements such that these workpiece receiving elements rotate about the outlet line. Owing to this rotation and to the fact that the toothed wheels engage with the fixed toothed wheel, the workpiece receiving elements themselves rotate about their central axes. 
     DE 78 28 466 U1 relates to the coating of workpieces, in particular tools. A central inlet line is rotated here, but not, however, the stationary workpiece receiving element. 
     SUMMARY OF THE INVENTION 
     In view of the statements made above, the object of the present invention is therefore to further develop the CVD reactor described above in connection with  FIG. 3  in such a manner that larger amounts of gas can flow in whilst maintaining consistent layer properties and a consistent homogeneity (layer thickness) so as to provide a uniform yet more rapid coating of the workpieces as compared to the prior art and to reduce the sealing problems. 
     This object is solved by a CVD reactor having the features of patent claim  1 . Advantageous further developments of the present invention are mentioned in the sub-claims. 
     The idea forming the basis for the present invention is to introduce the reaction gas into the reactor chamber preferably substantially parallel to, in any case in the vicinity of the heated reactor wall such that the large surface of the reactor wall can be used to preheat the reaction gas. In order to maintain homogeneity and the other layer properties as compared to the prior art, the tiered workpiece receiving element is furthermore rotated in accordance with the invention. 
     The CVD reactor of the present invention comprises an elongated, vertically-extending reactor chamber that is delimited by an at least partially heatable reactor wall and a reactor base. It is preferred for the reactor chamber to be cylindrical, preferably with a circular cross-section (octagonal, polyhedral or other cross-sections are also conceivable). The reactor wall can thereby be composed of, for example, a hollow-cylindrical part and a dome sealing one end of the hollow cylinder. The reactor base is provided at the other end of the hollow cylinder. Furthermore, the CVD reactor according to the invention comprises one or more inlet lines for the inflow, in particular continuous inflow, of the reaction gas into the reactor chamber. The inlet line thereby enters the reactor chamber in the region of the reactor base, preferably though the reactor base. Also provided is a preferably central outlet line for the preferably continuous outflow of the used reaction gas (which thus includes both the unconverted educts and the reaction products) out of the reactor chamber, said outlet line also exiting the reactor chamber in the region of the reactor base, preferably through the reactor base. Furthermore, a tiered workpiece receiving element that is arranged preferably centrally in the reactor chamber is rotatable around its central axis, in this case preferably the central axis of the preferably central outlet line and, where applicable, of the reactor chamber. In this case, tiered means that one or more trays or tables can be provided, which, when arranged vertically one above the other, each form a tier and accommodate the workpieces to be coated in a horizontal position. They are also to be understood as frames that accommodate the workpieces in a hanging manner. It is apparent from this arrangement that the inlet line is disposed between the central outlet line, more precisely the tiered workpiece receiving element, and the heatable reactor wall such that the gas flows directly or indirectly along the reactor wall and is sufficiently preheated by the heat radiated by the reactor wall. Owing to this arrangement and to the larger surface of the reactor wall as compared to the preheating chamber, a more rapid heating can be achieved as compared to the use of the preheating chamber, and thus larger amounts of gas are able to flow in without reducing the flow rate by means of baffles and/or diversions. As mentioned above, baffles and/or diversions are associated with considerable deposition problems, in particular if Lewis acids and Lewis bases are present as starting substances. All in all, the resulting advantage is that a quicker coating can take place, whereby owing inter alia to the rotation of the tiered workpiece receiving element the homogeneity and other layer properties are at least consistent as compared to the prior art. Furthermore, the inner surface of the reactor wall is a substantially smooth surface which, as compared to the preheating chamber, is easily accessible and thus easy to clean and which also does not require time-intensive and complex dismantling. By attaching easily exchangeable “baffle plates” the availability of CVD systems can also be improved since no long cleaning periods between two batches are required. That is to say, the “baffle plates” are simply exchanged during cleaning. Furthermore, a good temperature control of the reaction gas over the entire height of the CVD reactor is possible, in particular if a plurality of heating elements that can be regulated separately are provided, which leads to a more uniform and more homogeneous layer distribution over the workpieces arranged in the various tiers. As described above, sealing an interface with rotating elements is difficult and thus complex. In the present invention, the tiered workpiece receiving element is rotated whereas the central outlet line is stationary. The inlet line is also stationary. There is thus substantially no interface between gas-carrying elements in the region of the reactor base, at least one of which rotates. 
