Abstract:
A method of growing epitaxial layers on a wafer is provided and includes providing a wafer carrier having a surface for retaining the wafer; placing the wafer on the wafer-retaining surface of the wafer carrier while the wafer carrier is in a loading position separated from a spindle; transporting the wafer carrier toward the spindle; detachably mounting the wafer carrier on the upper end of the spindle for rotation therewith; and rotating the spindle and the wafer carrier while introducing one or more reactants into the reaction chamber. The invention also described several embodiments and variants of the method of the invention.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a division of U.S. patent application Ser. No. 09/778,265, filed Feb. 7, 2001 now U.S. Pat. No. 6,506,252, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to making semiconductor components and more particularly relates to devices for growing epitaxial layers on substrates, such as wafers. 
     BACKGROUND OF THE INVENTION 
     Various industries employ processes to form thin layers on solid substrates. The substrates having deposited thin layers are widely used in microprocessors, electro-optical devices, communication devices and others. The processes for the deposition of the thin layers on solid substrates are especially important for the semiconductor industry. In the manufacturing of semiconductors, the coated solid substrates, such as substantially planar wafers made of silicon and silicon carbide, are used to produce semiconductor devices. After the deposition, the coated wafers are subjected to well-known further processes to form semiconductor devices such as lasers, transistors, light emitting diodes, and a variety of other devices. For example, in the production of the light-emitting diodes, the layers deposited on the wafer form the active elements of the diodes. 
     The materials deposited on the solid substrates include silicon carbide, gallium arsenide, complex metal oxides (e.g., YBa 2 Cu 3 O 7 ) and many others. The thin films of inorganic materials are typically deposited by the processes collectively known as chemical vapor deposition (CVD). It is known that the CVD processes, if properly controlled, produce thin films having organized crystal lattices. Especially important are the deposited thin films having the same crystal lattice structures as the underlying solid substrates. The layers by which such thin films grow are called the epitaxial layers. 
     In a typical chemical vapor deposition process, the substrate, usually a wafer, is exposed to gases inside a CVD reactor. Reactant chemicals carried by the gases are introduced over the wafer in controlled quantities and at controlled rates while the wafer is heated and usually rotated. The reactant chemicals, commonly referred to as precursors, are introduced into the CVD reactor by placing the reactant chemicals in a device known as a bubbler and then passing a carrier gas through the bubbler. The carrier gas picks up the molecules of the precursors to provide a reactant gas that is then fed into a reaction chamber of the CVD reactor. The precursors typically consist of inorganic components, which later form the epitaxial layers on the surface of the wafer (e.g., Si, Y, Nb, etc.), and organic components. Usually, the organic components are used to allow the volatilization of the precursors in the bubbler. While the inorganic components are stable to the high temperatures inside the CVD reactor, the organic components readily decompose upon heating to a sufficiently high temperature. When the reactant gas reaches the vicinity of a heated wafer, the organic components decompose, depositing the inorganic components on the surface of the wafer in the form of the epitaxial layers. 
     CVD reactors have various designs, including horizontal reactors in which wafers are mounted at an angle to the inflowing reactant gases; horizontal reactors with planetary rotation in which the reactant gases pass across the wafers; barrel reactors; and vertical reactors in which wafers are rotated at a relatively high speed within the reaction chamber as reactant gases are injected downwardly onto the wafers. The vertical reactors with high-speed rotation are among the most commercially important CVD reactors. 
     Among the desirable characteristics for any CVD reactor are heating uniformity, low reactor cycle time, good performance characteristics, longevity of the internal parts that are heated and/or rotated inside the reaction chamber, ease of temperature control and high temperature tolerance for component parts. Also important are the cost of the required component parts, ease of maintenance, energy efficiency and minimization of the heating assembly&#39;s thermal inertia. For example, if the heated components of a CVD reactor have high thermal inertia, certain reactor operations may be delayed until the heated components reach the desired temperatures. Therefore, lower thermal inertia of the heated components of the reactor increases the productivity since the throughput depends upon the reactor cycle time. Similarly, if the internal parts of the reactor that are rotated during the deposition undergo even a small degree of deformation, the reactor may exhibit excessive vibration during use, resulting in heightened maintenance requirements. 
     A typical prior art vertical CVD reactor is illustrated in FIG.  1 . As seen from FIG. 1, a wafer  10  is placed on a wafer carrier  12 , which is placed on a susceptor  14 . The wafer carrier  12  is usually made from a material that is relatively inexpensive and allows good manufacturing reproducibility. The wafer carrier may have to be replaced after a certain commercially suitable number of reactor cycles. The susceptor  14  is permanently mounted and supported by a rotatable spindle  16 , which enables rotation of the susceptor  14 , the wafer carrier  12  and the wafer  10 . The susceptor  14 , the wafer carrier  12  and the wafer  10  are located in an enclosed reactor chamber  18 . A heating assembly  20 , which may include one or more heating filaments  22 , is arranged below the susceptor  14 , and heated by passing an electric current through electrodes  25 . The heating assembly  20  heats the susceptor  14 , the wafer carrier  12  and, ultimately, the wafer  10 . The rotation of the wafer carrier  12  is intended to enhance the temperature uniformity across the deposition area, as well as the uniformity of the reactant gas introduced over the wafer  10  during the deposition. As the wafer-supporting assembly (spindle/susceptor/wafer carrier) rotates the heated wafer  10 , the reactant gas is introduced into the reaction chamber  18 , depositing a film on the surface of the wafer  10 . 
     The vertical CVD reactors having both the susceptor and the wafer carrier, similar to the reactor shown in FIG. 1, enjoy a widespread and successful use for a variety of CVD applications. For example, the Enterprise and Discovery reactors, made by Emcore Corporation of Somerset, N.J., are some of the most successful CVD reactors in the commercial marketplace. However, as discovered by the inventors of the present invention, the performance of such CVD reactors may be further improved for certain CVD applications. 
     First, the CVD reactor having both a susceptor and a wafer carrier contains at least two thermal interfaces. Referring to FIG. 1, these are the interfaces between the heating assembly  20  and the susceptor  14 , and between the susceptor  14  and the wafer carrier  12 . Research by the inventors of the present invention has shown that a substantial temperature gradient exists at these interfaces. For example, the temperature of the heating assembly  20  is higher than the temperature of the susceptor  14 , which, in turn, is higher than the temperature of the wafer carrier  12 . Consequently, the heating assembly  20  must be heated to a substantially higher temperature than the temperature desired for the wafer  10  during the deposition. The required higher temperatures of the heating assembly lead to higher energy consumption and faster deterioration of the heating assembly&#39;s components. In addition, the typical susceptor possesses a significant heat capacity, and thus a large thermal inertia, substantially increasing the time required to heat and cool down the wafer carrier  12 . This results in a longer reactor cycle and consequent reduction in the productivity of the reactor. Also, the inventors have determined that the longer reactor cycle time tends to result in a less precise and less flexible control of the wafer carrier&#39;s temperature, increasing the time necessary to stabilize the temperature of the wafer carrier prior to the deposition. 
     Second, in the CVD reactors similar to the reactor of FIG. 1, the susceptor  14  must withstand a large number of reactor cycles since it is permanently mounted in the reaction chamber, and typically may not be easily replaced without interrupting the reactor cycle, opening up the reactor and removing the parts that permanently attach the susceptor to the spindle, such as screws, bolts and the like. Therefore, the susceptors are usually made from highly temperature- and deformation-resistant materials, typically molybdenum. Such materials are very expensive and often exhibit a high thermal inertia. 
     Third, every additional interface in the wafer-supporting assembly increases the manufacturing tolerance requirements. For example, again with reference to FIG. 1, the spacing between the susceptor  14  and the wafer carrier  12  must be precise and uniform to produce the required uniform heating of the wafer. However, notwithstanding the high precision machining used in the manufacturing of the susceptors, the susceptor/wafer carrier spacing is likely to exhibit some non-uniformity due to both the over-the-time deformation of the susceptor and a certain unavoidable degree of deviation in the susceptor-to-susceptor manufacturing reproducibility. Further, a small degree of deformation of the susceptor is essentially unavoidable in the CVD reactors having both the susceptor and the wafer carrier due to the required non-uniform heating of the susceptor to produce the uniform heating of the wafer carrier. The accumulated deformation of the susceptor eventually may lead to an excessive vibration of the wafer-supporting assembly during rotation in the deposition process, and the resulting loss and destruction of coated wafers. 
     Fourth, in the CVD reactors with permanently mounted susceptors, the susceptor is typically rigidly attached to the spindle to minimize the vibration during the operation of the reactor. The spindle/susceptor connection is heated during the repeated operation of the reactor and sometimes becomes difficult to disassemble, complicating the maintenance and the replacement procedures. 
     Finally, the heavier is the wafer-supporting assembly, the larger is the mechanical inertia of the spindle. In turn, the high mechanical inertia increases the strain on the spindle-supporting assembly, reducing its lifetime. 
     Notwithstanding these limitations, the existing prior art CVD reactors having both a susceptor and a wafer carrier continue enjoying a successful and widespread use in the semiconductor industry. 
     Nevertheless, there exists a need for a CVD reactor that minimizes these limitations of the presently available CVD reactors while maintaining a high level of performance. 
     SUMMARY OF THE INVENTION 
     The present invention addresses this need by providing a novel CVD reactor in which the wafer carrier is placed on the rotatable spindle without a susceptor, and a related method of growing epitaxial layers in a CVD reactor. These novel reactors are likely to be used along with the presently available successful CVD reactors, such as the reactor shown in FIG.  1 . 