     In particular so as to reduce deposition on the reactor wall, it is particularly advantageous, optionally independently of the rotation of the tiered workpiece receiving element, to design the inlet line as a pipe that extends parallel to the heatable part of the reactor wall, for example parallel to a portion of the hollow cylindrical part or the entire hollow cylindrical part of the reactor wall. The pipe can be made of heat transmitting material in order to already preheat the gas or gases in the inlet line, or it can be made of heat-insulating material (see below). 
     It is furthermore advantageous if the inlet pipe extends from the reactor base substantially up to the uppermost tier of the workpiece receiving element and possibly also beyond, or at least up to the vicinity of the uppermost tier. This makes it possible to minimize in a particularly simple manner the length of the path leading to the workpieces to be coated that is taken by the reaction gas following its exit from the inlet pipe and at the same time to keep the distance to the reactor wall small so as to configure preheating in an optimal manner. 
     Based on this design, there are a number of implementation possibilities which are also combinable. 
     For example, it is advantageous if at least one opening per tier of the workpiece receiving unit is provided in the inlet pipe in line with the opening of the respective tier, and the tiers are radially accessible (there is a fluid connection between the reactor chamber and the tiers) so that reaction gas can flow out of the openings of the inlet pipe, radially along the tiers, i.e. over and past the workpieces to be coated, up to the central outlet flow. 
     The openings can thereby be directed towards the reactor wall so that the reaction gas flows (can flow) against the reactor wall and undergoes additional preheating, after which it can flow along the substrate (workpiece) up to the outlet line taking the shortest path. Undesirable deposits from side reactions with impurities in the reaction gas mixture are deposited on the hot reactor wall, more specifically on the “baffle plates”. Coatings with a high-quality surface finish can thereby be generated. 
     In order to be able to achieve a flow that is as laminar as possible and thus a uniform gassing of the workpieces, it is furthermore preferred for the outlet line to be a pipe that extends centrally from the reactor base to the uppermost tier and that comprises at least one opening per tier, at the level of the respective tier, for the inflow of the used reaction gas into the outlet pipe. That is to say, the outlet pipe comprises at least one opening per tier in order to create the fluid connection between the tiers and the outlet pipe. 
     Alternatively or in addition to the above embodiment, the inlet pipe can be open at its end facing away from the reactor base, i.e. in the region of the uppermost tier. Openings are thereby preferably provided in an end plate that closes the workpiece receiving element at its end facing away from the reactor base so that the reaction gas can flow out of the opening of the inlet pipe, through the opening in the end plate in the direction of the reactor base and out of the reactor chamber via the central outlet line. For this purpose, the outlet pipe may, as described above, be designed with openings provided in the outlet pipe from the reactor base to the uppermost tier, or it may be shorter and abut in a funnel shape the bottommost tier in the region of the bottommost tier and extend from there to the reactor base. 
     The outlet pipe can be provided with bore holes or openings that are of equal size, equally spaced or adapted specifically to the coating task in order to optimize the flows. 
     In the case of the latter design alone, it is furthermore advantageous if the reaction gas only comes into contact with the reactor wall in the region of the dome. The reactor dome could, for example, be detachable from the hollow cylindrical section of the reactor wall so that this part can be cleaned separately. The workpiece receiving element is also rotated in this embodiment. 
     When using a preheating chamber such as described in connection with  FIG. 3 , the following additional problem exists. In some CVD processes, the reactive species in the gas phase only form at higher temperatures. In the reactor shown in  FIG. 3 , this occurs in the preheating chamber, accompanied by the already described problems with depositions in particular on the baffles and diversions. According to the invention, it is therefore advantageous to provide a plurality of (in particular two or three) said inlet lines, via which a plurality of different reaction gases can flow separately into the reactor chamber and mix in the reactor chamber to form the actual reaction gas. That is to say, the mixing of the plurality of reaction gases to form the reaction gas does not take place until the reactor chamber, i.e. after leaving the inlet lines, whereby, as mentioned above, the path between the inlet line or lines and the workpieces is small such that the region in which deposits can form is also small. For the sake of simplicity, the gases that flow through the plurality of inlet lines and mix to form the actual reaction gas are also referred to as “reaction gases” in the present description even though they should be more precisely referred to as “starting gases” that mix in the reactor chamber to form the actual reaction gas. For their part, the starting gases can in turn consist of a gas mixture also containing, for example, inert gases such as nitrogen or argon. However, they are preferably single gases with a suitable purity. 