     It has been determined by the inventors that, in the prior art CVD reactors, for example, the prior art reactor shown in FIG. 1, substantial thermal losses occur at thermal interfaces in the wafer-supporting assembly. The research by the inventors also has shown that the increase in the temperature of the heating filament required to achieve the desired wafer temperature significantly reduces the lifetime of the heating filaments. 
     It has also been determined by the inventors that the presence of a permanently mounted susceptor in the prior art CVD reactors makes a significant contribution to the overall thermal and mechanical inertia of the wafer-supporting assembly. 
     The inventors have also determined that the rotatable spindle is a source of a substantial heat drain from the wafer-supporting assembly during the deposition. This heat drain may negatively affect the heating uniformity, the energy efficiency and the lifetime of the heating filaments. 
     Therefore, the present invention provides a novel CVD reactor, use of which minimizes these limitations of the presently available CVD reactors, as well as the limitations described in the Background section herein. 
     According to one aspect of the invention, an apparatus for growing epitaxial layers on one or more wafers by chemical wafer deposition is provided, and includes a reaction chamber, a rotatable spindle, a heating means for heating the wafers and a wafer carrier for supporting and transporting the wafers between a deposition position and a loading position. 
     In the loading position, the wafer carrier is separated from the rotatable spindle and the wafers may be placed on the wafer carrier for subsequent transfer to the deposition position. The loading position may be located inside the reaction chamber or outside the reaction chamber. Preferably, the loading position is located outside the reaction chamber. There may be one or more of such loading positions. 
     In the deposition position, the wafer carrier is detachably mounted on the rotatable spindle inside the reaction chamber, permitting chemical vapor deposition of the wafers placed on the wafer carrier. Preferably, in the deposition position, the wafer carrier is in direct contact with the spindle. Also, preferably, when in the deposition position, the wafer carrier is centrally mounted onto the spindle and supported only by the spindle. Most preferably, the wafer carrier is retained on the spindle by the force of friction, meaning that there exist no separate retaining means for retaining the wafer carrier on the spindle in the deposition position. However, the apparatus of the present invention may also include a separate retaining means for retaining the wafer carrier in the deposition position. The separate retaining means may be integral with the rotatable spindle or separate from both the spindle and the wafer carrier. 
     The wafer carrier of the invention may include a top surface and a bottom surface. The top surface of the wafer carrier may include one or more cavities for placing the wafers. The bottom surface may include a central recess for detachably mounting the wafer carrier onto the spindle. The central recess extends from the bottom surface of the wafer carrier toward the top surface of the wafer carrier to a recess end point. Preferably, the central recess does not reach the top surface of the wafer carrier and therefore the recess end point lies at a lower elevation than the top surface of the wafer carrier. 
     The rotatable spindle includes an upper end for mounting the wafer carrier inside the reaction chamber. In the deposition position, the upper end of the spindle is inserted into the central recess of the bottom surface of the wafer carrier. Preferably, to improve the rotational stability of the wafer carrier, the spindle supports the wafer carrier above the wafer carrier&#39;s center of gravity. 
     The apparatus of the invention may also include a mechanical means for transporting the wafer carrier between the deposition position and the loading position. The heating means of the apparatus of the invention may include one or more radiant heating elements. The apparatus of the invention may be used to process a single wafer or a plurality of wafers. 
     According to another aspect of the present invention, an apparatus for growing epitaxial layers on one or more wafers by chemical vapor deposition is provided; the apparatus including a reaction chamber, a rotatable spindle having an upper end located inside the reaction chamber, a wafer carrier and a radiant heating element disposed under the wafer carrier. The wafer carrier provides a support and transports the wafers. During the deposition, the wafer carrier is centrally and detachably mounted on the upper end of the spindle, where it is in a contact with the spindle. The wafer carrier is mounted in a manner that allows it to be readily removed from the upper end of the spindle. After the deposition is complete or at any other time, the wafer carrier may be removed from the upper end of the spindle and transported to a position for loading or unloading wafers. There may be one or a plurality of such loading positions. The loading position may be located inside the reaction chamber or outside the reaction chamber. Preferably, the wafer carrier is in a direct contact with the upper end of the spindle and has a top surface that includes one or a plurality of cavities for supporting a plurality of wafers. Therefore, either a single wafer or a plurality of wafers may be deposited in the reactor of the invention at the same time. The wafer carrier is transported between the position mounted onto the upper end of the spindle and the loading position by mechanical means, typically a robotic arm. 
     In a preferred embodiment of this aspect of the invention, the bottom surface of the wafer carrier includes a central recess, which extends upward from the bottom surface in a direction of the top surface of the wafer carrier, terminating in a recess end point. The central recess does not reach the top surface of the wafer carrier. Therefore, the recess end point is located at a lower elevation than the top surface of the wafer carrier. When the wafer carrier is mounted onto the upper end of the spindle, the upper end of the spindle is inserted into the central recess in the bottom surface of the wafer carrier. The insertion provides a point of conduct between the spindle and the wafer carrier, allowing the wafer carrier to be supported by the spindle. To improve the rotational stability of the wafer carrier, the point of contact between the spindle and the wafer carrier having the highest elevation is located above the center of gravity of the wafer carrier. 