     The inlet line can preferably comprise a plurality of channels made of heat-insulating material (for example, ceramic). This allows the gases and vapors to be supplied in a relatively cold state, followed by a rapid mixing in the hot area in the direct vicinity of the material to be coated. 
     A very well controlled process is ensured in this manner, in which undesirable side reactions are avoided. 
     In particular in the embodiment in which the inlet line is open at its end facing the reactor base, it is therefore advantageous to define a mixing area in the reactor chamber that is defined between the end plate and the dome, so that the mixing of the plurality of reaction gases (starting gases) to form the reaction gas can take place in this mixing space. 
     In the embodiment having the plurality of openings along the length of the inlet pipe at the level of the respective tiers, it is advantageous if the inlet lines comprise corresponding openings at the same level and the openings are so orientated, for example are facing one another or are directed at the same region of the reactor wall or baffle plates, that the outflowing reaction gases collide with one another and a good mixing of the gases and rapid bonding (reaction gas) occurs without lengthening the path to the workpieces that has the associated problem of depositions on undesired surfaces. 
     In order to be able to achieve rotation of the workpiece receiving element in the simplest mechanical manner, it is advantageous for the workpiece receiving element to have internal toothing in the region of the reactor base, with which a driven toothed wheel engages so as to rotate the workpiece receiving element. The toothed wheel may be a spur gear and the internal toothing may be a ring gear. However, other designs and other drives are also conceivable. An electromotor is preferably used as the drive, the speed of which can be infinitely variable such that the tiered workpiece receiving element is rotatable in the range of 1 to 10 rotations per minute. The direction of rotation can be reversible to prevent an agglomeration of possibly accumulating contaminations by alternating the direction of rotation. 
     Furthermore, one or more reaction gas generators may advantageously be integrated in the inlet line in the region of the reactor base and disposed in the reactor chamber. Such a reaction gas generator may be, for example, an evaporator unit in which the initially liquid starting product or reaction product is vaporized. Purely by way of an example, a metal chloride generator can alternatively be mentioned, in which, for example, HCl or Cl 2  (or metal halide MX n ) flows through a metal so as to form a metal chloride that is finally introduced into the reactor chamber via the inlet line. The integration of these evaporator units or metal chloride generators or reaction gas generators in general into the reactor chamber leads to the advantage that no additional sealing problems arise. It can furthermore be advantageous for these reaction gas generators to be heated and regulated separately (reactor temperature decoupling). 
     It may furthermore be preferred to be able to remove the tiered workpiece receiving element from the reactor either together with or separately from the central outlet line so that after a concluded process, the workpiece receiving element can simply be replaced by a new one with newly loaded workpieces, thereby keeping the idle time of the CVD reactor low. The outlet line can hereby be tightly coupled and decoupled by means of, for example, a detachable coupling, and sealing between the workpiece receiving element and the reactor base can occur, where necessary, via a labyrinth seal, i.e. a non-contact seal. 
     In addition, it can be advantageous to also determine the temperature in the inlet and/or outlet line, and thus a sensor can be integrated in each of these lines. These additional temperature measurements lead to a further improved process control. 
     Furthermore, a module for cleaving and/or decomposing and/or activating gases (such as hydrocarbons, nitrogen-hydrogen compounds and organometallic compounds) for the reactions in the reactor chamber  10  can be integrated in the inlet line  34 . 
     Further advantages and features of the present invention are apparent from the following description of preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description of the preferred embodiments is carried out with reference to the accompanying drawings, in which: 
         FIG. 1  shows a schematic drawing of a first embodiment of a CVD reactor according to the invention; 
         FIG. 2  shows a schematic drawing of a second embodiment of a CVD reactor according to the invention; 
         FIG. 3  shows a schematic drawing of a CVD reactor that forms the starting point for the present invention; and 
         FIG. 4  shows a schematic drawing of the bottom portion of a CVD reactor according to the present invention illustrating a gas inlet having a plurality of separate inlet lines. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the sake of simplicity, identical or similar elements are provided with the same reference numbers in the various drawings. 