     In the most preferred embodiment of this aspect of the invention, the wafer carrier has a substantially round shape. In this embodiment, the top surface and the bottom surface of the wafer carrier are substantially parallel to each other. Of course, the top surface of the wafer carrier may include cavities for placing the wafers, and the bottom surface of the wafer carrier includes a recess for mounting the wafer carrier onto the upper end of the spindle, and other indentations or raised features are not excluded on either the top surface or the bottom surface of the wafer carrier. 
     The spindle according to this embodiment of the invention has a substantially cylindrical shape and an axis of rotation. The bottom surface of the wafer carrier, when mounted on the spindle, is substantially perpendicular to the axis of rotation of the spindle. The upper end of the spindle preferably terminates in a substantially flat top surface, which is also substantially perpendicular to the axis of rotation of the spindle. Preferably, the upper end of the spindle narrows toward the substantially flat top surface of the spindle. Therefore, the narrow portion of the upper end of the spindle is located near the substantially flat top surface of the spindle, and the wide portion of the spindle is located distal from the substantially flat top surface of the spindle. 
     As has been stated, the spindle is a source of a significant heat drain from the wafer-supporting assembly. The present invention provides the novel way of reducing this heat drain. To this end, in a preferred embodiment, the spindle has a cavity extending vertically downward from the substantially flat top surface of the upper end of the spindle to a cavity end point, which is disposed at a predetermined depth. The cavity in the spindle has a substantially cylindrical shape and is substantially coaxial with the spindle. The predetermined depth of the cavity in the spindle is preferably from about 3 to about 4 spindle diameters. This hollow construction of the upper end of the spindle allows the reduction of the heat drain from the wafer-supporting assembly. 
     To further reduce the heat drain, a specific arrangement of the radiant heating elements is provided. In this arrangement, the radiant heating element includes a first radiant heating element that is substantially coaxial with the rotatable spindle and has a top surface proximal to the bottom surface of the wafer carrier, an internal circumference and an external circumference. The internal circumference of the first radiant heating element defines a round opening around the spindle. This arrangement of the radiant heating elements of the invention may also include a second radiant heating element substantially coaxial with the first radiant heating element and the spindle, and located between the first radiant heating element and the spindle. The second radiant heating element defines an external circumference, the radius of which is smaller than the radius of the internal circumference of the first radiant heating element. Most preferably, the top surface of the second radiant heating element is located at substantially the same elevation as the top surface of the first radiant heating element, and the bottom surface of the second radiant heating element is located at the same elevation as the cavity end point of the rotatable spindle. The second radiant heating element allows heating of the upper end of the spindle, which along with the hollow construction of the upper end of the spindle reduces the heat drain from the wafer-supporting assembly. The reactor of the invention may also include a radiant heating shield. 
     According to yet another aspect of the invention, a method of growing epitaxial layers on one or more wafers by chemical wafer deposition is provided. According to the method of the invention, the chemical wafer deposition is carried out in a reactor chamber that includes a rotatable spindle having an upper end disposed inside the reaction chamber. To carry out the deposition, the method includes 
     a) providing a wafer carrier having a surface for retaining one or more wafers; 
     b) placing one or more wafers on the surface of the wafer carrier in a loading position, in which the wafer carrier is separated from the spindle; 
     c) transporting the wafer carrier towards the spindle; 
     d) detachably mounting the wafer carrier on the upper end of the spindle for rotation therewith; and 
     e) rotating the spindle and the wafer carrier located thereon while introducing one or more reactants to the reaction chamber and heating the wafer carrier. 