     The CVD reactor of the first embodiment, which is shown in  FIG. 1 , comprises a cylindrical and vertical reactor chamber  10 . This reactor chamber  10  is delimited by the reactor base  12  and the reactor wall  11 , with the reactor wall  11  being composed of a hollow cylindrical section  13  and a dome  14 . 
     A tiered workpiece receiving element  21  is accommodated centrally in the reactor chamber  10  (the central axis M of the reactor chamber and the workpiece receiving element are congruent) and extends over substantially the entire height of at least the hollow cylindrical section  13  of the reactor wall  11 . If the reactor is configured with reaction generators in the reactor chamber  10 , the workpiece receiving element and the later described outlet pipe  31  can also be accommodated off-centre. The tiered workpiece receiving element is composed of a plurality of parallel and horizontally arranged tray-shaped receiving elements  25 , on which the workpieces or substrates to be coated can be placed. Each of the receiving elements  25  defines a tier. Circumferential walls  24  that connect the receiving elements  25  are disposed between the individual receiving elements  25 . These circumferential walls which, as described below, are provided with inflow openings  26  can also be replaced by individual webs that connect the receiving elements  25  provided that the respective tiers, more precisely the spaces formed between two adjacent receiving elements  25 , are radially in fluid connection with the reactor chamber. An end plate  22  is provided at the end of the uppermost tier, which defines the final space in which a workpiece can be placed, namely the space between the uppermost receiving element  25  and the end plate  22 . A support  27  is attached to the lowermost tier, which comprises a ring gear  28  with internal toothing in the region of the reactor base  12 . The toothing of a spur gear  29  that is driven by the drive  18  engages with this internal toothing of the ring gear  28 . The support  27  is rotatably mounted on the reactor base  12  by means of, for example, ceramic slide bearing  30  or by means of rollers or ball bearings. A further bearing can take place in the region of the later described outlet pipe  31 , however this is not shown in  FIG. 1 . Owing to a rotation of the spur gear  29  and the engagement of the spur gear  29  with the internal toothing of the ring gear  28 , the tiered workpiece receiving element  21  can be rotated in the reactor chamber  10  about the central axis M of the ring gear  28 . The number of rotations is thereby dependent in particular on the number of inflow openings  26  in the outer walls  24  of the tiers and is preferably in the range of between 1 and 10 rotations per minute, and with, for example, two holes per tier is 4 rotations per minute. The drive  18  is preferably infinitely variable such that the speed of rotation of the tiered workpiece receiving element can be adapted as desired and the same drive can also be used for different embodiments. 
     The reactor wall  11  of the shown CVD reactor is surrounded by a heater that is composed of heating elements  1  to  4  (not necessarily 4 heating elements have to be provided. Less (1) or more (&gt;4) may be provided) and forms a space  16  surrounding the reactor wall  11 . The individual heating elements can be regulated separately, with the output gradually increasing from heater  1  to heater  4 . As a result of this separate regulation, an optimum temperature distribution in the reactor chamber  10  and thus an optimal preheating of the reaction gases can be set. 