     Preferably, the method of the invention further includes removing the wafer carrier from the upper end of the spindle to unload the wafers. The step of detachably mounting the wafer carrier may include directly mounting the wafer carrier, and/or centrally mounting the wafer carrier on the upper end of the spindle. Preferably, the wafer carrier is mounted on the upper end of the spindle above the wafer carrier&#39;s center of gravity and retained therein only by a force of friction. Preferably, the loading position is located outside the reaction chamber. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     A more accurate appreciation of the subject matter of the present invention and the various advantages thereof can be realized by reference to the following detailed description, which makes reference to the accompanying drawings in which: 
     FIG. 1 is a highly schematic front cross-sectional view of a CVD reactor of the prior art; 
     FIG. 2 is a highly schematic front cross-sectional view of a wafer-supporting assembly of the present invention, showing that the wafer carrier may be transported between the loading position and the deposition position, where it is placed on the spindle without a susceptor; 
     FIGS. 3A and 3B are highly schematic views of an apparatus of the present invention, showing that the wafer carrier may be transferred between a loading position and a deposition position through a gate valve; 
     FIG. 4 is a highly schematic diagram of the wafer-supporting assembly of the prior art, showing a susceptor permanently attached to the upper end of the spindle, the wafer carrier, the heating element and the radiant heating shield; 
     FIG. 5A is a highly schematic front cross-sectional view of the wafer-supporting assembly of the present invention, showing the wafer carrier mounted on the upper end of the spindle in the deposition position; 
     FIG. 5B is a top perspective view of the wafer carrier of the variant of the invention shown in FIG. 5A; 
     FIG. 5C is a top perspective view of the wafer-supporting assembly of the variant of invention shown in FIGS. 5A and 5B, with the wafer carrier being in the loading position, in which the wafer carrier is removed from the spindle, showing the upper end of the spindle and the primary heating element; 
     FIG. 5D is an elevated bottom view of the wafer carrier of the variant of the invention shown in FIGS. 5A-5C; 
     FIG. 6A is a highly schematic front cross-sectional view of another variant of the invention; 
     FIG. 6B is a top perspective top view of the spindle of the variant of the invention shown in FIG. 6A; 
     FIG. 7A is a highly schematic cross-sectional view of the wafer-supporting assembly of another variant of the invention, showing a cavity in the upper end of the spindle for reducing the heat drain from the wafer-supporting assembly through the spindle; 
     FIG. 7B is a top perspective view of an upper end of the spindle according to the variant shown in FIG. 7A; 
     FIG. 7C is a highly schematic front cross-sectional view of the spindle of the variant of the invention shown in FIGS. 7A and 7B; 
     FIG. 7D is a highly schematic front cross-sectional view of the relationship between the spindle and the wafer carrier of the variant of the invention shown in FIGS. 7A-7C; 
     FIG. 8A is a highly schematic front cross-sectional view of the wafer-supporting assembly of the invention showing a novel arrangement of the spindle and the radiant heating elements, use of produces a decrease in the heat drain from the wafer-supporting assembly through the spindle; 
     FIG. 8B is a top perspective top view of the wafer-supporting assembly of the invention, with the wafer carrier being in the loading position, showing the spindle/heating element arrangement for a variant of the invention shown in FIGS. 7A-7C; 
     FIGS. 9A,  9 B and  9 C show possible variants of the retaining means of the invention for retaining the wafer carrier on the upper end of the spindle in the deposition position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The general concept of the invention is shown in FIG.  2 . The reactor of the invention includes a reaction chamber  100 , a wafer carrier  110 , a rotatable spindle  120  and heating means  170 . The wafer carrier  110  is transported between a loading position L and a deposition position D. In the position L, the wafer carrier  110  is separated from the spindle  120 . In the position D, the wafer carrier  110  is mounted on the rotatable spindle  120 . Preferably, the wafer carrier  110  is mounted on an upper end  180  of the spindle  120 . 
     According to the invention, in position D, the wafer carrier is mounted in any manner that would allow it to be readily separated from the spindle  120  in the normal course of operating the reactor of the invention during the reactor cycle. Such manner of mounting the wafer carrier  110  excludes such means of attaching the wafer carrier  110  to the spindle  120  as screws, bolts and the like, the use of which would necessitate the opening of the reactor and the removal of such parts or pieces that would permanently attach the wafer carrier  110  to the spindle  120 . Preferably, in position D, the wafer carrier  110  is retained on the spindle  120  only by a force of friction, with no separate retaining means. 
     In contrast to the prior art CVD reactor shown in FIG. 1, the reactor of the present invention does not include a susceptor. Preferably, the wafer carrier  110  is directly mounted onto the spindle  120 , i.e., in the position D, a direct contact is established between the wafer carrier  110  and the spindle  120 . The invention does not exclude the possibility that intermediate elements may be present between the spindle  120  and the wafer carrier  110 , for example the elements that would facilitate retaining the wafer carrier  110  on the spindle  120 , such as rings, retainers and the like, as long as these intermediate elements do not interfere with the removal or detachment of the wafer carrier from the position D in the normal course of the operation of the reactor. 
     In the position L, wafers  130  are loaded onto the wafer carrier  110  prior to the transfer of the wafer carrier  110  and the wafer  130  to the reaction chamber  100 . The loading position L may be located inside or outside of the reaction chamber  100 . Although only one position L is shown in FIG. 2, there may be one or more such positions. 
     The wafer carrier  110  may include a top surface  111  for placing wafers. The reactor of the invention may be used for coating a single wafer or a plurality of wafers. Accordingly, the top surface  111  of the wafer carrier  110  may be adopted either for a single wafer or a plurality of wafers in any manner known in the art. Preferably, the top surface  111  has a plurality of cavities for placing a plurality of wafers  130 . 
     FIGS. 3A and 3B show an example of the transporting operation for the wafer carrier  110 . As can be seen with reference to FIG. 3A, the loading position L for the wafer carrier  110  is located in a separate loading chamber  150  that is connected to reaction chamber  100  by a gate valve  160 . The loading chamber  150  has an exhaust opening  108  that allows for separate ventilation of the loading chamber  150  without interrupting the reactor cycle. In position L, the wafer carrier  110  is loaded with uncoated wafers  130 . Thereafter, the wafer carrier  110  is transported through the gate valve  160  to the reaction chamber  100 . 