     As is apparent from  FIG. 1 , a narrow clearance is formed between the outer circumference of the tiered workpiece receiving element  21  and the reactor wall  11 . A gas inlet pipe  34  is provided in this clearance. The inlet pipe  34  passes through the reactor base  12  into the reactor chamber in a region of the reactor base  12  that is disposed radially outwards relative to the central axis M. The inlet pipe  34  is designed in terms of length and height such that it extends up to the uppermost tier of the workpiece receiving element  21  and a little bit beyond. The upper end of the inlet pipe  34  substantially ends at the lower surface of the end plate  22 . Along the length of the inlet pipe  34  an outflow opening  36  is provided at each tier. As is apparent from the arrows in  FIG. 1  that represent the flow of the reaction gas, these outflow openings  36  are directed towards the reactor wall  11  such that the outflowing reaction gas impinges upon the reactor wall. The reactor wall  11  is heated up and warmed by the elements  1  to  4  such that it on the one hand radiates heat into the reactor chamber  10  and on the other hand is itself hot. The reaction gas that flows out of the openings  36  and impinges upon the reactor wall  11  is thereby preheated to the desired reaction temperature. The inlet pipe  34  is stationary, and thus no sealing problems arise at the site where it passes through the reactor base  12  and a simple seal is possible. The cross-section of the inlet pipe  34  is so dimensioned that the rate of the inflowing reaction gas is such that a premature reaction and deposition in the inlet pipe  34  or on the inner surface thereof is prevented. However, the rate is at the same time selected such that sufficient preheating occurs via heating elements  1  to  4  that heat the reactor wall  11 . It is also conceivable to introduce several (for example two or three) different reaction gases (starting gases) into the reactor chamber  10  via a corresponding number of separate inlet lines  34  that should extend parallel to one another. This is advantageous in particular in the case of gases that are to be mixed at temperatures of greater than 200° C. to form the reaction gas, whereby mixing is, however, not supposed to take place until just before the workpieces to be coated. It is thereby advantageous for the outflow openings  36  of the inlet pipes  34  to be facing one another or to be directed at the same point of the reactor wall  11 . 
     An outlet pipe  31  is furthermore arranged centrally in the reactor chamber  10  (the central axis M of the outlet pipe is congruent with the central axis M of the reactor chamber) and is also stationary in the present embodiment so that it is easy to seal the site of passage through the reactor base  12 . A corresponding seal (not shown) is provided for this purpose in this interface region. With its central axis congruent with the central axis of the tiered workpiece receiving element  21 , the outlet pipe  31  extends upwards from the reactor base  12  into the uppermost tier, more specifically into the space between the uppermost receiving element  25  and the end plate  22 . The inlet pipe  31  is open at its upper end  33 . Furthermore, at least one but preferably more openings  32  (preferably at least two) are provided in the inlet pipe  31  at each tier. Used reaction gas (the unreacted starting gases and by-products of the reactions forming the basis for vapor-phase deposition) can flow through these inflow openings  32  into the central outlet pipe  31 . The used reaction gas flows out of the uppermost tier, through the upper opening  33  and into the outlet pipe  31 . The used reaction gas flows out of the reactor chamber  10  via the outlet pipe  31 . 
     The operation of the embodiment shown in  FIG. 1  will be explained in the following. The direction of flow of the reaction gas is thereby shown by the arrows in this figure. 
     The inflowing reaction gas or gases are introduced into the inlet pipe  34  and flow along the inlet pipe  34 . The reactor wall  11  that is heated by heating elements  1  to  4 , in particular in hollow cylindrical section  13 , leads to a heating of the inlet pipe  34  such that preheating of the gas already occurs as it flows through the inlet pipe  34 . The reaction gas furthermore flows out of the inlet pipe  34  via outflow openings  36  and, as already mentioned, impinges upon the reactor wall  11 , as a result of which a further preheating to the required reaction temperature takes place. It must be mentioned in this regard that in a CVD reactor according to the invention, the reaction gas for the chemical deposition preferably reaches a temperature of greater than 800° C. The examples described below are also to be taken into consideration in this respect. 
     Following impingement on the reactor wall  11 , the reaction gas is diverted (“rebounds”) and flows radially from the inner surface of the reactor wall  11  in the direction of the central axis M. The reaction gas thereby flows through the openings  26  in the outer wall  24  of the workpiece receiving element  21 , which are arranged between two adjacent receiving elements  25  and open out into the space that forms the respective tiers and that is formed between two adjacent receiving elements  25 . That is to say, the reaction gas flows into the space, on the base of which (lower receiving element  25 ) a workpiece to be coated is disposed. Owing to the vapor-phase deposition, the gaseous substances from the reaction gas are deposited on the surfaces of the workpiece, which can be, for example, cutting tips, saw blades, etc., and form the desired layer. The used reaction gas then flows through the inlet openings  32  of the outlet pipe  31  as well as through the end opening  33  of the outlet pipe  31  into the outlet pipe  31  and out of the reactor chamber  10 . During this entirely continuous inflow and outflow process, the tiered workpiece receiving element  21  is rotated about the central axis M of the ring gear  28  by the drive  18  of the toothed wheel  29 . In the preferred embodiment, two holes are provided in the circumference of the wall  24  of the workpiece receiving element and the workpiece receiving element  21  is rotated at 4 rotations per minute. 