     The reaction chamber  100  may include a top flange  104  and a bottom plate  102 . The spindle  120  is inserted through an opening in the base plate  102  so that the upper end  180  of the spindle  120  is inside the reaction chamber  100 . The spindle  120  may be connected to rotating means  109 , such as an electric motor. The reaction chamber  100  may also include an exhaust opening  106  and other elements known in the art. 
     As shown in FIG. 3B, in the deposition position D, the wafer carrier  110  with uncoated wafers  130  is mounted on the upper end  180  of the spindle  120 , and may be rotated together with the spindle  120  during the operation of the reactor. The precursor chemicals then may be supplied to the reaction chamber  100  through the top flange  104 , while the wafer carrier  110  and the wafers  130  are rotated by the spindle  120  and heated by the heating means  140 . Preferably, only the spindle  120  supports the wafer carrier  110  in the position D. 
     After the deposition is complete, the wafer carrier  110  is transported back to the position L to unload the coated wafers and to load new uncoated wafers for subsequent transfer to the position D in the reaction chamber  100 . This reactor cycle may be repeated to process a larger quantity of wafers. 
     The wafer carrier  110  may be transported between the positions D and L in any manner known in the art. For example, the reactor of the invention may include a mechanical means for the transfer, for example, a robotic arm or an autoloader. For example, the suitable mechanical means for transferring the wafer carrier of the present invention is described in co-assigned U.S. Pat. No. 6,001,183, which is incorporated herein by reference in its entirety. 
     Preferably, the wafer carrier  110  has a round or a rectangular shape; most preferably the wafer carrier  110  has a round shape. The wafer carrier may be made from any suitable material capable of withstanding the high temperatures inside the reaction chamber of the CVD reactor, such as graphite or molybdenum. Of course, cost considerations may affect the choice of the suitable material. The absence of the susceptor/wafer carrier interface, as explained above, broadens the choice of the suitable materials to include less expensive alternatives. 
     The heating means  140  preferably include one or more radiant heating elements. Use of a plurality of radiant heating elements permits multi-zone heating of the wafer carrier  110 , better temperature control and coating uniformity. The radiant heating elements may be arranged in any manner known to those skilled in the art. The preferred arrangement will be shown with reference to the specific embodiments of he invention. 
     The CVD reactor of the present invention has a number of important advantages. The absence of a permanently mounted susceptor reduces the thermal inertia of the wafer-supporting assembly, resulting in a reduction of the reactor cycle time and a better control over the wafer temperatures. Also, the elimination of one of the thermal interfaces present in the prior art reactors (i.e., heating element/susceptor interface) reduces the temperature gradient between the heating element or elements and the wafer, increasing the energy efficiency of the reactor and the lifetime of the heating elements. Further, the lower weight of the wafer-supporting assembly reduces its mechanical inertia and therefore the strain on the spindle. The elimination of the contact between the susceptor and the wafer carrier that requires high precision machining and still may exhibit some non-uniformity results in lower manufacturing tolerance requirements and better wafer-to-wafer temperature uniformity. For the same reasons, the wafer carrier of the present invention may be made of less expensive materials, reducing the overall cost of the reactor. Also, the possibility of the vibration of the wafer-supporting assembly is minimized due to the good rotational stability of the wafer carrier of the invention. For the same reasons, the lower vibration leads to lower losses of the coated wafers. These and other advantages of the invention will be explained with reference to the specific embodiments and variants of the invention. 
     For the purpose of illustration, the present invention will be described with reference to the specific embodiments. It should be understood that these embodiments are not limiting and the present invention encompasses any subject matter that is within the scope of the appended claims. 
     FIG. 4 shows a wafer-supporting assembly of the prior art. The susceptor  14  is permanently mounted onto the spindle  16  by screws  70 . During the deposition, the wafer carrier  12  is placed onto the susceptor  14 . The heating arrangement may include a primary heating element  25  and secondary heating elements  26  and  27 . As described above, the inventors have discovered that the presence of the susceptor  14  and the resulting heating element/susceptor and susceptor wafer carrier interfaces effect the performance of the reactor. 
     Therefore, all embodiments of the reactor of the invention do not include a permanently mounted susceptor. FIGS. 5A,  5 B,  5 C and  5 D show a variant of the wafer-supporting assembly for an embodiment of the reactor of the invention. As seen from FIG. 5A, the reactor includes the reaction chamber  100 , a spindle  250  having an upper end  280  located inside the reaction chamber  100 , a wafer carrier  200  and a radiant heating element  140 . FIG. 5A shows the wafer carrier  200  in the deposition position. 
     The wafer carrier  200  has a top surface  201  and a bottom surface  202 . The top surface  201  includes cavities  220  for placing wafers. As shown in FIG. 5B, the wafer carrier  200  has a round shape. The bottom surface  202  is parallel to the top surface  201 , except in the regions defined by the cavities  220 . As seen from FIG. 5D, the bottom surface  202  of the wafer carrier  200  includes a central recess  290 . The central recess  290  extends upwards from the bottom surface  202  and terminates in a flat surface  291  surrounded by recess walls  292 . 
     The spindle  250  has a cylindrical shape and an axis of rotation  255 . FIG. 5C shows the upper end  280  of the spindle  250  and the radiant heating element  140  when the wafer carrier  200  is separated from the spindle, such as when the wafer carrier is in the loading position L. As seen from FIG. 5C, the upper end  280  of the spindle  250  has spindle walls  282  that terminate in a top surface  281 . FIG. 5C also shows the radiant heating element  140  having a top surface  141 . The radiant heating element  140  is positioned in such a manner that, during the deposition, the top surface  141  is capable of heating the wafer carrier  200 , which is mounted on the upper end  280  of the spindle  250  above the radiant heating element  140 . 
     In the deposition position D, the upper end  280  of the spindle  250  is inserted in the central recess  290  of the wafer carrier  200 . The flat surface  281  of the spindle  250  lies adjacent to the flat surface  291  of the recess  290 , while the spindle wall  282  is in a direct contact with the recess wall  292 . Upon a complete insertion, the flat surface  281  of the upper end  280  of the spindle  250  is placed in a direct contact with the flat surface  291  of the central recess  290 . Preferably, the highest point or points of contact between the wafer carrier  200  and spindle  250  (in this variant of the invention, the area of contact between the surfaces  291  and  281 ) lies above the center of gravity of the wafer carrier  200 , contributing to the rotational stability of the wafer carrier. 
     The insertion of the upper end  280  of the spindle  250  into the recess  290  creates a friction fit between the spindle wall  282  and the recess wall  292  that allows the rotation of the wafer carrier  200  by the spindle  250  without separate retaining means. During the deposition, the spindle is rotated thereby rotating the wafer carrier  200  and the wafers placed in the cavities  220 . Retaining the wafer carrier on the spindle only by friction allow the minimization of the mechanical inertia of the carrier-spindle assembly and the resulting decrease of the strain on the spindle. If the spindle  250  have to be suddenly stopped and the force of inertia exerted upon the wafer carrier exceeds the force of friction between the upper end  280  of the spindle  250 , the wafer carrier  200  may rotate independently from the spindle, reducing the strain on the spindle. 
     However, the present invention also contemplates the use of a separate retaining means in the wafer-supporting assembly. Examples of such separate retaining means are shown in FIGS. 9A,  9 B and  9 C. As shown in FIG. 9A, the upper end  280  of the spindle  250  may include indentations  289 , extending vertically downward from the flat surface  281 . The wafer carrier  200  may have matching indentations  299  in the flat surface  291  of the recess  290 . The indentations  299  extend vertically upwards from the flat surface  291 . Fingers  800  may then be inserted in the indentations  289  and  299 , tying the wafer carrier  200  and the spindle  250  together. Alternatively, as seen in FIG. 9B, the flat surface  281  of the upper end  280  of the spindle  250  may include raised features  900 , which are integral with the upper end of the spindle. In the deposition position of the wafer carrier  200 , the features  900  are inserted into matching indentations  299  in the flat surface  291  of the recess  290 . Preferably, as seen from FIG. 9C, the retaining means include two fingers  800  or two raised features  900 , and the corresponding number of matching indentations. 
     Another variant of the wafer-supporting assembly is shown in FIGS. 6A and 6B. This variant is similar to the variant shown in FIGS. 5A-5D, with the exception of the wafer carrier/spindle relationship in the deposition position of the wafer carrier. According to this variant of the invention, a bottom surface  302  of wafer carrier  300  has a central recess  390 . The recess  390  includes a narrow portion  392  and a broad portion  391 . The narrow portion  392  terminates in a flat surface  395 . 
     As seen in FIG. 6B, an upper end  480  of the spindle  400  includes a narrow portion  485  and a broad portion  486 . The narrow portion  485 , that includes the spindle wall  482 , terminates in a top surface  481 . In the deposition position, the top surface  481  of the upper end  480  of the spindle  400  is inserted into the central recess  390  of the wafer carrier  300 . The difference between this variant of the wafer-supporting assembly and the previously described variant shown in FIGS. 5A-5D is principally in the shape of the central recess  390  and the upper end  480  of the spindle  400 . Similarly to the variant of the invention shown in FIGS. 5A-5D, the wafer carrier  300  is retained on the upper end  480  of the spindle  400  by the force of friction. In mounting the wafer carrier  300  in the deposition position, the upper end  480  of the spindle  400  is inserted into the central recess  390  until there is a tight fit between the spindle wall  482  and the walls of the recess  390 , which creates a force of friction for retaining the wafer carrier  300  in the deposition position. It also should be noted that the top surface  481  of the spindle  400  may or may be in a direct contact with the surface  395  of the central recess  390 , as will be shown below with reference to FIG. 7A describing another, but similar variant of the wafer-supporting assembly. 