     Since the dwelling time of the reaction gases in the lower region of the inlet pipe  34  is shorter, i.e. the time available for heating to the reaction temperature is shorter than for a gas that flows out of the uppermost outflow opening  36  and serves to gas a workpiece in the upper tier, the output of the heater  4  is greater than the output of heater  1  in order to be able to provide the respectively required heat. Owing to the separate regulation of the heating elements it is possible to optimally heat the reaction gases. Adaptations can furthermore be carried out in order to achieve the smallest possible deposition of the reaction gas on undesired surfaces. The latter is furthermore also controlled by adapting the speed of the reaction gas in the inlet pipe  34 . Owing to this embodiment according to the invention, it is possible, whilst maintaining consistent homogeneity and consistent layer properties, to introduce larger amounts of gas over the same period of time and to thus accelerate the coating process. The coating costs are thereby significantly reduced. Furthermore, in the shown embodiment, the reaction gas only comes into contact with the reactor wall  11  in selected areas and then only briefly. The path between the outflow openings  36  and the workpieces is otherwise kept short so that depositions on undesired surfaces, i.e. not on the workpieces to be coated, in particular on the reactor wall  11 , can be reduced. An additional deposition on the inner wall of the inlet pipe  34  can be controlled by setting a “high” flow rate of the reaction gas when flowing through the inlet pipe  34 . According to the invention, it is generally preferred for the flow rate of the reaction gas or plurality of starting gases to be in the laminar range. The dwell time of the reaction gas in the reactor is normally in the second range. The rotation of the workpiece receiving element  21  furthermore leads to a coating that is largely homogeneous and that retains the layer properties as compared to the embodiment in  FIG. 3 . 
     A second embodiment of the present invention is shown in  FIG. 2 . In order to avoid repetition, identical elements have not been described again. It is furthermore noted at this point that the embodiments of  FIGS. 1 and 2  can also be combined. That is to say, the inlet pipe  34  of the embodiment in  FIG. 1  can additionally be open at its upper end  35  and the end plate  22  can comprise openings  40 . In the embodiment of  FIG. 2  on the other hand, additional outflow openings  36  can also be provided in the inlet pipe that is then designed as in  FIG. 1 , and inflow openings  26  can be provided in the outer wall  24  of the workpiece receiving element  21 . 
     In the embodiment shown in  FIG. 2 , the inlet pipe has the same design as in  FIG. 1  but with just an open end  35 . The outflow openings  36  are not provided. 
     Furthermore, the outlet pipe  31  extends upwards through the reactor base  12  for a certain distance in the direction of the workpiece receiving element  21  and opens out into a funnel-shaped space  41  that abuts the lowermost tier of the workpiece receiving element  21 , i.e. that is disposed underneath the lowermost receiving element  25 . The individual receiving elements  25  are fluid-permeable, for instance are designed with a plurality of openings or in the form of a lattice. This design of the receiving elements  25  can, of course, also be applied to the embodiment in  FIG. 1 . Furthermore, a plurality of openings  40  are provided in the end plate  22  and a mixing space  42  is formed between the upper side of the end plate  22  and the underside of the dome  14 . Owing to the rotation of the workpiece receiving element  21 , movement of the reaction gas in the mixing space  42  is supported or induced. 
     The operation of the CVD reactor of  FIG. 2  will be explained in the following. A reaction gas or, in the case of separate introduction, a plurality of reaction gases is introduced into the inlet pipe or pipes  34 . Therein, the gases flow up to the opening  35  in the inlet pipe  34 . The wall of the inlet pipe  34  is heated by the heat radiated by the reactor wall  11  such that the reaction gas is also heated during its flow. As already mentioned, the reactor wall  11  is heated by means of heating elements  1  to  4 . At the open end  35  that is facing away from the reactor base  12 , the reaction gas flows out of the inlet pipe  34  and into the mixing space  42 . In the case of a plurality of separately introduced gases, these gases are mixed in the mixing space  42 . Otherwise, an additional final heating of the reaction gas to the desired reaction temperature can take place in this area since also this area is additionally heated by heating element  1 . The reaction gas  40  exits the mixing space  42  through the openings and flows from the uppermost tier downwards in the direction of the funnel-shaped space  41  through the individual receiving elements  25  of the workpiece receiving element  21 , on which the individual workpieces are disposed. The three-dimensional workpieces are thereby coated as desired. Finally, the used reaction gas flows through the funnel-shaped space  41  that opens out into the outlet pipe  31  and out of the reactor chamber  10  via the outlet pipe  31 . During this process, the workpiece receiving element  21  is continuously or intermittently rotated by the drive  18 . 