     As explained above, the spindle itself is often a source of a heat drain from the wafer-supporting assembly. Where a wafer carrier for processing a single wafer is mounted on a rotatable spindle, the presence of the spindle has an effect on the temperature of the wafers. The wafer carrier is centrally mounted on the spindle so that the central region of the single wafer cavity on the top surface of the wafer carrier overlies the rotatable spindle. As the spindle draws heat away from the region of the wafer carrier in the central region, the temperature gradient created in the wafer carrier is transferred to the overlying single wafer cavity, resulting in a non-uniform temperature distribution across the surface of the wafer being processed. It is a lesser problem where a plurality of wafers are processed simultaneously using a single wafer carrier since, as can be seen from FIG. 5B, such wafer carrier includes a plurality of wafer cavities arranged symmetrically around the center of the wafer carrier, and no one wafer cavity overlies the axial center of the wafer carrier where the spindle is connected. Hence, the fact that the spindle draws heat away from the center portion of the wafer carrier interferes with the temperature of the wafers positioned in the wafer cavities to a lesser degree than with a single wafer processing. However, even with wafer carriers such as shown in FIG. 5B, the heat drain may create some heating non-uniformity across the wafer carrier&#39;s surface. This non-uniformity may be increased for the reactors of the present invention since the wafer carrier is placed on the upper end of the spindle without an intermediate susceptor that is present in the prior art reactors. 
     Therefore, the present invention provides a variant of the wafer-supporting assembly that minimizes the heat drain through the rotatable spindle. This variant is shown in FIGS. 7A,  7 B,  7 C and  7 D. The upper end  580  of the spindle  500  includes a cavity  550 , extending downwards from the top surface  581 . The cavity  550  is substantially coaxial with the spindle  500 . FIG. 7B shows the upper end  580  of the spindle  500  without the wafer carrier  300 . The cavity  550  extends to a cavity end point  570 , which may constitute a flat surface  560  or otherwise. The depth h of the cavity  550  is preferably equal to from about 3 to about 4 of the spindle cavity diameters d (FIG.  7 C). As seen from FIGS. 7B and 7C, the upper end  580  of the spindle  500  has a hollow construction, and the contact area between the top surface  581  and the surfaces of the recess  390  is minimized. This reduces the heat drain from the wafer carrier  300  through the spindle  500 . Further reduction to the heat drain is obtained if the flat surface  395  of the recess  390  is not in contact with the top surface  581  of the spindle  500 , as shown in FIG.  7 A. 
     FIG. 7D shows a preferred relationship between the spindle and the wafer carrier for this variant of the invention. As stated earlier, the point of contact between the wafer carrier and the spindle is preferably above the center of gravity of the wafer carrier. As seen from FIG. 7D, this arrangement may be achieved via an adjustment in the manufacturing tolerances for the upper end of the spindle and the central recess of the wafer carrier. In general, it is difficult to avoid the presence of a small degree of deviation from the intended angle α (FIG.  7 D). However, the bias of the manufacturing tolerance A may be manipulated. Thus, preferably, in the manufacturing process, the angle α for the central recess of the wafer carrier and for the upper end of the spindle is set identically. However, for the central recess of the wafer carrier, the manufacturing tolerance A is given a positive bias, whereas for the upper end of the spindle, the manufacturing tolerance A is given a negative bias. Together with the appropriate choice of the depth for the central recess of the wafer carrier, this minimizes the contact between the wafer carrier and the spindle, and allows the point of contact between the wafer carrier and the spindle to be above the center of gravity of the wafer carrier. 
     To yet further reduce the heat drain through the spindle, the reactors of the invention may be equipped with a novel arrangement of radiant heating elements shown in FIGS. 8A and 8B. FIG. 8A shows a primary radiant heating element  140  and a secondary heating element  700 . The secondary heating element  700  has a top surface  701  and a bottom surface  702 , and is shaped around the hollow upper end  680  of the spindle  600 . The bottom surface  702  of the secondary heating element  700  is located at the same elevation as the endpoint  570  of the cavity  550 , thereby, upon heating, creating a heat barrier against the heat drain from the wafer-supporting assembly. Thus, the hollow upper end  680  of spindle  600  is heated by the secondary heating element  700 , further reducing the heat drain through the spindle. The top surface  701  of the secondary heating element  700  is located at the same elevation as the top surface  141  of the primary radiant heating element  140 . As seen from FIG. 8B, the upper end  680  of the spindle  600  may be the same as the upper end of the spindle in the variant of the invention shown in FIGS. 6A and 6B. 
     Although the present invention has been described herein with reference to the particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.