     As already mentioned above, the embodiments in  FIGS. 1 and 2  can be combined, with various combination possibilities, such as mentioned, being conceivable. 
     It shall also be understood that even without the rotation of the workpiece receiving element  21 , the design of the inlet pipe  34 , as described, leads to the specified advantages, in particular with respect to a reduced deposition on undesirable surfaces. In other words, the arrangement of the inlet pipe along the heatable reactor wall  11  is a separate aspect of the invention that can also be implemented independently of the possibility of rotating the tiered workpiece receiving element. It is only important in this regard that the length of the inlet pipe  34  is such that sufficient preheating of the reaction gas before gassing of the workpieces is possible and furthermore that the shortest possible path to the workpieces is provided. 
     The present CVD reactor allows a reaction temperature and/or deposition temperature of greater than 720° C. It can therefore also be referred to as a medium-temperature and high-temperature CVD reactor. The CVD reactor according to the invention can be operated in this regard both under negative pressure as well as overpressure or possibly even at atmospheric pressure. Furthermore, the components of the reactor are selected such that they can withstand the high temperatures even in the presence of corrosive compounds (for example HCl), which is important in particular as regards the selection of the materials and in particular the selection of the materials for the seals and bearings. 
     The CVD reactor according to the invention is suitable, for instance, for the application of various CVD coatings, in particular hard material coatings, to workpieces such as blades (in particular cutting tips) and saw blades. Suitable hard materials are, for example, carbides, nitrides, carbonitrides of the transition metals titanium, tantalum, tungsten, molybdenum and chromium, borides of the ferrous metals (Ni and Fe) and oxides of aluminum, zirconium, hafnium and silicon. The CVD reactor according to the invention can be used in particular for the deposition of TiC, TiN, Ti(C,N), Cr7C3, Ni and Fe borides and Al2O3, individually or combined in layers with stepwise or continuous transitions. It shall be understood that this list is not conclusive but is rather only provided as an example. 
     Depending on the desired coating, the person skilled in the art will select a suitable reaction gas and introduce it into the reactor chamber  10  via an inlet line  34  or, in the case of a plurality of reaction gases (starting gases), via a plurality of inlet lines  34 . In order to generate a Ti(C,N) coating on the workpieces, a reaction gas containing TiCl4, acetonitrile and hydrogen can, for example, be used. 
     Further examples of parameter ranges and cases of use are cited, without being conclusive, in the following: 
     Examples: 
                                                                                       Range in       Theoretically               Parameter unit   practice       conceivable range                                        Temperature ° C.   500   1100   200   1600           Pressure mbar   40   P atm    1   P atm           Dwell time sec   0.01    100    10 −5      10 4                          
Carrier and Reactive Gases:
 
     Hydrogen, nitrogen, hydrocarbons, (saturated, unsaturated, aromatics) amines, ammonia, hydrazines carbon dioxide, carbon monoxide, nitrogen monoxide, silanes, boranes, halides. BC13, SiCl4, SiCl3CH3, WF6. 
     Starting Materials that are Liquid at Room Temperature: 
     TiCl4, BBr3, CH3CN, CH3OH, organometallic compounds, for example trimethylaluminum, platinum-actonylacetonate, VC14. 
     Solid Compounds (Low Vapor Pressure): 
     CrCl2, MoCl5, TiJ4, AlCl3, HfCl4, NbCl5. 
     Layers: 
     Carbides, nitrides, oxides, borides, silicides, phosphides, the transition metals, IVB-VIIB, as well as covalent compounds of Si, N, Al, C . . . as monolayers, multi-layer coatings, gradients mixtures, inclusions, multilayers. 
     Base Materials: 
     Pure metals, alloys, steels, hard metals, super alloys, ceramics, graphites, etc.